Hydrogen sulphide (H 2 S) in the hidden half: Role in root growth, stress signalling and rhizospheric interactions.

Apart from nitric oxide (NO) and carbon monoxide (CO), hydrogen sulphide (H 2 S) has emerged as a potential gasotransmitter that has regulatory roles in root differentiation, proliferation and stress signalling. H 2 S metabolism in plants exhibits spatio-temporal differences that are intimately associated with sulphide signalling in the cytosol and other subcellular components, e.g. chloroplast and mitochondria. H 2 S biosynthesis in plant organs uses both enzymatic and non-enzymatic pathways. H 2 S generation in roots and aerial organs is modulated by developmental phase and changes in environmental stimuli. H 2 S has an influential role in root development and in the nodulation process. Studies have revealed that H 2 S is a part of the auxin and NO signalling pathways in roots, which induce lateral root formation. At the molecular level, exogenous application of H 2 S regulates expression of several transcription factors, viz. LBD (Lateral organ Boundaries Domain), MYB (myeloblastosis) and AP2/ERF (Apetala 2/ Ethylene Response Factor), which stimulate upregulation of PpLBD16 (Lateral organ boundaries domain 16), thereby significantly increasing the number of lateral roots. Concomitantly, H 2 S acts as a crucial signalling molecule in roots during various abiotic stresses, e.g. drought, salinity heavy metals (HMs), etc., and augments stress tolerance in plants. Interestingly, extensive crosstalk exists between H 2 S, NO, ABA, calcium and ethylene during stress, which escalate plant defence and regulate plant growth and productivity. Hence, the present review will elaborate the role of H 2 S in root development, stress alleviation, legume-Rhizobium symbiosis and rhizosphere signalling. The review also examines the mechanism of H 2 S-mediated abiotic stress mitigation and cross-talk with other signaling molecules.