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Glutathione (GSH) functions as a major sulfur repository and hence occupies an important position in primary sulfur metabolism. GSH degradation results in sulfur reallocation and is believed to be carried out mainly by γ-glutamyl cyclotransferases (GGCT2;1, GGCT2;2, and GGCT2;3), which, however, do not fully explain the rapid GSH turnover. Here, we discovered that γ-glutamyl peptidase 1 (GGP1) contributes to GSH degradation through a yeast complementation assay. Recombinant proteins of GGP1, as well as GGP3, showed high degradation activity of GSH, but not of oxidized glutathione (GSSG), in vitro. Notably, the GGP1 transcripts were highly abundant in rosette leaves, in agreement with the ggp1 mutants constantly accumulating more GSH regardless of nutritional conditions. Given the lower energy requirements of the GGP- than the GGCT-mediated pathway, the GGP-mediated pathway could be a more efficient route for GSH degradation than the GGCT-mediated pathway. Therefore, we propose a model wherein cytosolic GSH is degraded chiefly by GGP1 and likely also by GGP3. Another noteworthy fact is that GGPs are known to process GSH conjugates in glucosinolate and camalexin synthesis; indeed, we confirmed that the ggp1 mutant contained higher levels of O-acetyl-l-Ser, a signaling molecule for sulfur starvation, and lower levels of glucosinolates and their degradation products. The predicted structure of GGP1 further provided a rationale for this hypothesis. In conclusion, we suggest that GGP1 and possibly GGP3 play vital roles in both primary and secondary sulfur metabolism.

Sulfur (S) is an essential element for plants, and S deficiency causes severe growth retardation. Although the catabolic process of glucosinolates (GSLs), the major S-containing metabolites specific to Brassicales including Arabidopsis, has been recognized as one of the S deficiency (−S) responses in plants, the physiological function of this metabolic process is not clear. Two β-glucosidases (BGLUs), BGLU28 and BGLU30, are assumed to be responsible for this catabolic process as their transcript levels were highly upregulated by −S. To clarify the physiological function of BGLU28 and BGLU30 and their roles in GSL catabolism, we analyzed the accumulation of GSLs and other S-containing compounds in the single and double mutant lines of BGLU28 and BGLU30 and in wild-type plants under different S conditions. GSL levels were highly increased, while the levels of sulfate, cysteine, glutathione and protein were decreased in the double mutant line of BGLU28 and BGLU30 (bglu28/30) under −S. Furthermore, transcript level of Sulfate Transporter1;2, the main contributor of sulfate uptake from the environment, was increased in bglu28/30 mutants under −S. With these metabolic and transcriptional changes, bglu28/30 mutants displayed obvious growth retardation under −S. Overall, our results indicate that BGLU28 and BGLU30 are required for −S-induced GSL catabolism and contribute to sustained plant growth under −S by recycling sulfate to primary S metabolism.

High-salinity stress represses plant growth by inhibiting various metabolic processes. In contrast to the well-studied mechanisms mediating tolerance to high levels of salt, the effects of low levels of salts have not been well studied. In this study, we examined the growth of Arabidopsis thaliana plants under different NaCl concentrations. Interestingly, both shoot and root biomass increased in the presence of 5 mM NaCl, whereas more than 10 mM NaCl decreased plant biomass. To clarify the biological mechanism by which a low level of NaCl stimulated plant growth, we analyzed element accumulation in plants grown under different NaCl concentrations. In addition to the Na and Cl contents, C, S, Zn, and Cu contents were increased under 5 mM NaCl in shoots, this was not observed at higher NaCl concentrations. Adverse effects of high salinity, such as decreased levels of nitrate, phosphate, sulfate, and some cations, did not occur in the presence of 5 mM NaCl. An increase in C was possibly attributed to increased photosynthesis supported by Cl, Zn, and Cu, which also increased in shoots after NaCl application. Salt stress-responsive gene expression was enhanced under 20 mM NaCl but not at lower doses. Among the S metabolites analyzed, cysteine (Cys) was increased by 5 mM NaCl, suggesting that S assimilation was promoted by this dose of NaCl. These results indicate the usefulness of NaCl for plant growth stimulation.

Recent studies have shown various metabolic and transcriptomic interactions between sulfur (S) and phosphorus (P) in plants. However, most studies have focused on the effects of phosphate (Pi) availability and P signaling pathways on S homeostasis, whereas the effects of S availability on P homeostasis remain largely unknown. In this study, we investigated the interactions between S and P from the perspective of S availability. We investigated the effects of S availability on Pi uptake, transport, and accumulation in Arabidopsis thaliana grown under sulfur sufficiency (+S) and deficiency (−S). Total P in shoots was significantly increased under −S owing to higher Pi accumulation. This accumulation was facilitated by increased Pi uptake under −S. In addition, −S increased root-to-shoot Pi transport, which was indicated by the increased Pi levels in xylem sap under −S. The −S-increased Pi level in the xylem sap was diminished in the disruption lines of PHT1;9 and PHO1, which are involved in root-to-shoot Pi transport. Our findings indicate a new aspect of the interaction between S and P by listing the increased Pi accumulation as part of −S responses and by highlighting the effects of −S on Pi uptake, transport, and homeostasis.

Highlights / • Multiple mechanisms to up-regulate sulfate uptake in low sulfur environment. / • Sulfate transporters act as sulfate receptors. / • Contribution of SLIM1 transcription factor to controlling metabolic flow of sulfur in response to sulfur deficiency. / • Sulfur deficiency-induced repression of aliphatic glucosinolates biosynthesis. / • Catabolism of organic sulfur compounds accelerated under sulfur deficiency. /, Plants assimilate inorganic sulfate into various organic sulfur (S) compounds, which contributes to the global sulfur cycle in the environment as well as the nutritional supply of this essential element to animals. Plants, to sustain their lives, adapt the flow of their S metabolism to respond to external S status by activating S assimilation and catabolism of stored S compounds, and by repressing the synthesis of secondary S metabolites like glucosinolates. The molecular mechanism of this response has been gradually revealed, including the discovery of several regulatory proteins and enzymes involved in S deficiency responses. Recent progress in this research area and the remaining issues are reviewed here.

Root hairs often contribute to nutrient uptake from environments, but the contribution varies among nutrients. In Arabidopsis, two high-affinity sulfate transporters, SULTR1, 1 and SULTR1, 2, are responsible for sulfate uptake by roots. Their increased expression under sulfur deficiency (&minus, S) stimulates sulfate uptake. Inspired by the higher and lower expression, respectively, of SULTR1, 1 in mutants with more (werwolf [wer]) and fewer (caprice [cpc]) root hairs, we examined the contribution of root hairs to sulfate uptake. Sulfate uptake rates were similar among plant lines under both sulfur sufficiency (+S) and &minus, S. Under &minus, S, the expression of SULTR1, 2 was negatively correlated with the number of root hairs. These results suggest that both &minus, S-induced SULTR expression and sulfate uptake rates were independent of the number of root hairs. In addition, we observed (1) a negative correlation between primary root lengths and number of root hairs and (2) a greater number of root hairs under &minus, S than under +S. These observations suggested that under both +S and &minus, S, sulfate uptake was influenced by the root biomass rather than the number of root hairs.

Plants take up sulfur (S), an essential element for all organisms, as sulfate, which is mainly attributed to the function of SULTR1;2 in Arabidopsis. A disruption mutant of SULTR1;2, sel1-10, has been characterized with phenotypes similar to plants grown under sulfur deficiency (−S). Although the effects of −S on S metabolism were well investigated in seedlings, no studies have been performed on mature Arabidopsis plants. To study further the effects of −S on S metabolism, we analyzed the accumulation and distribution of S-containing compounds in different parts of mature sel1-10 and of the wild-type (WT) plants grown under long-day conditions. While the levels of sulfate, cysteine, and glutathione were almost similar between sel1-10 and WT, levels of glucosinolates (GSLs) differed between them depending on the parts of the plant. GSLs levels in the leaves and stems were generally lower in sel1-10 than those in WT. However, sel1-10 seeds maintained similar levels of aliphatic GSLs to those in WT plants. GSL accumulation in reproductive tissues is likely to be prioritized even when sulfate supply is limited in sel1-10 for its role in S storage and plant defense.

Selenium and chromium are heavy metals that are highly toxic to living organisms when they are present in excess amounts. The major forms of selenium and chromium in the environment are selenate and chromate, which act as toxic analogs of sulfate in plants, inhibiting its uptake and causing sulfur deficiency. To provide simple and inexpensive means of environmental analysis for selenate and chromate, we previously developed a model system for detecting and quantifying their levels by measuring the accumulation of green fluorescent protein (GFP) in transgenic plants under the control of the sulfur-responsive promoter region of the high-affinity sulfate transporter SULTR1;2 from Arabidopsis (PSULTR1;2-GFP). However, the detection limit for both selenate and chromate was 10 μmol L−1, which was higher than the environmental standards. Here, we report a more sensitive monitoring system for selenium and chromium by combining PSULTR1;2 with another sulfur-responsive region of the sulfate transporter SULTR2;1 located in the 3ʹ-non-transcribed intergenic region (TSULTR2;1). The fusion gene construct consisting of PSULTR1;2, GFP and TSULTR2;1 (PSULTR1;2-GFP-TSULTR2;1) was introduced into Arabidopsis plants, and PSULTR1;2-GFP-TSULTR2;1 transgenic plants showed significant increases in GFP under sulfur-deficient conditions or with the addition of selenate or chromate to the growth medium. The increase in GFP depended on selenate concentration. Overall, the use of PSULTR1;2-GFPTSULTR2; 1 plants decreased the detection limits to 1 and 3 μmol L−1 for selenate and chromate, respectively, providing a highly sensitive plant-based system for detecting selenium and chromium in the environment.

Sulfur is an essential element required for plant growth. It can be found as a thiol group of proteins or non-protein molecules, and as various sulfur-containing small biomolecules, including iron-sulfur (Fe/S) clusters, molybdenum cofactor (Moco), and sulfur-modified nucleotides. Thiol-mediated redox regulation has been well investigated, whereas biosynthesis pathways of the sulfur-containing small biomolecules have not yet been clearly described. In order to understand overall sulfur transfer processes in plant cells, it is important to elucidate the relationships among various sulfur delivery pathways as well as to investigate their interactions. In this review, we summarize the information from recent studies on the biosynthesis pathways of several sulfur-containing small biomolecules and the proteins participating in these processes. In addition, we show characteristic features of gene expression in Arabidopsis at the early stage of sulfate depletion from the medium, and we provide insights into sulfur transfer processes in plant cells.

Recent studies have shown various metabolic and transcriptomic interactions between sulfur (S) and phosphorus (P) in plants. However, most studies have focused on the effects of phosphate (Pi) availability and P signaling pathways on S homeostasis, whereas the effects of S availability on P homeostasis remain largely unknown. In this study, we investigated the interactions between S and P from the perspective of S availability. We investigated the effects of S availability on Pi uptake, transport, and accumulation in Arabidopsis thaliana grown under sulfur sufficiency (+S) and deficiency (&minus, S). Total P in shoots was significantly increased under &minus, S owing to higher Pi accumulation. This accumulation was facilitated by increased Pi uptake under &minus, S. In addition, &minus, S increased root-to-shoot Pi transport, which was indicated by the increased Pi levels in xylem sap under &minus, S. The &minus, S-increased Pi level in the xylem sap was diminished in the disruption lines of PHT1, 9 and PHO1, which are involved in root-to-shoot Pi transport. Our findings indicate a new aspect of the interaction between S and P by listing the increased Pi accumulation as part of &minus, S responses and by highlighting the effects of &minus, S on Pi uptake, transport, and homeostasis.

Under sulfur deficiency (-S), plants induce expression of the sulfate transport systems in roots to increase uptake and root-to-shoot transport of sulfate. The low-affinity sulfate transporter SULTR2;1 is predominantly expressed in xylem parenchyma and pericycle cells in Arabidopsis thaliana roots under -S. The mechanisms underlying -S-inducible expression of SULTR2;1 in roots have remained unclear, despite the possible significance of SULTR2;1 for acclimation to low-sulfur conditions. In this investigation, examination of deletions and base substitutions in the 3'-intergenic region of SULTR2;1 revealed novel sulfur-responsive elements, SURE21A (5'-CAATGTATC-3') and SURE21B (5'-CTAGTAC-3'), located downstream of the SULTR2;1 3'-untranslated region. SURE21A and SULTR21B effectively induced reporter gene expression from fusion constructs under -S in combination with minimal promoters or promoters not inducible by -S, suggesting their versatility in controlling transcription. T-DNA insertions near SURE21A and SULTR21B abolished -S-inducible expression of SULTR2;1 in roots and reduced the uptake and root-to-shoot transport of sulfate. In addition, these mutations partially suppressed SULTR2;1 expression in shoots, without changing its -S-responsive expression. These findings indicate that SULTR2;1 contributes to the increase in uptake and internal translocation of sulfate driven by gene expression induced under the control of sulfur-responsive elements in the 3'-nontranscribed intergenic region of SULTR2;1.

Cadmium (Cd) is a highly toxic element for living organisms, hence plants have evolved a variety of detoxification mechanisms. Phytochelatins and glutathione are low-molecular-weight sulfur compounds that function as chelators and play important roles in Cd detoxification. Previous studies have shown that the transcription of the genes involved in sulfate uptake and sulfur assimilation was increased in response to Cd stress. Recently, we reported that Cd-induced sulfate uptake is mainly attributed to the function of SULTR1;2, a high affinity sulfate transporter involved in sulfate uptake from the roots. Another distinct change in sulfate distribution induced by Cd treatment was preferential accumulation of sulfate to the shoots, which is due to the induction of root-to-shoot sulfate transport through xylem. In this study, we compared previous transcriptome data taken with Cd-treated plants to get suggestions about the SULTRs involved in Cd-induced sulfate distribution to shoots. In addition to the induction of SULTR1;1 and SULTR1;2 expressions, we found that the expression of SULTR2;1, a transporter involved in root-to-shoot transport of sulfate, and SULTR4;1 and SULTR4;2, exporters of stored sulfate in vacuole, were increased in roots upon Cd treatment. These SULTRs were suggested as contributors to the increased distribution of sulfate to shoots under Cd exposure.

Sulfur is an essential element for all organisms. Plants utilize soil sulfate to synthesize an amino acid, cysteine, which is used for a variety of sulfur-containing compounds such as glutathione (GSH), methionine, proteins, lipids, coenzymes, and various secondary metabolites. Since animals cannot synthesize organic sulfur compounds from inorganic ones, sulfate assimilation in plants is important for the global sulfur cycle.

Cadmium (Cd) is a highly toxic and non-essential element for plants, whereas phytochelatins and glutathione are low-molecular-weight sulfur compounds that function as chelators and play important roles in detoxification. Cadmium exposure is known to induce the expression of sulfur-assimilating enzymes and sulfate uptake by roots. However, the molecular mechanism underlying Cd-induced changes remains largely unknown. Accordingly, we analyzed the effects of Cd treatment on the uptake and translocation of sulfate and accumulation of thiols in Arabidopsis thaliana Both wild type (WT) and null mutant (sel1-10 and sel1-18) plants of the sulfate transporter SULTR1;2 exhibited growth inhibition when treated with CdCl2 However, the mutant plants exhibited a lower growth rate and lower Cd accumulation. Cadmium treatment also upregulated the transcription of SULTR1;2 and sulfate uptake activity in WT plants, but not in mutant plants. In addition, the sulfate, phytochelatin and total sulfur contents were preferentially accumulated in the shoots of both WT and mutant plants treated with CdCl2, and sulfur K-edge XANES spectra suggested that sulfate was the main compound responsible for the increased sulfur content in the shoots of CdCl2-treated plants. Our results demonstrate that Cd-induced sulfate uptake depends on SULTR1;2 activity, and that CdCl2 treatment greatly shifts the distribution of sulfate to shoots, increases the sulfate concentration of xylem sap and upregulates the expression of SULTRs involved in root-to-shoot sulfate transport. Therefore, we conclude that root-to-shoot sulfate transport is stimulated by Cd and suggest that the uptake and translocation of sulfate in CdCl2-treated plants are enhanced by demand-driven regulatory networks.

5-Aminolevulinic acid (ALA), a key precursor of porphyrin biosynthesis, promotes plant growth and crop yields. Although ALA is known to promote carbon fixation and nitrogen assimilation in plants, the effects of ALA on sulfur assimilation have not been determined. In the present study, we analyzed the effect of ALA on sulfur assimilation. We used a fusion gene construct consisting of a promoter region of the high-affinity sulfate transporter SULTR1;2 from Arabidopsis and green fluorescent protein ([GFP] P SULTR1;2 -GFP) to determine whether ALA treatment influences the expression of the sulfur transport gene. The GFP levels in P SULTR1;2 -GFP plants were significantly increased by 0.3 and 1 mmol L−1 ALA under both sulfur-sufficient and sulfur-deficient conditions. Real-time reverse transcription-polymerase chain reaction experiments revealed that these concentrations of ALA also increased the mRNA levels of other key sulfur transport and assimilatory genes, such as SULTR, adenosine 5′-phosphosulf...

Plants play an important role in the global sulphur cycle because they assimilate sulphur from the environment and build it into methionine and cysteine. Several genes of the sulphur assimilation pathway are regulated by microRNA-395 (miR395) that is itself induced by a low-sulphur (-S) environment. Here, we show that the six Arabidopsis miR395 loci are induced differently. We find that MIR395 loci are expressed in the vascular system of roots and leaves and root tips. Induction of miR395 by a -S environment in both roots and leaves suggests that translocation of miR395 from leaves to roots through the phloem is not necessary for plants growing on -S soil/medium. We also demonstrate that induction of miR395 is controlled by SLIM1, a key transcription factor in the sulphur assimilation pathway. Unexpectedly, the mRNA level of a miR395 target gene, SULTR2;1, strongly increases during miR395 induction in roots. We show that the spatial expression pattern of MIR395 transcripts in the vascular system does not appear to overlap with the expression pattern previously reported for SULTR2;1 mRNA. These results illustrate that negative temporal correlation between the expression level of a miRNA and its target gene in a complex tissue cannot be a requirement for target gene validation.

Sulfur is an essential macronutrient required for plant growth. To identify key transcription factors regulating the sulfur assimilatory pathway, we screened Arabidopsis thaliana mutants using a fluorescent reporter gene construct consisting of the sulfur limitation-responsive promoter of the SULTR1;2 sulfate transporter and green fluorescent protein as a background indicator for monitoring plant sulfur responses. The isolated mutant, sulfur limitation1 (slim1), was unable to induce SULTR1;2 transcripts under low-sulfur (–S) conditions. Mutations causing the sulfur limitation responseless phenotypes of slim1 were identified in an EIL family transcription factor, ETHYLENE-INSENSITIVE3-LIKE3 (EIL3), whose functional identity with SLIM1 was confirmed by genetic complementation. Sulfate uptake and plant growth on –S were significantly reduced by slim1 mutations but recovered by overexpression of SLIM1. SLIM1 functioned as a central transcriptional regulator, which controlled both the activation of sulfate acquisition and degradation of glucosinolates under –S conditions. Metabolite analysis indicated stable accumulation of glucosinolates in slim1 mutants, even under –S conditions, particularly in the molecular species with methylsulfinylalkyl side chains beneficial to human health. Overexpression of SLIM1 and its rice (Oryza sativa) homologs, but no other EIL genes of Arabidopsis, restored the sulfur limitation responseless phenotypes of slim1 mutants, suggesting uniqueness of the SLIM1/EIL3 subgroup members as sulfur response regulators.

Sulfate is a macronutrient required for cell growth and development. Arabidopsis has two high-affinity sulfate transporters (SULTR1;1 and SULTR1;2) that represent the sulfate uptake activities at the root surface. Sulfur limitation (–S) response relevant to the function of SULTR1;2 was elucidated in this study. We have isolated a novel T-DNA insertion allele defective in the SULTR1;2 sulfate transporter. This mutant, sel1-10, is allelic with the sel1 mutants identified previously in a screen for increased tolerance to selenate, a toxic analog of sulfate (Shibagaki et al., 2002). The abundance of SULTR1;1 mRNA was significantly increased in the sel1-10 mutant; however, this compensatory up-regulation of SULTR1;1 was not sufficient to restore the growth. The sulfate content of the mutant was 10% to 20% of the wild type, suggesting that induction of SULTR1;1 is not fully complementing the function of SULTR1;2 and that SULTR1;2 serves as the major facilitator for the acquisition of sulfate in Arabidopsis roots. Transcriptome analysis of approximately 8,000 Arabidopsis genes in the sel1-10 mutant suggested that dysfunction of the SULTR1;2 transporter can mimic general –S symptoms. Hierarchal clustering of sulfur responsive genes in the wild type and mutant indicated that sulfate uptake, reductive sulfur assimilation, and turnover of secondary sulfur metabolites are activated under –S. The profiles of –S-responsive genes further suggested induction of genes that may alleviate oxidative damage and generation of reactive oxygen species caused by shortage of glutathione.

A key precursor of porphyrin biosynthesis, 5-aminolevulinic acid (ALA), is used as a fertilizer to promote plant growth and crop yields. Carbon fixation and nitrogen assimilation are promoted by ALA in plants, but the effect on other metabolic pathway has not been elucidated. In this study, we analyzed the effect of ALA on sulfur assimilation. Treatment with 0.3 and 1 mmol−1 ALA significantly increased the sulfate uptake and the transcript levels of key sulfur transport and assimilatory genes. The accumulation of cysteine and glutathione was increased in shoots but decreased in roots. These data demonstrated a new role of ALA that regulates positively transcript levels of some sulfur assimilatory genes, sulfate uptake and contents of cysteine and glutathione.

SULTR1;1 high-affinity sulfate transporter is highly regulated in the epidermis and cortex of Arabidopsis roots responding to sulfur deficiency (-S). We identified a novel cis-acting element involved in the -S-inducible expression of sulfur-responsive genes in Arabidopsis. The promoter region of SULTR1;1 was dissected for deletion and gain-of-function analysis using luciferase (LUC) reporter gene in transgenic Arabidopsis. The 16-bp sulfur-responsive element (SURE) from -2777 to -2762 of SULTR1;1 promoter was sufficient and necessary for the -S-responsive expression, which was reversed when supplied with cysteine and glutathione (GSH). The SURE sequence contained an auxin response factor (ARF) binding sequence (GAGACA). However, SURE was not responsive to naphthalene acetic acid, indicating its specific function in the sulfur response. The base substitution analysis indicated the significance of a 5-bp sequence (GAGAC) within the conserved ARF binding site as a core element for the -S response. Microarray analysis of early -S response in Arabidopsis roots indicated the presence of SURE core sequences in the promoter regions of -S-inducible genes on a full genome GeneChip array. It is suggested that SURE core sequences may commonly regulate the expression of a gene set required for adaptation to the -S environment.

Plants require the function of plasma membrane-bound sulphate transporters for the initial uptake of inorganic sulphate. Part of this fundamental process is the energy-dependent proton/sulphate co-transport systems that are located in the surface cell layers of roots. During sulphur limitation, plants are able to activate the expression of sulphate transporters that facilitate the uptake of sulphate in roots. SULTR1;1 and SULTR1;2 are suggested to be the essential components of the sulphate uptake system in Arabidopsis roots. The physiological importance of SULTR1;1 and SULTR1;2 is supported by characteristics that can cope with sulphur deficiency: they were (i) functional high-affinity sulphate transporters; (ii) induced by sulphur limitation at the mRNA levels; and (iii) predominantly localized in the root hairs, epidermis, and cortex. The expression of high-affinity sulphate transporters was primarily regulated by sulphur in a promoter-dependent manner. Aside from the sulphur-specific regulation, the induction of SULTR1;1 and SULTR1;2 high-affinity sulphate transporters by sulphur limitation was dependent on the supply of carbon and nitrogen. In this review, the application of SULTR promoter-GFP systems for the analysis of regulatory pathways of sulphate acquisition in plants is described.

SULTR1;1 high-affinity sulfate transporter is highly regulated by sulfur deficiency (–S) in the epidermis and cortex of Arabidopsis roots. The regulatory mechanism of SULTR1;1 expression was studied using inhibitors for transcription, translation, protein phosphorylation and dephosphorylation. The induction of SULTR1;1 mRNA during –S was blocked by the addition of actinomycin D in the medium, suggesting that SULTR1;1 is transcriptionally regulated. Cycloheximide repressed the –S induction of SULTR1;1, but enhanced the basal mRNA level of SULTR1;1 under sulfur replete (+S) condition. In addition, the induction of SULTR1;1 by –S was significantly blocked by okadaic acid (OKA) and calyculin A (CalyA). Regulation of SULTR1;1 was further confirmed in transgenic plants expressing green fluorescent protein (GFP) under the control of SULTR1;1 promoter. Accumulation of GFP during –S was dependent to SULTR1;1 promoter, and the effects of OKA and CalyA were reproducible in the SULTR1;1 promoter-GFP plants. These results suggested that the up-regulation of SULTR1;1 by –S requires protein phosphatase as an upstream regulatory factor.

Plant-based assays for monitoring contaminated environments provide inexpensive and nontechnical means of environmental analysis. Here we report a model system for monitoring selenium and chromium, which are highly toxic heavy metals for living organisms. The major forms of selenium and chromium in nature are selenate and chromate. As toxic analogs of sulfate, they cause sulfur deficiency in plants by inhibiting the uptake of sulfate from the environment. We used a fusion gene construct consisting of a sulfur-responsive promoter region of the high-affinity sulfate transporter SULTR1;2 from Arabidopsis and green fluorescent protein (GFP; PSULTR1;2-GFP) to quantify the levels of selenate and chromate by GFP accumulation. The PSULTR1;2-GFP transgenic Arabidopsis plants showed drastic increases in GFP with the addition of selenate or chromate to the medium. The increase in GFP was concentration-dependent relative to the amounts of contaminants in the medium, suggesting the potential of PSULTR1;2-GFP plants as indicators in quantifying environmental selenate and chromate.

The effects of allyl, sulfur and cyanogenic compounds on the germination of upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds were examined. Mercaptoethanol and methylmercaptan as well as KCN, substrates for pcyanoalanine synthase (CAS), and H2S and thiocyanate, the products of the CAS catalyzing reaction, were effective in promoting germination, suggesting the involvement of CAS in germination. Most of allyl compounds, especially allylthiourea, as well as ethylene which activated CAS [Hasegawa et al. (1994) Physiol. Plant. 91: 141], promoted the germination in an abnormal type which occurred by the predominant growth of cotyledons as did C2H4 [Katoh and Esashi (1975) Plant Cell Physiol. 16: 687]. However, they failed to activate CAS unlike ethylene, and to liberate free ethylene during an incubation period. It was thus possible that an C2H4-like double bond within allyl compounds can act to promote seed germination.

Sulfur is one of the essential macronutrients required for plant growth. Since the expression of several sulfur-assimilatory genes is stimulated under the condition of sulfur deficiency (−S), transcriptional regulation of these genes is considered to be critical for the control of sulfur assimilation. In the last several years, the author and coworkers have been investigated molecular mechanisms of −S-inducible expression of high-affinity sulfate transporters, SULTR1;1 and SULTR1;2 in Arabidopsis thaliana. SULTR1;1 and SULTR1;2 facilitate sulfate uptake in roots. This review summarizes the recent progress about the transcriptional regulation of these sulfate transporters by focusing on three major topics. 1) Identification of a cis-acting element involved in the −S-inducible expression of SULTR1;1. 2) Cytokinin-dependent repression of SULTR expression and sulfate uptake. 3) Central transcription regulator SLIM1 controlling −S responsive gene expression including sulfur assimilation and metabolism.