The secondary metabolism glycosyltransferases UGT73B3 and UGT73B5 are components of redox status in resistance of Arabidopsis to Pseudomonas syringae pv. tomato

Secondary metabolism plant glycosyltransferases (UGTs) ensure conjugation of sugar moieties to secondary metabolites (SMs) and glycosylation contributes to the great diversity, reactivity and regulation of SMs. UGT73B3 and UGT73B5, two UGTs of Arabidopsis thaliana (Arabidopsis), are involved in the hypersensitive response (HR) to the avirulent bacteria Pseudomonas syringae pv. tomato (Pst-AvrRpm1), but their function in planta is unknown. Here, we report that ugt73b3, ugt73b5 and ugt73b3 ugt73b5 T-DNA insertion mutants exhibited an accumulation of reactive oxygen species (ROS), an enhanced cell death during the HR to Pst-AvrRpm1, whereas glutathione levels increased in the single mutants. In silico analyses indicate that UGT73B3 and UGT73B5 belong to the early salicylic acid (SA)-induced genes whose pathogen-induced expression is co-regulated with genes related to cellular redox homeostasis and general detoxification. Analyses of metabolic alterations in ugt mutants reveal modification of SA and scopoletin contents which correlate with redox perturbation, and indicate quantitative modifications in the pattern of tryptophan-derived SM accumulation after Pst-AvrRpm1 inoculation. Our data suggest that UGT73B3 and UGT73B5 participate in regulation of redox status and general detoxification of ROS-reactive SMs during the HR to Pst-AvrRpm1, and that decreased resistance to Pst-AvrRpm1 in ugt mutants is tightly linked to redox perturbation.

Functional analysis of Arabidopsis mutants points to novel roles for glutathione in coupling H(2)O(2) to activation of salicylic acid accumulation and signaling

AIMS

Through its interaction with H(2)O(2), glutathione is a candidate for transmission of signals in plant responses to pathogens, but identification of signaling roles is complicated by its antioxidant function. Using a genetic approach based on a conditional catalase-deficient Arabidopsis mutant, cat2, this study aimed at establishing whether GSH plays an important functional role in the transmission of signals downstream of H(2)O(2).

RESULTS

Introducing the cad2 or allelic mutations in the glutathione synthesis pathway into cat2 blocked H(2)O(2)-triggered GSH oxidation and accumulation. While no effects on NADP(H) or ascorbate were observed, and H(2)O(2)-induced decreases in growth were maintained, blocking GSH modulation antagonized salicylic acid (SA) accumulation and SA-dependent responses. Other novel double and triple mutants were produced and compared with cat2 cad2 at the levels of phenotype, expression of marker genes, nontargeted metabolite profiling, accumulation of SA, and bacterial resistance. Most of the effects of the cad2 mutation on H(2)O(2)-triggered responses were distinct from those produced by mutations for GLUTATHIONE REDUCTASE1 (GR1) or NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1), and were linked to compromised induction of ISOCHORISMATE SYNTHASE1 (ICS1) and ICS1-dependent SA accumulation.

INNOVATION

A novel genetic approach was used in which GSH content or antioxidative capacity was independently modified in an H(2)O(2) signaling background. Analysis of new double and triple mutants allowed us to infer previously undescribed regulatory roles for GSH.

CONCLUSION

In parallel to its antioxidant role, GSH acts independently of NPR1 to allow increased intracellular H(2)O(2) to activate SA signaling, a key defense response in plants.

Functional analysis of Arabidopsis mutants points to novel roles for glutathione in coupling H(2)O(2) to activation of salicylic acid accumulation and signaling

AIMS

Through its interaction with H(2)O(2), glutathione is a candidate for transmission of signals in plant responses to pathogens, but identification of signaling roles is complicated by its antioxidant function. Using a genetic approach based on a conditional catalase-deficient Arabidopsis mutant, cat2, this study aimed at establishing whether GSH plays an important functional role in the transmission of signals downstream of H(2)O(2).

RESULTS

Introducing the cad2 or allelic mutations in the glutathione synthesis pathway into cat2 blocked H(2)O(2)-triggered GSH oxidation and accumulation. While no effects on NADP(H) or ascorbate were observed, and H(2)O(2)-induced decreases in growth were maintained, blocking GSH modulation antagonized salicylic acid (SA) accumulation and SA-dependent responses. Other novel double and triple mutants were produced and compared with cat2 cad2 at the levels of phenotype, expression of marker genes, nontargeted metabolite profiling, accumulation of SA, and bacterial resistance. Most of the effects of the cad2 mutation on H(2)O(2)-triggered responses were distinct from those produced by mutations for GLUTATHIONE REDUCTASE1 (GR1) or NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1), and were linked to compromised induction of ISOCHORISMATE SYNTHASE1 (ICS1) and ICS1-dependent SA accumulation.

INNOVATION

A novel genetic approach was used in which GSH content or antioxidative capacity was independently modified in an H(2)O(2) signaling background. Analysis of new double and triple mutants allowed us to infer previously undescribed regulatory roles for GSH.

CONCLUSION

In parallel to its antioxidant role, GSH acts independently of NPR1 to allow increased intracellular H(2)O(2) to activate SA signaling, a key defense response in plants.

Regulation of basal and oxidative stress-triggered jasmonic acid-related gene expression by glutathione

Glutathione is a determinant of cellular redox state with roles in defence and detoxification. Emerging concepts suggest that this compound also has functions in cellular signalling. Here, we report evidence that glutathione plays potentially important roles in setting signalling strength through the jasmonic acid (JA) pathway. Firstly, we show that basal expression of JA-related genes is correlated with leaf glutathione content when the latter is manipulated either genetically or pharmacologically. Secondly, analyses of an oxidative stress signalling mutant, cat2, reveal that up-regulation of the JA pathway triggered by intracellular oxidation requires accompanying glutathione accumulation. Genetically blocking this accumulation in a cat2 cad2 line largely annuls H2 O2 -induced expression of JA-linked genes, and this effect can be rescued by exogenously supplying glutathione. While most attention on glutathione functions in biotic stress responses has been focused on the thiol-regulated protein NPR1, a comparison of JA-linked gene expression in cat2 cad2 and cat2 npr1 double mutants provides evidence that glutathione acts through other components to regulate the response of this pathway to oxidative stress. Our study provides new information implicating glutathione as a factor determining basal JA gene expression and suggests novel glutathione-dependent control points that regulate JA signalling in response to intracellular oxidation.

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.

AtRbohF is a crucial modulator of defence-associated metabolism and a key actor in the interplay between intracellular oxidative stress and pathogenesis responses in Arabidopsis

This work investigated the contribution of AtRbohD and AtRbohF to regulating defence-associated metabolism during three types of interaction: (i) incompatible and (ii) compatible interaction with Pseudomonas syringae; and (iii) intracellular oxidative stress in the catalase-deficient cat2 background. In all three cases, loss of function of either gene modulated the response of defence compounds. AtRbohF gene function was necessary for rapid and full induction of salicylic acid (SA) during compatible and incompatible interactions, and for resistance to virulent bacteria. Both artrboh mutations modulated the effects of intracellular ROS in the cat2 background, although the predominant effect was mediated by atrbohF. Loss of this gene function increased lesion formation in cat2 but uncoupled this effect from cat2-triggered induction of SA and camalexin, accumulation of glutathione and disease resistance, all of which were much lower in cat2 artbohF than in cat2. A detailed comparison of GC-TOF-MS profiles produced by the three interactions revealed considerable overlap between cat2 effects and those produced by bacterial infection in the wild-type background. Analysis of the impact of the two atrboh mutations on these profiles provided further evidence that AtRbohF interacts closely with intracellular oxidative stress to tune dynamic metabolic responses during infection. Thus, AtRbohF appears to be a key player not only in HR-related cell death but also in regulating metabolomic responses and resistance. Based on the results obtained during the three types of interaction, a model is proposed of how NADPH oxidases and intracellular ROS interact to determine the outcome of pathogen defence responses.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

Myo-inositol abolishes salicylic acid-dependent cell death and pathogen defence responses triggered by peroxisomal hydrogen peroxide

• Signalling between reactive oxygen species (ROS) and salicylic acid (SA)-dependent programmed cell death (PCD) and defence responses is complex and much remains to be discovered. Recent reports have implicated myo-inositol (MI) in defence responses, but the relationships between MI, ROS and SA remain to be elucidated. • This question was investigated in catalase-deficient Arabidopsis thaliana plants (cat2), in which a peroxisomal H(2) O(2) trigger induces SA-dependent lesion formation and a wide range of pathogen responses. • GC-MS analysis revealed that leaf MI contents were markedly decreased in cat2 independently of SA accumulation. Supplying MI to cat2 blocked lesion formation, SA accumulation and associated defence responses in a manner that closely mimicked the effect of genetically blocking SA synthesis through isochorismate synthase 1 (ICS1). The effects of MI were linked to repression of ICS1 transcripts but not decreased oxidative stress or signalling, and caused loss of resistance to virulent bacteria. The antagonistic effects of MI on lesion formation and resistance could be partly restored by supplying SA. • Our findings demonstrate a role for MI in cell death triggered by peroxisomal H(2) O(2) , and suggest that the tissue content of this compound is a key factor determining whether oxidative stress induces or opposes defence responses.

Catalase function in plants: a focus on Arabidopsis mutants as stress-mimic models

Hydrogen peroxide (H(2)O(2)) is an important signal molecule involved in plant development and environmental responses. Changes in H(2)O(2) availability can result from increased production or decreased metabolism. While plants contain several types of H(2)O(2)-metabolizing proteins, catalases are highly active enzymes that do not require cellular reductants as they primarily catalyse a dismutase reaction. This review provides an update on plant catalase genes, function, and subcellular localization, with a focus on recent information generated from studies on Arabidopsis. Original data are presented on Arabidopsis catalase single and double mutants, and the use of some of these lines as model systems to investigate the outcome of increases in intracellular H(2)O(2) are discussed. Particular attention is paid to interactions with cell thiol-disulphide status; the use of catalase-deficient plants to probe the apparent redundancy of reductive H(2)O(2)-metabolizing pathways; the importance of irradiance and growth daylength in determining the outcomes of catalase deficiency; and the induction of pathogenesis-related responses in catalase-deficient lines. Within the context of strategies aimed at understanding and engineering plant stress responses, the review also considers whether changes in catalase activities in wild-type plants are likely to be a significant part of plant responses to changes in environmental conditions or biotic challenge.

Peroxisomal hydrogen peroxide is coupled to biotic defense responses by ISOCHORISMATE SYNTHASE1 in a daylength-related manner

While it is well established that reactive oxygen species can induce cell death, intracellularly generated oxidative stress does not induce lesions in the Arabidopsis (Arabidopsis thaliana) photorespiratory mutant cat2 when plants are grown in short days (SD). One interpretation of this observation is that a function necessary to couple peroxisomal hydrogen peroxide (H(2)O(2))-triggered oxidative stress to cell death is only operative in long days (LD). Like lesion formation, pathogenesis-related genes and camalexin were only induced in cat2 in LD, despite less severe intracellular redox perturbation compared with SD. Lesion formation triggered by peroxisomal H(2)O(2) was modified by introducing secondary mutations into the cat2 background and was completely absent in cat2 sid2 double mutants, in which ISOCHORISMATE SYNTHASE1 (ICS1) activity is defective. In addition to H(2)O(2)-induced salicylic acid (SA) accumulation, the sid2 mutation in ICS1 abolished a range of LD-dependent pathogen responses in cat2, while supplementation of cat2 with SA in SD activated these responses. Nontargeted transcript and metabolite profiling identified clusters of genes and small molecules associated with the daylength-dependent ICS1-mediated relay of H(2)O(2) signaling. The effect of oxidative stress in cat2 on resistance to biotic challenge was dependent on both growth daylength and ICS1. We conclude that (1) lesions induced by intracellular oxidative stress originating in the peroxisomes can be genetically reverted; (2) the isochorismate pathway of SA synthesis couples intracellular oxidative stress to cell death and associated disease resistance responses; and (3) camalexin accumulation was strictly dependent on the simultaneous presence of both H(2)O(2) and SA signals.

The differential spatial distribution of secondary metabolites in Arabidopsis leaves reacting hypersensitively to Pseudomonas syringae pv. tomato is dependent on the oxidative burst

Secondary metabolites (SMs) play key roles in pathogen responses, although knowledge of their precise functions is limited by insufficient characterization of their spatial response. The present study addressed this issue in Arabidopsis leaves by non-targeted and targeted metabolite profiling of Pseudomonas syringae pv. tomato (Pst-AvrRpm1) infected and adjacent uninfected leaf tissues. While overlap was observed between infected and uninfected areas, the non-targeted metabolite profiles of these regions differed quantitatively and clustering analysis underscores a differential distribution of SMs within distinct metabolic pathways. Targeted metabolite profiling revealed that infected tissues accumulate more salicylic acid and the characteristic phytoalexin of Arabidopsis, camalexin, than uninfected adjacent areas. On the contrary, the antioxidant coumarin derivative, scopoletin, was induced in infected tissues while its glucoside scopolin predominated in adjacent tissues. To elucidate the still unclear relationship between the accumulation of SMs and reactive oxygen species (ROS) accumulation and signalling, a catalase-deficient line (cat2) in which ROS signalling is up-regulated, was used. Metabolic analysis of cat2 suggests that some SMs have important interactions with ROS in redox homeostasis during the hypersensitive response to Pst-AvrRpm1. Overall, the study demonstrates that ROS availability influences both the amount and the pattern of infection-induced SM accumulation.

Arabidopsis GLUTATHIONE REDUCTASE1 plays a crucial role in leaf responses to intracellular hydrogen peroxide and in ensuring appropriate gene expression through both salicylic acid and jasmonic acid signaling pathways

Glutathione is a major cellular thiol that is maintained in the reduced state by glutathione reductase (GR), which is encoded by two genes in Arabidopsis (Arabidopsis thaliana; GR1 and GR2). This study addressed the role of GR1 in hydrogen peroxide (H(2)O(2)) responses through a combined genetic, transcriptomic, and redox profiling approach. To identify the potential role of changes in glutathione status in H(2)O(2) signaling, gr1 mutants, which show a constitutive increase in oxidized glutathione (GSSG), were compared with a catalase-deficient background (cat2), in which GSSG accumulation is conditionally driven by H(2)O(2). Parallel transcriptomics analysis of gr1 and cat2 identified overlapping gene expression profiles that in both lines were dependent on growth daylength. Overlapping genes included phytohormone-associated genes, in particular implicating glutathione oxidation state in the regulation of jasmonic acid signaling. Direct analysis of H(2)O(2)-glutathione interactions in cat2 gr1 double mutants established that GR1-dependent glutathione status is required for multiple responses to increased H(2)O(2) availability, including limitation of lesion formation, accumulation of salicylic acid, induction of pathogenesis-related genes, and signaling through jasmonic acid pathways. Modulation of these responses in cat2 gr1 was linked to dramatic GSSG accumulation and modified expression of specific glutaredoxins and glutathione S-transferases, but there is little or no evidence of generalized oxidative stress or changes in thioredoxin-associated gene expression. We conclude that GR1 plays a crucial role in daylength-dependent redox signaling and that this function cannot be replaced by the second Arabidopsis GR gene or by thiol systems such as the thioredoxin system.

Confirmation of associations between ion channel gene SNPs and QTc interval duration in healthy subjects

Population-based association studies have identified several polymorphic variants in genes encoding ion channel subunits associated with the electrocardiographic heart-rate-corrected QT (QTc) length in healthy populations of Caucasian origin (KCNH2 rs1,805,123 (K897 T) and rs3,815,459, SCN5A rs1,805,126 (D1,819D), 1,141-3 C>A, rs1,805,124 (H558R), and IVS24+116 G>A, KCNQ1 rs757,092, KCNE1 IVS2-128 G>A and rs1,805,127 (G38S), and KCNE2 rs2,234,916 (T8A)). However, few of these results have been replicated in independent populations. We tested the association of SNPs KCNQ1 rs757,092, KCNH2 rs3,815,459, SCN5A IVS24+116 G>A, KCNE1 IVS2-128 G>A and KCNE2 rs2,234,916 with QTc length in two groups of 200 subjects presenting the shortest and the longest QTc from a cohort of 2,008 healthy subjects. All polymorphisms were in Hardy-Weinberg equilibrium in both groups. The minor allele SCN5A IVS24+116 A was more frequent in the group of subjects with the shortest QTc, whereas the minor alleles KCNQ1 rs757,092 G and KCNH2 rs3,815,459 A were more frequent in the group with the longest QTc. There was no significant difference for KCNE1 IVS2-128 G>A and KCNE2 rs2,234,916 between the two groups. Haplotype analysis showed a twofold increased risk of QTc lengthening for carriers of the haplotype, combining alleles C and A of the two common KCNE1 SNPs, IVS2-129 C>T (rs2,236,609) and rs1,805,127 (G38S), respectively. In conclusion, our study confirms the reported associations between QTc length and KCNQ1 rs757,092 and KCNH2 rs3,815,459.