A comparison between water uptake and root length density in winter wheat: effects of root density and rhizosphere properties

Physiological and genomic insights into a psychrotrophic drought-tolerant bacterial consortium for crop improvement in cold, semiarid regions

The agricultural land in the Indian Himalayan region (IHR) is susceptible to various spells of snowfall, which can cause nutrient leaching, low temperatures, and drought conditions. The current study, therefore, sought an indigenous psychrotrophic plant growth-promoting (PGP) bacterial inoculant with the potential to alleviate crop productivity under cold and drought stress. Psychrotrophic bacteria preisolated from the night-soil compost of the Lahaul Valley of northwestern Himalaya were screened for phosphate (P) and potash (K) solubilization, nitrogen fixation, indole acetic acid (IAA) production, siderophore and HCN production) in addition to their tolerance to drought conditions for consortia development. Furthermore, the effects of the selected consortium on the growth and development of wheat (Triticum aestivum L.) and maize (Zea mays L.) were assessed in pot experiments under cold semiarid conditions (50 % field capacity). Among 57 bacteria with P and K solubilization, nitrogen fixation, IAA production, siderophore and HCN production, Pseudomonas protegens LPH60, Pseudomonas atacamensis LSH24, Psychrobacter faecalis LUR13, Serratia proteamaculans LUR44, Pseudomonas mucidolens LUR70, and Glutamicibacter bergerei LUR77 exhibited tolerance to drought stress (-0.73 MPa). The colonization of wheat and maize seeds with these drought-tolerant PGP strains resulted in a germination index >150, indicating no phytotoxicity under drought stress. Remarkably, a particular strain, Pseudomonas sp. LPH60 demonstrated antagonistic activity against three phytopathogens Ustilago maydis, Fusarium oxysporum, and Fusarium graminearum. Treatment with the consortium significantly increased the foliage (100 % and 160 %) and root (200 % and 133 %) biomasses of the wheat and maize plants, respectively. Furthermore, whole-genome sequence comparisons of LPH60 and LUR13 with closely related strains revealed genes associated with plant nutrient uptake, phytohormone synthesis, siderophore production, hydrogen cyanide (HCN) synthesis, volatile organic compound production, trehalose and glycine betaine transport, cold shock response, superoxide dismutase activity, and gene clusters for nonribosomal peptide synthases and polyketide synthetases. With their PGP qualities, biocontrol activity, and ability to withstand environmental challenges, the developed consortium represents a promising cold- and drought-active PGP bioinoculant for cereal crops grown in cold semiarid regions.

The role of drought response genes and plant growth promoting bacteria on plant growth promotion under sustainable agriculture: A review

Drought is a major stressor that poses significant challenges for agricultural practices. It becomes difficult to meet the global demand for food crops and fodder. Plant physiology, physico-chemistry and morphology changes in plants like decreased photosynthesis and transpiration rate, overproduction of reactive oxygen species, repressed shoot and root shoot growth and modified stress signalling pathways by drought, lead to detrimental impacts on plant development and output. Coping with drought stress requires a variety of adaptations and mitigation techniques. Crop yields could be effectively increased by employing plant growth-promoting rhizobacteria (PGPR), which operate through many mechanisms. These vital microbes colonise the rhizosphere of crops and promote drought resistance by producing exopolysaccharides (EPS), 1-aminocyclopropane-1-carboxylate (ACC) deaminase and phytohormones including volatile compounds. The upregulation or downregulation of stress-responsive genes causes changes in root architecture due to acquiring drought resistance. Further, PGPR induces osmolyte and antioxidant accumulation. Another key feature of microbial communities associated with crops includes induced systemic tolerance and the production of free radical-scavenging enzymes. This review is focused on detailing the role of PGPR in assisting plants to adapt to drought stress.

Plant growth promoting bacteria (PGPB)-induced plant adaptations to stresses: an updated review

Plants and bacteria are co-evolving and interact with one another in a continuous process. This interaction enables the plant to assimilate the nutrients and acquire protection with the help of beneficial bacteria known as plant growth-promoting bacteria (PGPB). These beneficial bacteria naturally produce bioactive compounds that can assist plants' stress tolerance. Moreover, they employ various direct and indirect processes to induce plant growth and protect plants against pathogens. The direct mechanisms involve phytohormone production, phosphate solubilization, zinc solubilization, potassium solubilization, ammonia production, and nitrogen fixation while, the production of siderophores, lytic enzymes, hydrogen cyanide, and antibiotics are included under indirect mechanisms. This property can be exploited to prepare bioformulants for biofertilizers, biopesticides, and biofungicides, which are convenient alternatives for chemical-based products to achieve sustainable agricultural practices. However, the application and importance of PGPB in sustainable agriculture are still debatable despite its immense diversity and plant growth-supporting activities. Moreover, the performance of PGPB varies greatly and is dictated by the environmental factors affecting plant growth and development. This review emphasizes the role of PGPB in plant growth-promoting activities (stress tolerance, production of bioactive compounds and phytohormones) and summarises new formulations and opportunities.

Evaluation of soil nutrients and berry quality characteristics of Cabernet Gernischet (Vitis vinifera L.) vineyards in the eastern foothills of the Helan Mountains, China

Soil is the basis of the existence of fruit tree and soil nutrients plays a crucial role in plant growth and berry quality. To investigate the characteristics and interrelationships between soil nutrients and berry quality in Cabernet Gernischet vineyards, this study focused on seven representative vineyards in the eastern foothills of the Helan Mountains. Fifteen soil physicochemical factors and 10 berry quality factors were measured, followed by variation analysis, correlation analysis, multiple linear regression (MLR), partial-least squares regression (PLSR), principal component analysis (PCA), and systematic cluster analysis. We identified the main soil nutrient indicators influencing berry quality and developed linear regression equations. Utilizing PCA, a comprehensive evaluation model for berry quality was constructed, which enabled the calculation and ranking of integrated berry quality scores. The results indicated that soil nutrients in the vineyards of the eastern foothills of the Helan Mountains are relatively deficient and alkaline. The coefficient of variation for soil nutrient factors ranged from 3.19 to 118.08% and for berry quality factors 2.41-26.37%. Correlation analysis revealed varying degrees of correlation between soil nutrient indicators and fruit quality indicators. PCA extracted four principal components with a cumulative contribution rate of 91.506%. Based on the scores of these components and their corresponding weights, a comprehensive model for evaluating the quality of Cabernet Gernischet berries was established. The vineyards were ranked from the highest to the lowest combined scores as Zhenbeibu (ZBB), Yuquanying (YQY), Dawukou (DWK), Beihaizi (BHZ), Shuxin (SX), Huangyangtan (HYT), and Hongde (HD). These findings provide insights into soil nutrient management and comprehensive quality assessment of vineyards in the eastern foothills of the Helan Mountains. In conclusion, this study offers a theoretical foundation for vineyard managers to enhance grape berries quality through soil nutrient management. This will aid in the diagnosis of vineyard soil nutrition and the efficient use of fertilizers, with critical practical and theoretical implications for the meticulous management of vineyards and the production of high-quality wines.

Impact of Drought Stress on Plant Growth and Its Management Using Plant Growth Promoting Rhizobacteria

Drought stress is a significant environmental challenge affecting global agriculture, leading to substantial reductions in crop yields and overall plant productivity. It induces a cascade of physiological and biochemical changes in plants, including reduced water uptake, stomatal closure, and alterations in hormonal balance, all of which contribute to impaired growth and development. Drought stress diminishes crop production by impacting crucial plant metabolic pathways. Plants possess the ability to activate or deactivate specific sets of genes, leading to changes in their physiological and morphological characteristics. This adaptive response enables plants to evade, endure, or prevent the effects of drought stress. Drought stress triggers the activation of various genes, transcription factors, and signal transduction pathways in plants. In this context, imposing plant growth-promoting rhizobacteria (PGPR) emerges as a promising strategy. PGPR, employing diverse mechanisms such as osmotic adjustments, antioxidant activity, and phytohormone production, not only ensures the plant's survival during drought conditions but also enhances its overall growth. This comprehensive review delves into the various mechanisms through which PGPR enhances drought stress resistance, offering a thorough exploration of recent molecular and omics-based approaches to unravel the role of drought-responsive genes. The manuscript encompasses a detailed mechanistic analysis, along with the development of PGPR-based drought stress management in plants.

Osmolyte-producing microbial biostimulants regulate the growth of Arachis hypogaea L. under drought stress

Globally, drought stress poses a significant threat to crop productivity. Improving the drought tolerance of crops with microbial biostimulants is a sustainable strategy to meet a growing population's demands. This research aimed to elucidate microbial biostimulants' (Plant Growth Promoting Rhizobacteria) role in alleviating drought stress in oil-seed crops. In total, 15 bacterial isolates were selected for drought tolerance and screened for plant growth-promoting (PGP) attributes like phosphate solubilization and production of indole-3-acetic acid, siderophore, hydrogen cyanide, ammonia, and exopolysaccharide. This research describes two PGPR strains: Acinetobacter calcoaceticus AC06 and Bacillus amyloliquefaciens BA01. The present study demonstrated that these strains (AC06 and BA01) produced abundant osmolytes under osmotic stress, including proline (2.21 and 1.75 µg ml- 1), salicylic acid (18.59 and 14.21 µg ml- 1), trehalose (28.35 and 22.74 µg mg- 1 FW) and glycine betaine (11.35 and 7.74 mg g- 1) respectively. AC06 and BA01 strains were further evaluated for their multifunctional performance by inoculating in Arachis hypogaea L. (Groundnut) under mild and severe drought regimes (60 and 40% Field Capacity). Inoculation with microbial biostimulants displayed distinct osmotic-adjustment abilities of the groundnut, such as growth parameters, plant biomass, photosynthetic pigments, relative water content, proline, and soluble sugar in respective to control during drought. On the other hand, plant sensitivity indexes such as electrolyte leakage and malondialdehyde (MDA) contents were decreased as well as cooperatively conferred plant drought tolerance by induced alterations in stress indicators such as catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD). Thus, Acinetobacter sp. AC06 and Bacillus sp. BA01 can be considered as osmolyte producing microbial biostimulants to simultaneously induce osmotic tolerance and metabolic changes in groundnuts under drought stress.

Multi-trait efficiency and interactivity of bacterial consortia used to enhance plant performance under water stress conditions

Water stress is a major limiting factor for agricultural production under current and projected climate change scenarios. As a sustainable strategy, plant growth-promoting bacterial consortia have been used to reduce plant water stress. However, few studies have examined the effects of stress on multi-trait efficiency and interactivity of bacterial species. In this study, we used several in-vitro experiments, plant assays and greenhouse trials to investigate the effects of stress and bacterial consortia on 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) activities, indole-3-acetic acid (IAA) production and plant growth-promoting traits (Phosphate-solubilization, starch hydrolysis, siderophores and ammonium production). We further assessed biofilm formation and the chemotactic behaviour in response to ACC. A total of fifteen ACCD rhizobacteria with multiple growth-promoting traits from the dominant plant species from the hyperseasonal Aripo Savannas were screened in this study. Five of the isolates were further analyzed based on their ACCD activities and were tested in single and dual consortium to assess their abilities in promoting growth under simulated drought stress (-0.35 MPa) and chemically induced ACC conditions (0.03 mM). Our findings showed that bacteria which produce high concentrations of IAA affected the isolates' ability to promote growth under stress, irrespective of microbial combination with ACCD activity above the minimal threshold of 20 nmol α-ketobutyrate mg-1 h-1. Biofilm production with co-culture interaction varied greatly across treatments, however, the general trend showed an increase in biofilm under stress induce conditions. The best performing co-culture, UWIGT-83 and UWIGT-120 (Burkholderia sp.) showed enhanced growth in germination assays and in greenhouse trials with Capsicum chinense (Moruga red hot peppers) under drought stress, when compared to non-inoculated treatments. The findings highlight the importance of testing interactivity of bacterial species with multiple growth promoting traits under stress conditions; and proposed the use of ACC growth media as a novel biofilm screening method for selecting potential stress plant growth-promoting bacteria. Better screening strategies for appropriate plant growth-promoting bacteria may narrow the inconsistency observed between laboratory and field trials.

Serratia spp. as plant growth-promoting bacteria alleviating salinity, drought, and nutrient imbalance stresses

In agricultural environments, plants are often exposed to abiotic stresses including temperature extremes, salt stress, drought, and heavy metal soil contamination, which leads to significant economic losses worldwide. Especially salt stress and drought pose serious challenges since they induce ionic toxicity, osmotic stress, and oxidative stress in plants. A potential solution can be the application of bacteria of the Serratia spp. known to promote plant growth under normal conditions Thus the mini-review aims to summarize the current knowledge on plant growth promotion by Serratia spp. (under the conditions of salinity stress, drought, and nutrient deficit) and highlight areas for development in the field. So far, it has been proven that Serratia spp. strains exhibit a variety of traits contributing to enhanced plant growth and stress tolerance, such as phytohormone production, ACC deaminase activity, nitrogen fixation, P and Zn solubilization, antioxidant properties improvement, and modulation of gene expression. Nevertheless, further research on Serratia spp. is needed, especially on two subjects: elucidating its mechanisms of action on plants at the molecular level and the effects of Serratia spp. on the indigenous soil and plant microbiota and, particularly, the rhizosphere. In both cases, it is advisable to use omics techniques to gain in-depth insights into the issues. Additionally, some strains of Serratia spp. may be phytopathogens, therefore studies to rule out this possibility are recommended prior to field trials. It is believed that by improving said knowledge the potential of Serratia spp. to stimulate plant growth will increase and strains from the genus will serve as an eco-friendly biofertilizer in sustainable agriculture more often.

Alleviation of water-deficit stress in turmeric plant (Curcuma longa L.) using phosphate solubilizing rhizo-microbes inoculation

The objective of this study was to assess the effects of phosphate solubilizing rhizo-microbes inoculants on nutrient balance, physiological adaptation, growth characteristics, and rhizome yield traits as well as curcuminoids yield at the secondary-rhizome initiation stage of turmeric plants, subsequently subjected to water-deficit (WD) stress. Phosphorus contents in the leaf tissues of Talaromyces aff. macrosporus and Burkholderia sp. (Bruk) inoculated plants peaked at 0.33 and 0.29 mg g-1 DW, respectively, under well-watered (WW) conditions; however, phosphorus contents declined when subjected to WD conditions (p ≤ 0.05). Similarly, potassium and calcium contents reached their maximum values at 5.33 and 3.47 mg g-1 DW, respectively, in Burk inoculated plants under WW conditions, which contributed to sustained rhizome fresh weight even when exposed to WD conditions (p ≤ 0.05). There was an increase in free proline content in T. aff. macrosporus and Burk inoculated plants under WD conditions, which played a crucial role in controlling leaf osmotic potential, thereby stabilizing leaf greenness and maximum quantum yield of PSII. As indicators of drought stress, there were noticeable restrictions in stomatal gas exchange parameters, including net photosynthetic rate, stomatal conductance, and transpiration rate, accompanied by an increase in leaf temperature. These changes resulted in reduced total soluble sugar levels. Interestingly, total curcuminoids and curcuminoids yield in Burk inoculated plants under WD conditions were retained, especially in relation to rhizome biomass. Burk inoculation in turmeric plants is recommended as a promising technique as it alleviates water-deficit stress, sustains rhizome biomass, and stabilizes curcuminoids yield.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-03922-x.

Effects of water volume of drip irrigation on soil bacterial communities and its association with soil properties in jujube cultivation

Introduction

Jujube is one of an important crop in Xinjiang, China, a area suffered by water scarcity and DI has been proven as a suitable mode for jujube cultivation. Soil bacterial community play a vital role in biogeochemical cycles to support the crop growth, and water content is considered as one of the important factors for them. However, limited research has explored the optimum irrigation strategies, such as water volume of DI, to maximize the benefits of jujube cultivation by regulating the soil bacterial communities.

Methods

Therefore, in this study, we conducted DI experiments on jujube fields in Xinjiang with three different water volume levels, and measured the soil properties and bacterial communities of the flowering and fruit setting (FFS) and end of growth (EG) stages.

Results and discussion

Significant lower jujube yield and soil available nutrients were observed in samples with low water amount. In addition, we discovered significant effects of the water amount of DI and jujube growth stages on soil bacterial communities. Based on the compare of samples among different growth stages and water amounts some growth stage related bacterial genera (Mycobacterium, Bradyrhizobium, and Bacillus) and water amount-related bacterial phyla (Chloroflexi, Nitrospirota, and Myxococcota) were recognized. Moreover, according to the results of null model, soil bacterial communities were governed by stochastic and deterministic processes under middle and low water volumes of DI, respectively. Finally, we deduced that middle water amount (600 mm) could be the optimal condition of DI for jujube cultivation because the higher jujube yield, deterministic assembly, and stronger correlations between soil properties and bacterial community under this condition. Our findings provide guidance for promoting the application of DI in jujube cultivation, and further research is needed to investigate the underlying mechanisms of soil bacterial community to promote the jujube yield.

Juvenile Plant-Microbe Interactions Modulate the Adaptation and Response of Forest Seedlings to Rapid Climate Change

The negative impacts of climate change on native forest ecosystems have created challenging conditions for the sustainability of natural forest regeneration. These challenges arise primarily from abiotic stresses that affect the early stages of forest tree development. While there is extensive evidence on the diversity of juvenile microbial symbioses in agricultural and fruit crops, there is a notable lack of reports on native forest plants. This review aims to summarize the critical studies conducted on the diversity of juvenile plant-microbe interactions in forest plants and to highlight the main benefits of beneficial microorganisms in overcoming environmental stresses such as drought, high and low temperatures, metal(loid) toxicity, nutrient deficiency, and salinity. The reviewed studies have consistently demonstrated the positive effects of juvenile plant-microbiota interactions and have highlighted the potential beneficial attributes to improve plantlet development. In addition, this review discusses the beneficial attributes of managing juvenile plant-microbiota symbiosis in the context of native forest restoration, including its impact on plant responses to phytopathogens, promotion of nutrient uptake, facilitation of seedling adaptation, resource exchange through shared hyphal networks, stimulation of native soil microbial communities, and modulation of gene and protein expression to enhance adaptation to adverse environmental conditions.

The role of plant growth promoting rhizobacteria in strengthening plant resistance to fluoride toxicity: a review

A wide variety of bacteria are present in soil but in rhizospheric area, the majority of microbes helps plant in defending diseases and facilitate nutrient uptake. These microorganisms are supported by plants and they are known as plant growth-promoting rhizobacteria (PGPR). The PGPRs have the potential to replace chemical fertilizers in a way that is more advantageous for the environment. Fluoride (F) is one of the highly escalating, naturally present contaminants that can be hazardous for PGPRs because of its antibacterial capacity. The interactions of F with different bacterial species in groundwater systems are still not well understood. However, the interaction of PGPR with plants in the rhizosphere region reduces the detrimental effects of pollutants and increases plants' ability to endure abiotic stress. Many studies reveal that PGPRs have developed F defense mechanisms, which include efflux pumps, Intracellular sequestration, enzyme modifications, enhanced DNA repair mechanism, detoxification enzymes, ion transporter/antiporters, F riboswitches, and genetic mutations. These resistance characteristics are frequently discovered by isolating PGPRs from high F-contaminated areas or by exposing cells to fluoride in laboratory conditions. Numerous studies have identified F-resistant microorganisms that possess additional F transporters and duplicates of the well-known targets of F. Plants are prone to F accumulation despite the soil's low F content, which may negatively affect their growth and development. PGPRs can be used as efficient F bioremediators for the soil environment. Environmental biotechnology focuses on creating genetically modified rhizobacteria that can degrade F contaminants over time. The present review focuses on a thorough systemic analysis of contemporary biotechnological techniques, such as gene editing and manipulation methods, for improving plant-microbe interactions for F remediation and suggests the importance of PGPRs in improving soil health and reducing the detrimental effects of F toxicity. The most recent developments in the realm of microbial assistance in the treatment of F-contaminated environments are also highlighted.

Recent advances in PGPR-mediated resilience toward interactive effects of drought and salt stress in plants

The present crisis at hand revolves around the need to enhance plant resilience to various environmental stresses, including abiotic and biotic stresses, to ensure sustainable agriculture and mitigate the impact of climate change on crop production. One such promising approach is the utilization of plant growth-promoting rhizobacteria (PGPR) to mediate plant resilience to these stresses. Plants are constantly exposed to various stress factors, such as drought, salinity, pathogens, and nutrient deficiencies, which can significantly reduce crop yield and quality. The PGPR are beneficial microbes that reside in the rhizosphere of plants and have been shown to positively influence plant growth and stress tolerance through various mechanisms, including nutrient solubilization, phytohormone production, and induction of systemic resistance. The review comprehensively examines the various mechanisms through which PGPR promotes plant resilience, including nutrient acquisition, hormonal regulation, and defense induction, focusing on recent research findings. The advancements made in the field of PGPR-mediated resilience through multi-omics approaches (viz., genomics, transcriptomics, proteomics, and metabolomics) to unravel the intricate interactions between PGPR and plants have been discussed including their molecular pathways involved in stress tolerance. Besides, the review also emphasizes the importance of continued research and implementation of PGPR-based strategies to address the pressing challenges facing global food security including commercialization of PGPR-based bio-formulations for sustainable agricultural.

Pseudomonas spp. can help plants face climate change

Climate change is increasingly affecting agriculture through droughts, high salinity in soils, heatwaves, and floodings, which put intense pressure on crops. This results in yield losses, leading to food insecurity in the most affected regions. Multiple plant-beneficial bacteria belonging to the genus Pseudomonas have been shown to improve plant tolerance to these stresses. Various mechanisms are involved, including alteration of the plant ethylene levels, direct phytohormone production, emission of volatile organic compounds, reinforcement of the root apoplast barriers, and exopolysaccharide biosynthesis. In this review, we summarize the effects of climate change-induced stresses on plants and detail the mechanisms used by plant-beneficial Pseudomonas strains to alleviate them. Recommendations are made to promote targeted research on the stress-alleviating potential of these bacteria.

Beneficial Microorganisms Improve Agricultural Sustainability under Climatic Extremes

The challenging alterations in climate in the last decades have had direct and indirect influences on biotic and abiotic stresses that have led to devastating implications on agricultural crop production and food security. Extreme environmental conditions, such as abiotic stresses, offer great opportunities to study the influence of different microorganisms in plant development and agricultural productivity. The focus of this review is to highlight the mechanisms of plant growth-promoting microorganisms (especially bacteria and fungi) adapted to environmental induced stresses such as drought, salinity, heavy metals, flooding, extreme temperatures, and intense light. The present state of knowledge focuses on the potential, prospective, and biotechnological approaches of plant growth-promoting bacteria and fungi to improve plant nutrition, physio-biochemical attributes, and the fitness of plants under environmental stresses. The current review focuses on the importance of the microbial community in improving sustainable crop production under changing climatic scenarios.

A Look at Plant-Growth-Promoting Bacteria

Bacteria have been used to increase crop yields. For their application on crops, bacteria are provided in inoculant formulations that are continuously changing, with liquid- and solid-based products. Bacteria for inoculants are mainly selected from natural isolates. In nature, microorganisms that favor plants exhibit various strategies to succeed and prevail in the rhizosphere, such as biological nitrogen fixation, phosphorus solubilization, and siderophore production. On the other hand, plants have strategies to maintain beneficial microorganisms, such as the exudation of chemoattractanst for specific microorganisms and signaling pathways that regulate plant-bacteria interactions. Transcriptomic approaches are helpful in attempting to elucidate plant-microorganism interactions. Here, we present a review of these issues.

Cold-tolerant phosphate-solubilizing Pseudomonas strains promote wheat growth and yield by improving soil phosphorous (P) nutrition status

It is well-known that phosphate-solubilizing bacteria (PSB) promote crop growth and yield. The information regarding characterization of PSB isolated from agroforestry systems and their impact on wheat crops under field conditions is rarely known. In the present study, we aim to develop psychrotroph-based P biofertilizers, and for that, four PSB strains (Pseudomonas sp. L3, Pseudomonas sp. P2, Streptomyces sp. T3, and Streptococcus sp. T4) previously isolated from three different agroforestry zones and already screened for wheat growth under pot trial conditions were evaluated on wheat crop under field conditions. Two field experiments were employed; set 1 includes PSB + recommended dose of fertilizers (RDF) and set 2 includes PSB – RDF. In both field experiments, the response of the PSB-treated wheat crop was significantly higher compared to the uninoculated control. In field set 1, an increase of 22% in grain yield (GY), 16% in biological yield (BY), and 10% in grain per spike (GPS) was observed in consortia (CNS, L3 + P2) treatment, followed by L3 and P2 treatments. Inoculation of PSB mitigates soil P deficiency as it positively influences soil alkaline phosphatase (AP) and soil acid phosphatase (AcP) activity which positively correlated with grain NPK %. The highest grain NPK % was reported in CNS-treated wheat with RDF (N–0.26%, P–0.18%, and K-1.66%) and without RDF (N-0.27, P-0.26, and K-1.46%), respectively. All parameters, including soil enzyme activities, plant agronomic data, and yield data were analyzed by principal component analysis (PCA), resulting in the selection of two PSB strains. The conditions for optimal P solubilization, in L3 (temperature-18.46, pH–5.2, and glucose concentration–0.8%) and P2 (temperature-17°C, pH–5.0, and glucose concentration–0.89%), were obtained through response surface methodology (RSM) modeling. The P solubilizing potential of selected strains at <20°C makes them a suitable candidate for the development of psychrotroph-based P biofertilizers. Low-temperature P solubilization of the PSB strains from agroforestry systems makes them potential biofertilizers for winter crops.

Exogenous Melatonin Spray Enhances Salinity Tolerance in Zizyphus Germplasm: A Brief Theory

Fruit orchards are frequently irrigated with brackish water. Irrigation with poor quality water is also a major cause of salt accumulation in soil. An excess of salts results in stunted growth, poor yield, inferior quality and low nutritional properties. Melatonin is a low molecular weight protein that shows multifunctional, regulatory and pleiotropic behavior in the plant kingdom. Recently, its discovery brought a great revolution in sustainable fruit production under salinity-induced environments. Melatonin contributed to enhanced tolerance in Zizyphus fruit species by improving the plant defense system's potential to cope with the adverse effects of salinity. The supplemental application of melatonin has improved the generation of antioxidant assays and osmolytes involved in the scavenging of toxic ROS. The tolerance level of the germplasm is chiefly based on the activation of the defense system against the adverse effects of salinity. The current study explored the contribution of melatonin against salinity stress and provides information regarding which biochemical mechanism can be effective and utilized for the development of salt-tolerant germplasm in Zizyphus.

Enhancing plant growth promoting rhizobacterial activities through consortium exposure: A review

Plant Growth Promoting Rhizobacteria (PGPR) has gained immense importance in the last decade due to its in-depth study and the role of the rhizosphere as an ecological unit in the biosphere. A putative PGPR is considered PGPR only when it may have a positive impact on the plant after inoculation. From the various pieces of literature, it has been found that these bacteria improve the growth of plants and their products through their plant growth-promoting activities. A microbial consortium has a positive effect on plant growth-promoting (PGP) activities evident by the literature. In the natural ecosystem, rhizobacteria interact synergistically and antagonistically with each other in the form of a consortium, but in a natural consortium, there are various oscillating environmental conditions that affect the potential mechanism of the consortium. For the sustainable development of our ecological environment, it is our utmost necessity to maintain the stability of the rhizobacterial consortium in fluctuating environmental conditions. In the last decade, various studies have been conducted to design synthetic rhizobacterial consortium that helps to integrate cross-feeding over microbial strains and reveal their social interactions. In this review, the authors have emphasized covering all the studies on designing synthetic rhizobacterial consortiums, their strategies, mechanism, and their application in the field of environmental ecology and biotechnology.

Orchard Management and Incorporation of Biochemical and Molecular Strategies for Improving Drought Tolerance in Fruit Tree Crops

Water scarcity is one of the greatest concerns for agronomy worldwide. In recent years, many water resources have been depleted due to multiple factors, especially mismanagement. Water resource shortages lead to cropland expansion, which likely influences climate change and affects global agriculture, especially horticultural crops. Fruit yield is the final aim in commercial orchards; however, drought can slow tree growth and/or decrease fruit yield and quality. It is therefore necessary to find approaches to solve this problem. The main objective of this review is to discuss the most recent horticultural, biochemical, and molecular strategies adopted to improve the response of temperate fruit crops to water stress. We also address the viability of cultivating fruit trees in dry areas and provide precise protection methods for planting fruit trees in arid lands. We review the main factors involved in planting fruit trees in dry areas, including plant material selection, regulated deficit irrigation (DI) strategies, rainwater harvesting (RWH), and anti-water stress materials. We also provide a detailed analysis of the molecular strategies developed to combat drought, such as Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) through gene overexpression or gene silencing. Finally, we look at the molecular mechanisms associated with the contribution of the microbiome to improving plant responses to drought.

Bioinoculant mediated regulation of signalling cascades in various stress responses in plants

Bio-inoculation involves the association of plant with some beneficial microorganisms, and among these microbiotas, those bacteria which can promote plant growth and development are known as Plant Growth Promoting Rhizobacteria (PGPR). It can help a plant directly or indirectly, which includes root development, biological nitrogen (N₂) fixation, stress tolerance, cell division and elongation, solubilization of Zinc, Phosphate, Potassium, soil health improvement and many more. PGPR have gained attention as it can be used as biofertilizers and helpful in bioremediation techniques, which in turn can reduce the chemical dependency in agriculture. PGPR mediated plant growth and stress management is developed by the virtue of the interaction of plant and microbial signalling pathways. On the other hand, environmental stresses are something to which a plant is always exposed irrespective of other factors. The present review is all about the better understanding of the convergence strategies of these signalling molecules and the ambiguities of signalling activities occurring in the host due to the interaction with PGPR under environmental stressed conditions.

Bio-Organic Fertilizer Promotes Pear Yield by Shaping the Rhizosphere Microbiome Composition and Functions

Bio-organic fertilizers (BOF) containing both organic amendments and beneficial microorganisms have been consistently shown to improve soils fertility and yield. However, the exact mechanisms which link amendments and yields remain disputed, and the complexity of bio-organic fertilizers may work in parallel in several ways. BOF may directly improve yield by replenishing soil nutrients or introducing beneficial microbial genes or indirectly by altering the soil microbiome to enrich native beneficial microorganisms. In this work, we aim to disentangle the relative contributions of direct and indirect effects on pear yield. We treated pear trees with either chemical fertilizer or organic fertilizer with/without the plant-beneficial bacterium Bacillus velezensis SQR9. We then assessed, in detail, soil physicochemical and biological properties (metagenome sequencing) as well as pear yield. We then evaluated the relative importance of direct and indirect effects of soil amendments on pear yield. Both organic treatments increased plant yield by up to 20%, with the addition of bacteria tripling the increase driven by organic fertilizer alone. This increase could be linked to alterations in soil physicochemical properties, bacterial community function, and metabolism. Supplementation of organic fertilizer SQR9 increased rhizosphere microbiome richness and functional diversity. Fertilizer-sensitive microbes and functions responded as whole guilds. Pear yield was most positively associated with the Mitsuaria- and Actinoplanes-dominated ecological clusters and with gene clusters involved in ion transport and secondary metabolite biosynthesis. Together, these results suggested that bio-organic fertilizers mainly act indirectly on plant yield by creating soil chemical properties which promote a plant-beneficial microbiome. IMPORTANCE Bio-organic fertilization is a widely used, eco-friendly, sustainable approach to increasing plant productivity in the agriculture and fruit industries. However, it remains unclear whether the promotion of fruit productivity is related to specific changes in microbial inoculants, the resident microbiome, and/or the physicochemical properties of rhizosphere soils. We found that bio-organic fertilizers alter soil chemical properties, thus manipulating specific microbial taxa and functions within the rhizosphere microbiome of pear plants to promote yield. Our work unveils the ecological mechanisms which underlie the beneficial impacts of bio-organic fertilizers on yield promotion in fruit orchards, which may help in the design of more efficient biofertilizers to promote sustainable fruit production.

The effect of rhizobia in improving the protective mechanisms of wheat under drought and supplementary irrigation conditions

Introduction

Wheat (Triticum aestivum L.) is a strategic crop and one of the world's most essential cereals, providing most of the world's calories and protein needs. Drought stress is one of the main limitations for crop production such as wheat in arid and semi-arid regions. Plants can accumulate antioxidants, carbohydrates, and stress hormones that stimulate cell and molecular regeneration under stress conditions. Irrigation saves water, improves crop photosynthesis, and increases plant ability to absorb water and elements from soil. Therefore, irrigation at the right time or supplementary irrigation can help plant growth and crop yield under drought conditions. Appropriate nutrition with fertilizers increases plants' stress tolerance. Bio-fertilizers are restorative elements used in soil to improve tolerance to stresses such as drought stress. A well-known class of bio-fertilizers is plant growth promoting rhizobacteria (PGPR). These rhizosphere bacteria affect plant development and productivity by interacting with roots. Arbuscular mycorrhizal fungi (AMF) alleviate drought stress in plants by enhancing their ability to absorb water and nutrients from the soil. Seaweed extract bio-fertilizer is organic matter used to increase crop growth and soil fertility. This bio-fertilizer is utilized as growth stimulants and food supplements. Our research analyzed the effects of rhizobia and seaweed extracts on wheat's drought resistance mechanisms.

Materials and methods

This research was conducted in Iran in the crop years of 2017–2018 and 2018–2019 in the research farm of Kurdistan University Faculty of Agriculture located in Dehgolan with coordinates 47°18′ 55″ East and 35°19′ 10″ North with an altitude of 1866 meters above sea level, 45 kilometers east It was done on the wheat plant in Sanandaj city. The experiment was conducted in the form of a split-split plot in the form of a randomized complete block design with four replications. Irrigation treatments as the main factor (no irrigation or dry-land, one irrigation in the booting stage, two irrigations in the booting and spike stages), two wheat cultivars (Sardari and Sirvan) as secondary factors, and the application of biological fertilizers at eight levels including Mycorrhiza + Nitrozist and Phosphozist, Seaweed extract + Nitrozist and Phosphozist, Mycorrhiza + Seaweed extract, Mycorrhiza + Nitrozist and Phosphozist and no application of biological fertilizers (control) as Sub-sub-factors were considered.

Results and discussion

According to the study, when bio-fertilizer was applied with once and twice supplementary irrigation levels, leaf relative water content (RWC) and soluble protein content (SPC) increased, while lack of irrigation increased malondialdehyde (MDA). In both years, bio-fertilizers, especially their combinations, increased the amount and activity of enzymatic and non-enzymatic antioxidants, including peroxidase (POD), superoxide dismutase (SOD), phenol (Phe), flavonoid (Fla), and anthocyanin (Anth). Also, it enhanced the inhibition of free radicals by 2-2-Diphenyl picryl hydrazyl (DPPH) and cleared active oxygen species. It was found that malondialdehyde (MDA) levels were very low in wheat under two times irrigation with averages of 3.3909 and 3.3865 μmol g−1 FW. The results indicated a significant positive relationship between non-enzymatic and enzymatic antioxidants such as Phe, Fla, Anth, DPPH, POD, and SOD enzymes and their role in improving stress under dry-land conditions, especially in the Sardari variety. Biological fertilizers (Mycorrhiza + Nitrozist and Phosphozist + Seaweed extract) increased wheat yield compared to the control. Furthermore, Mycorrhiza + Nitrozist and Phosphozist + Seaweed extract improved grain yield by 8.04% and 6.96% in the 1st and 2nd years, respectively. Therefore, appropriate combinations of microorganisms, beneficial biological compounds, and supplementary irrigation can reduce the adverse effects of drought stress in arid and semi-arid regions.

Plant Growth-Promoting Activities of Bacteria Isolated from an Anthropogenic Soil Located in Agrigento Province

Bacteria producers of plant growth-promoting (PGP) substances are responsible for the enhancement of plant development through several mechanisms. The purpose of the present work was to evaluate the PGP traits of 63 bacterial strains that were isolated from an anthropogenic soil, and obtained by modification of vertisols in the Sicily region (Italy) seven years after creation. The microorganisms were tested for the following PGP characteristics: indole acetic acid (IAA), NH3, HCN and siderophore production, 1-aminocyclopropane-1-carboxylate deaminase activity (ACC) and phosphate solubilization. The results of principal component analysis (PCA) showed that Bacillus tequilensis SI 319, Brevibacterium frigoritolerans SI 433, Pseudomonas lini SI 287 and Pseudomonas frederiksbergensis SI 307 expressed high levels of IAA and production of ACC deaminase enzyme, while for the rest of traits analyzed the best performances were registered with Pseudomonas genus, in particular for the strains Pseudomonas atacamensis SI 443, Pseudomonas reinekei SI 441 and Pseudomonas granadensis SI 422 and SI 450. The in vitro screening provided enough evidence for future in vivo growth promotion tests of these eight strains.

Single and Associated Effects of Drought and Heat Stresses on Physiological, Biochemical and Antioxidant Machinery of Four Eggplant Cultivars

The impact of heat and drought stresses, either individually or combined, on physiological and biochemical parameters of four eggplant varieties (Solanum melongena L.) was investigated. The results showed that associated stress generated the highest increment in proline content, MDA concentration, and H2O2 accumulation and generated the lowest increment in RWC. In addition, ‘Bonica’ and ‘Galine’ exhibited higher starch accumulation and lower electrolyte leakage (EL) under combined stress. Moreover, drought and heat stresses applied individually contributed to a substantial decline in Chla, Chlb, total Chl, Chla/b, and carotenoids (p > 0.05) in ‘Adriatica’ and ‘Black Beauty’. The decreasing level of pigments was more substantial under associated drought and heat stresses. The simultaneous application of drought and heat stresses reduced PSII efficiency (Fv/Fm), quantum yield (ΦPSII), and photochemical efficiency (qp) and boosted non-photochemical quenching (NPQ) levels. However, the change recorded in the chlorophyll fluorescence parameters was less pronounced in ‘Bonica’ and ‘Galine’. In addition, the gas exchange parameters, transpiration rate (E), CO2 assimilation rate (A), and net photosynthesis (Pn) were decreased in all varieties under all stress conditions. However, the reduction was more pronounced in ‘Adriatica’ and ‘Black Beauty’. Under associated stress, antioxidant enzymes, SOD, APX, CAT, and GR exhibited a significant increment in all eggplant cultivars. However, the rising was more elevated in ‘Bonica’ and ‘Galine’ (higher than threefold increase) than in ‘Adriatica’ and ‘Black Beauty’ (less than twofold increase). Furthermore, ‘Bonica’ and ‘Galine’ displayed higher non-enzyme scavenging activity (AsA and GSH) compared to ‘Adriatica’ and ‘Black Beauty’ under associated stress. Under stressful conditions, nutrient uptake was affected in all eggplant cultivars; however, the root, stem, and leaf N, P, and K contents, in ‘Adriatica’ and ‘Black Beauty’ were lower than in ‘Bonica’ and ‘Galine’, thereby showing less capacity in accumulating nutrients. The coexistence of drought and heat stresses caused more damage on eggplant varieties than the single appearance of drought or heat stress separately. ‘Bonica’ and ‘Galine’ showed better distinguished performance compared to ‘Adriatica’ and ‘Black Beauty’. The superiority of ‘Bonica’ and ‘Galine’ in terms of tolerance to heat and drought stresses was induced by more effective antioxidant scavenging potential, enhanced osmolyte piling-up, and prominent ability in keeping higher photosynthetic efficiency and nutrient equilibrium compared with ‘Adriatica’ and ‘Black Beauty’.

Insight into Recent Progress and Perspectives in Improvement of Antioxidant Machinery upon PGPR Augmentation in Plants under Drought Stress: A Review

Agriculture has a lot of responsibility as the rise in the world’s population demands more food requirements. However, more than one type of biotic and abiotic stress continually impacts agricultural productivity. Drought stress is a major abiotic stress that significantly affects agricultural productivity every year as the plants undergo several morphological, biochemical, and physiological modifications, such as repressed root and shoot growth, reduced photosynthesis and transpiration rate, excessive production of reactive oxygen species (ROS), osmotic adjustments, and modified leaf senescence regulating and stress signaling pathways. Such modifications may permanently damage the plants; therefore, mitigation strategies must be developed. The use of drought resistant crop cultivars is more expensive and labor-intensive with few advantages. However, exploiting plant growth promoting rhizobacteria (PGPR) is a proven alternative with numerous direct and indirect advantages. The PGPR confers induced systemic tolerance (IST) mechanisms in plants in response to drought stress via multiple mechanisms, including the alteration of root architecture, maintenance of high relative water content, improvement of photosynthesis rate, production of phytohormones, exopolysaccharides, ACC deaminase, carotenoids and volatiles, induction of antioxidant defense system, and alteration in stress-responsive gene expression. The commercial application of PGPR as bioinoculants or biostimulants will remain contingent on more robust strain selection and performance under unfavorable environmental conditions. This review highlights the possible mechanisms of PGPR by activating the plant adaptive defense systems for enhancing drought tolerance and improving overall growth and yield.

Role of plant growth-promoting rhizobacteria in boosting the phytoremediation of stressed soils: Opportunities, challenges, and prospects

Soil is considered as a vital natural resource equivalent to air and water which supports growth of the plants and provides habitats to microorganisms. Changes in soil properties, productivity, and, inevitably contamination/stress are the result of urbanisation, industrialization, and long-term use of synthetic fertiliser. Therefore, in the recent scenario, reclamation of contaminated/stressed soils has become a potential challenge. Several customized, such as, physical, chemical, and biological technologies have been deployed so far to restore contaminated land. Among them, microbial-assisted phytoremediation is considered as an economical and greener approach. In recent decades, soil microbes have successfully been used to improve plants' ability to tolerate biotic and abiotic stress and strengthen their phytoremediation capacity. Therefore, in this context, the current review work critically explored the microbial assisted phytoremediation mechanisms to restore different types of stressed soil. The role of plant growth-promoting rhizobacteria (PGPR) and their potential mechanisms that foster plants' growth and also enhance phytoremediation capacity are focussed. Finally, this review has emphasized on the application of advanced tools and techniques to effectively characterize potent soil microbial communities and their significance in boosting the phytoremediation process of stressed soils along with prospects for future research.

Unlocking the potential of co-applied biochar and plant growth-promoting rhizobacteria (PGPR) for sustainable agriculture under stress conditions

Sustainable food security is a major challenge in today's world, particularly in developing countries. Among many factors, environmental stressors, i.e., drought, salinity and heavy metals are major impediments in achieving sustainable food security. This calls for finding environment-friendly and cheap solutions to address these stressors. Plant growth-promoting rhizobacteria (PGPR) have long been established as an environment-friendly means to enhance agricultural productivity in normal and stressed soils and are being applied at field scale. Similarly, pyrolyzing agro-wastes into biochar with the aim to amend soils is being proposed as a cheap additive for enhancement of soil quality and crop productivity. Many pot and some field-scale experiments have confirmed the potential of biochar for sustainable increase in agricultural productivity. Recently, many studies have combined the PGPR and biochar for improving soil quality and agricultural productivity, under normal and stressed conditions, with the assumption that both of these additives complement each other. Most of these studies have reported a significant increase in agricultural productivity in co-applied treatments than sole application of PGPR or biochar. This review presents synthesis of these studies in addition to providing insights into the mechanistic basis of the interaction of the PGPR and biochar. Moreover, this review highlights the future perspectives of the research in order to realize the potential of co-application of the PGPR and biochar at field scale.

Graphical Abstract

Effect of copper stress on Phaseolus coccineus in the presence of exogenous methyl jasmonate and/or Serratia plymuthica from the Spitsbergen soil

Copper stress in the presence of exogenous methyl jasmonate and Serratia plymuthica in a complete trifactorial design with copper (0, 50 µM), methyl jasmonate (0, 1, 10 µM) and Serratia plymuthica (without and with inoculation) was studied on the physiological parameters of Phaseolus coccineus. Copper application reduced biomass and allantoin content, but increased chlorophyll and carotenoids contents as well as catalase and peroxidases activities. Jasmonate did not modify biomass and organic acids levels under copper treatment, but additional inoculation elevated biomass and content of tartrate, malate and succinate. Jasmonate used alone or in combination with bacteria increased superoxide dismutase activity in copper application. With copper, allantoin content elevated at lower jasmonate concentration, but with additional inoculation - at higher jasmonate concentration. Under copper stress, inoculation resulted in higher accumulation of tartrate, malate and citrate contents in roots, which corresponded with lower allantoin concentration in roots. Combined with copper, inoculation reduced catalase and guaiacol peroxidase activities, whereas organic acids content was higher. Under metal stress, with bacteria, jasmonate reduced phenolics content, elevated superoxide dismutase and guaiacol peroxidase activities. The data indicate that jasmonate and S. plymuthica affected most physiological parameters of P. coccineus grown with copper and revealed some effect on biomass.

Plant Growth-Promoting Rhizobacteria Eliminate the Effect of Drought Stress in Plants: A Review

Plants evolve diverse mechanisms to eliminate the drastic effect of biotic and abiotic stresses. Drought is the most hazardous abiotic stress causing huge losses to crop yield worldwide. Osmotic stress decreases relative water and chlorophyll content and increases the accumulation of osmolytes, epicuticular wax content, antioxidant enzymatic activities, reactive oxygen species, secondary metabolites, membrane lipid peroxidation, and abscisic acid. Plant growth-promoting rhizobacteria (PGPR) eliminate the effect of drought stress by altering root morphology, regulating the stress-responsive genes, producing phytohormones, osmolytes, siderophores, volatile organic compounds, and exopolysaccharides, and improving the 1-aminocyclopropane-1-carboxylate deaminase activities. The use of PGPR is an alternative approach to traditional breeding and biotechnology for enhancing crop productivity. Hence, that can promote drought tolerance in important agricultural crops and could be used to minimize crop losses under limited water conditions. This review deals with recent progress on the use of PGPR to eliminate the harmful effects of drought stress in traditional agriculture crops.

Perspective of ACC-deaminase producing bacteria in stress agriculture

The 1-aminocyclopropane-1-carboxylate deaminase (ACCD) enzyme plays an important role in stress alleviation of both biotic and abiotic stressors in plants and thereby enhances their growth under harsh environmental conditions. In-depth analysis of AcdS gene encoding for ACC deaminase reveals its presence in diverse microorganisms including bacteria and fungi. Particularly, plant growth-promoting bacteria (PGPB) containing ACCD supports plant growth by modulating the level of 'stress ethylene' and cleaving its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) into α-ketobutyrate and ammonia, enabling PGPB to utilize ACC as a carbon and nitrogen source. The reduced synthesis of ethylene in plants further relieves the ethylene inhibition of plant growth and development, and improves plant resistance to various stressors. Therefore, the dual role of microbial ACCD makes it a cost-effective and eco-friendly biocatalyst for sustainable agricultural productions. The inducible ACCD encoding gene AcdS is differentially regulated by varying environmental conditions. Successful generation of transgenic plants with microbial AcdS gene enhanced biotic and abiotic stress tolerance in plants. In the present review, we discuss the importance of ACCD-producing PGPB for their ability to reduce ethylene production and the promotion of plant growth under stress conditions. We also highlighted the development of transgenic plants by overexpressing bacterial AcdS gene to improve their performance under stress conditions.

Drought adaptive microbes as bioinoculants for the horticultural crops

Drought stress is among the most destructive stresses for agricultural productivity. It interferes with normal metabolic activities of the plants resulting, a negative impact on physiology and morphology of the plants. The management of drought stress requires various adaptive and alleviation strategies in which stress adaptive microbiomes are exquisite bioresources for plant growth and alleviation of drought stress. Diverse drought adaptive microbes belonging to genera Achromobacter, Arthrobacter, Aspergillus, Bacillus, Pseudomonas, Penicillium and Streptomyces have been reported worldwide. These bioresources exhibit a wide range of mechanisms such as helping plant in nutrient acquisition, producing growth regulators, lowering the levels of stress ethylene, increasing the concentration of osmolytes, and preventing oxidative damage under water deficit environmental conditions. Horticulture is one of the potential agricultural sectors to speed up the economy, poverty and generation of employment for livelihood. The applications of drought adaptive plant growth promoting (PGP) microbes as biofertilizers and biopesticides for horticulture is a potential strategy to improve the productivity and protection of horticultural crops from abiotic and biotic stresses for agricultural sustainability.

The legacy of microbial inoculants in agroecosystems and potential for tackling climate change challenges

Microbial inoculations contribute to reducing agricultural systems' environmental footprint by supporting sustainable production and regulating climate change. However, the indirect and cascading effects of microbial inoculants through the reshaping of soil microbiome are largely overlooked. By discussing the underlying mechanisms of plant- and soil-based microbial inoculants, we suggest that a key challenge in microbial inoculation is to understand their legacy on indigenous microbial communities and the corresponding impacts on agroecosystem functions and services relevant to climate change. We explain how these legacy effects on the soil microbiome can be understood by building on the mechanisms driving microbial invasions and placing inoculation into the context of ecological succession and community assembly. Overall, we advocate that generalizing field trials to systematically test inoculants' effectiveness and developing knowledge anchored in the scientific field of biological/microbial invasion are two essential requirements for applying microbial inoculants in agricultural ecosystems to tackle climate change challenges.

Plant growth-promoting rhizobacteria Shewanella putrefaciens and Cronobacter dublinensis enhance drought tolerance of pearl millet by modulating hormones and stress-responsive genes

Drought is a major abiotic stress that affects crop productivity. Endophytic bacteria have been found to alleviate the adverse effects of drought on plants. In the present study, we evaluated the effects of two endophytic bacteria Shewanella putrefaciens strain MCL-1 and Cronobacter dublinensis strain MKS-1 on pearl millet (Pennisetum glaucum (L.) R. Br.) under drought stress conditions. Pearl millet plants were grown under three water levels: field capacity (FC), mild drought stress (MD), and severe drought stress (SD). The effects of inoculation on plant growth, physiological attributes, phytohormone content, and drought stress-responsive genes were assessed. The inoculation of pearl millet seeds with endophytes significantly improved shoot and root dry weight and root architecture of plants grown under FC and drought stress conditions. There was a significant increase in relative water content and proline accumulation in the inoculated plants. Among the phytohormones analyzed, the content of ABA and IAA was significantly higher in endophyte-treated plants under all moisture regimes than in uninoculated plants. C. dublinensis-inoculated plants had higher GA content than uninoculated plants under all moisture regimes. The expression level of genes involved in phytohormone biosynthesis (SbNCED, SbGA20oX, and SbYUC) and coding drought-responsive transcription factors (SbAP2, SbSNAC1 and PgDREB2A) was significantly higher under SD in endophyte-inoculated plants than in uninoculated plants. Thus, these endophytic bacteria presumably enhanced the tolerance of pearl millet to drought stress by modulating root growth, plant hormones, physiology and the expression of genes involved in drought tolerance.

Drought-tolerant Bacillus megaterium isolated from semi-arid conditions induces systemic tolerance of wheat under drought conditions

A detailed study of the response of wheat plants, inoculated with drought-tolerant PGPR is studied which would be beneficial to achieve genetic improvement of wheat for drought tolerance. Drought stress, a major challenge under current climatic conditions, adversely affects wheat productivity. In the current study, we observed the response of wheat plants, inoculated with drought-tolerant plant growth-promoting rhizobacteria (PGPR) Bacillus megaterium (MU2) and Bacillus licheniformis (MU8) under induced drought stress. In vitro study of 90 rhizobacteria exhibited 38 isolates showed one or more plant growth-promoting properties, such as solubilization of phosphorus, potassium, and exopolysaccharide production. Four strains revealing the best activities were tested for their drought-tolerance ability by growing them on varying water potentials (- 0.05 to - 0.73 MPa). Among them, two bacterial strains Bacillus megaterium and Bacillus licheniformis showed the best drought-tolerance potential, ACC deaminase activities, IAA production, and antagonistic activities against plant pathogens. Additionally, these strains when exposed to drought stress (- 0.73 MPa) revealed the induction of three new polypeptides (18 kDa, 35 kDa, 30 kDa) in Bacillus megaterium. We determined that 10⁶ cells/mL of Bacillus megaterium and Bacillus licheniformis were enough to induce drought tolerance in wheat under drought stress. These drought-tolerant strains increased the germination index (11-46%), promptness index (16-50%), seedling vigor index (11-151%), fresh weight (35-192%), and dry weight (58-226%) of wheat under irrigated and drought stress. Moreover, these strains efficiently colonized the wheat roots and increased plant biomass, relative water content, photosynthetic pigments, and osmolytes. Upon exposure to drought stress, Bacillus megaterium inoculated wheat plants exhibited improved tolerance by enhancing 59% relative water content, 260, 174 and 70% chlorophyll a, b and carotenoid, 136% protein content, 117% proline content and 57% decline in MDA content. Further, activities of defense-related antioxidant enzymes were also upregulated. Our results revealed that drought tolerance was more evident in Bacillus megaterium as compared to Bacillus licheniformis. These strains could be effective bioenhancer and biofertilizer for wheat cultivation in arid and semi-arid regions. However, a detailed study at the molecular level to deduce the mechanism by which these strains alleviate drought stress in wheat plants needs to be explored.

Drought-tolerant Pseudomonas sp. showed differential expression of stress-responsive genes and induced drought tolerance in Arabidopsis thaliana

The growth and persistence of rhizobacteria in soils are highly impacted by moisture stress. In this study, we report the first transcript analysis of four Pseudomonas strains (PS1, PS2, PS3, and PS4) isolated from the root-soil interface of rice and maize associated with different moisture levels during water deprivation. Filtered Pseudomonas sp. cells incubated at low (RH10%) and high (RH85%) relative humidity showed decreased survival of all Pseudomonas sp. at RH10% when compared with RH85%. RT-PCR showed differential expression of treS (trehalose synthase), rpoS (sigma factor), mucA (alginate regulatory gene), and fliM (flagellar motor switch protein gene) in response to exposure to RH10%. However, molecular fingerprinting and nutrient assimilation profile of Pseudomonas strains demonstrated genetic and physiological variation between the four strains irrespective of water regime and host. In vitro testing of these strains showed ACC deaminase activity and gibberellic acid, abscisic acid, indole acetic acid, and exopolysaccharide production. We determined that 50 μl of 1.2 × 10³  CFU ml^-1 of these Pseudomonas strains was enough to protect Arabidopsis plants against drought stress in a pot experiment. Inoculated plants increased their root colonization ability and biomass; however, PS2 showed higher survival (95%), relative water content (59%), chlorophyll (30%), glycine betaine (38%), proline (23%), and reduced MDA (43%) in shoots than irrigated control under induced water deprivation. It can be concluded that all Pseudomonas strains were effective in mitigating drought stress, however, PS2 appears to impart more resistance to drought than the other strains by upregulating key defense mechanisms.

Overexpression of the tyrosine decarboxylase gene MdTyDC in apple enhances long-term moderate drought tolerance and WUE

Drought stress affects the apple yield and quality. Tyrosine decarboxylase (TyDC) plays a fundamental role in many secondary metabolite reactions in higher plants (including those involving dopamine). Our aims of this study are: 1) to identify the role of TyDC in dopamine derivative biosynthesis and its function in long-term moderate drought conditions; and 2) to explore the role of MdTyDC in plant growth and development as well as the drought stress response. Wild type and three independently apple plants overexpression of MdTyDC were treated for long-term moderate drought stress. The growth and physiological parameters of apple plant, photosynthetic capacity, antioxidant enzymes activity, water use efficiency (WUE), stomatal behavior, amino acid content and dopamine content were detected under long-term moderate drought stress. Overexpression of MdTyDC (OE) in apple showed better growth performance, higher photosynthetic capacity and higher capacity for photochemical reactions than wild type lines (WT). Under long-term moderate drought stress, OE lines showed higher WUE, increased ABA content, decreased stomatal aperture, higher antioxidant activity, lower accumulation of ROS and increases in amino acids, such as proline, phenylalanine and lysine. In addition, qRT-PCR revealed higher gene expression of MdTyDC and dopamine content in OE compared with WT lines under long-term moderate drought stress. These results indicate that MdTyDC confers long-term moderate drought tolerance by improving photosynthetic capacity, WUE, antioxidant activity, dopamine content and changing the contents of amino acids (such as proline accumulation).

The ACC deaminase-producing plant growth-promoting bacteria: Influences of bacterial strains and ACC deaminase activities in plant tolerance to abiotic stress

Global climate change results in frequent occurrences and/or long durations of abiotic stress. Field grown plants are affected by abiotic stress, and they modulate ethylene in response to abiotic stress exposure and use it as a signaling molecule in stress tolerance mechanisms. However, frequent occurrences and/or long durations of stress conditions can cause plants to induce ethylene levels higher than their thresholds, resulting in a reduction of plant growth and crop productivity. The use of plant growth-promoting bacteria (PGPB) that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase has increased in various plant species to ameliorate the deleterious effects of stress-induced ethylene and promote plant growth despite abiotic stress conditions. Unfortunately, there are restrictions that limit the use of ACC deaminase-producing PGPB to protect plants from abiotic stresses. This review describes how abiotic stress induces ethylene and how stress-induced ethylene adversely affects plant growth. In addition, this review emphasizes the importance of the compatibility of PGPB strains and specific host plants and ACC deaminase activities in the reduction of stress ethylene and the promotion of plant growth, based on the research published in the last 10 years. Moreover, due to the restrictions in PGPB use, this review highlights the potential generation of transgenic plants expressing the AcdS gene that encodes the ACC deaminase enzyme as a substitute for PGPB in the future to support and uplift agricultural sustainability and food security globally.

Drought Tolerant Enterobacter sp./Leclercia adecarboxylata Secretes Indole-3-acetic Acid and Other Biomolecules and Enhances the Biological Attributes of Vigna radiata (L.) R. Wilczek in Water Deficit Conditions

Drought or water stress is a limiting factor that hampers the growth and yield of edible crops. Drought-tolerant plant growth-promoting rhizobacteria (PGPR) can mitigate water stress in crops by synthesizing multiple bioactive molecules. Here, strain PAB19 recovered from rhizospheric soil was biochemically and molecularly characterized, and identified as Enterobacter sp./Leclercia adecarboxylata (MT672579.1). Strain PAB19 tolerated an exceptionally high level of drought (18% PEG-6000) and produced indole-3-acetic acid (176.2 ± 5.6 µg mL-1), ACC deaminase (56.6 ± 5.0 µg mL-1), salicylic acid (42.5 ± 3.0 µg mL-1), 2,3-dihydroxy benzoic acid (DHBA) (44.3 ± 2.3 µg mL-1), exopolysaccharide (204 ± 14.7 µg mL-1), alginate (82.3 ± 6.5 µg mL-1), and solubilized tricalcium phosphate (98.3 ± 3.5 µg mL-1), in the presence of 15% polyethylene glycol. Furthermore, strain PAB19 alleviated water stress and significantly (p ≤ 0.05) improved the overall growth and biochemical attributes of Vigna radiata (L.) R. Wilczek. For instance, at 2% PEG stress, PAB19 inoculation maximally increased germination, root dry biomass, leaf carotenoid content, nodule biomass, leghaemoglobin (LHb) content, leaf water potential (ΨL), membrane stability index (MSI), and pod yield by 10%, 7%, 14%, 38%, 9%, 17%, 11%, and 11%, respectively, over un-inoculated plants. Additionally, PAB19 inoculation reduced two stressor metabolites, proline and malondialdehyde, and antioxidant enzymes (POD, SOD, CAT, and GR) levels in V. radiata foliage in water stress conditions. Following inoculation of strain PAB19 with 15% PEG in soil, stomatal conductance, intercellular CO2 concentration, transpiration rate, water vapor deficit, intrinsic water use efficiency, and photosynthetic rate were significantly improved by 12%, 8%, 42%, 10%, 9% and 16%, respectively. Rhizospheric CFU counts of PAB19 were 2.33 and 2.11 log CFU g-1 after treatment with 15% PEG solution and 8.46 and 6.67 log CFU g-1 for untreated controls at 40 and 80 DAS, respectively. Conclusively, this study suggests the potential of Enterobacter sp./L. adecarboxylata PAB19 to alleviate water stress by improving the biological and biochemical features and of V. radiata under water-deficit conditions.

Multifarious effect of ACC deaminase and EPS producing Pseudomonas sp. and Serratia marcescens to augment drought stress tolerance and nutrient status of wheat

Drought is the prime abiotic stress that rigorously influences plant growth, yield and quality of crops. The current investigation illustrated the bio-protective characters of Serratia marcescens and Pseudomonas sp. to ameliorate drought stress tolerance, plant growth and nutrient status of wheat. The present study aimed for search of potential drought tolerant plant growth-promoting rhizobacteria (PGPR). All screened bacterial isolates exhibited potential plant growth promoting (PGP) attributes such as production of ACC deaminase, exo-polysaccharide, siderophore, ammonia, IAA, and efficiently solubilized zinc and phosphate under in vitro conditions. To assess the in situ plant growth promotion potential of PGPR, a greenhouse experiment was conducted by priming wheat seeds with screened plant PGPR. Improved water status, reactive oxygen species, osmolyte accumulation, chlorophyll and carotenoids content in plant leaves confirmed the excellent drought tolerance conferring ability of RRN II 2 and RRC I 5. Among all PGPR, RRN II 2 and RRC I 5 inoculated plants not only demonstrated greater harvest index but also exhibited more micronutrient (zinc and iron) content in wheat grains. Further, RRN II 2 and RRC I 5 were identified through 16S rDNA sequencing as S. marcescens and Pseudomonas sp., respectively. Furthermore, amplification of acdS gene (Amplified band size of acdS gene was ~ 1.8 Kb) also confirmed ACC deaminase enzyme producing ability of Pseudomonas sp. Moreover, correlation coefficient, principal component analysis and cluster analysis also demonstrated that nutrient status and values of agronomical parameters of wheat primed with S. marcescens and Pseudomonas sp. were at par with the positive control. Thus, the outcome of this comparative investigation indicates that Pseudomonas sp. and S. marcescens could be utilized as bioinoculant in wheat since they can improve the physiological status, productivity and nutrient status in wheat crop under drought.

Plant Growth-Promoting Rhizobacteria as a Green Alternative for Sustainable Agriculture

Environmental stress is a major challenge for sustainable food production as it reduces yield by generating reactive oxygen species (ROS) which pose a threat to cell organelles and biomolecules such as proteins, DNA, enzymes, and others, leading to apoptosis. Plant growth-promoting rhizobacteria (PGPR) offers an eco-friendly and green alternative to synthetic agrochemicals and conventional agricultural practices in accomplishing sustainable agriculture by boosting growth and stress tolerance in plants. PGPR inhabit the rhizosphere of soil and exhibit positive interaction with plant roots. These organisms render multifaceted benefits to plants by several mechanisms such as the release of phytohormones, nitrogen fixation, solubilization of mineral phosphates, siderophore production for iron sequestration, protection against various pathogens, and stress. PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. Genetically engineered PGPR strains play significant roles to alleviate the abiotic stress to improve crop productivity. Thus, the present review will focus on the impact of PGPR on stress resistance, plant growth promotion, and induction of antioxidant systems in plants.

1-Aminocyclopropane-1-Carboxylate Deaminase-Producing Plant Growth-Promoting Rhizobacteria Improve Drought Stress Tolerance in Grapevine (Vitis vinifera L.)

Plant growth-promoting rhizobacteria (PGPRs) that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase are capable of reducing limits to plant growth due to water-deficient conditions. Here, seven PGPR strains that can produce ACC deaminase were successfully obtained from the rhizosphere soil of grapevine (Vitis vinifera L.) in arid regions of China. The strains belonged to three different genera: Pseudomonas, Enterobacter, and Achromobacter, according to their 16S rDNA sequencing analysis. A drought tolerance experiment revealed two PGPR strains (DR3 and DR6) with exceptionally high phosphate solubilization, nitrogen fixation, indoleacetic acid (IAA), and exopolysaccharides secretion potential. Both strains were selected for use in a pot experiment to evaluate their growth-promoting effects on grapevines under drought conditions. Each of these two PGPRs and their mixed inoculation into grapevines were expected to alleviate the comprehensive growth inhibition of grapevines caused by drought stress. The mixed inoculation was hypothesized to elicit the best growth-promoting effects. Inoculation with the PGPRs not only enhanced the root-adhering soil/root tissue ratios and soil aggregate stability, but it also increased the nitrogen and phosphorus levels in the soil and plant leaves. Further, inoculation with PGPRs significantly altered the plant height, biomass of shoot and root organs, relative water contents, and net photosynthetic rate of leaves, enabling grapevines to better cope with drought. Moreover, the contents of IAA, abscisic acid, and malondialdehyde in these grapevines under drought stress were significantly changed by PGPRs. They indirectly affected biochemical and physiological properties of grapevines to alleviate their drought stress. Taken together, these results demonstrate that the DR3 and DR6 PGPRs might be useful for effectively weakening the growth inhibition caused by drought in grapevines. The strains might also be applied as effective bioinoculants to maintain the quality of wine grapes.

Phosphate solubilizing rhizobacteria isolated from jujube ziziphus lotus plant stimulate wheat germination rate and seedlings growth

Jujube plant (Ziziphus lotus (L.) Desf.) can survive in arid climates and tolerates both biotic and abiotic stresses. Here, we isolated, for the first time in Morocco, nine phosphate solubilizing bacteria strains from jujube rhizosphere, designated J10 to J13, J15, & J153 to J156. Genotypic identification based on 16S rDNA sequencing, revealed six strains that belong to Pseudomonas (J10, J12, J13, J15, J153 and J154), two to Bacillus (J11 and J156), and one to Paenibacillus J155. Siderophores were produced by all strains. Proteases activity was missing in Pseudomonas sp. J153 & J154, whereas cellulase was restricted only to Pseudomonas sp. J10, Paenibacillus xylanexedens J155 and Bacillus cereus J156. Indole-3- acetic acid and ammonia were also produced by all strains, with a maxima of 204.28 µg mL⁻¹ in Bacillus megaterium J11 and 0.33 µmol mL⁻¹ in Pseudomonas sp. J153, respectively. Pseudomonas sp. J10 and B. cereus J156 grew on plates containing 1,500 µg mL⁻¹ of nickel nitrate, while Pseudomonas sp. J153 withstood 1,500 µg mL⁻¹ of either copper sulfate or cadmium sulfate. Phenotypic analysis of the potential of the isolates to promote early plant growth showed that wheat seeds inoculated with either P. moraviensis J12 or B. cereus J156 remarkably increased germination rate and seedlings growth. Lastly, antibiotic resistance profiling revealed that except for Pseudomonas sp. J11 and B. cereus J156, remaining strains displayed resistance at least to one of tested antibiotics. Collectively, Pseudomonas sp. J10, P. moraviensis J12, Pseudomonas sp. J153 and B. cereus J156, represent potential biofertilizers suitable for soils that are poor in P, and/or heavy metals contaminated.

Genomic Analysis of Serratia plymuthica MBSA-MJ1: A Plant Growth Promoting Rhizobacteria That Improves Water Stress Tolerance in Greenhouse Ornamentals

Water stress decreases the health and quality of horticulture crops by inhibiting photosynthesis, transpiration, and nutrient uptake. Application of plant growth promoting rhizobacteria (PGPR) can increase the growth, stress tolerance, and overall quality of field and greenhouse grown crops subjected to water stress. Here, we evaluated Serratia plymuthica MBSA-MJ1 for its ability to increase plant growth and quality of Petunia × hybrida (petunia), Impatiens walleriana (impatiens), and Viola × wittrockiana (pansy) plants recovering from severe water stress. Plants were treated weekly with inoculum of MBSA-MJ1, and plant growth and quality were evaluated 2 weeks after recovery from water stress. Application of S. plymuthica MBSA-MJ1 increased the visual quality and shoot biomass of petunia and impatiens and increased the flower number of petunia after recovery from water stress. In addition, in vitro characterizations showed that MBSA-MJ1 is a motile bacterium with moderate levels of antibiotic resistance that can withstand osmotic stress. Further, comprehensive genomic analyses identified genes putatively involved in bacterial osmotic and oxidative stress responses and the synthesis of osmoprotectants and vitamins that could potentially be involved in increasing plant water stress tolerance. This work provides a better understanding of potential mechanisms involved in beneficial plant-microbe interactions under abiotic stress using a novel S. plymuthica strain as a model.

Two PGPR strains from the rhizosphere of Haloxylon ammodendron promoted growth and enhanced drought tolerance of ryegrass

Haloxylon ammodendron, a typical xerophyte, tolerates various abiotic stresses and is widely distributed in desert areas. Two PGPR strains, Bacillus sp. WM13-24 and Pseudomonas sp. M30-35, were previously isolated from the rhizosphere of H. ammodendron in Tengger Desert, Gansu province, northwest China. The aim of this study was to investigate the role of M30-35 and WM13-24 in drought stress alleviation of ryegrass (Lolium perenne L.). Under normal condition, both M30-35 and WM13-24 increased shoot fresh and dry weight, chlorophyll content, total nitrogen and phosphorus contents and altered phytohormone distribution compared to control. Moreover, after 7 days of drought stress, WM13-24 and M30-35 enhanced photosynthetic capacity, relative water content, the activities of catalase (CAT) and peroxidase (POD) and proline content, resulted in decreased malondialdehyde (MDA) content, relative membrane permeability (RMP) and H₂O₂ accumulation; interestingly, the two strains decreased ABA content in leaves. This study demonstrated that the two PGPR strains promoted ryegrass growth and root development via regulating plant hormone distribution and enhanced drought tolerance of ryegrass through improving the activities of antioxidant enzymes, regulating ABA signaling and maintaining plant growth. Our results indicated that PGPR strains from rhizosphere of the desert plant species could be considered as promising bioinoculants for grass plants.

The Foliar Application of Rice Phyllosphere Bacteria induces Drought-Stress Tolerance in Oryza sativa (L.)

This study assessed the potential of Bacillus endophyticus PB3, Bacillus altitudinis PB46, and Bacillus megaterium PB50 to induce drought tolerance in a susceptible rice cultivar. The leaves of the potted rice plants subjected to physical drought stress for 10 days during the flowering stage were inoculated with single-strain suspensions. Control pots of irrigated and drought-stressed plants were included in the experiment for comparison. In all treatments, the plant stress-related physiochemical and biochemical changes were examined and the expression of six stress-responsive genes in rice leaves was evaluated. The colonization potential on the surface of the rice leaves and stomata of the most successful strain in terms of induced tolerance was confirmed in the gnotobiotic experiment. The plants sprayed with B. megaterium PB50 showed an elevated stress tolerance based on their higher relative water content and increased contents of total sugars, proteins, proline, phenolics, potassium, calcium, abscisic acid, and indole acetic acid, as well as a high expression of stress-related genes (LEA, RAB16B, HSP70, SNAC1, and bZIP23). Moreover, this strain improved yield parameters compared to other treatments and also confirmed its leaf surface colonization. Overall, this study indicates that the foliar application of B. megaterium PB50 can induce tolerance to drought stress in rice.

PGPR Mediated Alterations in Root Traits: Way Toward Sustainable Crop Production

The above ground growth of the plant is highly dependent on the belowground root system. Rhizosphere is the zone of continuous interplay between plant roots and soil microbial communities. Plants, through root exudates, attract rhizosphere microorganisms to colonize the root surface and internal tissues. Many of these microorganisms known as plant growth promoting rhizobacteria (PGPR) improve plant growth through several direct and indirect mechanisms including biological nitrogen fixation, nutrient solubilization, and disease-control. Many PGPR, by producing phytohormones, volatile organic compounds, and secondary metabolites play important role in influencing the root architecture and growth, resulting in increased surface area for nutrient exchange and other rhizosphere effects. PGPR also improve resource use efficiency of the root system by improving the root system functioning at physiological levels. PGPR mediated root trait alterations can contribute to agroecosystem through improving crop stand, resource use efficiency, stress tolerance, soil structure etc. Thus, PGPR capable of modulating root traits can play important role in agricultural sustainability and root traits can be used as a primary criterion for the selection of potential PGPR strains. Available PGPR studies emphasize root morphological and physiological traits to assess the effect of PGPR. However, these traits can be influenced by various external factors and may give varying results. Therefore, it is important to understand the pathways and genes involved in plant root traits and the microbial signals/metabolites that can intercept and/or intersect these pathways for modulating root traits. The use of advanced tools and technologies can help to decipher the mechanisms involved in PGPR mediated determinants affecting the root traits. Further identification of PGPR based determinants/signaling molecules capable of regulating root trait genes and pathways can open up new avenues in PGPR research. The present review updates recent knowledge on the PGPR influence on root architecture and root functional traits and its benefits to the agro-ecosystem. Efforts have been made to understand the bacterial signals/determinants that can play regulatory role in the expression of root traits and their prospects in sustainable agriculture. The review will be helpful in providing future directions to the researchers working on PGPR and root system functioning.

Overexpression of 1-Aminocyclopropane-1-Carboxylic Acid Deaminase (acdS) Gene in Petunia hybrida Improves Tolerance to Abiotic Stresses

Abiotic stress induces the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) in plants, which consequently enhances ethylene production and inhibits plant growth. The bacterial ACC deaminase enzyme encoded by the acdS gene reduces stress-induced ethylene production and improves plant growth in response to stress. In this study, overexpression of acdS in Petunia hybrida ('Mirage Rose') significantly reduced expression of the ethylene biosynthesis gene ACC oxidase 1 (ACO1) and ethylene production relative to those in wild type (WT) under various abiotic stresses (cold, drought, and salt). The higher reduction of stress-induced ethylene in the transgenic plants, which was due to the overexpression of acdS, led to a greater tolerance to the stresses compared to that in the WT plants. The greater stress tolerances were proven based on better plant growth and physiological performance, which were linked to stress tolerance. Moreover, expression analysis of the genes involved in stress tolerance also supported the increased tolerance of transgenics relative to that with the WT. These results suggest the possibility that acdS is overexpressed in ornamental plants, particularly in bedding plants normally growing outside the environment, to overcome the deleterious effect of ethylene on plant growth under different abiotic stresses. The development of stress-tolerant plants will be helpful to advance the floricultural industry.

Co-application of bio-fertilizer and salicylic acid improves growth, photosynthetic pigments and stress tolerance in wheat under drought stress

Drought stress hampers the growth and productivity of wheat crop worldwide. Thus far, different strategies have been proposed to improve drought tolerance in wheat but the combined application of plant growth-promoting rhizobacteria formulated bio-fertilizer (BF) and salicylic acid (SA) has not been thoroughly explored yet. Therefore, a pot experiment was conducted to observe the effect of SA, BF, and their combination on wheat plants under optimal and drought stress conditions. Seeds priming was done with BF (10⁷ CFU mL^-1). After 2 weeks of germination, SA (one mM) was applied as a foliar spray. Drought stress was applied by withholding water supply at three-leaf stage (30 d old plants) for the next 15 d until soil moisture dropped to 10%. Foliar application of SA increased the bacterial population of BF significantly compared to the sole application of BF under irrigated as well as drought stress conditions. Co-application of BF and foliar spray of SA induced drought tolerance in wheat plants by enhancing plant biomass, photosynthetic pigments, relative water content and osmolytes, and activities of the defense-related system. Plants treated with SA and BF together under drought stress had significantly increased leaf water status, Chl a, Chl b, and carotenoids synthesis by 238%, 125%, 167%, and 122%, respectively. Moreover, the co-application of SA and BF showed maximum SOD, POD, APX, and CAT activities by 165%, 85%, 156%, and 169% in the leaves while 153%, 86%, 116% and 200% in roots under drought stress. Similarly, the combined treatment exhibited a pronounced decrease in MDA content by 54% while increased production of proteins and proline by 145% and 149%, respectively. Our results showed that the co-application of SA and BF induced better drought tolerance as compared with the sole application of SA or BF. The results obtained herein suggest that combined application of BF and SA can be applied to the wheat crop to greatly improve drought tolerance in field conditions.