Cloning and characterization of a biotic-stress-inducible glutathione transferase from Phaseolus vulgaris

Biochemical and Structural Insights on the Poplar Tau Glutathione Transferase GSTU19 and 20 Paralogs Binding Flavonoids

Glutathione transferases (GSTs) constitute a widespread superfamily of enzymes notably involved in xenobiotic detoxification and/or in specialized metabolism. Populus trichocarpa genome (V4.1 assembly, Phytozome 13) consists of 74 genes coding for full-length GSTs and ten likely pseudogenes. These GSTs are divided into 11 classes, in which the tau class (GSTU) is the most abundant with 54 isoforms. PtGSTU19 and 20, two paralogs sharing more than 91% sequence identity (95% of sequence similarity), would have diverged from a common ancestor of P. trichocarpa and P. yatungensis species. These enzymes display the distinctive glutathione (GSH)-conjugation and peroxidase activities against model substrates. The resolution of the crystal structures of these proteins revealed significant structural differences despite their high sequence identity. PtGSTU20 has a well-defined deep pocket in the active site whereas the bottom of this pocket is disordered in PtGSTU19. In a screen of potential ligands, we were able to identify an interaction with flavonoids. Some of them, previously identified in poplar (chrysin, galangin, and pinocembrin), inhibited GSH-conjugation activity of both enzymes with a more pronounced effect on PtGSTU20. The crystal structures of PtGSTU20 complexed with these molecules provide evidence for their potential involvement in flavonoid transport in P. trichocarpa.

Genomic Analysis of the Glutathione S-Transferase Family in Pear (Pyrus communis) and Functional Identification of PcGST57 in Anthocyanin Accumulation

Anthocyanin accumulation in vacuoles results in red coloration in pear peels. Glutathione S-transferase (GST) proteins have emerged as important regulators of anthocyanin accumulation. Here, a total of 57 PcGST genes were identified in the European pear ‘Bartlett’ (Pyrus communis) through comprehensive genomic analysis. Phylogenetic analysis showed that PcGST genes were divided into 10 subfamilies. The gene structure, chromosomal localization, collinearity relationship, cis-elements in the promoter region, and conserved motifs of PcGST genes were analyzed. Further research indicated that glutamic acid (Glu) can significantly improve anthocyanin accumulation in pear peels. RNA sequencing (RNA-seq) analysis showed that Glu induced the expression of most PcGST genes, among which PcGST57 was most significantly induced. Further phylogenetic analysis indicated that PcGST57 was closely related to GST genes identified in other species, which were involved in anthocyanin accumulation. Transcript analysis indicated that PcGST57 was expressed in various tissues, other than flesh, and associated with peel coloration at different developmental stages. Silencing of PcGST57 by virus-induced gene silencing (VIGS) inhibited the expression of PcGST57 and reduced the anthocyanin content in pear fruit. In contrast, overexpression of PcGST57 improved anthocyanin accumulation. Collectively, our results demonstrated that PcGST57 was involved in anthocyanin accumulation in pear and provided candidate genes for red pear breeding.

Overexpression of A Biotic Stress-Inducible Pvgstu Gene Activates Early Protective Responses in Tobacco under Combined Heat and Drought

Drought and heat stresses are major factors limiting crop growth and productivity, and their effect is more devastating when occurring concurrently. Plant glutathione transferases (GSTs) are differentially expressed in response to different stimuli, conferring tolerance to a wide range of abiotic stresses. GSTs from drought-tolerant Phaseolus vulgaris var. "Plake Megalosperma Prespon" is expected to play an important role in the response mechanisms to combined and single heat and drought stresses. Herein, we examined wild-type N. tabacum plants (cv. Basmas Xanthi) and T1 transgenic lines overexpressing the stress-induced Pvgstu3-3 and Pvgstu2-2 genes. The overexpression of Pvgstu3-3 contributed to potential thermotolerance and greater plant performance under combined stress. Significant alterations in the primary metabolism were observed in the transgenic plants between combined stress and stress-free conditions. Stress-responsive differentially expressed genes (DEGs) and transcription factors (TFs) related to photosynthesis, signal transduction, starch and sucrose metabolism, osmotic adjustment and thermotolerance, were identified under combined stress. In contrast, induction of certain DEGs and TF families under stress-free conditions indicated that transgenic plants were in a primed state. The overexpression of the Pvgstu3-3 is playing a leading role in the production of signaling molecules, induction of specific metabolites and activation of the protective mechanisms for enhanced protection against combined abiotic stresses in tobacco.

Comparative structural and functional analysis of phi class glutathione transferases involved in multiple-herbicide resistance of grass weeds and crops

Multiple-herbicide resistant (MHR) weeds are a global problem and a looming threat to weed control in crops. MHR weeds express a specific phi class glutathione transferase (MHR-GSTF) which seems to contribute to herbicide resistance. The present work aims to investigate the structure and catalytic properties of the MHR-GSTFs from different grass weeds and crops (Alopecurus myosuroides, Lolium rigidum, Hordeum vulgare, Triticum aestivum). Recombinant MHR-GSTFs were expressed in E. coli and purified by affinity chromatography. Kinetic analysis of substrate specificity using a range of thiol substrates and xenobiotic compounds suggested that all enzymes display a broad range of specificity and are capable of detoxifying major stress-induced toxic products. Notably, all tested enzymes exhibited high activity towards organic hydroperoxides. The crystal structure of MHR-GSTF from Alopecurus myosuroides (AmGSTF) was determined by molecular replacement at 1.33 Å resolution. The enzyme was resolved with bound glutathione sulfenic acid (GSOH) at the G-site and succinic acid at the H-site. The enzyme shows conserved structural features compared to other Phi class GSTs. However, some differences were observed at the C-terminal helix H9 that may affect substrate specificity. The structural and functional features of AmGSTF were compared with those of the homologue crop enzymes (HvGSTF and TaGSTF) and discussed in light of their contribution to the MHR mechanism.

Tolerance of Transplastomic Tobacco Plants Overexpressing a Theta Class Glutathione Transferase to Abiotic and Oxidative Stresses

Chloroplasts are organelles subjected to extreme oxidative stress conditions. Biomolecules produced in the chloroplasts act as signals guiding plant metabolism toward stress tolerance and play a major role in regulating gene expression in the nucleus. Herein, we used transplastomic plants as an alternative approach to expression of transgenes in the nucleus for conferring stress tolerance to abiotic stresses and herbicides. To investigate the morphophysiological and molecular mechanisms and the role of plastid expressed GSTs in tobacco stress detoxification and stress tolerance, we used transplastomic tobacco lines overexpressing a theta class glutathione transferase (GST) in chloroplasts. The transplastomic plants were tested under drought (0, 100, and 200 mM mannitol) and salinity (0, 150, and 300 mM NaCl) in vitro, and under herbicide stress (Diquat). Our results suggest that pt ^AtGSTT lines were tolerant to herbicide-induced oxidative and salinity stresses and showed enhanced response tolerance to mannitol-induced osmotic stress compared to WT plants. Overexpression of the Arabidopsis thaliana AtGSTT in the chloroplasts resulted in enhanced photo-tolerance and turgor maintenance under stress. Whole-genome transcriptome analysis revealed that genes related to stress tolerance, were upregulated in pt ^AtGSTT2a line under both control and high mannitol stress conditions. Transplastomic plants overexpressing the pt ^AtGSTT2a in the chloroplast showed a state of acclimation to stress, as only limited number of genes were upregulated in the pt ^AtGSTT2a transplastomic line compared to WT under stress conditions while at the same time genes related to stress tolerance were upregulated in pt ^AtGSTT2a plants compared to WT in stress-free conditions. In parallel, the metabolic profile indicated limited perturbations of the metabolic homeostasis in the transplastomic lines and greater accumulation of mannitol, and soluble sugars under high mannitol stress. Therefore, transplastomic lines seem to be in a state of acclimation to stress under stress-free conditions, which was maintained even under high mannitol stress. The results help to elucidate the role of GSTs in plant abiotic stress tolerance and the underlying mechanisms of the GSTs expressed in the chloroplast, toward environmental resilience of cultivated crops.

Exploring priming responses involved in peach fruit acclimation to cold stress

Cold storage of fruit may induce the physiological disorder chilling injury (CI); however, the molecular basis of CI development remains largely unexplored. Simulated conditions of CI priming and suppression provided an interesting experimental system to study cold response in fruit. Peaches (cv. June Gold) at the commercial harvest (CH) or tree-ripe (TR) stages were immediately exposed to cold treatment (40 d, 0 °C) and an additional group of CH fruits were pre-conditioned 48 h at 20 °C prior to low-temperature exposure (pre-conditioning, PC). Following cold treatment, the ripening behaviour of the three groups of fruits was analysed (3 d, 20 °C). Parallel proteomic, metabolomic and targeted transcription comparisons were employed to characterize the response of fruit to CI expression. Physiological data indicated that PC suppressed CI symptoms and induced more ethylene biosynthesis than the other treatments. Differences in the protein and metabolic profiles were identified, both among treatments and before and after cold exposure. Transcriptional expression patterns of several genes were consistent with their protein abundance models. Interestingly, metabolomic and gene expression results revealed a possible role for valine and/or isoleucine in CI tolerance. Overall, this study provides new insights into molecular changes during fruit acclimation to cold environment.

Glutathione transferase-mediated benzimidazole-resistance in Fusarium graminearum

Fusarium graminearum laboratory mutants moderately (MR) and highly (HR) benzimidazole-resistant, carrying or not target-site mutations at the β₂-tubulin gene were utilized in an attempt to elucidate the biochemical mechanism(s) underlying the unique BZM-resistance paradigm of this fungal plant pathogen. Relative expression analysis in the presence or absence of carbendazim (methyl-2-benzimidazole carbamate) using a quantitative Real Time qPCR (RT-qPCR) revealed differences between resistant and the wild-type parental strain although no differences in expression levels of either β₁- or β₂-tubulin homologue genes were able to fully account for two of the highly resistant phenotypes. Glutathione transferase (GST)-mediated detoxification was shown to be -at least partly- responsible for the elevated resistance levels of a HR isolate bearing the β₂-tubulin Phe200Tyr resistance mutation compared with another MR isolate carrying the same mutation. This benzimidazole-resistance mechanism is reported for the first time in F. graminearum. No indications of detoxification involved in benzimidazole resistance were found for the rest of the isolates as revealed by GST and glutathione peroxidase (GPx) activities and bioassays using monoxygenase and hydrolase detoxification enzyme inhibiting synergists. Interestingly, besides the Phe200Tyr mutation-carrying HR isolate, the remaining highly-carbendazim resistant phenotypes could not be associated with any of the target site modification/overproduction, detoxification or reduced uptake-increased efflux mechanisms.

Plant glutathione transferase-mediated stress tolerance: functions and biotechnological applications

Plant glutathione transferases (EC 2.5.1.18, GSTs) are an ancient, multimember and diverse enzyme class. Plant GSTs have diverse roles in plant development, endogenous metabolism, stress tolerance, and xenobiotic detoxification. Their study embodies both fundamental aspects and agricultural interest, because of their ability to confer tolerance against biotic and abiotic stresses and to detoxify herbicides. Here we review the biotechnological applications of GSTs towards developing plants that are resistant to biotic and abiotic stresses. We integrate recent discoveries, highlight, and critically discuss the underlying biochemical and molecular pathways involved. We elaborate that the functions of GSTs in abiotic and biotic stress adaptation are potentially a result of both catalytic and non-catalytic functions. These include conjugation of reactive electrophile species with glutathione and the modulation of cellular redox status, biosynthesis, binding, and transport of secondary metabolites and hormones. Their major universal functions under stress underline the potential in developing climate-resilient cultivars through a combination of molecular and conventional breeding programs. We propose that future GST engineering efforts through rational and combinatorial approaches, would lead to the design of improved isoenzymes with purpose-designed catalytic activities and novel functional properties. Concurrent GST-GSH metabolic engineering can incrementally increase the effectiveness of GST biotechnological deployment.

Characterization and functional analysis of a recombinant tau class glutathione transferase GmGSTU2-2 from Glycine max

The plant tau class glutathione transferases (GSTs) perform diverse catalytic as well as non-catalytic roles in detoxification of xenobiotics, prevention of oxidative damage and endogenous metabolism. In the present work, the tau class isoenzyme GSTU2-2 from Glycine max (GmGSTU2-2) was characterized. Gene expression analysis of GmGSTU2 suggested a highly specific and selective induction pattern to osmotic stresses, indicating that gene expression is controlled by a specific mechanism. Purified, recombinant GmGSTU2-2 was shown to exhibit wide-range specificity towards xenobiotic compounds and ligand-binding properties, suggesting that the isoenzyme could provide catalytic flexibility in numerous metabolic conditions. Homology modeling and phylogenetic analysis suggested that the catalytic and ligand binding sites of GmGSTU2-2 are well conserved compared to other tau class GSTs. Structural analysis identified key amino acid residues in the hydrophobic binding site and provided insights into the substrate specificity of this enzyme. The results established that GmGSTU2-2 participates in a broad network of catalytic and regulatory functions involved in the plant stress response.

Biochemical Characterization of the Detoxifying Enzyme Glutathione Transferase P1-1 from the Camel Camelus Dromedarius

Glutathione transferase (GST, EC 2.5.1.18) is a primary line of defense against toxicities of electrophile compounds and oxidative stress and therefore is involved in stress-response and cell detoxification. In the present study, we investigated the catalytic and structural properties of the glutathione transferase (GST) isoenzyme P1-1 from Camelus dromedarius (CdGSTP1-1). Recombinant CdGSTP1-1 was produced in Escherichia coli BL21(DE3) and purified to electrophoretic homogeneity. Kinetic analysis revealed that CdGSTP1-1 displays broad substrate specificity and shows high activity towards halogenated aryl-compounds, isothiocyanates and hydroperoxides. Computation analysis and structural comparison of the catalytic and ligand binding sites of CdGSTP1-1 with other pi class GSTs allowed the identification of major structural variations that affect the active site pocket and the catalytic mechanism., Affinity labeling and kinetic inhibition studies identified key regions that form the ligandin-binding site (L-site) and gave further insights into the mechanism of non-substrate ligand recognition. The results of the present study provide new information into camelid detoxifying mechanism and new knowledge into the diversity and complex enzymatic functions of GST superfamily.

In planta characterization of a tau class glutathione S-transferase gene from Juglans regia (JrGSTTau1) involved in chilling tolerance

KEY MESSAGE

JrGSTTau1 is an important candidate gene for plant chilling tolerance regulation. A tau subfamily glutathione S-transferase (GST) gene from Juglans regia (JrGSTTau1, GeneBank No.: KT351091) was cloned and functionally characterized. JrGSTTau1 was induced by 16, 12, 10, 8, and 6 °C stresses. The transiently transformed J. regia showed much greater GST, glutathione peroxidase (GPX), superoxide dismutase (SOD), and peroxidase (POD) activities and lower H2O2, malondialdehyde (MDA), reactive oxygen species (ROS), and electrolyte leakage (EL) rate than prokII (empty vector control) and RNAi::JrGSTTau1 under cold stress, indicating that JrGSTTau1 may be involved in chilling tolerance. To further confirm the role of JrGSTTau1, JrGSTTau1 was heterologously expressed in tobacco, transgenic Line5, Line9, and Line12 were chosen for analysis. The germinations of WT, Line5, Line9, and Line12 were similar, but the fresh weight, primary root length, and total chlorophyll content (tcc) of the transgenic lines were significantly higher than those of WT under cold stress. When cultivated in soil, the GST and SOD activities of transgenic tobacco were significantly higher than those of WT; however, the MDA and H2O2 contents of WT were on average 1.47- and 1.96-fold higher than those of Line5, Line9, and Line12 under 16 °C. The DAB, Evans blue, and PI staining further confirmed these results. Furthermore, the abundances of NtGST, MnSOD, NtMAPK9, and CDPK15 were elevated in 35S::JrGSTTau1 tobacco compared with WT. These results suggested that JrGSTTau1 improves the plant chilling tolerance involved in protecting enzymes, ROS scavenging, and stress-related genes, indicating that JrGSTTau1 is a candidate gene for the potential application in molecular breeding to enhance plant abiotic stress tolerance.

Physiological and Molecular Alterations Promoted by Schizotetranychus oryzae Mite Infestation in Rice Leaves

Infestation of phytophagous mite Schizotetranychus oryzae in rice causes critical yield losses. To better understand this interaction, we employed Multidimensional Protein Identification Technology (MudPIT) approach to identify differentially expressed proteins. We detected 18 and 872 unique proteins in control and infested leaves, respectively, along with 32 proteins more abundant in control leaves. S. oryzae infestation caused decreased abundance of proteins related to photosynthesis (mostly photosystem II-related), carbon assimilation and energy production, chloroplast detoxification, defense, and fatty acid and gibberellin synthesis. On the contrary, infestation caused increased abundance of proteins involved in protein modification and degradation, gene expression at the translation level, protein partitioning to different organelles, lipid metabolism, actin cytoskeleton remodeling, and synthesis of jasmonate, amino acid, and molecular chaperones. Our results also suggest that S. oryzae infestation promotes cell-wall remodeling and interferes with ethylene biosynthesis in rice leaves. Proteomic data were positively correlated with enzymatic assays and RT-qPCR analysis. Our findings describe the protein expression patterns of infested rice leaves and suggest that the acceptor side of PSII is probably the major damaged target in the photosynthetic apparatus. These data will be useful in future biotechnological approaches aiming to induce phytophagous mite resistance in rice.

Tobacco plants over-expressing the sweet orange tau glutathione transferases (CsGSTUs) acquire tolerance to the diphenyl ether herbicide fluorodifen and to salt and drought stresses

The glutathione transferases (GSTs) are members of a superfamily of enzymes with pivotal role in the detoxification of both xenobiotic and endogenous compounds. In this work, the generation and characterization of transgenic tobacco plants over-expressing tau glutathione transferases from Citrus sinensis (CsGSTU1 and CsGSTU2) and several cross-mutate forms of these genes are reported. Putative transformed plants were verified for the presence of the transgenes and the relative quantification of transgene copy number was evaluated by Taqman real time PCR. The analysis of gene expression revealed that transformed plants exhibit high levels of CsGSTU transcription suggesting that the insertion of the transgenes occurred in transcriptional active regions of the tobacco genome. In planta studies demonstrate that transformed tobacco plants gain tolerance against fluorodifen. Simultaneously, the wild type CsGSTU genes were in vitro expressed and their kinetic properties were determined using fluorodifen as substrate. The results show that CsGSTU2 follows a Michaelis-Menten hyperbolic kinetic, whereas CsGSTU1 generates a sigmoid plot typical of the regulatory enzymes, thus suggesting that when working at sub-lethal fluorodifen concentrations CsGSTU2 can counteract the herbicide injury more efficiently than the CsGSTU1. Moreover, the transgenic tobacco plant over-expressing CsGSTs exhibited both drought and salinity stress tolerance. However, as we show that CsGSTUs do not function as glutathione peroxidase in vitro, the protective effect against salt and drought stress is not due to a direct scavenging activity of the oxidative stress byproducts. The transgenic tobacco plants, which are described in the present study, can be helpful for phytoremediation of residual xenobiotics in the environment and overall the over-expression of CsGSTUs can be helpful to develop genetically modified crops with high resistance to abiotic stresses.

Substrate specificities of two tau class glutathione transferases inducible by 2,4,6-trinitrotoluene in poplar

BACKGROUND

The genome of poplar (Populus trichocarpa) encodes 81 glutathione transferases (GSTs) annotated in eight distinct classes. The tau class is considered the most versatile in the biotransformation of xenobiotics and is composed of 58 GSTs. Two of the enzymes, GSTU16 and GSTU45, have particular interest since their expression is induced by exposure of poplar tissues to 2,4,6-trinitrotoluene (TNT) and could potentially be involved in the metabolism of this toxic environmental contaminant.

RESULTS

DNA encoding these GSTs was synthesized and the proteins were heterologously expressed in Escherichia coli and the purified enzymes were characterized.

MAJOR CONCLUSIONS

GSTU16 assayed with a number of conventional GST substrates showed the highest specific activity (60μmolmin⁻¹ mg⁻¹) with phenethyl isothiocyanate, 150-fold higher than that with CDNB. By contrast, GSTU45 showed CDNB as the most active substrate (3.3μmolmin⁻¹ mg⁻¹) whereas all of the 16 alternative substrates tested yielded significantly lower activities. Homology modeling suggested that the aromatic residues Phe10 and Tyr107 in the active site of GSTU16 are promoting the high activity with PEITC and other substrates with aromatic side-chains. Nonetheless, TNT was a poor substrate for GSTU16 as well as for GSTU45 with a specific activity of 0.05nmolmin⁻¹ mg⁻¹ for both enzymes.

GENERAL SIGNIFICANCE

GSTU16 and GSTU45 do not play a major role in the degradation of TNT in poplar.

Catalytic features and crystal structure of a tau class glutathione transferase from Glycine max specifically upregulated in response to soybean mosaic virus infections

The plant tau class glutathione transferases (GSTs) play important roles in biotic and abiotic stress tolerance in crops and weeds. In this study, we systematically examined the catalytic and structural features of a GST isoenzyme from Glycine max (GmGSTU10-10). GmGSTU10-10 is a unique isoenzyme in soybean that is specifically expressed in response to biotic stress caused by soybean mosaic virus (SMV) infections. GmGSTU10-10 was cloned, expressed in Escherichia coli, purified and characterized. The results showed that GmGSTU10-10 catalyzes several different reactions and exhibits wide substrate specificity. Of particular importance is the finding that the enzyme shows high antioxidant catalytic function and acts as hydroperoxidase. In addition, its Km for GSH is significantly lower, compared to other plant GSTs, suggesting that GmGSTU10-10 is able to perform efficient catalysis under conditions where the concentration of reduced glutathione is low (e.g. oxidative stress). The crystal structure of GmGSTU10-10 was solved by molecular replacement at 1.6Å resolution in complex with glutathione sulfenic acid (GSOH). Structural analysis showed that GmGSTU10-10 shares the same overall fold and domain organization as other plant cytosolic GSTs; however, major variations were identified in helix H9 and the upper part of helix H4 that affect the size of the active site pockets, substrate recognition and the catalytic mechanism. The results of the present study provide new information into GST diversity and give further insights into the complex regulation and enzymatic functions of this plant gene superfamily.

Plant GSTome: structure and functional role in xenome network and plant stress response

Glutathione transferases (GSTs) represent a major group of detoxification enzymes. All plants possess multiple cytosolic GSTs, each of which displays distinct catalytic as well as non-catalytic binding properties. The progress made in recent years in the fields of genomics, proteomics and protein crystallography of GSTs, coupled with studies on their molecular evolution, diversity and substrate specificity has provided new insights into the function of these enzymes. In plants, GSTs appear to be implicated in an array of different functions, including detoxification of xenobiotics and endobiotics, primary and secondary metabolism, stress tolerance, and cell signalling. This review focuses on plant GSTome and attempts to give an overview of its catalytic and functional role in xenome and plant stress regulatory networks.