The vesicular transport system is important for substance transport in plants. In recent years, the regulatory relationship between the vesicular transport system and plant disease resistance has received widespread attention; however, the underlying mechanism remains unclear. MdSYP121 is a key protein in the vesicular transport system. The overexpression of MdSYP121 decreased the B. dothidea resistance of apple, while silencing MdSYP121 resulted in the opposite phenotype. A metabolome and transcriptome dataset analysis showed that MdSYP121 regulated apple disease resistance by significantly affecting sugar metabolism. HPLC results showed that the levels of many soluble sugars were significantly higher in the MdSYP121-OE calli. Furthermore, the expression levels of genes related to sugar transport were significantly higher in the MdSYP121-OE calli after B. dothidea inoculation. In addition, the relationships between the MdSYP121 expression level, the soluble sugar content, and apple resistance to B. dothidea were verified in an F1 population derived from a cross between ‘Golden Delicious’ and ‘Fuji Nagafu No. 2’. In conclusion, these results suggested that MdSYP121 negatively regulated apple resistance to B. dothidea by influencing the soluble sugar content. These technologies and methods allow us to investigate the molecular mechanism of the vesicular transport system regulating apple resistance to B. dothidea.

Apple ring rot, caused by Botryosphaeria dothidea, represents a major challenge for apple cultivation worldwide. However, the genetic factors that confer resistance in apples are not well understood. This study utilized time-series transcriptome profiling, combined with weighted gene co-expression network analysis and previously identified quantitative trait loci, leading to the identification of MdNHL8 as a candidate gene linked to resistance. A genome-wide examination of the apple NDR1/HIN1-like gene family revealed 57 members with diverse conserved domains, confirming that MdNHL8 features a distinct RBR-type E3 ligase domain. Subcellular localization experiments showed that MdNHL8 is localized in the nucleus and plasma membrane. Functional assays indicated that overexpression of MdNHL8 enhances resistance to apple ring rot and significantly elevates the expression of MdPR1, whereas silencing this gene does not enhance resistance and exhibits a trend toward increased susceptibility. Furthermore, additional transcriptome sequencing analysis of MdNHL8 transgenic calli highlighted a turquoise module closely linked to MdNHL8-mediated resistance, identifying MdCYCB3, MdEIX2, and MdABC4 as candidate downstream genes with expression patterns tightly correlated with MdNHL8. This research identifies MdNHL8 as a positive regulator in apple defense against B. dothidea and provides a basis for further exploring the regulatory network centered on MdNHL8 in disease resistance, offering insights for future breeding efforts aimed at enhancing resistance to apple ring rot.

Dwarfing rootstocks are widely used in apple production to control tree vigor and improve fruit quality. However, their adoption in ‘Fuji’ orchards remains limited by high production costs and unstable field performance. In this study, five rootstock materials, namely GM256, GM310, MD001, LiaoZhen 2, and Malus hupehensis Rehd (CK), were evaluated for their effects on cold tolerance, tree growth, and fruit quality in ‘Fuji’ apple. Cold tolerance was assessed using electrolyte leakage and related physiological indicators under low-temperature stress. MD001 and GM256 exhibited significantly stronger cold tolerance than the control, as indicated by lower LT₅₀ values and more favorable physiological responses (p < 0.05). Regarding tree growth, GM310 showed the weakest dwarfing effect, while LiaoZhen 2 had a relatively high proportion of long shoots, suggesting excessive vegetative growth. Meanwhile, GM256 produced the highest yield among all combinations. Principal component analysis of fruit-quality traits indicated that GM256 and GM310 performed better overall than the other combinations. In addition, GM256 produced significantly higher soluble solids, vitamin C content, and sugar–acid ratio than the control and most other rootstocks (p < 0.05). Overall, GM256 combined strong cold tolerance, a balanced shoot composition, moderate tree vigor, and high fruit quality, suggesting that it is a promising rootstock for ‘Fuji’ apple cultivation in Shandong Province.