Genome-Wide Identification and Functional Characterization of the Carboxylesterase (CXE) Gene Family in Melon (Cucumis melo L.): CmCXE7 as a Cold-Responsive Regulator of Ester Metabolism

Melon fruit aroma depends on the balance between ester biosynthesis and hydrolysis, yet the carboxylesterase (CXE) family responsible for hydrolysis remains uncharacterized in this crop. Here, a genome-wide identification of the CXE gene family in melon revealed 22 CmCXE genes. Phylogenetic analysis revealed that Group III members cluster with functionally validated ester-hydrolyzing CXEs from peach, tomato, and strawberry, and their expression peaks during the climacteric and post-climacteric stages—the same window when melon aroma volatiles accumulate. We further functionally characterized CmCXE7, a cold-responsive member, through subcellular localization and heterologous overexpression in tomato. CmCXE7 localized predominantly to the plasma membrane with a weak nuclear signal. Non-targeted metabolomics revealed 776 differentially accumulated metabolites (DAMs) based on VIP > 1 and p < 0.05, with 11 of 14 high-magnitude ester-related changes being downregulated. These results identify Group III genes as candidate regulators of melon ester metabolism and identify CmCXE7 as a cold-responsive CXE candidate that may participate in ester metabolism. The potential relevance of CmCXE7 to fruit ripening and postharvest storage warrants further investigation, providing a candidate gene for future functional studies and molecular breeding of melon aroma quality.