Regulation of Sulfur Nutrition in Wild-Type and Transgenic Poplar Over-Expressing γ-Glutamylcysteine Synthetase in the Cytosol as Affected by Atmospheric H2S

Abstract

This study with poplar (Populus tremula ϫ Populus alba) cuttings was aimed to test the hypothesis that sulfate uptake is regulated by demand-driven control and that this regulation is mediated by phloem-transported glutathione as a shoot-toroot signal. Therefore, sulfur nutrition was investigated at (a) enhanced sulfate demand in transgenic poplar over-expressing ␥-glutamylcysteine (␥-EC) synthetase in the cytosol and (b) reduced sulfate demand during short-term exposure to H 2 S. H 2 S taken up by the leaves increased cysteine, ␥-EC, and glutathione concentrations in leaves, xylem sap, phloem exudate, and roots, both in wild-type and transgenic poplar. The observed reduced xylem loading of sulfate after H 2 S exposure of wild-type poplar could well be explained by a higher glutathione concentration in the phloem. In transgenic poplar increased concentrations of glutathione and ␥-EC were found not only in leaves, xylem sap, and roots but also in phloem exudate irrespective of H 2 S exposure. Despite enhanced phloem allocation of glutathione and its accumulation in the roots, sulfate uptake was strongly enhanced. This finding is contradictory to the hypothesis that glutathione allocated in the phloem reduces sulfate uptake and its transport to the shoot. Correlation analysis provided circumstantial evidence that the sulfate to glutathione ratio in the phloem may control sulfate uptake and loading into the xylem, both when the sulfate demand of the shoot is increased and when it is reduced.

Differential Regulation of the Expression of Two High-Affinity Sulfate Transporters, SULTR1.1 and SULTR1.2, in Arabidopsis

“…Glutathione and sulfate are both translocated within the sieve sap (Rennenberg et al, 1979), although glutathione is the major form of long-distance transport of reduced S in plants (Rennenberg et al, 1979;Herschbach et al, 2000). Interestingly, split-root experiments revealed that glutathione content in the 2S side of the split-roots, unlike sulfate content, remains high, and is comparable to the glutathione content of 1S/1S control roots (Fig.…”

“…However, such a negative effect of glutathione on the root sulfate uptake capacity was not observed in poplar after an exposure of aerial organs to H 2 S, although this treatment was shown to significantly increase leaf and root glutathione contents (Herschbach et al, 2000;Westerman et al, 2001). Also, the overexpression of the key enzyme for glutathione synthesis, g-glutamyl-Cys synthase, which resulted in the overaccumulation of glutathione, did not induce any down-regulation of the expression of sulfate transporters (Herschbach et al, 2000;Hartmann et al, 2004). Finally, increasing the glutathione content by providing O-acetyl-Ser to barley (Hordeum vulgare) plants did not result in a reduction in either the transcript accumulation of sulfate transporters or the sulfate uptake .…”

“…In rapeseed (Brassica napus) seedlings, split-root experiments showed that glutathione acts as a long-distance signal from the shoot to the root: the negative regulation of sulfate uptake resulting from the application of glutathione in one part of the root system was also demonstrated in the untreated part of the root (Lappartient and Touraine, 1996). However, such a negative effect of glutathione on the root sulfate uptake capacity was not observed in poplar after an exposure of aerial organs to H 2 S, although this treatment was shown to significantly increase leaf and root glutathione contents (Herschbach et al, 2000;Westerman et al, 2001). Also, the overexpression of the key enzyme for glutathione synthesis, g-glutamyl-Cys synthase, which resulted in the overaccumulation of glutathione, did not induce any down-regulation of the expression of sulfate transporters (Herschbach et al, 2000;Hartmann et al, 2004).…”

Maturation of Arabidopsis Seeds Is Dependent on Glutathione Biosynthesis within the Embryo

“…The demand of developing seeds for sulfur can potentially be satisfied either through supply of sulfate or, alternatively, in the form of reduced phloemmobile sulfur compounds. Potential transport metabolites for reduced sulfur are GSH (Rennenberg, 1982;Herschbach et al, 2000) and S-methylmethionine (Bourgis et al, 1999). It has been proposed that glutathione translocated from shoot to root via the phloem acts as a signal responsible for repression of sulfate uptake in the roots (Lappartient and Touraine, 1997;Lappartient et al, 1999).…”

Phloem-Localizing Sulfate Transporter, Sultr1;3, Mediates Re-Distribution of Sulfur from Source to Sink Organs in Arabidopsis

“…Furthermore, glutathione translocated in the phloem is suggested to mediate transmission of the interorgan signal of sulfur status in vascular plants (Lappartient et al, 1999). Studies with poplar more recently suggested a correlation between the demand of sulfur in shoots and the sulfate to glutathione ratio in the phloem sap (Herschbach et al, 2000).…”