Feng-Wang Ma

MdSPL1-MdNF-YB4-MdCKX7 Module Positively Regulates Saline-Alkali Tolerance by Mediating Cytokinin Levels in Apple.

Saline-alkali stress is a major abiotic factor that compromises plant growth, crop yield, and quality. Cytokinin plays an essential role in plant responses to saline-alkali stress; however, the underlying molecular mechanisms remain unclear in apple (Malus domestica). In this study, CYTOKININ OXIDASE/DEHYDROGENASE7 (MdCKX7) was identified as a positive regulator of saline-alkali tolerance in apple, encoding a functional CKX enzyme that mediates cytokinin degradation. Consistently, applying exogenous lovastatin, an inhibitor of cytokinin biosynthesis, alleviated the saline-alkali-sensitive phenotype in MdCKX7-silenced plants by promoting stomatal closure through cytokinin degradation. Molecular genetic assays revealed that apple transcription factor MdSPL1 is involved in the response to saline-alkali stress by directly activating the expression of MdCKX7. Overexpression of MdSPL1 enhanced saline-alkali tolerance by modulating photosynthetic efficiency and stomatal aperture, whereas its silencing resulted in increased sensitivity. Furthermore, overexpression of MdCKX7 in the MdSPL1-RNAi background rescued the saline-alkali-sensitive phenotype. Further analysis revealed that MdNF-YB4, a positive regulator of saline-alkali tolerance, interacts with MdSPL1 to facilitate its transcriptional activation of MdCKX7. Collectively, our study reveals the MdSPL1-MdNF-YB4-MdCKX7 regulatory module that orchestrates cytokinin-mediated saline-alkali tolerance in apple, providing valuable molecular tools for the breeding of saline-alkali-tolerance apple varieties.

MdJUB1 positively regulates salt stress in apple by increasing GABA contents and stabilizing Na +/K+ homeostasis

Soil salinization represents a significant constraint on global agricultural development. Although several NAC gene families have been implicated in the response to salt stress, the mechanisms underlying salt stress tolerance remain unclear, particularly in perennial fruit trees. In this study, we identify MdJUB1, a NAC transcription factor, as a key regulator of salt stress response in apple. Under salt stress, MdJUB1 increases γ-aminobutyric acid (GABA) content, thereby positively regulating salt stress tolerance in apple. Further analysis using yeast one-hybrid, luciferase assays, and electrophoretic mobility shift assays demonstrated that MdJUB1 directly binds to the promoter of MdGAD1, promoting GABA synthesis. Additionally, under salt stress, overexpression of MdJUB1 increased the Na+/K+ ratio in apple. MdJUB1 also binds to the promoters of MdNHX1, MdHKT1, and MdSOS3, activating their expression. Using yeast two-hybrid screening, we identified MdDOF2.4 as an interactor of MdJUB1 and found that MdDOF2.4 positively regulates salt stress tolerance. Importantly, the interaction between MdDOF2.4 and MdJUB1 enhances the expression of downstream genes, including MdGAD1, MdNHX1, MdHKT1, and MdSOS3. Our results demonstrate that MdJUB1 is involved in the regulation of GABA levels and the maintenance of Na+/K+ homeostasis, which may contribute to enhanced salt tolerance in apple.

MdXERICO negatively regulates salt tolerance in apple through ubiquitination and degradation of MdJAZ10.