Transcriptional regulation of lycopene metabolism mediated by rootstock during the ripening of grafted watermelons

Rootstocks have comprehensive effects on lycopene accumulation in grafted watermelon fruits. However, little is known about lycopene metabolic regulation in grafted watermelon. To address this problem, parallel changes in lycopene contents and the expression of its metabolic genes were analyzed during the fruit ripening of nongrafted watermelon and watermelon grafted onto bottle gourd, pumpkin, and wild watermelon. Results showed that rootstocks mediated the transcriptional regulations of lycopene accumulation in different ways. Bottle gourd and wild watermelon promoted lycopene accumulation in grafted watermelon fruits by upregulating the biosynthetic genes phytoene synthase (PSY) and ζ-carotene desaturase (ZDS), and downregulating the catabolic genes β-carotene hydroxylase (CHYB), zeaxanthin epoxidase (ZEP), 9-cis-epoxycarotenoid dioxygenase (NCED), and carotenoid cleavage dioxygenase (CCD). However, pumpkin did not affect lycopene accumulation by upregulating both biosynthetic and catabolic genes. The rootstock-dependent characteristic of lycopene accumulation in grafted watermelon fruits provided an alternative model for investigating lycopene metabolic regulation.