Hydrogen sulfide (H2S) as a protagonist to regulate metal stress in plants: an overview of physiological steps

Hydrogen sulfide (H2S) is one of the gaseous molecules that act as a signaling agent at lower concentrations. It performs significant action in protecting the plants against adverse conditions by being part of the signaling network. In the current review, heavy metal stress has been considered as one of the major stress factors, as it affects the vital function of plants. The heavy metals also activate the expression of transcription factors and protein signaling pathways to regulate the expression of several stress-responsive genes. Further, H2S signaling triggers gene expression in the antioxidant defense system. Along with the endogenous level, the exogenous treatment with H2S up to a certain concentration maintains the redox balance and dynamic regulation of antioxidant enzyme systems to regulate the metal toxicity. It promotes more expression of chelators synthetic genes like PCS1, PCS2, MT1A, MT1B, and MT2B to produce more phytochelatins (PCs) and Metallothioneins (MTs), to scavenge the metals. H2S manages metal toxicity through various signaling processes by modulating extracellular signals into intracellular responses. The present review is based on the understanding of metal toxicity regulation with the intrusion of H2S signaling molecules. Overall, it will try to provide mechanistic action of H2S signaling on physiological performance and ultra-structure changes in the plant in order to regulate the metal stress.