Abstract
The ripening of grape (Vitis vinifera L.) berries is associated with a large accumulation of glucose and fructose in the vacuoles of the fruit cells. These hexoses are derived from sucrose, which is released from the phloem and may be taken up by parenchyma cells prior to hydrolysis. We have expressed two putative ripening-related sucrose transporters from grape berries, VvSUC11 (synonymous with VvSUT1) and VvSUC12, in an invertase deficient yeast strain to characterize their transport activities. Sucrose was taken up by yeast transformed with either transporter at an optimum pH of <4.5 and with a Michaelis constant (Km) of 0.9–1.4m M. The uptake of sucrose through VvSUC11 and VvSUC12 was inhibited by protonophores and by vanadate. This is consistent with an active uptake mechanism involving proton cotransport, typical of sucrose/H+symporters. The transporters from grape berries were functionally similar to Scr1, a sucrose transporter from Ricinus cotyledons. It is likely that in grape berries VvSUC11 and VvSUC12 facilitate the loading of sucrose from the apoplast into the parenchyma cells.