Acrolein metabolism and its relationship to the occurrence of water-soaked brown flesh in Japanese pear fruits

Water-soaked brown flesh (WSBF) of pear (Pyrus pyrifolia var. culta) is a physiological disorder in which the flesh of the fruit turns brown in the late stage of maturation. Since it is not possible to determine the presence or absence of the disorder from the external appearance, it is difficult to remove the affected fruit during harvesting, sorting, and preparation work, resulting in significant losses not only for producers but also for consumers. In this study, we compared the metabolite contents and gene expression levels involved in acrolein (ACR) production in mature pear fruit of WSBF-susceptible and -resistant cultivars. WSBF occurred in the ripe fruit of the susceptible cultivar, but no WSBF was observed in the resistant cultivar, even in overripe fruit. ACR was significantly accumulated in the flesh tissue of ripe susceptible cultivar fruit, but little ACR accumulation was observed in the resistant cultivar, even in overripe fruit. The analysis of metabolite contents and gene expression levels showed that the gene expression levels of the polyamine metabolism and ethylene synthesis pathway were higher in the susceptible cultivar than in the resistant cultivar, indicating that these metabolic pathways were activated. Furthermore, the expression levels of genes involved in the decomposition of polyamines and lipids were also maintained at a higher level in the former compared to the latter. The results revealed that both the activation of the ethylene synthesis pathway and the subsequent active synthesis and degradation of polyamines leading to the production of ACR, and the increased activity of lipid-degrading enzymes due to activation of the ethylene synthesis pathway may be closely related to the occurrence of WSBF disorder in pear fruit.