Latest news about the sulfurtransferase protein family of higher plants

Time-Dependent and Coating Modulation of Tomato Response upon Sulfur Nanoparticle Internalization and Assimilation: An Orthogonal Mechanistic Investigation

Nanoenabled strategies have recently attracted attention as a sustainable platform for agricultural applications. Here, we present a mechanistic understanding of nanobiointeraction through an orthogonal investigation. Pristine (nS) and stearic acid surface-modified (cS) sulfur nanoparticles (NPs) as a multifunctional nanofertilizer were applied to tomato (Solanum lycopersicumL.) through soil. Both nS and cS increased root mass by 73% and 81% and increased shoot weight by 35% and 50%, respectively, compared to the untreated controls. Bulk sulfur (bS) and ionic sulfate (iS) had no such stimulatory effect. Notably, surface modification of S NPs had a positive impact, as cS yielded 38% and 51% greater shoot weight compared to nS at 100 and 200 mg/L, respectively. Moreover, nS and cS significantly improved leaf photosynthesis by promoting the linear electron flow, quantum yield of photosystem II, and relative chlorophyll content. The time-dependent gene expression related to two S bioassimilation and signaling pathways showed a specific role of NP surface physicochemical properties. Additionally, a time-dependent Global Test and machine learning strategy applied to understand the NP surface modification domain metabolomic profiling showed that cS increased the contents of IA, tryptophan, tomatidine, and scopoletin in plant leaves compared to the other treatments. These findings provide critical mechanistic insights into the use of nanoscale sulfur as a multifunctional soil amendment to enhance plant performance as part of nanoenabled agriculture.

Therapeutic Delivery of Nanoscale Sulfur to Suppress Disease in Tomatoes: In Vitro Imaging and Orthogonal Mechanistic Investigation

Nanoscale sulfur can be a multifunctional agricultural amendment to enhance crop nutrition and suppress disease. Pristine (nS) and stearic acid coated (cS) sulfur nanoparticles were added to soil planted with tomatoes (Solanum lycopersicum) at 200 mg/L soil and infested with Fusarium oxysporum. Bulk sulfur, ionic sulfate, and healthy controls were included. Orthogonal end points were measured in two greenhouse experiments, including agronomic and photosynthetic parameters, disease severity/suppression, mechanistic biochemical and molecular end points including the time-dependent expression of 13 genes related to two S bioassimilation and pathogenesis-response, and metabolomic profiles. Disease reduced the plant biomass by up to 87%, but nS and cS amendment significantly reduced disease as determined by area-under-the-disease-progress curve by 54 and 56%, respectively. An increase in planta S accumulation was evident, with size-specific translocation ratios suggesting different uptake mechanisms. In vivo two-photon microscopy and time-dependent gene expression revealed a nanoscale-specific elemental S bioassimilation pathway within the plant that is separate from traditional sulfate accumulation. These findings correlate well with time-dependent metabolomic profiling, which exhibited increased disease resistance and plant immunity related metabolites only with nanoscale treatment. The linked gene expression and metabolomics data demonstrate a time-sensitive physiological window where nanoscale stimulation of plant immunity will be effective. These findings provide mechanistic understandings of nonmetal nanomaterial-based suppression of plant disease and significantly advance sustainable nanoenabled agricultural strategies to increase food production.

Genome-Wide Association Studies Identifying Multiple Loci Associated With Alfalfa Forage Quality

Autotetraploid alfalfa is a major hay crop planted all over the world due to its adaptation in different environments and high quality for animal feed. However, the genetic basis of alfalfa quality is not fully understood. In this study, a diverse panel of 200 alfalfa accessions were planted in field trials using augmented experimental design at three locations in 2018 and 2019. Thirty-four quality traits were evaluated by Near Infrared Reflectance Spectroscopy (NIRS). The plants were genotyped using a genotyping by sequencing (GBS) approach and over 46,000 single nucleotide polymorphisms (SNPs) were obtained after variant calling and filtering. Genome-wide association studies (GWAS) identified 28 SNP markers associated with 16 quality traits. Among them, most of the markers were associated with fiber digestibility and protein content. Phenotypic variations were analyzed from three locations and different sets of markers were identified by GWAS when using phenotypic data from different locations, indicating that alfalfa quality traits were also affected by environmental factors. Among different sets of markers identified by location, two markers were associated with nine traits of fiber digestibility. One marker associated with lignin content was identified consistently in multiple environments. Putative candidate genes underlying fiber-related loci were identified and they are involved in the lignin and cell wall biosynthesis. The DNA markers and associated genes identified in this study will be useful for the genetic improvement of forage quality in alfalfa after the validation of the markers.

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.

Novel insights into the diversity of the sulfurtransferase family in photosynthetic organisms with emphasis on oak

Sulfurtransferases (STRs) constitute a large and complex protein family characterized by the presence of a rhodanese domain and implicated in diverse molecular and signaling processes as sulfur carriers. Although sulfurtransferases are present in the three domains of life and share evolutionary relationships, a high variability exists at different levels including the protein length and active site sequence, the presence of an indispensable catalytic cysteine residue, the domain arrangement and the subcellular localization. Because only Arabidopsis thaliana sequences have been inventoried so far, this paper aims at providing a detailed classification and inventory of evolutionary features of this family in photosynthetic organisms using comparative genomics, focusing on the oak genome. Based on the expansion of STRs in higher photosynthetic organisms, we classified the STR family in nine clusters depending on their primary sequence and domain arrangement. We found that oak possesses at least one isoform in all defined clusters and that clusters IV, V and VI contain plant-specific isoforms that are located mostly in chloroplasts. The novel classification proposed here provides the basis for functional genomics approaches in order to dissect the biochemical characteristics and physiological functions of individual STR representatives.

Rhodanese domain-containing sulfurtransferases: multifaceted proteins involved in sulfur trafficking in plants

Sulfur is an essential element for the growth and development of plants, which synthesize cysteine and methionine from the reductive assimilation of sulfate. Besides its incorporation into proteins, cysteine is the building block for the biosynthesis of numerous sulfur-containing molecules and cofactors. The required sulfur atoms are extracted either directly from cysteine by cysteine desulfurases or indirectly after its catabolic transformation to 3-mercaptopyruvate, a substrate for sulfurtransferases (STRs). Both enzymes are transiently persulfidated in their reaction cycle, i.e. the abstracted sulfur atom is bound to a reactive cysteine residue in the form of a persulfide group. Trans-persulfidation reactions occur when sulfur atoms are transferred to nucleophilic acceptors such as glutathione, proteins, or small metabolites. STRs form a ubiquitous, multigenic protein family. They are characterized by the presence of at least one rhodanese homology domain (Rhd), which usually contains the catalytic, persulfidated cysteine. In this review, we focus on Arabidopsis STRs, presenting the sequence characteristics of all family members as well as their biochemical and structural features. The physiological functions of particular STRs in the biosynthesis of molybdenum cofactor, thio-modification of cytosolic tRNAs, arsenate tolerance, cysteine catabolism, and hydrogen sulfide formation are also discussed.

Identification and Expression Analysis of Genes Induced in Response to Tomato chlorosis virus Infection in Tomato

Tomato (Solanum lycopersicum) is one of the most widely grown and economically important vegetable crops in the world. Tomato chlorosis virus (ToCV) is one of the recently emerged viruses of tomato distributed worldwide. ToCV-tomato interaction was investigated at the molecular level for determining changes in the expression of tomato genes in response to ToCV infection in this study. A cDNA library enriched with genes induced in response to ToCV infection were constructed and 240 cDNAs were sequenced from this library. The macroarray analysis of 108 cDNAs revealed that the expression of 92 non-redundant tomato genes was induced by 1.5-fold or greater in response to ToCV infection. The majority of ToCV-induced genes identified in this study were associated with a variety of cellular functions including transcription, defense and defense signaling, metabolism, energy, transport facilitation, protein synthesis and fate and cellular biogenesis. Twenty ToCV-induced genes from different functional groups were selected and induction of 19 of these genes in response to ToCV infection was validated by RT-qPCR assay. Finally, the expression of 6 selected genes was analyzed in different stages of ToCV infection from 0 to 45 dpi. While the expression of three of these genes was only induced by ToCV infection, others were induced both by ToCV infection and wounding. The result showed that ToCV induced the basic defense response and activated the defense signaling in tomato plants at different stages of the infection. Functions of these defense related genes and their potential roles in disease development and resistance to ToCV are also discussed.

Sulfite Reductase Co-suppression in Tobacco Reveals Detoxification Mechanisms and Downstream Responses Comparable to Sulfate Starvation

Sulfite reductase (SIR) is a key enzyme in higher plants in the assimilatory sulfate reduction pathway. SIR, being exclusively localized in plastids, catalyzes the reduction of sulfite (SO3 2-) to sulfide (S2-) and is essential for plant life. We characterized transgenic plants leading to co-suppression of the SIR gene in tobacco (Nicotiana tabacum cv. Samsun NN). Co-suppression resulted in reduced but not completely extinguished expression of SIR and in a reduction of SIR activity to about 20-50% of the activity in control plants. The reduction of SIR activity caused chlorotic and necrotic phenotypes in tobacco leaves, but with varying phenotype strength even among clones and increasing from young to old leaves. In transgenic plants compared to control plants, metabolite levels upstream of SIR accumulated, such as sulfite, sulfate and thiosulfate. The levels of downstream metabolites were reduced, such as cysteine, glutathione (GSH) and methionine. This metabolic signature resembles a sulfate deprivation phenotype as corroborated by the fact that O-acetylserine (OAS) accumulated. Further, chlorophyll contents, photosynthetic electron transport, and the contents of carbohydrates such as starch, sucrose, fructose, and glucose were reduced. Amino acid compositions were altered in a complex manner due to the reduction of contents of cysteine, and to some extent methionine. Interestingly, sulfide levels remained constant indicating that sulfide homeostasis is crucial for plant performance and survival. Additionally, this allows concluding that sulfide does not act as a signal in this context to control sulfate uptake and assimilation. The accumulation of upstream compounds hints at detoxification mechanisms and, additionally, a control exerted by the downstream metabolites on the sulfate uptake and assimilation system. Co-suppression lines showed increased sensitivity to additionally imposed stresses probably due to the accumulation of reactive compounds because of insufficient detoxification in combination with reduced GSH levels.

Enzymatic reactions involving the heteroatoms from organic substrates

Several enzymatic reactions of heteroatom-containing compounds have been explored as unnatural substrates. Considerable advances related to the search for efficient enzymatic systems able to support a broader substrate scope with high catalytic performance are described in the literature. These reports include mainly native and mutated enzymes and whole cells biocatalysis. Herein, we describe the historical background along with the progress of biocatalyzed reactions involving the heteroatom(S, Se, B, P and Si) from hetero-organic substrates.

A single-domain rhodanese homologue MnRDH1 helps to maintain redox balance in Macrobrachium nipponense

Rhodaneses are known to catalyze in vitro the transfer of a sulfane sulfur atom from thiosulfate to cyanide with concomitant formation of thiocyanate, however, their biological functions remain speculative despite the main role is considered as detoxifying cyanide especially in animal livers. In this study, we characterized a single-domain rhodanese homologue, MnRDH1, from Macrobrachium nipponense. We found MnRDH1 with the highest expression in hemocytes. Upon Aeromonas hydrophila challenge, expression of MnRDH1 was up-regulated in various tissues, including hepatopancreas, gill, intestine and hemocytes. RNAi knockdown of MnRDH1 led to rapid increases of malondialdehyde content, which reveals that MnRDH1 deficiency causes oxidative stress. The expression of MnRDH1 in hepatopancreas was significantly increased in response to the doxorubicin-induced oxidative stress, indicating the gene is oxidative stress inducible. We transformed E. coli with MnRDH1 and the mutant MnRDH1^C75A, and found significant rhodanese activity of the recombinant protein of MnRDH1 in vitro, but detected no enzyme activity of the mutant MnRDH1^C75A. When under the oxidative insult by H₂O₂, the MnRDH1 transformed E. coli had significantly enhanced survival rates compared to those bacteria transformed with MnRDH1^C75A. In conclusion, our study demonstrates that rhodanese in M. nipponense confers oxidative stress tolerance, and thus renders an evidence for the notion that rhodanese family genes act a critical role in antioxidant defenses.

Exogenous Supplementation of Silicon Improved the Recovery of Hyperhydric Shoots in Dianthus caryophyllus L. by Stabilizing the Physiology and Protein Expression

Hyperhydricity is one of the major problems hindering in vitro propagation of Dianthus caryophyllus L. Silicon (Si) is a well-known beneficial element renowned for its stress amelioration properties in plants. This study has demonstrated the physiological and molecular mechanism behind the Si-mediated recovery from hyperhydricity in D. caryophyllus L. 'Green Beauty'. Four weeks old hyperhydric shoots obtained from temporary immersion system were cultured on the Murashige and Skoog medium supplemented with 0 (control), 1.8 mM, or 3.6 mM of potassium silicate (K2SiO3). After 2 weeks of culture, we observed only 20% of hyperhydric shoots were recovered in control. On the other hand hyperhydricity, shoot recovery percentage in 1.8 mM and 3.6 mM of Si were 44% and 36%, respectively. Shoots in control possessed higher lipid peroxidation rate compared to the Si treatments. Similarly, damaged stomata were detected in the control, while Si treatments restored the normal stomatal development. Expressions of superoxide dismutase, guaiacol peroxidase, and catalase varied between the control and Si treatments. Furthermore, a proteomic analysis showed that as compared with the control Si up-regulated 17 and 10 protein spots in abundance at 1.8 and 3.6 mM of Si, respectively. In comparison to the 3.6 mM, 1.8 mM of Si treatment up-regulated 19 proteins and down-regulated 7 proteins. Identified proteins were categorized into six groups according to their biological roles such as ribosomal binding, oxido-reduction, hormone/cell signaling, metal/ion binding, defense, and photosynthesis. The proteomic results revealed that Si actively involved in the various metabolisms to accelerate the recovery of the shoots from hyperhydricity. Thus, the outcomes of this study can be utilized for addressing the molecular insight of hyperhydricity and its recovery mechanism by the supplementation of Si. Therefore, we conclude that active involvement of Si in the regulation and signaling process of proteins at 1.8 mM concentration could be efficient to trigger the reclamation process of hyperhydric carnation shoots.

The β-cyanoalanine synthase pathway: beyond cyanide detoxification

Production of cyanide through biological and environmental processes requires the detoxification of this metabolic poison. In the 1960s, discovery of the β-cyanoalanine synthase (β-CAS) pathway in cyanogenic plants provided the first insight on cyanide detoxification in nature. Fifty years of investigations firmly established the protective role of the β-CAS pathway in cyanogenic plants and its role in the removal of cyanide produced from ethylene synthesis in plants, but also revealed the importance of this pathway for plant growth and development and the integration of nitrogen and sulfur metabolism. This review describes the β-CAS pathway, its distribution across and within higher plants, and the diverse biological functions of the pathway in cyanide assimilation, plant growth and development, stress tolerance, regulation of cyanide and sulfide signalling, and nitrogen and sulfur metabolism. The collective roles of the β-CAS pathway highlight its potential evolutionary and ecological importance in plants.

S-Nitrosylation Induces Structural and Dynamical Changes in a Rhodanese Family Protein

S-Nitrosylation is well established as an important post-translational regulator in protein function and signaling. However, relatively little is known about its structural and dynamical consequences. We have investigated the effects of S-nitrosylation on the rhodanese domain of the Escherichia coli integral membrane protein YgaP by NMR, X-ray crystallography, and mass spectrometry. The results show that the active cysteine in the rhodanese domain of YgaP is subjected to two competing modifications: S-nitrosylation and S-sulfhydration, which are naturally occurring in vivo. It has been observed that in addition to inhibition of the sulfur transfer activity, S-nitrosylation of the active site residue Cys63 causes an increase in slow motion and a displacement of helix 5 due to a weakening of the interaction between the active site and the helix dipole. These findings provide an example of how nitrosative stress can exert action at the atomic level.

Proteomic Insight into the Response of Arabidopsis Chloroplasts to Darkness

Chloroplast function in photosynthesis is essential for plant growth and development. It is well-known that chloroplasts respond to various light conditions. However, it remains poorly understood about how chloroplasts respond to darkness. In this study, we found 81 darkness-responsive proteins in Arabidopsis chloroplasts under 8 h darkness treatment. Most of the proteins are nucleus-encoded, indicating that chloroplast darkness response is closely regulated by the nucleus. Among them, 17 ribosome proteins were obviously reduced after darkness treatment. The protein expressional patterns and physiological changes revealed the mechanisms in chloroplasts in response to darkness, e.g., (1) inhibition of photosystem II resulted in preferential cyclic electron flow around PSI; (2) promotion of starch degradation; (3) inhibition of chloroplastic translation; and (4) regulation by redox and jasmonate signaling. The results have improved our understanding of molecular regulatory mechanisms in chloroplasts under darkness.

Chilling-responsive mechanisms in halophyte Puccinellia tenuiflora seedlings revealed from proteomics analysis

Alkali grass (Puccinellia tenuiflora), a monocotyledonous perennial halophyte species, is a good pasture with great nutritional value for livestocks. It can thrive under low temperature in the saline-alkali soil of Songnen plain in northeastern China. In the present study, the chilling-responsive mechanism in P. tenuiflora leaves was investigated using physiological and proteomic approaches. After treatment of 10°C for 10 and 20days, photosynthesis, biomass, contents of osmolytes and antioxidants, and activities of reactive oxygen species scavenging enzymes were analyzed in leaves of 20-day-old seedlings. Besides, 89 chilling-responsive proteins were revealed from proteomic analysis. All the results highlighted that the growth of seedlings was inhibited due to chilling-decreased enzymes in photosynthesis, carbohydrate metabolism, and energy supplying. The accumulation of osmolytes (i.e., proline, soluble sugar, and glycine betaine) and enhancement of ascorbate-glutathione cycle and glutathione peroxidase/glutathione S-transferase pathway in leaves could minimize oxidative damage of membrane and other molecules under the chilling conditions. In addition, protein synthesis and turnover in cytoplasm and chloroplast were altered to cope with the chilling stress. This study provides valuable information for understanding the chilling-responsive and cross-tolerant mechanisms in monocotyledonous halophyte plant species.

Sulfite Oxidase Activity Is Essential for Normal Sulfur, Nitrogen and Carbon Metabolism in Tomato Leaves

Plant sulfite oxidase [SO; E.C.1.8.3.1] has been shown to be a key player in protecting plants against exogenous toxic sulfite. Recently we showed that SO activity is essential to cope with rising dark-induced endogenous sulfite levels in tomato plants (Lycopersicon esculentum/Solanum lycopersicum Mill. cv. Rheinlands Ruhm). Here we uncover the ramifications of SO impairment on carbon, nitrogen and sulfur (S) metabolites. Current analysis of the wild-type and SO-impaired plants revealed that under controlled conditions, the imbalanced sulfite level resulting from SO impairment conferred a metabolic shift towards elevated reduced S-compounds, namely sulfide, S-amino acids (S-AA), Co-A and acetyl-CoA, followed by non-S-AA, nitrogen and carbon metabolite enhancement, including polar lipids. Exposing plants to dark-induced carbon starvation resulted in a higher degradation of S-compounds, total AA, carbohydrates, polar lipids and total RNA in the mutant plants. Significantly, a failure to balance the carbon backbones was evident in the mutants, indicated by an increase in tricarboxylic acid cycle (TCA) cycle intermediates, whereas a decrease was shown in stressed wild-type plants. These results indicate that the role of SO is not limited to a rescue reaction under elevated sulfite, but SO is a key player in maintaining optimal carbon, nitrogen and sulfur metabolism in tomato plants.

Physiological Implications of Hydrogen Sulfide in Plants: Pleasant Exploration behind Its Unpleasant Odour

Recently, overwhelming evidence has proven that hydrogen sulfide (H₂S), which was identified as a gasotransmitter in animals, plays important roles in diverse physiological processes in plants as well. With the discovery and systematic classification of the enzymes producing H₂S in vivo, a better understanding of the mechanisms by which H₂S influences plant responses to various stimuli was reached. There are many functions of H₂S, including the modulation of defense responses and plant growth and development, as well as the regulation of senescence and maturation. Additionally, mounting evidence indicates that H₂S signaling interacts with plant hormones, hydrogen peroxide, nitric oxide, carbon monoxide, and other molecules in signaling pathways.

Mechanisms on boron-induced alleviation of aluminum-toxicity in Citrus grandis seedlings at a transcriptional level revealed by cDNA-AFLP analysis

The physiological and biochemical mechanisms on boron (B)-induced alleviation of aluminum (B)-toxicity in plants have been examined in some details, but our understanding of the molecular mechanisms underlying these processes is very limited. In this study, we first used the cDNA-AFLP to investigate the gene expression patterns in Citrus grandis roots responsive to B and Al interactions, and isolated 100 differentially expressed genes. Results showed that genes related to detoxification of reactive oxygen species (ROS) and aldehydes (i.e., glutathione S-transferase zeta class-like isoform X1, thioredoxin M-type 4, and 2-alkenal reductase (NADP+-dependent)-like), metabolism (i.e., carboxylesterases and lecithin-cholesterol acyltransferase-like 4-like, nicotianamine aminotransferase A-like isoform X3, thiosulfate sulfurtransferase 18-like isoform X1, and FNR, root isozyme 2), cell transport (i.e., non-specific lipid-transfer protein-like protein At2g13820-like and major facilitator superfamily protein), Ca signal and hormone (i.e., calcium-binding protein CML19-like and IAA-amino acid hydrolase ILR1-like 4-like), gene regulation (i.e., Gag-pol polyprotein) and cell wall modification (i.e., glycosyl hydrolase family 10 protein) might play a role in B-induced alleviation of Al-toxicity. Our results are useful not only for our understanding of molecular processes associated with B-induced alleviation of Al-toxicity, but also for obtaining key molecular genes to enhance Al-tolerance of plants in the future.

Spatial gene expression analysis in tomato hypocotyls suggests cysteine as key precursor of vascular sulfur accumulation implicated in Verticillium dahliae defense

Verticillium dahliae is a prominent generator of plant vascular wilting disease and sulfur (S)-enhanced defense (SED) mechanisms contribute to its in-planta elimination. The accumulation of S-containing defense compounds (SDCs) including elemental S (S(0) ) has been described based on the comparison of two near-isogenic tomato (Solanum lycopersicum) lines differing in fungal susceptibility. To better understand the effect of S nutrition on V. dahliae resistance both lines were supplied with low, optimal or supraoptimal sulfate-S. An absolute quantification demonstrated a most effective fungal elimination due to luxury plant S nutrition. High-pressure liquid chromatography (HPLC) showed a strong regulation of Cys levels and an S-responsive GSH pool rise in the bulk hypocotyl. High-frequency S peak accumulations were detected in vascular bundles of resistant tomato plants after fungal colonization by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Global transcriptomic analysis suggested that early steps of the primary S metabolism did not promote the SDCs synthesis in the whole hypocotyl as gene expression was downregulated after infection. Enhanced S fertilization mostly alleviated the repressive fungal effect but did not reverse it. Upregulation of glutathione (GSH)-associated genes in bulk hypocotyls but not in vascular bundles indicated a global antioxidative role of GSH. To finally assign the contribution of S metabolism-associated genes to high S(0) accumulations exclusively found in the resistant tomato line, a spatial gene expression approach was applied. Laser microdissection of infected vascular bundles revealed a switch toward transcription of genes connected with cysteine (Cys) synthesis. The upregulation of LeOASTLp1 suggests a role for Cys as key precursor for local S accumulations (possibly S(0) ) in the vascular bundles of the V. dahliae-resistant tomato line.

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.

Solution NMR structure and functional analysis of the integral membrane protein YgaP from Escherichia coli

The solution NMR structure of the α-helical integral membrane protein YgaP from Escherichia coli in mixed 1,2-diheptanoyl-sn-glycerol-3-phosphocholine/1-myristoyl-2-hydroxy-sn-glycero-3-phospho-(1'-rac-glycerol) micelles is presented. In these micelles, YgaP forms a homodimer with the two transmembrane helices being the dimer interface, whereas the N-terminal cytoplasmic domain includes a rhodanese-fold in accordance to its sequence homology to the rhodanese family of sulfurtransferases. The enzymatic sulfur transfer activity of full-length YgaP as well as of the N-terminal rhodanese domain only was investigated performing a series of titrations with sodium thiosulfate and potassium cyanide monitored by NMR and EPR. The data indicate the thiosulfate concentration-dependent addition of several sulfur atoms to the catalytic Cys-63, which process can be reversed by the addition of potassium cyanide. The catalytic reaction induces thereby conformational changes within the rhodanese domain, as well as on the transmembrane α-helices of YgaP. These results provide insights into a potential mechanism of YgaP during the catalytic thiosulfate activity in vivo.

Identification of photoperiod-regulated gene in soybean and functional analysis in Nicotiana benthamiana

Soybean (Glycine max) is a short-day crop and the photoperiod is a crucial factor regulating its flowering time. To investigate the molecular mechanism controlling the flowering time by photoperiod in soybean, cDNA-amplified fragment length polymorphism (cDNA-AFLP) was used to identify photoperiod-regulated genes in leaves of soybean growing under short-day length, neutral photoperiod and long-day length. A total of 36 transcript-derived fragments (TDFs) were identified to be regulated by photoperiod. Among them, 26 TDFs were homologues of genes with known function. These genes are involved in secondary metabolism, cellular metabolism, cell wall components metabolism, ion transport and hormone signalling. Silencing of the homologue genes in Nicotiana benthamiana for 14 TDFs was conducted by virus-induced gene silencing. The flowering time was delayed by silencing of the genes encoding rhodanese and 40S ribosomal protein S4 (RPS4). The results indicated that rhodanese and RPS4 probably play important roles in regulating flowering time.

Ferredoxin:thioredoxin reductase (FTR) links the regulation of oxygenic photosynthesis to deeply rooted bacteria

Uncovered in studies on photosynthesis 35 years ago, redox regulation has been extended to all types of living cells. We understand a great deal about the occurrence, function, and mechanism of action of this mode of regulation, but we know little about its origin and its evolution. To help fill this gap, we have taken advantage of available genome sequences that make it possible to trace the phylogenetic roots of members of the system that was originally described for chloroplasts-ferredoxin, ferredoxin:thioredoxin reductase (FTR), and thioredoxin as well as target enzymes. The results suggest that: (1) the catalytic subunit, FTRc, originated in deeply rooted microaerophilic, chemoautotrophic bacteria where it appears to function in regulating CO(2) fixation by the reverse citric acid cycle; (2) FTRc was incorporated into oxygenic photosynthetic organisms without significant structural change except for addition of a variable subunit (FTRv) seemingly to protect the Fe-S cluster against oxygen; (3) new Trxs and target enzymes were systematically added as evolution proceeded from bacteria through the different types of oxygenic photosynthetic organisms; (4) an oxygenic type of regulation preceded classical light-dark regulation in the regulation of enzymes of CO(2) fixation by the Calvin-Benson cycle; (5) FTR is not universally present in oxygenic photosynthetic organisms, and in certain early representatives is seemingly functionally replaced by NADP-thioredoxin reductase; and (6) FTRc underwent structural diversification to meet the ecological needs of a variety of bacteria and archaea.

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.

Mitochondrial sulfide detoxification requires a functional isoform O-acetylserine(thiol)lyase C in Arabidopsis thaliana

In non-cyanogenic species, the main source of cyanide derives from ethylene and camalexin biosyntheses. In mitochondria, cyanide is a potent inhibitor of the cytochrome c oxidase and is metabolized by the β-cyanoalanine synthase CYS-C1, catalyzing the conversion of cysteine and cyanide to hydrogen sulfide and β-cyanoalanine. The hydrogen sulfide released also inhibits the cytochrome c oxidase and needs to be detoxified by the O-acetylserine(thiol)lyase mitochondrial isoform, OAS-C, which catalyzes the incorporation of sulfide to O-acetylserine to produce cysteine, thus generating a cyclic pathway in the mitochondria. The loss of functional OAS-C isoforms causes phenotypic characteristics very similar to the loss of the CYS-C1 enzyme, showing defects in root hair formation. Genetic complementation with the OAS-C gene rescues the impairment of root hair elongation, restoring the wild-type phenotype. The mitochondria compromise their capacity to properly detoxify cyanide and the resulting sulfide because the latter cannot re-assimilate into cysteine in the oas-c null mutant. Consequently, we observe an accumulation of sulfide and cyanide and of the alternative oxidase, which is unable to prevent the production of reactive oxygen species probably due to the accumulation of both toxic molecules. Our results allow us to suggest that the significance of OAS-C is related to its role in the proper sulfide and cyanide detoxification in mitochondria.

Involvement of the Azotobacter vinelandii rhodanese-like protein RhdA in the glutathione regeneration pathway

The phenotypic features of the Azotobacter vinelandii RhdA mutant MV474 (in which the rhdA gene was deleted) indicated that defects in antioxidant systems in this organism were related to the expression of the tandem-domain rhodanese RhdA. In this work, further insights on the effects of the oxidative imbalance generated by the absence of RhdA (e.g. increased levels of lipid hydroperoxides) are provided. Starting from the evidence that glutathione was depleted in MV474, and using both in silico and in vitro approaches, here we studied the interaction of wild-type RhdA and Cys²³⁰Ala site-directed RhdA mutant with glutathione species. We found that RhdA was able to bind in vitro reduced glutathione (GSH) and that RhdA-Cys²³⁰ residue was mandatory for the complex formation. RhdA catalyzed glutathione-disulfide formation in the presence of a system generating the glutathione thiyl radical (GS^•, an oxidized form of GSH), thereby facilitating GSH regeneration. This reaction was negligible when the Cys²³⁰Ala RhdA mutant was used. The efficiency of RhdA as catalyst in GS^•-scavenging activity is discussed on the basis of the measured parameters of both interaction with glutathione species and kinetic studies.

Rhodanese functions as sulfur supplier for key enzymes in sulfur energy metabolism

How microorganisms obtain energy is a challenging topic, and there have been numerous studies on the mechanisms involved. Here, we focus on the energy substrate traffic in the hyperthermophilic bacterium Aquifex aeolicus. This bacterium can use insoluble sulfur as an energy substrate and has an intricate sulfur energy metabolism involving several sulfur-reducing and -oxidizing supercomplexes and enzymes. We demonstrate that the cytoplasmic rhodanese SbdP participates in this sulfur energy metabolism. Rhodaneses are a widespread family of proteins known to transfer sulfur atoms. We show that SbdP has also some unusual characteristics compared with other rhodaneses; it can load a long sulfur chain, and it can interact with more than one partner. Its partners (sulfur reductase and sulfur oxygenase reductase) are key enzymes of the sulfur energy metabolism of A. aeolicus and share the capacity to use long sulfur chains as substrate. We demonstrate a positive effect of SbdP, once loaded with sulfur chains, on sulfur reductase activity, most likely by optimizing substrate uptake. Taken together, these results lead us to propose a physiological role for SbdP as a carrier and sulfur chain donor to these key enzymes, therefore enabling channeling of sulfur substrate in the cell as well as greater efficiency of the sulfur energy metabolism of A. aeolicus.

Molecular Biology, Biochemistry and Cellular Physiology of Cysteine Metabolism in Arabidopsis thaliana

Cysteine is one of the most versatile molecules in biology, taking over such different functions as catalysis, structure, regulation and electron transport during evolution. Research on Arabidopsis has contributed decisively to the understanding of cysteine synthesis and its role in the assimilatory pathways of S, N and C in plants. The multimeric cysteine synthase complex is present in the cytosol, plastids and mitochondria and forms the centre of a unique metabolic sensing and signaling system. Its association is reversible, rendering the first enzyme of cysteine synthesis active and the second one inactive, and vice-versa. Complex formation is triggered by the reaction intermediates of cysteine synthesis in response to supply and demand and gives rise to regulation of genes of sulfur metabolism to adjust cellular sulfur homeostasis. Combinations of biochemistry, forward and reverse genetics, structural- and cell-biology approaches using Arabidopsis have revealed new enzyme functions and the unique pattern of spatial distribution of cysteine metabolism in plant cells. These findings place the synthesis of cysteine in the centre of the network of primary metabolism.