Brassinosteroid enhances tolerance to alkaline salt stress in malus domestica via the MdBSK1,MdSTOP1,andMdAHA2 pathway

Alkaline salt soils are spreading in orchards and, through the combined stresses of ionic toxicity and high pH, markedly reduce apple productivity by perturbing cellular ion and pH homeostasis and redox equilibrium. We show that exogenous brassinosteroid (BR) mitigates alkalinesalt stress in Malus hupehensis by engaging a BSK1, STOP1, andAHA2 axis that strengthens plasma-membrane H extrusion and alleviates oxidative stress. At the whole plant level, BR pretreatment under Alkaline salt soils improved survival and biomass, preserved membrane integrity (lower electrolyte leakage and malondialdehyde), tempered reactive oxygen species while elevating antioxidant enzymes, and increased H ATPase activity. Transcriptome coexpression pointed to MdSTOP1 as a central hub; its transcripts and protein rose during stress and were further promoted by BR, and MdSTOP1 overexpression recapitulated the physiological benefits. Mechanistically, MdBSK1 associated with MdSTOP1 in vivo and in vitro, and BR activated MdBSK1 kinase to phosphorylate and stabilize MdSTOP1, thereby raising its abundance. Genomic and biochemical assays showed that MdSTOP1 directly binds the MdAHA2 promoter and activates its transcription; consistent with this, MdAHA2 overexpression enhanced H ATPase activity, medium acidification, and stress tolerance. Transient epistasis placed MdBSK1 upstream of MdSTOP1 in activating MdAHA2, with BR producing the strongest response. Together, these results define a BR-activated MdBSK1and MdSTOP1 pathway that transcriptionally upregulates the H pump MdAHA2 to drive proton extrusion and enhance alkaline salt tolerance, providing a mechanistic framework and actionable target for improving saltalkalinity resilience in woody crops.