The extent to which vacuolar sugar transport activity affects molecular, cellular, and developmental processes in Arabidopsis (Arabidopsis thaliana) is unknown. Electrophysiological analysis revealed that overexpression of the tonoplast monosaccharide transporter TMT1 in a tmt1-2::tDNA mutant led to increased proton-coupled monosaccharide import into isolated mesophyll vacuoles in comparison with wild-type vacuoles. TMT1 overexpressor mutants grew faster than wild-type plants on soil and in high-glucose (Glc)-containing liquid medium. These effects were correlated with increased vacuolar monosaccharide compartmentation, as revealed by nonaqueous fractionation and by chlorophyll ab -binding protein1 and nitrate reductase1 gene expression studies. Soil-grown TMT1 overexpressor plants respired less Glc than wild-type plants and only about half the amount of Glc respired by tmt1-2::tDNA mutants. In sum, these data show that TMT activity in wild-type plants limits vacuolar monosaccharide loading. Remarkably, TMT1 overexpressor mutants produced larger seeds and greater total seed yield, which was associated with increased lipid and protein content. These changes in seed properties were correlated with slightly decreased nocturnal CO 2 release and increased sugar export rates from detached source leaves. The SUC2 gene, which codes for a sucrose transporter that may be critical for phloem loading in leaves, has been identified as Glc repressed. Thus, the observation that SUC2 mRNA increased slightly in TMT1 overexpressor leaves, characterized by lowered cytosolic Glc levels than wild-type leaves, provided further evidence of a stimulated source capacity. In summary, increased TMT activity in Arabidopsis induced modified subcellular sugar compartmentation, altered cellular sugar sensing, affected assimilate allocation, increased the biomass of Arabidopsis seeds, and accelerated early plant development.Sugars fulfill an extraordinarily wide range of functions in plants as well as in other organisms. They serve as valuable energy resources that are easy to store and remobilize. Sugars are required for the synthesis of cell walls and carbohydrate polymers. They are also necessary for starch accumulation and serve as precursors for a range of primary and secondary plant intermediates. From a chemical point of view, sugars represent a large class of metabolites. Among the prominent members in higher plants are the monosaccharides Glc and Fru and the disaccharide Suc (ap Rees, 1994).

SUMMARYVacuolar solute accumulation is an important process during plant development, growth and stress responses. Although several vacuolar carriers have been identified recently, knowledge regarding the regulation of transport is still limited. Solute accumulation may be controlled by various factors, such as alterations in carrier abundance or activity. Phosphorylation via kinases is a well-known principle for activation or deactivation of proteins. Several phosphorylated proteins have been identified in the tonoplast proteome; however, kinases that catalyse the phosphorylation of tonoplast proteins are currently unknown. The tonoplast monosaccaride transporter from Arabidopsis (AtTMT1) and its homologue from barley have multiple phosphorylation sites in their extremely large loops. Here we demonstrate that the loop of AtTMT1 interacts with a mitogen-activated triple kinase-like protein kinase (VIK), that an aspartate-rich loop domain is required for effective interaction, and that the presence of VIK stimulates glucose import into isolated vacuoles. Furthermore, the phenotype of VIK loss-of-function plants strikingly resembles that of plants lacking AtTMT1/ 2. These data suggest that VIK-mediated phosphorylation of the AtTMT1 loop enhances carrier activity and consequently vacuolar sugar accumulation. As many phosphorylated proteins have been identified in the tonoplast, differential phosphorylation may be a general mechanism regulating vacuolar solute import.