Combined analysis of carotenoid and chlorophyll metabolite and the transcriptome to analyze the peel color formation mechanism in pumpkin (Cucurbita maxima)

The peel color of pumpkin (Cucurbita maxima) gives it its delightful appearance and contributes towards it's economic value. The peel color of Cucurbita maxima changes with the development process of the fruit, and the molecular mechanism of pigment accumulation in pumpkin fruit remains to be explored. The fruit peel of '2013-12' was yellow at 0 DAP, and gradually turned from orange to red, and the fruit peel of '9-6' was green at 0 DAP with no stripe, and gradually turned to gray with green stripes during fruit development. The present findings reveal that the accumulation and ratio of carotenoid and chlorophyll are the key factors affecting the coloration of pumpkin. The results showed that the main pigment accumulated in '2013-12' was carotenoid. With fruit development, the increase of zeaxanthin content and ratio cause the peel color to change from yellow to red. The deepening of '9-6' peel color is mainly related to the pigment content rather than the proportion. Additionally, the expression levels of CmaPSY1, CmaBCH2, CmaVDE, and other genes in the two branches of carotenoid biosynthesis were upregulated, or had high abundance, which are important genes for the formation of red peel. The upregulated expressed CmaCHLH gene, and downregulated expressed chlorophyll degradation pathway-related genes regulated high chlorophyll accumulation in '9-6'. Combining the metabolome data with the transcriptome data, the regulatory pathways of different peel color formation in pumpkin were speculated. The findings in this study contribute to elucidating the novel mechanism that underpins the coloration of pumpkin peel.