Light regulation of assimilatory sulphate reduction in Arabidopsis thaliana

Catalysts for sulfur: understanding the intricacies of enzymes orchestrating plant sulfur anabolism

MAIN CONCLUSION

This review highlights the sulfur transporters, key enzymes and their encoding genes involved in plant sulfur anabolism, focusing on their occurrence, chemistry, location, function, and regulation within sulfur assimilation pathways. Sulfur, a vital element for plant life, plays diverse roles in metabolism and stress response. This review provides a comprehensive overview of the sulfur assimilation pathway in plants, highlighting the intricate network of enzymes and their regulatory mechanisms. The primary focus is on the key enzymes involved: ATP sulfurylase (ATPS), APS reductase (APR), sulfite reductase (SiR), serine acetyltransferase (SAT), and O-acetylserine(thiol)lyase (OAS-TL). ATPS initiates the process by activating sulfate to form APS, which is then reduced to sulfite by APR. SiR further reduces sulfite to sulfide, a crucial step that requires significant energy. The cysteine synthase complex (CSC), formed by SAT and OAS-TL, facilitates the synthesis of cysteine, thereby integrating serine metabolism with sulfur assimilation. The alternative sulfation pathway, catalyzed by APS kinase and sulfotransferases, is explored for its role in synthesizing essential secondary metabolites. This review also delves into the regulatory mechanism of these enzymes such as environmental stresses, sulfate availability, phytohormones, as well as translational and post-translational regulations. Understanding the key transporters and enzymes in sulfur assimilation pathways and their corresponding regulation mechanisms can help researchers grasp the importance of sulfur anabolism for the life cycle of plants, clarify how these enzymes and their regulatory processes are integrated to balance plant life systems in response to changes in both external conditions and intrinsic signals.

Sulfur metabolism and response to light in Ulva prolifera green tides

The outbreak of Ulva prolifera blooms causes significant changes in the coastal sulfur cycle due to the high production of dimethylsulfoniopropionate (DMSP) and the emission of dimethylsulfide (DMS). However, the sulfur metabolism mechanism of U. prolifera has not been thoroughly investigated. In this study, we examined the levels of intracellular and extracellular sulfate (SO42--S), total sulfur (TS), DMSP, and DMS in fresh U. prolifera under different light intensity conditions (54, 108 and 162 μmol photons m-2 s-1) during algal growth. We also conducted transcriptome analyses to investigate sulfur uptake and metabolism. When the light intensity increased by 50% (from 108 to 162 μmol photons m-2 s-1), the amount of absorbed SO42--S increased by 3.5 times after 24 h, while the fresh weight of U. prolifera increased by 16%, and the average release rates of DMS and DMSP increased by 136% and 100%, respectively. However, the expression of sulfate transporter and assimilation-related genes did not show significant up- or down-regulation in response to the light intensity changes. Therefore, it is speculated that the key gene responsible for DMSP synthesis in U. prolifera has not yet been identified. The sulfate metabolic pathway of U. prolifera was established, and four Alma genes, including DMSP lyase, were identified. During the bloom period, it is estimated that U. prolifera releases a maximum of approximately 0.4 tons of sulfur and 0.3 tons of carbon in the form of DMS into the atmosphere per day. Additionally, biogenic sulfur dissolved in seawater or within algae could potentially impact the regional climate and environment.

Coordinated metabolic adaptation of Arabidopsis thaliana to high light

In plants, exposure to high light irradiation induces various stress responses, which entail complex metabolic rearrangements. To explore these dynamics, we conducted time‐course experiments spanning 2 min to 72 h with Arabidopsis thaliana under high and control light. Comparative metabolomics, transcriptomics, redox proteomics, and stable isotope labeling on leaf rosettes identified a series of synchronous and successive responses that provide a deeper insight into well‐orchestrated mechanisms contributing to high‐light acclimation. We observed transient transcriptome downregulation related to light harvesting and electron flow before the profound remodeling of the photosynthetic apparatus. Throughout the entire time course, redox homeostasis is tightly balanced between downregulation of production and enhanced transformation of NADPH accompanied by redistribution of reducing equivalents across several subcellular compartments. In both light conditions, C4 acids such as malate and fumarate are produced via anaplerosis. In carbon units, their accumulation in vacuoles surpasses plastidic levels of starch and intensifies notably under high light. In parallel, citrate synthesis from pyruvate is significantly hindered diurnally. Isotopic labeling in 2‐oxoglutarate and glutamate suggests a moderate de novo synthesis of C5 acids from a vacuolar citrate reservoir during the light phase while they are largely renewed during the night. In the absence of a diurnal clockwise flow through the tricarboxylic acid (TCA) cycle, increased oxidation of photorespiratory glycine takes over as a source of reductants to fuel mitochondrial ATP production. These findings, along with previous research, contribute to a model integrating redox balance and linking increased carbon assimilation and nitrogen metabolism, especially in the context of an incomplete TCA cycle.

The sulfate assimilation and reduction of marine microalgae and the regulation of illumination

To examine the sulfate assimilation and reduction process and the regulation of illumination, diatom Phaeodactylum tricornutum and dinoflagellate Amphidinium carterae were selected for continuous simulation incubation under different photon flux densities (PFDs) (54, 108 and 162 μmol photons m-2 s-1), and concentration variations of related sulfur compounds sulfate, dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS) and acrylic acid (AA) in the culture system were observed. The optimal PFD for the growth of two microalgae was 108 μmol photons m-2 s-1. However, the maximum sulfate absorption occurred at 162 μmol photons m-2 s-1 for P. tricornutum and at 54 μmol photons m-2 s-1 for A. carterae. With the increase of PFD, the release of DMSP by P. tricornutum decreased while A. carterae increased. The largest release amount of DMS was 0.59 ± 0.05 fmol cells-1 for P. tricornutum and 2.61 ± 0.89 fmol cells-1 for A. carterae under their optimum growth light condition. The sulfate uptake of P. tricornutum was inhibited by the addition of amino acids, cysteine had a greater inhibitory effect than methionine, and the absorption process was controlled by light. The intermediate products of sulfur metabolism had an up-control effect on the sulfate uptake process of P. tricornutum. However, the addition of amino acids had no obvious effect on the sulfate absorption of A. carterae.

New insights into the regulation of plant metabolism by O-acetylserine: sulfate and beyond

Under conditions of sulfur deprivation, O-acetylserine (OAS) accumulates, which leads to the induction of a common set of six genes, called OAS cluster genes. These genes are induced not only under sulfur deprivation, but also under other conditions where OAS accumulates, such as shift to darkness and stress conditions leading to reactive oxygen species (ROS) or methyl-jasmonate accumulation. Using the OAS cluster genes as a query in ATTED-II, a co-expression network is derived stably spanning several hundred conditions. This allowed us not only to describe the downstream function of the OAS cluster genes but also to score for functions of the members of the co-regulated co-expression network and hence the effects of the OAS signal on the sulfate assimilation pathway and co-regulated pathways. Further, we summarized existing knowledge on the regulation of the OAS cluster and the co-expressed genes. We revealed that the known sulfate deprivation-related transcription factor EIL3/SLIM1 exhibits a prominent role, as most genes are subject to regulation by this transcription factor. The role of other transcription factors in response to OAS awaits further investigation.

Transcriptome dynamics provides insights into divergences of the photosynthesis pathway between Saccharum officinarum and Saccharum spontaneum

Saccharum spontaneum and Saccharum officinarum contributed to the genetic background of modern sugarcane cultivars. Saccharum spontaneum has shown a higher net photosynthetic rate and lower soluble sugar than S. officinarum. Here, we analyzed 198 RNA-sequencing samples to investigate the molecular mechanisms for the divergences of photosynthesis and sugar accumulation between the two Saccharum species. We constructed gene co-expression networks based on differentially expressed genes (DEGs) both for leaf developmental gradients and diurnal rhythm. Our results suggested that the divergence of sugar accumulation may be attributed to the enrichment of major carbohydrate metabolism and the oxidative pentose phosphate pathway. Compared with S. officinarum, S. spontaneum DEGs showed a high enrichment of photosynthesis and contained more complex regulation of photosynthesis-related genes. Noticeably, S. spontaneum lacked gene interactions with sulfur assimilation stimulated by photorespiration. In S. spontaneum, core genes related to clock and photorespiration displayed a sensitive regulation by the diurnal rhythm and phase-shift. Small subunit of Rubisco (RBCS) displayed higher expression in the source tissues of S. spontaneum. Additionally, it was more sensitive under a diurnal rhythm, and had more complex gene networks than that in S. officinarum. This indicates that the differential regulation of RBCS Rubisco contributed to photosynthesis capacity divergence in both Saccharum species.

Far-Red Light Coordinates the Diurnal Changes in the Transcripts Related to Nitrate Reduction, Glutathione Metabolism and Antioxidant Enzymes in Barley

Spectral quality, intensity and period of light modify many regulatory and stress signaling pathways in plants. Both nitrate and sulfate assimilations must be synchronized with photosynthesis, which ensures energy and reductants for these pathways. However, photosynthesis is also a source of reactive oxygen species, whose levels are controlled by glutathione and other antioxidants. In this study, we investigated the effect of supplemental far-red (735 nm) and blue (450 nm) lights on the diurnal expression of the genes related to photoreceptors, the circadian clock, nitrate reduction, glutathione metabolism and various antioxidants in barley. The maximum expression of the investigated four photoreceptor and three clock-associated genes during the light period was followed by the peaking of the transcripts of the three redox-responsive transcription factors during the dark phase, while most of the nitrate and sulfate reduction, glutathione metabolism and antioxidant-enzyme-related genes exhibited high expression during light exposure in plants grown in light/dark cycles for two days. These oscillations changed or disappeared in constant white light during the subsequent two days. Supplemental far-red light induced the activation of most of the studied genes, while supplemental blue light did not affect or inhibited them during light/dark cycles. However, in constant light, several genes exhibited greater expression in blue light than in white and far-red lights. Based on a correlation analysis of the gene expression data, we propose a major role of far-red light in the coordinated transcriptional adjustment of nitrate reduction, glutathione metabolism and antioxidant enzymes to changes of the light spectrum.

Light Intensity- and Spectrum-Dependent Redox Regulation of Plant Metabolism

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

Adenosine 5' phosphosulfate reductase and sulfite oxidase regulate sulfite-induced water loss in Arabidopsis

Chloroplast-localized adenosine-5'-phosphosulphate reductase (APR) generates sulfite and plays a pivotal role in reduction of sulfate to cysteine. The peroxisome-localized sulfite oxidase (SO) oxidizes excess sulfite to sulfate. Arabidopsis wild type, SO RNA-interference (SO Ri) and SO overexpression (SO OE) transgenic lines infiltrated with sulfite showed increased water loss in SO Ri plants, and smaller stomatal apertures in SO OE plants compared with wild-type plants. Sulfite application also limited sulfate and abscisic acid-induced stomatal closure in wild type and SO Ri. The increases in APR activity in response to sulfite infiltration into wild type and SO Ri leaves resulted in an increase in endogenous sulfite, indicating that APR has an important role in sulfite-induced increases in stomatal aperture. Sulfite-induced H2O2 generation by NADPH oxidase led to enhanced APR expression and sulfite production. Suppression of APR by inhibiting NADPH oxidase and glutathione reductase2 (GR2), or mutation in APR2 or GR2, resulted in a decrease in sulfite production and stomatal apertures. The importance of APR and SO and the significance of sulfite concentrations in water loss were further demonstrated during rapid, harsh drought stress in root-detached wild-type, gr2 and SO transgenic plants. Our results demonstrate the role of SO in sulfite homeostasis in relation to water consumption in well-watered plants.

Targets and Mechanisms of Geminivirus Silencing Suppressor Protein AC2

Geminiviruses are plant DNA viruses that infect a wide range of plant species and cause significant losses to economically important food and fiber crops. The single-stranded geminiviral genome encodes a small number of proteins which act in an orchestrated manner to infect the host. The fewer proteins encoded by the virus are multifunctional, a mechanism uniquely evolved by the viruses to balance the genome-constraint. The host-mediated resistance against incoming virus includes post-transcriptional gene silencing, transcriptional gene silencing, and expression of defense responsive genes and other cellular regulatory genes. The pathogenicity property of a geminiviral protein is linked to its ability to suppress the host-mediated defense mechanism. This review discusses what is currently known about the targets and mechanism of the viral suppressor AC2/AL2/transcriptional activator protein (TrAP) and explore the biotechnological applications of AC2.

Level of Sulfite Oxidase Activity Affects Sulfur and Carbon Metabolism in Arabidopsis

Molybdenum cofactor containing sulfite oxidase (SO) enzyme is an important player in protecting plants against exogenous toxic sulfite. It was also demonstrated that SO activity is essential to cope with rising dark-induced endogenous sulfite levels and maintain optimal carbon and sulfur metabolism in tomato plants exposed to extended dark stress. The response of SO and sulfite reductase to direct exposure of low and high levels of sulfate and carbon was rarely shown. By employing Arabidopsis wild-type, sulfite reductase, and SO-modulated plants supplied with excess or limited carbon or sulfur supply, the current study demonstrates the important role of SO in carbon and sulfur metabolism. Application of low and excess sucrose, or sulfate levels, led to lower biomass accumulation rates, followed by enhanced sulfite accumulation in SO impaired mutant compared with wild-type. SO-impairment resulted in the channeling of sulfite to the sulfate reduction pathway, resulting in an overflow of organic S accumulation. In addition, sulfite enhancement was followed by oxidative stress contributing as well to the lower biomass accumulation in SO-modulated plants. These results indicate that the role of SO is not limited to protection against elevated sulfite toxicity but to maintaining optimal carbon and sulfur metabolism in Arabidopsis plants.

Identification of Small RNAs During High Light Acclimation in Arabidopsis thaliana

The biological significance of non-coding RNAs (ncRNAs) has been firmly established to be important for the regulation of genes involved in stress acclimation. Light plays an important role for the growth of plants providing the energy for photosynthesis; however, excessive light conditions can also cause substantial defects. Small RNAs (sRNAs) are a class of non-coding RNAs that regulate transcript levels of protein-coding genes and mediate epigenetic silencing. Next generation sequencing facilitates the identification of small non-coding RNA classes such as miRNAs (microRNAs) and small-interfering RNAs (siRNAs), and long non-coding RNAs (lncRNAs), but changes in the ncRNA transcriptome in response to high light are poorly understood. We subjected Arabidopsis plants to high light conditions and performed a temporal in-depth study of the transcriptome data after 3 h, 6 h, and 2 days of high light treatment. We identified a large number of high light responsive miRNAs and sRNAs derived from NAT gene pairs, lncRNAs and TAS transcripts. We performed target predictions for differentially expressed miRNAs and correlated their expression levels through mRNA sequencing data. GO analysis of the targets revealed an overrepresentation of genes involved in transcriptional regulation. In A. thaliana, sRNA-mediated regulation of gene expression in response to high light treatment is mainly carried out by miRNAs and sRNAs derived from NAT gene pairs, and from lncRNAs. This study provides a deeper understanding of sRNA-dependent regulatory networks in high light acclimation.

The Arabidopsis APR2 positively regulates cadmium tolerance through glutathione-dependent pathway

Cadmium (Cd) is a dangerous environmental pollutant with high toxicity to plants. The adenosine 5'-phosphosulfate reductase 2 (APR2) is the dominant APRs in Arabidopsis and plays an important role in reductive sulfate assimilation pathway. However, whether the involvement of plant APRs in Cd stress response is largely unclear. Herein, we report that APR2 functions in Cd accumulation and tolerance in Arabidopsis. The transcript levels of APR2 were markedly induced by Cd exposure. Transgenic plants overexpressing APR2 improved Cd tolerance, whereas knockout of APR2 reduced Cd tolerance. APR2-overexpressing plants with increased Cd accumulation and tolerance showed higher glutathione (GSH) and phytochelatin (PC) levels than the wild type and apr2 mutant plants, but lower H₂O₂ and TBARS contents upon Cd exposure. Moreover, exogenous GSH application effectively rescued Cd hypersensitivity in APR2-knockout plants. Further analysis showed that buthionine sulfoximine (BSO, an inhibitor of GSH synthesis) treatment completely eliminated the enhanced Cd tolerance phenotypes of APR2-overexpressing plants, implying that APR2-mediated enhanced Cd tolerance is GSH dependent. In addition, over-expression of the APR2 led to elevated expressions of the GSH/PC synthesis-related genes under Cd stress. Taken together, our results indicated that APR2 regulated Cd accumulation and tolerance possibly through modulating GSH-dependent antioxidant capability and Cd-chelation machinery in Arabidopsis. APR2 could be exploited for engineering heavy metal-tolerant plants in phytoremediation.

Copper-Induced Activation of MAPKs, CDPKs and CaMKs Triggers Activation of Hexokinase and Inhibition of Pyruvate Kinase Leading to Increased Synthesis of ASC, GSH and NADPH in Ulva compressa

In order to analyze whether copper induces activation of CaMK, CDPK and/or MAPK signaling pathways leading to carbon flux reprogramming and to the synthesis of ascorbate (ASC), glutathione (GSH) and NADPH in order to buffer copper-induced oxidative stress, U. compressa was initially cultivated with 10 µM copper for 0 to 10 days. The activities of hexokinase (HK), pyruvate kinase (PK), L-galactone 1,4 lactone dehydrogenase (L-GLDH) and glucose 6-P dehydrogenase (G6PDH) were analyzed. HK activity was increased whereas PK was inhibited, and L-GLDH and G6PDH activities were increased indicating a copper-induced modulation of glycolysis leading to carbon flux reprogramming. Then, the alga was cultivated with an inhibitor of CaMs and CaMKs, CDPKs and MAPKs, and with 10 µM of copper for 5 days and the activities of HK, PK, L-GLDH, G6PDH and glutathione synthase (GS), the levels of ASC/DHA, GSG/GSSG and NADPH/NADP, the levels of superoxide anions (SA) and hydrogen peroxide (HP) and the integrity of plasma membrane were determined. The activation of HK was dependent on MAPKs, the inhibition of PK on CDPKs/MAPKs, the activation of L-GLDH on MAPKs, the activation GS on CDPKs/MAPKs, and the activation of G6PDH on MAPKs. Increases in the level of ASC/DHA were dependent on activation of CaMKs/CDPKs/MAPKs, those of GSG/GSSG on MAPKs and those NADPH/NADP on CaMKs/CDPKs/MAPKs. The accumulation of superoxide anions and hydrogen peroxide and the integrity of plasma membrane were dependent on CaMKs/CDPKs/MAPKs. Thus, copper induced the activation of MAPKs, CDPKs and CaMKs leading to the modulation of glycolysis and carbon flux reprogramming which trigger an increase in ASC, GSH and NADPH syntheses and the activation of antioxidant enzymes in order to buffer copper-induced oxidative stress in U. compressa.

Thiol redox-regulation for efficient adjustment of sulfur metabolism in acclimation to abiotic stress

Sulfur assimilation and sulfur metabolism are tightly controlled at the transcriptional, post-transcriptional, and post-translational levels in order to meet the demand for reduced sulfur in growth and metabolism. These regulatory mechanisms coordinate the cellular sulfhydryl supply with carbon and nitrogen assimilation in particular. Redox homeostasis is an important cellular parameter intimately connected to sulfur by means of multiple thiol modifications. Post-translational thiol modifications such as disulfide formation, sulfenylation, S-nitrosylation, persulfidation, and S-glutathionylation allow for versatile switching and adjustment of protein functions. This review focuses on redox-regulation of enzymes involved in the sulfur assimilation pathway, namely adenosine 5´-phosphosulfate reductase (APR), adenosine 5´-phosphosulfate kinase (APSK), and γ-glutamylcysteine ligase (GCL). The activity of these enzymes is adjusted at the transcriptional and post-translational level depending on physiological requirements and the state of the redox and reactive oxygen species network, which are tightly linked to abiotic stress conditions. Hormone-dependent fine-tuning contributes to regulation of sulfur assimilation. Thus, the link between oxylipin signalling and sulfur assimilation has been substantiated by identification of the so-called COPS module in the chloroplast with its components cyclophilin 20-3, O-acetylserine thiol lyase, 2-cysteine peroxiredoxin, and serine acetyl transferase. We now have a detailed understanding of how regulation enables the fine-tuning of sulfur assimilation under both normal and abiotic stress conditions.

Sulfonate-based networks between eukaryotic phytoplankton and heterotrophic bacteria in the surface ocean

In the surface ocean, phytoplankton transform inorganic substrates into organic matter that fuels the activity of heterotrophic microorganisms, creating intricate metabolic networks that determine the extent of carbon recycling and storage in the ocean. Yet, the diversity of organic molecules and interacting organisms has hindered detection of specific relationships that mediate this large flux of energy and matter. Here, we show that a tightly coupled microbial network based on organic sulfur compounds (sulfonates) exists among key lineages of eukaryotic phytoplankton producers and heterotrophic bacterial consumers in the North Pacific Subtropical Gyre. We find that cultured eukaryotic phytoplankton taxa produce sulfonates, often at millimolar internal concentrations. These same phytoplankton-derived sulfonates support growth requirements of an open-ocean isolate of the SAR11 clade, the most abundant group of marine heterotrophic bacteria. Expression of putative sulfonate biosynthesis genes and sulfonate abundances in natural plankton communities over the diel cycle link sulfonate production to light availability. Contemporaneous expression of sulfonate catabolism genes in heterotrophic bacteria highlights active cycling of sulfonates in situ. Our study provides evidence that sulfonates serve as an ecologically important currency for nutrient and energy exchange between microbial autotrophs and heterotrophs, highlighting the importance of organic sulfur compounds in regulating ecosystem function.

Atypical Myrosinase as a Mediator of Glucosinolate Functions in Plants

Glucosinolates (GLSs) are a well-known class of specialized plant metabolites, distributed mostly in the order Brassicales. A vast research field in basic and applied sciences has grown up around GLSs owing to their presence in important agricultural crops and the model plant Arabidopsis thaliana, and their broad range of bioactivities beneficial to human health. The major purpose of GLSs in plants has been considered their function as a chemical defense against predators. GLSs are physically separated from a specialized class of beta-thioglucosidases called myrosinases, at the tissue level or at the single-cell level. They are brought together as a consequence of tissue damage, primarily triggered by herbivores, and their interaction results in the release of toxic volatile chemicals including isothiocyanates. In addition, recent studies have suggested that plants may adopt other strategies independent of tissue disruption for initiating GLS breakdown to cope with certain biotic/abiotic stresses. This hypothesis has been further supported by the discovery of an atypical class of GLS-hydrolyzing enzymes possessing features that are distinct from those of the classical myrosinases. Nevertheless, there is only little information on the physiological importance of atypical myrosinases. In this review, we focus on the broad diversity of the beta-glucosidase subclasses containing known atypical myrosinases in A. thaliana to discuss the hypothesis that numerous members of these subclasses can hydrolyze GLSs to regulate their diverse functions in plants. Also, the increasingly broadening functional repertoires of known atypical/classical myrosinases are described with reference to recent findings. Assessment of independent insights gained from A. thaliana with respect to (1) the phenotype of mutants lacking genes in the GLS metabolic/breakdown pathways, (2) fluctuation in GLS contents/metabolism under specific conditions, and (3) the response of plants to exogenous GLSs or their hydrolytic products, will enable us to reconsider the physiological importance of GLS breakdown in particular situations, which is likely to be regulated by specific beta-glucosidases.

Knock-Down of the Phosphoserine Phosphatase Gene Effects Rather N- Than S-Metabolism in Arabidopsis thaliana

The aim of present study was to elucidate the significance of the phosphorylated pathway of Ser production for Cys biosynthesis in leaves at day and night and upon cadmium (Cd) exposure. For this purpose, Arabidopsis wildtype plants as control and its psp mutant knocked-down in phosphoserine phosphatase (PSP) were used to test if (i) photorespiratory Ser is the dominant precursor of Cys synthesis in autotrophic tissue in the light, (ii) the phosphorylated pathway of Ser production can take over Ser biosynthesis in leaves at night, and (iii) Cd exposure stimulates Cys and glutathione (GSH) biosynthesis and effects the crosstalk of S and N metabolism, irrespective of the Ser source. Glycine (Gly) and Ser contents were not affected by reduction of the psp transcript level confirming that the photorespiratory pathway is the main route of Ser synthesis. The reduction of the PSP transcript level in the mutant did not affect day/night regulation of sulfur fluxes while day/night fluctuation of sulfur metabolite amounts were no longer observed, presumably due to slower turnover of sulfur metabolites in the mutant. Enhanced contents of non-protein thiols in both genotypes and of GSH only in the psp mutant were observed upon Cd treatment. Mutation of the phosphorylated pathway of Ser biosynthesis caused an accumulation of alanine, aspartate, lysine and a decrease of branched-chain amino acids. Knock-down of the PSP gene induced additional defense mechanisms against Cd toxicity that differ from those of WT plants.

The Photorespiratory BOU Gene Mutation Alters Sulfur Assimilation and Its Crosstalk With Carbon and Nitrogen Metabolism in Arabidopsis thaliana

This study was aimed at elucidating the significance of photorespiratory serine (Ser) production for cysteine (Cys) biosynthesis. For this purpose, sulfur (S) metabolism and its crosstalk with nitrogen (N) and carbon (C) metabolism were analyzed in wildtype Arabidopsis and its photorespiratory bou-2 mutant with impaired glycine decarboxylase (GDC) activity. Foliar glycine and Ser contents were enhanced in the mutant at day and night. The high Ser levels in the mutant cannot be explained by transcript abundances of genes of the photorespiratory pathway or two alternative pathways of Ser biosynthesis. Despite enhanced foliar Ser, reduced GDC activity mediated a decline in sulfur flux into major sulfur pools in the mutant, as a result of deregulation of genes of sulfur reduction and assimilation. Still, foliar Cys and glutathione contents in the mutant were enhanced. The use of Cys for methionine and glucosinolates synthesis was reduced in the mutant. Reduced GDC activity in the mutant downregulated Calvin Cycle and nitrogen assimilation genes, upregulated key enzymes of glycolysis and the tricarboxylic acid (TCA) pathway and modified accumulation of sugars and TCA intermediates. Thus, photorespiratory Ser production can be replaced by other metabolic Ser sources, but this replacement deregulates the cross-talk between S, N, and C metabolism.

Central Role of Adenosine 5'-Phosphosulfate Reductase in the Control of Plant Hydrogen Sulfide Metabolism

Hydrogen sulfide (H₂S) has been postulated to be the third gasotransmitter in both animals and plants after nitric oxide (NO) and carbon monoxide (CO). In this review, the physiological roles of H₂S in plant growth, development and responses to biotic, and abiotic stresses are summarized. The enzymes which generate H₂S are subjected to tight regulation to produce H₂S when needed, contributing to delicate responses of H₂S to environmental stimuli. H₂S occupies a central position in plant sulfur metabolism as it is the link of inorganic sulfur to the first organic sulfur-containing compound cysteine which is the starting point for the synthesis of methionine, coenzyme A, vitamins, etc. In sulfur assimilation, adenosine 5'-phosphosulfate reductase (APR) is the rate-limiting enzyme with the greatest control over the pathway and probably the generation of H₂S which is an essential component in this process. APR is an evolutionarily conserved protein among plants, and two conserved domains PAPS_reductase and Thioredoxin are found in APR. Sulfate reduction including the APR-catalyzing step is carried out in chloroplasts. APR, the key enzyme in sulfur assimilation, is mainly regulated at transcription level by transcription factors in response to sulfur availability and environmental stimuli. The cis-acting elements in the promoter region of all the three APR genes in Solanum lycopersicum suggest that multiple factors such as sulfur starvation, cytokinins, CO₂, and pathogens may regulate the expression of SlAPRs. In conclusion, as a critical enzyme in regulating sulfur assimilation, APR is probably critical for H₂S generation during plants' response to diverse environmental factors.

Impact of Sulfur Starvation in Autotrophic and Heterotrophic Cultures of the Extremophilic Microalga Galdieria phlegrea (Cyanidiophyceae)

In plants and algae, sulfate assimilation and cysteine synthesis are regulated by sulfur (S) accessibility from the environment. This study reports the effects of S deprivation in autotrophic and heterotrophic cultures of Galdieria phlegrea (Cyanidiophyceae), a unicellular red alga isolated in the Solfatara crater located in Campi Flegrei (Naples, Italy), where H2S is the prevalent form of gaseous S in the fumarolic fluids and S is widespread in the soils near the fumaroles. This is the first report on the effects of S deprivation on a sulfurous microalga that is also able to grow heterotrophically in the dark. The removal of S from the culture medium of illuminated cells caused a decrease in the soluble protein content and a significant decrease in the intracellular levels of glutathione. Cells from heterotrophic cultures of G. phlegrea exhibited high levels of internal proteins and high glutathione content, which did not diminish during S starvation, but rather glutathione significantly increased. The activity of O-acetylserine(thiol)lyase (OASTL), the enzyme synthesizing cysteine, was enhanced under S deprivation in a time-dependent manner in autotrophic but not in heterotrophic cells. Analysis of the transcript abundance of the OASTL gene supports the OASTL activity increase in autotrophic cultures under S deprivation.

Glucose enhances indolic glucosinolate biosynthesis without reducing primary sulfur assimilation

The effect of glucose as a signaling molecule on induction of aliphatic glucosinolate biosynthesis was reported in our former study. Here, we further investigated the regulatory mechanism of indolic glucosinolate biosynthesis by glucose in Arabidopsis. Glucose exerted a positive influence on indolic glucosinolate biosynthesis, which was demonstrated by induced accumulation of indolic glucosinolates and enhanced expression of related genes upon glucose treatment. Genetic analysis revealed that MYB34 and MYB51 were crucial in maintaining the basal indolic glucosinolate accumulation, with MYB34 being pivotal in response to glucose signaling. The increased accumulation of indolic glucosinolates and mRNA levels of MYB34, MYB51, and MYB122 caused by glucose were inhibited in the gin2-1 mutant, suggesting an important role of HXK1 in glucose-mediated induction of indolic glucosinolate biosynthesis. In contrast to what was known on the function of ABI5 in glucose-mediated aliphatic glucosinolate biosynthesis, ABI5 was not required for glucose-induced indolic glucosinolate accumulation. In addition, our results also indicated that glucose-induced glucosinolate accumulation was due to enhanced sulfur assimilation instead of directed sulfur partitioning into glucosinolate biosynthesis. Thus, our data provide new insights into molecular mechanisms underlying glucose-regulated glucosinolate biosynthesis.

Hormonal control of sulfate uptake and assimilation

Plant hormones have a plethora of functions in control of plant development, stress response, and primary metabolism, including nutrient homeostasis. In the plant nutrition, the interplay of hormones with responses to nitrate and phosphate deficiency is well described, but relatively little is known about the interaction between phytohormones and regulation of sulfur metabolism. As for other nutrients, sulfate deficiency results in modulation of root architecture, where hormones are expected to play an important role. Accordingly, sulfate deficiency induces genes involved in metabolism of tryptophane and auxin. Also jasmonate biosynthesis is induced, pointing to the need of increase the defense capabilities of the plants when sulfur is limiting. However, hormones affect also sulfate uptake and assimilation. The pathway is coordinately induced by jasmonate and the key enzyme, adenosine 5'-phosphosulfate reductase, is additionally regulated by ethylene, abscisic acid, nitric oxid, and other phytohormones. Perhaps the most intriguing link between hormones and sulfate assimilation is the fact that the main regulator of the response to sulfate starvation, SULFATE LIMITATION1 (SLIM1) belongs to the family of ethylene related transcription factors. We will review the current knowledge of interplay between phytohormones and control of sulfur metabolism and discuss the main open questions.

Phylogenetic analysis and photoregulation of siroheme biosynthesis genes: uroporphyrinogen III methyltransferase and sirohydrochlorin ferrochelatase of Arabidopsis thaliana

Uroporphyrinogen III methyl transferase (UPM1) and Sirohydrochlorin ferrochelatase (SIRB) are the important genes involved in the biosynthesis of siroheme, the prosthetic group of nitrite reductases (NiR) and sulfite reductases (SiR) involved in nitrogen and sulfur assimilation. Both UPM1 and SIRB could be potential candidate genes targeted for sustainable agriculture especially in N-deficient soil. The phylogenetic analysis revealed that these genes are highly conserved among algae, bryophytes and vascular plants including dicots and monocots. The Arabidopsis proteins UPM1 and SIRB have close similarity with Camelina sativa followed by Brassica napus, Brassica rapa, and Brassica oleracea of the family brassicaceae. The tissue specific expression studies revealed that both the gene are expressed in stem, flower and silique and have highest expression in leaves where the protein content is quite high. The in silico promoter analysis revealed the presence of several light-responsive elements like GATA box, G box, I box, SORLIP2, SORLIP5, SORLREP3 and SORLREP4. Therefore, expression of both the genes was minimal in etiolated seedlings and was upregulated in light. Photo-regulation of transcript abundance of UPM1 and SIRB involved in the biosynthesis of siroheme the cofactor involved in 6 electron reduction of NO₂^- and SO₃^2- by NiR and SiR is crucial as the gene expression of latter two enzymes along with other N and S assimilatory enzymes are also modulated by light.

Compartmentalization and Regulation of Sulfate Assimilation Pathways in Plants

Plants utilize sulfate to synthesize primary and secondary sulfur-containing metabolites required for growth and survival in the environment. Sulfate is taken up into roots from the soil and distributed to various organs through the functions of membrane-bound sulfate transporters, while it is utilized as the primary substrate for synthesizing sulfur-containing metabolites in the sulfate assimilation pathways. Transporters and enzymes for the assimilative conversion of sulfate are regulated in highly organized manners depending on changes in sulfate supply from the environment and demand for biosynthesis of reduced sulfur compounds in the plant systems. Over the past few decades, the effect of sulfur nutrition on gene expression of sulfate transporters and assimilatory enzymes has been extensively studied with the aim of understanding the full landscape of regulatory networks.

Multifunctionality of plastid nucleoids as revealed by proteome analyses

Protocols aimed at the isolation of nucleoids and transcriptionally active chromosomes (TACs) from plastids of higher plants have been established already decades ago, but only recent improvements in the mass spectrometry methods enabled detailed proteomic characterization of their components. Here we present a comprehensive analysis of the protein compositions obtained from two proteomic studies of TAC fractions isolated from Arabidopsis/mustard and spinach chloroplasts, respectively, as well as nucleoid fractions from Arabidopsis, maize and pea. Interestingly, different approaches as well as the use of diverse starting materials resulted in the detection of varying protein catalogues with a number of shared proteins. Possible reasons for the discrepancies between the protein repertoires and for missing out some of the nucleoid proteins that have been identified previously by other means than mass spectrometry as well as the repeated identification of "unexpected" proteins indicating potential links between DNA/RNA-associated nucleoid core functions and energy metabolism as well as biosynthetic activities of plastids will be discussed. In accordance with the nucleoid association of proteins involved in key functions of plastids including photosynthesis, the phenotypes of mutants lacking one or the other plastid nucleoid-associated protein (ptNAP) show the importance of nucleoid proteins for overall plant development and growth. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Uncovering miRNAs involved in crosstalk between nutrient deficiencies in Arabidopsis

Integrating carbon (C), nitrogen (N), and sulfur (S) metabolism is essential for the growth and development of living organisms. MicroRNAs (miRNAs) play key roles in regulating nutrient metabolism in plants. However, how plant miRNAs mediate crosstalk between different nutrient metabolic pathways is unclear. In this study, deep sequencing of Arabidopsis thaliana small RNAs was used to reveal miRNAs that were differentially expressed in response to C, N, or S deficiency. Comparative analysis revealed that the targets of the differentially expressed miRNAs are involved in different cellular responses and metabolic processes, including transcriptional regulation, auxin signal transduction, nutrient homeostasis, and regulation of development. C, N, and S deficiency specifically induced miR169b/c, miR826 and miR395, respectively. In contrast, miR167, miR172, miR397, miR398, miR399, miR408, miR775, miR827, miR841, miR857, and miR2111 are commonly suppressed by C, N, and S deficiency. In particular, the miRNAs that are induced specifically by a certain nutrient deficiency are often suppressed by other nutrient deficiencies. Further investigation indicated that the modulation of nutrient-responsive miRNA abundance affects the adaptation of plants to nutrient starvation conditions. This study revealed that miRNAs function as important regulatory nodes of different nutrient metabolic pathways.

Brassica napus L. cultivars show a broad variability in their morphology, physiology and metabolite levels in response to sulfur limitations and to pathogen attack

Under adequate sulfur supply, plants accumulate sulfate in the vacuoles and use sulfur-containing metabolites as storage compounds. Under sulfur-limiting conditions, these pools of stored sulfur-compounds are depleted in order to balance the nitrogen to sulfur ratio for protein synthesis. Stress conditions like sulfur limitation and/or pathogen attack induce changes in the sulfate pool and the levels of sulfur-containing metabolites, which often depend on the ecotypes or cultivars. We are interested in investigating the influence of the genetic background of canola (Brassica napus) cultivars in sulfur-limiting conditions on the resistance against Verticillium longisporum. Therefore, four commercially available B. napus cultivars were analyzed. These high-performing cultivars differ in some characteristics described in their cultivar pass, such as several agronomic traits, differences in the size of the root system, and resistance to certain pathogens, such as Phoma and Verticillium. The objectives of the study were to examine and explore the patterns of morphological, physiological and metabolic diversity in these B. napus cultivars at different sulfur concentrations and in the context of plant defense. Results indicate that the root systems are influenced differently by sulfur deficiency in the cultivars. Total root dry mass and length of root hairs differ not only among the cultivars but also vary in their reaction to sulfur limitation and pathogen attack. As a sensitive indicator of stress, several parameters of photosynthetic activity determined by PAM imaging showed a broad variability among the treatments. These results were supported by thermographic analysis. Levels of sulfur-containing metabolites also showed large variations. The data were interrelated to predict the specific behavior during sulfur limitation and/or pathogen attack. Advice for farming are discussed.

Are changes in sulfate assimilation pathway needed for evolution of C4 photosynthesis?

C4 photosynthesis characteristically features a cell-specific localization of enzymes involved in CO2 assimilation in bundle sheath cells (BSC) or mesophyll cells. Interestingly, enzymes of sulfur assimilation are also specifically present in BSC of maize and many other C4 species. This localization, however, could not be confirmed in C4 species of the genus Flaveria. It was, therefore, concluded that the bundle sheath localization of sulfate assimilation occurs only in C4 monocots. However, recently the sulfate assimilation pathway was found coordinately enriched in BSC of Arabidopsis, opening new questions about the significance of such cell-specific localization of the pathway. In addition, next generation sequencing revealed expression gradients of many genes from C3 to C4 species and mathematical modeling proposed a sequence of adaptations during the evolutionary path from C3 to C4. Indeed, such gradient, with higher expression of genes for sulfate reduction in C4 species, has been observed within the genus Flaveria. These new tools provide the basis for reexamining the intriguing question of compartmentalization of sulfur assimilation. Therefore, this review summarizes the findings on spatial separation of sulfur assimilation in C4 plants and Arabidopsis, assesses the information on sulfur assimilation provided by the recent transcriptomics data and discusses their possible impact on understanding this interesting feature of plant sulfur metabolism to find out whether changes in sulfate assimilation are part of a general evolutionary trajectory toward C4 photosynthesis.

Plant sulfur nutrition: From Sachs to Big Data

Together with water and carbon dioxide plants require 14 essential mineral nutrients to finish their life cycle. The research in plant nutrition can be traced back to Julius Sachs, who was the first to experimentally prove the essentiality of mineral nutrients for plants. Among those elements Sachs showed to be essential is sulfur. Plant sulfur nutrition has been not as extensively studied as the nutrition of nitrogen and phosphate, probably because sulfur was not limiting for agriculture. However, with the reduction of atmospheric sulfur dioxide emissions sulfur deficiency has become common. The research in sulfur nutrition has changed over the years from using yeast and algae as experimental material to adopting Arabidopsis as the plant model as well as from simple biochemical measurements of individual parameters to system biology. Here the evolution of sulfur research from the times of Sachs to the current Big Data is outlined.

Predictive sulfur metabolism - a field in flux

The key role of sulfur metabolites in response to biotic and abiotic stress in plants, as well as their importance in diet and health has led to a significant interest and effort in trying to understand and manipulate the production of relevant compounds. Metabolic engineering utilizes a set of theoretical tools to help rationally design modifications that enhance the production of a desired metabolite. Such approaches have proven their value in bacterial systems, however, the paucity of success stories to date in plants, suggests that challenges remain. Here, we review the most commonly used methods for understanding metabolic flux, focusing on the sulfur assimilatory pathway. We highlight known issues with both experimental and theoretical approaches, as well as presenting recent methods for integrating different modeling strategies, and progress toward an understanding of flux at the whole plant level.

Oligo-carrageenan kappa increases NADPH, ascorbate and glutathione syntheses and TRR/TRX activities enhancing photosynthesis, basal metabolism, and growth in Eucalyptus trees

In order to analyze the effect of OC kappa in redox status, photosynthesis, basal metabolism and growth in Eucalyptus globulus, trees were treated with water (control), with OC kappa at 1 mg mL(-1), or treated with inhibitors of NAD(P)H, ascorbate (ASC), and glutathione (GSH) syntheses and thioredoxin reductase (TRR) activity, CHS-828, lycorine, buthionine sulfoximine (BSO), and auranofin, respectively, and with OC kappa, and cultivated for 4 months. Treatment with OC kappa induced an increase in NADPH, ASC, and GSH syntheses, TRR and thioredoxin (TRX) activities, photosynthesis, growth and activities of basal metabolism enzymes such as rubisco, glutamine synthetase (GlnS), adenosine 5'-phosphosulfate reductase (APR), involved in C, N, and S assimilation, respectively, Krebs cycle and purine/pyrimidine synthesis enzymes. Treatment with inhibitors and OC kappa showed that increases in ASC, GSH, and TRR/TRX enhanced NADPH synthesis, increases in NADPH and TRR/TRX enhanced ASC and GSH syntheses, and only the increase in NADPH enhanced TRR/TRX activities. In addition, the increase in NADPH, ASC, GSH, and TRR/TRX enhanced photosynthesis and growth. Moreover, the increase in NADPH, ASC and TRR/TRX enhanced activities of rubisco, Krebs cycle, and purine/pyrimidine synthesis enzymes, the increase in GSH, NADPH, and TRR/TRX enhanced APR activity, and the increase in NADPH and TRR/TRX enhanced GlnS activity. Thus, OC kappa increases NADPH, ASC, and GSH syntheses leading to a more reducing redox status, the increase in NADPH, ASC, GSH syntheses, and TRR/TRX activities are cross-talking events leading to activation of photosynthesis, basal metabolism, and growth in Eucalyptus trees.

Impairment in Sulfite Reductase Leads to Early Leaf Senescence in Tomato Plants

Sulfite reductase (SiR) is an essential enzyme of the sulfate assimilation reductive pathway, which catalyzes the reduction of sulfite to sulfide. Here, we show that tomato (Solanum lycopersicum) plants with impaired SiR expression due to RNA interference (SIR Ri) developed early leaf senescence. The visual chlorophyll degradation in leaves of SIR Ri mutants was accompanied by a reduction of maximal quantum yield, as well as accumulation of hydrogen peroxide and malondialdehyde, a product of lipid peroxidation. Interestingly, messenger RNA transcripts and proteins involved in chlorophyll breakdown in the chloroplasts were found to be enhanced in the mutants, while transcripts and their plastidic proteins, functioning in photosystem II, were reduced in these mutants compared with wild-type leaves. As a consequence of SiR impairment, the levels of sulfite, sulfate, and thiosulfate were higher and glutathione levels were lower compared with the wild type. Unexpectedly, in a futile attempt to compensate for the low glutathione, the activity of adenosine-5'-phosphosulfate reductase was enhanced, leading to further sulfite accumulation in SIR Ri plants. Increased sulfite oxidation to sulfate and incorporation of sulfite into sulfoquinovosyl diacylglycerols were not sufficient to maintain low basal sulfite levels, resulting in accumulative leaf damage in mutant leaves. Our results indicate that, in addition to its biosynthetic role, SiR plays an important role in prevention of premature senescence. The higher sulfite is likely the main reason for the initiation of chlorophyll degradation, while the lower glutathione as well as the higher hydrogen peroxide and malondialdehyde additionally contribute to premature senescence in mutant leaves.

Insights into the regulation of DMSP synthesis in the diatom Thalassiosira pseudonana through APR activity, proteomics and gene expression analyses on cells acclimating to changes in salinity, light and nitrogen

Despite the importance of dimethylsulphoniopropionate (DMSP) in the global sulphur cycle and climate regulation, the biological pathways underpinning its synthesis in marine phytoplankton remain poorly understood. The intracellular concentration of DMSP increases with increased salinity, increased light intensity and nitrogen starvation in the diatom Thalassiosira pseudonana. We used these conditions to investigate DMSP synthesis at the cellular level via analysis of enzyme activity, gene expression and proteome comparison. The activity of the key sulphur assimilatory enzyme, adenosine 5′-phosphosulphate reductase was not coordinated with increasing intracellular DMSP concentration. Under all three treatments coordination in the expression of sulphur assimilation genes was limited to increases in sulphite reductase transcripts. Similarly, proteomic 2D gel analysis only revealed an increase in phosphoenolpyruvate carboxylase following increases in DMSP concentration. Our findings suggest that increased sulphur assimilation might not be required for increased DMSP synthesis, instead the availability of carbon and nitrogen substrates may be important in the regulation of this pathway. This contrasts with the regulation of sulphur metabolism in higher plants, which generally involves up-regulation of several sulphur assimilatory enzymes. In T. pseudonana changes relating to sulphur metabolism were specific to the individual treatments and, given that little coordination was seen in transcript and protein responses across the three growth conditions, different patterns of regulation might be responsible for the increase in DMSP concentration seen under each treatment.

Molecular mechanisms of regulation of sulfate assimilation: first steps on a long road

The pathway of sulfate assimilation, which provides plants with the essential nutrient sulfur, is tightly regulated and coordinated with the demand for reduced sulfur. The responses of metabolite concentrations, enzyme activities and mRNA levels to various signals and environmental conditions have been well described for the pathway. However, only little is known about the molecular mechanisms of this regulation. To date, nine transcription factors have been described to control transcription of genes of sulfate uptake and assimilation. In addition, other levels of regulation contribute to the control of sulfur metabolism. Post-transcriptional regulation has been shown for sulfate transporters, adenosine 5'phosphosulfate reductase, and cysteine synthase. Several genes of the pathway are targets of microRNA miR395. In addition, protein-protein interaction is increasingly found in the center of various regulatory circuits. On top of the mechanisms of regulation of single genes, we are starting to learn more about mechanisms of adaptation, due to analyses of natural variation. In this article, the summary of different mechanisms of regulation will be accompanied by identification of the major gaps in knowledge and proposition of possible ways of filling them.

Diurnal and light regulation of sulphur assimilation and glucosinolate biosynthesis in Arabidopsis

Glucosinolates are a major class of sulphur-containing secondary metabolites involved in plant defence against pathogens. Recently many regulatory links between glucosinolate biosynthesis and sulphate assimilation were established. Since sulphate assimilation undergoes diurnal rhythm and is light regulated, this study analysed whether the same is true for glucosinolate biosynthesis. The levels of glucosinolates and glutathione were found to be higher during the day than during the night. This agreed with variation in sulphate uptake as well as activity of the key enzyme of the sulphate assimilation pathway, adenosine 5'-phosphosulphate reductase. Correspondingly, the flux through sulphate assimilation was higher during the day than during the night, with the maximum flux through primary assimilation preceding maximal incorporation into glucosinolates. Prolonged darkness resulted in a strong reduction in glucosinolate content. Re-illumination of such dark-adapted plants induced accumulation of mRNA for many genes of glucosinolate biosynthesis, leading to increased glucosinolate biosynthesis. The light regulation of the glucosinolate synthesis genes as well as many genes of primary sulphate assimilation was controlled at least partly by the LONG HYPOCOTYL5 (HY5) transcription regulator. Thus, glucosinolate biosynthesis is highly co-regulated with sulphate assimilation.

An essential role for tomato sulfite oxidase and enzymes of the sulfite network in maintaining leaf sulfite homeostasis

Little is known about the homeostasis of sulfite levels, a cytotoxic by-product of plant sulfur turnover. By employing extended dark to induce catabolic pathways, we followed key elements of the sulfite network enzymes that include adenosine-5'-phosphosulfate reductase and the sulfite scavengers sulfite oxidase (SO), sulfite reductase, UDP-sulfoquinovose synthase, and β-mercaptopyruvate sulfurtransferases. During extended dark, SO was enhanced in tomato (Solanum lycopersicum) wild-type leaves, while the other sulfite network components were down-regulated. SO RNA interference plants lacking SO activity accumulated sulfite, resulting in leaf damage and mortality. Exogenous sulfite application induced up-regulation of the sulfite scavenger activities in dark-stressed or unstressed wild-type plants, while expression of the sulfite producer, adenosine-5'-phosphosulfate reductase, was down-regulated. Unstressed or dark-stressed wild-type plants were resistant to sulfite applications, but SO RNA interference plants showed sensitivity and overaccumulation of sulfite. Hence, under extended dark stress, SO activity is necessary to cope with rising endogenous sulfite levels. However, under nonstressed conditions, the sulfite network can control sulfite levels in the absence of SO activity. The novel evidence provided by the synchronous dark-induced turnover of sulfur-containing compounds, augmented by exogenous sulfite applications, underlines the role of SO and other sulfite network components in maintaining sulfite homeostasis, where sulfite appears to act as an orchestrating signal molecule.

Kinetic assays for determining in vitro APS reductase activity in plants without the use of radioactive substances

Adenosine 5'-phosphosulfate (APS) reductase (APR; EC 1.8.4.9) catalyzes the two-electron reduction of APS to sulfite and AMP, a key step in the sulfate assimilation pathway in higher plants. In spite of the importance of this enzyme, methods currently available for detection of APR activity rely on radioactive labeling and can only be performed in a very few specially equipped laboratories. Here we present two novel kinetic assays for detecting in vitro APR activity that do not require radioactive labeling. In the first assay, APS is used as substrate and reduced glutathione (GSH) as electron donor, while in the second assay APS is replaced by an APS-regenerating system in which ATP sulfurylase catalyzes APS in the reaction medium, which employs sulfate and ATP as substrates. Both kinetic assays rely on fuchsin colorimetric detection of sulfite, the final product of APR activity. Incubation of the desalted protein extract, prior to assay initiation, with tungstate that inhibits the oxidation of sulfite by sulfite oxidase activity, resulted in enhancement of the actual APR activity. The reliability of the two methods was confirmed by assaying leaf extract from Arabidopsis wild-type and APR mutants with impaired or overexpressed APR2 protein, the former lacking APR activity and the latter exhibiting much higher activity than the wild type. The assays were further tested on tomato leaves, which revealed a higher APR activity than Arabidopsis. The proposed APR assays are highly specific, technically simple and readily performed in any laboratory.

Effects of fou8/fry1 mutation on sulfur metabolism: is decreased internal sulfate the trigger of sulfate starvation response?

The fou8 loss of function allele of adenosine bisphosphate phosphatase FIERY1 results in numerous phenotypes including the increased enzymatic oxygenation of fatty acids and increased jasmonate synthesis. Here we show that the mutation causes also profound alterations of sulfur metabolism. The fou8 mutants possess lower levels of sulfated secondary compounds, glucosinolates, and accumulate the desulfo-precursors similar to previously described mutants in adenosine 5′phosphosulfate kinase. Transcript levels of genes involved in sulfate assimilation differ in fou8 compared to wild type Col-0 plants and are similar to plants subjected to sulfate deficiency. Indeed, independent microarray analyses of various alleles of mutants in FIERY1 showed similar patterns of gene expression as in sulfate deficient plants. This was not caused by alterations in signalling, as the fou8 mutants contained significantly lower levels of sulfate and glutathione and, consequently, of total elemental sulfur. Analysis of mutants with altered levels of sulfate and glutathione confirmed the correlation of sulfate deficiency-like gene expression pattern with low internal sulfate but not low glutathione. The changes in sulfur metabolism in fou8 correlated with massive increases in 3′-phosphoadenosine 5′-phosphate levels. The analysis of fou8 thus revealed that sulfate starvation response is triggered by a decrease in internal sulfate as opposed to external sulfate availability and that the presence of desulfo-glucosinolates does not induce the glucosinolate synthesis network. However, as well as resolving these important questions on the regulation of sulfate assimilation in plants, fou8 has also opened an array of new questions on the links between jasmonate synthesis and sulfur metabolism.

Additional role of O-acetylserine as a sulfur status-independent regulator during plant growth

O-acetylserine (OAS) is one of the most prominent metabolites whose levels are altered upon sulfur starvation. However, its putative role as a signaling molecule in higher plants is controversial. This paper provides further evidence that OAS is a signaling molecule, based on computational analysis of time-series experiments and on studies of transgenic plants conditionally displaying increased OAS levels. Transcripts whose levels correlated with the transient and specific increase in OAS levels observed in leaves of Arabidopsis thaliana plants 5-10 min after transfer to darkness and with diurnal oscillation of the OAS content, showing a characteristic peak during the night, were identified. Induction of a serine-O-acetyltransferase gene (SERAT) in transgenic A. thaliana plants expressing the genes under the control of an inducible promoter resulted in a specific time-dependent increase in OAS levels. Monitoring the transcriptome response at time points at which no changes in sulfur-related metabolites except OAS were observed and correlating this with the light/dark transition and diurnal experiments resulted in identification of six genes whose expression was highly correlated with that of OAS (adenosine-5'-phosphosulfate reductase 3, sulfur-deficiency-induced 1, sulfur-deficiency-induced 2, low-sulfur-induced 1, serine hydroxymethyltransferase 7 and ChaC-like protein). These data suggest that OAS displays a signalling function leading to changes in transcript levels of a specific gene set irrespective of the sulfur status of the plant. Additionally, a role for OAS in a specific part of the sulfate response can be deduced.

Sulphur limitation and early sulphur deficiency responses in poplar: significance of gene expression, metabolites, and plant hormones

The influence of sulphur (S) depletion on the expression of genes related to S metabolism, and on metabolite and plant hormone contents was analysed in young and mature leaves, fine roots, xylem sap, and phloem exudates of poplar (Populus tremula×Populus alba) with special focus on early consequences. S depletion was applied by a gradual decrease of sulphate availability. The observed changes were correlated with sulphate contents. Based on the decrease in sulphate contents, two phases of S depletion could be distinguished that were denominated as 'S limitation' and 'early S deficiency'. S limitation was characterized by improved sulphate uptake (enhanced root-specific sulphate transporter PtaSULTR1;2 expression) and reduction capacities (enhanced adenosine 5'-phosphosulphate (APS) reductase expression) and by enhanced remobilization of sulphate from the vacuole (enhanced putative vacuolar sulphate transporter PtaSULTR4;2 expression). During early S deficiency, whole plant distribution of S was impacted, as indicated by increasing expression of the phloem-localized sulphate transporter PtaSULTR1;1 and by decreasing glutathione contents in fine roots, young leaves, mature leaves, and phloem exudates. Furthermore, at 'early S deficiency', expression of microRNA395 (miR395), which targets transcripts of PtaATPS3/4 (ATP sulphurylase) for cleavage, increased. Changes in plant hormone contents were observed at 'early S deficiency' only. Thus, S depletion affects S and plant hormone metabolism of poplar during 'S limitation' and 'early S deficiency' in a time series of events. Despite these consequences, the impact of S depletion on growth of poplar plants appears to be less severe than in Brassicaceae such as Arabidopsis thaliana or Brassica sp.

The key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase, is regulated by HY5 in Arabidopsis

Plant sulfate assimilation is regulated by demand for reduced sulfur, as is its key enzyme, adenosine 5'-phosphosulfate reductase (APR). In a genetic screen for mutants lacking this regulation, we identified the bZIP transcription factor LONG HYPOCOTYL 5 (HY5) as a necessary component of the regulatory circuit. Regulation of APR activity by the inhibitor of glutathione synthesis, buthionine sulfoximine, or by the precursor of cysteine, O-acetylserine, was disrupted in the hy5 mutant. When dark-adapted plants were re-illuminated, the rapid induction of APR1 and APR2 mRNA levels was attenuated in hy5 seedlings, but APR3 regulation was not affected. Chromatin immunoprecipitation revealed that HY5 binds directly to the APR1 and APR2 promoters but not to the APR3 promoter. Accordingly, the regulation of APR1 and APR2 by O-acetylserine was disturbed in hy5 roots. HY5 is also important for the coordination of nitrogen and sulfur assimilation, as, unlike the wild-type, hy5 mutants do not undergo a reduction in sulfate uptake and APR activity during nitrogen starvation. Altogether, these data show that HY5 plays an important role in regulation of APR gene expression and plant sulfate assimilation.

Adenosine 5′-phosphosulfate reductase (APR2) mutation in Arabidopsis implicates glutathione deficiency in selenate toxicity

APR2 is the dominant APR (adenosine 5′-phosphosulfate reductase) in the model plant Arabidopsis thaliana, and converts activated sulfate to sulfite, a key reaction in the sulfate reduction pathway. To determine whether APR2 has a role in selenium tolerance and metabolism, a mutant Arabidopsis line (apr2-1) was studied. apr2-1 plants had decreased selenate tolerance and photosynthetic efficiency. Sulfur metabolism was perturbed in apr2-1 plants grown on selenate, as observed by an increase in total sulfur and sulfate, and a 2-fold decrease in glutathione concentration. The altered sulfur metabolism in apr2-1 grown on selenate did not reflect typical sulfate starvation, as cysteine and methionine levels were increased. Knockout of APR2 also increased the accumulation of total selenium and selenate. However, the accumulation of selenite and selenium incorporation in protein was lower in apr2-1 mutants. Decreased incorporation of selenium in protein is typically associated with increased selenium tolerance in plants. However, because the apr2-1 mutant exhibited decreased tolerance to selenate, we propose that selenium toxicity can also be caused by selenate's disruption of glutathione biosynthesis leading to enhanced levels of damaging ROS (reactive oxygen species).

Interplay of SLIM1 and miR395 in the regulation of sulfate assimilation in Arabidopsis

MicroRNAs play a key role in the control of plant development and response to adverse environmental conditions. For example, microRNA395 (miR395), which targets three out of four isoforms of ATP sulfurylase, the first enzyme of sulfate assimilation, as well as a low-affinity sulfate transporter, SULTR2;1, is strongly induced by sulfate deficiency. However, other components of sulfate assimilation are induced by sulfate starvation, so that the role of miR395 is counterintuitive. Here, we describe the regulation of miR395 and its targets by sulfate starvation. We show that miR395 is important for the increased translocation of sulfate to the shoots during sulfate starvation. MiR395 together with the SULFUR LIMITATION 1 transcription factor maintain optimal levels of ATP sulfurylase transcripts to enable increased flux through the sulfate assimilation pathway in sulfate-deficient plants. Reduced expression of ATP sulfurylase (ATPS) alone affects both sulfate translocation and flux, but SULTR2;1 is important for the full rate of sulfate translocation to the shoots. Thus, miR395 is an integral part of the regulatory circuit controlling plant sulfate assimilation with a complex mechanism of action.

Transitions of gene expression induced by short-term blue light

Blue light modulates many processes in plants and plant cells. It influences global and long-term responses, such as seedling development and phototropism, and induces short-term adaptations like stomatal opening and chloroplast movement. Three genes were identified as important for the latter process, namely PHOT1, PHOT2 and CHUP1. The former two phototropin blue light receptors act in perception of the blue light signal. The protein CHUP1 is localised to the outer envelope membrane of chloroplasts and is involved in chloroplast movement. To explore whether short-term reactions required for chloroplast movement are under transcriptional control, we analysed the transcriptome in wild-type Arabidopsis thaliana, phot1, phot2 and chup1 with different blue light treatments for 5 or 30 min. Blue light-induced changes in transcription depended on illumination time and intensity. Illumination with 100 μmol·m(-2) · s(-1) blue light induced down-regulation of several genes and might point to cascades that could be important for sensing low levels of blue light. Analysis of the transcriptome of the mutants in response to the different light regimes suggests that the transcriptional response to blue light in the wild-type can be attributed to phot1 rather than phot2, suggesting that blue light-induced alteration of expression is a function of phot1. In contrast, the blue light response at the transcriptional level of chup1 plants was unique, and confirmed the higher light sensitivity of this mutant.

Control of sulfur partitioning between primary and secondary metabolism

Sulfur is an essential nutrient for all organisms. Plants take up most sulfur as inorganic sulfate, reduce it and incorporate it into cysteine during primary sulfate assimilation. However, some of the sulfate is partitioned into the secondary metabolism to synthesize a variety of sulfated compounds. The two pathways of sulfate utilization branch after activation of sulfate to adenosine 5'-phosphosulfate (APS). Recently we showed that the enzyme APS kinase limits the availability of activated sulfate for the synthesis of sulfated secondary compounds in Arabidopsis. To further dissect the control of sulfur partitioning between the primary and secondary metabolism, we analysed plants in which activities of enzymes that use APS as a substrate were increased or reduced. Reduction in APS kinase activity led to reduced levels of glucosinolates as a major class of sulfated secondary metabolites and an increased concentration of thiols, products of primary reduction. However, over-expression of this gene does not affect the levels of glucosinolates. Over-expression of APS reductase had no effect on glucosinolate levels but did increase thiol levels, but neither glucosinolate nor thiol levels were affected in mutants lacking the APR2 isoform of this enzyme. Measuring the flux through sulfate assimilation using [(35) S]sulfate confirmed the larger flow of sulfur to primary assimilation when APS kinase activity was reduced. Thus, at least in Arabidopsis, the interplay between APS reductase and APS kinase is important for sulfur partitioning between the primary and secondary metabolism.

Genes of primary sulfate assimilation are part of the glucosinolate biosynthetic network in Arabidopsis thaliana

Glucosinolates are plant secondary metabolites involved in responses to biotic stress. The final step of their synthesis is the transfer of a sulfo group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) onto a desulfo precursor. Thus, glucosinolate synthesis is linked to sulfate assimilation. The sulfate donor for this reaction is synthesized from sulfate in two steps catalyzed by ATP sulfurylase (ATPS) and adenosine 5'-phosphosulfate kinase (APK). Here we demonstrate that R2R3-MYB transcription factors, which are known to regulate both aliphatic and indolic glucosinolate biosynthesis in Arabidopsis thaliana, also control genes of primary sulfate metabolism. Using trans-activation assays we found that two isoforms of APK, APK1, and APK2, are regulated by both classes of glucosinolate MYB transcription factors; whereas two ATPS genes, ATPS1 and ATPS3, are differentially regulated by these two groups of MYB factors. In addition, we show that the adenosine 5'-phosphosulfate reductases APR1, APR2, and APR3, which participate in primary sulfate reduction, are also activated by the MYB factors. These observations were confirmed by analysis of transgenic lines with modulated expression levels of the glucosinolate MYB factors. The changes in transcript levels also affected enzyme activities, the thiol content and the sulfate reduction rate in some of the transgenic plants. Altogether the data revealed that the MYB transcription factors regulate genes of primary sulfate metabolism and that the genes involved in the synthesis of activated sulfate are part of the glucosinolate biosynthesis network.

Sulfite reductase defines a newly discovered bottleneck for assimilatory sulfate reduction and is essential for growth and development in Arabidopsis thaliana

The role of sulfite reductase (SiR) in assimilatory reduction of inorganic sulfate to sulfide has long been regarded as insignificant for control of flux in this pathway. Two independent Arabidopsis thaliana T-DNA insertion lines (sir1-1 and sir1-2), each with an insertion in the promoter region of SiR, were isolated. sir1-2 seedlings had 14% SiR transcript levels compared with the wild type and were early seedling lethal. sir1-1 seedlings had 44% SiR transcript levels and were viable but strongly retarded in growth. In mature leaves of sir1-1 plants, the levels of SiR transcript, protein, and enzymatic activity ranged between 17 and 28% compared with the wild type. The 28-fold decrease of incorporation of (35)S label into Cys, glutathione, and protein in sir1-1 showed that the decreased activity of SiR generated a severe bottleneck in the assimilatory sulfate reduction pathway. Root sulfate uptake was strongly enhanced, and steady state levels of most of the sulfur-related metabolites, as well as the expression of many primary metabolism genes, were changed in leaves of sir1-1. Hexose and starch contents were decreased, while free amino acids increased. Inorganic carbon, nitrogen, and sulfur composition was also severely altered, demonstrating strong perturbations in metabolism that differed markedly from known sulfate deficiency responses. The results support that SiR is the only gene with this function in the Arabidopsis genome, that optimal activity of SiR is essential for normal growth, and that its downregulation causes severe adaptive reactions of primary and secondary metabolism.