Microbial inoculation in rice regulates antioxidative reactions and defense related genes to mitigate drought stress

Selenium Nanomaterials Enhance Sheath Blight Resistance and Nutritional Quality of Rice: Mechanisms of Action and Human Health Benefit

In the current work, the foliar application of selenium nanomaterials (Se0 NMs) suppressed sheath blight in rice (Oryza sativa). The beneficial effects were nanoscale specific and concentration dependent. Specifically, foliar amendment of 5 mg/L Se0 NMs decreased the disease severity by 68.8% in Rhizoctonia solani-infected rice; this level of control was 1.57- and 2.20-fold greater than that of the Se ions with equivalent Se mass and a commercially available pesticide (Thifluzamide). Mechanistically, (1) the controlled release ability of Se0 NMs enabled a wider safe concentration range and greater bioavailability to Se0 NMs, and (2) transcriptomic and metabolomic analyses demonstrated that Se0 NMs simultaneously promoted the salicylic acid- and jasmonic-acid-dependent acquired disease resistance pathways, antioxidative system, and flavonoid biosynthesis. Additionally, Se0 NMs improved rice yield by 31.1%, increased the nutritional quality by 6.4-7.2%, enhanced organic Se content by 44.8%, and decreased arsenic and cadmium contents by 38.7 and 42.1%, respectively, in grains as compared with infected controls. Human simulated gastrointestinal tract model results showed that the application of Se0 NMs enhanced the bioaccessibility of Se in grains by 22.0% and decreased the bioaccessibility of As and Cd in grains by 20.3 and 13.4%, respectively. These findings demonstrate that Se0 NMs can serve as an effective and sustainable strategy to increase food quality and security.

Comparative analysis of root associated microbes in tropical cultivated and weedy rice (Oryza spp.) and temperate cultivated rice

Weedy rice is a major problem in paddy fields around the world. It is well known that weedy rice appears to grow faster and mature earlier than cultivated rice. It is possible that differences in the root microbial genetics are correlated with this characteristic. This study incorporated 16S rRNA amplicon sequencing to study the microbial composition in the rhizosphere and endosphere of rice root. No significant difference was found between the microbiota associated with weedy and cultivated rice lines grown in the same field. It was found that the endosphere had less microbial diversity compared to the rhizosphere. The major groups of bacteria found in the endosphere are from the phylum Proteobacteria, Myxococcota, Chloroflexota, and Actinobacteria. In addition, by analyzing the microbiome of japonica rice grown in the field in a temperate climate, we found that despite differences in genotype and location, some bacterial taxa were found to be common and these members of the putative rice core microbiome can also be detected by in situ hybridization. The delineation of a core microbiome in the endosphere of rice suggests that these bacterial taxa might be important in the life cycle of a wide range of rice types.

Plant growth-promoting and biocontrol traits of endophytic Bacillus licheniformis against soft rot causing Pythium myriotylum in ginger plant

Bacterial endophytes from plants harbor diverse metabolites that play major roles in biocontrol and improve plant growth. In this study, a total of 12 endophytic bacteria were isolated from the ginger rhizome. The strain K3 was highly effective in preventing mycelia growth of Pythium myriotylum (78.5 ± 1.5% inhibition) in dual culture. The cell-free extract (2.5%) of endophyte K3 inhibited 76.3 ± 4.8% mycelia growth, and 92.4 ± 4.2% inhibition was observed at a 5% sample concentration. The secondary metabolites produced by Bacillus licheniformis K3 showed maximum activity against Pseudomonas syringae (24 ± 1 mm zone of inhibition) and Xanthomonas campestris (28 ± 3 mm zone of inhibition). The strain K3 produced 28.3 ± 1.7 IU mL-1 protease, 28.3 ± 1.7 IU mL-1 cellulase, and 2.04 ± 0.13 IU mL-1 chitinase, respectively. The ginger rhizome treated with K3 in the greenhouse registered 53.8 ± 1.4% soft rot incidence, and the streptomycin-treated pot registered 78.3 ± 1.7% disease incidence. The selected endophyte K3 improved ascorbate peroxidase (1.37 ± 0.009 µmole ASC min-1 mg-1 protein), catalase (8.7 ± 0.28 µmole min-1 mg-1 protein), and phenylalanine ammonia-lyase (26.2 ± 0.99 Umg-1) in the greenhouse. In addition, K3 treatment in the field trial improved rhizome yield (730 ± 18.4 g) after 180 days (p < 0.01). The shoot length was 46 ± 8.3 cm in K3-treated plants, and it was about 31% higher than the control treatment (p < 0.01). The lytic enzyme-producing and growth-promoting endophyte is useful in sustainable crop production through the management of biotic stress.

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.

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.

Meta-analysis of plant growth-promoting rhizobacteria interaction with host plants: implications for drought stress response gene expression

Introduction

The molecular and physiological mechanisms activated in plants during drought stress tolerance are regulated by several key genes with both metabolic and regulatory roles. Studies focusing on crop gene expression following plant growth-promoting rhizobacteria (PGPR) inoculation may help understand which bioinoculant is closely related to the induction of abiotic stress responses.

Methods

Here, we performed a meta-analysis following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines to summarise information regarding plant-PGPR interactions, focusing on the regulation of nine genes involved in plant drought stress response. The literature research yielded 3,338 reports, of which only 41 were included in the meta-analysis based on the chosen inclusion criteria. The meta-analysis was performed on four genes (ACO, APX, ACS and DREB2); the other five genes (ERD15, MYB, MYC, acdS, WRKY) had an insufficient number of eligible articles.

Results

Forest plots obtained through each meta-analysis showed that the overexpression of ACO, APX, ACS and DREB2 genes was not statistically significant. Unlike the other genes, DREB2 showed statistically significant results in both the presence and absence of PGPR. Considering I2>75 %, the results showed a high heterogeneity among the studies included, and the cause for this was examined using subgroup analysis. Moreover, the funnel plot and Egger's test showed that the analyses were affected by strong publication bias.

Discussion

This study argues that the presence of PGPR may not significantly influence the expression of drought stress response-related crop genes. This finding may be due to high heterogeneity, lack of data on the genes examined, and significant publication bias.

Application of rhizobacteria to improve microbial community structure and maize (Zea mays L.) growth in saline soil

The utilization of plant growth-promoting rhizobacteria (PGPR) has emerged as a prominent focus in contemporary research on soil microbiology, microecology, and plant stress tolerance. However, how PGPR influence the soil bacterial community and related ecological functions remains unclear. The aim of this study was to investigate the effects of three natural PGPR inoculations (YL07, Planococcus soli WZYH02; YL10, Bacillus atrophaeus WZYH01; YL0710, Planococcus soli WZYH02 and Bacillus atrophaeus WZYH01) on maize (Zea mays L.) growth under two salt stress conditions (S1, ECe = 2.1 ~ 2.5 dS/m; S2, ECe = 5.5 ~ 5.9 dS/m). The results revealed that compared to the control (CK), the average plant height of maize seedlings significantly increased by 27%, 23%, and 29% with YL07, YL10, and YL0710 inoculation under S1 conditions, respectively, and increased by 30%, 20%, and 18% under S2 conditions, respectively. Moreover, PGPR inoculation positively influenced the content of superoxide dismutase, catalase, soluble sugar, and proline in maize under salt stress. Subsequent analysis of alpha diversity indices, relative microbial abundance, principal coordinate analysis, cladograms, and linear discriminant analysis effect size histograms indicated significant alterations in the rhizosphere microbial community due to PGPR inoculation. FAPROTAX analysis demonstrated that YL10 inoculation in S2 rhizosphere soil had a notable impact on carbon cycle functions, specifically chemoheterotrophy, fermentation, and phototrophy. Thus, this study provides evidence that PGPR inoculation improves soil microbial communities and plant indices under salt stress. These findings shed light on the potential of PGPR as a viable approach for enhancing plant stress tolerance and fostering sustainable agricultural practices.

Molecular insights and omics-based understanding of plant-microbe interactions under drought stress

The detrimental effects of adverse environmental conditions are always challenging and remain a major concern for plant development and production worldwide. Plants deal with such constraints by physiological, biochemical, and morphological adaptations as well as acquiring mutual support of beneficial microorganisms. As many stress-responsive traits of plants are influenced by microbial activities, plants have developed a sophisticated interaction with microbes to cope with adverse environmental conditions. The production of numerous bioactive metabolites by rhizospheric, endo-, or epiphytic microorganisms can directly or indirectly alter the root system architecture, foliage production, and defense responses. Although plant-microbe interactions have been shown to improve nutrient uptake and stress resilience in plants, the underlying mechanisms are not fully understood. "Multi-omics" application supported by genomics, transcriptomics, and metabolomics has been quite useful to investigate and understand the biochemical, physiological, and molecular aspects of plant-microbe interactions under drought stress conditions. The present review explores various microbe-mediated mechanisms for drought stress resilience in plants. In addition, plant adaptation to drought stress is discussed, and insights into the latest molecular techniques and approaches available to improve drought-stress resilience are provided.

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.

Halomonas ventosae JPT10 promotes salt tolerance in foxtail millet (Setaria italica) by affecting the levels of multiple antioxidants and phytohormones

Plant growth-promoting bacterias (PGPBs) can increase crop output under normal and abiotic conditions. However, the mechanisms underlying the plant salt tolerance-promoting role of PGPBs still remain largely unknown. In this study, we demonstrated that Halomonas ventosae JPT10 promoted the salt tolerance of both dicots and monocots. Physiological analysis revealed that JPT10 reduced reactive oxygen species accumulation by improving the antioxidant capability of foxtail millet seedlings. The metabolomic analysis of JPT10-inoculated foxtail millet seedlings led to the identification of 438 diversely accumulated metabolites, including flavonoids, phenolic acids, lignans, coumarins, sugar, alkaloids, organic acids, and lipids, under salt stress. Exogenous apigenin and chlorogenic acid increased the salt tolerance of foxtail millet seedlings. Simultaneously, JPT10 led to greater amounts of abscisic acid (ABA), indole-3-acetic acid (IAA), salicylic acid (SA), and their derivatives but lower levels of 12-oxo-phytodienoic acid (OPDA), jasmonate (JA), and JA-isoleucine (JA-Ile) under salt stress. Exogenous JA, methyl-JA, and OPDA intensified, whereas ibuprofen or phenitone, two inhibitors of JA and OPDA biosynthesis, partially reversed, the growth inhibition of foxtail millet seedlings caused by salt stress. Our results shed light on the response of foxtail millet seedlings to H. ventosae under salt stress and provide potential compounds to increase salt tolerance in foxtail millet and other crops.

The physiology of drought stress in two grapevine cultivars: Photosynthesis, antioxidant system, and osmotic regulation responses

Drought stress impedes viticultural plant growth and development by modifying various metabolic pathways. However, the regulatory network response underlying drought stress is not yet clear. In this study, the leaves and roots of "Shine Muscat" ("SM," Vitis labruscana × Vitis vinifera) and "Thompson Seedless" ("TS," V. vinifera L. cv.) were subjected to drought stress to study the regulatory network used by drought stress. Morphophysiological results showed that the malondialdehyde content after 28 days of drought stress increased more significantly in "TS" than "SM." Furthermore, the multiomics analysis studies showed that a total of 3036-6714 differentially expressed genes and 379-385 differentially abundant metabolites were identified in "SM" and "TS" grapevine cultivars under drought stress. Furthermore, the retained intron was the major form of differential alternative splicing event under drought stress. The photosynthesis pathway, antioxidant system, plant hormone signal transduction, and osmotic adjustment were the primary response systems in the two grapevine cultivars under drought stress. We have identified GRIK1, RFS2, and LKR/SDH as the hub genes in the coexpression network of drought stress. In addition, the difference in the accumulation of pheophorbide-a reveals different drought resistance mechanisms in the two grapevine cultivars. Our study explained the difference in drought response between cultivars and tissues and identified drought stress-responsive genes, which provides reference data for further understanding the regulatory network of drought tolerance in grapevine.

Evaluating the underlying physiological and molecular mechanisms in the system of rice intensification performance with Trichoderma-rice plant symbiosis as a model system

The system of rice intensification (SRI) is an extensively-researched and increasingly widely-utilized methodology for alleviating current constraints on rice production. Many studies have shown physiological and morphological improvements in rice plants induced by SRI management practices to be very similar to those that are associated with the presence of beneficial microbial endophytes in or around rice plants, especially their roots. With SRI methods, grain yields are increased by 25-100% compared to conventional methods, and the resulting plant phenotypes are better able to cope with biotic and abiotic stresses. SRI management practices have been shown to be associated with significant increases in the populations of certain microorganisms known to enhance soil health and plant growth, e.g., Azospirillum, Trichoderma, Glomus, and Pseudomonas. This article evaluates the effects of applying Trichoderma as a model microbe for assessing microbial growth-promotion, biological control activity, and modulation of gene expression under the conditions created by SRI practices. Information about the molecular changes and interactions associated with certain effects of SRI management suggests that these practices are enhancing rice plants' expression of their genetic potentials. More systematic studies that assess the effects of SRI methods respectively and collectively, compared with standard rice production methods, are needed to develop a more encompassing understanding of how SRI modifications of crops' growing environment elicit and contribute to more robust and more productive phenotypes of rice.

Effect of betanin synthesis on photosynthesis and tyrosine metabolism in transgenic carrot

BACKGROUND

Betalain is a natural pigment with important nutritional value and broad application prospects. Previously, we produced betanin biosynthesis transgenic carrots via expressing optimized genes CYP76AD1S, cDOPA5GTS and DODA1S. Betanin can accumulate throughout the whole transgenic carrots. But the effects of betanin accumulation on the metabolism of transgenic plants and whether it produces unexpected effects are still unclear.

RESULTS

The accumulation of betanin in leaves can significantly improve its antioxidant capacity and induce a decrease of chlorophyll content. Transcriptome and metabolomics analysis showed that 14.0% of genes and 33.1% of metabolites were significantly different, and metabolic pathways related to photosynthesis and tyrosine metabolism were markedly altered. Combined analysis showed that phenylpropane biosynthesis pathway significantly enriched the differentially expressed genes and significantly altered metabolites.

CONCLUSIONS

Results showed that the metabolic status was significantly altered between transgenic and non-transgenic carrots, especially the photosynthesis and tyrosine metabolism. The extra consumption of tyrosine and accumulation of betanin might be the leading causes.

Lanthanum Silicate Nanomaterials Enhance Sheath Blight Resistance in Rice: Mechanisms of Action and Soil Health Evaluation

In the current study, foliar spray with lanthanum (La) based nanomaterials (La10Si6O27 nanorods, La10Si6O27 nanoparticle, La(OH)3 nanorods, and La2O3 nanoparticle) suppressed the occurrence of sheath blight (Rhizoctonia solani) in rice. The beneficial effects were morphology-, composition-, and concentration-dependent. Foliar application of La10Si6O27 nanorods (100 mg/L) yielded the greatest disease suppression, significantly decreasing the disease severity by 62.4% compared with infected controls; this level of control was 2.7-fold greater than the commercially available pesticide (Thifluzamide). The order of efficacy was as follows: La10Si6O27 nanorods > La10Si6O27 nanoparticle > La(OH)3 nanorods > La2O3 nanoparticle. Mechanistically, (1) La10Si6O27 nanorods had greater bioavailability, slower dissolution, and simultaneous Si nutrient benefits; (2) transcriptomic and metabolomic analyses revealed that La10Si6O27 nanorods simultaneously strengthened rice systemic acquired resistance, physical barrier formation, and antioxidative systems. Additionally, La10Si6O27 nanorods improved rice yield by 35.4% and promoted the nutritional quality of the seeds as compared with the Thifluzamide treatment. A two-year La10Si6O27 nanorod exposure had no effect on soil health based on the evaluated chemical, physical, and biological soil properties. These findings demonstrate that La based nanomaterials can serve as an effective and sustainable strategy to safeguard crops and highlight the importance of nanomaterial composition and morphology in terms of optimizing benefit.

Phyto-microbiome to mitigate abiotic stress in crop plants

Plant-associated microbes include taxonomically diverse communities of bacteria, archaebacteria, fungi, and viruses, which establish integral ecological relationships with the host plant and constitute the phyto-microbiome. The phyto-microbiome not only contributes in normal growth and development of plants but also plays a vital role in the maintenance of plant homeostasis during abiotic stress conditions. Owing to its immense metabolic potential, the phyto-microbiome provides the host plant with the capability to mitigate the abiotic stress through various mechanisms like production of antioxidants, plant growth hormones, bioactive compounds, detoxification of harmful chemicals and toxins, sequestration of reactive oxygen species and other free radicals. A deeper understanding of the structure and functions of the phyto-microbiome and the complex mechanisms of phyto-microbiome mediated abiotic stress mitigation would enable its utilization for abiotic stress alleviation of crop plants and development of stress-resistant crops. This review aims at exploring the potential of phyto-microbiome to alleviate drought, heat, salinity and heavy metal stress in crop plants and finding sustainable solutions to enhance the agricultural productivity. The mechanistic insights into the role of phytomicrobiome in imparting abiotic stress tolerance to plants have been summarized, that would be helpful in the development of novel bioinoculants. The high-throughput modern approaches involving candidate gene identification and target gene modification such as genomics, metagenomics, transcriptomics, metabolomics, and phyto-microbiome based genetic engineering have been discussed in wake of the ever-increasing demand of climate resilient crop plants.

Rhizobacteria Increase the Adaptation Potential of Potato Microclones under Aeroponic Conditions

Adaptation ex vitro is strongly stressful for microplants. Plant-growth-promoting rhizobacteria (PGPR) help to increase the adaptation potential of microplants transplanted from test tubes into the natural environment. We investigated the mechanisms of antioxidant protection of PGPR-inoculated potato microclones adapting to ex vitro growth in an aeroponic system. Potato (Solanum tuberosum L. cv. Nevsky) microplants were inoculated in vitro with the bacteria Azospirillum baldaniorum Sp245 and Ochrobactrum cytisi IPA7.2. On days 1 and 7 of plant growth ex vitro, catalase and peroxidase activities in the leaves of inoculated plants were 1.5-fold higher than they were in non-inoculated plants. The activity of ascorbate peroxidase was reduced in both in vitro and ex vitro treatments, and this reduction was accompanied by a decrease in the leaf content of hydrogen peroxide and malondialdehyde. As a result, inoculation contributed to the regulation of the plant pro/antioxidant system, lowering the oxidative stress and leading to better plant survival ex vitro. This was evidenced by the higher values of measured morphological and physiological variables of the inoculated plants, as compared with the values in the control treatment. Thus, we have shown some PGPR-mediated mechanisms of potato plant protection from adverse environmental factors under aeroponic conditions.

Nitrogen fixation and phytohormone stimulation of sugarcane plant through plant growth promoting diazotrophic Pseudomonas

Diazotrophic microorganisms are free-living groups of organisms that can convert atmospheric nitrogen (N) into bioavailable nitrogen for plants, which increases crop development and production. The purpose of the current study was to ascertain how diazotrophic plant growth promoting (PGP) Pseudomonas strains (P. koreensis CY4 and P. entomophila CN11) enhanced nitrogen fixation, defense activity, and PGP attributes of sugarcane varieties; GT11 and G×B9. A 15N isotope-dilution study was conducted to confirm the sugarcane strains' capacity to fix nitrogen, and the results indicated that between 21 to 35% of plant, nitrogen is fixed biologically by selected rhizobacteria. In comparison to the control, after 30, 60, and 90 days, both CY4 and CN11 strains significantly increased defense-related enzymes (catalase, peroxidase, phenylalanine ammonia-lyase, superoxide dismutase, glucanase, and chitinase) and phytohormones (abscisic acid, ABA, cytokinin, etc.) in GT11 and GXB. Additionally, the expression of SuCHI, SuGLU, SuCAT, SuSOD, and SuPAL genes was found to be elevated in Pseudomonas strains inoculated plants using real-time quantitative polymerase chain reaction (RT-qPCR). Both bacterial strains increased all physiological parameters and chlorophyll content in sugarcane plants more than their control. The effects of P. koreensis CY4 and P. entomophila CN11 strains on sugarcane growth promotion and nitrogen fixation under greenhouse conditions are described here for the first time systematically. The results of confirmation studies demonstrated that P. koreensis CY4 and P. entomophila are PGP bacterial strains with the potential to be employed as a biofertilizer for sugarcane growth, nitrogen nutrient absorption, and reduced application of chemical nitrogenous fertilizers in agricultural fields.  .

Study on the Potential for Stimulating Mulberry Growth and Drought Tolerance of Plant Growth-Promoting Fungi

Drought stress often leads to heavy losses in mulberry planting, especially for fruits and leaves. Application of plant growth-promoting fungi (PGPF) endows various plant beneficial traits to overcome adverse environmental conditions, but little is known about the effects on mulberry under drought stress. In the present study, we isolated 64 fungi from well-growing mulberry trees surviving periodical drought stress, and Talaromyces sp. GS1, Pseudeurotium sp. GRs12, Penicillium sp. GR19, and Trichoderma sp. GR21 were screened out due to their strong potential in plant growth promotion. Co-cultivation assay revealed that PGPF stimulated mulberry growth, exhibiting increased biomass and length of stems and roots. Exogenous application of PGPF could alter fungal community structures in the rhizosphere soils, wherein Talaromyces was obviously enhanced after inoculation of Talaromyces sp. GS1, and Peziza was increased in the other treatments. Moreover, PGPF could promote iron and phosphorus absorption of mulberry as well. Additionally, the mixed suspensions of PGPF induced the production of catalase, soluble sugar, and chlorophyll, which in turn enhanced the drought tolerance of mulberry and accelerated their growth recovery after drought. Collectively, these findings might provide new insights into improving mulberry drought tolerance and further boosting mulberry fruit yields by exploiting interactions between hosts and PGPF.

Mitigation of water scarcity with sustained growth of Rice by plant growth promoting bacteria

Climate change augments the risk to food security by inducing drought stress and a drastic decline in global rice production. Plant growth-promoting bacteria (PGPB) have been known to improve plant growth under drought stress. Here in the present study, we isolated, identified, and well-characterized eight drought-tolerant bacteria from the rice rhizosphere that are tolerant to 20% PEG-8000. These strains exhibited multiple plant growth-promoting traits, i.e., 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, exopolysaccharide production, phosphate (P)-solubilizing activity (51-356 µg ml-1), indole-3 acetic acid (IAA) production (14.3-46.2 µg ml-1), and production of organic acids (72-178 µg ml-1). Inoculation of bacterial consortium (Bacillus subtilis NM-2, Brucella haematophilum NM-4, and Bacillus cereus NM-6) significantly improved seedling growth and vigor index (1009.2-1100) as compared to non-inoculated stressed plants (630-957). Through rhizoscanning, efficiency of the consortium was validated by improved root parameters such as root length (17%), diameter, and surface area (18%) of all tested genotypes as compared with respective non-inoculated stressed treatments. Furthermore, the response of consortium inoculation on three rice genotypes was positively correlated with improved plant growth and drought stress ameliorating traits by the accumulation of osmoprotectant, i.e., proline (85.8%-122%), relative water content (51%), membrane stability index (64%), and production of antioxidant enzymes to reduce oxidative damage by reactive oxygen species. A decrease in temperature and improved chlorophyll content of inoculated plants were found using infrared thermal imaging and soil plant analyzer development (SPAD), respectively. The key supporting role of inoculation toward stress responses was validated using robust techniques like infrared thermal imaging and an infrared gas analyzer. Furthermore, principal component analysis depicts the contribution of inoculation on stress responses and yield of tested rice genotypes under water stress. The integration of drought-tolerant rice genotype (NIBGE-DT02) and potential bacterial strains, i.e., NM-2, NM-4, and NM-6, can serve as an effective bioinoculant to cope with water scarcity under current alarming issues related to food security in fluctuating climate.

Overexpression of the FERONIA receptor kinase MdMRLK2 confers apple drought tolerance by regulating energy metabolism and free amino acids production

Drought is a major abiotic stress limiting the growth and production of apple trees worldwide. The receptor-like kinase FERONIA is involved in plant growth, development and stress responses; however, the function of FERONIA in apple under drought stress remains unclear. Here, the FERONIA receptor kinase gene MdMRLK2 from apple (Malus domestica) was shown to encode a plasma membrane-localized transmembrane protein and was significantly induced by abscisic acid and drought treatments. 35S::MdMRLK2 apple plants showed less photosystem damage and higher photosynthetic rates compared with wild-type (WT) plants, after withholding water for 7 days. 35S::MdMRLK2 apple plants also had enhanced energy levels, activated caspase activity and more free amino acids, than the WT, under drought conditions. By performing yeast two-hybrid screening, glyceraldehyde-3-phosphate dehydrogenase and MdCYS4, a member of cystatin, were identified as MdMRLK2 interaction partners. Moreover, under drought conditions, the 35S::MdMRLK2 apple plants were characterized by higher abscisic acid (ABA) content. Overall, these findings demonstrated that MdMRLK2 regulates apple drought tolerance, probably via regulating levels of energetic matters, free amino acids and ABA.

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.

Understanding plant-microbe interaction of rice and soybean with two contrasting diazotrophic bacteria through comparative transcriptome analysis

Understanding the beneficial plant-microbe interactions is becoming extremely critical for deploying microbes imparting plant fitness and achieving sustainability in agriculture. Diazotrophic bacteria have the unique ability to survive without external sources of nitrogen and simultaneously promote host plant growth, but the mechanisms of endophytic interaction in cereals and legumes have not been studied extensively. We have studied the early interaction of two diazotrophic bacteria, Gluconacetobacter diazotrophicus (GAB) and Bradyrhizobium japonicum (BRH), in 15-day-old seedlings of rice and soybean up to 120 h after inoculation (hai) under low-nitrogen medium. Root colonization of GAB in rice was higher than that of BRH, and BRH colonization was higher in soybean roots as observed from the scanning electron microscopy at 120 hai. Peroxidase enzyme was significantly higher at 24 hai but thereafter was reduced sharply in soybean and gradually in rice. The roots of rice and soybean inoculated with GAB and BRH harvested from five time points were pooled, and transcriptome analysis was executed along with control. Two pathways, "Plant pathogen interaction" and "MAPK signaling," were specific to Rice-Gluconacetobacter (RG), whereas the pathways related to nitrogen metabolism and plant hormone signaling were specific to Rice-Bradyrhizobium (RB) in rice. Comparative transcriptome analysis of the root tissues revealed that several plant-diazotroph-specific differentially expressed genes (DEGs) and metabolic pathways of plant-diazotroph-specific transcripts, viz., chitinase, brassinosteroid, auxin, Myeloblastosis (MYB), nodulin, and nitrate transporter (NRT), were common in all plant-diazotroph combinations; three transcripts, viz., nitrate transport accessory protein (NAR), thaumatin, and thionin, were exclusive in rice and another three transcripts, viz., NAC (NAM: no apical meristem, ATAF: Arabidopsis thaliana activating factor, and CUC: cup-shaped cotyledon), ABA (abscisic acid), and ammonium transporter, were exclusive in soybean. Differential expression of these transcripts and reduction in pathogenesis-related (PR) protein expression show the early interaction. Based on the interaction, it can be inferred that the compatibility of rice and soybean is more with GAB and BRH, respectively. We propose that rice is unable to identify the diazotroph as a beneficial microorganism or a pathogen from an early response. So, it expressed the hypersensitivity-related transcripts along with PR proteins. The molecular mechanism of diazotrophic associations of GAB and BRH with rice vis-à-vis soybean will shed light on the basic understanding of host responses to beneficial microorganisms.

Bacterial Siderophores: Classification, Biosynthesis, Perspectives of Use in Agriculture

Siderophores are synthesized and secreted by many bacteria, yeasts, fungi, and plants for Fe (III) chelation. A variety of plant-growth-promoting bacteria (PGPB) colonize the rhizosphere and contribute to iron assimilation by plants. These microorganisms possess mechanisms to produce Fe ions under iron-deficient conditions. Under appropriate conditions, they synthesize and release siderophores, thereby increasing and regulating iron bioavailability. This review focuses on various bacterial strains that positively affect plant growth and development through synthesizing siderophores. Here we discuss the diverse chemical nature of siderophores produced by plant root bacteria; the life cycle of siderophores, from their biosynthesis to the Fe-siderophore complex degradation; three mechanisms of siderophore biosynthesis in bacteria; the methods for analyzing siderophores and the siderophore-producing activity of bacteria and the methods for screening the siderophore-producing activity of bacterial colonies. Further analysis of biochemical, molecular-biological, and physiological features of siderophore synthesis by bacteria and their use by plants will allow one to create effective microbiological preparations for improving soil fertility and increasing plant biomass, which is highly relevant for sustainable agriculture.

Understanding the salinity stress on plant and developing sustainable management strategies mediated salt-tolerant plant growth-promoting rhizobacteria and CRISPR/Cas9

Soil salinity is a worldwide concern that decreases plant growth performance in agricultural fields and contributes to food scarcity. Salt stressors have adverse impacts on the plant's ionic, osmotic, and oxidative balance, as well as numerous physiological functions. Plants have a variety of coping strategies to deal with salt stress, including osmosensing, osmoregulation, ion-homeostasis, increased antioxidant synthesis, and so on. Not only does salt stress cause oxidative stress but also many types of stress do as well, thus plants have an effective antioxidant system to battle the negative effects of excessive reactive oxygen species produced as a result of stress. Rising salinity in the agricultural field affects crop productivity and plant development considerably; nevertheless, plants have a well-known copying mechanism that shields them from salt stress by facilitated production of secondary metabolites, antioxidants, ionhomeostasis, ABAbiosynthesis, and so on. To address this problem, various environment-friendly solutions such as salt-tolerant plant growth-promoting rhizobacteria, eco-friendly additives, and foliar applications of osmoprotectants/antioxidants are urgently needed. CRISPR/Cas9, a new genetic scissor, has recently been discovered to be an efficient approach for reducing salt stress in plants growing in saline soil. Understanding the processes underlying these physiological and biochemical responses to salt stress might lead to more effective crop yield control measures in the future. In order to address this information, the current review discusses recent advances in plant stress mechanisms against salinity stress-mediated antioxidant systems, as well as the development of appropriate long-term strategies for plant growth mediated by CRISPR/Cas9 techniques under salinity stress.

Biotechnological Advances to Improve Abiotic Stress Tolerance in Crops

The major challenges that agriculture is facing in the twenty-first century are increasing droughts, water scarcity, flooding, poorer soils, and extreme temperatures due to climate change. However, most crops are not tolerant to extreme climatic environments. The aim in the near future, in a world with hunger and an increasing population, is to breed and/or engineer crops to tolerate abiotic stress with a higher yield. Some crop varieties display a certain degree of tolerance, which has been exploited by plant breeders to develop varieties that thrive under stress conditions. Moreover, a long list of genes involved in abiotic stress tolerance have been identified and characterized by molecular techniques and overexpressed individually in plant transformation experiments. Nevertheless, stress tolerance phenotypes are polygenetic traits, which current genomic tools are dissecting to exploit their use by accelerating genetic introgression using molecular markers or site-directed mutagenesis such as CRISPR-Cas9. In this review, we describe plant mechanisms to sense and tolerate adverse climate conditions and examine and discuss classic and new molecular tools to select and improve abiotic stress tolerance in major crops.

Phytomicrobiome communications: Novel implications for stress resistance in plants

The agricultural sector is a foremost contributing factor in supplying food at the global scale. There are plethora of biotic as well as abiotic stressors that act as major constraints for the agricultural sector in terms of global food demand, quality, and security. Stresses affect rhizosphere and their communities, root growth, plant health, and productivity. They also alter numerous plant physiological and metabolic processes. Moreover, they impact transcriptomic and metabolomic changes, causing alteration in root exudates and affecting microbial communities. Since the evolution of hazardous pesticides and fertilizers, productivity has experienced elevation but at the cost of impeding soil fertility thereby causing environmental pollution. Therefore, it is crucial to develop sustainable and safe means for crop production. The emergence of various pieces of evidence depicting the alterations and abundance of microbes under stressed conditions proved to be beneficial and outstanding for maintaining plant legacy and stimulating their survival. Beneficial microbes offer a great potential for plant growth during stresses in an economical manner. Moreover, they promote plant growth with regulating phytohormones, nutrient acquisition, siderophore synthesis, and induce antioxidant system. Besides, acquired or induced systemic resistance also counteracts biotic stresses. The phytomicrobiome exploration is crucial to determine the growth-promoting traits, colonization, and protection of plants from adversities caused by stresses. Further, the intercommunications among rhizosphere through a direct/indirect manner facilitate growth and form complex network. The phytomicrobiome communications are essential for promoting sustainable agriculture where microbes act as ecological engineers for environment. In this review, we have reviewed our building knowledge about the role of microbes in plant defense and stress-mediated alterations within the phytomicrobiomes. We have depicted the defense biome concept that infers the design of phytomicrobiome communities and their fundamental knowledge about plant-microbe interactions for developing plant probiotics.

Unraveling the mechanism of sulfur nutrition in pigeonpea inoculated with sulfur-oxidizing bacteria

An investigation was carried out to understand the mechanism(s) involved in the uptake of sulfur (S) as sulfate in pigeonpea following single inoculation of two sulfur-oxidizing bacteria (SOB), Stenotrophomonas maltophilia and Stenotrophomonas pavanii in the treatments amended with either elemental sulfur (S0) or sulfate (S6). Colonization potential and biofilm formation were analyzed through confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM). Furthermore, the effect of seed inoculation on root architecture, expression of genes involved in sulfur oxidation (sox) in bacterial inoculants, and genes involved in sulfate transport in pigeonpea (PpSULTR) were analyzed to correlate with the higher uptake of S in roots and shoots of pigeonpea. Both the SOB exhibited a good colonization potential and biofilm formation on the roots of pigeonpea. Among the 11 sox genes targeted in rhizosphere of pigeonpea, expression was achieved for seven genes, which showed 2-fold increase in treatments inoculated with S. maltophilia and amended with either S6 or S0. The inoculation of S. maltophilia and amendment of S0 led to increased expression of PpSULTR genes by several folds in roots. The inoculation of SOB had a significant influence on non-enzymatic (osmolytes like proline) and enzymatic (PAL, peroxidase, superoxide dismutase, and catalase) levels. The results revealed a significant increase in sulfur uptake in roots and shoots in treatment inoculated with S. maltophilia and amended with S6. The investigation showed that the SOB-mediated over-expression of PpSULTR genes in roots of pigeonpea and sox genes in the rhizosphere were acting synergistically in facilitating higher uptake and translocation of S in roots and shoots of pigeonpea plants.

Improvement in the physiological and biochemical performance of strawberries under drought stress through symbiosis with Antarctic fungal endophytes

Strawberry is one of the most widely consumed fruit, but this crop is highly susceptible to drought, a condition strongly associated with climate change, causing economic losses due to the lower product quality. In this context, plant root-associated fungi emerge as a new and novel strategy to improve crop performance under water-deficiency stress. This study aimed to investigate the supplementation of two Antarctic vascular plant-associated fungal endophytes, Penicillium brevicompactum and Penicillium chrysogenum, in strawberry plants to develop an efficient, effective, and ecologically sustainable approach for the improvement of plant performance under drought stress. The symbiotic association of fungal endophytes with strawberry roots resulted in a greater shoot and root biomass production, higher fruit number, and an enhanced plant survival rate under water-limiting conditions. Inoculation with fungal endophytes provokes higher photosynthetic efficiency, lower lipid peroxidation, a modulation in antioxidant enzymatic activity, and increased proline content in strawberry plants under drought stress. In conclusion, promoting beneficial symbiosis between plants and endophytes can be an eco-friendly strategy to cope with drought and help to mitigate the impact of diverse negative effects of climate change on crop production.

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

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

Progress and Applications of Plant Growth-Promoting Bacteria in Salt Tolerance of Crops

Saline soils are a major challenge in agriculture, and salinization is increasing worldwide due to climate change and destructive agricultural practices. Excessive amounts of salt in soils cause imbalances in ion distribution, physiological dehydration, and oxidative stress in plants. Breeding and genetic engineering methods to improve plant salt tolerance and the better use of saline soils are being explored; however, these approaches can take decades to accomplish. A shorter-term approach to improve plant salt tolerance is to be inoculated with bacteria with high salt tolerance or adjusting the balance of bacteria in the rhizosphere, including endosymbiotic bacteria (living in roots or forming a symbiont) and exosymbiotic bacteria (living on roots). Rhizosphere bacteria promote plant growth and alleviate salt stress by providing minerals (such as nitrogen, phosphate, and potassium) and hormones (including auxin, cytokinin, and abscisic acid) or by reducing ethylene production. Plant growth-promoting rhizosphere bacteria are a promising tool to restore agricultural lands and improve plant growth in saline soils. In this review, we summarize the mechanisms of plant growth-promoting bacteria under salt stress and their applications for improving plant salt tolerance to provide a theoretical basis for further use in agricultural systems.

Biopriming of Maize Seeds with a Novel Bacterial Strain SH-6 to Enhance Drought Tolerance in South Korea

Maize is the third most common cereal crop worldwide, after rice and wheat, and plays a vital role in preventing global hunger crises. Approximately 50% of global crop yields are reduced by drought stress. Bacteria as biostimulants for biopriming can improve yield and enhance sustainable food production. Further, seed biopriming stimulates plant defense mechanisms. In this study, we isolated bacteria from the rhizosphere of Artemisia plants from Pohang beach, Daegu, South Korea. Twenty-three isolates were isolated and screened for growth promoting potential. Among them, bacterial isolate SH-6 was selected based on maximum induced tolerance to polyethylene glycol-simulated drought. SH-6 showed ABA concentration = 1.06 ± 0.04 ng/mL, phosphate solubilizing index = 3.7, and sucrose concentration = 0.51 ± 0.13 mg/mL. The novel isolate SH-6 markedly enhanced maize seedling tolerance to oxidative stress owing to the presence of superoxide dismutase, catalase, and ascorbate peroxidase activities in the culture media. Additionally, we quantified and standardized the biopriming effect of SH-6 on maize seeds. SH-6 significantly increased maize seedling drought tolerance by up to 20%, resulting in 80% germination potential. We concluded that the novel bacterium isolate SH-6 (gene accession number (OM757882) is a biostimulant that can improve germination performance under drought stress.

Salt stress mitigation in Lathyrus cicera by combining different microbial inocula

Arid and semi-arid areas are considered vulnerable to various environmental constraints which are further fortified by climate change. Salinity is one of the most serious abiotic factors affecting crop yield and soil fertility. Till now, no information is available on the effect of salinity on development and symbiotic nitrogen (N₂) fixation in the legume species Lathyrus cicera. Here, we evaluated the effect of different microbial inocula including nitrogen-fixing Rhizobium laguerreae, arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis, a complex mixed inoculum of AMF isolated from rhizospheric soil in "Al Aitha", and various plant growth-promoting bacteria (PGPB) including Bacillus subtilus, Bacillus simplex and Bacillus megaterium combined with Rhizobium, the AMF consortium, or R. irregularis on alleviating salt stress in this legume. A pot trial was conducted to evaluate the ability of different microbial inocula to mitigate adverse effects of salinity on L. cicera plants. The results showed that salinity (100 mM NaCl) significantly reduced L. cicera plant growth. However, inoculation with different inocula enhanced plant growth and markedly promoted various biochemical traits. Moreover, the combined use of PGPB and AMF was found to be the most effective treatment in mitigating deleterious effects of salinity stress on L. cicera. In addition, this co-inoculation upregulated the expression of two marker genes (LcHKT1 and LcNHX7) related to salinity tolerance. Our findings suggest that the AMF/PGPB formulation has a great potential to be used as a biofertilizer to improve L. cicera plant growth and productivity under saline conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01205-4.

Research Progress in the Field of Microbial Mitigation of Drought Stress in Plants

Plants defend themselves against ecological stresses including drought. Therefore, they adopt various strategies to cope with stress, such as seepage and drought tolerance mechanisms, which allow plant development under drought conditions. There is evidence that microbes play a role in plant drought tolerance. In this study, we presented a review of the literature describing the initiation of drought tolerance mediated by plant inoculation with fungi, bacteria, viruses, and several bacterial elements, as well as the plant transduction pathways identified via archetypal functional or morphological annotations and contemporary "omics" technologies. Overall, microbial associations play a potential role in mediating plant protection responses to drought, which is an important factor for agricultural manufacturing systems that are affected by fluctuating climate.

Pseudomonas simiae augments the tolerance to alkaline bauxite residue in Atriplex canescens by modulating photosynthesis, antioxidant defense enzymes, and compatible osmolytes

In situ revegetation is effective in improving water-stable aggregation, preserving structural stability, and decreasing groundwater pollution to reduce the environmental risks posed by alkaline bauxite residue (ABR). Pseudomonas simiae, a plant growth-promoting rhizobacteria (PGPR), was used to promote Atriplex canescens growth challenged by ABR. The mechanism of P. simiae-induced plant growth promotion and tolerance against ABR stresses has been investigated. P. simiae was shown to alleviate ABR-induced stress in A. canescens by regulating photosynthesis and transpiration, inducing antioxidant defense, causing osmolyte accumulation, and altering plant morphology. Shoot dry weight, root dry weight, and root length of A. canescens were increased by 5.9%, 6.7%, and 11.5%, respectively, after inoculation with P. simiae for 60 days. Thus, it seems that P. simiae systemically regulated physiological processes in A. canescens favoring its growth under ABR treatments.

Microbial Contributions for Rice Production: From Conventional Crop Management to the Use of 'Omics' Technologies

Rice, the main staple food for about half of the world's population, has had the growth of its production stagnate in the last two decades. One of the ways to further improve rice production is to enhance the associations between rice plants and the microbiome that exists around, on, and inside the plant. This article reviews recent developments in understanding how microorganisms exert positive influences on plant growth, production, and health, focusing particularly on rice. A variety of microbial species and taxa reside in the rhizosphere and the phyllosphere of plants and also have multiple roles as symbiotic endophytes while living within plant tissues and even cells. They alter the morphology of host plants, enhance their growth, health, and yield, and reduce their vulnerability to biotic and abiotic stresses. The findings of both agronomic and molecular analysis show ways in which microorganisms regulate the growth, physiological traits, and molecular signaling within rice plants. However, many significant scientific questions remain to be resolved. Advancements in high-throughput multi-omics technologies can be used to elucidate mechanisms involved in microbial-rice plant associations. Prospectively, the use of microbial inoculants and associated approaches offers some new, cost-effective, and more eco-friendly practices for increasing rice production.

The Adaptation and Tolerance of Major Cereals and Legumes to Important Abiotic Stresses

Abiotic stresses, including drought, extreme temperatures, salinity, and waterlogging, are the major constraints in crop production. These abiotic stresses are likely to be amplified by climate change with varying temporal and spatial dimensions across the globe. The knowledge about the effects of abiotic stressors on major cereal and legume crops is essential for effective management in unfavorable agro-ecologies. These crops are critical components of cropping systems and the daily diets of millions across the globe. Major cereals like rice, wheat, and maize are highly vulnerable to abiotic stresses, while many grain legumes are grown in abiotic stress-prone areas. Despite extensive investigations, abiotic stress tolerance in crop plants is not fully understood. Current insights into the abiotic stress responses of plants have shown the potential to improve crop tolerance to abiotic stresses. Studies aimed at stress tolerance mechanisms have resulted in the elucidation of traits associated with tolerance in plants, in addition to the molecular control of stress-responsive genes. Some of these studies have paved the way for new opportunities to address the molecular basis of stress responses in plants and identify novel traits and associated genes for the genetic improvement of crop plants. The present review examines the responses of crops under abiotic stresses in terms of changes in morphology, physiology, and biochemistry, focusing on major cereals and legume crops. It also explores emerging opportunities to accelerate our efforts to identify desired traits and genes associated with stress tolerance.

Effect of ZnO Nanoparticles on Growth and Biochemical Responses of Wheat and Maize

Zinc is an essential element that is also renowned for widespread contamination and toxicity at high concentrations. The present study was carried out to analyze the responses induced by lower, as well as higher, doses of zinc (0-200 mg/L), in the form of zinc oxide nanoparticles (ZnO NPs) in wheat and maize, for a period of 21 days. Accumulation of zinc increases with increasing Zn doses in both wheat and maize, with higher doses being in wheat (121 mg/kg in root and 66 mg/kg in shoot) than in maize (95 mg/kg in root and 48 mg/kg in shoot). The activity of alpha-amylase showed increase, while that of dehydrogenase decline, in response to ZnO NPs. The length and biomass of plants and photosynthetic pigments increased slightly upon ZnO NPs supply. Malondialdehyde content showed a progressive increase in root and shoot of both plants. However, in response, antioxidant enzymes (superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, and catalase) showed increase up to lower concentrations (100 mg/L) of ZnO NPs but decline variably at higher levels (150-200 mg/L) in wheat and maize. The results suggest that lower supply of ZnO NPs (100 mg/L) could be stimulatory to the growth of plants and can be recommended as a Zn fertilizer source for crop production.

Antioxidative Responses of Duckweed (Lemna minor L.) to Phenol and Rhizosphere-Associated Bacterial Strain Hafnia paralvei C32-106/3

Duckweed (L. minor) is a cosmopolitan aquatic plant of simplified morphology and rapid vegetative reproduction. In this study, an H. paralvei bacterial strain and its influence on the antioxidative response of the duckweeds to phenol, a recalcitrant environmental pollutant, were investigated. Sterile duckweed cultures were inoculated with H. paralvei in vitro and cultivated in the presence or absence of phenol (500 mg L⁻¹), in order to investigate bacterial effects on plant oxidative stress during 5 days. Total soluble proteins, guaiacol peroxidase expression, concentration of hydrogen peroxide and malondialdehyde as well as the total ascorbic acid of the plants were monitored. Moreover, bacterial production of indole-3-acetic acid (IAA) was measured in order to investigate H. paralvei’s influence on plant growth. In general, the addition of phenol elevated all biochemical parameters in L. minor except AsA and total soluble proteins. Phenol as well as bacteria influenced the expression of guaiacol peroxidase. Different isoforms were associated with phenol compared to isoforms expressed in phenol-free medium. Considering that duckweeds showed increased antioxidative parameters in the presence of phenol, it can be assumed that the measured parameters might be involved in the plant’s defense system. H. paralvei is an IAA producer and its presence in the rhizosphere of duckweeds decreased the oxidative stress of the plants, which can be taken as evidence that this bacterial strain acts protectively on the plants during phenol exposure.

Induction of drought tolerance in Pennisetum glaucum by ACC deaminase producing PGPR- Bacillus amyloliquefaciens through Antioxidant defense system

Rhizobacteria from pearl millet were screened to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase and to evaluate its role in alleviating drought stress. Amongst 96 isolates, 28 were positive for ACC deaminase production, with MMR04 offering maximum activity of 2196.23 nmol of α-ketobutyrate produced mg^-1 of protein h^-1. The ACC deaminase producing rhizobacteria with multiple beneficial properties along with root colonization and non-pathogenic were selected [Bacillus amyloliquefaciens (MMR04), Bacillus subtilis (MMR18) and Stenotrophomonas maltophilia (MMR36)] to confirm the presence of ACC deaminase gene. A significant enhancement in seed germination (91.75%) and seedling vigor (1213.73) was noted upon seed treatment with MMR04 and hence further evaluated for its ability to induce drought stress. The seed treatment with MMR04 improved plant growth parameters and total chlorophyll and RWC in plants grown under severe drought stress (G5) conditions compared to control plants. In addition, MMR04 seed treatment enhanced proline, APX and SOD activity while decreased the MDA content up to 2.3 fold compared to untreated plants (G5). Gene expression studies revealed a significant decrease of 3.3 and 1.8 fold in the relative expression of drought-responsive (DREB-1E) and ethylene-responsive factor (ERF-1B) marker genes, respectively and an increase of 2.2 and 2.9 fold in the relative expression of APX1 and SOD1, respectively in MMR04 treated plants grown under G5 conditions over control. The results confirmed that ACC deaminase producing B. amyloliquefaciens MMR04 could defend the pearl millet plants against drought stress through an antioxidative system, thereby warranting its application in drought stress management.

Effect of drought on photosynthesis, total antioxidant capacity, bioactive component accumulation, and the transcriptome of Atractylodes lancea

BACKGROUND

Atractylodes lancea (Thunb.) DC, a medicinal herb belonging to the Asteraceae family, often faces severe drought stress during its growth. Until now, there has been no research on the effect of drought stress on the quality formation of A. lancea. Therefore, the present study aimed to study the effects of drought stress on A. lancea through physical and chemical analysis, and to reveal the related molecular mechanisms via transcriptome analysis.

RESULTS

The photosynthesis was markedly inhibited under drought stress. There were alterations to photosynthetic parameters (Pn, Gs, Ci) and chlorophyll fluorescence (Fv/Fm, NPQ), and the chlorophyll content decreased. Twenty genes encoding important regulatory enzymes in light and dark reactions, including the Rubisco gene of the Calvin cycle, were significantly downregulated. After exposure to drought stress for more than 4 days, the activities of four antioxidative enzymes (SOD, POD CAT and APX) began to decrease and continued to decrease with longer stress exposure. Meanwhile, most of the genes encoding antioxidative enzymes were downregulated significantly. The downregulation of 21 genes related to the respiratory electron transport chain indicated that the blocked electron transfer accelerated excessive ROS. The MDA content was significantly elevated. The above data showed that 15 days of drought stress caused serious oxidative damage to A. lancea. Drought stress not only reduced the size and dry weight of A. lancea, but also lowered the amount of total volatile oil and the content of the main bioactive components. The total volatile oil and atractylodin content decreased slightly, whereas the content of atractylon and β-eudesmol decreased significantly. Moreover, ten significantly downregulated genes encoding sesquiterpene synthase were mainly expressed in rhizomes.

CONCLUSIONS

After exposed to drought stress, the process of assimilation was affected by the destruction of photosynthesis; stress tolerance was impaired because of the inhibition of the antioxidative enzyme system; and bioactive component biosynthesis was hindered by the downregulation of sesquiterpene synthase-related gene expression. All these had negative impacts on the quality formation of A. lancea under drought stress.

Recombinant Expression and Characterization of Lemon (Citrus limon) Peroxidase

BACKGROUND

Class III plant peroxidases play important role in a number of physiological processes in plants such as lignin biosynthesis, suberization, cell wall biosynthesis, reactive oxygen species metabolism and plant defense against pathogens. Peroxidases are also of significance in several industrial applications. In view of this, the production and identification of novel peroxidases having resistance towards temperature, pH, salts is desirable.

OBJECTIVE

The objective of the present work was to clone and characterize a novel plant peroxidase suitable for industrial application.

METHODS

A full length cDNA clone of lemon peroxidase was isolated using PCR and RACE approaches, characterized and heterologously expressed in Escherichia coli using standard protocols. The expressed peroxidase was purified using Ni-NTA agarose column and biochemically characterized using standard protocols. The peroxidase was also in-silico characterized at nucleotide as well as protein levels using standard protocols.

RESULTS

A full length cDNA clone of lemon peroxidase was isolated and expressed heterologously in E. coli. The expressed recombinant lemon peroxidase (LPRX) was activated by in-vitro refolding and purified. The purified LPRX exhibited pH and temperature optima of pH 7.0 and 50°C, respectively. The LPRX was found to be activated by metal ions (Na+, Ca2+, Mg2+ and Mn2+) at lower concentration. The expressional analysis of the transcripts suggested involvement of lemon peroxidase in plant defense. The lemon peroxidase was in silico modelled and docked with the substrates guaiacol, and pyrogallol and shown the favourability of pyrogallol over guaiacol, which is in agreement with the in-vitro findings. The protein function annotation analyses suggested the involvement of lemon peroxidase in the phenylpropanoid biosynthesis pathway and plant defense mechanisms.

CONCLUSION

Based on the biochemical characterization, the purified peroxidase was found to be resistant towards the salts and thus, might be a good candidate for industrial exploitation. The in-silico protein function annotation and transcript analyses highlighted the possible involvement of the lemon peroxidase in plant defense response.

Insights into the plant responses to drought and decoding the potential of root associated microbiome for inducing drought tolerance

Global increase in water scarcity is a serious problem for sustaining crop productivity. The lack of water causes the degeneration of the photosynthetic apparatus, an imbalance in key metabolic pathways, an increase in free radical generation as well as weakens the root architecture of plants. Drought is one of the major stresses that directly interferes with the osmotic status of plant cells. Abscisic acid (ABA) is known to be a key player in the modulation of drought responses in plants and involvement of both ABA-dependent and ABA-independent pathways have been observed during drought. Concomitantly, other phytohormones such as auxins, ethylene, gibberellins, cytokinins, jasmonic acid also confer drought tolerance and a crosstalk between different phytohormones and transcription factors at the molecular level exists. A number of drought-responsive genes and transcription factors have been utilized for producing transgenic plants for improved drought tolerance. Despite relentless efforts, biotechnological advances have failed to design completely stress tolerant plants until now. The root microbiome is the hidden treasure that possesses immense potential to revolutionize the strategies for inducing drought resistance in plants. Root microbiota consist of plant growth-promoting rhizobacteria, endophytes and mycorrhizas that form a consortium with the roots. Rhizospheric microbes are proliferous producers of phytohormones, mainly auxins, cytokinin, and ethylene as well as enzymes like the 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase) and metabolites like exopolysaccharides that help to induce systemic tolerance against drought. This review, therefore focuses on the major mechanisms of plant-microbe interactions under drought-stressed conditions and emphasizes the importance of drought-tolerant microbes for sustaining and improving the productivity of crop plants under stress.

Molecular Programming of Drought-Challenged Trichoderma harzianum-Bioprimed Rice (Oryza sativa L.)

Trichoderma biopriming enhances rice growth in drought-stressed soils by triggering various plant metabolic pathways related to antioxidative defense, secondary metabolites, and hormonal upregulation. In the present study, transcriptomic analysis of rice cultivar IR64 bioprimed with Trichoderma harzianum under drought stress was carried out in comparison with drought-stressed samples using next-generation sequencing techniques. Out of the 2,506 significant (p < 0.05) differentially expressed genes (DEGs), 337 (15%) were exclusively expressed in drought-stressed plants, 382 (15%) were expressed in T. harzianum-treated drought-stressed plants, and 1,787 (70%) were commonly expressed. Furthermore, comparative analysis of upregulated and downregulated genes under stressed conditions showed that 1,053 genes (42%) were upregulated and 733 genes (29%) were downregulated in T. harzianum-treated drought-stressed rice plants. The genes exclusively expressed in T. harzianum-treated drought-stressed plants were mostly photosynthetic and antioxidative such as plastocyanin, small chain of Rubisco, PSI subunit Q, PSII subunit PSBY, osmoproteins, proline-rich protein, aquaporins, stress-enhanced proteins, and chaperonins. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis states that the most enriched pathways were metabolic (38%) followed by pathways involved in the synthesis of secondary metabolites (25%), carbon metabolism (6%), phenyl propanoid (7%), and glutathione metabolism (3%). Some of the genes were selected for validation using real-time PCR which showed consistent expression as RNA-Seq data. Furthermore, to establish host-T. harzianum interaction, transcriptome analysis of Trichoderma was also carried out. The Gene Ontology (GO) analysis of T. harzianum transcriptome suggested that the annotated genes are functionally related to carbohydrate binding module, glycoside hydrolase, GMC oxidoreductase, and trehalase and were mainly upregulated, playing an important role in establishing the mycelia colonization of rice roots and its growth. Overall, it can be concluded that T. harzianum biopriming delays drought stress in rice cultivars by a multitude of molecular programming.

Evaluation of plant growth promotion properties and induction of antioxidative defense mechanism by tea rhizobacteria of Darjeeling, India

A total of 120 rhizobacteria were isolated from seven different tea estates of Darjeeling, West Bengal, India. Based on a functional screening of in vitro plant growth-promoting (PGP) activities, thirty potential rhizobacterial isolates were selected for in-planta evaluation of PGP activities in rice and maize crops. All the thirty rhizobacterial isolates were identified using partial 16S rRNA gene sequencing. Out of thirty rhizobacteria, sixteen (53.3%) isolates belong to genus Bacillus, five (16.6%) represent genus Staphylococcus, three (10%) represent genus Ochrobactrum, and one (3.3%) isolate each belongs to genera Pseudomonas, Lysinibacillus, Micrococcus, Leifsonia, Exiguobacterium, and Arthrobacter. Treatment of rice and maize seedlings with these thirty rhizobacterial isolates resulted in growth promotion. Besides, rhizobacterial treatment in rice triggered enzymatic [ascorbate peroxidase (APX), catalase (CAT), chitinase, and phenylalanine ammonia-lyase (PAL)], and non-enzymatic [proline and polyphenolics] antioxidative defense reactions indicating their possible role in the reduction of reactive oxygen species (ROS) burden and thereby priming of plants towards stress mitigation. To understand such a possibility, we tested the effect of rhizobacterial consortia on biotic stress tolerance of rice against necrotrophic fungi, Rhizoctonia solani AG1-IA. Our results indicated that the pretreatment with rhizobacterial consortia increased resistance of the rice plants towards the common foliar pathogen like R. solani AG1-IA. This study supports the idea of the application of plant growth-promoting rhizobacterial consortia in sustainable crop practice through the management of biotic stress under field conditions.

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.