Alteration of sugar and organic acid metabolism in postharvest granulation of Ponkan fruit revealed by transcriptome profiling

Granulation is a serious physiological disorder in citrus fruit, accompanied by deterioration of sugars and organic acids, of which the relevant mechanism remains largely unknown. Postharvest granulation was found to begin at the stem region and then gradually extend towards the stylar end of the segment in Ponkan fruit. Hence, stem and stylar juice vesicles within the same fruit with either non-granulation or granulation were analyzed by RNA-Sequencing. Through a comparison of incipient granulated and entire granulated fruit, the results demonstrated that 768 genes were reliably identified to be differentially expressed under granulation. A significant increase in transcript levels was observed in genes encoding enzymes involved in the degradation pathways of sugars and citric acid (e.g., invertase, hexokinase, aconitase, isocitrate dehydrogenase and α-ketoglutarate-dehydrogenase) and in the biosynthesis of cell wall materials including pro-pectin, cellulose and lignin (e.g., UDP-glucuronate 4-epimerase, UDP-glucose pyrophosphorylase and cinnamyl alcohol dehydrogenase) under granulation. Further, there was a distinct decrease in transcript levels of genes encoding enzymes involved in the synthesis pathway of sucrose and citric acid (e.g., sucrose phosphate synthase, sucrose phosphatase, aspartate aminotransferase and malate dehydrogenase) and the degradation pathway of cell wall components (e.g., pectin methylesterase and β-d-xylosidase) under granulation. Together with the decline in content of soluble sugars and acids (sucrose, glucose, fructose, citric acid and malic acid) and the increase in content of cell wall components (lignin, pro-pectin and cellulose) under granulation, the results suggested that sugar and organic acid metabolism adjusted to the synthesis pathway of cell wall components upon granulation at the expense of sugars and acids. This study is the first to unravel the global picture of the network of sugar and organic acid metabolism underlying fruit granulation.