PmASMT1, an N-acetylserotonin O-methyltransferase gene of Prunus mume, promotes melatonin biosynthesis and enhances drought tolerance

Prunus mume , one of the well-known flower and fruit tree species, is highly esteemed for its ornamental qualities and agricultural values. Drought significantly restricts the vigorous growth of P . mume and prevents its large-scale cultivation, especially in dry and semi-dry areas. To discover the mechanism by which P. mume responds to drought stress, melatonin was firstly detected in P. mume . We found a 61.04 % increase of melatonin content in leaves of P . mume under drought conditions, accompanied by enhanced antioxidant enzyme activity. Integration of transcriptome analysis and melatonin detection indicated a significant positive correlation (Rho = 0.927, P < 0.05) between the expression profile of the PmASMT1 gene and melatonin content. The expression level of PmASMT1 was quickly increased under drought stress by using qRT-PCR method. In vitro enzyme assay discovered that the purified PmASMT1 effectively facilitated melatonin biosynthesis. When PmASMT1 was transformed into tobacco, the transgenic lines exhibited a 2.77-fold increase of melatonin levels compared to the empty vector plants. Under drought stress, the PmASMT1 transgenic lines displayed no wilting phenotype, but the melatonin content in these transgenic lines was enhanced. Moreover, overexpression of PmASMT1 in tobacco protected cells from damage by elevating APX, CAT, POD, and SOD activity, meanwhile reducing malondialdehyde (MDA) content and relative electrolyte leakage (REL). Additionally, silencing PmASMT1 expression in P. mume decreased the drought tolerance, accompanied by significantly higher of H 2 O 2 and O 2− in pTRV- PmASMT1 than WT and pTRV leaves under drought stress. This study indicates PmASMT1 is an important gene in catalyzing melatonin biosynthesis and enhancing drought tolerance, which provides a foundation for genetic improvement of drought tolerance in plants.