Soil salinization is one of the major environmental factors that restrict plant growth and development. Zeaxanthin epoxidase (ZEP) functions in ABA biosynthesis and the xanthophyll cycle and has a vital role in plant responses to various environmental stresses. It was found by quantitative real-time PCR (qRT-PCR) that MhZEP responded to saline-alkali stress and showed the highest expression at 48 h of saline-alkali stress, which was 14.53-fold of 0 h. The MhZEP gene was cloned from the apple rootstock begonia (Malus halliana Koehne) and its protein physicochemical properties were analyzed. Subsequently, the functional characterization of MhZEP (ID: 103403091) was further investigated in Arabidopsis thaliana. The MhZEP contained a complete open reading frame with a length of 1998 bp, and encoded 665 amino acids with an isoelectric point of 7.18. Phylogenetic tree analysis showed that MhZEP was the most homologous and closely related to Glycine max. Compared with wild-type, transgenic plants grew better under saline-alkali stress and the MhZEP-OE line showed higher chlorophyll content, carotenoid content, enzyme activities (POD, SOD, CAT and APX) and K+ content, whereas they had lower chlorosis and Na(+ )content than the wild type (WT), which indicated that they had strong resistance to stress. The expression levels of saline-alkali stress-related genes in A. thaliana MhZEP-OE were examined by qRT-PCR, and it was found that the MhZEP improved the tolerance of A. thaliana to saline-alkali stress tolerance by regulating the expression of carotenoid synthesis genes (MhPSY, MhZDS, MhLYCB and MhVDE) and ABA biosynthesis genes (MhNCED5, MhABI1 and MhCYP707A2). And the potassium-sodium ratio in the cytoplasm was increased to maintain ionic homeostasis by modulating the expression of Na+ transporter genes (MhCHX15 and MhSOS1) and K+ transporter genes (MhHKT1;1, MhNHX1 and MhSKOR1). Moreover, the expression of H+-ATPase genes (MhAHA2 and MhAHA8) was increased to reduce the oxidative damage caused by saline-alkali stress. In summary, MhZEP acted as an essential role in plant resistance to saline-alkali stress, which lays the foundation for further studies on its function in apple.

ALDH (Aldehyde dehydrogenase), as an enzyme that encodes the dehydroxidization of aldehydes into corresponding carboxylic acids, played an important role inregulating gene expression in response to many kinds of biotic and abiotic stress, including saline-alkali stress. Saline-alkali stress was a common stress that seriously affected plant growth and productivity. Saline-alkali soil contained the characteristics of high salinity and high pH value, which could cause comprehensive damage such as osmotic stress, ion toxicity, high pH, and HCO3-/CO32- stress. In our study, 18 PaALDH genes were identified in sweet cherry genome, and their gene structures, phylogenetic analysis, chromosome localization, and promoter cis-acting elements were analyzed. Quantitative real-time PCR confirmed that PaALDH17 exhibited the highest expression compared to other members under saline-alkali stress. Subsequently, it was isolated from Prunus avium, and transgenic A. thaliana was successfully obtained. Compared with wild type, transgenic PaALDH17 plants grew better under saline-alkali stress and showed higher chlorophyll content, Superoxide dismutase (SOD), Peroxidase (POD) and Catalase (CAT) enzyme activities, which indicated that they had strong resistance to stress. These results indicated that PaALDH17 improved the resistance of sweet cherries to saline-alkali stress, which in turn improved quality and yields.