MdUGT88F4 involved in the synthesis of isosalipurposide positively regulates the salt stress response of apple plants

UGT (UDP-dependent glycosyltransferase) family members are mainly involved in the modification of secondary metabolites, as well as plant stress responses. Previously, we identified the UGT gene MdUGT88F4 from Malus spp. In order to verify whether it had biological function in apple plants, we generated transgenic apple plants overexpressed MdUGT88F4 and treated them with salt stress. The results showed that it conferred enhanced salt stress tolerance in transgenic ‘GL-3’ apple (Malus domestica) plants. The expression of MdUGT88F4 was induced by salt stress, and overexpression of this gene alleviated the inhibitory effects of salt stress on the growth of apple plants. After 15 days of salt stress treatment (100 mmol·L−1 NaCl), necrotic spots were present on the leaves of wild-type (WT) plants, and none were observed on the leaves of transgenic plants overexpressing MdUGT88F4 (OX). The relative electrolyte leakage and malondialdehyde content were lower and the total chlorophyll content and the relative water content were higher in OX plants than in WT plants. The photosynthetic efficiency was higher in OX plants than in WT plants under salt stress, which was consistent with their larger stomatal aperture; this might stem from a reduction in the content of abscisic acid. The production of reactive oxygen species was lower and the activities of antioxidant enzymes were higher in OX plants than in WT plants. In addition, OX plants accumulated less Na+ but more K+ than WT plants, and the expression of several genes involved in Na + transport was upregulated in OX plants. MdUGT88F4 failed to promote the accumulation of isosalipurposide in vivo, and it was involved in isosalipurposide synthesis. Most of the flavonoid metabolites accumulated to a greater degree in OX plants than in WT plants. In summary, our results show that MdUGT88F4 positively regulates the salt stress response in apple plants, possibly by affecting stomatal movement, as well as the accumulation of ions and flavonoids. Our findings enhance our understanding of the metabolic mechanisms by which UGT proteins ameliorate the effects of salt stress in plants.