Crosstalk between H2S and NO: an emerging signalling pathway during waterlogging stress in legume crops

In legumes, waterlogging is a major detrimental factor leading to huge yield losses. Generally, legumes lack tolerance to submergence, and conventional breeding to develop tolerant varieties are limited due to the lack of tolerant germplasm and potential target genes. Moreover, our understanding of the various signalling cascades, their interactions and key pathways induced during waterlogging is limited. Here, we focus on the role of two important plant signalling molecules, viz. hydrogen sulphide (H2S) and nitric oxide (NO), during waterlogging stress in legumes. Plants and soil microbes produce these signalling molecules both endogenously and exogenously under various stresses, including waterlogging. NO and H2S are known to regulate key physiological pathways, such as stomatal closure, leaf senescence and regulation of numerous stress signalling pathways, while NO plays a pivotal role in adventitious root formation during waterlogging. The crosstalk between H2S and NO is synergistic because of the resemblance of their physiological effects and proteomic functions, which mainly operate through cysteine-dependent post-translational modifications via S-nitrosation and persulfidation. Such knowledge has provided novel platforms for researchers to unravel the complexity associated with H2S-NO signalling and interactions with plant stress hormones. This review provides an overall summary on H2S and NO, including biosynthesis, biological importance, crosstalk, transporter regulation as well as understanding their role during waterlogging using 'multi-omics' approach. Understanding H2S and NO signalling will help in deciphering the metabolic interactions and identifying key regulatory genes that could be used for developing waterlogging tolerance in legumes.