Regulation of sulfur nutrition in wild-type and transgenic poplar over-expressing gamma-glutamylcysteine synthetase in the cytosol as affected by atmospheric H2S

This study with poplar (Populus tremula x Populus alba) cuttings was aimed to test the hypothesis that sulfate uptake is regulated by demand-driven co ntrol and that this regulation is mediated by phloem-transported glutathion e as a shoot-to-root signal. Therefore, sulfur nutrition was investigated a t (a) enhanced sulfate demand in transgenic poplar over-expressing gamma-gl utamylcysteine (gamma-EC) synthetase in the cytosol and (b) reduced sulfate demand during short-term exposure to H2S. H2S taken up by the leaves incre ased cysteine, gamma-EC, and glutathione concentrations in leaves, xylem sa p, phloem exudate, and roots, both in wild-type and transgenic poplar. The observed reduced xylem loading of sulfate after H2S exposure of wild-type p oplar could well be explained by a higher glutathione concentration in the phloem. In transgenic poplar increased concentrations of glutathione and ga mma-EC were found not only in leaves, xylem sap, and roots but also in phlo em exudate irrespective of H2S exposure. Despite enhanced phloem allocation of glutathione and its accumulation in the roots, sulfate uptake was stron gly enhanced. This finding is contradictory to the hypothesis that glutathi one allocated in the phloem reduces sulfate uptake and its transport to the shoot. Correlation analysis provided circumstantial evidence that the sulf ate to glutathione ratio in the phloem may control sulfate uptake and loadi ng into the xylem, both when the sulfate demand of the shoot is increased a nd when it is reduced.