Sulfur assimilation

Sulfate uptake by plants

Sulfate is taken up by the roots with high affinity and the maximal sulfate uptake rate is generally already reached at sulfate levels of 0.1 mM and lower. The uptake of sulfate by the roots and its transport to the shoot is strictly controlled and it appears to be one of the primary regulatory sites of sulfur assimilation. Sulfate is actively taken up across the plasma membrane of the root cells, subsequently loaded into the xylem vessels and transported to the shoot by the transpiration stream. The uptake and transport of sulfate is energy dependent (driven by a ATPases) through a proton/sulfate co-transport. In the shoot the sulfate is unloaded and transported to the chloroplasts where it is reduced. The remaining sulfate in plant tissue is predominantly present in the vacuole, since the concentration of sulfate in the cytoplasm is kept rather constant. Distinct sulfate transporter proteins mediate the uptake, transport and subcellular distribution of sulfate. According to their cellular and subcellular plastids prior to its reduction, whereas the function of Group 5 sulfate transporters is not known yet. Regulation and expression of the majority of sulfate transporters are controlled by the sulfur nutritional status of the plants. Upon sulfate deprivation, the rapid decrease in root sulfate is regularly accompanied by a strongly enhanced expression of most sulfate transporter genes (up to 100-fold), accompanied by a substantially enhanced sulfate uptake capacity. It is not yet solved, whether sulfate itself or metabolic products of the sulfur assimilation (O-acetyl-serine, glutathione) act as signals in the regulation of sulfate uptake by the root and its transport to the shoot, and in the expression of the sulfate transporters involved.

Sulfate reduction in plants

Even though root ferredoxin as a reductant. The remaining sulfate in plant tissue is transferred into the vacuole. The remobilization and redistribution of the vacuolar sulfate reserves appear to be rather slow and sulfur-deficient plants may still contain detectable levels of sulfate.

Synthesis and function of sulfur compounds in plants

Cysteine

Sulfide is incorporated into nitrogen assimilation in plants. Cysteine is sulfur donor for the synthesis of ferredoxins) and in regulatory proteins (e.g. thioredoxins).

Glutathione

NADPH-dependent glutathione reductase and the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) generally exceeds a value of 7. Glutathione fulfils various roles in plant functioning. In sulfur metabolism it functions as reductant in the reduction of APS to sulfite. It is also the major transport form of reduced sulfur in plants. Roots likely largely depend for their reduced sulfur supply on shoot/root transfer of glutathione via the phloem, since the reduction of sulfur occurs predominantly in the chloroplast. Glutathione is directly involved in the reduction and assimilation of flavonoids.

Sulfolipids

Sulfoquinovosyl diacylglycerol is the predominant sulfur-containing diacylglycerol is still under investigation. From recent studies it is evident that sulfite it the likely sulfur precursor for the formation of the sulfoquinovose group of this lipid.

Secondary sulfur compounds

Brassica species contain carcinogenic properties. Allium species contain γ-glutamylpeptides and alliins (S-alk(en)yl cysteine sulfoxides). The content of these sulfur-containing volatile sulfur-containing compounds. The physiological function of γ-glutamylpeptides and alliins is rather unclear.

Sulfur metabolism in plants and air pollution

The rapid economic growth, industrialization and urbanization are associated with a strong increase in energy demand and emissions of air pollutants including metabolism. Sulfur gases are potentially phytotoxic, however, they may also be metabolized and used as sulfur source and even be beneficial if the sulfur fertilization of the roots is not sufficient. Plant shoots form a sink for atmospheric radicals and subsequently reduced and assimilated again. Excessive sulfate is transferred into the vacuole; enhanced foliar sulfate levels are characteristic for exposed plants. The foliar uptake of hydrogen sulfide appears to be directly dependent on the rate of its metabolism into cysteine and subsequently into other sulfur compounds. There is strong evidence that O-acetyl-serine (thiol)lyase is directly responsible for the active fixation of atmospheric hydrogen sulfide by plants. Plants are able to transfer from sulfate to foliar absorbed atmospheric sulfur as sulfur source and levels of 60 ppb or higher appear to be sufficient to cover the sulfur requirement of plants. There is an interaction between atmospheric and pedospheric sulfur utilization. For instance, hydrogen sulfide exposure may result in a decreased activity of APS reductase and a depressed sulfate uptake.

Sources

Schnug, E. (1998) Sulfur in Agroecosystems. Kluwer Academic Publishers, Dordrecht, 221 pp, ISBN 0-7923-5123-1.

Grill, D., Tausz, M. and De Kok, L.J. (2001) Significance of Glutathione to Plant Adaptation to the Environment. Kluwer Academic Publishers, Dordrecht, ISBN 1-4020-0178-9.

Abrol Y.P. and Ahmad A. (2003) Sulphur in Plants. Kluwer Academic Publishers, Dordrecht, ISBN 1-4020-1247-0.

Saito, K., De Kok, L.J., Stulen, I., Hawkesford, M.J., Schnug, E., Sirko, A. and Rennenberg, H. (2005) Sulfur Transport and Assimilation in Plants in the Post Genomic Era. Backhuys Publishers, Leiden, ISBN 90-5782-166-4.

Hawkesford, M.J. and De Kok, L.J. (2006) Managing sulfur metabolism in plants. Plant Cell and Environment 29: 382-395.