Transcription Factor CmWRKY13 Regulates Cucurbitacin B Biosynthesis Leading to Bitterness in Oriental Melon Fruit (Cucumis melo var. Makuwa Makino)

Bitterness, caused by cucurbitacin B (CuB), is one of the important traits that affects melon fruit quality and consumer acceptance. Therefore, the detailed mechanism behind the regulation of CuB biosynthesis on melon fruit needs to be further explored. This study investigated CuB content and transcriptomes of "YMR" melon fruit treated by 5 and 20 mg L^-1 CPPU. The content of CuB reaches its peak in 5 days and then decreases. WGCNA identified the WRKY transcription factor (TF), CmWRKY13, coexpressed with CuB biosynthetic genes (Cm180, Cm170, Cm160, and CmACT). Yeast one-hybrid, dual-luciferase, and transient gene expression assays were conducted and suggested that the nucleus-localized CmWRKY13 transactivated the promoters of CuB biosynthetic genes and participated in the regulation of CuB biosynthesis. Furthermore, CmWRKY13 could interact with CmBt, the fruit bitterness-specific TF, which synergistically activated CuB biosynthetic gene expression. These findings provide a novel mechanistic insight for CuB biosynthesis and regulation in melon cultivation.