Magnesium promotes aluminum tolerance in apple by regulating cell wall remodeling through the MdMYB78-MdHCT/MdXTH2...

Soil acidification-induced aluminum (Al) toxicity severely restricts apple growth and development, threatening apple production and potentially human health. Magnesium (Mg) supplementation can alleviate Al toxicity in plants, but the underlying physiological and molecular mechanisms remain poorly understood. Here, increasing the Mg supply from 0.3 to 3 mM reduced root Al accumulation and increased root dry weight and root length in M26 apple rootstock seedlings exposed to Al stress (1.5 mM). Elemental analysis showed that approximately 60% of the accumulated Al was localized in the root cell wall, with hemicellulose 1 (HC1) exhibiting a greater Al-binding capacity than other cell wall components. Mg supplementation under Al stress reduced the cell wall hemicellulose (HC) and lignin contents, thereby decreasing cell wall thickness and Al accumulation. Integrated transcriptomic and metabolomic analyses identified MdHCT and MdXTH2 as key genes involved in Mg-mediated Al tolerance. Overexpression of MdHCT and MdXTH2 negatively and positively regulate Al tolerance in apple seedlings, respectively, by modulating cell wall modification. Transcription factor analysis identified the MYB family member MdMYB78, which directly binds the ACCAAC motif in promoters of MdHCT and MdXTH2, activating and repressing their transcription, respectively. Transgenic validation results indicate that MdMYB78 negatively regulates Al tolerance in apple seedlings. Collectively, these findings establish MdMYB78 as a key regulator of Mg-enhanced Al tolerance in apple seedlings by coordinately regulating MdHCT and MdXTH2 to remodel the root cell wall and alter its Al-binding capacity. Mg-mediated cell wall remodeling provides a potential strategy for mitigating Al toxicity in acidified soils.