The roles of three functional sulphate transporters involved in uptake andtranslocation of sulphate in Arabidopsis thaliana

To investigate the uptake and long-distance translocation of sulphate in pl ants, we have characterized three cell-type-specific sulphate transporters, Sultr1;1, Sultr2;1 and Sultr2;2 in Arabidopsis thaliana. Heterologous expr ession in the yeast sulphate transporter mutant indicated that Sultr1;1 enc odes a high-affinity sulphate transporter (K-m for sulphate 3.6 +/- 0.6 mu m), whereas Sultr2;1 and Sultr2;2 encode low-affinity sulphate transporters (K-m for sulphate 0.41 +/- 0.07 mm and greater than or equal to 1.2 mm, re spectively). In Arabidopsis plants expressing the fusion gene construct of the Sultr1;1 promoter and green fluorescent protein (GFP), GFP was localize d in the lateral root cap, root hairs, epidermis and cortex of roots. beta- glucuronidase (GUS) expressed with the Sultr2;1 promoter was specifically a ccumulated in the xylem parenchyma cells of roots and leaves, and in the ro ot pericycles and leaf phloem. Expression of the Sultr2;2 promoter-GFP fusi on gene showed specific localization of GFP in the root phloem and leaf vas cular bundle sheath cells. Plants continuously grown with low sulphate conc entrations accumulated high levels of Sultr1;1 and Sultr2;1 mRNA in roots a nd Sultr2;2 mRNA in leaves. The abundance of Sultr1;1 and Sultr2;1 mRNA was increased remarkably in roots by short-term stress caused by withdrawal of sulphate. Addition of selenate in the sulphate-sufficient medium increased the sulphate uptake capacity, tissue sulphate content and the abundance of Sultr1;1 and Sultr2;1 mRNA in roots. Concomitant decrease of the tissue th iol content after selenate treatment was consistent with the suggested role of glutathione (GSH) as a repressive effector for the expression of sulpha te transporter genes.