Hydrogen sulphide (H2S) in the hidden half: Role in root growth, stress signalling and rhizospheric interactions

Apart from nitric oxide (NO) and carbon monoxide (CO), hydrogen sulphide (H2S) has emerged as a potential gasotransmitter that has regulatory roles in root differentiation, proliferation and stress signalling. H2S 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. H2S biosynthesis in plant organs uses both enzymatic and non-enzymatic pathways. H2S generation in roots and aerial organs is modulated by developmental phase and changes in environmental stimuli. H2S has an influential role in root development and in the nodulation process. Studies have revealed that H2S is a part of the auxin and NO signalling pathways in roots, which induce lateral root formation. At the molecular level, exogenous application of H2S 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, H2S 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 H2S, 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 H2S in root development, stress alleviation, legume-Rhizobium symbiosis and rhizosphere signalling. The review also examines the mechanism of H2S-mediated abiotic stress mitigation and cross-talk with other signaling molecules.