Monosaccharide/proton symporter AtSTP1 plays a major role in uptake and response of Arabidopsis seeds and seedlings to sugars

The aim of this study was to investigate the in vivo properties and functio n of the high-affinity monosaccharide/proton symporter AtSTP1 of Arabidopsi s. We isolated an Atstp1 knock-out mutant and found that this plant grows a nd develops normally. The AtSTP1 gene is expressed in germinating seeds and seedlings, with AtSTP1 activity found mainly in the seedling root. The rat e of uptake of [C-14]-3-O-methylglucose and [C-14]-D-glucose is 60% less in Atstp1 seedlings than in the wild type, showing that AtSTP1 is the major m onosaccharide transporter in Arabidopsis seedlings. Transport of D-galactos e and D-mannose is also up to 60% less in Atstp1 seedlings compared to wild type, but transport of D-fructose, L-arabinose and sucrose is not reduced. Germination of Atstp1 seed shows reduced sensitivity to D-mannose, demonst rating that AtSTP1 is active before germination. Atstp1 seedlings grow effe ctively on concentrations of D-galactose that inhibit wild-type growth, eve n at up to 100 mM D-galactose, indicating that active transport by AtSTP1 p lays a major role at very high concentrations of exogenous sugar. These fin dings provide insight into the physiological function of AtSTP1 and clearly establish its importance in the uptake of extracellular sugars by the embr yo and in seedlings.