Exploitation of agro-climatic environment for selection of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase producing salt tolerant indigenous plant growth promoting rhizobacteria

Fostering climate-resilient agriculture with ACC-deaminase producing rhizobacterial biostimulants from the cold deserts of the Indian Himalayas

Climate change is one of the most significant threats to agricultural productivity, which necessitates a need for more resilient and sustainable farming practices. Rhizobacterial biostimulants that secrete 1-aminocyclopropane-1-carboxylate (ACC) deaminase and enhance crop resilience and yield can serve as a potential sustainable solution. The present study provides a comprehensive analysis of ACC-deaminase producing rhizobacteria (ACCD) isolated from cold deserts of the Indian trans-Himalayas and their efficacy to improve crop resilience and productivity under diverse climatic conditions. Thirty four efficient ACCD showed ACC deaminase activity ranging from 4.9 to 24484.3 nM α-ketobutyrate/h/mg/protein. These strains also exhibited broad-spectrum plant growth promotion (PGP) attributes, including tri-calcium phosphate (TCP) solubilization ranging from 2.4 to 687.5 μg/ml, siderophore production ranging from 62 to 224% and indole-3-acetic acid (IAA)-like auxin production ranging from 0.9 to 88.2 μg/ml. 16S rRNA gene sequencing of efficient strains showed their belonging to 10 genera, including Acinetobacter, Agrobacterium, Arthrobacter, Cellulomonas, Enterobacter, Microbacterium, Neomicrococcus, Priestia, Pseudomonas, and Rhizobium. Among these, Pseudomonas was the dominant genus with high ACC-deaminase activity and multiple PGP traits. These strains also showed growth under various stressed culture conditions, including acidity/alkalinity, different temperatures, desiccation, and salinity. Field applications of 4 efficient and stress-tolerant ACCD, including Pseudomonas geniculata, P. migulae, Priestia aryabhattai, and Rhizobium nepotum with reduced NPK dose under two different temperate climate conditions showed a significant improvement in growth and productivity of crops such as garlic, pea, potato, and wheat in slightly acidic soils and maize in saline-sodic alkaline soils. These findings indicated the broad-spectrum potential of these efficient and stress-tolerant ACCD strains to improve plant growth and productivity across diverse soil types and climatic conditions.

Priestia aryabhattai Improves Soil Environment and Promotes Alfalfa Growth by Enhancing Rhizosphere Microbial Carbon Sequestration Capacity Under Greenhouse Conditions

Plant Growth-Promoting Rhizobacteria (PGPR) are gaining increasing attention, but their interactions with indigenous rhizosphere microbiomes remain unclear. To address this issue, we isolated a strain of Priestia aryabhattai with a growth-promoting effect. Under greenhouse conditions, its growth-promoting effect on alfalfa was evaluated, and amplicon sequencing was used to analyze changes in the rhizosphere microbial community to explore the growth promotion mechanism. Our study shows that inoculation with Priestia aryabhattai increases the α-diversity index of the alfalfa rhizosphere microbiome and enhances the abundance of beneficial bacterial genera. This is likely because inoculation with Priestia aryabhattai increased the abundance of carbon-sequestering genera, particularly Gemmatimonas, thereby improving the soil environment. The increased abundance of beneficial bacteria stimulates root development in alfalfa and enhances nutrient uptake, particularly phosphorus, which in turn boosts photosynthesis and promotes alfalfa growth. In summary, Priestia aryabhattai improves soil environment and promotes alfalfa growth by enhancing the carbon sequestration capacity of the rhizosphere microbial community. This work provides theoretical support and insight for the development of PGPR inoculants and for further research on their mechanisms.

Endophytic bacilli from Cyamopsis tetragonoloba (L.) Taub. induces plant growth and drought tolerance

Cyamopsis tetragonoloba (L.) Taub. (guar) is a commercially important crop known for its galactomannan content in seeds. Drought stress is a significant global concern that compromises the productivity of major legumes including guar. The endophytic microbes associated with plants play a significant role in enhancing plant growth and modulating the impact of abiotic stress(s). The present study involved the isolation of 73 endophytic bacteria from the guar seeds of drought-tolerant (RGC-1002 and RGC-1066) and sensitive (Sarada and Varsha) varieties. Based on multiple PGP attributes and drought tolerance, at 50% PEG6000 w/v, 11 efficient isolates were selected and identified through 16S rRNA gene sequencing. Isolates belonging to ten different species of bacilli including Cytobacillus oceanisediminis, Mesobacillus fermenti, Peribacillus simplex from sensitive and Bacillus zanthoxyli, B. safensis, B. velezensis, B. altitudinis, B. licheniformis, B. tequilensis, and B. paralicheniformis isolated from tolerant varieties. A greenhouse experiment with a drought-sensitive guar variety demonstrated that inoculation of selected isolates showed comparatively better plant growth, higher relative water content (RWC), decreased carbon isotope discrimination ratio (Δ13C), increased chlorophyll, carotenoids, anthocyanin, and proline content, decreased malondialdehyde (MDA) and modulated defense enzymes as compared to their uninoculated controls. Tolerant variety isolates B. tequilensis NBRI14G and B. safensis NBRI10R showed the most promising results in improving plant growth and also drought stress tolerance in guar plants. This study represents for the first time that seed endophytic bacterial strains from guar can be utilized to develop the formulation for improving the productivity of guar under drought-stress conditions.

Biofilm forming, exopolysaccharide producing and halotolerant, bacterial consortium mitigates salinity stress in Triticum aestivum

Biofilm and EPS characterization of a rhizobacterial isolate BC-II-20 was done using biophysical techniques. SEM revealed surface morphology of EPS powder to be irregular porous web-like structure. FTIR spectra showed peaks of the polymeric carbohydrate functional groups with probable role in imparting biological properties to EPS. XRD analysis showed signal at 220 (2θ) and confirms its amorphous or semi-crystalline nature. EPS derived from bacterial consortium gradually increased under 200 mM, 400 mM, 600 mM and 800 mM NaCl and SEM-EDAX analysis of EPS showed increase in Na & Cl peaks under the above salt concentrations, depicting EPS-NaCl binding. Triticum aestivum plants under 200 mM NaCl stress with different combinations of treatments showed that bacterial consortium provides tolerance. Under 200 mM salt stress the shoot length was 7.74 cm and total chlorophyll was 4.16 mg g-1Fw of the uninoculated plants whereas inoculated ones were 9.94 cm and 5.62 mg g-1Fw respectively. Under salinity stress, membrane stability index was increased from 47 % to 61 % and electrolyte leakage was decreased to 48 % from 64 %, after inoculation with bacterial consortium. Therefore, consortium comprising of these halotolerant and biofilm forming, EPS producing bioinoculants provides salt tolerance and can be exploited as a sustainable alternative for stress tolerance.

Harnessing microbial interactions with rice: Strategies for abiotic stress alleviation in the face of environmental challenges and climate change

Rice, which feeds more than half of the world's population, confronts significant challenges due to environmental and climatic changes. Abiotic stressors such as extreme temperatures, drought, heavy metals, organic pollutants, and salinity disrupt its cellular balance, impair photosynthetic efficiency, and degrade grain quality. Beneficial microorganisms from rice and soil microbiomes have emerged as crucial in enhancing rice's tolerance to these stresses. This review delves into the multifaceted impacts of these abiotic stressors on rice growth, exploring the origins of the interacting microorganisms and the intricate dynamics between rice-associated and soil microbiomes. We highlight their synergistic roles in mitigating rice's abiotic stresses and outline rice's strategies for recruiting these microorganisms under various environmental conditions, including the development of techniques to maximize their benefits. Through an in-depth analysis, we shed light on the multifarious mechanisms through which microorganisms fortify rice resilience, such as modulation of antioxidant enzymes, enhanced nutrient uptake, plant hormone adjustments, exopolysaccharide secretion, and strategic gene expression regulation, emphasizing the objective of leveraging microorganisms to boost rice's stress tolerance. The review also recognizes the growing prominence of microbial inoculants in modern rice cultivation for their eco-friendliness and sustainability. We discuss ongoing efforts to optimize these inoculants, providing insights into the rigorous processes involved in their formulation and strategic deployment. In conclusion, this review emphasizes the importance of microbial interventions in bolstering rice agriculture and ensuring its resilience in the face of rising environmental challenges.

Endophytic Alkalotolerant Plant Growth-Promoting Bacteria Render Maize (Zea mays L.) Growth Under Alkaline Stress

This study investigates the role of bacterial endophytes from extreme alkaline environments in alleviating alkaline stress and plant development. Stressful environmental factors, such as soil acidity and alkalinity/sodicity, frequently affect plant development. In the present study, alkaline-tolerant endophytic strains were isolated from three plant species Saccharum munja, Calotropis procera, and Chenopodium album, and 15 out of the total of 48 isolates were selected for further examination of their abiotic stress tolerance. Molecular analysis based on 16S rRNA gene sequencing revealed strains from Enterobacter, Acinetobacter, Stenotrophomonas, Bacillus, Lysinibacillus, and Mammaliicoccus genera. Out of 15 isolates based on their quantitative PGP traits and abiotic stress tolerance, 6 were finally selected for greenhouse experiments. Under alkaline conditions, results demonstrated that the strains from the genera Enterobacter, Bacillus, Stenotrophomonas, and Lysinibacillus had beneficial effects on maize growth. These findings suggest that using a combination of bacteria with multiple plant growth-promoting attributes could be a sustainable approach to enhance agricultural yield, even in a challenging alkaline environment. The study concludes that the application of bacterial endophytes from plants growing in extremely alkaline environments might provide other plants with similar stress-tolerance abilities. The outcome of the study provides a basis for future exploration of the mechanisms underlying endophyte-induced stress tolerance.

Harnessing Rhizospheric Microbes for Eco-friendly and Sustainable Crop Production in Saline Environments

Soil salinization is a global issue that negatively impacts crop yield and has become a prime concern for researchers worldwide. Many important crop plants are susceptible to salinity-induced stresses, including ionic and osmotic stress. Approximately, 20% of the world's cultivated and 33% of irrigated land is affected by salt. While various agricultural practices have been successful in alleviating salinity stress, they can be costly and not environment-friendly. Therefore, there is a need for cost-effective and eco-friendly practices to improve soil health. One promising approach involves utilizing microbes found in the vicinity of plant roots to mitigate the effects of salinity stress and enhance plant growth as well as crop yield. By exploiting the salinity tolerance of plants and their associated rhizospheric microorganisms, which have plant growth-promoting properties, it is possible to reduce the adverse effects of salt stress on crop plants. The soil salinization is a common problem in the world, due to which we are unable to use the saline land. To make proper use of this land for different crops, microorganisms can play an important role. Looking at the increasing population of the world, this will be an appreciated effort to make the best use of the wasted land for food security. The updated information on this issue is needed. In this context, this article provides a concise review of the latest research on the use of salt-tolerant rhizospheric microorganisms to mitigate salinity stress in crop plants.

Ochrobactrum sp. NBRISH6 Inoculation Enhances Zea mays Productivity, Mitigating Soil Alkalinity and Plant Immune Response

Intensifying sodic land characterized by high alkaline pH is an incipient environmental hazard-limiting agricultural potential. In this study, we investigated the effects of plant growth-promoting bacteria Ochrobactrum sp. strain NBRISH6 on the growth and physiology of maize (Zea mays L.) grown under alkaline stress at two soil pH levels. Additionally, we also studied the effects of NBRISH6 on soil fertility parameters. A greenhouse experiment was designed using two live soils (pH 8.2 and 10.2) in earthen pots using maize as a host. Results revealed a significant increase in plant growth and a decrease in defense enzymes in both soil types due to NBRISH6 inoculation as compared to non-treated control. Furthermore, activities of all soil enzymes along with bacterial diversity increased in NBRISH6 treatment under normal as well as stressed conditions. In addition, field evaluation of NBRISH6 inoculation using maize was carried out under normal and alkaline conditions, which resulted in significant enhancement of all vegetative parameters as compared to respective controls. Therefore, the study suggested that Ochrobactrum sp. NBRISH6 can be used to develop a bioinoculant formulation to ameliorate abiotic stresses and enhanced crop productivity.

Bacillus Consortia Modulate Transcriptional and Metabolic Machinery of Arabidopsis Plants for Salt Tolerance

Rhizobacteria that are helpful to plants can lessen the impacts of salt stress, and they may hold promise for the development of sustainable agriculture in the future. The present study was intended to explicate consortia of salt-tolerant plant-beneficial rhizobacteria for the amelioration of salinity stress in Arabidopsis plants. Inoculation with both the consortia positively influenced the growth of plants as indicated by total chlorophyll content, MDA content, and antioxidant enzyme activities under stressful conditions. Both the multi-trait consortia altered the expression profiles of stress-related genes including CSD1, CAT1, Wrky, Ein, Etr, and ACO. Furthermore, the metabolomic analysis indicated that inoculated plants modulated the metabolic profiles to stimulate physiological and biochemical responses in Arabidopsis plants to mitigate salt stress. Our study affirms that the consortia of salt-tolerant bacterial strains modulate the transcriptional as well as metabolic machinery of plants to protect them from salinity stress. Nevertheless, the findings of this study revealed that consortia are composed of salt-tolerant bacterial strains viz. Bacillus safensis NBRI 12M, B. subtilis NBRI 28B, and B. subtilis NBRI 33N demonstrated significant improvement in Arabidopsis plants under saline stress conditions.

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.

Identification of endophytic fungi with ACC deaminase-producing isolated from halophyte Kosteletzkya Virginica

Seashore mallow (Kosteletzkya virginica), as a noninvasive perennial halophytic oilseed-producing dicot, is native from the Gulf to the Atlantic coasts of the U.S. The purpose of our research was to investigate 1-aminocyclopropane-1carboxylic acid deaminase (ACCD) producing endophytic fungi from K.virginica. A total of 59 endophytic fungal strains, isolated from roots in K.virginica of seedlings, were grouped into 12 genera including in Penicillium, Aspergillus, Fusarium, Trichoderma, Rhizopycnis sp., Ceriporia Donk, Trametes sp., Schizophyllum commune sp., Alternaria, Cladosporium, Cylindrocarpon, and Scytalidium according to sequences of ITS. The ACD activity of 10 endophytic fungi isolated was detected. T.asperellum had the highest ACC deaminase activity among all 10 isolated genera of fungal strains, followed by T. viride. Dry weight and fresh weight of plant, plant height, root length, SOD activity, and chlorophyll content of wheat and soybean inoculated with T.asperellum or T. viride was increased compared with non-inoculated control plants under non salt or salt stress. Further analysis showed that T.asperellum or T.viride strains induced downregulation of the expression of ethylene synthesis-related genes such as ACC oxidase (ACO) and ACC synthase (ACS), thereby reducing ethylene synthesis and damage to plants under salt stress. These endophytic fungi can be used as alternative bioinoculants to increase crop yield in saline soil.

Potassium Humate and Plant Growth-Promoting Microbes Jointly Mitigate Water Deficit Stress in Soybean Cultivated in Salt-Affected Soil

Lack of high-quality irrigation water and soil salinity are two main environmental factors that affect plant development. When both stressors are combined, the soil becomes sterile and constrains plant productivity. Consequently, two field trials were designed to assess whether plant growth-promoting microbes (PGPMs; Bradyrhizobium japonicum (USDA 110) and Trichoderma harzianum) and potassium humate (K-humate) can stimulate soybean growth, productivity, and seed quality under two different watering regimes as follows: (i) well-watered (WW), where plants were irrigated at 12-day intervals (recommended), and (ii) water stress (WS), where plants were irrigated at the 18-day intervals in salt-affected soil during 2020 and 2021 seasons. Results revealed that coupled application of PGPMs and K-humate resulted in a substantial improvement in K⁺ levels in the leaves compared to Na⁺ levels, which has a direct positive impact on an enhancement in the antioxidants defense system (CAT, POX, SOD), which caused the decline of the oxidative stress indicators (H₂O₂, MDA, and EL%) as well as proline content under water stress in salt-affected soil. Hence, a significant increase in root length, nodule weight, soybean relative water content (RWC), stomatal conductance, photosynthetic pigments, net photosynthetic rate, soluble protein, seed carbohydrate content as well as the number of pods plant^-1 and seed yield was reported. In conclusion, the combined application of PGPMs and K-humate might be recommended to maximize the soybean growth and productivity under harsh growth conditions (e.g., water stress and soil salinity).

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.

Role of ACC deaminase producing bacteria for abiotic stress management and sustainable agriculture production

Plants are immobile and are exposed to various biotic and abiotic stresses, including heat, cold, drought, flooding, nutrient deficiency, heavy metal exposure, phytopathogens, and pest attacks. The stressors significantly affect agricultural productivity when exceed a certain threshold. It has been reported that most of the stressed plants are reported to have increased ethylene synthesis from its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene is a plant hormone that plays a vital role in the regulation of various physiological processes, such as respiration, nitrogen fixation, and photosynthesis. The increment in the plant hormone ethylene would reduce plant growth and development, and if the ethylene level increased beyond the limit, it could also result in plant death. Therefore, plant growth-promoting bacteria (PGPB) possessing ACC deaminase activity play an essential role in the management of biotic and abiotic stresses by hydrolysing 1-aminocyclopropane-1-carboxylic acid using ACC deaminase. In this review, the importance of ACC deaminase-producing bacteria in promoting plant growth under various abiotic stressors is discussed.

N-acyl Homoserine Lactone Mediated Quorum Sensing Exhibiting Plant Growth-promoting and Abiotic Stress Tolerant Bacteria Demonstrates Drought Stress Amelioration

Multiple plant growth-promoting attributes with N-acyl homoserine lactone (AHL)-mediated quorum sensing exhibiting bacterial strains can help plants to withstand varying abiotic and biotic stress conditions for improving the plant health and productivity. In total, 306 bacterial isolates were isolated from diverse locations and sites. In our exploration, bacterial isolates were screened based on AHL production, plant growth-promoting attributes, abiotic stress tolerance, and antagonistic activity against phytopathogenic fungi. Among the screened 306 isolates, 4 (11VPKHP4, 7VP51.8, P51.10, NBRI N7) were selected based on their efficiency in AHL production, biofilm formation, enduring different abiotic stress conditions, exhibiting plant growth-promoting attributes, and antagonistic activity. Based on 16S rRNA gene sequencing analyses of the selected 4 isolates belong to Pseudomonas genera. Selected isolates 11VPKHP4, 7VP51.8, P51.10, and NBRI N7 were also proficient in biosurfactant production, emulsification, suggesting that all isolates fabricate emulsifiers. The plant growth promotion potential of selected 4 bacterial isolates showed significant growth enhancement in all the vegetative parameters of Zea mays under control as well as drought stress condition. Biochemical parameters and defense enzymes under drought stress conditions were also modulated in the PGPR treated plants as compared to their uninoculated respective controls. With quorum sensing, multiple PGPR attributes, stress tolerance, biofilm formation, and EPS production the selected isolates have the potential to facilitate enhanced plant growth, rhizosphere colonization, maintenance of soil moisture content under normal and diverse stresses.

Endophytic bacteria promote biomass production and mercury-bioaccumulation of Bermuda grass and Indian goosegrass

Plant growth-promoting endophytic bacteria can potentially improve the biomass production of Hg-accumulating grasses, resulting in improved Hg extraction from contaminated soils. This study aimed to analyze the effect of inoculation of Hg-resistant endophytic bacteria (i) Jeotgalicoccus huakuii (B1) and (ii) Bacillus amyloliquefaciens (B2), as single and consortium inoculant, on biomass production and Hg bioaccumulation of Bermuda grass (R1) and Indian goosegrass (R2) planted as monoculture and mixed cropping. The grass seeds were surface-sterilized before the inoculation. Both inoculated seeds with B1 and B2 (treatments) and uninoculated seeds were sown separately in sterilized sand. Grass seedlings of both treatments and control were replanted in the washed and sterilized sand medium, spiked with HgCl₂ (100 mg kg^-1). A subset of grass was harvested at 0, 4, 6, and 10 weeks after planting to measure biomass production and Hg bioaccumulation. The results showed that bacterial inoculation enhanced the grass biomass by 52.68% and Hg bioaccumulation by 47.76%. Mercury residue of Hg-spiked sand treated with the bacterial consortium was reduced by 80%. This suggests that endophytic bacteria can improve grass biomass production and enhance Hg bioaccumulation in grass biomass.

Genetic and non-genetic factors affecting conception rate of frozen semen in small holder dairy farmer system of rural India

Fertility traits are as important as production traits in crossbred bovine population. Assessment of the fertility of bull in any frozen semen production program and evaluation of the conception rate under actual field conditions provide valuable information. The objective of this retrospective study was to estimate the effect of genetic and non-genetic influence on conception rate of frozen semen bulls maintained at Bharatiya Agro-Industries Foundation (BAIF) Pune based on the conception rate in cattle population of small holder dairy farmer system. The data comprising of 1,08,238 insemination records pertaining to 83 Holstein Friesian pure and crossbred bulls available at BAIF Pune were used to analyze conception rate. The fixed effect solutions and covariance components were estimated by linear mixed model using the restricted maximum likelihood method in WOMBAT software. The genetic correlations were estimated using bivariate analysis between post thaw motility and conception rate. The study was based on fertility related information from cows maintained in different villages of India and thus reflects the actual fertility of frozen semen used. The study was suggestive of influence of very small fraction of genetic effect and higher impact of management effect on conception rate. Fertility-related information available from this study is an invaluable asset in decision making process of breeding policies.

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.

Streptomyces sp. S-9 promotes plant growth and confers resistance in Pigeon pea (Cajanus cajan) against Fusarium wilt

Streptomyces sp. strain S-9 was studied for its effect in inducing systemic resistance in Pigeon pea against the plant pathogen Fusarium udum causing wilt. The 16S rRNA gene sequencing and phylogenetic analysis indicated that S-9 is closely related to genus Streptomyces for which it was referred to as Streptomyces sp. S-9. Streptomyces sp. S-9 caused 85% inhibition of the pathogen and showed various attributes of plant growth-promoting such as the production of IAA, P-solubilization, and β -1, 3-Glucanase activity. Proline and malondialdehyde (MDA) content was significantly higher whereas the chlorophyll content decreased in the pathogen-infected plant when compared to S-9 treated Pigeon pea plants. The anatomical research assisted the biocontrol-mediated stress tolerance findings in the Pigeon pea plant through increased root epidermis and enhanced stress-related xylem tissues. Fungus inoculation elevated the antioxidative enzymatic activities of superoxide dismutase (SOD; 78%) and catalase (CAT; 56%). Marked reductions in antioxidant enzymes were associated with the antagonistic effects of the different treatments. Conclusions showed that S-9 bioinocula applied as a seed coating enhanced soil availability of nitrogen (N), phosphate (P), and potassium (K), indicating their suitability for direct application invigorating plant growth and persuade resistance in the plant Pigeon pea against Fusarium wilt.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02989-0.

Microbial methylglyoxal metabolism contributes towards growth promotion and stress tolerance in plants

Plant growth promotion by microbes is a cumulative phenomenon involving multiple traits, many of which are not explored yet. Hence, to unravel microbial mechanisms underlying growth promotion, we have analysed the genomes of two potential growth-promoting microbes, viz., Pseudomonas sp. CK-NBRI-02 (P2) and Bacillus marisflavi CK-NBRI-03 (P3) for the presence of plant-beneficial traits. Besides known traits, we found that microbes differ in their ability to metabolize methylglyoxal (MG), a ubiquitous cytotoxin regarded as general consequence of stress in plants. P2 exhibited greater tolerance to MG and possessed better ability to sustain plant growth under dicarbonyl stress. However, under salinity, only P3 showed a dose-dependent induction in MG detoxification activity in accordance with concomitant increase in MG levels, contributing to enhanced salt tolerance. Furthermore, salt-stressed transcriptomes of both the strains showed differences with respect to MG, ion and osmolyte homeostasis, with P3 being more responsive to stress. Importantly, application of either strain altered MG levels and subsequently MG detoxification machinery in Arabidopsis, probably to strengthen plant defence response and growth. We therefore, suggest a crucial role of microbial MG resistance in plant growth promotion and that it should be considered as a beneficial trait while screening microbes for stress mitigation in plants.

Is phosphate solubilizing ability in plant growth-promoting rhizobacteria isolated from chickpea linked to their ability to produce ACC deaminase?

AIMS

Since most phosphate solubilizing bacteria (PSB) also produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, we investigated if there was an association between these two plant growth-promoting properties under in vitro conditions.

METHODS AND RESULTS

A total of 841 bacterial isolates were obtained using selective and enrichment isolation methods. ACC deaminase was investigated using in vitro methods and by sequencing the acdS gene. The effect of ACC deaminase on P solubilization was investigated further using five efficient PSB. ACC deaminase production ability was found amongst a wide range of bacteria belonging to the genera Bacillus, Burkholderia, Pseudomonas and Variovorax. The amount of ACC deaminase produced by PSB was significantly associated with the liberation of Pi from Ca-P when ACC was the sole N source. Ca-P solubilization was associated with the degree of acidification of the medium. Additionally, the P solubilization potential of PSB with (NH₄ )₂ SO₄ was determined by the type of carboxylates produced. An in-planta experiment was conducted using Burkholderia sp. 12F on chickpea cv. Genesis-863 in sand : vermiculite (1 : 1 v/v) amended with rock phosphate and inoculation of this efficient PSB significantly increased growth, nodulation and P uptake of chickpea fertilized with rock phosphate.

CONCLUSION

ACC deaminase activity influenced the capacity of PSB to solubilize P from Ca-P when ACC was the sole N source and Burkholderia sp. 12F promoted the chickpea-Mesorhizobium symbiosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

ACC deaminase activity could enhance the P solubilizing activity of rhizobacteria that improve plant growth.

Beneficial Plant-Associated Microorganisms From Semiarid Regions and Seasonally Dry Environments: A Review

Semiarid regions are apparently low biodiversity environments; however, these environments may host a phylogenetically diverse microbial community associated with plants. Their microbial inhabitants are often recruited to withstand stressful settings and improve plant growth under harsh conditions. Thus, plant-associated microorganisms isolated from semiarid and seasonally dry environments will be detailed in the present review, focusing on plant growth promotion potential and the microbial ability to alleviate plant abiotic stress. Initially, we explored the role of microbes from dry environments around the world, and then, we focused on seasonally dry Brazilian biomes, the Caatinga and the Cerrado. Cultivable bacteria from semiarid and seasonally dry environments have demonstrated great plant growth promotion traits such as plant hormone production, mobilization of insoluble nutrients, and mechanisms related to plant abiotic stress alleviation. Several of these isolates were able to improve plant growth under stressful conditions commonly present in typical semiarid regions, such as high salinity and drought. Additionally, we highlight the potential of plants highly adapted to seasonal climates from the Caatinga and Cerrado biomes as a suitable pool of microbial inoculants to maintain plant growth under abiotic stress conditions. In general, we point out the potential for the exploitation of new microbial inoculants from plants growing in dry environments to ensure a sustainable increase in agricultural productivity in a future climate change scenario.

Evaluation of Pseudomonas sp. for its multifarious plant growth promoting potential and its ability to alleviate biotic and abiotic stress in tomato (Solanum lycopersicum) plants

1-Aminocyclopropane-1-carboxylate (ACC) deaminase activity is one of the most beneficial traits of plant growth promoting (PGP) rhizobacteria responsible for protecting the plants from detrimental effects of abiotic and biotic stress. The strain S3 with ACC deaminase activity (724.56 nmol α-ketobutyrate mg-1 protein hr-1) was isolated from rhizospheric soil of turmeric (Curcuma longa), a medicinal plant, growing in Motihari district of Indian state, Bihar. The halotolerant strain S3, exhibited optimum growth at 8% (w/v) NaCl. It also exhibited multiple PGP traits such as indole acetic acid production (37.71 μg mL-1), phosphate solubilization (69.68 mg L-1), siderophore, hydrocyanic acid (HCN) and ammonia production as well as revealed antagonism against Rhizoctonia solani. The potential of isolated strain to alleviate salinity stress in tomato plants was investigated through pots trials by inoculating strain S3 through-seed bacterization, soil drenching, root dipping as well as seed treatment + soil drenching. The strain S3 inoculated through seed treatment and soil drenching method led to improved morphological attributes (root/shoot length, root/shoot fresh weight and root/shoot dry weight), photosynthetic pigment content, increased accumulation of osmolytes (proline and total soluble sugar), enhanced activities of antioxidants (Catalase and Peroxidase) and phenolic content in salt stressed tomato plants. The biochemical characterisation, FAMEs analysis and 16S rRNA gene sequencing revealed that strain S3 belongs to the genus Pseudomonas. The overall findings of the study revealed that Pseudomonas sp. strain S3 can be explored as an effective plant growth promoter which stimulate growth and improve resilience in tomato plants under saline condition.

Microbial Diversity of Upland Rice Roots and Their Influence on Rice Growth and Drought Tolerance

Among abiotic stresses, drought is one of the most important factors limiting plant growth. To increase their drought tolerance and survival, most plants interact directly with a variety of microbes. Upland rice (Oryza sativa L.) is a rice ecotype that differs from irrigated ecotype rice; it is adapted to both drought-stress and aerobic conditions. However, its root microbial resources have not been explored. We isolated bacteria and fungi from roots of upland rice in Xishuangbanna, China. Four hundred sixty-two endophytic and rhizospheric isolates (337 bacteria and 125 fungi) were distributed. They were distributed among 43 genera on the basis of 16S rRNA and internal transcribed spacer (ITS) gene sequence analysis. Notably, these root microbes differed from irrigated rice root microbes in irrigated environments; for example, members of the Firmicutes phylum were enriched (by 28.54%) in the roots of the upland plants. The plant growth-promoting (PGP) potential of 217 isolates was investigated in vitro. The PGP ability of 17 endophytic and 10 rhizospheric isolates from upland rice roots was evaluated under well-irrigated and drought-stress conditions, and 9 fungal strains increased rice seedling shoot length, shoot and root fresh weight (FW), antioxidant capability, and proline (Pro) and soluble sugar contents. Our work suggests that fungi from upland rice roots can increase plant growth under irrigated and drought-stress conditions and can serve as effective microbial resources for sustainable agricultural production in arid regions.

Mechanistic Insights of the Interaction of Plant Growth-Promoting Rhizobacteria (PGPR) With Plant Roots Toward Enhancing Plant Productivity by Alleviating Salinity Stress

Agriculture plays an important role in a country's economy. The sector is challenged by many stresses, which led to huge loss in plant productivity worldwide. The ever-increasing population, rapid urbanization with shrinking agricultural lands, dramatic change in climatic conditions, and extensive use of agrochemicals in agricultural practices that caused environmental disturbances confront mankind of escalating problems of food security and sustainability in agriculture. Escalating environmental problems and global hunger have led to the development and adoption of genetic engineering and other conventional plant breeding approaches in developing stress-tolerant varieties of crops. However, these approaches have drawn flaws in their adoption as the process of generating tolerant varieties takes months to years in bringing the technology from the lab to the field. Under such scenario, sustainable and climate-smart agricultural practices that avail bacterial usage open the avenues in fulfilling the incessant demand for food for the global population. Ensuring stability on economic fronts, bacteria minimizes plant salt uptake by trapping ions in their exopolysaccharide matrix besides checking the expression of Na+/H+ and high-affinity potassium transporters. Herein we describe information on salinity stress and its effect on plant health as well as strategies adopted by plant growth-promoting rhizobacteria (PGPR) in helping plants to overcome salinity stress and in mitigating loss in overall plant productivity. It is believed that acquisition of advanced knowledge of plant-beneficial PGPR will help in devising strategies for sustainable, environment-friendly, and climate-smart agricultural technologies for adoption in agriculture to overcome the constrained environmental conditions.

Mitigation of drought stress in rice crop with plant growth-promoting abiotic stress-tolerant rice phyllosphere bacteria

In the search of effective drought-alleviating and growth-promoting phyllosphere bacteria, a total of 44 bacterial isolates were isolated from the leaf surface of drought-tolerant rice varieties, Mattaikar, Nootripattu, Anna R(4), and PMK3, and screened for their abiotic stress tolerance by subjecting their growth medium to temperature, salinity, and osmotic stress. Only eight isolates were found to grow and proliferate under different abiotic stress conditions. These isolates were identified using 16S ribosomal DNA gene sequence and submitted to the NCBI database. All the bacterial isolates were identified as Bacillus sp., except PB24, which was identified as Staphylococcus sp., and these isolates were further screened for plant growth-promoting (PGP) traits such as IAA production, GA production, ACC deaminase activity, and exopolysaccharide production under three different osmotic stress conditions adjusted using polyethylene glycol (PEG 6000). Additionally, mineral solubilization was measured under the normal condition. Bacillus endophyticus PB3, Bacillus altitudinis PB46, and Bacillus megaterium PB50 were found to have multifarious PGP traits. Consecutively, the performance of an individual strain to improve the plant growth was investigated under the osmotic stress (25% PEG 6000) and nonstress condition by inoculating them into rice seeds using hydroponics culture. Furthermore, the drought-alleviating potency of bacterial strains was assessed in the rice plants using pot experiment (-1.2 MPa) through bacterial foliar application during the reproductive stage. Finally, as a result of seed inoculation and foliar spray, the application of B. megaterium PB50 significantly improved the plant growth under osmotic stress, protected plants from physical drought through stomatal closure, and improved carotenoid, total soluble sugars, and total protein content. Metabolites of PB50 were profiled under both stress and nonstress conditions using gas chromatography-mass spectroscopy.

Drought tolerant Ochrobactrum sp. inoculation performs multiple roles in maintaining the homeostasis in Zea mays L. subjected to deficit water stress

Plant growth-promoting rhizobacteria (PGPR) improve plant health under various biotic and abiotic stresses. However, the underlying mechanisms of the protective effects of PGPR in deficit water stress (WS) remain less explored. This study aimed to characterize the role of Ochrobactrum sp. NBRISH6 inoculation on maize (Zea mays "Maharaja") under WS conditions using multiple approaches such as physiological, anatomical, metabolic, and molecular. The effect of NBRISH6 inoculation using maize as a host plant was characterized under greenhouse conditions in deficit water stress. Results from this study demonstrated that NBRISH6 significantly lowered the expression of genes involved in the abscisic acid cycle, deficit water stress-response, osmotic stress, and antioxidant enzyme activity (superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and polyphenol oxidase). Phytohormones, i.e. indole acetic acid (IAA) and salicylic acid (SA) levels, intercellular CO₂ concentration, metabolites such as simple sugars, amino acids, aliphatic hydrocarbons, and the number of shrunken pith cells modulated in maize roots inoculated with NBRISH6. The NBRISH6 inoculation also improved the plant vegetative properties (root length, 33.80%; shoot length, 20.68%; root dry weight, 39.21%; shoot dry weight, 61.95%), shoot nutrients, xylem cells, root hairs, vapor pressure deficit (75%), intrinsic water-use efficiency (41.67%), photosynthesis rate (83.33%), and total chlorophyll (16.15%) as compared to the respective stress controls. This study provides valuable insights into mechanistic functions of PGPR in WS amelioration and promoting plant physiological response.

Characterization of plant growth-promoting alkalotolerant Alcaligenes and Bacillus strains for mitigating the alkaline stress in Zea mays

Intensification of sodic soil due to increasing pH is an emerging environmental issue. The present study aimed to isolate and characterise alkaline stress-tolerant and plant growth-promoting bacterial strains from moderately alkaline soil (pH 8-9), strongly alkaline soil (pH 9-10), and very strongly alkaline soil (> 10). Total 68 bacteria were isolated, and screened for multiple plant growth promoting (PGP) attributes. Out of total, 42 isolates demonstrating at least three plant growth promoting PGP traits selected for further assays. Then out of 42, 15 bacterial isolates were selected based on enhanced maize plant growth under greenhouse experiment, and 16S rRNA gene sequencing revealed Bacillus spp. as a dominant genus. Furthermore, based on improved seed germination percentage and biomass of maize (Zea mays L.) under alkaline stress conditions Alcaligenes sp. NBRI NB2.5, Bacillus sp. NBRI YE1.3, and Bacillus sp. NBRI YN4.4 bacterial strains were selected, and evaluated for growth-promotion and alkaline stress amelioration under greenhouse condition. Amongst the selected 3 plant growth promoting rhizobacterial (PGPR) strains, Bacillus sp. NBRI YN4.4 significantly improved the photosynthetic pigments and soluble sugar content, and decreased proline level in inoculated maize plants as compared to uninoculated control under stress conditions. Moreover, significantly enhanced soil enzymes such as dehydrogenase, alkaline phosphatase and betaglucosidase due to inoculation of Bacillus sp. NBRI YN4.4 in maize plants grown in alkaline soil attributes to its role in improving the soil health. Therefore, alkaline stress-tolerant PGPR NBRI YN4.4 can be useful for developing strategies for the reclamation of saline/sodic soils and improving the plant growth and soil health in sustainable manner.

Research Advances of Beneficial Microbiota Associated with Crop Plants

Plants are associated with hundreds of thousands of microbes that are present outside on the surfaces or colonizing inside plant organs, such as leaves and roots. Plant-associated microbiota plays a vital role in regulating various biological processes and affects a wide range of traits involved in plant growth and development, as well as plant responses to adverse environmental conditions. An increasing number of studies have illustrated the important role of microbiota in crop plant growth and environmental stress resistance, which overall assists agricultural sustainability. Beneficial bacteria and fungi have been isolated and applied, which show potential applications in the improvement of agricultural technologies, as well as plant growth promotion and stress resistance, which all lead to enhanced crop yields. The symbioses of arbuscular mycorrhizal fungi, rhizobia and Frankia species with their host plants have been intensively studied to provide mechanistic insights into the mutual beneficial relationship of plant–microbe interactions. With the advances in second generation sequencing and omic technologies, a number of important mechanisms underlying plant–microbe interactions have been unraveled. However, the associations of microbes with their host plants are more complicated than expected, and many questions remain without proper answers. These include the influence of microbiota on the allelochemical effect caused by one plant upon another via the production of chemical compounds, or how the monoculture of crops influences their rhizosphere microbial community and diversity, which in turn affects the crop growth and responses to environmental stresses. In this review, first, we systematically illustrate the impacts of beneficial microbiota, particularly beneficial bacteria and fungi on crop plant growth and development and, then, discuss the correlations between the beneficial microbiota and their host plants. Finally, we provide some perspectives for future studies on plant–microbe interactions.

ACC deaminase-producing rhizosphere competent Bacillus spp. mitigate salt stress and promote Zea mays growth by modulating ethylene metabolism

Plant growth-promoting rhizobacteria (PGPR) are known for growth promotion and mitigating environmental stresses. Here, we examined the propitiousness of three indigenous salt-tolerant PGPR, i.e., Bacillus subtilis (NBRI 28B), B. subtilis (NBRI 33 N), and B. safensis (NBRI 12 M) for plant growth promotion and salt stress amelioration in Zea mays. Results of the in vitro plant growth-promoting attribute revealed NBRI 12 M demonstrated the highest values at 1 M salt (NaCl) concentration. Furthermore, the greenhouse experiment using three Bacillus strains confirmed plant growth-promoting and salt stress-ameliorating ability, through colonizing successfully and mitigating the adverse effects of ethylene by modulating 1-aminocyclopropane-1-carboxylic acid (ACC) accumulation, ACC-oxidase (ACO), and ACC-synthase (ACS) activities under salt stress. Bacillus sp. inoculation has also induced plant response for defense enzymes, chlorophyll, proline and soluble sugar under salt stress. Among three Bacillus strains, NBRI 12 M not only demonstrated higher values for plant growth-promoting (PGP) attributes but also the same was observed in the greenhouse experiment. Thus, the outcomes of this comparative study represent for the first time that salt-tolerant Bacillus strains exhibiting multiple PGP attributes under salt stress along with high rhizosphere competence can alleviate salt stress by reducing the stress ethylene level in the host plant.

Salt-Tolerant Plant Growth Promoting Rhizobacteria for Enhancing Crop Productivity of Saline Soils

Soil salinity has emerged as a serious issue for global food security. It is estimated that currently about 62 million hectares or 20 percent of the world's irrigated land is affected by salinity. The deposition of an excess amount of soluble salt in cultivable land directly affects crop yields. The uptake of high amount of salt inhibits diverse physiological and metabolic processes of plants even impacting their survival. The conventional methods of reclamation of saline soil which involve scraping, flushing, leaching or adding an amendment (e.g., gypsum, CaCl2, etc.) are of limited success and also adversely affect the agro-ecosystems. In this context, developing sustainable methods which increase the productivity of saline soil without harming the environment are necessary. Since long, breeding of salt-tolerant plants and development of salt-resistant crop varieties have also been tried, but these and aforesaid conventional approaches are not able to solve the problem. Salt tolerance and dependence are the characteristics of some microbes. Salt-tolerant microbes can survive in osmotic and ionic stress. Various genera of salt-tolerant plant growth promoting rhizobacteria (ST-PGPR) have been isolated from extreme alkaline, saline, and sodic soils. Many of them are also known to mitigate various biotic and abiotic stresses in plants. In the last few years, potential PGPR enhancing the productivity of plants facing salt-stress have been researched upon suggesting that ST-PGPR can be exploited for the reclamation of saline agro-ecosystems. In this review, ST-PGPR and their potential in enhancing the productivity of saline agro-ecosystems will be discussed. Apart from this, PGPR mediated mechanisms of salt tolerance in different crop plants and future research trends of using ST-PGPR for reclamation of saline soils will also be highlighted.

The influence of plant growth-promoting rhizobacteria in plant tolerance to abiotic stress: a survival strategy

Action is needed to face the global threat arising from inconsistent rainfall, rise in temperature, and salinization of farm lands which may be the product of climate change. As crops are adversely affected, man and animals may face famine. Plants are severely affected by abiotic stress (drought, salinity, alkalinity, and temperature), which impairs yield and results in loss to farmers and to the nation at large. However, microbes have been shown to be of great help in the fight against abiotic stress, via their biological activities at the rhizosphere of plants. The external application of chemical substances such as glycine betaine, proline, and nutrients has helped in sustaining plant growth and productive ability. In this review, we tried to understand the part played by bioinoculants in aiding plants to resist the negative consequences arising from abiotic stress and to suggest better practices that will be of help in today’s farming systems. The fact that absolute protection and sustainability of plant yield under stress challenges has not been achieved by microbes, nutrients, nor the addition of chemicals (osmo-protectants) alone suggests that studies should focus on the integration of these units (microbes, nutrients, chemical stimulants, and osmo-protectants) into a strategy for achieving a complete tolerance to abiotic stress. Also, other species of microbes capable of shielding plant from stress, boosting yield and growth, providing nutrients, and protecting the plants from harmful invading pathogens should be sought.