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

Abstract

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.