Melatonin-mediated regulation of anthocyanin biosynthesis and antioxidant defense enhances the resistance of tea plants (Camellia sinensis L) to fluoride stress

• Melatonin alleviated fluorine induced oxidative stress by increasing antioxidant defence. • Melatonin promoted the accumulation of precursor phenylalanine. • Melatonin facilitated phenylalanine conversion towards the anthocyanin pathways by upregulating key genes. • 100-200 μM melatonin had a significant fluoride-reducing effect in tea plantations. Excessive accumulation of fluoride impairs the safety of tea plants. Melatonin is a widely occurring hormone in plants. The effect of melatonin in enhancing the fluoride stress tolerance of tea plants and its underlying mechanisms remain unclear. Focused on two tea plant genotypes with extreme differences in anthocyanin contents, the fluoride accumulation, antioxidant capacity and anthocyanin-related secondary metabolite synthesis were compared in this study. The results showed that fluoride stress induced oxidative stress, marked by a significant reduction in the maximum photochemical efficiency of photosystem II (PSII). Melatonin could alleviate fluoride stress with increases in the maximum photochemical efficiency of photosystem II (PSII) and the antioxidant capacities. The effect was more pronounced in LJ43 (the low anthocyanin genotype), with the Ws-F accumulation decreases in the mature leaves, stems, and roots by 10%, 26%, and 61%, respectively. Melatonin promoted the precursor phenylalanine accumulation in LJ43 and facilitated conversion towards the anthocyanin pathways by regulating key genes ( CsMYB24, CsWD40, CsCHI, CsDFR , and CsANS ). Furthermore, the 100 μM melatonin significantly decreased the concentration of soluble fluoride in mature leaves by 18.1% for LJ43. This represented a new approach to reducing fluoride accumulation in tea plants.