Water Conservation and Plant Survival Strategies of Rhizobacteria under Drought Stress

Enhancing drought stress tolerance and growth promotion in chiltepin pepper (Capsicum annuum var. glabriusculum) through native Bacillus spp

The drought can cause a decrease in food production and loss of biodiversity. In northern Mexico, an arid region, the chiltepin grows as a semi-domesticated crop that has been affected in its productivity and yield. An alternative to mitigate the effect of drought and aid in its conservation could be using Plant Growth-Promoting Bacteria (PGPB). The present study evaluated the capacity of native Bacillus spp., isolated from arid soils, as PGPBs and drought stress tolerance inducers in chiltepin under controlled conditions. Chiltepin seeds and seedlings were inoculated with native strains of Bacillus spp. isolated from arid soils, evaluating germination, vegetative, and drought stress tolerance parameters. The PGPBs improved vegetative parameters such as height, stem diameter, root length, and slenderness index in vitro. B. cereus (Bc25-7) improved in vitro survival of stressed seedlings by 68% at -1.02 MPa. Under greenhouse conditions, seedlings treated with PGPBs exhibited increases in root length (9.6%), stem diameter (13.68%), leaf fresh weight (69.87%), and chlorophyll content (38.15%). Bc25-7 alleviated severe water stress symptoms (7 days of water retention stress), and isolates B. thuringiensis (Bt24-4) and B. cereus (Bc25-7, and Bc30-2) increased Relative Water Content (RWC) by 51%. Additionally, the treated seeds showed improved germination parameters with a 46.42% increase in Germination Rate (GR). These findings suggest that using PGPBs could be an alternative to mitigate the effect of drought on chiltepin.

Unveiling stress-adapted endophytic bacteria: Characterizing plant growth-promoting traits and assessing cross-inoculation effects on Populus deltoides under abiotic stress

In the face of the formidable environmental challenges precipitated by the ongoing climate change, Plant Growth-Promoting Bacteria (PGPB) are gaining widespread acknowledgement for their potential as biofertilizers, biocontrol agents, and microbial inoculants. However, a knowledge gap pertains to the ability of PGPB to improve stress tolerance in forestry species via cross-inoculation. To address this gap, the current investigation centres on PGPBs, namely, Acinetobacter johnsonii, Cronobacter muytjensii, and Priestia endophytica, selected from the phyllosphere of robust and healthy plants thriving in the face of stress-inducing conditions. These strains were selected based on their demonstrated adaptability to saline, arid, and nitrogen-deficient environments. The utilization of PGPB treatment resulted in an improvement of stomatal conductance (gs) and transpiration rate (E) in poplar plants exposed to both salt and drought stress. It also induced an increase in essential biochemical components such as proline (PRO), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD). These reactions were accompanied by a decrease in leaf malonaldehyde (MDA) content and electrolyte leakage (EL). Furthermore, the PGPB treatment demonstrated a notable enhancement in nutrient absorption, particularly nitrogen and carbon, achieved through the solubilization of nutrients. The estimation of canopy temperature via thermal imaging proved to be an efficient method for distinguishing stress reactions in poplar than conventional temperature recording techniques. In summation, the utilization of PGPB especially Cronobacter muytjensii in this study, yielded profound improvements in the stress tolerance of poplar plants, manifesting in reduced membrane lipid peroxidation, enhanced photosynthesis, and bolstered antioxidant capacity within the leaves.

Potential of Drought Tolerant Rhizobacteria Amended with Biochar on Growth Promotion in Wheat

Drought stress is the prime obstacle for worldwide agricultural production and necessitates innovative strategies for enhancing crop resilience. This study explores the efficacy of plant growth-promoting rhizobacteria (PGPR) and biochar (BC) as sustainable amendments for mitigating the effects of drought on wheat growth. Multiple experiments were carried out on isolated strains to assess their drought tolerance potential and multiple plant growth-promoting attributes. Experiments in the laboratory and natural environment were conducted to assess the impact of plant growth-promoting rhizobacteria, biochar, and their synergistic application on various growth parameters of wheat. The results revealed that the drought-tolerant PGPR strains (Bacillus subtilis and Bacillus tequilensis), alongside biochar (rice husk), alleviated the phytotoxic impact of drought by increasing the root length from 17.0% to 70.0% and shoot length from 30.0% to 82.0% as compared to un-inoculated stressed controls. The total chlorophyll and carotenoid contents of the plants were substantially increased to 477% and 423%, respectively, when biochar and PGPR were applied synergistically. Significant enhancements in membrane stability index, relative water content, proline, and sugar level were achieved by combining biochar and bacterial strains, resulting in increases of 19.5%, 37.9%, 219%, and 300%, respectively. The yield of wheat in terms of plant height, spike length, number of spikelets per spike, and number of grains per spike was enhanced from 26.7% to 44.6%, 23.5% to 62.7%, 91.5% to 154%, and 137% to 182%, respectively. It was concluded that the biochar-based application of PGPR induced drought tolerance in wheat under water deficit conditions, ultimately improving the production and yield of wheat.

Exploring Bacillus mycoides PM35 efficacy in enhancing rice (Oryza sativa L.) response to different types of microplastics through gene regulation and cellular fractionation

Soil contamination with microplastics (MPs) is a persistent threat to crop production worldwide. With a wide range of MP types, including polystyrene (PS), polyvinyl chloride (PVC) and polyethylene (PE), contaminating our environment, it is important to understand their impact on agricultural productivity. The present study was conducted to investigate the effects of different types of MPs (PS, PVC and PE) on various aspects of plant growth. Specifically, we examined growth and biomass, photosynthetic pigments, gas exchange attributes, oxidative stress responses, antioxidant compound activity (both enzymatic and non-enzymatic), gene expression, proline metabolism, the AsA-GSH cycle and cellular fractionation and nutritional status, in different parts of rice (Oryza sativa L.) seedlings, which were also exposed to plant growth promoting rhizobacteria (PGPR), i.e. Bacillus mycoides PM35, i.e. 20 μL. The research outcomes indicated that the different types of MPs in the soil notably reduced plant growth and biomass, photosynthetic pigments and gas exchange attributes. However, MP stress also induced oxidative stress in the roots and shoots of the plants by increasing malondialdehyde (MDA), hydrogen peroxide (H2O2) and electrolyte leakage (EL) which also induced increased compounds of various enzymatic and non-enzymatic antioxidants and also the gene expression. Furthermore, a significant increase in proline metabolism, the AsA-GSH cycle, and the fractionations of cellular components was observed. Although the application of B. mycoides PM35 showed a significant increase in plant growth and biomass, gas exchange characteristics, enzymatic and non-enzymatic compounds and their gene expression and also decreased oxidative stress. In addition, the application of B. mycoides PM35 enhanced cellular fractionation and decreased the proline metabolism and AsA-GSH cycle in O. sativa plants. These results open new insights for sustainable agriculture practices and hold immense promise in addressing the pressing challenges of MP contamination in agricultural soils.

Mitigating drought stress in wheat plants (Triticum Aestivum L.) through grain priming in aqueous extract of spirulina platensis

BACKGROUND

The study focuses on the global challenge of drought stress, which significantly impedes wheat production, a cornerstone of global food security. Drought stress disrupts cellular and physiological processes in wheat, leading to substantial yield losses, especially in arid and semi-arid regions. The research investigates the use of Spirulina platensis aqueous extract (SPAE) as a biostimulant to enhance the drought resistance of two Egyptian wheat cultivars, Sakha 95 (drought-tolerant) and Shandawel 1 (drought-sensitive). Each cultivar's grains were divided into four treatments: Cont, DS, SPAE-Cont, and SPAE + DS. Cont and DS grains were presoaked in distilled water for 18 h while SPAE-Cont and SPAE + DS were presoaked in 10% SPAE, and then all treatments were cultivated for 96 days in a semi-field experiment. During the heading stage (45 days: 66 days), two drought treatments, DS and SPAE + DS, were not irrigated. In contrast, the Cont and SPAE-Cont treatments were irrigated during the entire experiment period. At the end of the heading stage, agronomy, pigment fractions, gas exchange, and carbohydrate content parameters of the flag leaf were assessed. Also, at the harvest stage, yield attributes and biochemical aspects of yielded grains (total carbohydrates and proteins) were evaluated.

RESULTS

The study demonstrated that SPAE treatments significantly enhanced the growth vigor, photosynthetic rate, and yield components of both wheat cultivars under standard and drought conditions. Specifically, SPAE treatments increased photosynthetic rate by up to 53.4%, number of spikes by 76.5%, and economic yield by 190% for the control and 153% for the drought-stressed cultivars pre-soaked in SPAE. Leaf agronomy, pigment fractions, gas exchange parameters, and carbohydrate content were positively influenced by SPAE treatments, suggesting their effectiveness in mitigating drought adverse effects, and improving wheat crop performance.

CONCLUSION

The application of S. platensis aqueous extract appears to ameliorate the adverse effects of drought stress on wheat, enhancing the growth vigor, metabolism, and productivity of the cultivars studied. This indicates the potential of SPAE as an eco-friendly biostimulant for improving crop resilience, nutrition, and yield under various environmental challenges, thus contributing to global food security.

Mitigation of water stress in broccoli by soil application of humic acid

The main challenge to plant productivity is water scarcity, which is predicted to get worse with climate change, particularly in arid and semi-arid areas. Humic acid could improve plant tolerance to mitigate drought damage, which is an effective strategy to improve crop production and agriculture sustainability under limited water conditions in these regions, but its effective application rates should also be established. Thus, two field experiments were carried out at the Qaha Vegetable Research Farm in Qalubia Governorate, Egypt, during the two seasons of 2020-21 and 2021-22 on clay soil. The present study investigated the effect of three rates of humic acid application (0, 4.8, and 9.6 kg ha-1) on growth, yield, and quality of broccoli cv. Montop F1 hybrid under well-watered and drought conditions. Drought was induced by missing alternate irrigation. Soluble humic acid as potassium-humate was applied three times with irrigation water at the time of the first three irrigations of drought treatment. Water-stressed plants had a decrease in growth, yield, leaf chlorophyll, and nutrient content, while they showed an increase in the contents of leaf proline and curd dry matter and total soluble solids as well as water use efficiency, in both seasons. Soil application of humic acid was effective in mitigating the adverse effects of water deficit stress on the growth and yield of broccoli. Water-stressed plants had the highest WUE value (9.32 and 9.36 kg m3-1 in the first and second seasons, respectively) when the maximal humic acid rate was applied. Humic acid at a high level (9.6 kg ha-1) had the most promising results and represents an opportunity that must be applied to improve broccoli yield and its production sustainability in arid and semiarid regions.

Prospecting the Potential of Plant Growth-Promoting Microorganisms for Mitigating Drought Stress in Crop Plants

Drought is a global phenomenon affecting plant growth and productivity, the severity of which has impacts around the whole world. A number of approaches, such as agronomic, conventional breeding, and genetic engineering, are followed to increase drought resilience; however, they are often time consuming and non-sustainable. Plant growth-promoting microorganisms are used worldwide to mitigate drought stress in crop plants. These microorganisms exhibit multifarious traits, which not only help in improving plant and soil health, but also demonstrate capabilities in ameliorating drought stress. The present review highlights various adaptive strategies shown by these microbes in improving drought resilience, such as modulation of various growth hormones and osmoprotectant levels, modification of root morphology, exopolysaccharide production, and prevention of oxidative damage. Gene expression patterns providing an adaptive edge for further amelioration of drought stress have also been studied in detail. Furthermore, the practical applications of these microorganisms in soil are highlighted, emphasizing their potential to increase crop productivity without compromising long-term soil health. This review provides a comprehensive coverage of plant growth-promoting microorganisms-mediated drought mitigation strategies, insights into gene expression patterns, and practical applications, while also guiding future research directions.

A plant's perception of growth-promoting bacteria and their metabolites

Many recent studies have highlighted the importance of plant growth-promoting (rhizo)bacteria (PGPR) in supporting plant's development, particularly under biotic and abiotic stress. Most focus on the plant growth-promoting traits of selected strains and the latter's effect on plant biomass, root architecture, leaf area, and specific metabolite accumulation. Regarding energy balance, plant growth is the outcome of an input (photosynthesis) and several outputs (i.e., respiration, exudation, shedding, and herbivory), frequently neglected in classical studies on PGPR-plant interaction. Here, we discuss the primary evidence underlying the modifications triggered by PGPR and their metabolites on the plant ecophysiology. We propose to detect PGPR-induced variations in the photosynthetic activity using leaf gas exchange and recommend setting up the correct timing for monitoring plant responses according to the specific objectives of the experiment. This research identifies the challenges and tries to provide future directions to scientists working on PGPR-plant interactions to exploit the potential of microorganisms' application in improving plant value.

Fostering plant growth performance under drought stress using rhizospheric microbes, their gene editing, and biochar

Stress due to drought lowers crop yield and frequently leads to a rise in food scarcity. Plants' intricate metabolic systems enable them to tolerate drought stress, but they are unable to handle it well. Adding some external, environmentally friendly supplements can boost plant growth and productivity when it comes to drought-stressed plants. In order to prevent the detrimental effects of drought in agricultural regions, environmentally friendly practices must be upheld. Plant growth-promoting rhizobacteria (PGPR) can exhibit beneficial phytostimulation, mineralization, and biocontrol activities under drought stress. The significant impact of the PGPR previously reported has not been accepted as an effective treatment to lessen drought stress. Recent studies have successfully shown that manipulating microbes can be a better option to reduce the severity of drought in plants. In this review, we demonstrate how modifying agents such as biochar, PGPR consortia, PGPR, and mycorrhizal fungi can help overcome drought stress responses in crop plants. This article also discusses CRISPR/Cas9-modifiable genes, increase plant's effectiveness in drought conditions, and increase plant resistance to drought stress. With an eco-friendly approach in mind, there is a need for practical management techniques having potential prospects based on an integrated strategy mediated by CRISPR-Cas9 editing, PGPR, which may alleviate the effects of drought stress in crops and aid in achieving the United Nation Sustainable Development Goals (UN-SDGs-2030).

Relationships between Phyllosphere Bacterial Communities and Leaf Functional Traits in a Temperate Forest

As a vital component of biodiversity, phyllosphere bacteria in forest canopy play a critical role in maintaining plant health and influencing the global biogeochemical cycle. There is limited research on the community structure of phyllosphere bacteria in natural forests, which creates a gap in our understanding of whether and/or how phyllosphere bacteria are connected to leaf traits of their host. In this study, we investigated the bacterial diversity and composition of the canopy leaves of six dominant tree species in deciduous broad-leaved forests in northeastern China, using high-throughput sequencing. We then compare the differences in phyllosphere bacterial community structure and functional genes of dominant tree species. Fourteen key leaf functional traits of their host trees were also measured according to standard protocols to investigate the relationships between bacterial community composition and leaf functional traits. Our result suggested that tree species with closer evolutionary distances had similar phyllosphere microbial alpha diversity. The dominant phyla of phyllosphere bacteria were Proteobacteria, Actinobacteria, and Firmicutes. For these six tree species, the functional genes of phyllosphere bacteria were mainly involved in amino acid metabolism and carbohydrate metabolism processes. The redundancy and envfit analysis results showed that the functional traits relating to plant nutrient acquisition and resistance to diseases and pests (such as leaf area, isotope carbon content, and copper content) were the main factors influencing the community structure of phyllosphere bacteria. This study highlights the key role of plant interspecific genetic relationships and plant attributes in shaping phyllosphere bacterial diversity.

Unveiling novel insights into haloarchaea (Halolamina pelagica CDK2) for alleviation of drought stress in wheat

Plant growth promoting microorganisms have various implications for plant growth and drought stress alleviation; however, the roles of archaea have not been explored in detail. Herein, present study was aimed for elucidating potential of haloarchaea (Halolamina pelagica CDK2) on plant growth under drought stress. Results showed that haloarchaea inoculated wheat plants exhibited significant improvement in total chlorophyll (100%) and relative water content (30.66%) compared to the uninoculated water-stressed control (30% FC). The total root length (2.20-fold), projected area (1.60-fold), surface area (1.52-fold), number of root tips (3.03-fold), number of forks (2.76-fold) and number of links (1.45-fold) were significantly higher in the inoculated plants than in the uninoculated water stressed control. Additionally, the haloarchaea inoculation resulted in increased sugar (1.50-fold), protein (2.40-fold) and activity of antioxidant enzymes such as superoxide dismutase (1.93- fold), ascorbate peroxidase (1.58-fold), catalase (2.30-fold), peroxidase (1.77-fold) and glutathione reductase (4.70-fold), while reducing the accumulation of proline (46.45%), glycine betaine (35.36%), lipid peroxidation (50%), peroxide and superoxide radicals in wheat leaves under water stress. Furthermore, the inoculation of haloarchaea significantly enhanced the expression of stress-responsive genes (DHN, DREB, L15, and TaABA-8OH) and wheat vegetative growth under drought stress over the uninoculated water stressed control. These results provide novel insights into the plant-archaea interaction for plant growth and stress tolerance in wheat and pave the way for future research in this area.

Alleviation of banded leaf and sheath blight disease incidence in maize by bacterial volatile organic compounds and molecular docking of targeted inhibitors in Rhizoctonia solani

Rhizoctonia solani (RS) is a pathogenic fungus that affects maize (Zea mays L.) plants and causes banded leaf and sheath blight (BLSB) with severe consequences leading to significant economic losses. Contrarily, rhizobacteria produce numerous volatile organic compounds (VOCs) that help in devising the environment-friendly mechanism for promoting plant growth and stress alleviation without having physical contact with plants. In the present study, 15 rhizobacterial strains were tested for their antagonism against RS. The antagonistic potential of VOCs of the tested plant growth-promoting rhizobacteria (PGPR) strains ranged from 50% to 80% as compared to the control (without PGPR). Among these 15 strains, the maximum (80%) antagonistic activity was exhibited by Pseudomonas pseudoalcaligenes SRM-16. Thus, the potential of VOCs produced by P. pseudoalcaligenes SRM-16 to alleviate the BLSB disease in maize was evaluated. A pot experiment was conducted under greenhouse conditions to observe the effect of VOCs on disease resistance of BLSB-infected seedlings. Overall, maize seedlings exposed to VOCs showed a significant increase in disease resistance as indicated by a reduced disease score than that of unexposed infected plants. The VOCs-exposed maize exhibited lower (11.6%) disease incidence compared to the non-inoculated maize (14.1%). Moreover, plants exposed to VOCs displayed visible improvements in biomass, photosynthetic pigments, osmoregulation, and plant antioxidant and defense enzyme activities compared to the healthy but unexposed seedlings. Simultaneous application of RS and VOCs enhanced superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), phenylalanine ammonia lyase (PAL), ascorbate peroxidase (APX), and polyphenol oxidase (PPO) activities by 96.7%, 266.6%, 313.7%, 246.6%, 307%, and 149.7%, respectively, in the roots and by 81.6%, 246.4%, 269.5%, 269.6%, 329%, and 137.6%, respectively, in the shoots, relative to those of the control plants. The binding affinity of the VOCs (2-pentylfuran, 2,3-butanediol, and dimethyl disulfide) with CRZ1 and S9 protein receptors of RS was assessed by deploying in silico methods. Overall, 2-pentylfuran exhibited a binding affinity with both the selected receptors of RS, while 2,3-butanediol and dimethyl disulfide were able to bind S9 protein only. Hence, it can be deduced that S9 protein receptors are more likely the target RS receptors of bacterial VOCs to inhibit the proliferation of RS.

Halotolerant plant growth-promoting bacteria, Bacillus pumilus, modulates water status, chlorophyll fluorescence kinetics and antioxidant balance in salt and/or arsenic-exposed wheat

Seed priming is an effective and novel technique and the use of eco-friendly biological agents improves the physiological functioning in the vegetative stage of plants. This procedure ensures productivity and acquired stress resilience in plants against adverse conditions without contaminating the environment. Though the mechanisms of bio-priming-triggered alterations have been widely explained under induvial stress conditions, the interaction of combined stress conditions on the defense system and the functionality of photosynthetic apparatus in the vegetative stage after the inoculation to seeds has not been fully elucidated. After Bacillus pumilus inoculation to wheat seeds (Triticum aestivum), three-week-old plants were hydroponically exposed to the alone and combination of salt (100 mM NaCl) and 200 μM sodium arsenate (Na₂HAsO₄·7H₂O, As) for 72 h. Salinity and As pollutant resulted in a decline in growth, water content, gas exchange parameters, fluorescence kinetics and performance of photosystem II (PSII). On the other hand, the seed inoculation against stress provided the alleviation of relative growth rate (RGR), relative water content (RWC) and chlorophyll fluorescence. These negative impacts were reversed by B. pumilus inoculation. Since there was no effective antioxidant capacity, As and/or salinity caused induction of H₂O₂ accumulation and thiobarbituric acid reactive substances content (TBARS) in wheat leaves. The inoculated leaves had high activity of superoxide dismutase (SOD) under stress. B. pumilis decreased the NaCl-induced toxic H₂O₂ levels by increasing peroxidase (POX) and enzymes/non-enzymes related to ascorbate-glutathione (AsA-GSH) cycle. In the presence of As exposure, the inoculated plants exhibited an induction in CAT activity. On the other hand, for H₂O₂ scavenging, the improvement in the AsA-GSH cycle was observed in bacterium priming plants plus the combined stress treatment. Since B. pumilus inoculation reduced H₂O₂ levels against all stress treatments, lipid peroxidation subsequently decreased in wheat leaves. The findings obtained from our study explained that the seed inoculation with B. pumilus provided an activation in the defense system and protection in growth, water status, and gas exchange regulation in wheat leaves against the combination of salt and As.

Potential of growth-promoting bacteria in maize (Zea mays L.) varies according to soil moisture

Climate change has caused irregularities in water distribution, which affect the soil drying-wetting cycle and the development of economically important agricultural crops. Therefore, the use of plant growth-promoting bacteria (PGPB) emerges as an efficient strategy to mitigate negative impacts on crop yield. We hypothesized that the use of PGPB (in consortium or not) had potential to promote maize (Zea mays L.) growth under a soil moisture gradient in both non-sterile and sterile soils. Thirty PGPB strains were characterized for direct plant growth-promotion and drought tolerance induction mechanisms and were used in two independent experiments. Four soil water contents were used to simulate a severe drought (30% of field capacity [FC]), moderate drought (50% of FC), no drought (80% of FC) and, finally, a water gradient comprising the three mentioned soil water contents (80%, 50%, and 30% of FC). Two bacteria strains (BS28-7 Arthrobacter sp. and BS43 Streptomyces alboflavus), in addition to three consortia (BC2, BC4 and BCV) stood out in maize growth performance in experiment 1 and were used in experiment 2. Overall, under moderate drought, inoculation with BS43 surpassed the control treatment in root dry mass and nutrient uptake. Considering the water gradient treatment (80-50-30% of FC), the greatest total biomass was found in the uninoculated treatment when compared to BS28-7, BC2, and BCV. The greatest development of Z. mays L. was only observed under constant water stress conditions in the presence of PGPB. This is the first report that demonstrated the negative effect of individual inoculation of Arthrobacter sp. and the consortium of this strain with Streptomyces alboflavus on the growth of Z. mays L. based on a soil moisture gradient; however, future studies are needed for further validation.

Salinity stress endurance of the plants with the aid of bacterial genes

The application of plant growth-promoting bacteria (PGPB) is vital for sustainable agriculture with continuous world population growth and an increase in soil salinity. Salinity is one of the severe abiotic stresses which lessens the productivity of agricultural lands. Plant growth-promoting bacteria are key players in solving this problem and can mitigate salinity stress. The highest of reported halotolerant Plant growth-promoting bacteria belonged to Firmicutes (approximately 50%), Proteobacteria (40%), and Actinobacteria (10%), respectively. The most dominant genera of halotolerant plant growth-promoting bacteria are Bacillus and Pseudomonas. Currently, the identification of new plant growth-promoting bacteria with special beneficial properties is increasingly needed. Moreover, for the effective use of plant growth-promoting bacteria in agriculture, the unknown molecular aspects of their function and interaction with plants must be defined. Omics and meta-omics studies can unreveal these unknown genes and pathways. However, more accurate omics studies need a detailed understanding of so far known molecular mechanisms of plant stress protection by plant growth-promoting bacteria. In this review, the molecular basis of salinity stress mitigation by plant growth-promoting bacteria is presented, the identified genes in the genomes of 20 halotolerant plant growth-promoting bacteria are assessed, and the prevalence of their involved genes is highlighted. The genes related to the synthesis of indole acetic acid (IAA) (70%), siderophores (60%), osmoprotectants (80%), chaperons (40%), 1-aminocyclopropane-1-carboxylate (ACC) deaminase (50%), and antioxidants (50%), phosphate solubilization (60%), and ion homeostasis (80%) were the most common detected genes in the genomes of evaluated halotolerant plant growth-promoting and salinity stress-alleviating bacteria. The most prevalent genes can be applied as candidates for designing molecular markers for screening of new halotolerant plant growth-promoting bacteria.

Melatonin production by rhizobacteria native strains: Towards sustainable plant growth promotion strategies

In the current context of climate change and water deficit, the selection of native beneficial microorganisms, such as plant growth-promoting rhizobacteria (PGPR), has become a trend for sustainable agriculture due to their ability to improve plant-bacteria interaction with a minimal adverse effect on the soil microbiota compared to commercial PGPR. Until now, the production of phytohormones like melatonin (MT) by native PGPR and their effect on endogenous MT levels in plants have been poorly studied. MT is a ubiquitous phytohormone that protects plants against biotic and abiotic stress by improving the tolerance of stressed plants. In this work, the production of MT by two native PGPR, Enterobacter 64S1 and Pseudomonas 42P4, was evaluated and both PGPR were applied in Arabidopsis thaliana plants grown under drought conditions to assess the inoculation effects. Parameters such as plant growth, leaf cellular membrane damage, leaf protective compounds, and endogenous MT levels under drought and irrigation conditions were evaluated. The results demonstrated that the native strains Pseudomonas 42P4 and Enterobacter 64S1 produce MT and increase the content of endogenous MT in A. thaliana plants under drought. These native strains improved the tolerance of arabidopsis plants to drought by preventing oxidative and membrane damages and improving plant growth. To the best of our knowledge, this is the first report on MT production by native PGPR and their effects on endogenous MT levels in arabidopsis plants, setting the bases to elucidate the role of native PGPR on water deficit conditions.

Recent Developments in the Application of Plant Growth-Promoting Drought Adaptive Rhizobacteria for Drought Mitigation

Drought intensity that has increased as a result of human activity and global warming poses a serious danger to agricultural output. The demand for ecologically friendly solutions to ensure the security of the world's food supply has increased as a result. Plant growth-promoting rhizobacteria (PGPR) treatment may be advantageous in this situation. PGPR guarantees the survival of the plant during a drought through a variety of processes including osmotic adjustments, improved phytohormone synthesis, and antioxidant activity, among others and these mechanisms also promote the plant's development. In addition, new developments in omics technology have improved our understanding of PGPR, which makes it easier to investigate the genes involved in colonizing plant tissue. Therefore, this review addresses the mechanisms of PGPR in drought stress resistance to summarize the most current omics-based and molecular methodologies for exploring the function of drought-responsive genes. The study discusses a detailed mechanistic approach, PGPR-based bioinoculant design, and a potential roadmap for enhancing their efficacy in combating drought stress.

Bacterial bioinoculants adapted for sustainable plant health and soil fertility enhancement in Namibia

The increase in dryland agriculture elicits the need to develop sustainable practices that improve crop yield and protect soil fertility. The use of biofertilisers adapted to nutrient deficient soils and arid climates would help achieve this. In this review, the use of plant growth-promoting bacteria is explored as a possible solution to the current state of dryland agriculture and climate change threats to agriculture. Plant microbe interactions form the basis of this review as evidence has shown that these interactions often exist to improve the health of plants. This is achieved by the production of important biochemicals and enzymes like indole acetic acid and amino cyclopropane-1-carboxylate deaminase while also actively protecting plants from pathogens including fungal pathogens. Research, therefore, has shown that these plant-growth promoting bacteria may be exploited and developed into biofertilisers. These biofertilisers are both economically and environmentally sustainable while improving soil quality and crop yield. The literature presented in this review is in context of the Namibian climate and soil profiles.

Formulating Zwitterionic, Responsive Polymers for Designing Smart Soils

The design of new remediation strategies and materials for treating saline-alkaline soils is of fundamental and practical importantance for many applications. Conventional soil remediation strategies mainly focus on the development of fertilizers or additives for water, nutrient, and heavy metal managements in soils, but they often overlook a soil sensing function for early detection of salinization/alkalization levels toward optimal and timely soil remediation. Here, new smart soils, structurally consisting of the upper signal soil and the bottom hygroscopic bed and chemically including zwitterionic, thermo-responsive poly(NIPAM-co-VPES) and poly(NIPAM-co-SBAA) aerogels in each soil layer are formulated. Upon salinization, the resultant smart soils exhibit multiple superior capacities for reducing the soil salinity and alkalinity through ion exchange, controlling the water cycling, modulating the degradation of pyridine-base ligands into water-soluble, nitrogenous salts-rich ingredients for soil fertility, and real-time monitoring salinized soils via pH-induced allochroic color changes. Further studies of plant growth in smart soils with or without salinization treatments confirm a synergy effect of soil remediation and soil sensing on facilitating the growth of plants and increasing the saline-alkaline tolerance of plants. The esign concept of smart soils can be further expanded for soil remediation and assessment.

Biochar enhances wheat crop productivity by mitigating the effects of drought: Insights into physiological and antioxidant defense mechanisms

Drought stress is a major limitation in wheat production around the globe. Organic amendments could be the possible option in semi-arid climatic conditions to mitigate the adverse effects of drought at critical growth stages. Wheat straw biochar (BC₀ = Control, BC₁ = 3% biochar and BC₂ = 5% biochar) was used to alleviate the drought stress at tillering (DTS), flowering (DFS), and grain filling (DGFS) stages. Drought stress significantly reduced the growth and yield of wheat at critical growth stages, with DGFS being the most susceptible stage, resulting in significant yield loss. Biochar application substantially reduced the detrimental effects of drought by improving plant height (15.74%), fertile tiller count (17.14%), spike length (16.61%), grains per spike (13.89%), thousand grain weight (10.4%), and biological yield (13.1%) when compared with the control treatment. Furthermore, physiological parameters such as water use efficiency (38.41%), stomatal conductance (42.76%), chlorophyll a (19.3%), chlorophyll b (22.24%), transpiration rate (39.17%), photosynthetic rate (24.86%), electrolyte leakage (-42.5%) hydrogen peroxide (-18.03%) superoxide dismutase (24.66%), catalase (24.11%) and peroxidase (-13.14%) were also improved by biochar application. The use of principal component analysis linked disparate scales of our findings to explain the changes occurred in wheat growth and yield in response to biochar application under drought circumstances. In essence, using biochar at 5% rate could be a successful strategy to promote wheat grain production by reducing the hazardous impacts of drought stress.

Alleviation of drought stress by root-applied thiourea is related to elevated photosynthetic pigments, osmoprotectants, antioxidant enzymes, and tubers yield and suppressed oxidative stress in potatoes cultivars

The growth and productivity of plants are enhanced by the use of thiourea (TU) under stressful conditions. When TU is applied as a rooting medium, it improves plant growth characteristics and other physiological parameters in stressed environment. A pot experiment was conducted in the botanical garden of the Government College University, Faisalabad 38000, Pakistan to examine the TU-mediated fluctuations in some crucial physio-biochemical parameters and the oxidative defense of potatoes under a restricted water supply. For this purpose, two potato cultivars (potato-SH-5 and potato-FD-73) were sown in pots containing 10 kg of soil. Water was regularly applied to the pots until germination. After 2 weeks of germination, drought stress with 65% field capacity was imposed, while the control was subjected to 100% field capacity. TU, as a rooting medium, was applied at the vegetative stage (0 (no application), 0.5, 0.75 mM). A substantial reduction in the total number of leaves, leaf area, tuber biomass (fresh and dry weight), photosynthetic pigments, membrane permeability, and leaf relative water content (RWC) was recorded in plants under drought stress conditions as compared to control plants. The damaging effects of water stress were more critical for cv. potato-FD-73 as compared to cv. potato-SH-5. In contrast, drought stress enhanced the malondialdehyde (MDA) and hydrogen peroxide (H2O2) content while also increased antioxidant enzyme activities (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)) and triggered the accumulation of soluble proteins, soluble sugars, proline, and phenolic and anthocyanin contents. However, TU applied as rooting medium at 0.5 and 0.75 mM was effective in reducing the detrimental effects of water stress in both cultivars. Furthermore, increasing levels of TU enhanced chlorophyll pigments, dissolved proteins, complete dissolved sugars, and enzymatic capabilities of POD, SOD, and CAT, while reducing the MDA and H2O2 in both cultivars under stress conditions. In conclusion, TU improved the yield and chlorophyll pigments of potato plants by mitigating the adverse effects of drought stress through reduced EL, MDA, and H2O2 contents and improved activities of enzymatic and non-enzymatic antioxidants and osmoprotectants.

Decoding the potential of a new Pseudomonas putida strain for inducing drought tolerance of tomato (Solanum lycopersicum) plants through seed biopriming

A total of 11 potential plant growth promoting rhizobacteria previously isolated from naturally stressed environments were evaluated for various traits of interest for a beneficial symbiosis with plants, including colonization ability, biofilm formation, motility, exopolysaccharide production and salt tolerance. The vast majority of the strains were found to possess multiple plant growth promoting traits. Nevertheless, the intensity varied among isolates, with those originated from tomato plants being more efficient colonizers. The strain SAESo11, genetically characterized as a Pseudomonas putida member was selected for further investigation of its potential to alleviate drought stress in tomato seedlings. Inoculation with SAESo11 mitigated the negative effects of drought stress as indicated by growth and photosynthetic indices. Furthermore, bacterial inoculation enhanced H₂O₂ content and malondialdehyde levels in colonized plants. Drought treatment did not further alter the oxidative status of these plants. Similarly, total phenolic content and antioxidant enzyme activity were induced in plant tissues in response to drought stress only at the absence of inoculum. These results indicated that inoculation with the selected strain imposed plants at a priming state, that enabled them to respond more robustly at the exposure to drought stress and efficiently attenuated the drought-induced injury. This state of plant alertness mediated by SAESo11 occurred at no cost to growth, highlighting its role as a potential plant priming agent.

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.

Identification, characterization and optimization of phosphate solubilizing rhizobacteria (PSRB) from rice rhizosphere

Two billion people worldwide take rice (Oryza sativa L.) as a staple food. Phosphorus (P) and Nitrogen (N) are the major requirements of rice; although these are available in limited concentrations within rice growing regions. Among different types of Plant growth-promoting rhizobacteria (PGPR), Phosphate solubilizing rhizobacteria (PSRB) constitute an important class. These are known for plant growth promotion by enhancing P and N uptake. PSRB are nowadays used as biofertilizers to restore the soil health. Under the present investigation identification, characterization and optimization of phosphate solubilizing activity of these microbes at different pH, temperature and salt concentrations was carried out. Thirty-seven isolates were recovered from different regions of rice rhizosphere on Pikovskaya (PVK) agar among which 15 isolates were recovered from R.S. Pura, 12 isolates from Bishnah and 10 isolates were recovered from Akhnoor sector of Jammu, India. A prominent halo zone of clearance was developed around the colonies of 12 different isolates, indicating phosphate solubilization activity. Four distinct isolates were amplified, cloned and sequenced for taxonomic identification using 16S primers. The results indicated that PS 1, PS 2, PS 3, PS 4 were related to Pseudomonas aeruginosa, Bacillus subtilis strain 1, B. subtilis strain 2, B. subtilis strain 3, respectively. These strains when grown at a wide range of ecological factors showed maximum growth at pH between 6.8 and 8.8, temperature between 28 °C and 37 °C and salinity between 1% and 2%. Screening for phosphate solubilization activity revealed that the halo zone diameter formed by these isolates extended from 2.1 to 3.2 mm. The phosphate solubilizing efficiency (SE) ranged from 35.4 to 50.9 with highest value of 50.9 by PS4 and maximum P solubilization of 10.22 µg/ml was recorded by PS4 at 7th day. Phosphate solubilization activity of these identified PSRB strains can be utilized and explored in the rice growing belts of Jammu region which are deficient in phosphorus. MIC value for zinc sulphate heptahydrate in 12 isolates varied from 1 mg/ml to 6 mg/ml. Phosphate solubilization activity and MIC of these identified PSRB strains can be utilized and explored in the rice growing belts of Jammu region which are deficient in phosphorus.

Insights into the Bacterial and Nitric Oxide-Induced Salt Tolerance in Sugarcane and Their Growth-Promoting Abilities

Soil salinity causes severe environmental stress that affects agriculture production and food security throughout the world. Salt-tolerant plant-growth-promoting rhizobacteria (PGPR) and nitric oxide (NO), a distinctive signaling molecule, can synergistically assist in the alleviation of abiotic stresses and plant growth promotion, but the mechanism by which this happens is still not well known. In the present study, in a potential salt-tolerant rhizobacteria strain, ASN-1, growth up to 15% NaCl concentration was achieved with sugarcane rhizosphere soil. Based on 16S-rRNA gene sequencing analysis, the strain ASN-1 was identified as a Bacillus xiamenensis. Strain ASN-1 exhibits multiple plant-growth-promoting attributes, such as the production of indole-3-acetic acid, 1-aminocyclopropane-1-carboxylate deaminase, siderophores, HCN, ammonia, and exopolysaccharides as well as solubilized phosphate solubilization. Biofilm formation showed that NO enhanced the biofilm and root colonization capacity of the PGPR strain ASN-1 with host plants, evidenced by scanning electron microscopy. The greenhouse study showed that, among the different treatments, the combined application of PGPR and sodium nitroprusside (SNP) as an NO donor significantly (p ≤ 0.05) enhanced sugarcane plant growth by maintaining the relative water content, electrolyte leakage, gas exchange parameters, osmolytes, and Na⁺/K⁺ ratio. Furthermore, PGPR and SNP fertilization reduced the salinity-induced oxidative stress in plants by modulating the antioxidant enzyme activities and stress-related gene expression. Thus, it is believed that the acquisition of advanced information about the synergistic effect of salt-tolerant PGPR and NO fertilization will reduce the use of harmful chemicals and aid in eco-friendly sustainable agricultural production under salt stress conditions.

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.

Plant growth promoting bacteria induce anti-quorum-sensing substances in chickpea legume seedling bioassay

Microorganisms and their hosts communicate through an array of signals. Many physiological processes regulated in quorum sensing (QS) are dependent on auto-inducers, like N-acyl-homoserine lactones (AHLs) as in numerous groups of both gram-positive and gram-negative bacteria. In vitro grown seven-day old chickpea seedlings treated with plant growth promoting bacteria (PGPRs) were used to screen the AHL mimicking and for phytochemical substances like phytohormones and secondary metabolites such as phenolics and flavonoids. Potential anti-quorum sensing (anti-QS) activity surrounding the roots on semi-solid agar lawn of Chromobacterium violaceum (ATCC12742) was observed. Crude protein (4.46-8.30 μg/mL) and methanolic extracts (100 μg/mL) of seedling gave moderate anti-QS activity against CV12742 anti QS bioassay, respectively. Crude protein and methanolic extract of Bacillus amyloliquefaciens (34.00 ± 2.23; 34.00 ± 4.33 mm) and B. subtilis A (27.00 ± 2.10; 3.29 ± 2.16 mm) treated samples showed higher zone of inhibition due to anti-QS activity. Phytohormone analysis using LC-MS for zeatin, auxin and methyl jasmonate (MeJA) indicated that phytohormones were significantly upregulated by 1909.80 ng/g FW, 669.67 ng/g FW and 244.55 ng/g FW, respectively in Pseudomonas brassicacearum treated seedlings compared to control. UHPLC of PGPR treated seedlings showed overly expressed gallic acid, protocatechuic acid, catechin, p-hydroxybenzoic acid, caffeic acid, catechol, vanillin, and ferulic acid in B. amyloliquefaciens treated seedlings compared to others. Enrichment analysis identified significant pathways related to metabolism, biosynthesis of secondary metabolites. The present study indicates that chickpea neutralizes an extensive range of functional responses to AHLs that may play important role in legume host-microbe interactions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01034-x.

Insights into the Interactions among Roots, Rhizosphere, and Rhizobacteria for Improving Plant Growth and Tolerance to Abiotic Stresses: A Review

Abiotic stresses, such as drought, salinity, heavy metals, variations in temperature, and ultraviolet (UV) radiation, are antagonistic to plant growth and development, resulting in an overall decrease in plant yield. These stresses have direct effects on the rhizosphere, thus severely affect the root growth, and thereby affecting the overall plant growth, health, and productivity. However, the growth-promoting rhizobacteria that colonize the rhizosphere/endorhizosphere protect the roots from the adverse effects of abiotic stress and facilitate plant growth by various direct and indirect mechanisms. In the rhizosphere, plants are constantly interacting with thousands of these microorganisms, yet it is not very clear when and how these complex root, rhizosphere, and rhizobacteria interactions occur under abiotic stresses. Therefore, the present review attempts to focus on root–rhizosphere and rhizobacterial interactions under stresses, how roots respond to these interactions, and the role of rhizobacteria under these stresses. Further, the review focuses on the underlying mechanisms employed by rhizobacteria for improving root architecture and plant tolerance to abiotic stresses.

Biocontrol Potential of Bacillus amyloliquefaciens against Botrytis pelargonii and Alternaria alternata on Capsicum annuum

The aim of this study was to assess the ability of Bacillus amyloliquefaciens, to augment plant growth and suppress gray mold and leaf spot in pepper plants. Morphological modifications in fungal pathogen hyphae that expanded toward the PGPR colonies were detected via scanning electron microscope. Furthermore, preliminary screening showed that PGPR could produce various hydrolytic enzymes in its media. Treatments with B. amyloliquefaciens suppressed Botrytis gray mold and Alternaria leaf spot diseases on pepper caused by Botrytis pelargonii and Alternaria alternata, respectively. The PGPR strain modulated plant physio-biochemical processes. The inoculation of pepper with PGPR decreased protein, amino acid, antioxidant, hydrogen peroxide, lipid peroxidation, and abscisic acid levels but increased salicylic acid and sugar levels compared to those of uninoculated plants, indicating a mitigation of the adverse effects of biotic stress. Moreover, gene expression studies confirmed physio-biochemical findings. PGPR inoculation led to increased expression of the CaXTH genes and decreased expression of CaAMP1, CaPR1, CaDEF1, CaWRKY2, CaBI-1, CaASRF1, CaSBP11, and CaBiP genes. Considering its beneficial effects, the inoculation of B. amyloliquefaciens can be proposed as an eco-friendly alternative to synthetic chemical fungicides.

Volatile organic compounds produced by Pseudomonas pseudoalcaligenes alleviated drought stress by modulating defense system in maize (Zea mays L.)

Research on plant growth-promoting bacteria (PGPR) revealed an effective role of bacterial volatile organic compounds (VOCs) in stress alleviation. Out of 15 PGPR strains, infection with VOCs from Pseudomonas pseudoalcaligenes' resulted in maximum germination, growth promotion, and drought tolerance in maize plants. The VOCs of P. pseudoalcaligenes caused induced systemic tolerance in maize plants during 7 days of drought stress. The VOCs exposed plants displayed resistance to drought stress by reducing electrolyte leakage and malondialdehyde content and increasing the synthesis of photosynthetic pigments, proline, and phytohormones contents. Maize plants revealed enhanced resistance by showing higher activities of antioxidant defense enzymes both in shoots and roots under drought stress. Activities of antioxidant enzymes were more pronounced in shoots than roots. Gas chromatography and mass spectrophotometric (GC-MS) analysis comparing VOCs produced by the most efficient P. pseudoalcaligenes strain and inefficient strains of Pseudomonas sp. grown in culture media revealed nine compounds that they had in common. However, dimethyl disulfide, 2,3-butanediol, and 2-pentylfuran were detected only in P. pseudoalcaligenes, indicating these compounds are potential candidates for drought stress induction. Further studies are needed to unravel the molecular mechanisms of VOCs-mediated systemic drought tolerance in plants related to each identified VOC.

Delineation of mechanistic approaches employed by plant growth promoting microorganisms for improving drought stress tolerance in plants

Drought stress is expected to increase in intensity, frequency, and duration in many parts of the world, with potential negative impacts on plant growth and productivity. The plants have evolved complex physiological and biochemical mechanisms to respond and adjust to water-deficient environments. The physiological and biochemical mechanisms associated with water-stress tolerance and water-use efficiency have been extensively studied. Besides these adaptive and mitigating strategies, the plant growth-promoting rhizobacteria (PGPR) play a significant role in alleviating plant drought stress. These beneficial microorganisms colonize the endo-rhizosphere/rhizosphere of plants and enhance drought tolerance. The common mechanism by which these microorganisms improve drought tolerance included the production of volatile compounds, phytohormones, siderophores, exopolysaccharides, 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase), accumulation of antioxidant, stress-induced metabolites such as osmotic solutes proline, alternation in leaf and root morphology and regulation of the stress-responsive genes. The PGPR is an easy and efficient alternative approach to genetic manipulation and crop enhancement practices because plant breeding and genetic modification are time-consuming and expensive processes for obtaining stress-tolerant varieties. In this review, we will elaborate on PGPR's mechanistic approaches in enhancing the plant stress tolerance to cope with the drought stress.

Ameliorative Effects of Exogenous Proline on Photosynthetic Attributes, Nutrients Uptake, and Oxidative Stresses under Cadmium in Pigeon Pea (Cajanus cajan L.)

Proline plays a significant role in the plant response to stress conditions. However, its role in alleviating metal-induced stresses remains elusive. We conducted an experiment to evaluate the ameliorative role of exogenous proline on cadmium-induced inhibitory effects in pigeon pea subjected to different Cd treatments (4 and 8 mg/mL). Cadmium treatments reduced photosynthetic attributes, decreased chlorophyll contents, disturbed nutrient uptake, and affected growth traits. The elevated activity of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), in association with relatively high contents of hydrogen peroxide, thiobarbituric acid reactive substances, electrolyte leakage, and endogenous proline, was measured. Exogenous proline application (3 and 6 mM) alleviated cadmium-induced oxidative damage. Exogenous proline increased antioxidant enzyme activities and improved photosynthetic attributes, nutrient uptake (Mg²⁺, Ca²⁺, K⁺), and growth parameters in cadmium-stressed pigeon pea plants. Our results reveal that proline supplementation can comprehensively alleviate the harmful effects of cadmium on pigeon pea plants.

Root Morphogenesis of Arabidopsis thaliana Tuned by Plant Growth-Promoting Streptomyces Isolated From Root-Associated Soil of Artemisia annua

In this study, the capacity to tune root morphogenesis by a plant growth-promoting rhizobacterium, Streptomyces lincolnensis L4, was investigated from various aspects including microbial physiology, root development, and root endophytic microbial community. Strain L4 was isolated from the root-associated soil of 7-year plantation of Artemisia annua. Aiming at revealing the promotion mechanism of Streptomyces on root growth and development, this study first evaluated the growth promotion characters of S. lincolnensis L4, followed by investigation in the effect of L4 inoculation on root morphology, endophytic microbiota of root system, and expression of genes involved in root development in Arabidopsis thaliana. Streptomyces lincolnensis L4 is able to hydrolyze organic and inorganic phosphorus, fix nitrogen, and produce IAA, ACC deaminase, and siderophore, which shaped specific structure of endophytic bacterial community with dominant Streptomyces in roots and promoted the development of roots. From the observation of root development characteristics, root length, root diameter, and the number of root hairs were increased by inoculation of strain L4, which were verified by the differential expression of root development-related genes in A. thaliana. Genomic traits of S. lincolnensis L4 which further revealed its capacity for plant growth promotion in which genes involved in phosphorus solubilization, ACC deamination, iron transportation, and IAA production were identified. This root growth-promoting strain has the potential to develop green method for regulating plant development. These findings provide us ecological knowledge of microenvironment around root system and a new approach for regulating root development.