Arabidopsis root-abundant cytosolic methionine sulfoxide reductase B genes MsrB7 and MsrB8 are involved in tolerance to oxidative stress

Function analysis of transcription factor OSR1 regulating osmotic stress resistance in maize

BACKGROUND

Maize is a major cereal crop world widely, however, the yield of maize is frequently limited by dehydration and even death of plants, which resulted from osmotic stress such as drought and salinity. Dissection of molecular mechanisms controlling stress tolerance will enable plant scientists and breeders to increase crops yield by manipulating key regulatory components.

METHODS

The candidate OSR1 gene was identified by map-based cloning. The expression level of OSR1 was verified by qRT-PCR and digital PCR in WT and osr1 mutant. Electrophoretic mobility shift assay, transactivation activity assay, subcellular localization, transcriptome analysis and physiological characters measurements were conducted to analyze the function of OSR1 in osmotic stress resistance in maize.

RESULTS

The osr1 mutant was significantly less sensitive to osmotic stress than the WT plants and displayed stronger water-holding capacity, and the OSR1 homologous mutant in Arabidopsis showed a phenotype similar with maize osr1 mutant. Differentially expressed genes (DEGs) were identified between WT and osr1 under osmotic stress by transcriptome analysis, the expression levels of many genes, such as LEA, auxin-related factors, PPR family members, and TPR family members, changed notably, which may primarily involve in osmotic stress or promote root development.

CONCLUSIONS

OSR1 may serve as a negative regulatory factor in response to osmotic stress in maize. The present study sheds new light on the molecular mechanisms of osmotic stress in maize.

Genome-Wide Identification and Characterization of the Msr Gene Family in Alfalfa under Abiotic Stress

Alfalfa (Medicago sativa) is an important leguminous forage, known as the "The Queen of Forages". Abiotic stress seriously limits the growth and development of alfalfa, and improving the yield and quality has become an important research area. However, little is known about the Msr (methionine sulfoxide reductase) gene family in alfalfa. In this study, 15 Msr genes were identified through examining the genome of the alfalfa "Xinjiang DaYe". The MsMsr genes differ in gene structure and conserved protein motifs. Many cis-acting regulatory elements related to the stress response were found in the promoter regions of these genes. In addition, a transcriptional analysis and qRT-PCR (quantitative reverse transcription PCR) showed that MsMsr genes show expression changes in response to abiotic stress in various tissues. Overall, our results suggest that MsMsr genes play an important role in the response to abiotic stress for alfalfa.

Arabidopsis AtMSRB5 functions as a salt-stress protector for both Arabidopsis and rice

Salinity, drought and low temperature are major environmental factors that adversely affect crop productivity worldwide. In this study we adopted an activation tagging approach to identify salt tolerant mutants of Arabidopsis. Thousands of tagged Arabidopsis lines were screened to obtain several potential mutant lines resistant to 150 mM NaCl. Transcript analysis of a salt-stress tolerance 1 (sst1) mutant line indicated activation of AtMSRB5 and AtMSRB6 which encode methionine sulfoxide reductases. Overexpression of AtMSRB5 in Arabidopsis (B5OX) showed a similar salt tolerant phenotype. Furthermore, biochemical analysis indicated stability of the membrane protein, H+-ATPase 2 (AHA2) through regulation of Na+/K+ homeostasis which may be involved in a stress tolerance mechanism. Similarly, overexpression of AtMSRB5 in transgenic rice demonstrated a salt tolerant phenotype via the modulation of Na+/K+ homeostasis without a yield drag under salt and oxidative stress conditions.

Chromosome groups 5, 6 and 7 harbor major quantitative trait loci controlling root traits in bread wheat (Triticum aestivum L.)

Identifying genomic regions for root traits in bread wheat can help breeders develop climate-resilient and high-yielding wheat varieties with desirable root traits. This study used the recombinant inbred line (RIL) population of Synthetic W7984 × Opata M85 to identify quantitative trait loci (QTL) for different root traits such as rooting depth (RD), root dry mass (RM), total root length (RL), root diameter (Rdia) and root surface areas (RSA1 for coarse roots and RSA2 for fine roots) under controlled conditions in a semi-hydroponic system. We detected 14 QTL for eight root traits on nine wheat chromosomes; we discovered three QTL each for RD and RSA1, two QTL each for RM and RSA2, and one QTL each for RL, Rdia, specific root length and nodal root number per plant. The detected QTL were concentrated on chromosome groups 5, 6 and 7. The QTL for shallow RD (Q.rd.uwa.7BL: Xbarc50) and high RM (Q.rm.uwa.6AS: Xgwm334) were validated in two independent F₂ populations of Synthetic W7984 × Chara and Opata M85 × Cascade, respectively. Genotypes containing negative alleles for Q.rd.uwa.7BL had 52% shallower RD than other Synthetic W7984 × Chara population lines. Genotypes with the positive alleles for Q.rm.uwa.6AS had 31.58% higher RM than other Opata M85 × Cascade population lines. Further, we identified 21 putative candidate genes for RD (Q.rd.uwa.7BL) and 13 for RM (Q.rm.uwa.6AS); TraesCS6A01G020400, TraesCS6A01G024400 and TraesCS6A01G021000 identified from Q.rm.uwa.6AS, and TraesCS7B01G404000, TraesCS7B01G254900 and TraesCS7B01G446200 identified from Q.rd.uwa.7BL encoded important proteins for root traits. We found germin-like protein encoding genes in both Q.rd.uwa.7BL and Q.rm.uwa.6AS regions. These genes may play an important role in RM and RD improvement. The identified QTL, especially the validated QTL and putative candidate genes are valuable genetic resources for future root trait improvement in wheat.

Methionine sulfoxide reductase B5 plays a key role in preserving seed vigor and longevity in rice (Oryza sativa)

Oxidation of methionine leads to the formation of methionine S-sulfoxide and methionine R-sulfoxide, which can be reverted by two types of methionine sulfoxide reductase (MSR): MSRA and MSRB. Though the role of MSR enzymes has been elucidated in various physiological processes, the regulation and role of MSR in seeds remains poorly understood. In this study, through molecular, biochemical, and genetic studies using seed-specific overexpression and RNAi lines of OsMSRB5 in Oryza sativa, we demonstrate the role of OsMSRB5 in maintaining seed vigor and longevity. We show that an age-induced reduction in the vigor and viability of seeds is correlated with reduced MSR activity and increased methionine sulfoxide (MetSO) formation. OsMSRB5 expression increases during seed maturation and is predominantly localized to the embryo. Further analyses on transgenic lines reveal the role of OsMSRB5 in modulating reactive oxygen species (ROS) homeostasis to preserve seed vigor and longevity. We show that ascorbate peroxidase and PROTEIN l-ISOASPARTYL METHYLTRANSFERASE undergo MetSO modification in seeds that affects their functional competence. OsMSRB5 physically interacts with these proteins and reverts this modification to facilitate their functions and preserve seed vigor and longevity. Our results thus illustrate the role of OsMSRB5 in preserving seed vigor and longevity by modulating ROS homeostasis in seeds.

Are Methionine Sulfoxide-Containing Proteins Related to Seed Longevity? A Case Study of Arabidopsisthaliana Dry Mature Seeds Using Cyanogen Bromide Attack and Two-Dimensional-Diagonal Electrophoresis

In recent years, several reports pointed out the role of protein oxidation in seed longevity, notably regarding the oxidation of methionine (Met) residues to methionine sulfoxide (MetO) in proteins. To further consider this question, we present a handy proteomic method based on the use of two-dimensional diagonal electrophoresis (2Dd) and cyanogen bromide (CNBr) cleavage, which we refer to as 2Dd-CNBr. CNBr treatment of proteins causes the non-enzymatic hydrolysis of peptide bonds on the carboxyl side of reduced Met residues. However, Met oxidation causes a lack of cleavage, thus modifying the electrophoretic mobility of CNBr-induced peptides. This approach was first validated using bovine serum albumin as a model protein, which confirmed the possibility of distinguishing between oxidized and non-oxidized forms of Met-containing peptides in gels. Then, the 2Dd-CNBr method was applied to the Arabidopsis thaliana seed protein extract in a control (non-oxidized) condition and in an oxidized one (as obtained following hypochlorous acid treatment). Twenty-four oxidized Met residues in 19 proteins identified by mass spectrometry were found to be surface exposed in these proteins. In the three-dimensional environment of the oxidized Met, we detected amino acid residues that could be converted by oxidation (carbonylation) or by phosphorylation, suggesting a possible interplay between Met oxidation and the other protein modifications. The identification of the proteins oxidatively modified in Met residues revealed the finding that MetO-containing proteins are related to seed longevity. Based on these results, we suggest that the method presently described also has the potential for wider applications.

Improving oxidative stress resilience in plants

Originally conceived as harmful metabolic byproducts, reactive oxygen species (ROS) are now recognized as an integral part of numerous cellular programs. Thanks to their diverse physicochemical properties, compartmentalized production, and tight control exerted by the antioxidant machinery they activate signaling pathways that govern plant growth, development, and defense. Excessive ROS levels are often driven by adverse changes in environmental conditions, ultimately causing oxidative stress. The associated negative impact on cellular constituents have been a major focus of decade-long research efforts to improve the oxidative stress resilience by boosting the antioxidant machinery in model and crop species. We highlight the role of enzymatic and non-enzymatic antioxidants as integral factors of multiple signaling cascades beyond their mere function to prevent oxidative damage under adverse abiotic stress conditions.

Spider mite egg extract modifies Arabidopsis response to future infestations

Transcriptional plant responses are an important aspect of herbivore oviposition studies. However, most of our current knowledge is derived from studies using Lepidopteran models, where egg-laying and feeding are separate events in time. Little is known regarding plant response to pests where females feed and oviposit simultaneously. The present study characterized oviposition-induced transcriptomic response of Arabidopsis to Tetranychus urticae egg extracts. Transcriptional evidence indicates that early events in plant response to the egg extract involve responses typical to biotic stresses, which include the alteration in the levels of Ca²⁺ and ROS, the modification of pathways regulated by the phytohormones jasmonic acid and ethylene, and the production of volatiles and glucosinolates as defence mechanisms. These molecular changes affect female fertility, which was significantly reduced when mites fed on plants pre-exposed to the egg extract. However, longer periods of plant exposure to egg extract cause changes in the transcriptional response of the plant reveal a trend to a decrease in the activation of the defensive response. This alteration correlated with a shift at 72 h of exposition in the effect of the mite feeding. At that point, plants become more susceptible and suffer higher damage when challenged by the mite.

Proteomics of developing pea seeds reveals a complex antioxidant network underlying the response to sulfur deficiency and water stress

Pea is a legume crop producing protein-rich seeds and is increasingly in demand for human consumption and animal feed. The aim of this study was to explore the proteome of developing pea seeds at three key stages covering embryogenesis, the transition to seed-filling, and the beginning of storage-protein synthesis, and to investigate how the proteome was influenced by S deficiency and water stress, applied either separately or combined. Of the 3184 proteins quantified by shotgun proteomics, 2473 accumulated at particular stages, thus providing insights into the proteome dynamics at these stages. Differential analyses in response to the stresses and inference of a protein network using the whole proteomics dataset identified a cluster of antioxidant proteins (including a glutathione S-transferase, a methionine sulfoxide reductase, and a thioredoxin) possibly involved in maintaining redox homeostasis during early seed development and preventing cellular damage under stress conditions. Integration of the proteomics data with previously obtained transcriptomics data at the transition to seed-filling revealed the transcriptional events associated with the accumulation of the stress-regulated antioxidant proteins. This transcriptional defense response involves genes of sulfate homeostasis and assimilation, thus providing candidates for targeted studies aimed at dissecting the signaling cascade linking S metabolism to antioxidant processes in developing seeds.

Role of electrophilic nitrated fatty acids during development and response to abiotic stress processes in plants

Nitro-fatty acids are generated from the interaction of unsaturated fatty acids and nitric oxide (NO)-derived molecules. The endogenous occurrence and modulation throughout plant development of nitro-linolenic acid (NO2-Ln) and nitro-oleic acid (NO2-OA) suggest a key role for these molecules in initial development stages. In addition, NO2-Ln content increases significantly in stress situations and induces the expression of genes mainly related to abiotic stress, such as genes encoding members of the heat shock response family and antioxidant enzymes. The promoter regions of NO2-Ln-induced genes are also involved mainly in stress responses. These findings confirm that NO2-Ln is involved in plant defense processes against abiotic stress conditions via induction of the chaperone network and antioxidant systems. NO2-Ln signaling capacity lies mainly in its electrophilic nature and allows it to mediate a reversible post-translational modification called nitroalkylation, which is capable of modulating protein function. NO2-Ln is a NO donor that may be involved in NO signaling events and is able to generate S-nitrosoglutathione, the major reservoir of NO in cells and a key player in NO-mediated abiotic stress responses. This review describes the current state of the art regarding the essential role of nitro-fatty acids as signaling mediators in development and abiotic stress processes.

Localization and Dynamics of the Methionine Sulfoxide Reductases MsrB1 and MsrB2 in Beech Seeds

Beech seeds are produced irregularly, and there is a need for long-term storage of these seeds for forest management practices. Accumulated reactive oxygen species broadly oxidize molecules, including amino acids, such as methionine, thereby contributing to decreased seed viability. Methionine oxidation can be reversed by the activity of methionine sulfoxide reductases (Msrs), which are enzymes involved in the regulation of many developmental processes and stress responses. Two types of Msrs, MsrB1 and MsrB2, were investigated in beech seeds to determine their abundance and localization. MsrB1 and MsrB2 were detected in the cortical cells and the outer area of the vascular cylinder of the embryonic axes as well as in the epidermis and parenchyma cells of cotyledons. The abundances of MsrB1 and MsrB2 decreased during long-term storage. Ultrastructural analyses have demonstrated the accumulation of these proteins in protein storage vacuoles and in the cytoplasm, especially in close proximity to the cell membrane. In silico predictions of possible Msr interactions supported our findings. In this study, we investigate the contribution of MsrB1 and MsrB2 locations in the regulation of seed viability and suggest that MsrB2 is linked with the longevity of beech seeds via association with proper utilization of storage material.

A Methionine Sulfoxide Reductase B Is Required for the Establishment of Astragalus sinicus-Mesorhizobium Symbiosis

Methionine sulfoxide reductase B (MsrB) is involved in oxidative stress or defense responses in plants. However, little is known about its role in legume-rhizobium symbiosis. In this study, an MsrB gene was identified from Astragalus sinicus and its function in symbiosis was characterized. AsMsrB was induced under phosphorus starvation and displayed different expression patterns under symbiotic and nonsymbiotic conditions. Hydrogen peroxide or methyl viologen treatment enhanced the transcript level of AsMsrB in roots and nodules. Subcellular localization showed that AsMsrB was localized in the cytoplasm of onion epidermal cells and co-localized with rhizobia in nodules. Plants with AsMsrB-RNAi hairy roots exhibited significant decreases in nodule number, nodule nitrogenase activity and fresh weight of the aerial part, as well as an abnormal nodule and symbiosome development. Statistical analysis of infection events showed that plants with AsMsrB-RNAi hairy roots had significant decreases in the number of root hair curling events, infection threads and nodule primordia compared with the control. The content of hydrogen peroxide increased in AsMsrB-RNAi roots but decreased in AsMsrB overexpression roots at the early stage of infection. The transcriptome analysis showed synergistic modulations of the expression of genes involved in reactive oxygen species generation and scavenging, defense and pathogenesis and early nodulation. In addition, a candidate protein interacting with AsMsrB was identified and confirmed by bimolecular fluorescence complementation. Taken together, our results indicate that AsMsrB plays an essential role in nodule development and symbiotic nitrogen fixation by affecting the redox homeostasis in roots and nodules.

Characterization of rhizome transcriptome and identification of a rhizomatous ER body in the clonal plant Cardamine leucantha

The rhizome is a plant organ that develops from a shoot apical meristem but penetrates into belowground environments. To characterize the gene expression profile of rhizomes, we compared the rhizome transcriptome with those of the leaves, shoots and roots of a rhizomatous Brassicaceae plant, Cardamine leucantha. Overall, rhizome transcriptomes were characterized by the absence of genes that show rhizome-specific expression and expression profiles intermediate between those of shoots and roots. Our results suggest that both endogenous developmental factors and external environmental factors are important for controlling the rhizome transcriptome. Genes that showed relatively high expression in the rhizome compared to shoots and roots included those related to belowground defense, control of reactive oxygen species and cell elongation under dark conditions. A comparison of transcriptomes further allowed us to identify the presence of an ER body, a defense-related belowground organelle, in epidermal cells of the C. leucantha rhizome, which is the first report of ER bodies in rhizome tissue.

Wheat methionine sulfoxide reductase A4.1 interacts with heme oxygenase 1 to enhance seedling tolerance to salinity or drought stress

Here, a functional characterization of a wheat MSR has been presented: this protein makes a contribution to the plant's tolerance of abiotic stress, acting through its catalytic capacity and its modulation of ROS and ABA pathways. The molecular mechanism and function of certain members of the methionine sulfoxide reductase (MSR) gene family have been defined, however, these analyses have not included the wheat equivalents. The wheat MSR gene TaMSRA4.1 is inducible by salinity and drought stress and in this study, we demonstrate that its activity is restricted to the Met-S-SO enantiomer, and its subcellular localization is in the chloroplast. Furthermore, constitutive expression of TaMSRA4.1 enhanced the salinity and drought tolerance of wheat and Arabidopsis thaliana. In these plants constitutively expressing TaMSRA4.1, the accumulation of reactive oxygen species (ROS) was found to be influenced through the modulation of genes encoding proteins involved in ROS signaling, generation and scavenging, while the level of endogenous abscisic acid (ABA), and the sensitivity of stomatal guard cells to exogenous ABA, was increased. A yeast two-hybrid screen, bimolecular fluorescence complementation and co-immunoprecipitation assays demonstrated that heme oxygenase 1 (HO1) interacted with TaMSRA4.1, and that this interaction depended on a TaHO1 C-terminal domain. In plants subjected to salinity or drought stress, TaMSRA4.1 reversed the oxidation of TaHO1, activating ROS and ABA signaling pathways, but not in the absence of HO1. The aforementioned properties advocate TaMSRA4.1 as a candidate for plant genetic enhancement.

Regulation of Gene Expression of Methionine Sulfoxide Reductases and Their New Putative Roles in Plants

Oxidation of methionine to methionine sulfoxide is a type of posttranslational modification reversed by methionine sulfoxide reductases (Msrs), which present an exceptionally high number of gene copies in plants. The side-form general antioxidant function-specific role of each Msr isoform has not been fully studied. Thirty homologous genes of Msr type A (MsrA) and type B (MsrB) that originate from the genomes of Arabidopsis thaliana, Populus trichocarpa, and Oryza sativa were analyzed in silico. From 109 to 201 transcription factors and responsive elements were predicted for each gene. Among the species, 220 and 190 common transcription factors and responsive elements were detected for the MsrA and MsrB isoforms, respectively. In a comparison of 14 MsrA and 16 MsrB genes, 424 transcription factors and responsive elements were reported in both types of genes, with almost ten times fewer unique elements. The transcription factors mainly comprised plant growth and development regulators, transcription factors important in stress responses with significant overrepresentation of the myeloblastosis viral oncogene homolog (MYB) and no apical meristem, Arabidopsis transcription activation factor and cup-shaped cotyledon (NAC) families and responsive elements sensitive to ethylene, jasmonate, sugar, and prolamine. Gene Ontology term-based functional classification revealed that cellular, metabolic, and developmental process terms and the response to stimulus term dominated in the biological process category. Available experimental transcriptomic and proteomic data, in combination with a set of predictions, gave coherent results validating this research. Thus, new manners Msr gene expression regulation, as well as new putative roles of Msrs, are proposed.

Function of the evolutionarily conserved plant methionine-S-sulfoxide reductase without the catalytic residue

In plants, two types of methionine sulfoxide reductase (MSR) exist, namely methionine-S-sulfoxide reductase (MSRA) and methionine-R-sulfoxide reductase (MSRB). These enzymes catalyze the reduction of methionine sulfoxides (MetO) back to methionine (Met) by a catalytic cysteine (Cys) and one or two resolving Cys residues. Interestingly, a group of MSRA encoded by plant genomes does not have a catalytic residue. We asked that if this group of MSRA did not have any function (as fitness), why it was not lost during the evolutionary process. To challenge this question, we analyzed the gene family encoding MSRA in soybean (GmMSRAs). We found seven genes encoding GmMSRAs, which included three segmental duplicated pairs. Among them, a pair of duplicated genes, namely GmMSRA1 and GmMSRA6, was without a catalytic Cys residue. Pseudogenes were ruled out as their transcripts were detected in various tissues and their Ka/Ks ratio indicated a negative selection pressure. In vivo analysis in Δ3MSR yeast strain indicated that the GmMSRA6 did not have activity toward MetO, contrasting to GmMSRA3 which had catalytic Cys and had activity. When exposed to H2O2-induced oxidative stress, GmMSRA6 did not confer any protection to the Δ3MSR yeast strain. Overexpression of GmMSRA6 in Arabidopsis thaliana did not alter the plant's phenotype under physiological conditions. However, the transgenic plants exhibited slightly higher sensitivity toward salinity-induced stress. Taken together, this data suggested that the plant MSRAs without the catalytic Cys are not enzymatically active and their existence may be explained by a role in regulating plant MSR activity via dominant-negative substrate competition mechanism.

Hydrogen Peroxide: Its Role in Plant Biology and Crosstalk with Signalling Networks

Hydrogen peroxide (H₂O₂) is steadily gaining more attention in the field of molecular biology research. It is a major REDOX (reduction⁻oxidation reaction) metabolite and at high concentrations induces oxidative damage to biomolecules, which can culminate in cell death. However, at concentrations in the low nanomolar range, H₂O₂ acts as a signalling molecule and in many aspects, resembles phytohormones. Though its signalling network in plants is much less well characterized than are those of its counterparts in yeast or mammals, accumulating evidence indicates that the role of H₂O₂-mediated signalling in plant cells is possibly even more indispensable. In this review, we summarize hydrogen peroxide metabolism in plants, the sources and sinks of this compound and its transport via peroxiporins. We outline H₂O₂ perception, its direct and indirect effects and known targets in the transcriptional machinery. We focus on the role of H₂O₂ in plant growth and development and discuss the crosstalk between it and phytohormones. In addition to a literature review, we performed a meta-analysis of available transcriptomics data which provided further evidence for crosstalk between H₂O₂ and light, nutrient signalling, temperature stress, drought stress and hormonal pathways.

Physiological Roles of Plant Methionine Sulfoxide Reductases in Redox Homeostasis and Signaling

Oxidation of methionine (Met) leads to the formation of two S- and R-diastereoisomers of Met sulfoxide (MetO) that are reduced back to Met by methionine sulfoxide reductases (MSRs), A and B, respectively. Here, we review the current knowledge about the physiological functions of plant MSRs in relation with subcellular and tissue distribution, expression patterns, mutant phenotypes, and possible targets. The data gained from modified lines of plant models and crop species indicate that MSRs play protective roles upon abiotic and biotic environmental constraints. They also participate in the control of the ageing process, as shown in seeds subjected to adverse conditions. Significant advances were achieved towards understanding how MSRs could fulfil these functions via the identification of partners among Met-rich or MetO-containing proteins, notably by using redox proteomic approaches. In addition to a global protective role against oxidative damage in proteins, plant MSRs could specifically preserve the activity of stress responsive effectors such as glutathione-S-transferases and chaperones. Moreover, several lines of evidence indicate that MSRs fulfil key signaling roles via interplays with Ca²⁺- and phosphorylation-dependent cascades, thus transmitting ROS-related information in transduction pathways.

Harpin-inducible defense signaling components impair infection by the ascomycete Macrophomina phaseolina

Soybean (Glycine max) infection by the charcoal rot (CR) ascomycete Macrophomina phaseolina is enhanced by the soybean cyst nematode (SCN) Heterodera glycines. We hypothesized that G. max genetic lines impairing infection by M. phaseolina would also limit H. glycines parasitism, leading to resistance. As a part of this M. phaseolina resistance process, the genetic line would express defense genes already proven to impair nematode parasitism. Using G. max[DT97-4290/PI 642055], exhibiting partial resistance to M. phaseolina, experiments show the genetic line also impairs H. glycines parasitism. Furthermore, comparative studies show G. max[DT97-4290/PI 642055] exhibits induced expression of the effector triggered immunity (ETI) gene NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) that functions in defense to H. glycines as compared to the H. glycines and M. phaseolina susceptible line G. max[Williams 82/PI 518671]. Other defense genes that are induced in G. max[DT97-4290/PI 642055] include the pathogen associated molecular pattern (PAMP) triggered immunity (PTI) genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), NONEXPRESSOR OF PR1 (NPR1) and TGA2. These observations link G. max defense processes that impede H. glycines parasitism to also potentially function toward impairing M. phaseolina pathogenicity. Testing this hypothesis, G. max[Williams 82/PI 518671] genetically engineered to experimentally induce GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 expression leads to impaired M. phaseolina pathogenicity. In contrast, G. max[DT97-4290/PI 642055] engineered to experimentally suppress the expression of GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 by RNA interference (RNAi) enhances M. phaseolina pathogenicity. The results show components of PTI and ETI impair both nematode and M. phaseolina pathogenicity.

Delay of Postharvest Browning in Litchi Fruit by Melatonin via the Enhancing of Antioxidative Processes and Oxidation Repair

Melatonin acts as a crucial signaling and antioxidant molecule with multiple physiological functions in organisms. To explore effects of exogenous melatonin on postharvest browning and its possible mechanisms in litchi fruit, 'Ziniangxi' litchi fruits were treated with an aqueous solution of melatonin at 0.4 mM and then stored at 25 °C for 8 days. The results revealed that melatonin strongly suppressed pericarp browning and delayed discoloration during storage. Melatonin treatment reduced relative membrane-leakage rate and inhibited the generation of superoxide radicals (O₂^-·), hydrogen peroxide (H₂O₂), and malondialdehyde (MDA). Melatonin treatment markedly promoted the accumulation of endogenous melatonin; delayed loss of total phenolics, flavonoids, and anthocyanins; and enhanced the activities of antioxidant enzymes, including superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and glutathione reductase (GR, EC 1.6.4.2). By contrast, the activities of browning-related enzymes including polyphenoloxidase (PPO, EC 1.10.3.1) and peroxidase (POD, EC 1.11.1.7) were reduced. In addition, melatonin treatment up-regulated the expression of four genes encoding enzymes for repair of oxidized proteins, including LcMsrA1, LcMsrA2, LcMsrB1, and LcMsB2. These findings indicate that the delay of pericarp browning and senescence by melatonin in harvested litchi fruit could be attributed to the maintenance of redox homeostasis by the improvement of the antioxidant capacity and modulation of the repair of oxidatively damaged proteins.

Difference in oxidative stress tolerance between rice cultivars estimated with chlorophyll fluorescence analysis

BACKGROUND

Oxidative stress is considered to be involved in growth retardation of plants when they are exposed to a variety of biotic and abiotic stresses. Despite its potential importance in improving crop production, comparative studies on oxidative stress tolerance between rice (Oryza sativa L.) cultivars are limited. This work describes the difference in term of oxidative stress tolerance between 72 rice cultivars.

METHODS

72 rice cultivars grown under naturally lit greenhouse were used in this study. Excised leaf discs were subjected to a low concentration of methyl viologen (paraquat), a chemical reagent known to generate reactive oxygen species in chloroplast. Chlorophyll fluorescence analysis using a two-dimensional fluorescence meter, ion leakage analysis as well as the measurement of chlorophyll contents were used to evaluate the oxidative stress tolerance of leaf discs. Furthermore, fluorescence intensities were finely analyzed based on new fluorescence theories that we have optimized.

RESULTS

Treatment of leaf discs with methyl viologen caused differential decrease of maximum quantum yield of photosystem II (Fv/Fm) between cultivars. Decrease of Fv/Fm was also closely correlated with increase of ion leakage and decrease of chlorophyll a/b ratio. Fv/Fm was factorized into photochemical and non-photochemical parameters to classify rice cultivars into sensitive and tolerant ones. Among the 72 compared rice cultivars, the traditional cultivar Co13 was identified as the most tolerant to oxidative stress. Koshihikari, a dominant modern Japonica cultivar in Japan as well as IR58, one of the modern Indica breeding lines exhibited a strong tolerance to oxidative stress.

CONCLUSIONS

Close correlation between Fv/Fm and chlorophyll a/b ratio provides a simple method to estimate oxidative stress tolerance, without measurement of chlorophyll fluorescence with special equipment. The fact that modern cultivars, especially major cultivars possessed tolerance to oxidative stress suggests that oxidative stress tolerance is one of the agricultural traits prerequisite for improvement of modern rice cultivars. Data presented in this study would enable breeding of rice cultivars having strong tolerance to oxidative stress.

Arabidopsis thaliana methionine sulfoxide reductase B8 influences stress-induced cell death and effector-triggered immunity

Reactive oxygen species (ROS) oxidize methionine to methionine sulfoxide (MetSO) and thereby inactivate proteins. Methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Our results show that Arabidopsis thaliana MSR enzyme coding gene MSRB8 is required for effector-triggered immunity and containment of stress-induced cell death in Arabidopsis. Plants activate pattern-triggered immunity (PTI), a basal defense, upon recognition of evolutionary conserved molecular patterns present in the pathogens. Pathogens release effector molecules to suppress PTI. Recognition of certain effector molecules activates a strong defense, known as effector-triggered immunity (ETI). ETI induces high-level accumulation of reactive oxygen species (ROS) and hypersensitive response (HR), a rapid programmed death of infected cells. ROS oxidize methionine to methionine sulfoxide (MetSO), rendering several proteins nonfunctional. The methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Though a few plant MSR genes are known to provide tolerance against oxidative stress, their role in plant-pathogen interaction is not known. We report here that activation of cell death by avirulent pathogen or UV treatment induces expression of MSRB7 and MSRB8 genes. The T-DNA insertion mutant of MSRB8 exaggerates HR-associated and UV-induced cell death and accumulates a higher level of ROS than wild-type plants. The negative regulatory role of MSRB8 in HR is further supported by amiRNA and overexpression lines. Mutants and overexpression lines of MSRB8 are susceptible and resistant respectively, compared to the wild-type plants, against avirulent strains of Pseudomonas syringae pv. tomato DC3000 (Pst) carrying AvrRpt2, AvrB, or AvrPphB genes. However, the MSRB8 gene does not influence resistance against virulent Pst or P. syringae pv. maculicola (Psm) pathogens. Our results altogether suggest that MSRB8 function is required for ETI and containment of stress-induced cell death in Arabidopsis.

Genome-Wide Analysis of Genes Encoding Methionine-Rich Proteins in Arabidopsis and Soybean Suggesting Their Roles in the Adaptation of Plants to Abiotic Stress

Oxidation and reduction of methionine (Met) play important roles in scavenging reactive oxygen species (ROS) and signaling in living organisms. To understand the impacts of Met oxidation and reduction in plants during stress, we surveyed the genomes of Arabidopsis and soybean (Glycine max L.) for genes encoding Met-rich proteins (MRPs). We found 121 and 213 genes encoding MRPs in Arabidopsis and soybean, respectively. Gene annotation indicated that those with known function are involved in vital cellular processes such as transcriptional control, calcium signaling, protein modification, and metal transport. Next, we analyzed the transcript levels of MRP-coding genes under normal and stress conditions. We found that 57 AtMRPs were responsive either to drought or to high salinity stress in Arabidopsis; 35 GmMRPs were responsive to drought in the leaf of late vegetative or early reproductive stages of soybean. Among the MRP genes with a known function, the majority of the abiotic stress-responsive genes are involved in transcription control and calcium signaling. Finally, Arabidopsis plant which overexpressed an MRP-coding gene, whose transcripts were downregulated by abiotic stress, was more sensitive to paraquat than the control. Taken together, our report indicates that MRPs participate in various vital processes of plants under normal and stress conditions.

A Glycine soja methionine sulfoxide reductase B5a interacts with the Ca(2+) /CAM-binding kinase GsCBRLK and activates ROS signaling under carbonate alkaline stress

Although research has extensively illustrated the molecular basis of plant responses to salt and high-pH stresses, knowledge on carbonate alkaline stress is poor and the specific responsive mechanism remains elusive. We have previously characterized a Glycine soja Ca(2+) /CAM-dependent kinase GsCBRLK that could increase salt tolerance. Here, we characterize a methionine sulfoxide reductase (MSR) B protein GsMSRB5a as a GsCBRLK interactor by using Y2H and BiFc assays. Further analyses showed that the N-terminal variable domain of GsCBRLK contributed to the GsMSRB5a interaction. Y2H assays also revealed the interaction specificity of GsCBRLK with the wild soybean MSRB subfamily proteins, and determined that the BoxI/BoxII-containing regions within GsMSRBs were responsible for their interaction. Furthermore, we also illustrated that the N-terminal basic regions in GsMSRBs functioned as transit peptides, which targeted themselves into chloroplasts and thereby prevented their interaction with GsCBRLK. Nevertheless, deletion of these regions allowed them to localize on the plasma membrane (PM) and interact with GsCBRLK. In addition, we also showed that GsMSRB5a and GsCBRLK displayed overlapping tissue expression specificity and coincident expression patterns under carbonate alkaline stress. Phenotypic experiments demonstrated that GsMSRB5a and GsCBRLK overexpression in Arabidopsis enhanced carbonate alkaline stress tolerance. Further investigations elucidated that GsMSRB5a and GsCBRLK inhibited reactive oxygen species (ROS) accumulation by modifying the expression of ROS signaling, biosynthesis and scavenging genes. Summarily, our results demonstrated that GsCBRLK and GsMSRB5a interacted with each other, and activated ROS signaling under carbonate alkaline stress.

Oxidative stress-induced toxicity of CuO nanoparticles and related toxicogenomic responses in Arabidopsis thaliana

Microarray analysis of toxicogenomic effects of CuO NPs on Arabidopsis thaliana was conducted. Arabidopsis growth was significantly inhibited by CuO NPs (10 and 20 mg/L). CuO NPs (10 and 20 mg/L) caused significant root damage after short-time (0-2 h) exposure while their corresponding Cu(2+) ions (0.80 and 1.35 mg/L) did not show any root damage. After longer exposure times (1 and 2 days), Cu(2+) ions induced obvious root damage, indicating that released Cu(2+) ions from CuO NPs contributed partial toxicity during CuO NPs exposure. After CuO NPs (10 mg/L) exposure for 2 h, reactive oxygen species (ROS) generation in root tips was much higher than that in the corresponding Cu(2+) ions (0.8 mg/L) treatment. The gene ontology categories identified from microarray analysis showed that CuO NPs (10 mg/L) caused 1658 differentially expressed genes (p < 0.01, fold change>3). Of these, 1035 and 623 genes were up-regulated and down-regulated, respectively. 47 genes among all the up-regulated genes were response to oxidative stress, in which 19 genes were also related to "response to abiotic stimulus" and 12 genes were involved in the phenylpropanoid biosynthesis of the KEGG metabolic pathway. The expression of all the selected genes (RHL41, MSRB7, BCB, PRXCA, and MC8) measured using quantitative RT-PCR was consistent with the microarray analysis. CuO NPs contributed much stronger up-regulation of oxidative stress-related genes than the corresponding Cu(2+) ions.

Involvement of thiol-based mechanisms in plant development

BACKGROUND

Increasing knowledge has been recently gained regarding the redox regulation of plant developmental stages.

SCOPE OF VIEW

The current state of knowledge concerning the involvement of glutathione, glutaredoxins and thioredoxins in plant development is reviewed.

MAJOR CONCLUSIONS

The control of the thiol redox status is mainly ensured by glutathione (GSH), a cysteine-containing tripeptide and by reductases sharing redox-active cysteines, glutaredoxins (GRXs) and thioredoxins (TRXs). Indeed, thiol groups present in many regulatory proteins and metabolic enzymes are prone to oxidation, ultimately leading to post-translational modifications such as disulfide bond formation or glutathionylation. This review focuses on the involvement of GSH, GRXs and TRXs in plant development. Recent studies showed that the proper functioning of root and shoot apical meristems depends on glutathione content and redox status, which regulate, among others, cell cycle and hormone-related processes. A critical role of GRXs in the formation of floral organs has been uncovered, likely through the redox regulation of TGA transcription factor activity. TRXs fulfill many functions in plant development via the regulation of embryo formation, the control of cell-to-cell communication, the mobilization of seed reserves, the biogenesis of chloroplastic structures, the metabolism of carbon and the maintenance of cell redox homeostasis. This review also highlights the tight relationships between thiols, hormones and carbon metabolism, allowing a proper development of plants in relation with the varying environment and the energy availability.

GENERAL SIGNIFICANCE

GSH, GRXs and TRXs play key roles during the whole plant developmental cycle via their antioxidant functions and the redox-regulation of signaling pathways. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

MSRB7 reverses oxidation of GSTF2/3 to confer tolerance of Arabidopsis thaliana to oxidative stress

Methionine sulfoxide reductases (MSRs) catalyse the reduction of oxidized methionine residues, thereby protecting proteins against oxidative stress. Accordingly, MSRs have been associated with stress responses, disease, and senescence in a taxonomically diverse array of organisms. However, the cytosolic substrates of MSRs in plants remain largely unknown. Here, we used a proteomic analysis strategy to identify MSRB7 substrates. We showed that two glutathione transferases (GSTs), GSTF2 and GSTF3, had fewer oxidized methionine (MetO) residues in MSRB7-overexpressing Arabidopsis thaliana plants than in wild-type plants. Conversely, GSTF2 and GSTF3 were highly oxidized and unstable in MSRB7-knockdown plants. MSRB7 was able to restore the MetO-GSTF2M100/104 and MetO-GSTF3M100 residues produced during oxidative stress. Furthermore, both GSTs were specifically induced by the oxidative stress inducer, methyl viologen. Our results indicate that specific GSTs are substrates of MSRs, which together provide a major line of defence against oxidative stress in A. thaliana.

Differential transcriptomic analysis by RNA-Seq of GSNO-responsive genes between Arabidopsis roots and leaves

S-Nitrosoglutathione (GSNO) is a nitric oxide-derived molecule that can regulate protein function by a post-translational modification designated S-nitrosylation. GSNO has also been detected in different plant organs under physiological and stress conditions, and it can also modulate gene expression. Thirty-day-old Arabidopsis plants were grown under hydroponic conditions, and exogenous 1 mM GSNO was applied to the root systems for 3 h. Differential gene expression analyses were carried out both in roots and in leaves by RNA sequencing (RNA-seq). A total of 3,263 genes were identified as being modulated by GSNO. Most of the genes identified were associated with the mechanism of protection against stress situations, many of these having previously been identified as target genes of GSNO by array-based methods. However, new genes were identified, such as that for methionine sulfoxide reductase (MSR) in leaves or different miscellaneous RNA (miscRNA) genes in Arabidopsis roots. As a result, 1,945 GSNO-responsive genes expressed differently in leaves and roots were identified, and 114 of these corresponded exclusively to one of these organs. In summary, it is demonstrated that RNA-seq extends our knowledge of GSNO as a signaling molecule which differentially modulates gene expression in roots and leaves under non-stress conditions.

The physiological role of reversible methionine oxidation

Sulfur-containing amino acids such as cysteine and methionine are particularly vulnerable to oxidation. Oxidation of cysteine and methionine in their free amino acid form renders them unavailable for metabolic processes while their oxidation in the protein-bound state is a common post-translational modification in all organisms and usually alters the function of the protein. In the majority of cases, oxidation causes inactivation of proteins. Yet, an increasing number of examples have been described where reversible cysteine oxidation is part of a sophisticated mechanism to control protein function based on the redox state of the protein. While for methionine the dogma is still that its oxidation inhibits protein function, reversible methionine oxidation is now being recognized as a powerful means of triggering protein activity. This mode of regulation involves oxidation of methionine to methionine sulfoxide leading to activated protein function, and inactivation is accomplished by reduction of methionine sulfoxide back to methionine catalyzed by methionine sulfoxide reductases. Given the similarity to thiol-based redox-regulation of protein function, methionine oxidation is now established as a novel mode of redox-regulation of protein function. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

Cloning, expression, and characterization of a methionine sulfoxide reductase B gene from Nicotiana tabacum

Reactive oxygen species (ROS) are generated during normal aerobic metabolism and in plants exposed to environmental stress. Methionine (Met) residues are particularly sensitive to ROS-mediated oxidation, leading to the formation of methionine sulfoxide (MetSO) under mild oxidative conditions. Methionine sulfoxide reductase (MSR) repairs oxidized Met and protects plants from oxidative damage. Herein, we identified a tobacco (Nicotiana tabacum) MSRB gene, referred to as NtMSRB3. Analysis of the sequence showed that the NtMSRB3 protein had four typical motifs in a SelR domain, which is known as the catalytic region of MSRBs. NtMSRB3 specifically reduced the R epimer of MetSO and converted either free MetSO or Dabsyl-MetSO in the presence of dithiothreitol. Escherichia coli cells harboring NtMSRB3 displayed relative high viability under H₂O₂ stress. Subcellular localization of NtMSRB3 revealed that it was a plastid-targeted protein. Furthermore, the semi-quantitative reverse transcription polymerase chain reaction assay showed that NtMSRB3 was upregulated apparently by abscisic acid, salt, cold, and methyl viologen stress within 24 h of treatment.