Rootstock mediates transcriptional regulation of citrulline metabolism in grafted watermelon

Citrulline is a non-essential amino acid, involved in key biological functions in plants and humans. Rootstocks have a major impact on citrulline accumulation in grafted watermelon. Information regarding rootstock induced changes in citrulline metabolism is elusive. To understand the regulatory mechanism, parallel changes in the expression profiles of citrulline metabolic genes and citrulline content of watermelon were monitored during the development of self-rooted watermelon and watermelon grafted onto pumpkin, wild and bottle gourd rootstocks. Results demonstrated that rootstocks regulated the expression profiles in different ways to influence the citrulline content. GAT, NAGPR, ASS3 ASS2 and Asl2 showed the negative correlation with citrulline content in pumpkin grafted watermelon. Pumpkin rootstock promoted the citrulline content by high down-regulation and synergistic effect of ASS2, ASS3, ASL1 and ASl2 genes. In wild grafted watermelon, citrulline was accumulated as a result of down regulation of GAT, NAGS and ASL2 genes, which showed an inverse correlation with citrulline. In gourd grafted watermelon, changes in citrulline content were observed to be linked with lower expressions of GAT, NAGK, ASS2, ASS3, ASL1 and ARG which were negatively correlated with citrulline content. Our study will provide the basis to understand the molecular mechanism of citrulline accumulation in various rootstocks.