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

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

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