Hydrogen sulfide: An emerging signaling molecule regulating drought stress response in plants.

Hydrogen sulfide (H S) is a small, reactive signaling molecule that is produced within chloroplasts of plant cells as an intermediate in the assimilatory sulfate reduction pathway by the enzyme sulfite reductase. In addition, H S is also produced in cytosol and mitochondria by desulfhydration of l-cysteine catalyzed by l-cysteine desulfhydrase (DES1) in the cytosol and from β-cyanoalanine in mitochondria, in a reaction catalyzed by β-cyano-Ala synthase C1 (CAS-C1). H S exerts its numerous biological functions by post-translational modification involving oxidation of cysteine residues (RSH) to persulfides (RSSH). At lower concentrations (10-1000 μmol L ), H S shows huge agricultural potential as it increases the germination rate, the size, fresh weight, and ultimately the crop yield. It is also involved in abiotic stress response against drought, salinity, high temperature, and heavy metals. H S donor, for example, sodium hydrosulfide (NaHS), has been exogenously applied on plants by various researchers to provide drought stress tolerance. Exogenous application results in the accumulation of polyamines, sugars, glycine betaine, and enhancement of the antioxidant enzyme activities in response to drought-induced osmotic and oxidative stress, thus, providing stress adaptation to plants. At the biochemical level, administration of H S donors reduces malondialdehyde content and lipoxygenase activity to maintain the cell integrity, causes abscisic acid-mediated stomatal closure to prevent water loss through transpiration, and accelerates the photosystem II repair cycle. Here, we review the crosstalk of H S with secondary messengers and phytohormones towards the regulation of drought stress response and emphasize various approaches that can be addressed to strengthen research in this area.