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Increasing evidence shows that nitric oxide (NO), a typical signaling molecule plays important role in development of plant and in bacteria-plant interaction. In the present study, we tested the effect of sodium nitroprusside (SNP)-a nitric oxide donor, on bacterial metabolism and its role in establishment of PGPR-plant interaction under salinity condition. In the present study, we adopted methods namely, biofilm formation assay, GC-MS analysis of bacterial volatiles, chemotaxis assay of root exudates (REs), measurement of electrolyte leakage and lipid peroxidation, and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) for gene expression. GC-MS analysis revealed that three new volatile organic compounds (VOCs) were expressed after treatment with SNP. Two VOCs namely, 4-nitroguaiacol and quinoline were found to promote soybean seed germination under 100 mM NaCl stress. Chemotaxis assay revealed that SNP treatment, altered root exudates profiling (SS-RE), found more attracted to Pseudomonas simiae bacterial cells as compared to non-treated root exudates (S-RE) under salt stress. Expression of Peroxidase (POX), catalase (CAT), vegetative storage protein (VSP), and nitrite reductase (NR) genes were up-regulated in T6 treatment seedlings, whereas, high affinity K transporter (HKT1), lipoxygenase (LOX), polyphenol oxidase (PPO), and pyrroline-5-carboxylate synthase (P5CS) genes were down-regulated under salt stress. The findings suggest that NO improves the efficiency and establishment of PGPR strain in the plant environment during salt condition. This strategy may be applied on soybean plants to increase their growth during salinity stress.

Plant growth-promoting bacteria (PGPB) are capable of alleviating environmental stress and eliciting tolerance in plants to promote their growth. Several PGPB elicit physical and/or chemical changes related to plant defense in the form of induced systemic resistance (ISR) under biotic stress. Researchers emphasized that PGPBelicited ISR has suppressed plant diseases caused by a range of pathogens in both the greenhouse and field. PGPB-elicited physical and chemical changes in plants result in enhanced tolerance to drought, salt, and other factors that have been described as a form of induced systemic tolerance under abiotic stress. This review will focus on recent research concerning interactions between PGPB and plants under biotic and abiotic stresses. The use of PGPB requires precise understanding of the interactions between plant-bacteria, among bacteria-microbiota, and how biotic and abiotic factors influence these relationships. Consequently, continued research is needed to develop new approaches to ameliorate the efficiency of PGPB and to understand the ecological, genetic, and biochemical relationships in their habitat.

Salt tolerant bacteria can be helpful in improving a plant's tolerance to salinity. Although plant-bacteria interactions in response to salt stress have been characterized, the precise molecular mechanisms by which bacterial inoculation alleviates salt stress in plants are still poorly explored. In the present study, we aimed to determine the role of a salt-tolerant plant growth-promoting rhizobacteria (PGPR) Sphingobacterium BHU-AV3 for improving salt tolerance in tomato through investigating the physiological responses of tomato roots and leaves under salinity stress. Tomato plants inoculated with BHU-AV3 and challenged with 200 mM NaCl exhibited less senescence, positively correlated with the maintenance of ion balance, lowered reactive oxygen species (ROS), and increased proline content compared to the non-inoculated plants. BHU-AV3-inoculated plant leaves were less affected by oxidative stress, as evident from a reduction in superoxide contents, cell death, and lipid peroxidation. The reduction in ROS level was associated with the increased antioxidant enzyme activities along with multiple-isoform expression [peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD)] in plant roots. Additionally, BHU-AV3 inoculation induced the expression of proteins involved in (i) energy production [ATP synthase], (ii) carbohydrate metabolism (enolase), (iii) thiamine biosynthesis protein, (iv) translation protein (elongation factor 1 alpha), and the antioxidant defense system (catalase) in tomato roots. These findings have provided insight into the molecular mechanisms of bacteria-mediated alleviation of salt stress in plants. From the study, we can conclude that BHU-AV3 inoculation effectively induces antioxidant systems and energy metabolism in tomato roots, which leads to whole plant protection during salt stress through induced systemic tolerance.

The identification of such differentially expressed proteins provide new targets for future studies that will allow assessment of their physiological roles and significance in the response of glycophytes to stresses. Further work should uncover more about the chemical side of VOC compounds and a detailed study about their molecular mechanism responsible for plant growth.

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