Alteration of pectin metabolism in blood orange fruit (Citrus sinensis cv. Tarocco) in response to vesicle collapse

Segment drying is a severe physiological disorder of citrus fruit, and vesicles become granulated or collapsed. Aside from the hypothesis that alteration of cell wall metabolism is the main factor of citrus granulation, little is known about vesicle collapse. This study aimed to elucidate the changes in pectin metabolism during vesicle collapse in blood orange. Vesicle collapse was characterized by decreased nutrients while increased chelate- and sodium carbonate-soluble pectin and calcium content. The nanostructure of chelate-soluble pectin got complex and developed multi-branching upon collapse. The activity of pectin methylesterase increased, while that of polygalacturonase and pectate lyase decreased upon collapse. Genome-wide transcriptional analysis revealed an increasing pattern of genes encoding pectin methylesterase and other enzymes involved in pectin synthesis and de-acetylation upon collapse. Drying vesicles were characterized by increased abscisic acid content and relevant gene expressions. In conclusion, we discovered alteration of pectin metabolism underlying citrus vesicle collapse, mainly promoting pectin demethylesterification, remodeling pectin structures, and further inhibiting pectin degradation, which was hypothesized to be a main factor for the citrus collapse. This is the first to disclose the potential intrinsic mechanism underlying vesicle collapse in orange fruit.