New insights into vesicle granulation in Citrus grandis revealed by systematic analysis of sugar- and acid-related genes and metabolites

Abstract

Vesicle granulation is a common internal physiological disorder in citrus fruit, associated with excessive consumption of sugar and acid. Aside from cell wall metabolism, it remains largely unexplored how the sugar and acid metabolic pathways altered as vesicle granulation progressed. The present study performed systematic analysis of sugar- and acid-related genes and metabolites in vesicles during pomelo granulation. Eighty-one genes were identified from the pomelo genome, belonging to 19 families, including 37 and 44 genes for sugar and acid metabolism, respectively. Vesicle granulation was characterized by increasing expression levels of genes of sucrose and citrate degradation while decreasing expression levels of genes of sucrose and citrate synthesis, according to the genome-wide transcript analysis. Further, those genes of citrate degradation were mainly related to glutamine and acetyl-CoA pathways. Systematic analysis of metabolites suggested that various metabolites, including glutamine and other metabolites of the acetyl-CoA pathway, were accumulated dramatically upon granulation. Interestingly, the γ-aminobutyrate shunt, the pivotal pathway for citrate degradation in citrus fruit upon ripening, may not be activated during pomelo granulation. In conclusion, the present study discovered novel metabolic pathways underlying citrus granulation, including shifting citrate flux to the glutamine pathway and acetyl-CoA pathway, but not the γ-aminobutyrate shunt.