Starch-sugar metabolism in response to wound stress in apple fruit

The sugar molecule is essential for plant response to abiotic stress. Leveraging previously obtained transcriptome data, this study investigated the changes in starch and cellulose degradation as well as sucrose metabolism in post-harvest mechanically wounded apples. The findings demonstrated that wounding up-regulated the expression levels of MdAMY1.1, MdAMY2, MdBAM1, and MdBAM3, which in turn enhanced the enzymatic activities of α-amylase (AMY) and β-amylase (BMY), thereby accelerating the degradation of total starch, amylose, and amylopectin. Conversely, it down-regulated the expression levels of MdSS3 and MdSS4, reduced the enzymatic activities of sucrose synthase synthesis (SS-s) and sucrose synthase cleavage (SS-c). Wounding also up-regulated the expression levels of cellulase and β-glucosidase genes, thus facilitating the degradation of cellulose. Moreover, wound stress induced the up-regulation of MdSPS1, MdCWINV1, MdINV∗DC4, and MdTIV1 expression levels, thereby maintaining the balance of sucrose metabolism. Consequently, this leads to an increase in the biosynthesis of glucose, fructose, sucrose, and reducing sugars at the wound sites. All these evidences indicate that starch and cellulose at wound sites are degraded to produce sugars, which facilitate the wound healing process in apple fruit, thereby helping to reduce microbial infections and minimize post-harvest losses.