Hypothesis of cell wall metabolism disorder in segment drying: Evidence from vesicle collapse in ‘Dayagan’ hybrid citrus fruit

Segment drying is a common physiological disorder in citrus fruit during ripening and postharvest storage. The underlying mechanisms of this disorder are beginning to be understood, with the "Disorder of cell wall metabolism" hypothesis proposed as a major driving factor for nutrient loss and vesicle disorder during granulation-type segment drying. However, the extent to which this hypothesis applies to vesicle collapse-type segment drying is not well established. Thus, this study aimed to investigate whether the data on vesicle collapse support this hypothesis using 'Dayagan' fruit as material. The results showed that the collapsed vesicles exhibited deterioration of sugars and acids, while accumulating cell wall components such as protopectin and cellulose. Metabolic pathways altered during segment drying included the activation of sucrose and citrate degradation and synthesis of cell wall materials. Enzyme activity analyses revealed that the glutamine synthesis and acetyl-CoA pathways, but not the GABA shunt, were responsible for citrate depletion and that pectin de-methylesterification might be activated in vesicles during collapse. Systematic changes in gene expression patterns further revealed the complex regulation of metabolic pathways at the transcription level, where sucrose degradation and pectin metabolism may be regulated but not the acetyl-CoA and lignin synthesis pathways. These findings provide important insights into metabolic pathway changes that are conserved during vesicle collapse and granulation, supporting the hypothesis that disorder of cell wall metabolism may result in nutrient depletion and segment drying. This study provides a conceptual foundation for further research on vesicle collapse.