Vacuolar Phosphate Transporter1 (VPT1) may transport sugar in response to soluble sugar status of grape fruits.

Vacuolar Phosphate Transporter1 (VPT1)-mediated phosphate uptake in the vacuoles is essential to plant development and fruit ripening. Interestingly, here we find that the VPT1 may transport sugar in response to soluble sugar status of fruits. The VvVPT1 protein isolated from grape (Vitis vinifera) berries was tonoplast-localized and contains SPX (Syg1/Pho81/XPR1) and MFS (major facilitator superfamily) domains. Its mRNA expression was significantly increased during fruit ripening and induced by sucrose. Functional analyses based on transient transgenic systems in grape berry showed that VvVPT1 positively regulated berry ripening and significantly affected hexose contents, fruit firmness, and ripening-related gene expression. The VPT1 proteins (Grape VvVPT1, strawberry FaVPT1, and Arabidopsis AtVPT1) all showed low affinity for phosphate verified in yeast system, while they appear different in sugar transport capacity, consistent with fruit sugar status. Thus, our findings reveal a role for VPT1 in fruit ripening, associated to its SPX and MFS domains in direct transport of soluble sugar available into the vacuole, and open potential avenues for genetic improvement in fleshy fruit.

The Proteome and Phosphoproteome Uncovers Candidate Proteins Associated With Vacuolar Phosphate Signal Multipled by Vacuolar Phosphate Transporter 1 (VPT1) in Arabidopsis.

Plant vacuoles serve as the primary intracellular compartments for inorganic phosphate (Pi) storage. Passage of Pi across vacuolar membranes plays a critical role in buffering the cytoplasmic Pi level against fluctuations of external Pi and metabolic activities. To gain new insights into the proteins and processes, vacuolar Pi level regulated by vacuolar phosphate transporter 1 (VPT1) in Arabidopsis, we carried out tandem mass tag labeling proteome and phosphoproteome profiling of Arabidopsis WT and vpt1 loss-of-function mutant plants. The vpt1 mutant had a marked reduced vacuolar Pi level and a slight increased cytosol Pi level. The mutant was stunted as reflected in the reduction of the fresh weight compared with WT plants and bolting earlier under normal growth conditions in soil. Over 5566 proteins and 7965 phosphopeptides were quantified. About 146 and 83 proteins were significantly changed at protein abundance or site-specific phosphorylation levels, but only six proteins were shared between them. Functional enrichment analysis revealed that the changes of Pi states in vpt1 are associated with photosynthesis, translation, RNA splicing, and defense response, consistent with similar studies in Arabidopsis. Except for PAP26, EIN2, and KIN10, which were reported to be associated with phosphate starvation signal, we also found that many differential proteins involved in abscisic acid signaling, such as CARK1, SnRK1, and AREB3, were significantly changed in vpt1. Our study illuminates several new aspects of the phosphate response and identifies important targets for further investigation and potential crop improvement.