Enhanced vacuolar invertase activity and capability for carbohydrate import in GA-treated inflorescence correlate with increased fruit set in grapevine

Abstract

Application of exogenous gibberellin (GA) has been widely used to improve fruit set in grapevine, although the underlying molecular mechanism remains unknown. In this work, GA application at 10 days before anthesis promoted fruit set of grapevine “Fenghou” (Vitis vinifera × V. labrusca) and increased sugar content and acid invertase activity in the ovary, especially vacuolar invertase (VIN) activity during fruit set, compared with the control. However, the GA biosynthesis inhibitor paclobutrazol (PAC) had the opposite effect. Tissue-specific expression analysis and in situ hybridization showed that transcripts of three acid invertase genes were primarily detected in the phloem of the flower and fruitlet. In contrast to the expression of the cell wall invertase gene, which was only induced before blooming, transcript accumulation of the VIN genes VvGIN1 and VvGIN2 was enhanced during fruit set by GA application. Nevertheless, PAC treatment reduced VvGIN1 transcript accumulation after anthesis, but had no significant effect on the VvGIN2 transcript level at the same time point. Interestingly, the effect of GA and PAC application on expression of VvPIN1-like and VvPIN3-like (auxin transporters) was similar to that on VvGIN1 after blooming. In addition, the inducibility of the VvGIN1 and VvGIN2 promoters by GA was demonstrated using transient expression assays in tobacco leaves. These results indicated that enhanced VIN activity and carbohydrate import into grapevine fruitlets by GA application contribute to the increased fruit set through increasing sink strength and sugar signaling activities, possibly by regulating polar auxin transport.