Sucrose accumulation during grape berry and strawberry fruit ripening is controlled predominantly by sucrose synthase activity

Phloem unloading and the metabolism of sucrose largely determine the content and type of sugars which have important roles in the improvement of fruit quality. However, the molecular mechanisms underlying the accumulation of sucrose in grape berry and strawberry fruit remain unclear. The relative contents of sucrose (Suc), glucose (Glu), and fructose (Fru) were analysed, as well as the levels of expression of key genes involved in sucrose accumulation, including those for soluble acid invertase (SAT), cell wall invertase (CWI), sucrose synthase (SS), sucrose phosphate synthase (SPS), and sucrose transporter 1 (SUT1) during the ripening of both grape berry and strawberry fruit. The results showed that 'Hamburg' grape berry accumulated low amounts of sucrose, whereas, 'Hongyan' strawberry fruit accumulated high levels of sucrose. Transcription of CWI, SPS, and SUT1 increased rapidly in both fruit, while SAI transcript levels increased only slightly, and remained low, except for a large increase during early ripening in strawberry fruit. Interestingly, the levels of transcription and translation of the SS gene showed opposite patterns in the two types of fruit. High SS enzyme activity with low sucrose contents in grape berry, and low SS enzyme activity with high sucrose contents in strawberry fruit, suggest that SS plays a dominant role in sucrose accumulation during ripening in both types of fruit.