A role for ‘futile cycles’ involving invertase and sucrose synthase in sucrose metabolism of tomato fruit

Current concepts of the factors determining sink strength and the subsequent regulation of carbohydrate metabolism in tomato fruit are based upon an understanding of the relative roles of sucrose synthase, sucrose phosphate synthase and invertase, derived from studies in mutants and transformed plants. These enzymes participate in at least four futile cycles that involve sugar transport between the cytosol, vacuole and apoplast. Key reactions are (1) the continuous rapid degradation of sucrose in the cytosol by sucrose synthase (SuSy), (2) sucrose re-synthesis via either SuSy or sucrose phosphate synthase (SPS), (3) sucrose hydrolysis in the vacuole or apoplast by acid invertase, (4) subsequent transport of hexoses to the cytosol where they are once more converted into sucrose, and (5) rapid synthesis and breakdown of starch in the amyloplast. In this way futile cycles of sucrose/hexose interchange govern fruit sugar content and composition. The major function of the high and constant invertase activity in red tomato fruit is, therefore, to maintain high cellular hexose concentrations, the hydrolysis of sucrose in the vacuole and in the intercellular space allowing more efficient storage of sugar in these compartments. Vacuolar sugar storage may be important in sustaining fruit cell growth at times when less sucrose is available for the sink organs because of exhaustion of the carbohydrate pools in source leaves.