Identification of the G2-like transcriptional factor gene family reveals their critical roles in controlling chlorophyll accumulation of melon fruit

The melon (Cucumis melo L.) is an economically important fruit crop around the world. Its colorful and diverse fruit peels also possess remarkable aesthetic value. The G2-like transcription factors (GLKs) play a pivotal role in regulating the fruit peel color. However, the melon's GLKs have not been comprehensively analyzed. In this study, a total of 37 CmGLK genes were identified and further categorized into four distinct subfamilies: I, II, III, and IV. The organ-specific gene expression revealed that CmAPRR2;1 specifically and highly expressed in fruit tissues. It belongs to Cluster I subfamily. This subfamily contains three key genes CmGLK1, CmAPRR2;1 and CmAPRR2;2, which have been proved to be involved in chlorophyll accumulation. Furthermore, our findings indicated that the fruit bagging treatment harmed the color formation of melon peel and flesh by significantly reducing the accumulation of chlorophyll a (Chla), chlorophyll b (Chlb), and carotenoid (Car). Additionally, the expression level of CmAPRR2;1 is directly linked to the intensity of peel color. The subcellular localization and transactivation assays demonstrated that CmAPRR2;1 was a transcriptional activator localized within the nucleus. Therefore, we conclude that CmAPRR2;1 plays a pivotal role in regulating chlorophyll accumulation of melon fruit. These findings offer valuable insights for further exploring the molecular mechanisms underlying peel color formation in melon.