Demand-driven control of root ATP sulfurylase activity and SO42- uptake in intact canola - The role of phloem-translocated glutathione

The activity of ATP sulfurylase extracted from roots of intact canola (Brassica napus L. cv Drakkar) increased after withdrawal of the S source from the nutrient solution and declined after refeeding SO42- to S-starved plants. The rate of SO42- uptake by the roots was similarly influenced. identical responses were obtained in SO42--fed roots when one-half of the root system was starved for S. The internal levels of SO42- and glutathione (GSH) declined after S starvation of the whole root system, but only GSH concentration declined in +S roots of plants from split root experiments. The concentration of GSH in phloem exudates decreased upon transfer of plants to S-free solution. Supplying GSH or cysteine to roots, either exogenously or internally via phloem sap, inhibited both ATP sulfurylase activity and SO42- uptake. Buthionine sulfoximine, an inhibitor of GSH synthesis, reversed the inhibitory effect of cysteine on ATP sulfurylase. It is hypothesized that CSH is responsible for mediating the responses to S availability. ATP sulfurylase activity and the SO42- uptake rate are regulated by similar demand-driven processes that involve the translocation of a phloem-transported message (possibly CSH) to the roots that provides information concerning the nutritional status of the leaves.