Melatonin improves cadmium tolerance in Salix viminalis by enhancing amino acid and saccharide metabolism, and increasing polyphenols

As a short-rotation woody plant, Salix viminalis has the potential for phytoremediation of cadmium (Cd), but it has poor tolerance to high Cd concentrations. Melatonin (MEL), a candidate bio-promoter, was considered to play an active role in plant responses to Cd. However, the molecular mechanism by which MEL regulates metabolic processes in plants to defend against Cd stress remain unclear. Transcriptomics and global untargeted metabolomic sequencing were used to investigate the rapid response of S. viminalis to high Cd concentrations during initial growth stage after foliar application of MEL. Four treatments were set up in a pot experiment involving foliar application of MEL on the first day, followed by irrigation with a Cd solution the next day. Significant variations in the relevant defence genes and metabolites in leaves exposed to Cd were observed between willows treated with and without MEL. Foliar application of MEL upregulated sulphur metabolism-related genes such as methionine and S-adenosylmethionine synthases in leaves exposed to Cd; glutamine content, which is the key point of nitrogen assimilation, also increased. Additionally, glycolysis and sucrose metabolic genes, including hexokinase, sucrose synthase, invertase, and the inositol phosphate metabolic gene myo-inositol-1-phosphate synthase were also upregulated in leaves. Moreover, MEL also upregulated genes related to the synthesis of flavonoids, anthocyanins, and proanthocyanins in the leaves. These results demonstrated that MEL improved amino acid and saccharide metabolism in the leaves of S. viminalis in response to Cd. It also improved the antioxidant capacity and Cd tolerance in S. viminalis leaves by enhancing synthetic capacity of polyphenol compounds. MEL may be involved in processes of photorespiration, ethylene metabolism, GABA shunt, nitric oxide metabolism, osmotic adjustment, and the synthesis of glutathione and ascorbate in S. viminalis under Cd stress. This series of metabolic changes in S. viminalis occurred within 24 h of the foliar application of MEL, which provided a sufficient substrate for subsequent defence reactions to cope with Cd stress. Our findings will help elucidate the molecular mechanism by which MEL regulates metabolic processes in plants in response to Cd challenges and guide the application of MEL to improve Cd phytoremediation efficiency.