Heterologous overexpression of HIOMT alleviates alkaline stress in apple plants by increasing melatonin concentration

Abstract

Melatonin is an indole derivative of tryptophan that has an important function in alleviating abiotic stress. Previous studies have reported that exogenous melatonin increases the tolerance of plants to saline-alkali stress in a concentration-dependent manner, but the role of endogenous melatonin metabolism in the response of apple (Malus domestica) plants to alkali stress is unclear. In this study, the human melatonin biosynthesis-related enzyme N-acetyl-5-hydroxytryptamine methyltransferase gene (HIOMT) transformed in ‘GL-3’ apple was used to study the regulation of HIOMT in alkaline stress of apple plants. The heterologous overexpression of HIOMT under alkaline stress increased the concentration of endogenous melatonin in apple leaves, alleviated the stress-induced growth inhibition of the plants, enhanced root development, leaf photosynthesis, and antioxidant capacity, and stabilized cell membrane permeability and ionic balance. In addition, the transgenic lines also increased the concentrations of organic acids, such as oxalic acid, malic acid, malonic acid, citric acid, and succinic acid, which alleviated the high pH stress. These results provide evidence for the critical role of heterologous overexpression of HIOMT in the alkaline stress tolerance of apple plants.