Regulation of sulfate uptake and xylem loading of poplar roots (Populus tremula x P. alba)

Sulfate transport processes and its regulation were studied in roots of poplar trees (Populus tremula x P. alba). From the exponential increase in sulfate uptake with temperature an activation energy (Ea) of 9.0±0.8 kJ mol−1 was calculated. In the concentration range 0.005–10 mM sulfate uptake showed biphasic Michaelis-Menten kinetics with a Km of 3.2±3.4 μM and a Vmax of 49±11 nmol SO42− g−1 FW h−1 for the high-affinity uptake system (phase 1) and a Km of 1.33±0.41 mM and a Vmax of 255±25 nmol SO42− g−1 FW h−1 for the low-affinity system (phase 2). Xylem loading decreased linearly with temperature and remained unchanged within the sulfate concentration range studied. Regulation of sulfate uptake and xylem loading by O-acetyl serine (OAS), Cys, reduced glutathione (GSH), Met and S-methylmethionine (SMM) were tested by perfusion into the xylem sap with the pressure probe and by addition to the incubation medium. When added directly to the transport medium, Cys and GSH repressed, and OAS stimulated sulfate uptake; xylem loading was stimulated by Cys, repressed by GSH and only slightly affected by OAS. When perfused into the xylem, none of the compounds tested affected sulfate uptake of excised roots, but xylem loading was stimulated by SMM and OAS and repressed by Met. Apparently, the site of application strongly determined the effect of regulatory compounds of sulfate transport processes.