Integrated analysis of postharvest storage characteristics of seven apple cultivars and transcriptome data identifies MdBBX25 as a negative regulator of fruit softening during storage in apples

Apple is classified as a climacteric fruit, characterized by a rapid surge in ethylene levels at the onset of ripening. However, there is significant variation in the degree and timing of ripening and softening among different apple cultivars. Understanding the molecular mechanisms underlying fruit softening and the associated changes in cell wall integrity is crucial for improving fruit quality and extending shelf life. In this study, we conducted a comparative analysis of fruit firmness and ethylene production among seven apple cultivars with different softening patterns during storage. We also examined the expression patterns of genes and enzyme activities involved in cell wall metabolism. Our results showed that fruit firmness was negatively correlated with ethylene production rate, respiration rate, soluble pectin content, cellulase activity, polygalacturonase activity, and β-glucosidase activity, while it was positively correlated with protopectin content and cellulose content. Furthermore, we identified a BBX transcription factor gene, MdBBX25, that was negatively correlated with fruit softening during storage. Overexpression of MdBBX25 resulted in delayed fruit softening by suppressing ethylene biosynthesis and cell wall-related genes, whereas silencing of MdBBX25 accelerated fruit softening by promoting ethylene biosynthesis and cell wall-related genes in apple. Our findings provide insights into the role of MdBBX25 as a negative regulator of fruit softening through the modulation of ethylene biosynthesis and cell wall degradation during storage.