Vacuoles release sucrose via tonoplast‐localised SUC4‐type transporters

Abstract

Arabidopsis thaliana has seven genes for functionally active sucrose transporters. Together with sucrose transporters from other dicot and monocot plants, these proteins form four separate phylogenetic groups. Group-IV includes the Arabidopsis protein SUC4 (synonym SUT4) and related proteins from monocots and dicots. These Group-IV sucrose transporters were reported to be either tonoplast- or plasma membrane-localised, and in heterologous expression systems were shown to act as sucrose/H(+) symporters. Here, we present comparative analyses of the subcellular localisation of the Arabidopsis SUC4 protein and of several other Group-IV sucrose transporters, studies on tissue specificity of the Arabidopsis SUC4 promoter, phenotypic characterisations of Atsuc4.1 mutants and AtSUC4 overexpressing (AtSUC4-OX) plants, and functional comparisons of Atsuc4.1 and AtSUC4-OX vacuoles. Our data show that SUC4-type sucrose transporters from different plant families (Brassicaceae, Cucurbitaceae and Solanaceae) localise exclusively to the tonoplast, demonstrating that vacuolar sucrose transport is a common theme of all SUC4-type proteins. AtSUC4 expression is confined to the stele of Arabidopsis roots, developing anthers and meristematic tissues in all aerial parts. Analyses of the carbohydrate content of WT and mutant seedlings revealed reduced sucrose content in AtSUC4-OX seedlings. This is in line with patch-clamp analyses of AtSUC4-OX vacuoles that characterise AtSUC4 as a sucrose/H(+) symporter directly in the tonoplast membrane.

Sugar Potentiation of Fatty Acid and Triacylglycerol Accumulation

“…Unlike Suc storage organs, which have efficient sugar transporters and sugar-metabolizing enzymes that compartmentalize Suc into vacuoles, elevated levels of Suc accumulate in the cytosol of adg1suc2. This is consistent with the observation that adg1suc2 and the wild type have similar expression levels of TMT1/2, which encodes the protoncoupled antiporter capable of high-capacity vacuolar Glc and Suc loading (Schulz et al, 2011), and similar levels of SUC4, a proton-coupled symporter that exports Suc from the vacuole (Schneider et al, 2012).…”

Routes to the Tonoplast: The Sorting of Tonoplast Transporters in Arabidopsis Mesophyll Protoplasts

“…By contrast, another tonoplast-localized transporter, the Suc transporter SUC4 (Endler et al, 2006;Schneider et al, 2011;Schulz et al, 2011), is transported via a second, AP-3-dependent route. (Bonifacino and Traub, 2003).…”

“…As controls, we compared the localization of GFP fusions to the Arabidopsis Suc transporter SUC4 (GFP-SUC4) and to the Arabidopsis Glc transporter ESL1 (ESL1-GFP), which were previously shown to localize to the tonoplast (Endler et al, 2006;Yamada et al, 2010;Schneider et al, 2011). Whereas ESL1 has an N-terminal [D/E]XXXL[L/I]-type motif involved in the sorting to the tonoplast (Yamada et al, 2010), SUC4 does not contain any of the putative sorting signals known from animals.…”

“…Transformed Arabidopsis protoplasts were incubated for 24 to 72 h in the dark at 228C prior to confocal analysis. Osmotic lysis of protoplasts was performed as described by Schneider et al (2011).…”

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

“…Loss of function of this type of carrier in Arabidopsis, poplar (Populus spp. ), or rice (Oryza sativa) leads to an accumulation of Suc in leaves (Eom et al, 2011;Payyavula et al, 2011;Schneider et al, 2012), elegantly proving that this type of carrier fulfills an export function under in vivo conditions.…”