Melatonin-mediated regulation of anthocyanin biosynthesis and antioxidant defense confer tolerance to arsenic stress in Camellia sinensis L

Arsenic is a toxic metalloid for both animals and plants. The signaling molecule melatonin can enhance abiotic stress tolerance, but the effects of As and melatonin on tea plants and the mechanisms of resilience remain unclear. Here we report that excess As causes severe oxidative stress in tea leaves as revealed by significantly reduced maximal photochemical efficiency of photosystem-II, and increased reactive oxygen species accumulation and lipid peroxidation. However, exogenous melatonin application alleviated the As phytotoxicity and increased the anthocyanin content upto 69.4 % by selectively upregulating the expression of its biosynthetic genes such as CsCHS and CsANS. Comparison of As tolerance between two tea genotypes differing in basal levels of anthocyanin revealed that a tea cultivar with increased anthocyanin content, Zijuan (ZJ), showed enhanced tolerance to As stress compared with Longjing 43 (LJ43) that contained relatively low levels of anthocyanin. Interestingly, exogenous anthocyanin also enhanced As tolerance in LJ43, but exogenous melatonin did not improve As tolerance in ZJ genotype. Analysis of As content in tea leaves revealed that melatonin significantly reduced As content in LJ43 but not in ZJ, suggesting that melatonin-enhanced tolerance to As stress is largely dependent on the basal levels of anthocyanin in tea plants.