Cross-talk between sulfur assimilation and ethylene signaling in plants

Sulfur (S) deficiency is prevailing all over the world and becoming an important issue for crop improvement through maximising its utilization efficiency by plants for sustainable agriculture. Its interaction with other regulatory molecules in plants is necessary to improve our understanding on its role under changing environment. Our knowledge on the influence of S on ethylene signaling is meagre although it is a constituent of cysteine (Cys) required for the synthesis of reduced glutathione (GSH) and S-adenosyl methionine (SAM), a precursor of ethylene biosynthesis. Thus, there may be an interaction between S assimilation, ethylene signaling and plant responses under optimal and stressful environmental conditions. The present review emphasizes that responses of plants to S involve ethylene action. This evaluation will provide an insight into the details of interactive role of S and ethylene signaling in regulating plant processes and prove profitable for developing sustainability under changing environmental conditions.     

Molecular mechanisms of plant metal tolerance and homeostasis

Transition metals such as copper are essential for many physiological processes yet can be toxic at elevated levels. Other metals (e.g. lead) are nonessential and potentially highly toxic. Plants – like all other organisms – possess homeostatic mechanisms to maintain the correct concentrations of essential metal ions in different cellular compartments and to minimize the damage from exposure to nonessential metal ions. A regulated network of metal transport, chelation, trafficking and sequestration activities functions to provide the uptake, distribution and detoxification of metal ions. Some of the components of this network have now been identified: a number of uptake transporters have been cloned as well as candidate transporters for the vacuolar sequestration of metals. Chelators and chaperones are known, and evidence for intracellular metal trafficking is emerging. This recent progress in the molecular understanding of plant metal homeostasis and tolerance is reviewed. Received: 14 July 2000 / Accepted: 22 September 2000     

Thiosulfate, polythionates and elemental sulfur assimilation and reduction in the bacterial world

Abstract Among sulfur compounds, thiosulfate and polythionates are present at least transiently in many environments. These compounds have a similar chemical structure and their metabolism appears closely related. They are commonly used as energy sources for photoautotrophic or chemolithotrophic microorganisms, but their assimilation has been seldom studied and their importance in bacterial physiology is not well understood. Almost all bacterial strains are able to cleave these compounds since they possess thiosulfate sulfur transferase, thiosulfate reductace or S -sulfocysteine synthase activities. However, the role of these enzymes in the assimilation of thiosulfate or polythionates has not always been clearly established.
Elemental sulfur is, on the contrary, very common in the environmental. It is an energy source for sulfur-reducing eubacteria and archaebacteria and many sulfur-oxidizing archaebacteria. A phenomenon still not well understood is the 'excessive assimilatory sulfur metabolism' as observed in methanogens which perform a sulfur reduction which exceeds their anabolic needs without any apparent benefit. In heterotrophs, assimilation of elemental sulfur is seldom described and it is uncertain whether this process actually has a physiological significance.
Thus, reduction of thiosulfate and elemental sulfur is a common by incompletely understood feature among bacteria. These activities could give bacteria a selective advantage, but futher investigations are needed to clarify this possibility. Presence of thiosulfate, polythionates and sulfur reductase activities does not imply obligatorily that these activities play a role in thiosulfate, polythionates or sulfur assimilation as these compounds could be merely intermediates in bacterial metabolism. The possibility also exists that the assimilation of these sulfur compounds is just a side effect of an enzymatic activity with a completely different function.     

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).     

Integrating fluctuating nitrate uptake and assimilation to robust homeostasis

Nitrate is an important nitrogen source used by plants. Despite of the considerable variation in the amount of soil nitrate, plants keep cytosolic nitrate at a homeostatic controlled level. Here we describe a set of homeostatic controller motifs and their interaction that can maintain robust cytosolic nitrate homeostasis at fluctuating external nitrate concentrations and nitrate assimilation levels. The controller motifs are divided into two functional classes termed as inflow and outflow controllers. In the presence of high amounts of environmental nitrate, the function of outflow controllers is associated to efflux mechanisms removing excess of nitrate from the cytosol that is taken up by low-affinity transporter systems (LATS). Inflow controllers on the other hand maintain homeostasis in the presence of a high demand of nitrate by the cell relative to the amount of available environmental nitrate. This is achieved by either remobilizing nitrate from a vacuolar store, or by taking up nitrate by means of high-affinity transporter systems (HATS). By combining inflow and outflow controllers we demonstrate how nitrate uptake, assimilation, storage and efflux are integrated to a regulatory network that maintains cytosolic nitrate homeostasis at changing environmental conditions.     

The production and utilization of intermediary elemental sulfur during the oxidation of reduced sulfur compounds by Thiobacillus ferrooxidans

The intermediary production of elemental sulfur during the microbial oxidation of reduced sulfur compounds has frequently been reported. Thiobacillus ferrooxidans, an acidophilic chemolithoautotroph, was found to produce an insoluble sulfur compound, primarily elemental sulfur, during the oxidation of thiosulfate, trithionate, tetrathionate and sulfide. This was confirmed by light and electron microscopy. Sulfur was produced from sulfide by an oxidative step, while the production from tetrathionate was initiated by a hydrolytic step, probably followed by a series of chemical reactions. The oxidation of intermediary sulfur was severely inhibited by sulfhydryl-binding reagents such as N-ethylmaleimide, by the addition of uncouplers or after freezing and thawing of the cells, which probably damaged the cell membrane. The mechanisms behind these inhibitions have not yet been clarified. Finally, it was observed that elemental sulfur oxidation by whole cells depended on the medium composition. The absence of sulfate or selenate reduced the sulfur oxidation rate.Non-standard abbreviations NEM N-ethylmaleimide - CCCP carbonyl cyanide m-chlorophenyl hydrazone     

Alterations in nitrogen assimilation and partitioning in nitrogen stressed plants

Although nutrient stress is known to alter partitioning between shoots and roots, the physiological basis for the phenomenon is unresolved. Experiments were conducted to examine assimilation of ¹⁵NO₃ by N-stressed plants and to determine whether apparent changes in assimilation in the root contributed to alterations in whole-plant partitioning of reduced-N. Tobacco plants (Nicotiana tabacum L. cv. NC 2326) were exposed to a low concentration of NO₃^? in solution (80 μM) for 9 days to effect a N-stress response. Exposure of plants to 1000 μM¹⁵NO₃^? for 12 h on selected days revealed that roots of N-stressed plants developed an increased capacity to absorb NO₃^?, and accumulation of reduced-¹⁵N in the root increased to an even greater extent. When plants were exposed to 80 or 1000 μM¹⁵NO₃^? in steady-state, ¹⁵NO₃^? uptake over a 12 h period was noticeably restricted at the lower concentration, but a larger proportion of the absorbed ¹⁵N still accumulated as reduced-¹⁵N in the root. The alteration in reduced-¹⁵N partitioning was maintained in N-stressed plants during the subsequent 3-day “chase” period when formation of insoluble reduced-¹⁵N in the root was quantitatively related to the disappearance of ¹⁵NO₃^? and soluble reduced-¹⁵N. The results indicate that increased assimilation of absorbed NO₃^?, in the root may contribute significantly to the altered reduced-N partitioning which occurs in N-stressed plants.     

Endocrine regulation of trace element homeostasis in the rat

Conclusion The mechanisms responsible for the modifications of the concentrations of elements in tissues induced by hormonal influences are complex, because each hormone has pleiotropic actions that are sometimes opposite. Among these modifications, it is often difficult to distinguish between the adaptive defensive changes and the changes responsible for disorders or for exacerbating them, for example, Fe accumulation in tissues after HPX, TPTX, and castration. Moreover, in clinical conditions, hormonal disturbances are often enhanced by other defects, such as an excessive or a deficient intake of elements.     

Assimilatory sulfur metabolism in Rhodopseudomonas sulfoviridis

Rhodopseudomonas sulfoviridis is unable to grow with sulfate as sole sulfur source. Radioactively labelled sulfate is not incorporated into the cells. Growth only occurs in the presence of reduced sulfur compounds, such as sulfide, thiosulfate, elemental sulfur and cysteine. ATP sulfurylase, adenylylsulfate kinase, O-acetylserine sulfhydrylase and cysteine desulfhydrase are present. Adenylylsulfate sulfotransferase and thiosulfonate reductase are lacking. The enzymes of the sulfate-activating system are not derepressed by O-acetylserine.Non common Abbreviations APS Adenosine 5-phosphosulfate - PAPS 3-phosphoadenosine 5-phosphosulfate     

Effect of N source during soybean pod filling on nitrogen and sulfur assimilation and remobilization

During pod filling, a grain legume remobilizes vegetative nitrogen and sulfur to its developing fruit. This study was conducted to determine whether different nitrogen sources affected N and S assimilation and remobilization during pod filling. Well-nodulated plants fed 1.0 mM KNO₃, 0.5 mM urea, or 2.5 mM urea assimilated 0%, 37%, or 114% more N, respectively, and 25%, 46%, or 56% more S, respectively, than did the average non-nodulated control plant fed 5.0 mM KNO₃. Thus, N source during pod filling greatly affected both N and S assimilation. Depending upon N source, plant N concentration during pod filling decreased from 2.96% to between 1.36% and 1.82%. Non-nodulated control plants fed 5.0 mM KNO₃ had the highest residual N at harvest. During the same treatments, plant S concentration decreased from 0.246% to a relatively uniform 0.215%. Thus, during pod filling, vegetative N was seemingly remobilized more efficiently (38–54%) than was S (13%). N source also affected seed yield and seed quality. Non-nodulated control plants fed 5.0 mM KNO₃ produced the lowest yield (21.1 g seeds plant^-1), whereas well nodulated plants fed 1.0 mM KNO₃, 0.5 mM urea, or 2.5 mM urea produced yields of 26.2 g, 31.8 g, and 36.7 g seeds plant^-1, respectively. Non-nodulated plants fed 2.5 mM urea yielded 28.6 g of seeds plant^-1. Seed N concentrations of non-nodulated plants and nodulated plants fed 2.5 mM urea were high, 6.30% and 6.11% N, respectively, whereas their seed S concentrations were low, 0.348% and 0.330% S, respectively. N sources that produced both a relatively high seed yield and seed N concentration (i.e., a relatively high total seed N plant^-1) produced a proportionately smaller increase in total seed sulfur. Consequently, seed quality, as judged solely by seed S concentration, was lowered.     

The role of nickel in urea assimilation by algae

Nickel is required for urease synthesis by Phaeodactylum tricornutum and Tetraselmis subcordiformis and for growth on urea by Phaeodactylum. There is no requirement for nickel for urea amidolyase synthesis by Chlorella fusca var. vacuolata. Neither copper nor palladium can substitute for nickel but cobalt partially restored urease activity in Phaeodactylum. The addition of nickel to nickel-deficient cultures of Phaeodactylum or Tetraselmis resulted in a rapid increase of urease activity to 7–30 times the normal level; this increase was not inhibited by cycloheximide. It is concluded that nickel-deficient cells over-produce a non-functional urease protein and that either nickel or the functional urease enzyme participates in the regulation of the production of urease protein.Abbreviation UALase ATP; urea amidolyase     

The role of the sulfur globule proteins of Allochromatium vinosum: mutagenesis of the sulfur globule protein genes and expression studies by real-time RT-PCR

During oxidation of reduced sulfur compounds, the purple sulfur bacterium Allochromatium vinosum stores sulfur in the periplasm in the form of intracellular sulfur globules. The sulfur in the globules is enclosed by a protein envelope that consists of the homologous 10.5-kDa proteins SgpA and SgpB and the smaller 8.5-kDa SgpC. Reporter gene fusions of sgpA and alkaline phosphatase showed the constitutive expression of sgpA in A. vinosum and yielded additional evidence for the periplasmic localization of the sulfur globules. Expression analysis of the wild-type sgp genes by quantitative RT-PCR using the LightCycler system showed the constitutive expression of all three sgp genes. The expression of sgpB and sgpC is significantly enhanced under photolithotrophic conditions. Interestingly, sgpB is expressed ten times less than sgpA and sgpC implying that SgpA and SgpC are the main proteins of the sulfur globule envelope. Mutants with inactivated sgpA or sgpB did not show any differences in comparison with the wild-type, i.e., the encoded proteins can replace each other, whereas inactivation of sgpC leads to the formation of considerably smaller sulfur globules. This indicates a role of SgpC for globule expansion. A sgpBC double mutant was unable to grow on sulfide and could not form sulfur globules, showing that the protein envelope is indispensible for the formation and deposition of intracellular sulfur.The paper is dedicated to Prof. Dr. Dr. h.c. mult. Hans Günter Schlegel, Göttingen, on the occasion of his 80th birthday on October 24th, 2004, with great gratitude, as our interest in microbial sulfur metabolism goes back to the early 1960s, when HGT worked in Prof. Schlegels laboratory and in 1972 established this field in Bonn.     

Characteristics of inorganic nitrogen assimilation of plants in fire-prone Mediterranean-type vegetation

A range of approaches was used to investigate how species within a fire-prone Banksia woodland in South West Australia exploited inorganic soil nitrogen sources and how this changes through the development of the fire chronosequence. Nitrate and ammonium were present in soil solution throughout the chronosequence but nitrate predominated in recently burnt sites. Mean shoot nitrate reductase activities were high for all species in recently burnt sites and showed little increase when nitrate was supplied via the transpiration stream. Nitrate reductase of shoots of most species was low at sites not burnt for several years, but following transpirational induction with nitrate, developed activities similar to those at recently burnt sites. The principal amino compounds transported in the xylem were species specific, including asparagine, glutamine and citrulline-dominated species, and changed little in relative composition across the chronosequence. Species most active in leaf nitrate reduction transported the largest amounts of nitrate in their xylem sap and proportional amounts of nitrate in xylem tended to be greatest in recently burnt sites. Most of the species examined appeared to be shoot rather than root nitrate assimilators, but marked differences were recorded in potential of leafy shoots of different species to reduce nitrate. As a general rule, shallow-rooted herbaceous, non-mycorrhizal or VAM-positive species had the highest capacity to reduce nitrate, whereas woody species with ericoid mycorrhizae or combined vesicular arbuscular/ectomycorrhizal associations exhibited little capacity to reduce nitrate in roots or shoots. It seems likely that this latter group utilize ammonium or even organic forms of nitrogen rather than nitrate. Some putative nitrogen-fixing species were active in reducing and transporting nitrate, others were virtually inactive in these respects.     

The use of scanning electron microscopy and an electron microprobe in studies on oxidation of sulfur compounds by bacteria and bioleaching of heavy metals

Abstract: We present results obtained during studies on oxidation of reduced sulfur compounds by Thiobacillus neapolitanus and bioleaching of copper from limestone and sandstone ore. We have used simple and cheap methods for visualisation of chemical and biological phenomena in microscale involving examination with scanning electron microscope and electron microprobe.