Methylglyoxal levels in plants under salinity stress are dependent on glyoxalase I and glutathione

Two D-2-hydroxy-acid dehydrogenases in Arabidopsis thaliana with catalytic capacities to participate in the last reactions of the methylglyoxal and beta-oxidation pathways

The Arabidopsis thaliana locus At5g06580 encodes an ortholog to Saccharomyces cerevisiae d-lactate dehydrogenase (AtD-LDH). The recombinant protein is a homodimer of 59-kDa subunits with one FAD per monomer. A substrate screen indicated that AtD-LDH catalyzes the oxidation of d- and l-lactate, d-2-hydroxybutyrate, glycerate, and glycolate using cytochrome c as an electron acceptor. AtD-LDH shows a clear preference for d-lactate, with a catalytic efficiency 200- and 2000-fold higher than that for l-lactate and glycolate, respectively, and a K(m) value for d-lactate of approximately 160 microm. Knock-out mutants showed impaired growth in the presence of d-lactate or methylglyoxal. Collectively, the data indicated that the protein is a d-LDH that participates in planta in the methylglyoxal pathway. Web-based bioinformatic tools revealed the existence of a paralogous protein encoded by locus At4g36400. The recombinant protein is a homodimer of 61-kDa subunits with one FAD per monomer. A substrate screening revealed highly specific d-2-hydroxyglutarate (d-2HG) conversion in the presence of an organic cofactor with a K(m) value of approximately 580 microm. Thus, the enzyme was characterized as a d-2HG dehydrogenase (AtD-2HGDH). Analysis of knock-out mutants demonstrated that AtD-2HGDH is responsible for the total d-2HGDH activity present in A. thaliana. Gene coexpression analysis indicated that AtD-2HGDH is in the same network as several genes involved in beta-oxidation and degradation of branched-chain amino acids and chlorophyll. It is proposed that AtD-2HGDH participates in the catabolism of d-2HG most probably during the mobilization of alternative substrates from proteolysis and/or lipid degradation.

Arabidopsis MSI1 is required for negative regulation of the response to drought stress

Arabidopsis MSI1 has fundamental functions in plant development. MSI1 is a subunit of Polycomb group protein complexes and Chromatin assembly factor 1, and it interacts with the Retinoblastoma-related protein 1. Altered levels of MSI1 result in pleiotropic phenotypes, reflecting the complexity of MSI1 protein functions. In order to uncover additional functions of MSI1, we performed transcriptional profiling of wild-type and plants with highly reduced MSI1 levels (msi1-cs). Surprisingly, the known functions of MSI1 could only account for a minor part of the transcriptional changes in msi1-cs plants. One of the most striking unexpected observations was the up-regulation of a subset of ABA-responsive genes eliciting the response to drought and salt stress. We report that MSI1 can bind to the chromatin of the drought-inducible downstream target RD20 and suggest a new role for MSI1 in the negative regulation of the Arabidopsis drought-stress response.

Molecular cloning and characterization of a novel glyoxalase I gene TaGly I in wheat (Triticum aestivum L.)

Methylglyoxal is a kind of poisonous metabolite that can react with RNA, DNA and protein, which generally results in a number of side advert effects to cell. Glyoxalase I is a member of glyoxalase system that can detoxify methylglyoxal. An EST encoding a glyoxalase I was isolated from a SSH (suppression subtractive hybridization)-cDNA library of wheat spike inoculated by Fusarium graminearum. The corresponding full length gene, named TaGly I, was cloned, sequenced and characterized. Its genomic sequence consists of 2,719 bp, including seven exons and six introns, and its coding sequence is 929 bp with an open reading frame encoding 291 amino acids. Sequence alignment showed that there were two glyoxalase I domains in the deduced protein sequence. By using specific primers, TaGly I was mapped to chromosome 7D of wheat via a set of durum wheat 'Langdon' D-genome disomic-substitution lines. The result of Real-time quantitative polymerase chain reaction demonstrated that TaGly I was induced by the inoculation of Fusarium graminearum in wheat spikes. Additionally, it was also induced by high concentration of NaCl and ZnCl2. When TaGly I was overexpressed in tobacco leaves via Agrobacterium tumefaciens infection, the transgenic tobacco showed stronger tolerance to ZnCl2 stress relative to transgenic control with GFP. The above facts indicated that TaGly I might play a role in response to diverse stresses in plants.

Chilling stress-induced proteomic changes in rice roots

Roots are highly sensitive organs in plants. To gain a better knowledge of the chilling stress responses of plants, it is imperative to analyze the tissue-specific proteome patterns under chilling stress. In the present study, two-dimensional gel electrophoresis (2-DE) coupled with mass spectrometry, has been adopted to investigate the protein expression patterns of rice roots in response to chilling stress. Rice seedlings were subjected to 10 degrees C and samples were collected 24 and 72h after treatment. To identify the low-abundant proteins in root tissues, samples were fractionated by 15% polyethylene glycol (PEG), separated by 2-DE, and visualized by silver or CBB staining. A total of 27 up-regulated proteins were identified by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry or electrospray ionization-tandem mass spectrometry (ESI-MS/MS) analysis. Together with the previously identified cold-stress-responsive proteins, a group of novel proteins were identified including acetyl transferase, phosphogluconate dehydrogenase, NADP-specific isocitrate dehydrogenase, fructokinase, PrMC3, putative alpha-soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein, and glyoxalase 1. These proteins are involved in several cellular processes, including energy production and metabolism, vesicular trafficking, and detoxification. Gene expression at the mRNA level of some selected proteins revealed that transcription levels are not always concomitant to the translational level. Thus, investigation of root proteome expression and identification of some novel proteins could be useful in better understanding the molecular basis of chilling stress responses in plants.

Potato (Solanum tuberosum L.) tuber ageing induces changes in the proteome and antioxidants associated with the sprouting pattern

During post-harvest storage, potato tubers age as they undergo an evolution of their physiological state influencing their sprouting pattern. In the present study, physiological and biochemical approaches were combined to provide new insights on potato (Solanum tuberosum L. cv. Désirée) tuber ageing. An increase in the physiological age index (PAI) value from 0.14 to 0.83 occurred during storage at 4 degrees C over 270 d. Using this reference frame, a proteomic approach was followed based on two-dimensional electrophoresis. In the experimental conditions of this study, a marked proteolysis of patatin occurred after the PAI reached a value of 0.6. In parallel, several glycolytic enzymes were up-regulated and cellular components influencing protein conformation and the response to stress were altered. The equilibrium between the 20S and 26S forms of the proteasome was modified, the 20S form that recycles oxidized proteins being up-regulated. Two proteins belonging to the cytoskeleton were also differentially expressed during ageing. As most of these changes are also observed in an oxidative stress context, an approach focused on antioxidant compounds and enzymes as well as oxidative damage on polyunsaturated fatty acids and proteins was conducted. All the changes observed during ageing seemed to allow the potato tubers to maintain their radical scavenging activity until the end of the storage period as no accumulation of oxidative damage was observed. These data are interpreted considering the impact of reactive oxygen species on the development and the behaviour of other plant systems undergoing ageing or senescence processes.

Salt stress-induced alterations in the root proteome of barley genotypes with contrasting response towards salinity

In addition to drought and extreme temperatures, soil salinity represents a growing threat to crop productivity. Among the cereal crops, barley is considered as notably salt tolerant, and cultivars show considerable variation for tolerance towards salinity stress. In order to unravel the molecular mechanisms underlying salt stress tolerance and to utilize the natural genetic variation of barley accessions, a series of hydroponics-based salinity stress experiments was conducted using two genetic mapping parents, cvs Steptoe and Morex, which display contrasting levels of salinity tolerance. The proteome of roots from both genotypes was investigated as displayed by two-dimensional gel electrophoresis, and comparisons were made between plants grown under non-saline and saline conditions. Multivariate analysis of the resulting protein patterns revealed cultivar-specific and salt stress-responsive protein expression. Mass spectrometry-based identification was successful for 26 out of 39 selected protein spots. Hierarchical clustering was applied to detect similar protein expression patterns. Among those, two proteins involved in the glutathione-based detoxification of reactive oxygen species (ROS) were more abundant in the tolerant genotype, while proteins involved in iron uptake were expressed at a higher level in the sensitive one. This study emphasizes the role of proteins involved in ROS detoxification during salinity stress, and identified potential candidates for increasing salt tolerance in barley.

A glutathione-specific aldose reductase of Leishmania donovani and its potential implications for methylglyoxal detoxification pathway

Methylglyoxal is mainly catabolized by two major enzymatic pathways. The first is the ubiquitous detoxification pathway, the glyoxalase pathway. In addition to the glyoxalase pathway, aldose reductase pathway also plays a crucial role in lowering the levels of methylglyoxal. The gene encoding aldose reductase (ALR) has been cloned from Leishmania donovani, a protozoan parasite causing visceral leishmaniasis. DNA sequence analysis revealed an open reading frame (ORF) of approximately 855 bp encoding a putative protein of 284 amino acids with a calculated molecular mass of 31.7 kDa and a predicted isoelectric point of 5.85. The sequence identity between L. donovani ALR (LdALR) and mammals and plants is only 36-44%. The ORF is a single copy gene. A protein with a molecular mass that matched the estimated approximately 74 kDa according to the amino acid composition of LdALR with a maltose binding tag present at its N-terminal end was induced by heterologous expression of LdALR in Escherichia coli. In the presence of glutathione, recombinant LdALR reduced methylglyoxal with a K(m) of approximately 112 microM. Comparative structural analysis of the human ALR structure with LdALR model suggests that the active site anchoring the N-terminal end of the glutathione is highly conserved. However, the C-terminal end of the glutathione backbone is expected to be exposed in LdALR, as the residues anchoring the C-terminal end of the glutathione backbone come from the three loop regions in human, which are apparently shortened in the LdALR structure. Thus, the computational analysis provides clues about the expected mode of glutathione binding and its interactions with the protein. This is the first report of the role of an ALR in the metabolic disposal of methylglyoxal in L. donovani and of thiol binding to a kinetoplastid aldose reductase.

An overview on the role of methylglyoxal and glyoxalases in plants

Methylglyoxal (MG) is a highly reactive cytotoxic alpha-oxoaldehyde compound and is formed endogenously via different enzymatic and non-enzymatic reactions. In plants MG is detoxified mainly via the glyoxalase system that is comprised of two enzymes, glyoxalase I and glyoxalase II. Glyoxalase I converts MG to S-D-lactoylglutathione by utilizing glutathione, while glyoxalase II converts S-D-lactoylglutathione to D-lactic acid, and during this reaction glutathione is regenerated. The presence and characterization of both glyoxalase I and II has been reported in many plants and the genes encoding these have been cloned and found to be regulated under various environmental conditions. In plants, MG has been found to be present during normal growth conditions and it accumulates to higher levels under various environmental stresses. Abiotic and heavy metal stresses induce reactive oxygen species (ROS) and MG. Overexpression of the glyoxalase pathway in transgenic tobacco and rice plants has been found to check an increase of ROS and MG under stress conditions by maintaining glutathione homeostasis and antioxidant enzyme levels. There is also evidence that in addition to glyoxalase, other pathways, such as the aldose reductase pathway, may also be involved in MG detoxification in plants. To unravel the role of MG and the glyoxalase pathway in signal transduction during environmental stress conditions in plants is a topic of future research interest. In this paper we review work on plant glyoxalases especially with respect to their role under abiotic stresses.

Identification and quantification of methylglyoxal as the dominant antibacterial constituent of Manuka (Leptospermum scoparium) honeys from New Zealand

The 1,2-dicarbonyl compounds 3-deoxyglucosulose (3-DG), glyoxal (GO), and methylglyoxal (MGO) were measured as the corresponding quinoxalines after derivatization with orthophenylendiamine using RP-HPLC and UV-detection in commercially available honey samples. Whereas for most of the samples values for 3-DG, MGO, and GO were comparable to previously published data, for six samples of New Zealand Manuka (Leptospermum scoparium) honey very high amounts of MGO were found, ranging from 38 to 761 mg/kg, which is up to 100-fold higher compared to conventional honeys. MGO was unambigously identified as the corresponding quinoxaline via photodiodearry detection as well as by means of mass spectroscopy. Antibacterial activity of honey and solutions of 1,2-dicarbonyl towards Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were analyzed using an agar well diffusion assay. Minimum concentrations needed for inhibition of bacterial growth (minimum inhibitory concentration, MIC) of MGO were 1.1 mM for both types of bacteria. MIC for GO was 6.9 mM (E. coli) or 4.3 mM (S. aureus), respectively. 3-DG showed no inhibition in concentrations up to 60 mM. Whereas most of the honey samples investigated showed no inhibition in dilutions of 80% (v/v with water) or below, the samples of Manuka honey exhibited antibacterial activity when diluted to 15-30%, which corresponded to MGO concentrations of 1.1-1.8 mM. This clearly demonstrates that the pronounced antibacterial activity of New Zealand Manuka honey directly originates from MGO.

Proline and glycinebetaine enhance antioxidant defense and methylglyoxal detoxification systems and reduce NaCl-induced damage in cultured tobacco cells

Salt stress impairs reactive oxygen species (ROS) and methylglyoxal (MG) detoxification systems, and causes oxidative damage to plants. Up-regulation of the antioxidant and glyoxalase systems provides protection against NaCl-induced oxidative damage in plants. Thiol-disulfide contents, glutathione content and its associated enzyme activities involved in the antioxidant defense and glyoxalase systems, and protein carbonylation in tobacco Bright Yellow-2 cells grown in suspension culture were investigated to assess the protection offered by proline and glycinebetaine against salt stress. Salt stress increased protein carbonylation, contents of thiol, disulfide, reduced (GSH) and oxidized (GSSG) forms of glutathione, and the activity of glutathione-S-transferase and glyoxalase II enzymes, but decreased redox state of both thiol-disulfide and glutathione, and the activity of glutathione peroxidase and glyoxalase I enzymes involved in the ROS and MG detoxification systems. Exogenous application of proline or glycinebetaine resulted in a reduction of protein carbonylation, and in an increase in glutathione redox state and activity of glutathione peroxidase, glutathione-S-transferase and glyoxalase I under salt stress. Neither proline nor glycinebetaine, however, had any direct protective effect on NaCl-induced GSH-associated enzyme activities. The present study, therefore, suggests that both proline and glycinebetaine provide a protective action against NaCl-induced oxidative damage by reducing protein carbonylation, and enhancing antioxidant defense and MG detoxification systems.

Proteomic analysis of Chelidonium majus milky sap using two-dimensional gel electrophoresis and tandem mass spectrometry

Milky sap, a milky-like orange fluid, isolated from the Greater Celandine (Chelidonium majus L.), family Papaveraceae, serves as a rich source of various biologically active substances such as alkaloids, several flavonoids, phenolic acids and proteins. The objective of this study was to separate Ch. majus milky sap extract proteins using two-dimensional gel electrophoresis (2-DE) to demonstrate for the first time the protein composition in the sap and to identify them using liquid chromatography-tandem mass spectrometry analysis (LC-ESI-MS/MS). It was possible to identify 21 proteins, which comprise disease/defence-related, signalling, Krebs cycle, nucleic acid binding and other proteins. The majority of the identified proteins can be linked to direct and indirect stress and defence reactions, e.g. against different pathogens. The specific protein composition of the milky sap suggests an important role of these proteins for the whole plant physiology and development.

Enhancing salt tolerance in a crop plant by overexpression of glyoxalase II

Earlier we have shown the role of glyoxalase overexpression in conferring salinity tolerance in transgenic tobacco. We now demonstrate the feasibility of same in a crop like rice through overproduction of glyoxalase II. The rice glyoxalase II was cloned in pCAMBIA1304 and transformed into rice (Oryza sativa cv PB1) via Agrobacterium. The transgenic plants showed higher constitutive activity of glyoxalase II that increased further upon salt stress, reflecting the upregulation of endogenous glyoxalase II. The transgenic rice showed higher tolerance to toxic concentrations of methylglyoxal (MG) and NaCl. Compared with non-transgenics, transgenic plants at the T1 generation exhibited sustained growth and more favorable ion balance under salt stress conditions.

Methylglyoxal in food and living organisms

Methylglyoxal (MG) is a highly reactive alpha-oxoaldehyde formed endogenously in numerous enzymatic and nonenzymatic reactions. It modifies arginine and lysine residues in proteins forming advanced glycation end-products such as N(delta)-(5-methyl-4-imidazolon-2-yl)-L-ornithine (MG-H1), 2-amino-5-(2-amino-5-hydro-5-methyl-4-imidazolon-1-yl)pentanoic acid (MG-H2), 2-amino-5-(2-amino-4-hydro-4-methyl-5-imidazolon-1-yl)pentanoic acid (MG-H3), argpyrimidine, N(delta)-(4-carboxy-4,6-dimethyl-5,6-dihydroxy-1,4,5,6-tetrahydropyrimidine-2-yl)-L-ornithine (THP), N(epsilon)-(1-carboxyethyl)lysine (CEL), MG-derived lysine dimer (MOLD), and 2-ammonio-6-({2-[4-ammonio-5-oxido-5-oxopently)amino]-4-methyl-4,5-dihydro-1H-imidazol-5-ylidene}amino)hexanoate (MODIC), which have been identified in vivo and are associated with complications of diabetes and some neurodegenerative diseases. In foodstuffs and beverages, MG is formed during processing, cooking, and prolonged storage. Fasting and metabolic disorders and/or defects in MG detoxification processes cause accumulation of this reactive dicarbonyl in vivo. In addition, the intake of low doses of MG over a prolonged period of time can cause degenerative changes in different tissues, and can also exert anticancer activity. MG in biological samples can be quantified by HPLC or GC methods with preliminary derivatization into more stable chromophores and/or fluorophores, or derivatives suitable for determination by MS by use of diamino derivatives of benzene and naphthalene, 6-hydroxy-2,4,5-triaminopyrimidine, cysteamine, and o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine. The methods include three basic steps: deproteinization, incubation with derivatization agent, and chromatographic analysis with or without preliminary extraction of the formed products.

Proteomics Uncovers a Role for Redox in Drought Tolerance in Wheat

Proteomic analysis offers a new approach to identify a broad spectrum of genes that are expressed in living systems. We applied a proteomic approach to study changes in wheat grain in response to drought, a major environmental parameter adversely affecting development and crop yield. Three wheat genotypes differing in genetic background were cultivated in field under well-watered and drought conditions by following a randomized complete block design with four replications. The overall effect of drought was highly significant as determined by grain yield and total dry matter. About 650 spots were reproducibly detected and analyzed on 2-DE gels. Of these, 121 proteins showed significant change under drought condition in at least one of the genotypes. Mass spectrometry analysis using MALDI-TOF/TOF led to the identification of 57 proteins. Two-thirds of identified proteins were thioredoxin (Trx) targets, in accordance with the link between drought and oxidative stress. Further, because of contrasting changes in the tolerant and susceptible genotypes studied, several proteins emerge as key participants in the drought response. In addition to providing new information on the response to water deprivation, the present study offers opportunities to pursue the breeding of wheat with enhanced drought tolerance using identified candidate genetic markers. The 2-DE database of wheat seed proteins is available for public access at http://www.proteome.ir.

The chloroplast lumen and stromal proteomes of Arabidopsis thaliana show differential sensitivity to short- and long-term exposure to low temperature

Cold acclimation and over-wintering by herbaceous plants are energetically expensive and are dependent on functional plastid metabolism. To understand how the stroma and the lumen proteomes adapt to low temperatures, we have taken a proteomic approach (difference gel electrophoresis) to identify proteins that changed in abundance in Arabidopsis chloroplasts during cold shock (1 day), and short- (10 days) and long-term (40 days) acclimation to 5 degrees C. We show that cold shock (1 day) results in minimal change in the plastid proteomes, while short-term (10 days) acclimation results in major changes in the stromal but few changes in the lumen proteome. Long-term acclimation (40 days) results in modulation of the proteomes of both compartments, with new proteins appearing in the lumen and further modulations in protein abundance occurring in the stroma. We identify 43 differentially displayed proteins that participate in photosynthesis, other plastid metabolic functions, hormone biosynthesis and stress sensing and signal transduction. These findings not only provide new insights into the cold response and acclimation of Arabidopsis, but also demonstrate the importance of studying changes in protein abundance within the relevant cellular compartment.

Characterization and functional validation of glyoxalase II from rice

Glyoxalase II, one of the enzymes of the glyoxalase pathway, cDNA cloned from rice (OsglyII) consists of 1623 nucleotides with an open reading frame of 1010 bp encoding a polypeptide of 336 amino acids and an estimated isoelectric point of 8.08. The recombinant protein purified from Escherichia coli using Ni-NTA affinity chromatography showed molecular mass of approximately 37 kDa. Catalytic parameters of the protein were determined using S-D-lactoylglutathione as a thioester substrate. The K(m) (61 microM) and K(cat) (301 s(-1)) values were lower than those reported for Arabidopsis, human and yeast and showed pH optima at 7.2. The E. coli overexpressing OsglyII were able to grow on higher concentration of methylglyoxal. Transcript analysis in rice showed that OsglyII gene expression is stimulated within 15 min in response to various abiotic stresses as well as treatment with abscisic acid or salicylic acid. This multistress response of OsglyII gene documents its future utility in developing tolerance to various stresses in crop plants.

Transgenic tobacco overexpressing glyoxalase pathway enzymes grow and set viable seeds in zinc-spiked soils

We reported earlier that engineering of the glyoxalase pathway (a two-step reaction mediated through glyoxalase I and II enzymes) enhances salinity tolerance. Here we report the extended suitability of this engineering strategy for improved heavy-metal tolerance in transgenic tobacco (Nicotiana tabacum). The glyoxalase transgenics were able to grow, flower, and set normal viable seeds in the presence of 5 mm ZnCl2 without any yield penalty. The endogenous ion content measurements revealed roots to be the major sink for excess zinc accumulation, with negligible amounts in seeds in transgenic plants. Preliminary observations suggest that glyoxalase overexpression could confer tolerance to other heavy metals, such as cadmium or lead. Comparison of relative tolerance capacities of transgenic plants, overexpressing either glyoxalase I or II individually or together in double transgenics, evaluated in terms of various critical parameters such as survival, growth, and yield, reflected double transgenics to perform better than either of the single-gene transformants. Biochemical investigations indicated restricted methylglyoxal accumulation and less lipid peroxidation under high zinc conditions in transgenic plants. Studies employing the glutathione biosynthetic inhibitor, buthionine sulfoximine, suggested an increase in the level of phytochelatins and maintenance of glutathione homeostasis in transgenic plants during exposure to excess zinc as the possible mechanism behind this tolerance. Together, these findings presents a novel strategy to develop multiple stress tolerance via glyoxalase pathway engineering, thus implicating its potential use in engineering agriculturally important crop plants to grow on rapidly deteriorating lands with multiple unfavorable edaphic factors.

Transgenic tobacco plants overexpressing glyoxalase enzymes resist an increase in methylglyoxal and maintain higher reduced glutathione levels under salinity stress

The mechanism behind enhanced salt tolerance conferred by the overexpression of glyoxalase pathway enzymes was studied in transgenic vis-à-vis wild-type (WT) plants. We have recently documented that salinity stress induces higher level accumulation of methylglyoxal (MG), a potent cytotoxin and primary substrate for glyoxalase pathway, in various plant species [Yadav, S.K., Singla-Pareek, S.L., Ray, M., Reddy, M.K. and Sopory, S.K. (2005) MG levels in plants under salinity stress are dependent on glyoxalase I and glutathione. Biochem. Biophys. Res. Commun. 337, 61-67]. The transgenic tobacco plants overexpressing glyoxalase pathway enzymes, resist an increase in the level of MG that increased to over 70% in WT plants under salinity stress. These plants showed enhanced basal activity of various glutathione related antioxidative enzymes that increased further upon salinity stress. These plants suffered minimal salinity stress induced oxidative damage measured in terms of the lipid peroxidation. The reduced glutathione (GSH) content was high in these transgenic plants and also maintained a higher reduced to oxidized glutathione (GSH:GSSG) ratio under salinity. Manipulation of glutathione ratio by exogenous application of GSSG retarded the growth of non-transgenic plants whereas transgenic plants sustained their growth. These results suggest that resisting an increase in MG together with maintaining higher reduced glutathione levels can be efficiently achieved by the overexpression of glyoxalase pathway enzymes towards developing salinity stress tolerant plants.