Genetic and Gene Expression Analysis of Berry Sugar Composition in a Sucrose-accumulating Grapevine

Abstract

We examined hexose- and sucrose-accumulating grapevines to elucidate genetic and transcriptional control of berry sugar composition in table grapes. Sugar composition was classified into two types: a hexose accumulator that accumulates fructose, glucose, and no or trace amounts of sucrose and a sucrose accumulator that accumulates fructose, glucose, and a large amount of sucrose. Segregation ratios among diploid seedling populations of S₁ and F₂ indicate that the trait of sucrose accumulation is controlled by a single major recessive gene. Transcriptional levels of sugar accumulation-related key genes were monitored during berry ripening using two hexose-accumulating cultivar/selections and one sucrose-accumulating cultivar. Gene expression of sucrose transporter and sucrose phosphate synthase coincided with a rapid increase in sugar accumulation after the onset of berry ripening; gene expressions of those enzymes and cell wall invertase were not significantly different between sucrose- and hexose-accumulators and gene expression of vacuolar acid invertase decreased after the onset of berry ripening in both sucrose- and hexose-accumulators. Gene expression of sucrose synthase (catalyzes the reaction of sucrose cleavage) in the hexose accumulators remained constant throughout berry development, whereas the expression in sucrose accumulators was significantly lower, with a concomitant sharp rise in sucrose accumulation. These results suggest that sucrose synthase may play a critical role in the sugar metabolism of sucrose-accumulating grape berries.