MdBACT5 and MdBACT8 contribute to the formation of branched-chain volatiles in apple

Branched-chain amino acid aminotransferases (BCATs) catalyze both the final anabolic step and the initial catabolic step of branched-chain amino acids (BCAAs), which are pivotal for the formation of plant branched-chain volatiles (BCVs). However, the members of BCAT family in apple (Malus domestica Borkh.) remain poorly characterized. In the current study, we identified nine BCAT genes in the apple genome. Phylogenetic analysis classified these MdBCATs into two groups distributed across five chromosomes, with conserved gene structures within each group. Physicochemical analysis revealed coding sequence (CDS) lengths ranging from 852 to 1248 bp, encoding proteins with molecular weights of 31.13 45-41 kDa and isoelectric points (pI) of 5.86-8.35. Collinearity analysis indicated that segmental duplication predominantly drove the expansion of the apple BCAT family. Promoter regions of MdBCATs harbored cis-acting elements associated with growth and development, stress responses, and hormone signaling. RT-qPCR analysis demonstrated differential expression patterns of MdBCATs in the peel tissue of 'Oregon Spur II' apples during ambient storage. Subcellular localization revealed plastid- and mitochondrial-targeting of specific MdBCATs. Notably, transient overexpression of mitochondrially-localized MdBACT5 and MdBACT8 significantly enhanced BCV biosynthesis. Taken together, this study provides critical insights into the role of BCATs in apple fruit aroma quality.