An integrated transcriptome and metabolome approach reveals the accumulation of taste-related metabolites and gene regulatory networks during watermelon fruit development

Main conclusion Accumulation patterns and gene regulatory networks of sugars and cucurbitacins and related primary and secondary metabolites during cultivated watermelon 'Cheng Lan' and wild watermelon 'PI 632,751' fruit development were identified. Metabolites are the end products of cellular regulatory processes and play important roles in fruit taste formation. However, comprehensive studies on the accumulation patterns of watermelon fruit metabolites and transcriptional regulatory networks are still scarce. In this study, 451 annotated metabolites were identified at four key fruit developmental stages in wild watermelon 'PI 632,751' and modern cultivated watermelon 'Cheng Lan'. Interestingly, 11 sugars and 25 major primary metabolites were mainly accumulated in 'Cheng Lan' during fruit development, which are considered to be the potential metabolites beneficial to the formation of watermelon taste. Cucurbitacins and the main flavonoids were mainly specifically accumulated in 'PI 632,751', not being considered to be responsible for the taste. Moreover, forty-seven genes involved in carbohydrate metabolism, glycolysis, and TCA cycle were highly expressed in 'Cheng Lan', which was positively correlated with the accumulation of major primary metabolites. Alternatively, seven UDP-glycosyltransferase genes are closely related to the glycosylation of cucurbitacins through co-expression analysis. Our findings established a global map of metabolite accumulation and gene regulation during fruit development in wild and cultivated watermelons and provided valuable information on taste formation in watermelon fruit.