JXB Advance Access originally published online on May 7, 2004
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Journal of Experimental Botany, Vol. 55, No. 401, pp. 1283-1292, June 1, 2004
© 2004 Oxford University Press
FOCUS PAPER |
Molecular analysis and control of cysteine biosynthesis: integration of nitrogen and sulphur metabolism
Received 4 December 2003; Accepted 24 February 2004
Max-Planck-Institut für Molekulare Pflanzenphysiologie, Department of Molecular Physiology, Am Muehlenberg 1, D-14476 Golm, Germany
*To whom the correspondence should be addressed. Fax: +49 331 56789 8247. E-mail: hesse{at}mpimp-golm.mpg.de
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Abstract |
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 Top Abstract IntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Since cysteine is the first committed molecule in plant metabolism containing both sulphur and nitrogen, the regulation of its biosynthesis is critically important. Cysteine itself is required for the production of an abundance of key metabolites in diverse pathways. Plants alter their metabolism to compensate for sulphur and nitrogen deficiencies as best as they can, but limitations in either nutrient not only curb a plant’s ability to synthesize cysteine, but also restrict protein synthesis. Nutrients such as nitrate and sulphate (and carbon) act as signals; they trigger molecular mechanisms that modify biosynthetic pathways and thereby have a profound impact on metabolite fluxes. Cysteine biosynthesis is modified by regulators acting at the site of uptake and throughout the plant system. Recent data point to the existence of nutrient-specific signal transduction pathways that relay information about external and internal nutrient concentrations, resulting in alterations to cysteine biosynthesis. Progress in this field has led to the cloning of genes that play pivotal roles in nutrient-induced changes in cysteine formation.
Key words: Cysteine biosynthesis, nitrate assimilation, O-acetylserine (thiol)lyase, serine acetyltransferase, sulphate, transcriptomics.
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Introduction |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Under natural conditions, plants are confronted with an environment continuously changing with respect to physical, chemical, and biotic parameters (Brunold et al., 1996). Plants have developed strategies resulting in optimal adaptation to environmental resources needed for growth, respiration, and propagation (Rennenberg and Brunold, 1994; Brunold et al., 1996). Among these adaptations, strategic co-ordination of macronutrient assimilation, i.e. carbon, nitrogen, phosphorous, water, and sulphur [‘CHONSP’] is especially important, as these pathways are dependent on the varying availability of mineral nutrients and CO2 and changes in light energy (Fig. 1). These assimilatory pathways interact in the formation of products and are regulated in a co-ordinate manner to balance macro- and micronutrients in possible synergistic or antagonistic effects (Brunold, 1993; Leustek et al., 2000; Saito, 2000). In the past, major efforts have been made to understand nitrogen metabolism in plants. However, since an increased number of reports of sulphur-deficiency affecting yield from various parts of the world became available this has triggered interest in sulphur nutrition. Nitrogen has a favourable position among the essential nutrients in soil. Crop productivity is to a great extent determined by it and hence soil fertility and soil nitrogen have become almost synonymous with each other (Fig. 1). Sulphur interacts with nitrogen in such a way that lack of one reduces the uptake and assimilation of the other (Fig. 2). Clarkson et al. (1989) observed a marked depression in the ability of cereal plants to take up nitrate and ammonium when plants were starved of sulphur, which was accompanied by an increased capacity for sulphur uptake. In turn, excess nitrogen provided as fertilizer remains in the soil and burdens the quality of the groundwater, which is a major problem, at least in Europe (Reuveny et al., 1980; Barney and Bush, 1985; Brunold and Suter, 1984; Bell et al., 1995; Ahmad et al., 1999). Under sulphur-deficient conditions, reduced protein synthesis is accompanied by the accumulation of organic and inorganic nitrogenous compounds. Plants starved of sulphur accumulate arginine and asparagine with reduced levels of sulphur-containing amino acids such as cysteine and methionine (Thomas et al., 2000; Prosser et al., 2001; McCallum et al., 2002; Nikiforova et al., 2003). Barney and Bush (1985) concluded that one nutrient accumulated when the other was limiting and that the accumulated nutrient was used in protein synthesis when the treatment was reversed. Sulphur uptake and assimilation has been shown to be dependent upon the constant supply of the precursor of cysteine, O-acetylserine, which, in turn, is dependent upon adequate nitrogen and carbon availability (Koprivova et al., 2000; Kopriva et al., 2002) (Figs 1, 2). Excess cysteine or another reduced sulphur-compound represses the uptake and assimilation of sulphur, when either sulphur is in excess, or nitrogen is limiting (Zhao et al., 1999). The regulatory interaction between sulphate assimilation and nitrate reduction is believed to occur at the transcriptional level (Prosser et al., 2001).
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Cysteine is incorporated into proteins and glutathione (GSH). Cysteine acts as sulphur donor for methionine (Met) synthesis and, subsequently, for S-adenosylmethionine and S-methylmethionine (Ravanel et al., 1998; Matthews, 1999; Hesse and Hoefgen, 2003). Further more, secondary compounds such as S-methylcysteine, S-alkylcysteine, glucosinolates, and phytoalexins are based on sulphur directly or on cysteine and Met, respectively (Schmidt and Jäger, 1992). Cysteine and Met residues play crucial roles in the secondary and tertiary structures of proteins and contribute to catalytic functions. In particular, cysteine, through the nucleophilic properties of its sulphur atom, acts as a general catalyst in redox reactions, which utilize dithiol–disulphide interchange, as displayed in the thioredoxin and the glutaredoxin systems (Schürmann and Jacquot, 2000; Jacquot et al., 2002). Cysteine, in the low-molecular-weight peptide GSH and derivatives (phytochelatin polymers), plays a critical role in protection against abiotic/biotic stresses (Leustek et al., 2000; Noctor et al., 2002). Finally, plant sulphate assimilation and sulphur amino acid synthesis (cysteine and Met content) are of nutritional importance for animals that lack this capability (Tabe and Higgins, 1998; Hesse et al., 2001; Hoefgen et al., 2001).
Cysteine formation is the result of successive steps starting with sulphate uptake by the respective sulphate transporter, activation of sulphate by covalent binding to ATP via an ATP-sulphurylase-catalysed reaction to form APS, its reduction to sulphite by APS-reductase (APR), and finally the reduction to sulphide by sulphite reductase. Sulphide is then transferred to activated serine by O-acetylserine(thiol)lyase (OASTL, also called cysteine synthase) to form cysteine (Fig. 3). The activated serine, O-acetylserine (OAS) is synthesized by serine acetyltransferase (SAT) which forms a complex with OASTL. Although for ATP-sulphurylase and sulphite reductase pivotal roles in the sulphate assimilation were described, three major processes performed by the sulphate transporter, APR, and SAT, respectively, affect the overall synthesis of cysteine. First, sulphate uptake by roots enables the plant to perceive the inner cellular homeostasis. Second, sulphate reduction increases the flux of intermediates to yield sufficient reduced sulphur; and third, serine trans-acetylation ensures the provision of a carbon/nitrogen backbone for cysteine formation. Alterations to any of these three processes can have profound effects on cysteine biosynthesis and on the capacity of plants to grow in soils in which nutrient resources are limiting.
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Plants that are grown with insufficient levels of sulphate develop symptoms of sulphur deficiency, which include chlorosis of young leaves, growth retardation, and altered root morphology (Hawkesford, 2000; López-Bucio et al., 2003; Nikiforova et al., 2003). This indicates that an insufficient sulphur supply firstly results in a reduced amount of cysteine being produced at the growing point of the plant where proteins are synthesized at high rates. Transport rates from source organs thus seem to be insufficient to compensate for this limitation. By contrast, the symptoms of nitrogen deficiency, i.e. chlorosis appearing first in older leaves, suggest that plants are able to mobilize and reallocate nitrogen from existing (protein) stores more efficiently than they are able to remobilize assimilated sulphur under sulphur deficiency.
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Sulphate transporter: uptake and translocation |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Sulphate transporters form a gene family classified into five groups depending on their apparent function (Hawkesford, 2003). Their functional characteristics and patterns of regulation, together with localization data, suggest that these groups have specific roles, such as high affinity uptake in the roots, translocation in vascular tissues, and cell to cell transfer in leaves and seeds. In addition, some members of the sulphate transporter family may have discrete subcellular locations in the plastid or tonoplast membranes. Due to their differential expression one might assume that sulphate transporters contribute to the control of sulphate fluxes in the plant throughout development. Furthermore, the existence of a family of sulphate transporters, with specific occurrence in different tissues, and with differential responsiveness to sulphur-supply, supports the idea that sulphate transporters have an important role in whole plant sulphur-management. Systematic analysis of these transporters in terms of function and expression is in progress. The expression of genes encoding different sulphate transporters is regulated by signals that respond to the nutrient status of the plants (sulphur-supply). When the external supply of sulphate is affecting the internal concentrations of sulphate, cysteine and glutathione decline, while a rapid increase in mRNA transcripts is observed. Sulphur re-supply decreased the gene expression of sulphate transporters (Smith et al., 1997; Bolchi et al., 1999; Lappartient et al., 1999; Vidmar et al., 1999; Takahashi et al., 2000). These data suggest that the expression of sulphate transporters is induced if the intracellular ‘sulphur status’ is low, giving a hint that reduced sulphur in the form of glutathione or cysteine might act negatively on the expression of sulphate transporters and ATP-sulphurylase in sulphur-starved plants. These data support a model in which, under sulphur-sufficient conditions, metabolites such as cysteine and glutathione act as regulators of sulphur-uptake and assimilation at the level of gene expression. While under sulphur-deficient conditions the decreasing levels of these compounds remove repression and thus result in increased transporter activity with maximized sulphate uptake.
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APS reductase: sulphate reduction |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Sulphate reduction is initiated and carried out by the enzyme APS reductase in leaf and root plastids. The amino acid structure of plant APS reductase revealed a multidomain composition (Suter et al., 2000). The amino terminal domain of the mature protein is homologous to PAPS reductase and the carboxyl terminal domain is homologous to thioredoxin, a redox enzyme. APS reductase is able to use GSH or dithiothreitol as an electron source. APS reductase is thought to be one of the key regulators of the sulphate reduction pathway. Its activity and steady-state mRNA level increased markedly and co-ordinately in response to sulphate starvation (Gutierrez-Marcos et al., 1996; Takahashi et al., 1997; Yamaguchi et al., 1999), oxidative stress (Leustek et al., 2000), or heavy metal exposure (Heiss et al., 1999). The latter two stresses increase the demand for glutathione, and hence, the cysteine necessary for glutathione synthesis. Other sulphate assimilatory enzymes are regulated to a lesser degree (ATP sulphurylase) or are constitutively expressed (sulphite reductase) (Bork et al., 1998). On the other hand, APR activity and transcript levels were decreased if cysteine and GSH were fed in excess (Vauclare et al., 2002). This result implies that increased internal cysteine and GSH levels might control sulphate assimilation. From split-root experiments it could be concluded that GSH, and not cysteine, is acting as a signal (Lappartient and Touraine, 1996; Lappartient et al., 1999).
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O-acetylserine (thiol)lyase/serine acetyltransferase complex formation |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Two enzymes catalyse the final step of cysteine biosynthesis. Serine acetyltransferase (SAT) generates an activated serine derivative, O-acetylserine (OAS) in the presence of acetyl-CoA and L-serine and O-acetylserine (thiol)lyase (OASTL), a pyridoxal phosphate-dependent enzyme, transfers reduced sulphide to OAS in a ß-replacement reaction that yields cysteine and acetate (Bogdanova and Hell, 1997; Leustek et al., 2000; Saito, 2000). Therefore, these two reactions represent the major link between nitrogen/carbon and sulphate assimilation (Figs 2, 3). Both enzymes have been demonstrated to be present in three plant cell compartments: (a) the chloroplast; (b) the cytosol; and (c) the mitochondrion (Hoefgen et al., 2001) (Fig. 3). Biochemical and molecular approaches showed that in plants, SAT and OASTL are associated in a multi-enzyme complex called cysteine synthase as first described in Salmonella thyphimurium and Escherichia coli, (Kredich, 1993; Bogdanova and Hell, 1997; Wirtz et al., 2001; Berkowitz et al., 2002). The current understanding of the complex formation is that OASTL bound to SAT is inactive in the synthesis of cysteine, but causes the stabilization of SAT while SAT is only active if bound in the complex. The OAS intermediate catalysed by the OASTL–bound SAT disrupts the bi-enzyme complex and OASTL is released to convert OAS to cysteine. The reversible formation of the cysteine synthase complex might be part of a sensor system and can thus be discussed as the regulatory centre of this pathway wherein increasing levels of OAS also regulate the complex formation and cysteine formation because only the free OASTL enzyme is able to do this efficiently (Hawkesford, 2000; Saito, 2000; Berkowitz et al., 2002; Hell et al., 2002). OAS was thought to be a dominant limiting factor for cysteine synthesis, and presumably, as in bacteria, a signal molecule for the regulation of the gene network from sulphate uptake to cysteine biosynthesis (Hawkesford and Wray, 2000; Leustek et al., 2000; Saito, 2000). These observations led to the proposal that SAT, catalysing production of the nitrogen/carbon precursor for cysteine synthesis, could be regulated at a metabolic level by cysteine concentrations in cellular compartments (Hawkesford and Wray, 2000; Leustek et al., 2000; Saito, 2000). From biochemical and molecular studies, the regulation of either cytosolic or chloroplastic SATs was reported for different plant species, including the plant model Arabidopsis thaliana and plants as Citrullis vulgaris, Allium tuberosum, Spinacia oleracea, and Phaseolus vulgaris (Smith, 1972; Brunold and Suter, 1982; Inoue et al., 1999; Noji et al., 1998, 2001; Urano et al., 2000; Noji and Saito, 2002). On the other hand, regulation through phosphorylation, involving a large family of calcium-regulated protein kinases, has been described for a soybean cytosolic SAT in response to cysteine levels (Yoo and Harmon, 1997; Saito, 2000).
A further aspect in understanding cysteine synthesis is the subcellular localization of both enzymes. Both enzymes exist in several compartments (Hoefgen et al., 2001). The presence of isoforms in the cytosol, plastids, and mitochondria suggests that the ability to form cysteine is essential for all compartments with the ability for protein biosynthesis (Fig. 3). However, their single contributions to the net cysteine synthesis and any functional interactions that may occur between these subcellular locations are unknown. Interestingly, only A. thaliana seems to possess a mitochondrial localized OASTL (Hesse et al., 1999), while in other plants, such as spinach, ß-cyanoalanine synthase (CAS) substitutes for this function (Saito et al., 1994; Hatzfeld et al., 2000; Warrilow and Hawkesford, 1998, 2000). In this context it is important to note that in A. thaliana mitochondria CAS exists in addition to OASTL (Hatzfeld et al., 2000). Further molecular–biochemical studies have to be performed to verify the interaction of CAS with SAT to prove the above conducted regulatory model of cysteine synthesis. So far there is evidence that the mitochondrial OASTL isoform is able to form a complex with SAT (Jost et al., 2000).
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OAS: co-ordination with nitrogen metabolism |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Recent publications have shown that there is regulatory interaction between assimilatory sulphate and nitrate reduction in plants that requires the cellular homeostasis of certain metabolites to be maintained (Yamaguchi et al., 1999; Koprivova et al., 2000). Early studies have shown that the activities of ATP sulphurylase, APR, and OASTL decreased in Lemna minor and cultured tobacco cells under nitrogen-deficient conditions (Reuveny et al., 1980; Smith, 1980; Brunold and Suter, 1984). The replenishment of the nitrogen-deficient medium by nitrate or ammonia restored the activities of these enzymes. Ammonia also increased the flow of sulphur-assimilation intermediates, measured as the radioactive sulphate incorporated into proteins (Brunold and Suter, 1984). Other nitrogen sources, such as amino acids (Arg, Asn, Gln), could restore up to 110% of the extractable APR activity, but had no impact on ATP sulphurylase or OASTL in L. minor (Brunold and Suter, 1984; Suter et al., 1986). Nitrogen-deprivation in A. thaliana for up to 3 d resulted in a decrease of APR activity to 30% in roots correlating with a decrease in mRNA and protein levels. Cysteine and glutathione contents were not affected by this treatment (Koprivova et al., 2000). Sulphur-deprivation resulted, in turn, in a reduction of nitrate reductase activity and an accumulation of amino acids (Reuveny et al., 1980; Migge et al., 2000; Prosser et al., 2001). However, changes of nitrate reductase activity and mRNA levels might be a late adaptation to sulphur-deficiency (Prosser et al., 2001).
Recent reports assign OAS an extraordinary position in the pathway connecting nitrogen with sulphur metabolism (Hawkesford and Wray, 2000; Leustek et al., 2000; Hawkesford, 2003) (Figs 2, 4). OAS serves not only as the carbon backbone for cysteine formation, but is also a positive regulator of sulphate uptake and assimilation. OAS seems to be limiting for cysteine synthesis in the presence of sulphate, as shown by overexpression of serine acetyltransferase resulting in increased levels of cysteine and glutathione in transgenic tobacco and potato plants (Blaszczyk et al., 1999; Harms et al., 2000; Wirtz and Hell, 2003). On the other hand, OAS accumulates during sulphur starvation (Kim et al., 1999; Awazuhara et al., 2000; Nikiforova et al., 2003) and acts positively on the transcript and activity levels of sulphate transporters, ATP sulphurylase, APR, sulphite reductase, plastidial OASTL, and cystosolic SAT, as shown in OAS-feeding experiments (Clarkson et al., 1999; Bolchi et al., 1999; Koprivova et al., 2000; Saito, 2000; Hawkesford, 2003). Based on these results, Hawkesford (2000) proposed a model in which the expression of genes involved in uptake and assimilation are under positive regulation by OAS (Fig. 4). In a later phase of sulphur-starvation, OAS starts to accumulate when insufficient sulphate is available to utilize OAS for cysteine synthesis. This model might reflect the metabolic regulation on a cellular level but not on a plant level. OAS fed to roots revealed that, for example, APR responded only locally to the inducer and not in other tissues (Hesse et al., 2003). Other sulphur-compounds than OAS, such as glutathione, are discussed as mediating the signal via the phloem (Lappartient et al., 1999).
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Transcriptome approaches |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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The regulation of plant metabolic processes in response to environmental and developmental signals is a complex interaction between optimization of enzyme activity and transcriptional regulation of gene expression. The development of new platform technologies such as transcriptomics, proteomics, and metabolomics have allowed a deeper insight into cellular mechanisms of metabolic adaptation on the molecular level, but also the reconstruction of cellular networks. In future, bioinformatic analysis of –omics data will allow modelling of various states that are characteristic of the respective nutrient status. Depictions of transcriptomes have recently been published with respect to altered concentrations of sulphur (Hirai et al., 2003; Maruyama-Nakashita et al., 2003; Nikiforova et al., 2003), nitrogen (Wang et al., 2000, 2001, 2002, 2003; Colebatch et al., 2002;), phosphate (Wang et al., 2002; Maathuis et al., 2003), iron (Thimm et al., 2001; Negishi et al., 2002), potassium (Wang et al., 2002), and others (Harmer et al., 2000; Kreps et al., 2002; Seki et al., 2002; Hammond et al., 2003; Oono et al., 2003).
Molecular mechanisms for the responses to sulphur deprivation in higher plants were recently investigated by using transcriptome approaches (Nikiforova et al., 2003; Hirai et al., 2003; Maruyama-Nakashita et al., 2003). Here, the focus is set on the adaptation of plants to changing sulphur and nitrogen supply. The data presented in Table 1 compile the expression levels of sulphur and nitrogen assimilatory pathway genes of A. thaliana in response to alterations in sulphate or in nitrate supply. The use of full genome arrays allows a complete network to be deduced in response to sulphate (B Gakière et al., personal communication, MPI-MP) and nitrate (Wang et al., 2003) availability.
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A set of genes of sulphur metabolism is induced by nitrate. Two putative sulphate transporter genes and an APR gene are induced in roots. The data in Table 1 suggest that nitrate itself can induce genes of sulphate uptake and assimilation and, thereby, may increase sulphate assimilation rates or capacity. They also suggest that, in roots, the absence of nitrate can down-regulate the capture and assimilation of sulphate by the roots.
A linkage between sulphur and nitrogen metabolism has been known for many years, with a deprivation of one leading to a reduction of the metabolism of the other (Reuveny et al., 1980; Prosser et al., 2001). It has also been shown in barley (Hordeum vulgare) that high levels of nitrate and ammonium can induce a high affinity sulphate transporter gene and hence sulphate uptake in nitrogen-fed plants suggesting that a nitrogen metabolite may affect sulphate transporter gene expression (Vidmar et al., 1999).
In shoots, as in roots, nitrate induces similar genes that are involved in nitrate uptake and assimilation, but to a lesser extent. Among the genes involved in sulphur metabolism, one encoding a SAT is induced by nitrate. This means that under nitrate re-supply leaf cells can synthesize OAS at high rates to ensure the amino acid balance under increased nitrogen-assimilation. Hence, it can be speculated that in leaves, cysteine synthesis is impaired under nitrogen deficiency.
Although deprivation of nitrogen leads to a disruption of sulphur metabolism, the effect of sulphur depletion on nitrogen metabolism is much less evident. In roots, sulphate depletion did not significantly affect nitrate uptake and assimilation when it occurs for a short period. Longer sulphate starvation stimulates the accumulation of glutamine as a nitrogen store probably because of limited protein synthesis under sulphur deficiency. It has to be noted for understanding the regulatory system that excess nitrogen, i.e. the cytotoxic ammonium as the end-product of the nitrogen reduction pathway must be stored as nitrogenous organic compounds. Sulphate on the other hand can be stored in vacuoles and reduced sulphate accumulates as glutathione to certain upper levels or can even be released as volatiles.
In leaves, therefore, long periods of sulphur starvation activate transcription of sulphate transporters that may be involved in sulphate remobilization through the vasculature. Glutamine synthesis is induced to counteract ammonium intoxication. Sulphate induces SAT and other genes involved in nitrate transport and assimilation. This means that when sulphur becomes available again, nitrate uptake is enhanced and reduced nitrogen is bound in transportable mass amino acids, such as asparagine, to be allocated to different plant organs. The latter case might also be due to the fact that the carbon backbone derived from aspartate is then no longer directed to the amino acids of the aspartate family but to asparagine synthesis. Furthermore, the branch of nitrogen metabolism leading to OAS synthesis is re-induced.
Generally, one can speculatively conclude that the interrelationship of sulphur and nitrogen metabolism is of a hierarchical nature, that of nitrogen having priority over that of sulphur. The difference is probably due to the relative molar needs of both elements, being higher for nitrogen, and for the different toxicity of the intermediates, end-products, and storage forms as well as subcellular allocations of the nutrient ions.
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Future prospects |
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TopAbstractIntroductionSulphate transporter: uptake and...APS reductase: sulphate...O-acetylserine...OAS: co-ordination with nitrogen...Transcriptome approachesFuture prospectsReferences |
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Cysteine biosynthesis has been studied for about two decades using whole plants, or excised tissues, cell cultures, and cell-free extracts. Molecular tools were developed leading to an improved understanding of biosynthetic mechanisms and regulation of cysteine formation. A prerequisite for such an approach were studies on E. coli, Neurospora crassa, yeast, and Chlamydomonas reinhardii from which regulatory genes governing sulphur assimilation were isolated and networks of genetic regulation have been partially clarified. It then could be shown that related regulatory networks are present in higher plants. Modern platform techniques such as transcriptomics, metabolomics, and proteomics will help to refine current models and to understand the interconnection between different pathways, such as sulphur and nitrogen. These techniques will allow the identification of common responses that are shared in primary signal-transduction pathways or which reflect changes in metabolism that secondarily affect other processes. It is clear, however, that conclusions based solely on, for example, microarray data will not be sufficient to unravel metabolic networks. In future, the compilation of various –omics data will enable interpretation and, subsequently allow cellular network modelling. This is what can be termed ‘systems biology’.
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Acknowledgements |
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We thank Megan McKenzie for editing the manuscript. The authors’ work has been supported by the European Union (Bio4CT 97-2182, QLRT-2000-00103 and QLK5-CT-2002-51590), the Deutsche Forschungsgemeinschaft, DFG, and the Max Planck Society.
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