Sulfur assimilation
[1] In plants, sulfate is absorbed by the roots and then transported to the chloroplasts by the transipration stream where the sulfur are reduced to sulfide with the help of a series of enzymatic reactions.
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 plasma membrane of the root cells, subsequently loaded into the xylem vessels and transported to the shoot by the transpiration stream.
Glutathione or its homologues, e.g. homoglutathione in Fabaceae; hydroxymethylglutathione in Poaceae are the major water-soluble non-protein thiol compounds present in plant tissue and account for 1-2% of the total sulfur.
Furthermore, glutathione is of great significance in the protection of plants against oxidative and environmental stress and it depresses/scavenges the formation of toxic reactive oxygen species, e.g. superoxide, hydrogen peroxide and lipid hydroperoxides.
Glutathione is also involved in the detoxification of xenobiotics, compounds without direct nutritional value or significance in metabolism, which at too high levels may negatively affect plant functioning.
Under natural conditions glutathione S-transferases are assumed to have significance in the detoxification of lipid hydroperoxides, in the conjugation of endogenous metabolites, hormones and DNA degradation products, and in the transport of flavonoids.
The synthesis of glucosinolates starts with the oxidation of the parent amino acid to an aldoxime, followed by the addition of a thiol group (through conjugation with glutathione) to produce thiohydroximate.
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 content of these sulfur-containing secondary compounds strongly depends on stage of development of the plant, temperature, water availability and the level of nitrogen and sulfur nutrition.
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 transpeptidases.
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.
Methionine is first converted to S-adenosylmethionine (SAM), a compound that is involved in many important biological processes, including DNA methylation and neurotransmitter synthesis.
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.
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.
The rapid economic growth, industrialization and urbanization are associated with a strong increase in energy demand and emissions of air pollutants including sulfur dioxide (see also acid rain) and hydrogen sulfide, which may affect plant metabolism.
The foliar uptake of sulfur dioxide is generally directly dependent on the degree of opening of the stomates, since the internal resistance to this gas is low.
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.
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.
For instance, hydrogen sulfide exposure may result in a decreased activity of APS reductase and a depressed sulfate uptake.
