MdWRKY40 Regulates Volatile Ester Degradation through a Carboxylesterase-Mediated Pathway in Apple Fruit

Volatile esters are critical for apple fruit aroma, yet the regulatory mechanism underlying their degradation remains poorly understood. In this study, we identified the transcription factor MdWRKY40 as a direct activator of MdCXE1, a key carboxylesterase gene responsible for ester hydrolysis. Through comparative analysis of 10 apple cultivars, we observed a significant negative correlation between MdCXE1 expression and ester content. Transient overexpression and silencing assays confirmed that MdCXE1 negatively regulates ester levels. Combined in vitro and in vivo binding assays demonstrated that MdWRKY40 binds to the W-box motif in the MdCXE1 promoter. Functional studies in apple calli and tomato fruits showed that MdWRKY40 overexpression reduces ester accumulation, while its knockout enhances it. Furthermore, dual silencing of MdWRKY40 and MdCXE1 attenuated ester degradation. Our findings complement the established biosynthetic machinery by demonstrating that the MdWRKY40-MdCXE1 module functions as a transcriptional regulator of ester catabolism, thereby revealing a potential target for molecular breeding of apple varieties with tailored aroma profiles.