Sulfur assimilation
Encyclopedia
Sulfur
is an essential element
for growth and physiological functioning of plant
s. However, its content strongly varies between plant species
and it ranges from 0.1 to 6 % of the plants' dry weight. Sulfates taken up by the root
s are the major sulfur source for growth, though it has to be reduced to sulfide
before it is further metabolized. Root plastids contain all sulfate
reduction enzymes, but the reduction of sulfate to sulfide
and its subsequent incorporation into cysteine predominantly takes place in the shoot, in the chloroplasts. Cysteine
is the precursor or reduced sulfur donor of most other organic sulfur compounds in plants. The predominant proportion of the organic sulfur is present in the protein
fraction (up to 70 % of total sulfur), as cysteine
and methionine
(two amino acids) residues. Cysteine
and methionine
are highly significant in the structure, conformation and function of proteins. Plants contain a large variety of other organic sulfur compounds, as thiols (glutathione
), sulfolipid
s and secondary sulfur compounds (alliin
s, glucosinolate
s, phytochelatins), which play an important role in physiology
and protection against environmental stress and pests. Sulfur compounds are also of great importance for food quality
and for the production of phyto-pharmaceutics
. Sulfur deficiency
will result in the loss of plant production, fitness and resistance to environmental stress and pests.
is taken up by the root
s that have high affinity. 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 proton gradient generated by 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 gene expression
, and possible functioning the sulfate transporter
s gene family
has been classified in up to 5 different groups. Some groups are expressed exclusively in the roots or shoots or expressed both in the roots and shoots. 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 vacuole
s, 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 nutrition
al 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-acetylserine
, cysteine
, 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.
plastids contain all sulfate reduction enzymes, sulfate reduction takes pre-dominantly place in the leaf chloroplast
s. 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 ATP
sulfurylase, which affinity for sulfate is rather low (Km approximately 1 mM) and the in situ sulfate concentration in the chloroplast is most likely one of the limiting/regulatory steps in sulfur reduction. Subsequently APS is reduced to sulfite, catalyzed by APS reductase with likely glutathione
as reductant. 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.
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 (sulphur 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 amino acids and are of great significance in the structure, conformation and function of proteins and enzymes, but high levels of these amino acids may also be present in seed storage proteins. The thiol groups of the cysteine residues in proteins can be oxidized resulting in disulfide
bridges with other cysteine side chain
s (and form cystine
) and/or linkage of polypeptides. Disulfide bridges (disulfide bond
s) make an important contribution to the structure of proteins. The thiol
groups are also of great importance in substrate binding of enzymes, in metal-sulfur clusters in proteins (e.g. ferredoxin
s) and in regulatory proteins (e.g. thioredoxin
s).
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. The content of glutathione in plant tissue ranges from 0.1 - 3 mM. Cysteine is the direct precursor for the synthesis of glutathione (and its homologues). First, γ-glutamylcysteine is synthesized from cysteine and glutamate catalyzed by gamma-glutamylcysteine synthetase
. Second, glutathione is synthesized from γ-glutamylcysteine and glycine
(in glutathione homologues, β-alanine
or serine
) catalyzed by glutathione synthetase. Both steps of the synthesis of glutathione are ATP dependent reactions. Glutathione is maintained in the reduced form by an 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 selenite
into selenocysteine
. 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 functions as reductant in the enzymatic detoxification of reactive oxygen species in the glutathione-ascorbate cycle and as thiol buffer in the protection of proteins via direct reaction with reactive oxygen species or by the formation of mixed disulfides. The potential of glutathione as protectant is related to the pool size of glutathione, its redox state (GSH/GSSG ratio) and the activity of glutathione reductase
. Glutathione is the precursor for the synthesis of phytochelatins, which are synthesized enzymatically by a constitutive phytochelatin synthase. The number of γ-glutamyl-cysteine residues in the phytochelatins may range from 2 - 5, sometimes up to 11. Despite the fact that the phytochelatins form complexes which a few heavy metals, viz. cadmium
, it is assumed that these compounds play a role in heavy metal homeostasis
and detoxification by buffering of the cytoplasmatic concentration of essential heavy metals. Glutathione is also involved in the detoxification of xenobiotic
s, compounds without direct nutritional value or significance in metabolism, which at too high levels may negatively affect plant functioning. Xenobiotics may be detoxified in conjugation reactions with glutathione catalyzed by glutathione S-transferase
, which activity is constitutive; different xenobiotics may induce distinct isoforms of the enzyme. Glutathione S-transferases have great significance in herbicide
detoxification and tolerance in agriculture and their induction by herbicide antidotes ('safeners') is the decisive step for the induction of herbicide tolerance in many crop plants. 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.
s 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
membrane
s 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.
species contain glucosinolate
s, which are sulfur-containing secondary compounds. Glucosinolates are composed of a β-thioglucose moiety, a sulfonated oxime and a side chain. 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 cysteine) to produce thiohydroximate. The transfer of a glucose
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 isothiocyanate
s, thiocyanate
s, nitrile
s 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-carcinogenic properties.
Allium
species contain γ-glutamylpeptides and alliin
s (S-alk(en)yl cysteine sulfoxides). 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. In onion bulbs 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 transpeptidase
s. 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.
(see also acid rain
) and hydrogen sulfide
, which may affect plant 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 sulfur
gases, which can directly be taken up by the foliage (dry deposition). 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. Sulfur is highly soluble in the apoplast
ic water of the mesophyll
, where it dissociates under formation of bisulfite
and sulfite
. 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 peroxidase
s 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 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.
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.
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
is an essential element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
for growth and physiological functioning of plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s. However, its content strongly varies between plant species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
and it ranges from 0.1 to 6 % of the plants' dry weight. Sulfates taken up by the root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s are the major sulfur source for growth, though it has to be reduced to sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
before it is further metabolized. Root plastids contain all sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
reduction enzymes, but the reduction of sulfate to sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
and its subsequent incorporation into cysteine predominantly takes place in the shoot, in the chloroplasts. Cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
is the precursor or reduced sulfur donor of most other organic sulfur compounds in plants. The predominant proportion of the organic sulfur is present in the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
fraction (up to 70 % of total sulfur), as cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
and methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
(two amino acids) residues. Cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
and methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
are highly significant in the structure, conformation and function of proteins. Plants contain a large variety of other organic sulfur compounds, as thiols (glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
), sulfolipid
Sulfolipid
Sulfolipids are a class of lipids which possess a sulfur-containing functional group. One of the most common consituents of sulfolipids is sulfoquinovose, which is acylated to form sulfoquinovosyl diacylglycerols. In plants, sulfolipids are important intermediates in the sulfur cycle....
s and secondary sulfur compounds (alliin
Alliin
Alliin is a sulfoxide that is a natural constituent of fresh garlic. It is a derivative of the amino acid cysteine. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic...
s, glucosinolate
Glucosinolate
The glucosinolates are a class of organic compounds that contain sulfur and nitrogen and are derived from glucose and an amino acid. They occur as secondary metabolites of almost all plants of the order Brassicales , but also in the genus Drypetes .-Chemistry:Glucosinolates are water-soluble...
s, phytochelatins), which play an important role in physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
and protection against environmental stress and pests. Sulfur compounds are also of great importance for food quality
Food quality
Food quality is the quality characteristics of food that is acceptable to consumers. This includes external factors as appearance , texture, and flavour; factors such as federal grade standards and internal .Food quality in the United States is enforced by the Food Safety Act 1990...
and for the production of phyto-pharmaceutics
Pharmaceutics
Pharmaceutics is the discipline of pharmacy that deals with all facets of the process of turning a new chemical entity into a medication able to be safely and effectively used by patients in the community. Pharmaceutics is the science of dosage form design...
. Sulfur deficiency
Deficiency
A deficiency is generally a lack of something. It may also refer to:*A deficient number, in mathematics, a number n for which σ A deficiency is generally a lack of something. It may also refer to:...
will result in the loss of plant production, fitness and resistance to environmental stress and pests.
Sulfate uptake by plants
SulfateSulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
is taken up by the root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s that have high affinity. 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
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
cells, subsequently loaded into the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
vessels and transported to the shoot by the transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...
stream. The uptake and transport of sulfate is energy dependent (driven by a proton gradient generated by ATPases) through a proton/sulfate co-transport
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...
. 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
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
, since the concentration of sulfate in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within 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 gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
, and possible functioning the sulfate transporter
Transporter
Transporter may refer to:* Transporter , types of vehicles designed to transport items* Volkswagen Transporter, a model of van- Film and television :...
s gene family
Gene family
A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions...
has been classified in up to 5 different groups. Some groups are expressed exclusively in the roots or shoots or expressed both in the roots and shoots. Group 1 are 'high affinity sulfate transporters', which are involved in the uptake of sulfate by the roots. Group 2 are vascular
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
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 vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s, 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 nutrition
Nutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....
al 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-acetylserine
O-Acetylserine
O-Acetylserine is the α-amino acid with the chemical formula HO2CCHCH2OCCH3. It is an intermediate in the biosynthesis of the common amino acid cysteine in bacteria and plants. O-Acetylserine is biosynthesized by acetylation of the serine by the enzyme serine transacetylase...
, cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
, glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
) 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 rootRoot
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
plastids contain all sulfate reduction enzymes, sulfate reduction takes pre-dominantly place in the leaf chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s. The reduction of sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
to sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
occurs in three steps. Sulfate needs to be activated to adenosine 5'-phosphosulfate (APS) prior to its reduction to sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
. The activation of sulfate is catalyzed by ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
sulfurylase, which affinity for sulfate is rather low (Km approximately 1 mM) and the in situ sulfate concentration in the chloroplast is most likely one of the limiting/regulatory steps in sulfur reduction. Subsequently APS is reduced to sulfite, catalyzed by APS reductase with likely glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
as reductant. 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
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
is with high affinity reduced by sulfite reductase
Sulfite reductase
Sulfite reductases are important enzymes in the process of sulfur metabolism. They catalyze the reduction of sulfite to hydrogen sulfide and water. Electrons for the reaction are provided by a dissociable molecule of either NADPH, bound flavins, or ferredoxins.Sulfite reductases belong to the...
to sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
with ferredoxin
Ferredoxin
Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
as a reductant. The remaining sulfate in plant tissue is transferred into the vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
. 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.
Cysteine
SulfideSulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
is incorporated into cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
, catalyzed by O-acetylserine (thiol)lyase, with O-acetylserine as substrate. The synthesis of O-acetylserine is catalyzed by serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
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 (sulphur metabolism) and nitrogen assimilation
Nitrogen assimilation
Nitrogen assimilation is a fundamental biological process that occurs in plants and algae that are incapable of independent nitrogen fixation. The assimilation of nitrogen has marked effects on plant productivity, biomass, and crop yield, and nitrogen deficiency leads to a decrease in structural...
in plants. This differs from the process in yeast, where sulfide must be incorporated first in homocysteine
Homocysteine
Homocysteine is a non-protein amino acid with the formula HSCH2CH2CHCO2H. It is a homologue of the amino acid cysteine, differing by an additional methylene group. It is biosynthesized from methionine by the removal of its terminal Cε methyl group...
then converted in two steps to cysteine.
Cysteine is sulfur donor for the synthesis of methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
, the major other sulfur-containing amino acid present in plants. This happens through the transsulfuration pathway
Transsulfuration pathway
The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine, through the intermediate cystathionine. In eukarotes, such as humans, the transsulfuration pathway is critical for creating cysteine from the essential amino acid methionine...
and the methylation of homocysteine
Homocysteine
Homocysteine is a non-protein amino acid with the formula HSCH2CH2CHCO2H. It is a homologue of the amino acid cysteine, differing by an additional methylene group. It is biosynthesized from methionine by the removal of its terminal Cε methyl group...
. Both cysteine and methionine are sulfur-containing amino acids and are of great significance in the structure, conformation and function of proteins and enzymes, but high levels of these amino acids may also be present in seed storage proteins. The thiol groups of the cysteine residues in proteins can be oxidized resulting in disulfide
Disulfide
In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. Disulfide usually refer to a chemical compound that contains a disulfide bond, such as diphenyl disulfide, C6H5S-SC6H5....
bridges with other cysteine side chain
Side chain
In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called "main chain" or backbone. The placeholder R is often used as a generic placeholder for alkyl group side chains in chemical structure diagrams. To indicate other non-carbon...
s (and form cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...
) and/or linkage of polypeptides. Disulfide bridges (disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
s) make an important contribution to the structure of proteins. The thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
groups are also of great importance in substrate binding of enzymes, in metal-sulfur clusters in proteins (e.g. ferredoxin
Ferredoxin
Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
s) and in regulatory proteins (e.g. thioredoxin
Thioredoxin
Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes. In humans, it is encoded by the TXN gene. Loss-of-function mutation of either of the two human thioredoxin genes is lethal at the four-cell stage of the...
s).
Glutathione
GlutathioneGlutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
or its homologues, e.g. homoglutathione in Fabaceae
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...
; hydroxymethylglutathione in Poaceae
Poaceae
The Poaceae is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes and sedges...
are the major water-soluble non-protein thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
compounds present in plant tissue and account for 1-2 % of the total sulfur. The content of glutathione in plant tissue ranges from 0.1 - 3 mM. Cysteine is the direct precursor for the synthesis of glutathione (and its homologues). First, γ-glutamylcysteine is synthesized from cysteine and glutamate catalyzed by gamma-glutamylcysteine synthetase
Gamma-glutamylcysteine synthetase
Gamma-glutamylcysteine synthetase is the first enzyme in the glutathione biosynthesis pathway.-Function:...
. Second, glutathione is synthesized from γ-glutamylcysteine and glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
(in glutathione homologues, β-alanine
Alanine
Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 amino acids encoded by the genetic code. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid...
or serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
) catalyzed by glutathione synthetase. Both steps of the synthesis of glutathione are ATP dependent reactions. Glutathione is maintained in the reduced form by an NADPH-dependent glutathione reductase
Glutathione reductase
Glutathione reductase, also known as GSR or GR, is an enzyme that reduces glutathione disulfide to the sulfhydryl form GSH, which is an important cellular antioxidant....
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
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
, since the reduction of sulfur occurs predominantly in the chloroplast. Glutathione is directly involved in the reduction and assimilation of selenite
Selenite (ion)
The selenite anion is a selenium oxoanion with the chemical formula SeO32−.A selenite is a compound that contains this ion....
into selenocysteine
Selenocysteine
Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...
. 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
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
, e.g. superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
, hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
and lipid hydroperoxides. Glutathione functions as reductant in the enzymatic detoxification of reactive oxygen species in the glutathione-ascorbate cycle and as thiol buffer in the protection of proteins via direct reaction with reactive oxygen species or by the formation of mixed disulfides. The potential of glutathione as protectant is related to the pool size of glutathione, its redox state (GSH/GSSG ratio) and the activity of glutathione reductase
Glutathione reductase
Glutathione reductase, also known as GSR or GR, is an enzyme that reduces glutathione disulfide to the sulfhydryl form GSH, which is an important cellular antioxidant....
. Glutathione is the precursor for the synthesis of phytochelatins, which are synthesized enzymatically by a constitutive phytochelatin synthase. The number of γ-glutamyl-cysteine residues in the phytochelatins may range from 2 - 5, sometimes up to 11. Despite the fact that the phytochelatins form complexes which a few heavy metals, viz. cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, it is assumed that these compounds play a role in heavy metal homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
and detoxification by buffering of the cytoplasmatic concentration of essential heavy metals. Glutathione is also involved in the detoxification of xenobiotic
Xenobiotic
A xenobiotic is a chemical which is found in an organism but which is not normally produced or expected to be present in it. It can also cover substances which are present in much higher concentrations than are usual...
s, compounds without direct nutritional value or significance in metabolism, which at too high levels may negatively affect plant functioning. Xenobiotics may be detoxified in conjugation reactions with glutathione catalyzed by glutathione S-transferase
Glutathione S-transferase
Enzymes of the glutathione S-transferase family are composed of many cytosolic, mitochondrial, and microsomal proteins. GSTs are present in eukaryotes and in prokaryotes, where they catalyze a variety of reactions and accept endogenous and xenobiotic substrates.GSTs can constitute up to 10% of...
, which activity is constitutive; different xenobiotics may induce distinct isoforms of the enzyme. Glutathione S-transferases have great significance in herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
detoxification and tolerance in agriculture and their induction by herbicide antidotes ('safeners') is the decisive step for the induction of herbicide tolerance in many crop plants. 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
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
degradation products, and in the transport of flavonoids.
Sulfolipids
SulfolipidSulfolipid
Sulfolipids are a class of lipids which possess a sulfur-containing functional group. One of the most common consituents of sulfolipids is sulfoquinovose, which is acylated to form sulfoquinovosyl diacylglycerols. In plants, sulfolipids are important intermediates in the sulfur cycle....
s 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
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s and likely is involved in chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
functioning. The route of biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
and physiological function of sulfoquinovosyl diacylglycerol
Diglyceride
A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....
is still under investigation. From recent studies it is evident that sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
it the likely sulfur precursor for the formation of the sulfoquinovose
Sulfoquinovose
Sulfoquinovose, also known as 6-sulfoquinovose and 6-deoxy-6-sulfo-D-glucopyranose is a monosaccharide sugar that is found as a building block in the sulfolipid sulfoquinovosyl diacylglycerol . Sulfoquinovose is a sulfonic acid derivative of glucose, the sulfonic acid group is introduced into the...
group of this lipid.
Secondary sulfur compounds
BrassicaBrassica
Brassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
species contain glucosinolate
Glucosinolate
The glucosinolates are a class of organic compounds that contain sulfur and nitrogen and are derived from glucose and an amino acid. They occur as secondary metabolites of almost all plants of the order Brassicales , but also in the genus Drypetes .-Chemistry:Glucosinolates are water-soluble...
s, which are sulfur-containing secondary compounds. Glucosinolates are composed of a β-thioglucose moiety, a sulfonated oxime and a side chain. 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 cysteine) to produce thiohydroximate. The transfer of a glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
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
Myrosinase
Myrosinase is a family of enzymes involved in plant defense against herbivores. Their structure has been elucidated and is available.-Myrosinase activity:...
and may yield a variety of biologically active products such as isothiocyanate
Isothiocyanate
Isothiocyanate is the chemical group –N=C=S, formed by substituting sulfur for oxygen in the isocyanate group. Many natural isothiocyanates from plants are produced by enzymatic conversion of metabolites called glucosinolates. These natural isothiocyanates, such as allyl isothiocyanate, are also...
s, thiocyanate
Thiocyanate
Thiocyanate is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...
s, nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
s and oxazolidine-2-thiones. The glucosinolate-myrosinase system is assumed to play a role in plant-herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
and plant-pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
interactions. Furthermore, glucosinolates are responsible for the flavor properties of Brassicaceae
Brassicaceae
Brassicaceae, a medium sized and economically important family of flowering plants , are informally known as the mustards, mustard flowers, the crucifers or the cabbage family....
and recently have received attention in view of their potential anti-carcinogenic properties.
Allium
Allium
Allium is a monocot genus of flowering plants, informally referred to as the onion genus. The generic name Allium is the Latin word for garlic....
species contain γ-glutamylpeptides and alliin
Alliin
Alliin is a sulfoxide that is a natural constituent of fresh garlic. It is a derivative of the amino acid cysteine. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic...
s (S-alk(en)yl cysteine sulfoxides). 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. In onion bulbs 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 transpeptidase
Transpeptidase
A transpeptidase is a bacterial enzyme that cross-links the peptidoglycan chains to form rigid cell walls. This enzyme is also known by several other names including DD-peptidase, DD-transpeptidase, D-alanyl-D-alanine carboxypeptidase and serine-type D-Ala-D-Ala carboxypeptidase...
s. 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
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
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 sulfur dioxideSulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
(see also acid rain
Acid rain
Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...
) and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, which may affect plant metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. Sulfur gases are potentially phytotoxic
Phytotoxicity
Phytotoxicity is a term used to describe the degree of toxic effect by a compound on plant growth. Such damage may be caused by a wide variety of compounds, including trace metals, pesticides, salinity, phytotoxins or allelopathy.-Urea and urine:...
, 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 sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
gases, which can directly be taken up by the foliage (dry deposition). 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. Sulfur is highly soluble in the apoplast
Apoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
ic water of the mesophyll
Mesophyll
Mesophyll can refer to:* Mesophyll tissue, in plant anatomy, photosynthetic parenchyma cells that lie between the upper and lower epidermis layers of a leaf...
, where it dissociates under formation of bisulfite
Bisulfite
Bisulfite ion is the ion HSO3−. Salts containing the HSO3− ion are termed bisulfites also known as sulfite lyes...
and sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
. Sulfite may directly enter the sulfur reduction pathway and be reduced to sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
, incorporated into cysteine, and subsequently into other sulfur compounds. Sulfite may also be oxidized to sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
, extra- and intracellularly by peroxidase
Peroxidase
Peroxidases are a large family of enzymes that typically catalyze a reaction of the form:For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides...
s or non-enzymatically catalyzed by metal ions or superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
radicals
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
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
PPB
PPB can stand for:* Party political broadcast, a type of political programming in the United Kingdom* parts-per-billion, a unit of concentration* Portland Police Bureau, a police agency for the city of Portland...
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.