Patterns of enzyme activities and gene expressions in sucrose metabolism in relation to sugar accumulation and composition in the aril of Litchi chinensis Sonn.

Sucrose metabolism enzymes, including invertase (EC 3.2.1.26), sucrose synthase (SS, EC 2.4.1.13), and sucrose phosphate synthase (SPS, EC 2.4.1.14), are key factors that determine fruit sugar accumulation and composition. Sugar concentration and sugar composition in the arils of 42 litchi cultivars were determined at maturity. The cultivars were grouped into three types according to their hexose/sucrose ratio. Five cultivars of each type were selected to monitor the activities and gene expressions of enzymes related to sucrose metabolism. Pattern changes in the arils of four cultivars with different sugar concentrations and compositions were traced from around 40 d after anthesis to full maturity. Highly significant positive correlations were observed between hexose/sucrose ratios and the activities and expression levels of soluble acid invertase (SAI) and SS among the 15 cultivars tested. The increase in hexose/sucrose ratio was accompanied by enhanced acid invertase (AI) and SS activities and the expression of their genes in Feizixiao (FZX) and Heiye (HY). By contrast, no significant correlation was observed between hexose/sucrose ratio and SPS. These results indicate that the sugar composition in litchi aril depends mainly on the sucrose cleavage enzymes AI and SS and not on the sucrose synthetic enzyme SPS. The cultivar Nuomici, which had the highest sugar content among the cultivars studied, displayed significantly lower activities of cell wall acid invertase, SAI, neutral invertase, and SS and lower expression levels of SAI and SS compared with HY, the cultivar with the lowest sugar content. The inconsistent patterns of sugar accumulation and activities and expressions of sucrose metabolism enzymes suggest that these sucrose metabolism enzymes are not necessarily related to sugar accumulation.