Transcriptome and volatile analyses identify key genes involved in ester biosynthesis in durian (Durio zibethinus) during fruit ripening.

Durian (Durio zibethinus), which is an important economic crop in Southeast Asia, is known for its distinct aroma combining sulfurous and fruity notes. Volatile esters are the main contributors to the sweet, fruity aroma of ripe durian fruit. Volatile ester contents in fruit are generally regulated by the activities of alcohol acyltransferase (AAT) and enzymes that metabolize amino acids, fatty acids, and carbohydrates into aroma volatiles. However, the molecular regulation of ester biosynthesis in durian remains unexplored. In this study, we aimed to identify key durian genes involved in ester biosynthesis and investigate the association between their expression and volatile profiles. We analyzed the volatile profiles of 'Monthong' pulp samples from four ripening stages (unripe, midripe, ripe, and overripe), revealing the activation of ester production during fruit ripening. Additionally, a comparison of the volatile profiles of six durian cultivars at the ripe stage detected differences in ester contents among cultivars. We identified three pulp-specific AAT genes, of which DzAAT1 was strongly upregulated during ripening and its expression was correlated with ester content variation among cultivars. The distinct features of durian volatiles, especially the predominance of ethyl esters, are likely due to ethanolic fermentation under hypoxia-like ripening conditions. We identified two ripening-associated pyruvate decarboxylase genes (DzPDC1a/b) that are highly expressed in ripe durian pulp. Furthermore, we analyzed RNA-seq data of three durian cultivars, which showed distinct volatile profiles associated with ester biosynthesis based on partial least squares-discriminant analysis. An integrated volatile and transcriptome analysis indicated that differentially expressed genes in the ester biosynthesis pathway, including carboxylesterase, branched-chain amino acid aminotransferase, and methionine γ-lyase genes, were associated with cultivar-specific volatile profiles. The identification of key genes associated with ester biosynthesis provides a molecular foundation for improving postharvest durian fruit flavor quality and breeding new cultivars with optimized fruit flavors.