Salicylic acid-independent ENHANCED DISEASE SUSCEPTIBILITY1 signaling in Arabidopsis immunity and cell death is regulated by the monooxygenase FMO1 and the Nudix hydrolase NUDT7

Arabidopsis thaliana ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) controls defense activation and programmed cell death conditioned by intracellular Toll-related immune receptors that recognize specific pathogen effectors. EDS1 is also needed for basal resistance to invasive pathogens by restricting the progression of disease. In both responses, EDS1, assisted by its interacting partner, PHYTOALEXIN-DEFICIENT4 (PAD4), regulates accumulation of the phenolic defense molecule salicylic acid (SA) and other as yet unidentified signal intermediates. An Arabidopsis whole genome microarray experiment was designed to identify genes whose expression depends on EDS1 and PAD4, irrespective of local SA accumulation, and potential candidates of an SA-independent branch of EDS1 defense were found. We define two new immune regulators through analysis of corresponding Arabidopsis loss-of-function insertion mutants. FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) positively regulates the EDS1 pathway, and one member (NUDT7) of a family of cytosolic Nudix hydrolases exerts negative control of EDS1 signaling. Analysis of fmo1 and nudt7 mutants alone or in combination with sid2-1, a mutation that severely depletes pathogen-induced SA production, points to SA-independent functions of FMO1 and NUDT7 in EDS1-conditioned disease resistance and cell death. We find instead that SA antagonizes initiation of cell death and stunting of growth in nudt7 mutants.     

Layered basal defenses underlie non-host resistance of Arabidopsis to Pseudomonas syringae pv. phaseolicola

Arabidopsis is a non-host for Pseudomonas syringae pv. phaseolicola NPS3121 (Pph), a bacterial pathogen of bean. Pph does not induce a hypersensitive response in Arabidopsis. Here we show that Arabidopsis instead resists Pph with multi-layered basal defense. Our approach was: (i) to identify defense readouts induced by Pph; (ii) to determine whether mutations in known Arabidopsis defense genes disrupt Pph-induced defense signaling; (iii) to determine whether heterologous type III effectors from pathogens of Arabidopsis suppress Pph-induced defense signaling, and (iv) to ascertain how basal defenses contribute to resistance against Pph by individually or multiply disrupting defense signaling pathways with mutations and heterologous type III effectors. We demonstrate that Pph elicits a minimum of three basal defense-signaling pathways in Arabidopsis. These pathways have unique readouts, including PR-1 protein accumulation and morphologically distinct types of callose deposition. Further, they require distinct defense genes, including PMR4, RAR1, SID2, NPR1, and PAD4 . Finally, they are suppressed differentially by heterologous type III effectors, including AvrRpm1 and HopM1. Pph growth is enhanced only when multiple defense pathways are disrupted. For example, mutation of NPR1 or SID2 combined with the action of AvrRpm1 and HopM1 renders Arabidopsis highly susceptible to Pph. Thus, non-host resistance of Arabidopsis to Pph is based on multiple, individually effective layers of basal defense.     

Modulation of redox homeostasis under suboptimal conditions by Arabidopsis nudix hydrolase 7

Background  

Nudix hydrolases play a key role in maintaining cellular homeostasis by hydrolyzing various nuceloside diphosphate derivatives and capped mRNAs. Several independent studies have demonstrated that Arabidopsis nudix hydrolase 7 (AtNUDT7) hydrolyzes NADH and ADP-ribose. Loss of function Atnudt7-1 mutant plants (SALK_046441) exhibit stunted growth, higher levels of reactive oxygen species, enhanced resistance to pathogens. However, using the same T-DNA line, two other groups reported that mutant plants do not exhibit any visible phenotypes. In this study we analyze plausible factors that account for differences in the observed phenotypes in Atnudt7. Secondly, we evaluate the biochemical and molecular consequences of increased NADH levels due to loss of function of AtNUDT7 in Arabidopsis.     

Activation of NUDT5, an ADP-ribose pyrophosphatase, by nitric oxide-mediated ADP-ribosylation

The ADP-ribose (ADPR) pyrophosphatase (ADPRase) NUDT5, a member of a superfamily of Nudix hydrolases, hydrolyzes ADP-ribose (ADPR) to AMP and ribose 5'-phosphate. Nitric oxide (NO) enhances nonenzymatic ADP-ribosylation of proteins such as beta-actin and glyceraldehydes 3-phosphate dehydrogenase in the presence of free ADPR, suggesting a possibility that NUDT5 could also be ADP-ribosylated by its substrate, ADPR. Here, we show that NO stimulates nonenzymatic ADP-ribosylation of NUDT5 using ADP-ribose and consequently activates its ADPRase activity. We found that ADPRase activity in J774 macrophage cells is increased by the treatment with SNP, an exogenous NO generator or TNF-alpha/IFN-gamma, endogenous NO inducers. Anti-NUDT5 antibody pulled down most of the ADPRase activity increased by NO, indicating that the ADPRase regulated by NO is NUDT5. Using recombinant human NUDT5, we also demonstrated that the increase of ADPRase activity is mediated via ADP-ribosylation at cysteine residue(s) in the presence of reductant. This result suggests that NO activates NUDT5 through ADP-ribosylation at cysteine residues of the enzyme in macrophages.     

Salicylic acid antagonism of EDS1‐driven cell death is important for immune and oxidative stress responses in Arabidopsis

Reactive oxygen species (ROS) have emerged as signals in the responses of plants to stress. Arabidopsis Enhanced Disease Susceptibility1 (EDS1) regulates defense and cell death against biotrophic pathogens and controls cell death propagation in response to chloroplast‐derived ROS. Arabidopsis Nudix hydrolase7 (nudt7) mutants are sensitized to photo‐oxidative stress and display EDS1‐dependent enhanced resistance, salicylic acid (SA) accumulation and initiation of cell death. Here we explored the relationship between EDS1, EDS1‐regulated SA and ROS by examining gene expression profiles, photo‐oxidative stress and resistance phenotypes of nudt7 mutants in combination with eds1 and the SA‐biosynthetic mutant, sid2. We establish that EDS1 controls steps downstream of chloroplast‐derived O₂^?? that lead to SA‐assisted H₂O₂ accumulation as part of a mechanism limiting cell death. A combination of EDS1‐regulated SA‐antagonized and SA‐promoted processes is necessary for resistance to host‐adapted pathogens and for a balanced response to photo‐oxidative stress. In contrast to SA, the apoplastic ROS‐producing enzyme NADPH oxidase RbohD promotes initiation of cell death during photo‐oxidative stress. Thus, chloroplastic O₂^?? signals are processed by EDS1 to produce counter‐balancing activities of SA and RbohD in the control of cell death. Our data strengthen the idea that EDS1 responds to the status of O₂^?? or O₂^??‐generated molecules to coordinate cell death and defense outputs. This activity may enable the plant to respond flexibly to different biotic and abiotic stresses in the environment.     

激发子α-吡啶羧酸诱发拟南芥和水稻悬浮细胞中依赖于水杨酸、茉莉酸/乙烯和钙离子途径的防卫反应

α-吡啶羧酸(PA)是动物细胞程序化死亡的诱导物.我们前期的研究表明,PA可以激发单子叶模式植物水稻的过敏反应(HR).进一步用双子叶模式植物拟南芥(Arabidopsis thaliana)进行的研究表明,PA是一个广谱的植物HR反应的激发子,包括诱导氧进发和细胞死亡.我们探究了PA诱导的拟南芥防卫反应途径,利用不同信号途径标志基因PR-1,PR-2和PDF1.2受诱导剂量和时间激活的结果,表明PA可以同时激活水杨酸和茉莉酸/乙烯依赖的防卫途径.我们也发现PA诱导水稻悬浮细胞产生活性氧是钙离子依赖性的.综合所有结果,我们认为PA可以作为一个非专化性的植物防卫反应激发子,可望用于系统获得性抗性激发的细胞模型的建立.     

激发子α-吡啶羧酸诱发拟南芥和水稻悬浮细胞中依赖于水杨酸、茉莉酸/乙烯和钙离子途径的防卫反应(英文)

α-吡啶羧酸(PA)是动物细胞程序化死亡的诱导物。我们前期的研究表明,PA可以激发单子叶模式植物水稻的过敏反应(HR)。进一步用双子叶模式植物拟南芥(Arabidopsis thaliana)进行的研究表明,PA是一个广谱的植物HR反应的激发子,包括诱导氧进发和细胞死亡。我们探究了PA诱导的拟南芥防卫反应途径,利用不同信号途径标志基因PR-1,PR-2和PDF1.2受诱导剂量和时间激活的结果,表明PA可以同时激活水杨酸和茉莉酸/乙烯依赖的防卫途径。我们也发现PA诱导水稻悬浮细胞产生活性氧是钙离子依赖性的。综合所有结果,我们认为PA可以作为一个非专化性的植物防卫反应激发子,可望用于系统获得性抗性激发的细胞模型的建立。     

Distinct regulation of Arabidopsis ADP-ribose/NADH pyrophosphohydrolases,AtNUDX6 and 7, in biotic and abiotic stress responses

Among Arabidopsis Nudix hydrolases (AtNUDX1∼27), AtNUDX6 and AtNUDX7 having ADP-ribose/NADH pyrophosphohydrolase activities have been found to contribute to keeping the energy and redox homeostasis, and/or modulating defense responses against biotic and abiotic stress. Interestingly, AtNUDX6 had an opposite effect to AtNUDX7 on the regulation of immune responses. A comparison of the activities of ADP-ribose/NADH pyrophosphohydrolase among wild-type, knockout (KO)-nudx6, and KO-nudx7 plants revealed AtNUDX7 to contribute more than AtNUDX6 to the total pyrophosphohydrolase activity toward both ADP-ribose and NADH under normal conditions and oxidative stress, while AtNUDX6 accounted for the majority of total NADH pyrophosphohydrolase activity under salicylic acid treatment. These results support the idea that the metabolism of ADP-ribose and/or NADH needs to be finely tuned for accurate regulation of cellular responses to biotic and abiotic stress.Key words: nudix hydrolase, ADP-ribose/NADH pyrophosphohydrolases, biotic and abiotic stress responseNudix (nucleoside diphosphates linked to some moiety X) hydrolases distributed among all classes of organisms from archaea to vertebrates have the potential to hydrolyze a wide range of substrates such as dinucleoside polyphosphates, various coenzymes, nucleotide sugars, ribo- and deoxynucleoside triphophates, and alcohols.^1–3 Recently, Nudix hydrolases having hydrolysis activity toward other compounds containing pyrophosphate bounds, such as nucleoside diphophates, the mRNA cap, 5′triphosphorylated RNA, and guanosine 3′,5′-bispyrophosphate, and non-nucleoside substrates such as diphosphoinositol polyphosphates, 5-phosphoribosyl 1-diphosphate, thiamine pyrophosphate, and dihydroneopterin triphosphate, have been identified.³ Several of these substrates are potentially toxic compounds, cell signaling molecules, metabolic intermediates, or coenzymes. Nudix hydrolases are thus considered to be associated with various cellular processes by hydrolytically removing these substrates.Arabidopsis thaliana has 27 genes encoding Nudix hydrolase (AtNUDX1-27), which can be classified into three types by their predicted subcellular localization, the cytosol (AtNUDX1∼11 and 25), mitochondria (AtNUDX12∼18), or chloroplasts (AtNUDX19∼24, 26 and 27).^4,5 It is remarkable that there are a large number of AtNUDXs having ADP-ribose or NADH pyrophosphohydrolase activity (AtNUDX2, 6, 7, 10, 14, 19 and 23); the number (7) of enzymes in the subfamily is greater than that in humans, which have 5 genes encoding the putative ADP-ribose or NADH pyrophosphohydrolase. Recombinant forms of AtNUDX2, 6 and 7 have showed the pyrophosphohydrolase activity toward both ADP-ribose and NADH with high affinity in vitro.⁴ Recent studies have demonstrated that the modulation of ADP-ribose and/or NADH levels through the hydrolysis by AtNUDX2, 6 and 7 contributes to keeping the energy and redox homeostasis, and/or modulating defense responses to both biotic and abiotic stress,^6–10 indicating the diverse roles of Nudix hydrolases in plants. AtNUDX2 might not function physiologically, because of its low levels even under stressful conditions.⁶ It should be noted that the physiological role of AtNUDX6 differs considerably from that of AtNUDX7, although their enzymatic properties in vivo are partly the same: we previously demonstrated that AtNUDX7 acts in the hydrolysis of both ADP-ribose and NADH in cells, while AtNUDX6 acts only on NADH.^7,8It was demonstrated that AtNUDX7 acts as a negative regulator to prevent excessive stimulation of the defense response, which is dependent on and independent of Nonexpresser of Pathogenesis-Related genes 1, a master regulator of salicylic acid (SA)-induced defense genes, and SA accumulation,¹⁰ while AtNUDX6 acts as a positive regulator through NPR1-dependent SA signaling pathways.⁸ In addition, AtNUDX7, but not AtNUDX6, modulated the poly(ADP-ribosyl)ation reaction, which is one of the early responses to DNA damage caused by oxidative stress.⁷ These observations raise the question of how AtNUDXs control such different processes.To evaluate the physiological importance of each AtNUDX, here we compared AtNUDX6 and 7 in ADP-ribose and/or NADH pyrophosphohydrolase activity in Arabidopsis cells under various conditions. From the difference in activity of extracts prepared from the leaves of wild-type, knockout (KO)-nudx6, and KO-nudx7 plants grown under normal conditions for 2 weeks, it was estimated that AtNUDX7 accounts for 23% of the total ADP-ribose pyrophosphohydrolase activity, but AtNUDX6 barely contributes to the activity (Fig. 1). Oxidative stress caused by 3 µM paraquat (PQ) for 7 days caused an increase in the total ADP-ribose pyrophosphohydrolase activity. Under oxidative stress, the contribution of AtNUDX7 to the activity increased to 34%. Treatment with 0.5 mM SA, a signaling molecule necessary for the onset of systemic acquired resistance, had no effect on the activity.Open in a separate windowFigure 1Changes in the ADP-ribose/NADH pyrophosphohydrolase activity of AtNUDX6 and 7 in Arabidopsis leaves under treatment with PQ or SA. The activities of pyrophosphohydrolase toward ADP-ribose (A) and NADH (B) in the leaves of wild-type plants grown on MS medium for 2 weeks under long-day conditions [16 h of light (100 µmol photons m⁻² s⁻¹), 25°C/8 h of dark, 22°C] are shown as Control. PQ treatment was imposed by growing 2-week-old plants in MS medium containing the agent at 3 µm for 7 days under long-day conditions (PQ). SA treatment was imposed by growing 2-week-old plants in MS medium containing 0.5 mM SA for 24 h under long-day conditions (SA). The ADP-ribose and NADH pyrophosphohydrolase activities were measured as described previously.⁸ The contributions (%) of AtNUDX6 and AtNUDX7 to total ADP-ribose/NADH pyrophosphohydrolase activity under treatment with PQ and SA were estimated from the decrease in activity in the respective knockout mutants (KO-nudx6 and KO-nudx7)^7,8 and are indicated in parentheses. Data are the mean ± SD for three individual experiments (n = 3) using plants grown independently. Different letters indicate significant differences (p < 0.05).AtNUDX6 and 7 accounted for 25 and 53%, respectively, of the total pyrophosphohydrolase activity toward NADH under normal conditions (Fig. 1). The activity was increased by oxidative stress, with AtNUDX7 contributing 57%. On the other hand, under treatment with SA, the total NADH pyrophosphohydrolase activity was increased and AtNUDX6 accounted for 53% of the activity. These results indicated that AtNUDX7 contributed more than AtNUDX6 to the total pyrophosphohydrolase activity toward both ADP-ribose and NADH under normal conditions and oxidative stress.⁷ On the other hand, AtNUDX6 accounted for the majority of the total NADH pyrophosphohydrolase activity under SA treatment.⁸Plants are simultaneously exposed to abiotic and biotic hazards in nature. There is increasing evidence of crosstalk among the signaling pathways for biotic and abiotic stress.^12,13 It is worth noting that the expression of AtNUDX7, but not AtNUDX6, is regulated by intracellular levels of reactive oxygen species (ROS), since it is induced by not only pathogen infections but also oxidative stress including PQ treatment, all of which are known to cause the production of ROS in the cells.^8–11 On the other hand, the expression of AtNUDX6 was induced only by the application of SA and its analogues, 2,6-dichloroisonicotinic acid or acibenzolar-S-methyl benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester, and not by oxidative stress.^8,14,15 Therefore, the expression of AtNUDX6 was thought to be regulated by intracellular SA levels, although it was also induced by pathogenic attacks causing local excessive production of both ROS and SA.⁹ The differences in the regulation of AtNUDX6 and 7 and timing of production of ROS and SA in response to biotic stress raise the possibility that the total activity of ADP-ribose/NADH pyrophosphohydrolase and subsequent metabolism of ADP-ribose and/or NADH must be finely tuned for accurate regulation of such cellular responses. The activation of metabolism caused by the accumulation of either ROS or SA at specific phases in plant cells might have different effects on cellular responses through cooperation with other factors.     

酵母双杂交检测C17orf25蛋白与ADP-核糖焦磷酸水解酶NUDT9的相互作用

通过RACEPCR的方法从肝组织中分离得到C17orf2 5基因。利用酵母双杂交的方法以C17orf2 5为结合结构域筛选人HeLacDNA文库分离得到nudt9基因。NUDT9是一种焦磷酸酶 ,可以将ADP 核糖水解成AMP和核糖 5 磷酸。在大肠杆菌中直接表达了C17orf2 5蛋白、6×His tag与NUDT9的融合蛋白质 ,两者均以包涵体形式存在。蛋白质条带割胶纯化 ,并复性。之后的NTA Ni2 亲和柱层析实验表明这两种蛋白质在体外相互作用。将C17orf2 5与绿色荧光蛋白基因在SMMC772 1中融合表达 ,结果表明C17orf2 5蛋白可能定位在线粒体中 ,侧面印证了在细胞内与NUDT9作用的空间可能性。ADP 核糖在体内具有重要的生理作用 ,在细胞内的累积对细胞生长不利 ;同时 ,ADP 核糖化是一种重要的蛋白质修饰方式 ,与多种细胞凋亡的发生有关。因此从实验结果可以判断 ,C17orf2 5对细胞生长的抑制作用可能通过与NUDT9的相互作用来实现     

Suppression of edr2-mediated powdery mildew resistance, cell death and ethylene-induced senescence by mutations in ALD1 in Arabidopsis

EDR2 is a negative regulator of the defense response and cell death in Arabidopsis. Loss-of-function of EDR2 leads to enhanced resistance to powdery mildew. To identify new components in the EDR2 signal transduction pathway, mutations that suppress edr2 resistant phenotypes were screened. Three mutants, edts5-1, edts5-2 and edts5-3 (edr (t)wo (s)uppressor 5), were identified. The EDTS5 gene was identified by map-based cloning and previously was shown to encode an aminotransferase (ALD1). Therefore we renamed these three alleles ald1-10, ald1-11 and ald1-12, respectively. Mutations in ALD1 suppressed all edr2-mediated phenotypes, including powdery mildew resistance, programmed cell death and ethylene-induced senescence. Accumulation of hydrogen peroxide in edr2 was also suppressed by ald1 mutation. The expression of defense-related genes was up-regulated in the edr2 mutant, and the up-regulation of those genes in edr2 was suppressed in the edr2/ald1 double mutant. The ald1 single mutant displayed delayed ethylene-induced senescence. In addition, ald1 mutation suppressed edr1-mediated powdery mildew resistance, but could not suppress the edr1/edr2 double-mutant phenotype. These data demonstrate that ALD1 plays important roles in edr2-mediated defense responses and senescence, and revealed a crosstalk between ethylene and salicylic acid signaling mediated by ALD1 and EDR2.     

Downy mildew (Peronospora parasitica) resistance genes in Arabidopsis vary in functional requirements for NDR1, EDS1, NPR1 and salicylic acid accumulation

To better understand the genetic requirements for R gene-dependent defense activation in Arabidopsis, we tested the effect of several defense response mutants on resistance specified by eight RPP genes (for resistance to Peronospora parasitica) expressed in the Col-0 background. In most cases, resistance was not suppressed by a mutation in the SAR regulatory gene NPR1 or by expression of the NahG transgene. Thus, salicylic acid accumulation and NPR1 function are not necessary for resistance mediated by these RPP genes. In addition, resistance conferred by two of these genes, RPP7 and RPP8, was not significantly suppressed by mutations in either EDS1 or NDR1. RPP7 resistance was also not compromised by mutations in EIN2, JAR1 or COI1 which affect ethylene or jasmonic acid signaling. Double mutants were therefore tested. RPP7 and RPP8 were weakly suppressed in an eds1-2/ndr1-1 background, suggesting that these RPP genes operate additively through EDS1, NDR1 and as-yet-undefined signaling components. RPP7 was not compromised in coi1/npr1 or coi1/NahG backgrounds. These observations suggest that RPP7 initiates resistance through a novel signaling pathway that functions independently of salicylic acid accumulation or jasmonic acid response components.     

Evidence for regulation of resistance in Arabidopsis to Egyptian cotton worm by salicylic and jasmonic acid signaling pathways

Signaling cross-talk between wound- and pathogen-response pathways influences resistance of plants to insects and disease. To elucidate potential interactions between salicylic acid (SA) and jasmonic acid (JA) defense pathways, we exploited the availability of characterized mutants of Arabidopsis thaliana (L.) Heynh. and monitored resistance to Egyptian cotton worm (Spodoptera littoralis Boisd.; Lepidoptera: Noctuidae). This generalist herbivore is sensitive to induced plant defense pathways and is thus a useful model for a mechanistic analysis of insect resistance. As expected, treatment of wild-type Arabidopsis with JA enhanced resistance to Egyptian cotton worm. Conversely, the coil mutant, with a deficiency in the JA response pathway, was more susceptible to Egyptian cotton worm than wild-type Arabidopsis. By contrast, the nprl mutant, with defects in systemic disease resistance, exhibited enhanced resistance to Egyptian cotton worm. Pretreatment with SA significantly reduced this enhanced resistance of nprl plants but had no influence on the resistance of wild-type plants. However, exogenous SA reduced the amount of JA that Egyptian cotton worm induced in both npr1 mutant and wild-type plants. Thus, this generalist herbivore engages two different induced defense pathways that interact to mediate resistance in Arabidopsis.     

Overexpression of an ADP-ribose pyrophosphatase, AtNUDX2, confers enhanced tolerance to oxidative stress in Arabidopsis plants

Mutant pqr-216 from an Arabidopsis activation-tagged line showed a phenotype of increased tolerance to oxidative stress after treatment with 3 μ m paraquat (PQ). Based on the phenotype of transgenic plants overexpressing the genes flanking the T-DNA insert, it was clear that enhanced expression of a Nudix (nucleoside diphosphates linked to some moiety X) hydrolase gene, AtNUDX2 (At5g47650), was responsible for the tolerance. It has been reported that the AtNUDX2 protein has pyrophosphatase activities towards both ADP-ribose and NADH ( Ogawa et al ., 2005 ). Interestingly, the pyrophosphatase activity toward ADP-ribose, but not NADH, was increased in pqr-216 and Pro _35S :AtNUDX2 plants compared with control plants. The amount of free ADP-ribose was lower in the Pro _35S :AtNUDX2 plants, while the level of NADH was similar to those in control plants under both normal conditions and oxidative stress. Depletion of NAD⁺ and ATP resulting from activation of poly(ADP-ribosyl)ation under oxidative stress was observed in the control Arabidopsis plants. Such alterations in the levels of these molecules were significantly suppressed in the Pro _35S :AtNUDX2 plants. The results indicate that overexpression of AtNUDX2 , encoding ADP-ribose pyrophosphatase, confers enhanced tolerance of oxidative stress on Arabidopsis plants, resulting from maintenance of NAD⁺ and ATP levels by nucleotide recycling from free ADP-ribose molecules under stress conditions.     

Comprehensive analysis of cytosolic Nudix hydrolases in Arabidopsis thaliana

Nudix hydrolases are a family of proteins that catalyze the hydrolysis of a variety of nucleoside diphosphate derivatives. Twenty-four genes of the Nudix hydrolase homologues (AtNUDTs) with predicted localizations in the cytosol, chloroplasts, and mitochondria exist in Arabidopsis thaliana. Here, we demonstrated the comprehensive analysis of nine types of cytosolic AtNUDT proteins (AtNUDT1, -2, -4, -5, -6, -7, -9, -10, and -11). The recombinant proteins of AtNUDT2, -6, -7, and -10 showed both ADP-ribose and NADH pyrophosphatase activities with significantly high affinities compared with those of animal and yeast enzymes. The expression of each AtNUDT is individually regulated in different tissues. These findings suggest that most cytosolic AtNUDTs may substantially function in the sanitization of potentially hazardous ADP-ribose and the regulation of the cellular NADH/NAD(+) ratio in plant cells. On the other hand, the AtNUDT1 protein had the ability to hydrolyze 8-oxo-dGTP with a K(m) value of 6.8 mum and completely suppress the increased frequency of spontaneous mutations in the Escherichia coli mutT(-) strain, indicating that AtNUDT1 is a functional homologue of E. coli MutT in A. thaliana and is involved in the prevention of spontaneous mutation. The results obtained here suggest that the plant Nudix family has evolved in a specific manner that differs from that of yeast and humans.     

Constitutive activation of jasmonate signaling in an Arabidopsis mutant correlates with enhanced resistance to Erysiphe cichoracearum,Pseudomonas syringae,and Myzus persicae

In Arabidopsis spp., the jasmonate (JA) response pathway generally is required for defenses against necrotrophic pathogens and chewing insects, while the salicylic acid (SA) response pathway is generally required for specific, resistance (R) gene-mediated defenses against both biotrophic and necrotrophic pathogens. For example, SA-dependent defenses are required for resistance to the biotrophic fungal pathogen Erysiphe cichoracearum UCSC1 and the bacterial pathogen Pseudomonas syringae pv. maculicola, and also are expressed during response to the green peach aphid Myzus persicae. However, recent evidence indicates that the expression of JA-dependent defenses also may confer resistance to E. cichoracearum. To confirm and to extend this observation, we have compared the disease and pest resistance of wild-type Arabidopsis plants with that of the mutants coil, which is insensitive to JA, and cev1, which has constitutive JA signaling. Measurements of the colonization of these plants by E. cichoracearum, P. syringae pv. maculicola, and M. persicae indicated that activation of the JA signal pathway enhanced resistance, and was associated with the activation of JA-dependent defense genes and the suppression of SA-dependent defense genes. We conclude that JA and SA induce alternative defense pathways that can confer resistance to the same pathogens and pests.