Linguang Li

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.

Auxin is a central plant hormone, and organic acids are key determinants of fruit acidity and flavor; however, the molecular mechanism connecting auxin signaling to organic acids remains elusive. Here, we report a negative correlation between auxin levels and malate, the predominant organic acid in apple (Malus domestica), in progenies of “Gala” × “Mato 1” and across fruit developmental stages. This suggests that auxin reduces malate levels independently of Ma1, an aluminum-activated malate transporter (ALMT) gene that is the causal gene for the major quantitative trait locus (QTL) determining fruit acidity in apple. Integrated bulked segregant analysis and RNA-seq identified MdARF16 and MdARF17 as auxin-responsive transcription factors that repress malate accumulation. Auxin represses 2 tonoplast-localized malate transporter genes, the ALMT gene MdALMT9L and the gene MdMATEL2, which encodes a functionally distinct multidrug and toxic compound extrusion protein. However, auxin does not repress Ma1. This repression requires the cooperative action of MdARF16 and MdARF17, which directly bind the promoters of the malate transporter genes to inhibit transcription. Decreased malate transporter levels then restrict cytosolic-to-vacuolar malate transport. These findings reveal a Ma1-independent auxin-MdARF16/17 pathway controlling vacuolar malate transport in apple, offering a molecular framework for the auxin-responsive improvement of organic acid traits.

Russeting is a significant limiting factor affecting the superficial quality of apple fruits, and it is important to understand the formation mechanism of apple russeting for improving fruit appearance quality. In this study, six representative cultivars were selected to systematically investigate the russeting formation during early fruit development through multidimensional analyses. Findings demonstrated that russeting predominantly manifests at 50–65 d after flowering (DAF), a period coinciding with dramatic cuticular reorganization. Susceptible cultivars exhibited disintegration of wax crystals and disordered epidermal cell arrangements, whereas resistant cultivars maintained stable cuticular morphology. A critical redox imbalance was observed in susceptible cultivars, characterized by excessive hydrogen peroxide (H₂O₂) accumulation and dysregulated antioxidant enzyme activities, indicating a collapse of reactive oxygen species (ROS) scavenging systems. Notably, susceptible cultivars exhibited hyperactivation of lignin biosynthesis genes. While the resulting lignin deposition showed a positive correlation with the severity of epidermal damage, suggesting that such maladaptive or excessive lignification disrupted a relatively stable physiological state. Overall, these findings offer potential molecular markers for breeding programs and a theoretical foundation for developing targeted control strategies.

In rootstock-scion combinations, the rootstock is the key factor determining a plant's salt tolerance, primarily through its root physiology and soil interaction capabilities. However, the interaction between rootstock and scion in influencing salt tolerance in apple trees remains underexplored. In this study, three one-year-old dwarf rootstocks—'Qingzhen-1', 'M9-T337', and '(Malus hupehensis Rehd, var. Pingyiensis, PYTC)'—were selected, with 'Luli' used as the scion. The salt tolerance was compared in natural soil (1‰), and saline-alkali soil from Dongying (3‰, 6‰, 10‰). The results showed that salt stress inhibited plant growth and damaged root structure. Among the combinations, Luli/Qingzhen-1 exhibited superior growth performance, with reduced malondialdehyde content, relative conductivity, and mortality. Additionally, in the Luli/Qingzhen-1 combination, genes associated with antioxidant enzyme activity and stress signal transduction were significantly upregulated. In conclusion, Qingzhen-1 demonstrates greater salt tolerance than M9-T337 and PYTC, making it a promising rootstock for cultivating 'Luli' scions in saline-alkali soils.

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.

The mitogen-activated protein kinase (MAPK) cascade is a highly conserved signaling transduction module that transduces extracellular stimuli into intracellular responses in plants. Early studies of plant MAPKs focused on their functions in model plants. Based on the results of whole-genome sequencing, many MAPKs have been identified in horticultural plants, such as tomato and apple. Recent studies revealed that the MAPK cascade also plays crucial roles in the biotic and abiotic stress responses of horticultural plants. In this review, we summarize the composition and classification of MAPK cascades in horticultural plants and recent research on this cascade in responses to abiotic stresses (such as drought, extreme temperature and high salinity) and biotic stresses (such as pathogen infection). In addition, we discuss the most advanced research themes related to plant MAPK cascades, thus facilitating research on MAPK cascade functions in horticultural plants.