Melatonin enhances salt tolerance by promoting CcCAD10-mediated lignin biosynthesis in pigeon pea

Melatonin plays a crucial role in enhancing plant resistance to salt stress by regulating biosynthesis of specialized metabolites. Phenylpropanoids, especially lignin, contribute to all aspects of plant responses toward biotic and abiotic stresses. However, the crosstalk between melatonin and lignin is largely unknown in pigeon pea under salt stress. In this study, the cinnamyl alcohol dehydrogenase CcCAD10 was identified to be involved in melatonin treatment and salt stress. The content of lignin was significantly increased in CcCAD10 over-expression (OE) lines, the enhanced antioxidant enzyme activities, indicating enhanced salt resistance. As a parallel branch of the lignin synthesis pathway, the content of flavonoids was further determined. The accumulations of luteolin, genistin, genistein, biochain A, apigenin and isovitexin were down-regulated in CcCAD10-OE hairy root. The results indicate that CcCAD10-OE mediated carbon flow from the phenylalanine pathway is redirected to the lignin pathway at the expense of less carbon flow in the flavonoid pathway, enhancing the salt-tolerance. Furthermore, we found the exogenous melatonin stimulated endogenous melatonin production mainly by upregulating the expression of CcASMT2 gene. This study reveals a novel mechanism by which melatonin enhances salt tolerance in pigeon pea, which laid a foundation for exploring the molecular mechanism of melatonin in salt stress response.