ORCID IDs: 0000-0003-3694-6378 (P.S.); 0000-0003-0820-9936 (A.K.S.)Mitogen-activated protein kinase (MAPK) signal transduction networks have been extensively explored in plants; however, the connection between MAPK signaling cascades and submergence tolerance is currently unknown. The ethylene response factor-like protein SUB1A orchestrates a plethora of responses during submergence stress tolerance in rice (Oryza sativa). In this study, we report that MPK3 is activated by submergence in a SUB1A-dependent manner. MPK3 physically interacts with and phosphorylates SUB1A in a tolerant-allele-specific manner. Furthermore, the tolerant allele SUB1A1 binds to the MPK3 promoter and regulates its expression in a positive regulatory loop during submergence stress signaling. We present molecular and physiological evidence for the key role of the MPK3-SUB1A1 module in acclimation of rice seedlings to the adverse effects of submergence. Overall, the results provide a mechanistic understanding of submergence tolerance in rice.

MYC2, a bHLH TF, acts as regulatory hub within several signaling pathways by integration of various endogenous and exogenous signals which shape plant growth and development. However, its involvement in salt stress regulation is still elusive. This study has deciphered a novel role of MYC2 in imparting salt stress intolerance by regulating delta1 ‐pyrroline‐5‐carboxylate synthase1 (P5CS1) gene and hence proline synthesis. P5CS1 is a rate‐limiting enzyme in the biosynthesis of proline. Y‐1‐H and EMSA studies confirmed the binding of MYC2 with the 5′UTR region of P5CS1. Transcript and biochemical studies have revealed MYC2 as a negative regulator of proline biosynthesis. Proline is necessary for imparting tolerance toward abiotic stress; however, its overaccumulation is toxic for the plants. Hence, studying the regulation of proline biosynthesis is requisite to understand the mechanism of stress tolerance. We have also studied that MYC2 is regulated by mitogen‐activated protein kinase (MAPK) cascade mitogen‐activated protein kinase kinase 3‐MPK6 and vice versa. Altogether, this study demonstrates salt stress‐mediated activation of MYC2 by MAPK cascade, regulating proline biosynthesis and thus salt stress.

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SUMMARY Phytopathogens pose a severe threat to agriculture and strengthening the plant defense response is an important strategy for disease control. Here, we report that AtRAV1, an AP2 and B3 domain‐containing transcription factor, is required for basal plant defense in Arabidopsis thaliana. The atrav1 mutant lines demonstrate hyper‐susceptibility against fungal pathogens (Rhizoctonia solani and Botrytis cinerea), whereas AtRAV1 overexpressing lines exhibit disease resistance against them. Enhanced expression of various defense genes and activation of mitogen‐activated protein kinases (AtMPK3 and AtMPK6) are observed in the R. solani infected overexpressing lines, but not in the atrav1 mutant plants. An in vitro phosphorylation assay suggests AtRAV1 to be a novel phosphorylation target of AtMPK3. Bimolecular fluorescence complementation and yeast two‐hybrid assays support physical interactions between AtRAV1 and AtMPK3. Overexpression of the native as well as phospho‐mimic but not the phospho‐defective variant of AtRAV1 imparts disease resistance in the atrav1 mutant A. thaliana lines. On the other hand, overexpression of AtRAV1 fails to impart disease resistance in the atmpk3 mutant. These analyses emphasize that AtMPK3‐mediated phosphorylation of AtRAV1 is important for the elaboration of the defense response in A. thaliana. Considering that RAV1 homologs are conserved in diverse plant species, we propose that they can be gainfully deployed to impart disease resistance in agriculturally important crop plants. Indeed, overexpression of SlRAV1 (a member of the RAV1 family) imparts disease tolerance against not only fungal (R. solani and B. cinerea), but also against bacterial (Ralstonia solanacearum) pathogens in tomato, whereas silencing of the gene enhances disease susceptibility.