MdWRKY126 modulates malate accumulation in apple fruit by regulating cytosolic malate dehydrogenase (MdMDH5)

The content of organic acids greatly influences the taste and storage life of fleshy fruit. Our current understanding of the molecular mechanism of organic acid accumulation in apple (Malus domestica) fruit focuses on the aluminum-activated malate transporter 9/Ma1 gene. In this study, we identified a candidate gene, MdWRKY126, for controlling fruit acidity independent of Ma1 using homozygous recessive mutants of Ma1, namely Belle de Boskoop "BSKP" and Aifeng "AF." Analyses of transgenic apple calli and flesh and tomato (Solanum lycopersicum) fruit demonstrated that MdWRKY126 was substantially associated with malate content. MdWRKY126 was directly bound to the promoter of the cytoplasmic NAD-dependent malate dehydrogenase MdMDH5 and promoted its expression, thereby enhancing the malate content of apple fruit. In MdWRKY126 overexpressing calli, the mRNA levels of malate-associated transporters and proton pump genes also significantly increased, which contributed to the transport of malate accumulated in the cytoplasm to the vacuole. These findings demonstrated that MdWRKY126 regulates malate anabolism in the cytoplasm and coordinates the transport between cytoplasm and vacuole to regulate malate accumulation. Our study provides useful information to improve our understanding of the complex mechanism regulating apple fruit acidity.