The CmABF1‐CmBt/CmBr Module Regulates Cucurbitacin B Biosynthesis to Promote Cold Tolerance of Oriental Melon Seedlings

ABSTRACT Low temperature is a common abiotic stress in the early spring and winter cultivation of oriental melons, which seriously affects their normal growth and development, and plants can activate the defence mechanism related to reactive oxygen species through secondary metabolic pathways. Cucurbitacins are a defensive secondary metabolite that produces a bitter taste in Cucurbitaceae plants. However, the molecular regulatory mechanism between low temperature and biosynthesis of cucurbitacins, as well as whether cucurbitacins will enhance the plant's resistance to cold stress, has not been studied yet. In this study, cucurbitacin B (CuB) content in oriental melon seedlings increased under low‐temperature treatment at 10°C; the CuB biosynthetic genes and abscisic acid‐responsive element binding factor 1 (CmABF1) was significantly induced to express 1 day after treatment. Through analyzing the promoters of CuB biosynthetic genes and conducting various transcriptional activation experiments, it was found that CmABF1 could bind to and activate the promoters of CmBi , Cm160 , Cm170 , Cm180 and CmACT , respectively, and positively regulates their expression. Furthermore, CmABF1 interacted with bitterness‐specific transcription factors (CmBt/CmBr) in leaves/roots at the protein level and engaged in regulating CuB biosynthesis in response to cold stress. More importantly, we found exogenous spraying of CuB can alleviate the low‐temperature stress injury of melon and tomato seedlings. In brief, our research has identified the specific molecular mechanisms by which the CmABF1‐CmBt/CmBr modules regulate CuB biosynthesis in response to cold stress. This study clarified the interrelationship between low temperature and CuB biosynthesis, explored a new way to alleviate the abiotic stress injury of seedlings, which is of great significance for the early seedling development of oriental melons.