A member of the mitogen‐activated protein 3‐kinase family is involved in the regulation of plant vacuolar glucose uptake

Abstract

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.

Overexpression of the Vacuolar Sugar CarrierAtSWEET16Modifies Germination, Growth, and Stress Tolerance in Arabidopsis

“…As a consequence of this, the exact physiological impact of a modified activity of carriers like SWEET16 can vary from condition to condition tested, because the individual external stimuli influence the relative rates of sugar synthesis and degradation in the cytosol and in the vacuolar lumen. Moreover, we cannot exclude that SWEET16 is dependent upon the external stimulus posttranslationally modified, as shown for the vacuolar TMT1 protein (Wingenter et al, 2011), which might affect transport properties considerably when compared with the recombinant situation in the oocyte.…”

“…The activity of the main vacuolar monosaccharide importer TMT is even more complex regulated, since the TMT1 and TMT2 genes are under the control of sugars, salt, and temperature and both genes are cell type-specifically expressed (Wormit et al, 2006). Moreover, TMT activity is tuned by a recently identified protein kinase, VIK1, belonging to a subclade of the large MAP3K family of protein kinases (Wingenter et al, 2011). We show here that SWEET16 transports Glc, Fru, and Suc in the recombinant oocyte system (Fig.…”

“…S1). The growth of genotypes with altered expression of vacuolar sugar transporters in liquid culture conditions allows a quick check of whether vacuolar transporter mutants affect cellular sugar homeostasis (Wingenter et al, 2010(Wingenter et al, , 2011. Thus, in a first set of experiments, we searched for a putative effect of altered vacuolar SWEET16 expression during the growth of 35S Pro :SWEET16 compared with wild-type plants in standard liquid culture medium, harboring 0.5% Suc (Scheible et al, 2004 (Fig.…”

“…While vacuolar Glc transporter loss-of-function plants do not show significant changes in monosaccharide levels (Aluri and Büttner, 2007), decreased TMT activity correlates with impaired vacuolar sugar import and low levels of both Glc and Fru in leaves (Wormit et al, 2006). This fact and the observations that (1) TMT1 is a sugar/proton antiporter , (2) increased TMT activity provokes improved seed biomass (Wingenter et al, 2010), and (3) TMT activity is highly regulated via protein phosphorylation (Wingenter et al, 2011) clearly underline the superior function of TMT for monosaccharide loading into the plant vacuole.…”

A Novel Arabidopsis Vacuolar Glucose Exporter Is Involved in Cellular Sugar Homeostasis and Affects the Composition of Seed Storage Compounds

“…Only very recently, two previously unknown Glc transporter families, the tonoplast monosaccharide transporters (AtTMTs) and the vacuolar Glc transporters, have been identified in Arabidopsis (Arabidopsis thaliana), and direct or indirect transport assays support an import function for both types of transporters (Wormit et al, 2006;Aluri and Bü ttner, 2007). The activity of TMT limits seed yield in Arabidopsis (Wingenter et al, 2010) and is complex controlled by phosphorylation, catalyzed by a mitogen-activated protein triple kinase (Wingenter et al, 2011). However, Glc export from the vacuole, which was demonstrated biochemically, could not be attributed to one of the sugar transporters characterized so far.…”