Ectopic expression of AANAT or HIOMT improves melatonin production and enhances UV-B tolerance in transgenic apple plants

Melatonin is involved in plant responses to various environmental stresses. Although many studies have demonstrated that the tolerance of plants to stress is improved by exogenous melatonin, the role of endogenous melatonin metabolism in the response of apples to UV-B stress remains unclear. Here, the human melatonin biosynthesis-related enzyme genes AANAT or HIOMT were transformed into 'GL-3' apple, and the transgenic lines were treated with UV-B stress. The ectopic expression of AANAT or HIOMT significantly increased the melatonin content in apples. After UV-B stress, the tolerance of apple lines with ectopic expression of AANAT or HIOMT was markedly improved. The decrease in chlorophyll fluorescence, the generation of reactive oxygen species and the shrinkage of stomata caused by UV-B stress were alleviated by AANAT or HIOMT ectopic expression. In addition, the total phenolic content was markedly increased in the transgenic lines compared with the WT (wild type). The increase in phenolic compounds was related to the increase in benzoic acid, hydroxycinnamic acid, dihydrochalcones and flavanols, among which increases in chlorogenic acid, phloridzin and procyanidin B1 content were most prominent. Furthermore, the transgenic lines did not only promote the expression of genes related to phenolic synthesis under UV-B stress, but they also increased the accumulation of phenolic compounds by inhibiting the expression of MdPPO and MdPOD related to phenolic degradation. In summary, our results demonstrate that AANAT- or HIOMT-mediated melatonin synthesis improved the tolerance of apples to UV-B stress, mainly by scavenging reactive oxygen species, increasing photosynthetic capacity and increasing total phenolic content.