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

Analysis of temperature modulation of plant defense against biotrophic microbes

Plant-pathogen interactions are known to be affected by environmental factors including temperature; however, the temperature effects have not been systematically studied in plant disease resistance. Here, we characterized the effects of a moderate increase in temperature on resistance to bacterial pathogen Pseudomonas syringae and two viral elicitors in Arabidopsis thaliana and Nicotiana benthamiana. Both the basal and the resistance (R) gene-mediated defense responses to Pseudomonas syringae are found to be inhibited by a moderately high temperature, and hypersensitive responses induced by R genes against two viruses are also reduced by an increase of temperature. These indicate that temperature modulation of defense responses to biotrophic and hemibiotrophic pathogens might be a general phenomenon. We further investigated the roles of two small signaling molecules, salicylic acid and jasmonic acid, as well as two defense regulators, EDS1 and PAD4, in this temperature modulation. These components, though modulated by temperature or involved in temperature regulation or both, are not themselves determinants of temperature sensitivity in the defense responses analyzed. The inhibition of plant defense response by a moderately high temperature may thus be mediated by other defense signaling components or a combination of multiple factors.

Layers of defense responses to Leptosphaeria maculans below the RLM1- and camalexin-dependent resistances

Plants have evolved different defense components to counteract pathogen attacks. The resistance locus resistance to Leptosphaeria maculans 1 (RLM1) is a key factor for Arabidopsis thaliana resistance to L. maculans. The present work aimed to reveal downstream defense responses regulated by RLM1. Quantitative assessment of fungal colonization in the host was carried out using quantitative polymerase chain reaction (qPCR) and GUS expression analyses, to further characterize RLM1 resistance and the role of salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) in disease development. Additional assessments of A. thaliana mutants were performed to expand our understanding of this pathosystem. Resistance responses such as lignification and the formation of vascular plugs were found to occur in an RLM1-dependent manner, in contrast to the RLM1-independent increase in reactive oxygen species at the stomata and hydathodes. Analyses of mutants defective in hormone signaling in the camalexin-free rlm1(Ler)pad3 background revealed a significant influence of JA and ET on symptom development and pathogen colonization. The overall results indicate that the defense responses of primary importance induced by RLM1 are all associated with physical barriers, and that responses of secondary importance involve complex cross-talk among SA, JA and ET. Our observations further suggest that ET positively affects fungal colonization.

Salicylic Acid, a multifaceted hormone to combat disease

For more than 200 years, the plant hormone salicylic acid (SA) has been studied for its medicinal use in humans. However, its extensive signaling role in plants, particularly in defense against pathogens, has only become evident during the past 20 years. This review surveys how SA in plants regulates both local disease resistance mechanisms, including host cell death and defense gene expression, and systemic acquired resistance (SAR). Genetic studies reveal an increasingly complex network of proteins required for SA-mediated defense signaling, and this process is amplified by several regulatory feedback loops. The interaction between the SA signaling pathway and those regulated by other plant hormones and/or defense signals is also discussed.

Photorespiratory metabolism: genes, mutants, energetics, and redox signaling

Photorespiration is a high-flux pathway that operates alongside carbon assimilation in C(3) plants. Because most higher plant species photosynthesize using only the C(3) pathway, photorespiration has a major impact on cellular metabolism, particularly under high light, high temperatures, and CO(2) or water deficits. Although the functions of photorespiration remain controversial, it is widely accepted that this pathway influences a wide range of processes from bioenergetics, photosystem II function, and carbon metabolism to nitrogen assimilation and respiration. Crucially, the photorespiratory pathway is a major source of H(2)O(2) in photosynthetic cells. Through H(2)O(2) production and pyridine nucleotide interactions, photorespiration makes a key contribution to cellular redox homeostasis. In so doing, it influences multiple signaling pathways, particularly those that govern plant hormonal responses controlling growth, environmental and defense responses, and programmed cell death. The potential influence of photorespiration on cell physiology and fate is thus complex and wide ranging. The genes, pathways, and signaling functions of photorespiration are considered here in the context of whole plant biology, with reference to future challenges and human interventions to diminish photorespiratory flux.

Arabidopsis transcriptome reveals control circuits regulating redox homeostasis and the role of an AP2 transcription factor

Sensors and regulatory circuits that maintain redox homeostasis play a central role in adjusting plant metabolism and development to changing environmental conditions. We report here control networks in Arabidopsis (Arabidopsis thaliana) that respond to photosynthetic stress. We independently subjected Arabidopsis leaves to two commonly used photosystem II inhibitors: high light (HL) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Microarray analysis of expression patterns during the period of redox adjustment to these inhibitors reveals that 20% and 8% of the transcriptome are under HL and DCMU regulation, respectively. Approximately 6% comprise a subset of genes common to both perturbations, the redox responsive genes (RRGs). A redox network was generated in an attempt to identify genes whose expression is tightly coordinated during adjustment to homeostasis, using expression of these RRGs under HL conditions. Ten subnetworks were identified from the network. Hierarchal subclustering of subnetworks responding to the DCMU stress identified novel groups of genes that were tightly controlled while adjusting to homeostasis. Upstream analysis of the promoters of the genes in these clusters revealed different motifs for each subnetwork, including motifs that were previously identified with responses to other stresses, such as light, dehydration, or abscisic acid. Functional categorization of RRGs demonstrated involvement of genes in many metabolic pathways, including several families of transcription factors, especially those in the AP2 family. Using a T-DNA insertion in one AP2 transcription factor (redox-responsive transcription factor 1 [RRTF1]) from the RRGs, we showed that the genes predicted to be within the subnetwork containing RRTF1 were changed in this insertion line (Deltarrtf1). Furthermore, Deltarrtf1 showed greater sensitivity to photosynthetic stress compared to the wild type.

Plastidial localization of a potato 'Nudix' hydrolase of ADP-glucose linked to starch biosynthesis

Escherichia coli and potato (Solanum tuberosum) ADP-sugar pyrophosphatases (EcASPP and StASPP, respectively) are 'Nudix' hydrolases of the bacterial glycogen and starch precursor molecule, ADP-glucose (ADPG). We have previously shown that potato leaves expressing EcASPP either in the cytosol or in the chloroplast exhibited large reductions in the levels of starch, suggesting the occurrence of cytosolic and plastidial pools of ADPG linked to starch biosynthesis. In this work, we produced and characterized potato and Arabidopsis plants expressing EcASPP and StASPP fused with green fluorescent protein (GFP). Confocal fluorescence microscopy analyses of these plants confirmed that EcASPP-GFP has a cytosolic localization, whereas StASPP-GFP occurs in the plastid stroma. Both source leaves and potato tubers from EcASPP-GFP-expressing plants showed a large reduction of the levels of both ADPG and starch. In contrast, StASPP-GFP-expressing leaves and tubers exhibited reduced starch and normal ADPG contents when compared with control plants. With the exception of starch synthase in StASPP-GFP-expressing plants, no pleiotropic changes in maximum catalytic activities of enzymes closely linked to starch metabolism could be detected in EcASPP-GFP- and StASPP-GFP-expressing plants. The overall data (i) show that potato plants possess a plastidial ASPP that has access to ADPG linked to starch biosynthesis and (ii) are consistent with the occurrence of plastidic and cytosolic pools of ADPG linked to starch biosynthesis.

Molecular characterization of organelle-type Nudix hydrolases in Arabidopsis

Nudix (for nucleoside diphosphates linked to some moiety X) hydrolases act to hydrolyze ribonucleoside and deoxyribonucleoside triphosphates, nucleotide sugars, coenzymes, or dinucleoside polyphosphates. Arabidopsis (Arabidopsis thaliana) contains 27 genes encoding Nudix hydrolase homologues (AtNUDX1 to -27) with a predicted distribution in the cytosol, mitochondria, and chloroplasts. Previously, cytosolic Nudix hydrolases (AtNUDX1 to -11 and -25) were characterized. Here, we conducted a characterization of organelle-type AtNUDX proteins (AtNUDX12 to -24, -26, and -27). AtNUDX14 showed pyrophosphohydrolase activity toward both ADP-ribose and ADP-glucose, although its K(m) value was approximately 100-fold lower for ADP-ribose (13.0+/-0.7 microm) than for ADP-glucose (1,235+/-65 microm). AtNUDX15 hydrolyzed not only reduced coenzyme A (118.7+/-3.4 microm) but also a wide range of its derivatives. AtNUDX19 showed pyrophosphohydrolase activity toward both NADH (335.3+/-5.4 microm) and NADPH (36.9+/-3.5 microm). AtNUDX23 had flavin adenine dinucleotide pyrophosphohydrolase activity (9.1+/-0.9 microm). Both AtNUDX26 and AtNUDX27 hydrolyzed diadenosine polyphosphates (n=4-5). A confocal microscopic analysis using a green fluorescent protein fusion protein showed that AtNUDX15 is distributed in mitochondria and AtNUDX14 -19, -23, -26, and -27 are distributed in chloroplasts. These AtNUDX mRNAs were detected ubiquitously in various Arabidopsis tissues. The T-DNA insertion mutants of AtNUDX13, -14, -15, -19, -20, -21, -25, -26, and -27 did not exhibit any phenotypical differences under normal growth conditions. These results suggest that Nudix hydrolases in Arabidopsis control a variety of metabolites and are pertinent to a wide range of physiological processes.

Downstream targets of WRKY33

Innate immunity signaling pathways in both animals and plants are regulated by mitogen-activated protein kinase (MAPK) cascades. In a recent publication we show that MPK4 and its substrate MKS1 interact with WRKY33 in vivo, and that WRKY33 is released from complexes with MPK4 upon infection. Transcriptome analysis of a wrky33 loss-of-function mutant identified a subset of defense-related genes as putative targets of WRKY33. These genes include PAD3 and CYP71A13, which encode cytochrome P450 monoxygenases required for synthesis of the antimicrobial phytoalexin camalexin. Chromatin immunoprecipitation confirmed that WRKY33 bound the promoter of PAD3 when plants were inoculated with pathogens. Here we further discuss the involvement of two other targets of WRKY33, NUDT6 and ROF2 in defense responses against invading pathogens.

The genetic network controlling the Arabidopsis transcriptional response to Pseudomonas syringae pv. maculicola: roles of major regulators and the phytotoxin coronatine

Expression profiling of wild-type plants and mutants with defects in key components of the defense signaling network was used to model the Arabidopsis network 24 h after infection by Pseudomonas syringae pv. maculicola ES4326. Results using the Affymetrix ATH1 array revealed that expression levels of most pathogen-responsive genes were affected by mutations in coi1, ein2, npr1, pad4, or sid2. These five mutations defined a small number of different expression patterns displayed by the majority of pathogen-responsive genes. P. syringae pv. tomato strain DC3000 elicited a much weaker salicylic acid (SA) response than ES4326. Additional mutants were profiled using a custom array. Profiles of pbs3 and ndr1 revealed major effects of these mutations and allowed PBS3 and NDR1 to be placed between the EDS1/PAD4 node and the SA synthesis node in the defense network. Comparison of coi1, dde2, and jar1 profiles showed that many genes were affected by coi1 but very few were affected by dde2 or jar1. Profiles of coi1 plants infected with ES4326 were very similar to those of wild-type plants infected with bacteria unable to produce the phytotoxin coronatine, indicating that, essentially, all COI1-dependent gene expression changes in this system are caused by coronatine.

Manipulation of the eukaryotic transcriptional machinery by bacterial pathogens

Pathogenic bacteria inject a diverse set of effectors into eukaryotic host cells and manipulate the cellular environment to enhance bacterial fitness. There is mounting evidence that the transcription machinery in the nucleus of plant cells is a target of various bacterial effectors. These effectors seem to mimic the actions of host nuclear components. To monitor and counteract such virulence-promoting strategies, plants have acquired unique immune-sensing mechanisms.

Global analysis of Arabidopsis gene expression uncovers a complex array of changes impacting pathogen response and cell cycle during geminivirus infection

Geminiviruses are small DNA viruses that use plant replication machinery to amplify their genomes. Microarray analysis of the Arabidopsis (Arabidopsis thaliana) transcriptome in response to cabbage leaf curl virus (CaLCuV) infection uncovered 5,365 genes (false discovery rate <0.005) differentially expressed in infected rosette leaves at 12 d postinoculation. Data mining revealed that CaLCuV triggers a pathogen response via the salicylic acid pathway and induces expression of genes involved in programmed cell death, genotoxic stress, and DNA repair. CaLCuV also altered expression of cell cycle-associated genes, preferentially activating genes expressed during S and G2 and inhibiting genes active in G1 and M. A limited set of core cell cycle genes associated with cell cycle reentry, late G1, S, and early G2 had increased RNA levels, while core cell cycle genes linked to early G1 and late G2 had reduced transcripts. Fluorescence-activated cell sorting of nuclei from infected leaves revealed a depletion of the 4C population and an increase in 8C, 16C, and 32C nuclei. Infectivity studies of transgenic Arabidopsis showed that overexpression of CYCD3;1 or E2FB, both of which promote the mitotic cell cycle, strongly impaired CaLCuV infection. In contrast, overexpression of E2FA or E2FC, which can facilitate the endocycle, had no apparent effect. These results showed that geminiviruses and RNA viruses interface with the host pathogen response via a common mechanism, and that geminiviruses modulate plant cell cycle status by differentially impacting the CYCD/retinoblastoma-related protein/E2F regulatory network and facilitating progression into the endocycle.

Chloroplast signaling and LESION SIMULATING DISEASE1 regulate crosstalk between light acclimation and immunity in Arabidopsis

Plants are simultaneously exposed to abiotic and biotic hazards. Here, we show that local and systemic acclimation in Arabidopsis thaliana leaves in response to excess excitation energy (EEE) is associated with cell death and is regulated by specific redox changes of the plastoquinone (PQ) pool. These redox changes cause a rapid decrease of stomatal conductance, global induction of ASCORBATE PEROXIDASE2 and PATHOGEN RESISTANCE1, and increased production of reactive oxygen species (ROS) and ethylene that signals through ETHYLENE INSENSITIVE2 (EIN2). We provide evidence that multiple hormonal/ROS signaling pathways regulate the plant's response to EEE and that EEE stimulates systemic acquired resistance and basal defenses to virulent biotrophic bacteria. In the Arabidopsis LESION SIMULATING DISEASE1 (lsd1) null mutant that is deregulated for EEE acclimation responses, propagation of EEE-induced programmed cell death depends on the plant defense regulators ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4). We find that EDS1 and PAD4 operate upstream of ethylene and ROS production in the EEE response. The data suggest that the balanced activities of LSD1, EDS1, PAD4, and EIN2 regulate signaling of programmed cell death, light acclimation, and holistic defense responses that are initiated, at least in part, by redox changes of the PQ pool.

News from the frontline: recent insights into PAMP-triggered immunity in plants

Plants have developed a complex defence network to fight off invading pathogens. In recent years, the full importance of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) within this network became apparent. Several new PAMPs have been isolated and new pattern-recognition receptors (PRRs) identified. The discovery of the PRR-interacting protein BAK1 sheds light on the immediate downstream signalling events. Further, transcriptomic analyses identified a core set of approximately 100 PAMP-responsive genes. These studies also revealed a significant overlap with genes regulated during effector-triggered immunity (ETI). Strikingly, ETI seems to operate by alleviating the negative feedback regulation of PTI, leading to stronger defences. This review discusses recent findings in PTI recognition and signalling, and illustrates the need to discover new regulators of PTI responses for a full understanding of plant innate immunity.

Identification of regulatory pathways controlling gene expression of stress-responsive mitochondrial proteins in Arabidopsis

In this study we analyzed transcript abundance and promoters of genes encoding mitochondrial proteins to identify signaling pathways that regulate stress-induced gene expression. We used Arabidopsis (Arabidopsis thaliana) alternative oxidase AOX1a, external NADP H-dehydrogenase NDB2, and two additional highly stress-responsive genes, At2g21640 and BCS1. As a starting point, the promoter region of AOX1a was analyzed and functional analysis identified 10 cis-acting regulatory elements (CAREs), which played a role in response to treatment with H(2)O(2), rotenone, or both. Six of these elements were also functional in the NDB2 promoter. The promoter region of At2g21640, previously defined as a hallmark of oxidative stress, shared two functional CAREs with AOX1a and was responsive to treatment with H(2)O(2) but not rotenone. Microarray analysis further supported that signaling pathways induced by H(2)O(2) and rotenone are not identical. The promoter of BCS1 was not responsive to H(2)O(2) or rotenone, but highly responsive to salicylic acid (SA), whereas the promoters of AOX1a and NDB2 were unresponsive to SA. Analysis of transcript abundance of these genes in a variety of defense signaling mutants confirmed that BCS1 expression is regulated in a different manner compared to AOX1a, NDB2, and At2g21640. These mutants also revealed a pathway associated with programmed cell death that regulated AOX1a in a manner distinct from the other genes. Thus, at least three distinctive pathways regulate mitochondrial stress response at a transcriptional level, an SA-dependent pathway represented by BCS1, a second pathway that represents a convergence point for signals generated by H(2)O(2) and rotenone on multiple CAREs, some of which are shared between responsive genes, and a third pathway that acts via EDS1 and PAD4 regulating only AOX1a. Furthermore, posttranscriptional regulation accounts for changes in transcript abundance by SA treatment for some genes.

Arabidopsis MAP kinase 4 regulates gene expression through transcription factor release in the nucleus

Plant and animal perception of microbes through pathogen surveillance proteins leads to MAP kinase signalling and the expression of defence genes. However, little is known about how plant MAP kinases regulate specific gene expression. We report that, in the absence of pathogens, Arabidopsis MAP kinase 4 (MPK4) exists in nuclear complexes with the WRKY33 transcription factor. This complex depends on the MPK4 substrate MKS1. Challenge with Pseudomonas syringae or flagellin leads to the activation of MPK4 and phosphorylation of MKS1. Subsequently, complexes with MKS1 and WRKY33 are released from MPK4, and WRKY33 targets the promoter of PHYTOALEXIN DEFICIENT3 (PAD3) encoding an enzyme required for the synthesis of antimicrobial camalexin. Hence, wrky33 mutants are impaired in the accumulation of PAD3 mRNA and camalexin production upon infection. That WRKY33 is an effector of MPK4 is further supported by the suppression of PAD3 expression in mpk4-wrky33 double mutant backgrounds. Our data establish direct links between MPK4 and innate immunity and provide an example of how a plant MAP kinase can regulate gene expression by releasing transcription factors in the nucleus upon activation.

The oomycete response gene LURP1 is required for defense against Hyaloperonospora parasitica in Arabidopsis thaliana

LURP1 is a member of the LURP cluster and the PR1 regulon, two previously defined sets of co-expressed Arabidopsis thaliana genes that share a pronounced upregulation in response to infections by the pathogenic oomycete Hyaloperonospora parasitica. LURP1 shows the most extreme transcriptional inducibility by H. parasitica of all LURP and PR1 regulon genes. Using insertion mutants we found that LURP1 is required for full basal defense to H. parasitica and resistance to this pathogen mediated by the R-proteins RPP4 and RPP5. The LURP1 protein shows no obvious similarity to proteins of known molecular function. We identified a 39-bp region of the LURP1 promoter that mediates reporter gene expression in response to H. parasitica and salicylic acid. This promoter region contains a W box motif, W(LURP1), that interacts in vitro with nuclear factors producing two separate DNA-binding patterns. W(LURP1)-related sequences are strongly enriched in the promoters of the PR1 regulon, suggesting a role for this motif in the coordinated expression of genes inducible by H. parasitica and related defense conditions.

WRR4 encodes a TIR-NB-LRR protein that confers broad-spectrum white rust resistance in Arabidopsis thaliana to four physiological races of Albugo candida

White blister rust in the Brassicaceae is emerging as a superb model for exploring how plant biodiversity has channeled speciation of biotrophic parasites. The causal agents of white rust across a wide breadth of cruciferous hosts currently are named as variants of a single oomycete species, Albugo candida. The most notable examples include a major group of physiological races that each are economically destructive in a different vegetable or oilseed crop of Brassica juncea (A. candida race 2), B. rapa (race 7), or B. oleracea (race 9); or parasitic on wild crucifers such as Capsella bursa-pastoris (race 4). Arabidopsis thaliana is innately immune to these races of A. candida under natural conditions; however, it commonly hosts its own molecularly distinct subspecies of A. candida (A. candida subsp. arabidopsis). In the laboratory, we have identified several accessions of Arabidopsis thaliana (e.g.,. Ws-3) that can permit varying degrees of rust development following inoculation with A. candida races 2, 4, and 7, whereas race 9 is universally incompatible in Arabidopsis thaliana and nonrusting resistance is the most prevalent outcome of interactions with the other races. Subtle variation in resistance phenotypes is evident, observed initially with an isolate of A. candida race 4, indicating additional genetic variation. Therefore, we used the race 4 isolate for map-based cloning of the first of many expected white rust resistance (WRR) genes. This gene was designated WRR4 and encodes a cytoplasmic toll-interleukin receptor-like nucleotide-binding leucine-rich repeat receptor-like protein that confers a dominant, broad-spectrum white rust resistance in the Arabidopsis thaliana accession Columbia to representative isolates of A. candida races 2, 4, 7, and 9, as verified by transgenic expression of the Columbia allele in Ws-3. The WRR4 protein requires functional expression of the lipase-like protein EDS1 but not the paralogous protein PAD4, and confers full immunity that masks an underlying nonhypersensitive incompatibility in Columbia to A. candida race 4. This residual incompatibility is independent of functional EDS1.

Light regulation and daytime dependency of inducible plant defenses in Arabidopsis: phytochrome signaling controls systemic acquired resistance rather than local defense

We have examined molecular and physiological principles underlying the light dependency of defense activation in Arabidopsis (Arabidopsis thaliana) plants challenged with the bacterial pathogen Pseudomonas syringae. Within a fixed light/dark cycle, plant defense responses and disease resistance significantly depend on the time of day when pathogen contact takes place. Morning and midday inoculations result in higher salicylic acid accumulation, faster expression of pathogenesis-related genes, and a more pronounced hypersensitive response than inoculations in the evening or at night. Rather than to the plants' circadian rhythm, this increased plant defense capability upon day inoculations is attributable to the availability of a prolonged light period during the early plant-pathogen interaction. Moreover, pathogen responses of Arabidopsis double mutants affected in light perception, i.e. cryptochrome1cryptochrome2 (cry1cry2), phototropin1phototropin2 (phot1phot2), and phytochromeAphytochromeB (phyAphyB) were assessed. Induction of defense responses by either avirulent or virulent P. syringae at inoculation sites is relatively robust in leaves of photoreceptor mutants, indicating little cross talk between local defense and light signaling. In addition, the blue-light receptor mutants cry1cry2 and phot1phot2 are both capable of establishing a full systemic acquired resistance (SAR) response. Induction of SAR and salicylic-acid-dependent systemic defense reactions, however, are compromised in phyAphyB mutants. Phytochrome regulation of SAR involves the essential SAR component FLAVIN-DEPENDENT MONOOXYGENASE1. Our findings highlight the importance of phytochrome photoperception during systemic rather than local resistance induction. The phytochrome system seems to accommodate the supply of light energy to the energetically costly increase in whole plant resistance.

Light regulation and daytime dependency of inducible plant defenses in Arabidopsis: phytochrome signaling controls systemic acquired resistance rather than local defense

We have examined molecular and physiological principles underlying the light dependency of defense activation in Arabidopsis (Arabidopsis thaliana) plants challenged with the bacterial pathogen Pseudomonas syringae. Within a fixed light/dark cycle, plant defense responses and disease resistance significantly depend on the time of day when pathogen contact takes place. Morning and midday inoculations result in higher salicylic acid accumulation, faster expression of pathogenesis-related genes, and a more pronounced hypersensitive response than inoculations in the evening or at night. Rather than to the plants' circadian rhythm, this increased plant defense capability upon day inoculations is attributable to the availability of a prolonged light period during the early plant-pathogen interaction. Moreover, pathogen responses of Arabidopsis double mutants affected in light perception, i.e. cryptochrome1cryptochrome2 (cry1cry2), phototropin1phototropin2 (phot1phot2), and phytochromeAphytochromeB (phyAphyB) were assessed. Induction of defense responses by either avirulent or virulent P. syringae at inoculation sites is relatively robust in leaves of photoreceptor mutants, indicating little cross talk between local defense and light signaling. In addition, the blue-light receptor mutants cry1cry2 and phot1phot2 are both capable of establishing a full systemic acquired resistance (SAR) response. Induction of SAR and salicylic-acid-dependent systemic defense reactions, however, are compromised in phyAphyB mutants. Phytochrome regulation of SAR involves the essential SAR component FLAVIN-DEPENDENT MONOOXYGENASE1. Our findings highlight the importance of phytochrome photoperception during systemic rather than local resistance induction. The phytochrome system seems to accommodate the supply of light energy to the energetically costly increase in whole plant resistance.

Arabidopsis proteins important for modulating defense responses to Pseudomonas syringae that secrete HopW1-1

Plant infection responses result from the interaction of pathogen-derived molecules with host components. For the bacterial pathogen Pseudomonas syringae, these molecules are often effector proteins (Hops) that are injected into plant cells. P. syringae carrying hopW1-1 have restricted host range on some Arabidopsis thaliana accessions. At least two Arabidopsis genomic regions are important for the natural variation that conditions resistance to P. syringae/hopW1-1. HopW1-1 elicits a resistance response, and consequently the accumulation of the signal molecule salicylic acid (SA) and transcripts of HWI1 (HopW1-1-Induced Gene1). This work identified three HopW1-1-interacting (WIN) plant proteins: a putative acetylornithine transaminase (WIN1), a protein phosphatase (WIN2) and a firefly luciferase superfamily protein (WIN3). Importantly, WIN2 and WIN3 are partially required for HopW1-1-induced disease resistance, SA production and HWI1 expression. The requirement for WIN2 is specific for HopW1-1-induced resistance, whereas WIN3 is important for responses to several effectors. Overexpression of WIN2 or WIN3 confers resistance to virulent P. syringae, which is consistent with these proteins being defense components. Several known genes important for SA production or signaling are also partially (EDS1, NIM1/NPR1, ACD6 and ALD1) or strongly (PAD4) required for the robust resistance induced by HopW1-1, suggesting a key role for SA in the HopW1-1-induced resistance response. Finally, WIN1 is an essential protein, the overexpression of which over-rides the resistance response to HopW1-1 (and several other defense-inducing effectors), and delays SA and HWI1 induction. Thus, the WIN proteins have different roles in modulating plant defense.

A lesion-mimic syntaxin double mutant in Arabidopsis reveals novel complexity of pathogen defense signaling

The lesion-mimic Arabidopsis mutant, syp121 syp122, constitutively expresses the salicylic acid (SA) signaling pathway and has low penetration resistance to powdery mildew fungi. Genetic analyses of the lesion-mimic phenotype have expanded our understanding of programmed cell death (PCD) in plants. Inactivation of SA signaling genes in syp121 syp122 only partially rescues the lesion-mimic phenotype, indicating that additional defenses contribute to the PCD. Whole genome transcriptome analysis confirmed that SA-induced transcripts, as well as numerous other known pathogen-response transcripts, are up-regulated after inactivation of the syntaxin genes. A suppressor mutant analysis of syp121 syp122 revealed that FMO1, ALD1, and PAD4 are important for lesion development. Mutant alleles of EDS1, NDR1, RAR1, and SGT1b also partially rescued the lesion-mimic phenotype, suggesting that mutating syntaxin genes stimulates TIR-NB-LRR and CC-NB-LRR-type resistances. The syntaxin double knockout potentiated a powdery mildew-induced HR-like response. This required functional PAD4 but not functional SA signaling. However, SA signaling potentiated the PAD4-dependent HR-like response. Analyses of quadruple mutants suggest that EDS5 and SID2 confer separate SA-independent signaling functions, and that FMO1 and ALD1 mediate SA-independent signals that are NPR1-dependent. These studies highlight the contribution of multiple pathways to defense and point to the complexity of their interactions.

Discovery of ADP-ribosylation and other plant defense pathway elements through expression profiling of four different Arabidopsis-Pseudomonas R-avr interactions

A dissection of plant defense pathways was initiated through gene expression profiling of the responses of a single Arabidopsis thaliana genotype to isogenic Pseudomonas syringae strains expressing one of four different cloned avirulence (avr) genes. Differences in the expression profiles elicited by different resistance (R)-avr interactions were observed. A role for poly(ADP-ribosyl)ation in plant defense responses was suggested initially by the upregulated expression of genes encoding NUDT7 and poly(ADP-ribose) glycohydrolase in multiple R-avr interactions. Gene knockout plant lines were tested for 20 candidate genes identified by the expression profiling, and Arabidopsis NUDT7 mutants allowed less growth of virulent P. syringae (as previously reported) but also exhibited a reduced hypersensitive-response phenotype. Inhibitors of poly(ADP-ribose) polymerase (PARP) disrupted FLS2-mediated basal defense responses such as callose deposition. EIN2 (ethylene response) and IXR1 and IXR2 (cellulose synthase) mutants impacted the FLS2-mediated responses that occur during PARP inhibition, whereas no impacts were observed for NPR1, PAD4, or NDR1 mutants. In the expression profiling work, false-positive selection and grouping of genes was reduced by requiring simultaneous satisfaction of statistical significance criteria for each of three separate analysis methods, and by clustering genes based on statistical confidence values for each gene rather than on average fold-change of transcript abundance.

The origin and function of calmodulin regulated Ca2+ pumps in plants

While Ca2+ signaling plays an important role in both plants and animals, the machinery that codes and decodes these signals have evolved to show interesting differences and similarities. For example, typical plant and animal cells both utilize calmodulin (CaM)-regulated Ca2+ pumps at the plasma membrane to help control cytoplasmic Ca2+ levels. However, in flowering plants this family of pumps has evolved with a unique structural arrangement in which the regulatory domain is located at the N-terminal instead of C-terminal end. In addition, some of the plant isoforms have evolved to function at endomembrane locations. For the 14 Ca2+ pumps present in the model plant Arabidopsis, molecular genetic analyses are providing exciting insights into their function in diverse aspects of plant growth and development.

The role of AtNUDT7, a Nudix hydrolase, in the plant defense response

Nudix hydrolases constitute a large family of proteins that hydrolyze nucleoside diphosphate derivatives. Some Nudix hydrolases act as 'housecleaning' enzymes whereas others may function to sense and modulate the levels of their substrates to maintain physiological homeostasis. The Arabidopsis genome encodes 32 Nudix proteins (AtNUDTs). However, their physiological substrates and biological functions are little known. AtNUDT7 has been identified as a negative regulator of the defense response and its loss-of-function mutation leads to enhanced disease resistance and makes the plants hyper-responsive to inciting agents including pathogenic as well as nonpathogenic micro-organisms. Based on in vitro enzymatic characterization, it was speculated that ADP-ribose (ADPR) and/or NADH may be biologically significant substrates of AtNUDT7. However, our result from determination of the levels of ADPR and NAD(H) in the mutant and wild-type plants indicates neither of these nucleotide analogs likely is its physiological substrates. The Atnudt7 mutant does have a higher ratio of GSSG/GSH than wild-type plants. This alteration in redox homeostasis may prime the mutant plants for excessive cellular stimulation when being provoked by biotic stresses.

Activation of the indole-3-acetic acid-amido synthetase GH3-8 suppresses expansin expression and promotes salicylate- and jasmonate-independent basal immunity in rice

New evidence suggests a role for the plant growth hormone auxin in pathogenesis and disease resistance. Bacterial infection induces the accumulation of indole-3-acetic acid (IAA), the major type of auxin, in rice (Oryza sativa). IAA induces the expression of expansins, proteins that loosen the cell wall. Loosening the cell wall is key for plant growth but may also make the plant vulnerable to biotic intruders. Here, we report that rice GH3-8, an auxin-responsive gene functioning in auxin-dependent development, activates disease resistance in a salicylic acid signaling- and jasmonic acid signaling-independent pathway. GH3-8 encodes an IAA-amino synthetase that prevents free IAA accumulation. Overexpression of GH3-8 results in enhanced disease resistance to the rice pathogen Xanthomonas oryzae pv oryzae. This resistance is independent of jasmonic acid and salicylic acid signaling. Overexpression of GH3-8 also causes abnormal plant morphology and retarded growth and development. Both enhanced resistance and abnormal development may be caused by inhibition of the expression of expansins via suppressed auxin signaling.

Powdery mildew induces defense-oriented reprogramming of the transcriptome in a susceptible but not in a resistant grapevine

Grapevines exhibit a wide spectrum of resistance to the powdery mildew fungus (PM), Erysiphe necator (Schw.) Burr., but little is known about the transcriptional basis of the defense to PM. Our microscopic observations showed that PM produced less hyphal growth and induced more brown-colored epidermal cells on leaves of PM-resistant Vitis aestivalis 'Norton' than on leaves of PM-susceptible Vitis vinifera 'Cabernet sauvignon'. We found that endogenous salicylic acid levels were higher in V. aestivalis than in V. vinifera in the absence of the fungus and that salicylic acid levels increased in V. vinifera at 120 h postinoculation with PM. To test the hypothesis that gene expression differences would be apparent when V. aestivalis and V. vinifera were mounting a response to PM, we conducted a comprehensive Vitis GeneChip analysis. We examined the transcriptome at 0, 4, 8, 12, 24, and 48 h postinoculation with PM. We found only three PM-responsive transcripts in V. aestivalis and 625 in V. vinifera. There was a significant increase in the abundance of transcripts encoding ENHANCED DISEASE SUSCEPTIBILITY1, mitogen-activated protein kinase kinase, WRKY, PATHOGENESIS-RELATED1, PATHOGENESIS-RELATED10, and stilbene synthase in PM-infected V. vinifera, suggesting an induction of the basal defense response. The overall changes in the PM-responsive V. vinifera transcriptome also indicated a possible reprogramming of metabolism toward the increased synthesis of the secondary metabolites. These results suggested that resistance to PM in V. aestivalis was not associated with overall reprogramming of the transcriptome. However, PM induced defense-oriented transcriptional changes in V. vinifera.

Nuclear accumulation of the Arabidopsis immune receptor RPS4 is necessary for triggering EDS1-dependent defense

Recognition of specific pathogen molecules inside the cell by nucleotide-binding domain and leucine-rich repeat (NB-LRR) receptors constitutes an important layer of innate immunity in plants. Receptor activation triggers host cellular reprogramming involving transcriptional potentiation of basal defenses and localized programmed cell death. The sites and modes of action of NB-LRR receptors are, however, poorly understood. Arabidopsis Toll/Interleukin-1 (TIR) type NB-LRR receptor RPS4 recognizes the bacterial type III effector AvrRps4. We show that epitope-tagged RPS4 expressed under its native regulatory sequences distributes between endomembranes and nuclei in healthy and AvrRps4-triggered tissues. RPS4 accumulation in the nucleus, mediated by a bipartite nuclear localization sequence (NLS) at its C terminus, is necessary for triggering immunity through authentic activation by AvrRps4 in Arabidopsis or as an effector-independent "deregulated" receptor in tobacco. A strikingly conserved feature of TIR-NB-LRR receptors is their recruitment of the nucleocytoplasmic basal-defense regulator EDS1 in resistance to diverse pathogens. We find that EDS1 is an indispensable component of RPS4 signaling and that it functions downstream of RPS4 activation but upstream of RPS4-mediated transcriptional reprogramming in the nucleus.

Basal resistance against Pseudomonas syringae in Arabidopsis involves WRKY53 and a protein with homology to a nematode resistance protein

Basal resistance is the ultimately unsuccessful plant defense response to infection with a virulent pathogen. It is thought to be triggered by host recognition of pathogen-associated molecular patterns, with subsequent suppression of particular components by pathogen effectors. To identify novel components of Arabidopsis basal resistance against the bacterial pathogen Pseudomonas syringae pv. tomato, microarray expression profiling was carried out on the cirl mutant, which displays enhanced resistance against P. syringae pv. tomato. This identified two genes, At4g23810 and At2g40000, encoding the transcription factor WRKY53 and the nematode resistance protein-like HSPRO2, whose expression was upregulated in cir1 prior to pathogen infection and in wild-type plants after P. syringae pv. tomato infection. WRKY53 and HSPRO2 are positive regulators of basal resistance. Knockout mutants of both genes were more susceptible to P. syringae pv. tomato infection than complemented lines, with increased growth of the pathogen in planta. WRKY53 and HSPRO2 appear to function downstream of salicylic acid and to be negatively regulated by signaling through jasmonic acid and ethylene.

Lysigenous aerenchyma formation in Arabidopsis is controlled by LESION SIMULATING DISEASE1

Aerenchyma tissues form gas-conducting tubes that provide roots with oxygen under hypoxic conditions. Although aerenchyma have received considerable attention in Zea mays, the signaling events and genes controlling aerenchyma induction remain elusive. Here, we show that Arabidopsis thaliana hypocotyls form lysigenous aerenchyma in response to hypoxia and that this process involves H(2)O(2) and ethylene signaling. By studying Arabidopsis mutants that are deregulated for excess light acclimation, cell death, and defense responses, we find that the formation of lysigenous aerenchyma depends on the plant defense regulators LESION SIMULATING DISEASE1 (LSD1), ENHANCED DISEASE SUSCEPIBILITY1 (EDS1), and PHYTOALEXIN DEFICIENT4 (PAD4) that operate upstream of ethylene and reactive oxygen species production. The obtained results indicate that programmed cell death of lysigenous aerenchyma in hypocotyls occurs in a similar but independent manner from the foliar programmed cell death. Thus, the induction of aerenchyma is subject to a genetic and tissue-specific program. The data lead us to conclude that the balanced activities of LSD1, EDS1, and PAD4 regulate lysigenous aerenchyma formation in response to hypoxia.

Plant pathogen recognition mediated by promoter activation of the pepper Bs3 resistance gene

Plant disease resistance (R) proteins recognize matching pathogen avirulence proteins. Alleles of the pepper R gene Bs3 mediate recognition of the Xanthomonas campestris pv. vesicatoria (Xcv) type III effector protein AvrBs3 and its deletion derivative AvrBs3Deltarep16. Pepper Bs3 and its allelic variant Bs3-E encode flavin monooxygenases with a previously unknown structure and are transcriptionally activated by the Xcv effector proteins AvrBs3 and AvrBs3Deltarep16, respectively. We found that recognition specificity resides in the Bs3 and Bs3-E promoters and is determined by binding of AvrBs3 or AvrBs3Deltarep16 to a defined promoter region. Our data suggest a recognition mechanism in which the Avr protein binds and activates the promoter of the cognate R gene.

Rumble in the nuclear jungle: compartmentalization, trafficking, and nuclear action of plant immune receptors

Plants and animals have evolved structurally related innate immune sensors inside cells to detect the presence of microbial molecules. An evolutionary ancient folding machinery becomes engaged for the synthesis of autorepressed receptor forms in both kingdoms. The receptors act as regulatory signal transduction switches and are activated upon direct or indirect perception of non-self structures. Recent findings indicate that nucleo-cytoplasmic partitioning and nuclear activity is critical for the function of several plant immune sensors, thereby linking receptor function to transcriptional reprogramming of host cells for pathogen defense. This implies short signalling pathways and reveals parallels with regulatory control mechanisms of animal steroid receptors.

Phloem-based resistance to green peach aphid is controlled by Arabidopsis PHYTOALEXIN DEFICIENT4 without its signaling partner ENHANCED DISEASE SUSCEPTIBILITY1

Green peach aphid (GPA) Myzus persicae (Sülzer) is a phloem-feeding insect with an exceptionally wide host range. Previously, it has been shown that Arabidopsis thaliana PHYTOALEXIN DEFICIENT4 (PAD4), which is expressed at elevated levels in response to GPA infestation, is required for resistance to GPA in the Arabidopsis accession Columbia. We demonstrate here that the role of PAD4 in the response to GPA is conserved in Arabidopsis accessions Wassilewskija and Landsberg erecta. Electrical monitoring of aphid feeding behavior revealed that PAD4 modulates a phloem-based defense mechanism against GPA. GPA spends more time actively feeding from the sieve elements of pad4 mutants than from wild-type plants, and less time feeding on transgenic plants in which PAD4 is ectopically expressed. The activity of PAD4 in limiting phloem sap uptake serves as a deterrent in host-plant choice, and restricts aphid population size. In Arabidopsis defense against pathogens, all known PAD4 functions require its signaling and stabilizing partner EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1). Bioassays with eds1 mutants alone or in combination with pad4 and with plants conditionally expressing PAD4 under the control of a dexamethasone-inducible promoter reveal that PAD4-modulated defense against GPA does not involve EDS1. Thus, a PAD4 mode of action that is uncoupled from EDS1 determines the extent of aphid feeding in the phloem.

AtNUDT7, a negative regulator of basal immunity in Arabidopsis, modulates two distinct defense response pathways and is involved in maintaining redox homeostasis

Plants have evolved complicated regulatory systems to control immune responses. Both positive and negative signaling pathways interplay to coordinate development of a resistance response with the appropriate amplitude and duration. AtNUDT7, a Nudix domain-containing protein in Arabidopsis (Arabidopsis thaliana) that hydrolyzes nucleotide derivatives, was found to be a negative regulator of the basal defense response, and its loss-of-function mutation results in enhanced resistance to infection by Pseudomonas syringae. The nudt7 mutation does not cause a strong constitutive disease resistance phenotype, but it leads to a heightened defense response, including accelerated activation of defense-related genes that can be triggered by pathogenic and nonpathogenic microorganisms. The nudt7 mutation enhances two distinct defense response pathways: one independent of and the other dependent on NPR1 and salicylic acid accumulation. In vitro enzymatic assays revealed that ADP-ribose and NADH are preferred substrates of NUDT7, and the hydrolysis activity of NUDT7 is essential for its biological function and is sensitive to inhibition by Ca(2+). Further analyses indicate that ADP-ribose is not likely the physiological substrate of NUDT7. However, the nudt7 mutation leads to perturbation of cellular redox homeostasis and a higher level of NADH in pathogen-challenged leaves. The study suggests that the alteration in cellular antioxidant status caused by the nudt7 mutation primes the cells for the amplified defense response and NUDT7 functions to modulate the defense response to prevent excessive stimulation.

Flavin-containing monooxygenases in plants: looking beyond detox

Flavin-containing monooxygenases (FMOs) are known in bacteria, yeast and mammals where they catalyze the transfer of one atom of molecular O(2) to low molecular weight substrates. The predominant physiological function of animal FMOs appears to be detoxification of a vast spectrum of xenobiotics but until recently very little was known about the function of FMOs in plants. In the last two to three years, genetic and biochemical characterization has shown that plant FMOs can catalyze specific steps in the biosynthesis of auxin or in the metabolism of glucosinolates, and, furthermore, have a role in pathogen defence. Thus, plant FMOs hint that further FMO functions might be identified also in non-plant organisms and could stimulate novel research in this area.

Regulation of plant innate immunity by three proteins in a complex conserved across the plant and animal kingdoms

Innate immunity against pathogen infection is an evolutionarily conserved process among multicellular organisms. Arabidopsis SNC1 encodes a Resistance protein that combines attributes of multiple mammalian pattern recognition receptors. Utilizing snc1 as an autoimmune model, we identified a discrete protein complex containing at least three members--MOS4 (Modifier Of snc1, 4), AtCDC5, and PRL1 (Pleiotropic Regulatory Locus 1)--that are all essential for plant innate immunity. AtCDC5 has DNA-binding activity, suggesting that this complex probably regulates defense responses through transcriptional control. Since the complex components along with their interactions are highly conserved from fission yeast to Arabidopsis and human, they may also have a yet-to-be-identified function in mammalian innate immunity.

Comparative transcript profiling by cDNA-AFLP reveals similar patterns of Avr4/Cf-4- and Avr9/Cf-9-dependent defence gene expression

Tomato Cf genes confer resistance to the fungal pathogen Cladosporium fulvum. Although the Cf-4 and Cf-9 proteins are very similar, the Cf-4- and Cf-9-dependent hypersensitive responses (HRs) are distinct in cell death pattern, intensity and sensitivity to environmental conditions. To investigate the mechanism leading to these differences, comparative transcript profiling of Avr4/Cf-4- and Avr9/Cf-9-dependent defence gene expression was performed. To do this, cDNA-AFLP analysis was conducted on Avr/Cf tomato seedlings undergoing early HR. Both Avr4/Cf-4 and Avr9/Cf-9 signalling elicited the same spectrum of genes, referred to here as Avr/Cf-elicited (ACE) genes. Of approximately 25 000 transcript-derived fragments (TDFs), 367 (1.5%) showed significant differential expression between HR(+) and HR(-) seedlings (either Avr4/Cf-4- or Avr9/Cf-9-dependent). However, 42.8% of the ACE TDFs (157/367 in total) showed quantitatively different expression in the two types of HR(+) seedlings. The majority of these (135/157, 86.0%) displayed significantly greater differential expression (either induced or repressed) in Avr4/Cf-4 HR(+) seedlings than in Avr9/Cf-9 HR(+) seedlings. Our results are consistent with the previous observation that Avr4/Cf-4-dependent HR is more severe than Avr9/Cf-9-dependent HR, and indicate that the distinction between Avr4/Cf-4- and Avr9/Cf-9-dependent HR is most probably a result of events upstream of the defence responses. Sequencing of 189 ACE fragments identified genes associated with: defence and resistance (33.3%), signal transduction (7.4%), HR and cell death (5.3%), transcriptional regulation and post-transcriptional modification (4.3%). Expression data revealed that defence response, respiration and biological oxidation are strongly induced while photosynthesis is severely repressed in the HR(+) seedlings.

The WRKY70 transcription factor of Arabidopsis influences both the plant senescence and defense signaling pathways

Regulatory proteins play critical roles in controlling the kinetics of various cellular processes during the entire life span of an organism. Leaf senescence, an integral part of the plant developmental program, is fine-tuned by a complex transcriptional regulatory network ensuring a successful switch to the terminal life phase. To expand our understanding on how transcriptional control coordinates leaf senescence, we characterized AtWRKY70, a gene encoding a WRKY transcription factor that functions as a negative regulator of developmental senescence. To gain insight into the interplay of senescence and plant defense signaling pathways, we employed a collection of mutants, allowing us to specifically define the role of AtWRKY70 in the salicylic acid-mediated signaling cascades and to further dissect the cross-talk of signal transduction pathways during the onset of senescence in Arabidopsis thaliana. Our results provide strong evidence that AtWRKY70 influences plant senescence and defense signaling pathways. These studies could form the basis for further unraveling of these two complex interlinked regulatory networks.

Identification of a flavin-monooxygenase as the S-oxygenating enzyme in aliphatic glucosinolate biosynthesis in Arabidopsis

The cancer-preventive activity of cruciferous vegetables is commonly attributed to isothiocyanates resulting from the breakdown of the natural products glucosinolates (GSLs). Sulforaphane, the isothiocyanate derived from 4-methylsulfinylbutyl GSL, is thought to be the major agent conferring cancer-preventive properties, whereas the isothiocyanate of 4-methylthiobutyl GSL does not have the same activity. We report the identification of an Arabidopsis flavin-monooxygenase (FMO) enzyme, FMO(GS-OX1), which catalyzes the conversion of methylthioalkyl GSLs into methylsulfinylalkyl GSLs. This is evidenced by biochemical characterization of the recombinant protein, and analyses of the GSL content in FMO(GS-OX1) overexpression lines and an FMO(GS-OX1) knock-out mutant of Arabidopsis. The FMO(GS-OX1) overexpression lines show almost complete conversion of methylthioalkyl into methylsulfinylalkyl GSLs, with an approximately fivefold increase in 4-methylsulfinylbutyl GSL in seeds. Identification of FMO(GS-OX1) provides a molecular tool for breeding of Brassica vegetable crops with increased levels of this important GSL, which has implications for production of functional foods enriched with the cancer-preventive sulforaphane.

Expression of AtWRKY33 encoding a pathogen- or PAMP-responsive WRKY transcription factor is regulated by a composite DNA motif containing W box elements

WRKY transcription factors regulate distinct parts of the plant defense transcriptome. Expression of many WRKY genes themselves is induced by pathogens or pathogen-mimicking molecules. Here, we demonstrate that Arabidopsis WRKY33 responds to various stimuli associated with plant defense as well as to different kinds of phytopathogens. Although rapid pathogen-induced AtWRKY33 expression does not require salicylic acid (SA) signaling, it is dependent on PAD4, a key regulator upstream of SA. Activation of AtWRKY33 is independent of de novo protein synthesis, suggesting that it is at least partly under negative regulatory control. We show that a set of three WRKY-specific cis-acting DNA elements (W boxes) within the AtWRKY33 promoter is required for efficient pathogen- or PAMP-triggered gene activation. This strongly indicates that WRKY transcription factors are major components of the regulatory machinery modulating immediate to early expression of this gene in response to pathogen attack.

Impairment of cellulose synthases required for Arabidopsis secondary cell wall formation enhances disease resistance

Cellulose is synthesized by cellulose synthases (CESAs) contained in plasma membrane-localized complexes. In Arabidopsis thaliana, three types of CESA subunits (CESA4/IRREGULAR XYLEM5 [IRX5], CESA7/IRX3, and CESA8/IRX1) are required for secondary cell wall formation. We report that mutations in these proteins conferred enhanced resistance to the soil-borne bacterium Ralstonia solanacearum and the necrotrophic fungus Plectosphaerella cucumerina. By contrast, susceptibility to these pathogens was not altered in cell wall mutants of primary wall CESA subunits (CESA1, CESA3/ISOXABEN RESISTANT1 [IXR1], and CESA6/IXR2) or POWDERY MILDEW-RESISTANT5 (PMR5) and PMR6 genes. Double mutants indicated that irx-mediated resistance was independent of salicylic acid, ethylene, and jasmonate signaling. Comparative transcriptomic analyses identified a set of common irx upregulated genes, including a number of abscisic acid (ABA)-responsive, defense-related genes encoding antibiotic peptides and enzymes involved in the synthesis and activation of antimicrobial secondary metabolites. These data as well as the increased susceptibility of ABA mutants (abi1-1, abi2-1, and aba1-6) to R. solanacearum support a direct role of ABA in resistance to this pathogen. Our results also indicate that alteration of secondary cell wall integrity by inhibiting cellulose synthesis leads to specific activation of novel defense pathways that contribute to the generation of an antimicrobial-enriched environment hostile to pathogens.

Arabidopsis WRKY70 is required for full RPP4-mediated disease resistance and basal defense against Hyaloperonospora parasitica

AtWRKY70, encoding a WRKY transcription factor, is co-expressed with a set of Arabidopsis genes that share a pattern of RPP4- and RPP7-dependent late upregulation in response to Hyaloperonospora parasitica infection (LURP) genes. We show that AtWRKY70 is required for both full RPP4-mediated resistance and basal defense against H. parasitica. These two defense pathways are related to each other, because they require PAD4 and salicylic acid (SA). RPP7 function, which is independent from PAD4 and SA, is not affected by insertions in AtWRKY70. Although AtWRKY70 is required for RPP4-resistance, it appears not to contribute significantly to RPP4-triggered cell death. Furthermore, our data indicate that AtWRKY70 functions downstream of defense-associated reactive oxygen intermediates and SA. Constitutive and RPP4-induced transcript levels of two other LURP genes are reduced in AtWRKY70 T-DNA mutants, indicating a direct or indirect role for AtWRKY70 in their regulation. We propose that AtWRKY70 is a component of a basal defense mechanism that is boosted by engagement of either RPP4 or RPP7 and is required for RPP4-mediated resistance.

Oviposition by pierid butterflies triggers defense responses in Arabidopsis

Insect eggs represent a threat for the plant as hatching larvae rapidly start with their feeding activity. Using a whole-genome microarray, we studied the expression profile of Arabidopsis (Arabidopsis thaliana) leaves after oviposition by two pierid butterflies. For Pieris brassicae, the deposition of egg batches changed the expression of hundreds of genes over a period of 3 d after oviposition. The transcript signature was similar to that observed during a hypersensitive response or in lesion-mimic mutants, including the induction of defense and stress-related genes and the repression of genes involved in growth and photosynthesis. Deposition of single eggs by Pieris rapae caused a similar although much weaker transcriptional response. Analysis of the jasmonic acid and salicylic acid mutants coi1-1 and sid2-1 indicated that the response to egg deposition is mostly independent of these signaling pathways. Histochemical analyses showed that egg deposition is causing a localized cell death, accompanied by the accumulation of callose, and the production of reactive oxygen species. In addition, activation of the pathogenesis-related1::beta-glucuronidase reporter gene correlated precisely with the site of egg deposition and was also triggered by crude egg extract. This study provides molecular evidence for the detection of egg deposition by Arabidopsis plants and suggests that oviposition causes a localized response with strong similarity to a hypersensitive response.

Natural variation in partial resistance to Pseudomonas syringae is controlled by two major QTLs in Arabidopsis thaliana

BACKGROUND

Low-level, partial resistance is pre-eminent in natural populations, however, the mechanisms underlying this form of resistance are still poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, we used the model pathosystem Pseudomonas syringae pv. tomato DC3000 (Pst) - Arabidopsis thaliana to study the genetic basis of this form of resistance. Phenotypic analysis of a set of Arabidopsis accessions, based on evaluation of in planta pathogen growth revealed extensive quantitative variation for partial resistance to Pst. It allowed choosing a recombinant inbred line (RIL) population derived from a cross between the accessions Bayreuth and Shahdara for quantitative genetic analysis. Experiments performed under two different environmental conditions led to the detection of two major and two minor quantitative trait loci (QTLs) governing partial resistance to Pst and called PRP-Ps1 to PRP-Ps4. The two major QTLs, PRP-Ps1 and PRP-Ps2, were confirmed in near isogenic lines (NILs), following the heterogeneous inbred families (HIFs) strategy. Analysis of marker gene expression using these HIFs indicated a negative correlation between the induced amount of transcripts of SA-dependent genes PR1, ICS and PR5, and the in planta bacterial growth in the HIF segregating at PRP-Ps2 locus, suggesting an implication of PRP-Ps2 in the activation of SA dependent responses.

CONCLUSIONS/SIGNIFICANCE

These results show that variation in partial resistance to Pst in Arabidopsis is governed by relatively few loci, and the validation of two major loci opens the way for their fine mapping and their cloning, which will improve our understanding of the molecular mechanisms underlying partial resistance.

The Arabidopsis flavin-dependent monooxygenase FMO1 is an essential component of biologically induced systemic acquired resistance

Upon localized attack by necrotizing pathogens, plants gradually develop increased resistance against subsequent infections at the whole-plant level, a phenomenon known as systemic acquired resistance (SAR). To identify genes involved in the establishment of SAR, we pursued a strategy that combined gene expression information from microarray data with pathological characterization of selected Arabidopsis (Arabidopsis thaliana) T-DNA insertion lines. A gene that is up-regulated in Arabidopsis leaves inoculated with avirulent or virulent strains of the bacterial pathogen Pseudomonas syringae pv maculicola (Psm) showed homology to flavin-dependent monooxygenases (FMO) and was designated as FMO1. An Arabidopsis knockout line of FMO1 proved to be fully impaired in the establishment of SAR triggered by avirulent (Psm avrRpm1) or virulent (Psm) bacteria. Loss of SAR in the fmo1 mutants was accompanied by the inability to initiate systemic accumulation of salicylic acid (SA) and systemic expression of diverse defense-related genes. In contrast, responses at the site of pathogen attack, including increases in the levels of the defense signals SA and jasmonic acid, camalexin accumulation, and expression of various defense genes, were induced in a similar manner in both fmo1 mutant and wild-type plants. Consistently, the fmo1 mutation did not significantly affect local disease resistance toward virulent or avirulent bacteria in naive plants. Induction of FMO1 expression at the site of pathogen inoculation is independent of SA signaling, but attenuated in the Arabidopsis eds1 and pad4 defense mutants. Importantly, FMO1 expression is also systemically induced upon localized P. syringae infection. This systemic up-regulation is missing in the SAR-defective SA pathway mutants sid2 and npr1, as well as in the defense mutant ndr1, indicating a close correlation between systemic FMO1 expression and SAR establishment. Our findings suggest that the presence of the FMO1 gene product in systemic tissue is critical for the development of SAR, possibly by synthesis of a metabolite required for the transduction or amplification of a signal during the early phases of SAR establishment in systemic leaves.