Transcription Factor\nCmWRKY13 Regulates Cucurbitacin\nB Biosynthesis Leading to Bitterness in Oriental Melon Fruit (Cucumis melo var. Makuwa Makino)

Bitterness, caused by cucurbitacin B (CuB), is one of\nthe important\ntraits that affects melon fruit quality and consumer acceptance. Therefore,\nthe detailed mechanism behind the regulation of CuB biosynthesis on\nmelon fruit needs to be further explored. This study investigated\nCuB content and transcriptomes of “YMR” melon fruit\ntreated by 5 and 20 mg L<sup>–1</sup> CPPU. The content of\nCuB reaches its peak in 5 days and then decreases. WGCNA identified\nthe WRKY transcription factor (TF), CmWRKY13, coexpressed with CuB\nbiosynthetic genes (<i>Cm180</i>, <i>Cm170</i>, <i>Cm160</i>, and <i>CmACT</i>). Yeast one-hybrid,\ndual-luciferase, and transient gene expression assays were conducted\nand suggested that the nucleus-localized CmWRKY13 transactivated the\npromoters of CuB biosynthetic genes and participated in the regulation\nof CuB biosynthesis. Furthermore, CmWRKY13 could interact with CmBt,\nthe fruit bitterness-specific TF, which synergistically activated\nCuB biosynthetic gene expression. These findings provide a novel mechanistic\ninsight for CuB biosynthesis and regulation in melon cultivation.