Identification of a fourth family of lycopene cyclases in photosynthetic bacteria

A fourth and large family of lycopene cyclases was identified in photosynthetic prokaryotes. The first member of this family, encoded by the cruA gene of the green sulfur bacterium Chlorobium tepidum, was identified in a complementation assay with a lycopene-producing strain of Escherichia coli. Orthologs of cruA are found in all available green sulfur bacterial genomes and in all cyanobacterial genomes that lack genes encoding CrtL- or CrtY-type lycopene cyclases. The cyanobacterium Synechococcus sp. PCC 7002 has two homologs of CruA, denoted CruA and CruP, and both were shown to have lycopene cyclase activity. Although all characterized lycopene cyclases in plants are CrtL-type proteins, genes orthologous to cruP also occur in plant genomes. The CruA- and CruP-type carotenoid cyclases are members of the FixC dehydrogenase superfamily and are distantly related to CrtL- and CrtY-type lycopene cyclases. Identification of these cyclases fills a major gap in the carotenoid biosynthetic pathways of green sulfur bacteria and cyanobacteria.     

G6PD up-regulation promotes pancreatic beta-cell dysfunction

Increased reactive oxygen species (ROS) induce pancreatic β-cell dysfunction during progressive type 2 diabetes. Glucose-6-phosphate dehydrogenase (G6PD) is a reduced nicotinamide adenine dinucleotide phosphate-producing enzyme that plays a key role in cellular reduction/oxidation regulation. We have investigated whether variations in G6PD contribute to β-cell dysfunction through regulation of ROS accumulation and β-cell gene expression. When the level of G6PD expression in pancreatic islets was examined in several diabetic animal models, such as db/db mice and OLEFT rats, G6PD expression was evidently up-regulated in pancreatic islets in diabetic animals. To investigate the effect of G6PD on β-cell dysfunction, we assessed the levels of cellular ROS, glucose-stimulated insulin secretion and β-cell apoptosis in G6PD-overexpressing pancreatic β-cells. In INS-1 cells, G6PD overexpression augmented ROS accumulation associated with increased expression of prooxidative enzymes, such as inducible nitric oxide synthase and reduced nicotinamide adenine dinucleotide phosphate oxidase. G6PD up-regulation also caused decrease in glucose-stimulated insulin secretion in INS-1 cells and primary pancreatic islets. Moreover, elevated G6PD expression led to β-cell apoptosis, concomitant with the increase in proapoptotic gene expression. On the contrary, suppression of G6PD with small interference RNA attenuated palmitate-induced β-cell apoptosis. Together, these data suggest that up-regulation of G6PD in pancreatic β-cells would induce β-cell dysregulation through ROS accumulation in the development of type 2 diabetes.     

Carotenoids and cardiovascular disease

Carotenoids are a class of natural fat-soluble pigments found principally in plants. They have potential antioxidant biological properties due to their chemical structure and interaction with biological membranes. The most abundant carotenoids in the diet are β-carotene, lycopene, lutein, β-cryptoxanthin, zeaxanthin, and astaxanthin. Numerous epidemiologic studies have supported the hypothesis that antioxidants could be used as an inexpensive means of prevention, and possibly treatment, of cardiovascular diseases, even though findings from interventional trials have been mixed, with some positive findings, many null findings, and some suggestion of harm in certain high-risk populations. Recent smaller interventional studies with carefully chosen populations, such as those under high levels of oxidative stress, have yielded largely positive results. This suggests that we need more hypothesisdriven and rigorous clinical trial designs. The aim of this review is to examine the published studies about the use of carotenoids, especially lycopene and astaxanthin, in the treatment of cardiovascular diseases.     

Effects of simvastatin on carotenoid status in plasma

Background and aims

Carotenoids are potent antioxidants mainly transported in the low density lipoprotein (LDL) fraction. They may also influence the immune response and inverse associations with inflammatory markers have been reported. We investigated whether simvastatin, by exerting both lipid-lowering and anti-inflammatory effects, altered the carotenoid status in plasma.

Methods and results

A randomized, double-blind, placebo-controlled study design was applied. Eighty volunteers with mild to moderate hypercholesterolemia received either simvastatin 40 mg or placebo for 6 weeks. Lipids, oxidized LDL (ox-LDL), C-reactive protein (CRP), interleukin (IL)-6, oxygenated carotenoids (lutein, zeaxanthin, β-cryptoxanthin) and hydrocarbon carotenoids (α-carotene, β-carotene, lycopene) were measured in plasma. Simvastatin use was associated with significant reductions in total cholesterol, LDL, ox-LDL and CRP. Simvastatin therapy also resulted in reduced plasma levels of both oxygenated and hydrocarbon carotenoids. However, when adjusted for lipids, all carotenoids except β-cryptoxanthin showed significant increases after simvastatin therapy. Both crude and lipid-adjusted carotenoids were inversely correlated with CRP and IL-6 in plasma but the change in carotenoid status during simvastatin therapy was not specifically related to any changes in inflammatory markers.

Conclusions

To summarize, the change in carotenoid status during simvastatin therapy was mainly attributed to the lowering of cholesterol and not to the suppression of inflammatory activity. After adjustment for lipids, the levels of lutein, lycopene, α-carotene and β-carotene were significantly increased by simvastatin suggesting an increased ratio of carotenoids per particle.     

Role of β-adrenergic receptors in regulation of hepatic fat accumulation during aging

Excessive fat accumulation in liver (hepatic steatosis) predisposes to hepatic functional and structural impairment and overall metabolic risk. Previous studies noted an association between hepatic steatosis and age in humans and rodents. However, the mechanisms leading to age-associated hepatic fat accumulation remain unknown. Earlier work from our group showed that β-adrenergic receptor (β-AR) levels and β-AR-stimulated adenylyl cyclase activity increase in rat liver during aging. Here we investigated whether age-associated increases in β-AR signaling play a role in augmenting hepatic lipid accumulation. We demonstrate an increase in hepatic lipid content during senescence and a significant correlation between hepatic fat content and stimulation of adenylyl cyclase activity by the β-AR agonist isoproterenol in rat liver. Isoproterenol administration to young and old rodents in vivo increased hepatic lipid accumulation. Furthermore, in vitro overexpression of β1- and β2-AR subtypes in hepatocytes from young rodents increased cellular lipid content, whereas inhibition of β-ARs by receptor subtype-specific inhibitors reduced lipid levels in hepatocytes from senescent animals. Isoproterenol-induced hepatic lipid accumulation in vivo was prevented by the β-AR nonselective blocker propranolol, suggesting a novel therapeutic effect of this class of drugs in hepatic steatosis. Acipimox, which inhibits adipose tissue lipolysis, did not alter isoproterenol-mediated hepatic fat accumulation; thus β-AR responsive hepatic lipid accumulation does not appear to be related primarily to altered lipolysis. These findings suggest that augmented hepatic β-AR signaling during aging may increase lipid accumulation in liver and advocate a possible role for β-adrenergic blockers in preventing or retarding the development of hepatic steatosis.     

Low plasma levels of oxygenated carotenoids in patients with coronary artery disease

Background and aimsLow circulating levels of carotenoids have been associated with cardiovascular disease. The distribution of different carotenoids in blood may have an impact on the cardioprotective capacity. The aim of the present study was to determine the plasma levels of 6 major carotenoids in patients with coronary artery disease (CAD) and relate the findings to clinical, metabolic and immune parameters.Methods and resultsPlasma levels of oxygenated carotenoids (lutein, zeaxanthin, β-cryptoxanthin) and hydrocarbon carotenoids (α-carotene, β-carotene, lycopene) were determined in 39 patients with acute coronary syndrome, 50 patients with stable CAD and 50 controls. Serological assays for inflammatory activity and flow cytometrical analysis of lymphocyte subsets were performed. Both patient groups had significantly lower plasma levels of oxygenated carotenoids, in particular lutein + zeaxanthin, compared to controls. Low levels of oxygenated carotenoids were associated with smoking, high body mass index (BMI), low high density lipoprotein (HDL) cholesterol and, to a minor degree, inflammatory activity. Plasma levels of lutein + zeaxanthin were independently associated with the proportions of natural killer (NK) cells, but not with other lymphocytes, in blood.ConclusionAmong carotenoids, lutein + zeaxanthin and β-cryptoxanthin were significantly reduced in CAD patients independent of clinical setting. The levels were correlated to a number of established cardiovascular risk factors. In addition, the relationship between NK cells and lutein + zeaxanthin may indicate a particular role for certain carotenoids in the immunological scenario of CAD.     

Unusual carotenoid composition and a new type of xanthophyll cycle in plants

The capture of photons by the photosynthetic apparatus is the first step in photosynthesis in all autotrophic higher plants. This light capture is dominated by pigment-containing proteins known as light-harvesting complexes (LHCs). The xanthophyll–carotenoid complement of these LHCs (neoxanthin, violaxanthin, and lutein) is highly conserved, with no deletions and few, uncommon additions. We report that neoxanthin, considered an integral component of LHCs, is stoichiometrically replaced by lutein-5,6-epoxide in the parasitic angiosperm Cuscuta reflexa, without compromising the structural integrity of the LHCs. Lutein-5,6-epoxide differs from neoxanthin in that it is involved in a light-driven deepoxidation cycle similar to the deepoxidation of violaxanthin in the xanthophyll cycle, which is implicated in protection against photodamage. The absence of neoxanthin and its replacement by lutein-5,6-epoxide changes our understanding of the structure-function relationship in LHCs, has implications for biosynthetic pathways involving neoxanthin (such as the plant hormone abscisic acid), and identifies one of the early steps associated with the evolution of heterotrophy from autotrophy in plants.     

Carotenoid oxidation products are stress signals that mediate gene responses to singlet oxygen in plants

(1)O(2) (singlet oxygen) is a reactive O(2) species produced from triplet excited chlorophylls in the chloroplasts, especially when plants are exposed to excess light energy. Similarly to other active O(2) species, (1)O(2) has a dual effect: It is toxic, causing oxidation of biomolecules, and it can act as a signal molecule that leads to cell death or to acclimation. Carotenoids are considered to be the main (1)O(2) quenchers in chloroplasts, and we show here that light stress induces the oxidation of the carotenoid β-carotene in Arabidopsis plants, leading to the accumulation of different volatile derivatives. One such compound, β-cyclocitral, was found to induce changes in the expression of a large set of genes that have been identified as (1)O(2) responsive genes. In contrast, β-cyclocitral had little effect on the expression of H(2)O(2) gene markers. β-Cyclocitral-induced reprogramming of gene expression was associated with an increased tolerance to photooxidative stress. The results indicate that β-cyclocitral is a stress signal produced in high light that is able to induce defense mechanisms and represents a likely messenger involved in the (1)O(2) signaling pathway in plants.     

Somatic mtDNA mutations cause aging phenotypes without affecting reactive oxygen species production

The mitochondrial theory of aging proposes that reactive oxygen species (ROS) generated inside the cell will lead, with time, to increasing amounts of oxidative damage to various cell components. The main site for ROS production is the respiratory chain inside the mitochondria and accumulation of mtDNA mutations, and impaired respiratory chain function have been associated with degenerative diseases and aging. The theory predicts that impaired respiratory chain function will augment ROS production and thereby increase the rate of mtDNA mutation accumulation, which, in turn, will further compromise respiratory chain function. Previously, we reported that mice expressing an error-prone version of the catalytic subunit of mtDNA polymerase accumulate a substantial burden of somatic mtDNA mutations, associated with premature aging phenotypes and reduced lifespan. Here we show that these mtDNA mutator mice accumulate mtDNA mutations in an approximately linear manner. The amount of ROS produced was normal, and no increased sensitivity to oxidative stress-induced cell death was observed in mouse embryonic fibroblasts from mtDNA mutator mice, despite the presence of a severe respiratory chain dysfunction. Expression levels of antioxidant defense enzymes, protein carbonylation levels, and aconitase enzyme activity measurements indicated no or only minor oxidative stress in tissues from mtDNA mutator mice. The premature aging phenotypes in mtDNA mutator mice are thus not generated by a vicious cycle of massively increased oxidative stress accompanied by exponential accumulation of mtDNA mutations. We propose instead that respiratory chain dysfunction per se is the primary inducer of premature aging in mtDNA mutator mice.     

Mitochondrial alterations, cellular response to oxidative stress and defective degradation of proteins in aging

Respiratory function decline and increase ofoxidative stress in mitochondria have beenproposed as important contributors to humanaging. A wide spectrum of alterations in agedindividuals and senescent cells are similar andare correlated to cellular response tosublethal dose of oxidative stress. Thesealterations and responses include: (1) declinein mitochondrial respiratory function; (2)increase in the rate of production of reactiveoxygen species (ROS); (3) accumulation ofmitochondrial DNA (mtDNA) mutations; (4)increase in the levels of oxidative damage toDNA, protein, and lipids; and (5) decrease inthe capacities of degradation of oxidativelydamaged proteins and other macromolecules. Responses to oxidative stress and theirsubsequent interactions in tissues result inthe deleterious effect of ROS on the cellularfunction, which culminate in aging anddegenerative diseases. In this review, wefocus on the roles that ROS play in age-relatedoxidative damage to mtDNA and proteins andoxidative stress responses at the molecular andcellular levels. The alterations of geneexpression profiles elicited by oxidativestress in aging animals are discussed. Wesuggest that the increase in mitochondrialproduction of ROS and decline in the cellularcapacity to cope with oxidative stress andsubsequent accumulation of mtDNA mutations andoxidized proteins play an important role in theaging process.     

Comprehensive geriatric assessment of elderly highlanders in Qinghai,China, III: Oxidative stress and aging in Tibetan and Han elderly highlanders

Background: Although there are several factors which may contribute to oxidative stress at high altitude, little is known about the association between oxidative stress and aging in the community‐dwelling elderly in the Tibetan Plateau. Methods: Reactive oxygen species (ROS) and comprehensive geriatric functions were examined among 235 community‐dwelling elderly subjects aged 60 years or more (146 Hans and 89 Tibetans). As a marker of ROS, the levels of reactive oxygen metabolites (ROM) were measured using the d‐ROM test. Results: The rate of dependence of basic activities of daily living (basic ADL) among Tibetan elderly highlanders was significantly higher than that among Han elderly highlanders. The d‐ROM level was higher among the Tibetan elderly than those among the Han elderly (Tibetan 465.6 ± 97.9 Carr U, Han 415.3 ± 72.0 Carr U, P = 0.003). The ROM level was higher among women than those among men. Stepwise multiple regression analysis showed that being Tibetan, female, and oxygen saturation were independent predictors of increasing d‐ROM level (Tibetan β, 0.241; female β, 0.206; oxygen saturation β, 0.218). The high levels of ROM (d‐ROM >500 Carr U) were significantly associated with dependence of basic ADL after adjustment for age, sex and ethnicity (odds ratio = 2.51, P = 0.028). Conclusion: The findings of this study imply the possibility that ROS is higher among Tibetan elderly highlanders than that of Han, which related to the geriatric items. Further studies are needed to show the impact of oxidative stress on the aging of highlanders.     

Glucose regulation of β-cell stress in type 2 diabetes

In type 2 diabetes, the β-cell is exposed to chronic hyperglycaemia, which increases its metabolic activity, with excess generation of reactive oxygen species (ROS) as a consequence. ROS accumulation induces both oxidative and endoplasmic reticulum (ER) stress, which may lead to β-cell dysfunction and apoptosis. Recent data suggest that oxidative and ER stress are interconnected, although the mechanisms involved in nutrient regulation of the different stress pathways are dissimilar. Several components of the oxidative and ER stress machineries have important roles in the physiological response to glucose and are thus necessary for normal β-cell function. Glucose stimulates signalling pathways that provide crucial messages for β-cell adaptation to metabolic stress; however, the same pathways may eventually lead to apoptosis. Dynamic, temporally fluctuating activation of stress signalling is probably required for the maintenance of β-cell survival, whereas its persistent activation results in β-cell dysfunction and apoptosis. Thus, stress signalling is a 'double-edged sword' that may promote adaptation or apoptosis according to the balance between the divergent outputs of the various pathways. Developing new strategies for β-cell protection based on inhibition of oxidative and/or ER stress requires comprehensive understanding of the switch from β-cell adaptation to β-cell apoptosis under conditions of metabolic stress, such as occurs under hyperglycaemic conditions.     

A Carotenogenic Enzyme Aggregate in Phycomyces: Evidence from Quantitive Complementation

Wild-type Phycomyces blakesleeanus accumulates beta-carotene, while the mutant strain C2 is unable to synthesize carotenoids, and the mutant strain C9 accumulates lycopene. Heterokaryons containing a proportion, p, of C2 nuclei and (l - p) of C9 nuclei accumulate lycopene, gamma-carotene, and beta-carotene in the relative amounts (l - p), p(l - p), and p(2), respectively, for different values of p.It is shown that these results are expected from the operation of a carotenogenic enzyme aggregate that works as an assembly line and contains two copies of a cyclase, which is defective in strain C9, as well as other enzymes.     

Melatonin administration decreases adipogenesis in the liver of ob/ob mice through autophagy modulation

Despite efforts to curb the incidence of obesity and its comorbidities, this condition remains the fifth leading cause of death worldwide. To identify ways to reduce this global effect, we investigated the actions of daily melatonin administration on oxidative stress parameters and autophagic processes as a possible treatment of obesity in ob/ob mice. The involvement of melatonin in many physiological functions, such as the regulation of seasonal body weight variation, glucose uptake, or adiposity, and the role of this indoleamine as an essential antioxidant, has become the focus of numerous anti‐obesity studies. Here, we examined the oxidative status in the livers of obese melatonin‐treated and untreated mice, observing a decrease in the oxidative stress levels through elevated catalase activity. ROS‐mediated autophagy was downregulated in the liver of melatonin‐treated animals and was accompanied by significant accumulation of p62. Autophagy is closely associated with adipogenesis; in this study, we report that melatonin‐treated obese mice also showed reduced adiposity, as demonstrated by diminished body weight and reduced peroxisome proliferator‐activated receptor gamma expression. Based on these factors, it is reasonable to assume that oxidative stress and autophagy play important roles in obesity, and therefore, melatonin could be an interesting target molecule for the development of a potential therapeutic agent to curb body weight.     

Melatonin‐rich transgenic rice plants exhibit resistance to herbicide‐induced oxidative stress

To examine whether melatonin‐rich plants can defend against oxidative stress, we subjected melatonin‐rich transgenic (MRT) rice plants to the singlet‐oxygen‐generating herbicide butafenacil. Both MRT and transgenic control (TC; expressing the vector only) rice seeds germinated and grew equally well in continuous dark on half‐strength Murashige and Skoog (MS) medium containing 0.1 μm butafenacil. However, after transferring the seedlings to light, the TCs rapidly necrotized, whereas the MRT seedlings showed resistant phenotypes. Seven‐day‐old MRT seedlings treated with 0.1 μm butafenacil were resistant to the herbicide and contained high chlorophyll levels and low malondialdehyde and hydrogen peroxide contents compared with the TCs. As they did before the herbicide treatment, the MRT plants also produced much more melatonin after the herbicide treatment than the TCs. In addition, the MRT plants exhibited higher superoxide dismutase and catalase activities before and after the herbicide treatment compared with the TCs. This is the first report showing that MRT plants exhibit resistance against a peroxidizing herbicide that acts by generating reactive oxygen species (ROS) that kill plants. This result indicates that melatonin scavenges ROS efficiently in vivo in the transgenic plants, leading to oxidative stress resistance.     

Progressive reduction in circulating levels of carotenoids and other micronutrients in patients with chronic pancreatitis

BackgroundAlthough micronutrients modulate immunity and inflammation, it remains elusive whether they are implicated in the development and progression of chronic pancreatitis (CP). This study aimed to investigate differences in the circulating levels of selected carotenoids and vitamins between CP and controls and trends in the levels of these micronutrients across controls, early CP, and definite CP.MethodsDemographic and lifestyle data were extracted from medical records for 53 patients with CP (13 early and 38 definite) and obtained using a questionnaire for 52 controls. Plasma β-carotene, lycopene, cryptoxanthin, zeaxanthin, and α-tocopherol and serum 25(OH)D, folate, IL-6, TNF-α, and MCP-1 were measured with state-of-the-art methods.ResultsThe levels of all micronutrients (except folate) were significantly lower in CP than in controls. There was a progressive decrease in the levels of these micronutrients across controls, early CP, and definite CP (all p values for trend: ≤0.0012); e.g., plasma lycopene was 36.6, 21.5, and 14.5 μg/dL for controls, early CP, and definite CP, respectively. After adjustment for confounders, there were strong, inverse associations between the levels of all micronutrients (except folate) and CP (e.g., OR (95% CI) for ≥ median vs. <median: 0.10 (0.04, 0.27) for lycopene, 0.15 (0.05, 0.38) for α-tocopherol, and 0.24 (0.09, 0.64) for 25(OH)D). These associations became weaker after additional adjustment for inflammation markers (IL-6, TNF-α, and MCP-1).ConclusionsThe circulating levels of some carotenoids, α-tocopherol, and vitamin D were reduced in CP patients compared with controls and this reduction was more pronounced in definite CP than in early CP.     

The effect of carotenoids on the expression of cell surface adhesion molecules and binding of monocytes to human aortic endothelial cells

Several large epidemiological studies have shown a correlation between elevated plasma carotenoid levels and decreased risk of cardiovascular disease (CVD). One proposed mechanism for the beneficial effect of carotenoids is through functional modulation of potentially atherogenic processes associated with the vascular endothelium. To test this, we incubated confluent human aortic endothelial cell (HAEC) cultures (passages 4–8) for 24 h with each of the five most prevalent carotenoids in human plasma, which are -carotene, β-carotene, β-cryptoxanthin, lutein, and lycopene, at an approximate concentration of 1 μmol/l. Carotenoids were solubilized in 0.7% (v/v) tetrahydrofuran and incorporated into FBS before adding to cell culture medium. Due to disparate solubilities in aqueous medium, final concentrations of -carotene, β-carotene, β-cryptoxanthin, lutein, and lycopene were 1.7, 1.1, 0.7, 0.9, and 0.3 μmol/l and monolayers accumulated 647, 158, 7, 113, and 9 pmol/mg protein, respectively. Monolayers were then stimulated with IL-1β (5 ng/ml) for 6 h with subsequent determination of cell surface expression of adhesion molecules as measured by an enzyme-linked immunosorbent assay (ELISA). To assess endothelial cell adhesion to monocytes, IL-1β-stimulated monolayers were incubated for 10 min with ⁵¹Cr-labeled U937 monocytic cells and adhesion determined by isotope counting. Pre-incubation of HAEC with β-carotene, lutein and lycopene significantly reduced VCAM-1 expression by 29, 28, and 13%, respectively. Pre-incubation with β-carotene and lutein significantly reduced E-selectin expression by 38 and 34%, respectively. Pre-treatment with β-carotene, lutein and lycopene significantly reduced the expression of ICAM-1 by 11, 14, and 18%, respectively. While other carotenoids were ineffective, lycopene attenuated both IL-1β-stimulated and spontaneous HAEC adhesion to U937 monocytic cells by 20 and 25%, respectively. Thus, among the carotenoids, lycopene appears to be most effective in reducing both HAEC adhesion to monocytes and expression of adhesion molecules on the cell surface.     

NDP kinase 2 interacts with two oxidative stress-activated MAPKs to regulate cellular redox state and enhances multiple stress tolerance in transgenic plants

NDP kinases (NDPKs) are multifunctional proteins that regulate a variety of eukaryotic cellular activities, including cell proliferation, development, and differentiation. However, much less is known about the functional significance of NDPKs in plants. We show here that NDPK is associated with H(2)O(2)-mediated mitogen-activated protein kinase signaling in plants. H(2)O(2) stress strongly induces the expression of the NDPK2 gene in Arabidopsis thaliana (AtNDPK2). Proteins from transgenic plants overexpressing AtNDPK2 showed high levels of autophosphorylation and NDPK activity, and they have lower levels of reactive oxygen species (ROS) than wild-type plants. Mutants lacking AtNDPK2 had higher levels of ROS than wild type. H(2)O(2) treatment induced the phosphorylation of two endogenous proteins whose molecular weights suggested they are AtMPK3 and AtMPK6, two H(2)O(2)-activated A. thaliana mitogen-activated protein kinases. In the absence of H(2)O(2) treatment, phosphorylation of these proteins was slightly elevated in plants overexpressing AtNDPK2 but markedly decreased in the AtNDPK2 deletion mutant. Yeast two-hybrid and in vitro protein pull-down assays revealed that AtNDPK2 specifically interacts with AtMPK3 and AtMPK6. Furthermore, AtNDPK2 also enhances the myelin basic protein phosphorylation activity of AtMPK3 in vitro. Finally, constitutive overexpression of AtNDPK2 in Arabidopsis plants conferred an enhanced tolerance to multiple environmental stresses that elicit ROS accumulation in situ. Thus, AtNDPK2 appears to play a previously uncharacterized regulatory role in H(2)O(2)-mediated MAPK signaling in plants.