MdHMGB15 enhances zinc tolerance by activating multiple metal homeostasis genes in apple

Zinc (Zn) imbalance severely impairs plant growth, especially in perennial fruit crops chronically exposed to soilborne metal accumulation. However, the regulatory mechanisms underlying Zn detoxification in apple (Malus domestica) remain largely unknown. Here, we identified a high-mobility group protein MdHMGB15, transcript level and protein stability of which was increased under Zn treatment, suggesting that MdHMGB15 may play special roles in Zn stress tolerance. Subsequently, MdHMGB15 transgenic apple calli, Arabidopsis and tomato were generated, which showed that overexpression of MdHMGB15 significantly enhanced Zn tolerance, which was reflected by improved biomass, reduced oxidative damage, and lower electrolyte leakage under excess Zn. Furthermore, electrophoretic mobility shift assays (EMSA), yeast one-hybrid analyses (Y1H), and dual-luciferase reporter assays demonstrated that MdHMGB15 directly bound to the AT-rich cis-elements within the promoters of genes related to metal homeostasis (Heavy metal-associated isoprenylated plant protein 37 (MdHIP37), Heavy metal ATPase 3 (MdHMA3), Heavy metal ATPase 5 (MdHMA5), Metallothionein 3 (MdMT3), Metal tolerance protein 1 (MdMTP1), and Natural resistance-associated macrophage protein 5 (MdNRAMP5)) and activated their transcription. Collectively, these findings revealed MdHMGB15 as an upstream transcriptional activator that coordinated Zn sequestration, efflux, and oxidative stress mitigation through direct induction of metal transporter genes. This study provides novel mechanistic insights into functions of MdHMGB15 and identifies a promising target for improving Zn tolerance in apple and other fruit crops.