Sulfur assimilation

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Sulfur assimilation is the process by which living organisms incorporate sulfur into their biological molecules.^[1] In plants, sulfate is absorbed by the roots and then be transported to the chloroplasts by the transipration stream where the sulfur are reduced to sulfide with the help of a series of enzymatic reactions. Furthermore, the reduced sulfur is incorporated into cysteine,^[2] an amino acid that is a precursor to many other sulfur-containing compounds. In animals, sulfur assimilation occurs primarily through the diet, as animals cannot produce sulfur-containing compounds directly. Sulfur is incorporated into amino acids such as cysteine and methionine, which are used to build proteins and other important molecules.^[2]

Sulfate uptake by plants

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Sulfate uptake first occurs in roots that have a high affinity for sulfur. ^[3] 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.^[3]

Sulfate is actively taken up across the

is kept rather constant.

Distinct sulfate transporter proteins mediate the uptake, transport and subcellular distribution of sulfate.[6] The sulfate transporters gene family has been classified in up to 5 different groups according to their cellular and sub-cellular gene expression, and possible functioning.^[7] Each group of transporter proteins may be expressed exclusively in the roots or shoots of the plant, or both.

• Group 1 are 'high affinity sulfate transporters', which are involved in the uptake of sulfate by the roots.
• Group 2 are vascular transporters and are 'low affinity sulfate transporters'.
• Group 3 is the so-called 'leaf group', however, still little is known about the characteristics of this group.
• Group 4 transporters are involved in the efflux of sulfate from the vacuoles, whereas the function of Group 5 sulfate transporters is not known yet, and likely function only as molybdate transporters.

Regulation and expression of the majority of sulfate transporters are controlled by the sulfur

) act as signals in the regulation of sulfate uptake and transport, or in the expression of the sulfate transporters involved.

Sulfate reduction in plants

Sulfate reduction predominantly takes place in the leaf chloroplasts. Here, the reduction of sulfate to sulfide occurs in three steps. Sulfate needs to be activated to adenosine 5'-phosphosulfate (APS) prior to its reduction to sulfite.

The activation of sulfate is catalyzed by

.

The latter reaction is assumed to be one of the primary regulation points in the sulfate reduction, since the activity of APS reductase is the lowest of the enzymes of the sulfate reduction pathway and it has a fast turnover rate. Sulfite is with high affinity reduced by sulfite reductase to sulfide with 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 cysteine, catalyzed by O-acetylserine (thiol)lyase, with O-acetylserine as substrate. The synthesis of O-acetylserine is catalyzed by serine acetyltransferase and together with O-acetylserine (thiol)lyase it is associated as enzyme complex named cysteine synthase.

The formation of cysteine is the direct coupling step between sulfur (sulfur metabolism) and nitrogen assimilation in plants. This differs from the process in yeast, where sulfide must be incorporated first in homocysteine then converted in two steps to cysteine.

Cysteine is sulfur donor for the synthesis of methionine, the major other sulfur-containing amino acid present in plants. This happens through the transsulfuration pathway and the methylation of homocysteine.

Both cysteine and methionine are sulfur-containing

.

Disulfide bridges (

).

Glutathione

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

flavonoids

.

Sulfolipids

Sulfolipids are sulfur containing lipids. Sulfoquinovosyl diacylglycerols are the predominant sulfolipids present in plants. In leaves its content comprises up to 3 - 6% of the total sulfur present. This sulfolipid is present in plastid membranes and likely is involved in chloroplast functioning. The route of biosynthesis and physiological function of sulfoquinovosyl 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

and a sulfate moiety completes the formation of the glucosinolates.

The physiological significance of glucosinolates is still ambiguous, though they are considered to function as sink compounds in situations of sulfur excess. Upon tissue disruption glucosinolates are enzymatically degraded by myrosinase and may yield a variety of biologically active products such as isothiocyanates, thiocyanates, nitriles and oxazolidine-2-thiones. The glucosinolate-myrosinase system is assumed to play a role in plant-herbivore and plant-pathogen interactions.

Furthermore, glucosinolates are responsible for the flavor properties of Brassicaceae and recently have received attention in view of their potential anti-

properties. their content may account for up to 80% of the organic sulfur fraction. Less is known about the content of secondary sulfur compounds in the seedling stage of the plant.

It is assumed that alliins are predominantly synthesized in the leaves, from where they are subsequently transferred to the attached bulb scale. The biosynthetic pathways of synthesis of γ-glutamylpeptides and alliins are still ambiguous. γ-Glutamylpeptides can be formed from cysteine (via γ-glutamylcysteine or glutathione) and can be metabolized into the corresponding alliins via oxidation and subsequent hydrolyzation by γ-glutamyl

.

However, other possible routes of the synthesis of γ-glutamylpeptides and alliins may not be excluded. Alliins and γ-glutamylpeptides are known to have therapeutic utility and might have potential value as phytopharmaceutics. The alliins and their breakdown products (e.g. allicin) are the flavor precursors for the odor and taste of species. Flavor is only released when plant cells are disrupted and the enzyme alliinase from the vacuole is able to degrade the alliins, yielding a wide variety of volatile and non-volatile sulfur-containing compounds. The physiological function of γ-glutamylpeptides and alliins is rather unclear.

Sulfur assimilation in animal

Unlike in plants, animals do not have a pathway for the direct assimilation of inorganic sulfate into organic compounds. In animals, the primary source of sulfur is dietary methionine, an essential amino acid that contains a sulfur atom. Methionine is first converted to S-adenosylmethionine (SAM), a compound that is involved in many important biological processes, including DNA methylation and neurotransmitter synthesis.

SAM can then be used to synthesize other important sulfur-containing compounds such as cysteine, taurine, and glutathione. Cysteine is a precursor for the synthesis of several important proteins and peptides, as well as glutathione, a powerful antioxidant that protects cells from oxidative stress. Taurine is involved in a variety of physiological processes, including osmoregulation, modulation of calcium signaling, and regulation of mitochondrial function.

Sulfur assimiation in microorganisms

In bacteria and fungi, the sulfur assimilation pathway is similar to that in plants, where inorganic sulfate is reduced to sulfide, and then incorporated into cysteine and other sulfur-containing compounds.

Bacteria and fungi can absorb inorganic sulfate from the environment through a sulfate transporter, which is regulated by the presence of sulfate in the medium. Once inside the cell, sulfate is activated by

(APS), which is then reduced to sulfite by APS reductase. Sulfite is further reduced to sulfide by sulfite reductase, which is then incorporated into cysteine by enzyme.

Cysteine, once synthesized, can be used for the biosynthesis of methionine and other important biomolecules. In addition, microorganisms also use sulfur-containing compounds for various other purposes, such as the synthesis of antibiotics.

Sulfur assimilation in microorganisms is regulated by a variety of environmental factors, including the availability of sulfur in the medium and the presence of other nutrients. The activity of key enzymes in the sulfur assimilation pathway is also regulated by feedback inhibition from downstream products, similar to the regulation seen in plants.

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

of the roots is not sufficient.

Plant shoots form a sink for atmospheric

.

Sulfite may directly enter the sulfur reduction pathway and be reduced to sulfide, incorporated into cysteine, and subsequently into other sulfur compounds. Sulfite may also be oxidized to sulfate, extra- and intracellularly by peroxidases or non-enzymatically catalyzed by metal ions or superoxide 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

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.

See also

• Plant nutrition
• Sulfonucleotide reductase
• Sulfur metabolism

References

• 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.