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.

Drought stress significantly impedes apple growth, development, and yield, leading to substantial economic losses within the global apple industry. Malus prunifolia (Mp), a commonly utilized apple rootstock, has shown promise in augmenting cultivated apple resistance to abiotic stress. Although Alfin-like (ALs) proteins have demonstrated pivotal roles in dicotyledonous plants' response to abiotic stresses, knowledge about AL genes in apple rootstocks is limited, and their functions remain largely elusive. In this study, we identified and characterized 10 MpAL gene members in the apple rootstock genome, confirming their localization within the nucleus. Our investigation revealed the significant regulation of MpALs' expression under drought and abscisic acid (ABA) stresses in M. prunifolia. In this study, one of the members, MpAL1, was selected for further exploration in Arabidopsis and apple to explore its potential function in response to drought and ABA stresses. The results showed that overexpression-MpAL1 transgenic apple calli grew significantly better than WT and MpAL1-RNAi lines, which regulates the accumulation of H₂O₂ and O₂^.- levels. Additionally, transgenic Arabidopsis plants overexpressing MpAL1 exhibited positively regulating anti-oxidant enzymes activities under stress treatments. Further study showed that silencing MpAL1 in apple plants showed obvious chlorosis in leaves, and accumulation of reactive oxygen species under drought stress. Moreover, our detailed analysis established that MpAL1 regulates several drought and ABA-responsive genes, exerting an influence on their expression in transgenic apple. Collectively, our findings identify MpAL1 as a positive regulator that increases drought stress in apple, shedding light on its potential significance in bolstering drought resistance in this fruit crop.

Auxin is throughout the entire life process of plants and is involved in the crosstalk with other hormones, yet its role in apple disease resistance remains unclear. In this study, we investigated the function of auxin/indole-3-acetic acid (IAA) gene MdIAA24 overexpression in enhancing apple resistance to Glomerella leaf spot (GLS) caused by Colletotrichum fructicola (Cf). Analysis revealed that, upon Cf infection, 35S::MdIAA24 plants exhibited enhanced superoxide dismutase (SOD) and peroxidase (POD) activity, as well as a greater amount of glutathione (reduced form) and ascorbic acid accumulation, resulting in less H₂O₂ and superoxide anion (O₂^.−) in apple leaves. Furthermore, 35S::MdIAA24 plants produced more protocatechuic acid, proanthocyanidins B1, proanthocyanidins B2 and chlorogenic acid when infected with Cf. Following Cf infection, 35S::MdIAA24 plants presented lower levels of IAA and jasmonic acid (JA), but higher levels of salicylic acid (SA), along with the expression of related genes. The overexpression of MdIAA24 was observed to enhance the activity of chitinase and β-1,3-glucanase in Cf-infected leaves. The results indicated the ability of MdIAA24 to regulate the crosstalk between IAA, JA and SA, and to improve reactive oxygen species (ROS) scavenging and defense-related enzymes activity. This jointly contributed to GLS resistance in apple.

The red flesh in apple fruit is a desired trait by consumers and it is associated to the anthocyanin content, which is mainly controlled by MdMYB10 with a R6 promoter. In this study, a high-density linkage group was constructed using the ‘Fuji’ x ‘Red3’ population which contained homozygous alleles R1R1 and R6R6, respectively. The linkage group consists of 7630 SNPs along 17 linkage groups, spanning 2270.21 cM, with an average density of 0.30 cM per marker. The cyanidin-3-galactoside concentration was used as the phenotypic data in QTL analysis. Moreover, one QTL peak which was flaked by two markers, marker2187260 to marker2173766, with LOD scores of 4.49 was detected. This QTL ranged from 0 to 40.79 cM on the top of linkage group (LG16). In addition one candidate molecular marker (marker2175442) in this QTL was identified, which was significant correlated with the flesh cyanidin-3-galactoside concentration. These genetic findings enrich the breeding basis of fruit flesh coloration in apple.