Cell Wall Composition and Ethylene Synthesis Are Altered in Transgenic Apple with Decreased Sorbitol Synthesis

In transgenic ‘Greensleeves’ apple trees with decreased sorbitol synthesis, sucrose accounts for a larger proportion of the carbon exported from leaves to fruit whereas leaf photosynthesis remains largely the same. In response to this increased sucrose supply, sucrose metabolism is up-regulated in fruit, leading to homeostasis of both fructose and sucrose but much higher concentrations of glucose and galactose at harvest. However, it is not known whether cell wall composition and fruit ripening are affected. In this study, fruit samples were taken at regular intervals from 5 weeks before harvest to 3 weeks after harvest to measure cell wall composition, fruit firmness, internal ethylene concentrations and the expression levels of key genes involved in ethylene synthesis. Compared with the untransformed control, transgenic fruit had greater firmness, but higher levels of internal ethylene. These corresponded to higher concentrations of cellulose and semicellulose, and higher expression levels of both ACC synthase 1 (ACS1) and ACS3 and ACC oxidase 1 (ACO1). These results indicate that enhanced sucrose metabolism in apple fruit alters cell wall composition and ethylene synthesis, thereby affecting its firmness and ripening process.