The Arabidopsis NPR1 gene that controls systemic acquired resistance encodes a novel protein containing ankyrin repeats

Ubiquitination-mediated protein degradation and modification: an emerging theme in plant-microbe interactions

Post-translational modification is central to protein stability and to the modulation of protein activity. Various types of protein modification, such as phosphorylation, methylation, acetylation, myristoylation, glycosylation, and ubiquitination, have been reported. Among them, ubiquitination distinguishes itself from others in that most of the ubiquitinated proteins are targeted to the 26S proteasome for degradation. The ubiquitin/26S proteasome system constitutes the major protein degradation pathway in the cell. In recent years, the importance of the ubiquitination machinery in the control of numerous eukaryotic cellular functions has been increasingly appreciated. Increasing number of E3 ubiquitin ligases and their substrates, including a variety of essential cellular regulators have been identified. Studies in the past several years have revealed that the ubiquitination system is important for a broad range of plant developmental processes and responses to abiotic and biotic stresses. This review discusses recent advances in the functional analysis of ubiquitination-associated proteins from plants and pathogens that play important roles in plant-microbe interactions.

An essential role for salicylic acid in AtMYB30-mediated control of the hypersensitive cell death program in Arabidopsis

Salicylic acid (SA) plays a central role in resistance and defense induction in response to pathogen attack, but its role in the activation of the hypersensitive response (HR), a form of programmed cell death associated with resistance of plants, remains to be elucidated. AtMYB30, a R2R3-MYB transcriptional factor which acts as a positive regulator of the HR, is a good model for studying the role of SA in programmed cell death. Here, we demonstrate that AtMYB30 expression in response to an HR-inducing bacterial pathogen is dependent on SA accumulation, but NPR1-independent. Alterations of AtMYB30 expression (overexpression, depletion by antisense strategy, T-DNA insertion mutant) modulate SA levels and SA-associated gene expression. Additionally, mutants or transgenic lines altered in SA accumulation (nahG, sid1, sid2), but not those affected in SA signalling (npr1), abolish the accelerated cell death phenotype conferred by over-expression of AtMYB30. These results suggest that AtMYB30 is involved in an amplification loop or signalling cascade that modulates SA synthesis, which in turn modulates cell death.

WRKY70 modulates the selection of signaling pathways in plant defense

Cross-talk between signal transduction pathways is a central feature of the tightly regulated plant defense signaling network. The potential synergism or antagonism between defense pathways is determined by recognition of the type of pathogen or pathogen-derived elicitor. Our studies have identified WRKY70 as a node of convergence for integrating salicylic acid (SA)- and jasmonic acid (JA)-mediated signaling events during plant response to bacterial pathogens. Here, we challenged transgenic plants altered in WRKY70 expression as well as WRKY70 knockout mutants of Arabidopsis with the fungal pathogens Alternaria brassicicola and Erysiphe cichoracearum to elucidate the role of WRKY70 in modulating the balance between distinct defense responses. Gain or loss of WRKY70 function causes opposite effects on JA-mediated resistance to A. brassicicola and the SA-mediated resistance to E. cichoracearum. While the up-regulation of WRKY70 caused enhanced resistance to E. cichoracearum, it compromised plant resistance to A. brassicicola. Conversely, down-regulation or insertional inactivation of WRKY70 impaired plant resistance to E. cichoracearum. Over-expression of WRKY70 resulted in the suppression of several JA responses including expression of a subset of JA- and A. brassicicola-responsive genes. We show that this WRKY70-controlled suppression of JA-signaling is partly executed by NPR1. The results indicate that WRKY70 has a pivotal role in determining the balance between SA-dependent and JA-dependent defense pathways.

Physical and functional interactions between pathogen-induced Arabidopsis WRKY18, WRKY40, and WRKY60 transcription factors

Limited information is available about the roles of specific WRKY transcription factors in plant defense. We report physical and functional interactions between structurally related and pathogen-induced WRKY18, WRKY40, and WRKY60 transcription factors in Arabidopsis thaliana. The three WRKY proteins formed both homocomplexes and heterocomplexes and DNA binding activities were significantly shifted depending on which WRKY proteins were present in these complexes. Single WRKY mutants exhibited no or small alterations in response to the hemibiotrophic bacterial pathogen Pseudomonas syringae and the necrotrophic fungal pathogen Botrytis cinerea. However, wrky18 wrky40 and wrky18 wrky60 double mutants and the wrky18 wrky40 wrky60 triple mutant were substantially more resistant to P. syringae but more susceptible to B. cinerea than wild-type plants. Thus, the three WRKY proteins have partially redundant roles in plant responses to the two distinct types of pathogens, with WRKY18 playing a more important role than the other two. The contrasting responses of these WRKY mutants to the two pathogens correlated with opposite effects on pathogen-induced expression of salicylic acid-regulated PATHOGENESIS-RELATED1 and jasmonic acid-regulated PDF1.2. While constitutive expression of WRKY18 enhanced resistance to P. syringae, its coexpression with WRKY40 or WRKY60 made plants more susceptible to both P. syringae and B. cinerea. These results indicate that the three WRKY proteins interact both physically and functionally in a complex pattern of overlapping, antagonistic, and distinct roles in plant responses to different types of microbial pathogens.

Plum pox virus induces differential gene expression in the partially resistant stone fruit tree Prunus armeniaca cv. Goldrich

We investigated the changes in the expression profiles of the partially resistant apricot (Prunus armeniaca L.) cultivar Goldrich following inoculation with Plum pox virus (PPV) using cDNA-amplification fragment length polymorphism (AFLP). Altered expression patterns were detected and twenty-one differentially expressed cDNA had homologies with genes in databases coding for proteins involved in metabolism, signal transduction, defense, stress and intra/intercellular connections. Seven of the modified expressed patterns were further investigated by semi-quantitative RT-PCR or Northern blotting. The expression patterns of five of these genes were confirmed in the partially resistant P. armeniaca cv. 'Goldrich' and assessed in a susceptible genotype. One of these cDNAs, coding for a putative class III chitinase, appeared to be repressed in infected plants of the partially resistant genotype and expressed in the susceptible one which could be related to the partially resistant phenotype. On the contrary, the expression patterns of the genes coding for a transketolase, a kinesin-like and an ankyrin-like protein, were clearly linked to the susceptible interaction. These candidate genes could play a role either in the compatible interaction leading to virus invasion or to the quantitative resistance of apricot to PPV.

Ozone-induced expression of the Arabidopsis FAD7 gene requires salicylic acid, but not NPR1 and SID2

The Arabidopsis FAD7 gene encodes a plastid omega-3 fatty acid desaturase that catalyzes the desaturation of dienoic fatty acids to trienoic fatty acids in chloroplast membrane lipids. The expression of FAD7 was rapidly and locally induced by ozone exposure, which causes oxidative responses equivalent to pathogen-induced hypersensitive responses and subsequently activates various defense-related genes. This induction was reduced in salicylic acid (SA)-deficient NahG plants expressing SA hydroxylase, but was unaffected in etr1 and jar1 mutants, which are insensitive to ethylene and jasmonic acid (JA), respectively. The SA dependence of the FAD7 induction was confirmed by the exogenous application of SA. SA-induced expression of FAD7 in the npr1 mutant which is defective in an SA signaling pathway occurred to the same extent as in the wild type. Furthermore, in the sid2 mutant which lacks an enzyme required for SA biosynthesis, the expression of FAD7 was induced by ozone exposure. These results suggest that the ozone-induced expression of FAD7 gene requires SA, but not ethylene, JA, NPR1 and SID2.

Fine-Tuning Plant Defence Signalling: Salicylate versus Jasmonate

Plant defences against pathogens and herbivorous insects form a comprehensive network of interacting signal transduction pathways. The signalling molecules salicylic acid (SA) and jasmonic acid (JA) play important roles in this network. SA is involved in signalling processes providing systemic acquired resistance (SAR), protecting the plant from further infection after an initial pathogen attack. SAR is long-lasting and provides broad spectrum resistance to biotrophic pathogens that feed on a living host cell. The regulatory protein NPR1 is a central positive regulator of SAR. SA-activated NPR1 localizes to the nucleus where it interacts with TGA transcription factors to induce the expression of a large set of pathogenesis-related proteins that contribute to the enhanced state of resistance. In a distinct signalling process, JA protects the plant from insect infestation and necrotrophic pathogens that kill the host cell before feeding. JA activates the regulatory protein COI1 that is part of the E3 ubiquitin ligase-containing complex SCFCOI1, which is thought to derepress JA-responsive genes involved in plant defence. Both synergistic and antagonistic interactions have been observed between SA- and JA-dependent defences. NPR1 has emerged as a critical modulator of cross-talk between the SA and JA signal and is thought to aid in fine tuning defence responses specific to the encountered attacker. Here we review SA- and JA-dependent signal transduction and summarize our current understanding of the molecular mechanisms of cross-talk between these defences.

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a-beta-glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10-100 microm) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, alpha-linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10-beta-glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

The membrane-anchored BOTRYTIS-INDUCED KINASE1 plays distinct roles in Arabidopsis resistance to necrotrophic and biotrophic pathogens

Plant resistance to disease is controlled by the combination of defense response pathways that are activated depending on the nature of the pathogen. We identified the Arabidopsis thaliana BOTRYTIS-INDUCED KINASE1 (BIK1) gene that is transcriptionally regulated by Botrytis cinerea infection. Inactivation of BIK1 causes severe susceptibility to necrotrophic fungal pathogens but enhances resistance to a virulent strain of the bacterial pathogen Pseudomonas syringae pv tomato. The response to an avirulent bacterial strain is unchanged, limiting the role of BIK1 to basal defense rather than race-specific resistance. The jasmonate- and ethylene-regulated defense response, generally associated with resistance to necrotrophic fungi, is attenuated in the bik1 mutant based on the expression of the plant defensin PDF1.2 gene. bik1 mutants show altered root growth, producing more and longer root hairs, demonstrating that BIK1 is also required for normal plant growth and development. Whereas the pathogen responses of bik1 are mostly dependent on salicylic acid (SA) levels, the nondefense responses are independent of SA. BIK1 is membrane-localized, suggesting possible involvement in early stages of the recognition or transduction of pathogen response. Our data suggest that BIK1 modulates the signaling of cellular factors required for defense responses to pathogen infection and normal root hair growth, linking defense response regulation with that of growth and development.

Whole-Genome Microarray in Arabidopsis Facilitates Global Analysis of Retained Introns

Alternative splicing (AS) is an important post-transcriptional regulatory mechanism that can increase protein diversity and affect mRNA stability. Different types of AS have been observed; these include exon skipping, alternative donor or acceptor site and intron retention. In humans, exon skipping is the most common type while intron retention is rare. In contrast, in Arabidopsis, intron retention is the most prevalent AS type (approximately 40%). Here we show that direct transcript expression analysis using high-density oligonucleotide-based whole-genome microarrays (WGAs) is particularly amenable for assessing global intron retention in Arabidopsis. By applying a novel algorithm retained introns are detected in 8% of the transcripts examined. A sampling of 14 transcripts showed that 86% can be confirmed by RT-PCR. This rate of detection predicts an overall total AS rate of 20% for Arabidopsis compared with 10-22% based on EST/cDNA-based analysis. These findings will facilitate monitoring constitutive and dynamic whole-genome splicing on the next generation WGA slides.

Role of salicylic acid and fatty acid desaturation pathways in ssi2-mediated signaling

Stearoyl-acyl carrier protein desaturase-mediated conversion of stearic acid to oleic acid (18:1) is the key step that regulates the levels of unsaturated fatty acids (FAs) in cells. Our previous work with the Arabidopsis (Arabidopsis thaliana) ssi2/fab2 mutant and its suppressors demonstrated that a balance between glycerol-3-phosphate (G3P) and 18:1 levels is critical for the regulation of salicylic acid (SA)- and jasmonic acid-mediated defense signaling in the plant. In this study, we have evaluated the role of various genes that have an impact on SA, resistance gene-mediated, or FA desaturation (FAD) pathways on ssi2-mediated signaling. We show that ssi2-triggered resistance is dependent on EDS1, PAD4, EDS5, SID2, and FAD7 FAD8 genes. However, ssi2-triggered defects in the jasmonic acid pathway, morphology, and cell death phenotypes are independent of the EDS1, EDS5, PAD4, NDR1, SID2, FAD3, FAD4, FAD5, DGD1, FAD7, and FAD7 FAD8 genes. Furthermore, the act1-mediated rescue of ssi2 phenotypes is also independent of the FAD2, FAD3, FAD4, FAD5, FAD7, and DGD1 genes. Since exogenous application of glycerol converts wild-type plants into ssi2 mimics, we also studied the effect of exogenous application of glycerol on mutants impaired in resistance-gene signaling, SA, or fad pathways. Glycerol increased SA levels and induced pathogenesis-related gene expression in all but sid2, nahG, fad7, and fad7 fad8 plants. Furthermore, glycerol-induced phenotypes in various mutant lines correlate with a concomitant reduction in 18:1 levels. Inability to convert glycerol into G3P due to a mutation in the nho1-encoded glycerol kinase renders plants tolerant to glycerol and unable to induce the SA-dependent pathway. A reduction in the NHO1-derived G3P pool also results in a partial age-dependent rescue of the ssi2 morphological and cell death phenotypes in the ssi2 nho1 plants. The glycerol-mediated induction of defense was not associated with any major changes in the lipid profile and/or levels of phosphatidic acid. Taken together, our results suggest that glycerol application and the ssi2 mutation in various mutant backgrounds produce similar effects and that restoration of ssi2 phenotypes is not associated with the further desaturation of 18:1 to linoleic or linolenic acids in plastidal or extraplastidal lipids.

Identification of NPR1-dependent and independent genes early induced by salicylic acid treatment in Arabidopsis

Salicylic acid (SA) plays a crucial role in stress resistance in plants by modifying the expression of a battery of genes. In this paper, we report the identification of a group of early SA-regulated genes of Arabidopsis (activated between 0.5-2.5 h), using the cDNA-amplified fragment length polymorphism technique (cDNA-AFLP). Using 128 different primer combinations, we identified several genes based on their differential expression during SA treatment. Among these, we identified 12 genes up-regulated by SA whose patterns of induction were confirmed by Northern analysis. The identified genes can be grouped into two functional groups: Group 1: genes involved in cell protection (i.e. glycosyltransferases, glutathion S-transferases), and Group 2: genes involved in signal transduction (protein kinases and transcription factors). We also evaluated NPR1 requirement for the induction of the 12 up-regulated genes, and found that only those belonging to Group 2 require this co-activator for their expression. In silico analysis of the promoter sequences of the up-regulated genes, allowed us to identify putative cis-elements over-represented in these genes. Interestingly, as-1-like elements, previously characterized as SA-responsive elements, were specifically over-represented in Group 1 genes. The identification of early SA-regulated genes is an important step towards understanding the complex role of this hormone in plant stress resistance.

Structure-function analysis of the plasma membrane- localized Arabidopsis defense component ACD6

The ACCELERATED CELL DEATH 6 (ACD6) protein, composed of an ankyrin-repeat domain and a predicted transmembrane region, is a necessary positive regulator of Arabidopsis defenses. ACD6 overexpression confers enhanced disease resistance by priming stronger and quicker defense responses during pathogen infection, plant development or treatment with an agonist of the key defense regulator salicylic acid (SA). Modulation of ACD6 affects both SA-dependent and SA-independent defenses. ACD6 localizes to the plasma membrane and is an integral membrane protein with a cytoplasmic ankyrin domain. An activated version of ACD6 with a predicted transmembrane helix mutation called ACD6-1 has the same localization and overall topology as the wild-type protein. A genetic screen for mutants that suppress acd6-1-conferred phenotypes identified 17 intragenic mutations of ACD6. The majority of these mutations reside in the ankyrin domain and in predicted transmembrane helices, suggesting that both ankyrin and transmembrane domains are important for ACD6 function. One mutation (S638F) also identified a key residue in a putative loop between two transmembrane helices. This mutation did not alter the stability or localization of ACD6, suggesting that S635 is a critical residue for ACD6 function. Based on structural modeling, two ankyrin domain mutations are predicted to be in surface-accessible residues. As ankyrin repeats are protein interaction modules, these mutations may disrupt protein-protein interactions. A plausible scenario is that information exchange between the ankyrin and transmembrane domains is involved in activating defense signaling.

High humidity represses Cf-4/Avr4- and Cf-9/Avr9-dependent hypersensitive cell death and defense gene expression

Gene-for-gene resistance is a well-known type of plant disease resistance. It is governed by plant resistance (R) genes and their matching pathogen avirulence (Avr) genes. This resistance is characterized by a hypersensitive response (HR) resulting from the interaction between products of a complementary R and Avr gene pair. The pathosystem of tomato (Lycopersicon esculentum Mill.) and its leaf mold fungal pathogen Cladosporium fulvum is a model system to study gene-for-gene resistance. HR occurs in tomato seedlings carrying a tomato Cf resistance gene and a matching C. fulvum Avr gene, including, for example, Cf-4/Avr4 and Cf-9/Avr9. Employing Cf/Avr tomato seedlings that both express a Cf gene and the matching Avr gene, here we report that both Cf-4/Avr4- and Cf-9/Avr9-dependent HR is delayed and reduced under high humidity (95%), and that Cf-9/Avr9-dependent HR is more sensitive to high humidity when compared to Cf-4/Avr4-dependent HR. Furthermore, high humidity acts synergistically with high temperature on HR suppression, resulting in complete blocking of the HR. The transcript profile of over 60 genes, related to HR, signaling and defense, in Cf/Avr seedlings grown under high humidity and thus showing no HR, significantly differed from that of the same seedlings grown under normal humidity and thus showing HR. These results demonstrate that high humidity probably acts at a very early point of the Cf downstream signaling pathway, or alternatively influences the interaction between the Cf and the Avr proteins.

No role for bacterially produced salicylic Acid in rhizobacterial induction of systemic resistance in Arabidopsis

ABSTRACT The role of bacterially produced salicylic acid (SA) in the induction of systemic resistance in plants by rhizobacteria is far from clear. The strong SA producer Pseudomonas fluorescens WCS374r induces resistance in radish but not in Arabidopsis thaliana, whereas application of SA leads to induction of resistance in both plant species. In this study, we compared P. fluorescens WCS374r with three other SA-producing fluorescent Pseudomonas strains, P. fluorescens WCS417r and CHA0r, and P. aeruginosa 7NSK2 for their abilities to produce SA under different growth conditions and to induce systemic resistance in A. thaliana against bacterial speck, caused by P. syringae pv. tomato. All strains produced SA in vitro, varying from 5 fg cell(-1) for WCS417r to >25 fg cell(-1) for WCS374r. Addition of 200 muM FeCl(3) to standard succinate medium abolished SA production in all strains. Whereas the incubation temperature did not affect SA production by WCS417r and 7NSK2, strains WCS374r and CHA0r produced more SA when grown at 33 instead of 28 degrees C. WCS417r, CHA0r, and 7NSK2 induced systemic resistance apparently associated with their ability to produce SA, but WCS374r did not. Conversely, a mutant of 7NSK2 unable to produce SA still triggered induced systemic resistance (ISR). The possible involvement of SA in the induction of resistance was evaluated using SA-nonaccumulating transgenic NahG plants. Strains WCS417r, CHA0r, and 7NSK2 induced resistance in NahG Arabidopsis. Also, WCS374r, when grown at 33 or 36 degrees C, triggered ISR in these plants, but not in ethylene-insensitive ein2 or in non-plant pathogenesis- related protein-expressing npr1 mutant plants, irrespective of the growth temperature of the bacteria. These results demonstrate that, whereas WCS374r can be manipulated to trigger ISR in Arabidopsis, SA is not the primary determinant for the induction of systemic resistance against bacterial speck disease by this bacterium. Also, for the other SAproducing strains used in this study, bacterial determinants other than SA must be responsible for inducing resistance.

Genetic analysis of developmentally regulated resistance to downy mildew (Hyaloperonospora parasitica) in Arabidopsis thaliana

Although developmentally regulated disease resistance has been observed in a variety of plant-pathogen interactions, the molecular basis of this phenomenon is not well understood. Arabidopsis thaliana ecotype Columbia-0 (Col-0) expresses a developmentally regulated resistance to Hyaloperonospora parasitica isolate Emco5. Col-0 seedlings support profuse mycelial growth and asexual spore formation in the cotyledons. In contrast, Emco5 growth and reproduction is dramatically (but not completely) restricted in the first set of true leaves. Subsequent leaves exhibit progresssively increased resistance. This adult resistance is strongly suppressed by expression of the salicylic acid-degrading transgene NahG and by loss-of-function mutations in the defense-response regulators PAD4, NDR1, RAR1, PBS3, and NPR1. In contrast to Col-0, the Wassilewskija-0 (Ws-0) ecotype supports profuse growth of Emco5 at all stages of development. Gene-dosage experiments and segregation patterns indicate that adult susceptibility in Ws-0 is incomepletely dominant to adult resistance in Col-0. Genetic mapping in a Col x Ws F2 population revealed a major locus on the bottom arm of chromosome 5, which we named RPP31. Analysis of T-DNA insertion lines indicated that the Columbia allele of RPP8, though tightly linked to RPP31, is not necessary for adult resistance.

An importin alpha homolog, MOS6, plays an important role in plant innate immunity

Plant disease resistance is the consequence of an innate defense mechanism mediated by Resistance (R) genes [1]. The conserved structure of one class of R protein is reminiscent of Toll-like receptors (TLRs) and Nucleotide binding oligomerization domain (NOD) proteins-immune-response perception modules in animal cells [2, 3, and 4]. The Arabidopsis snc1 (suppressor of npr1-1, constitutive, 1) mutant contains a mutation in a TIR-NBS-LRR-type of R gene that renders resistance responses constitutively active without interaction with pathogens [5]. Few components of the downstream signaling network activated by snc1 are known. To search for regulators of R-gene-mediated resistance, we screened for genetic suppressors of snc1. Three alleles of the mutant mos6 (modifier of snc1, 6) partially suppressed constitutive-resistance responses and immunity to virulent pathogens in snc1. Furthermore, the mos6-1 single mutant exhibited enhanced disease susceptibility to a virulent oomycete pathogen. MOS6, identified by positional cloning, encodes importin alpha3, one of eight alpha importins in Arabidopsis [6]. alpha importins mediate the import of specific proteins across the nuclear envelope. We previously reported that MOS3, a protein homologous to human nucleoporin 96, is required for constitutive resistance in snc1 [7]. Our data highlight an essential role for nucleo-cytoplasmic trafficking, especially protein import, in plant innate immunity.

Down regulation of virulence factors of Pseudomonas aeruginosa by salicylic acid attenuates its virulence on Arabidopsis thaliana and Caenorhabditis elegans

Salicylic acid (SA) is a phenolic metabolite produced by plants and is known to play an important role in several physiological processes, such as the induction of plant defense responses against pathogen attack. Here, using the Arabidopsis thaliana-Pseudomonas aeruginosa pathosystem, we provide evidence that SA acts directly on the pathogen, down regulating fitness and virulence factor production of the bacteria. Pseudomonas aeruginosa PA14 showed reduced attachment and biofilm formation on the roots of the Arabidopsis mutants lox2 and cpr5-2, which produce elevated amounts of SA, as well as on wild-type Arabidopsis plants primed with exogenous SA, a treatment known to enhance endogenous SA concentration. Salicylic acid at a concentration that did not inhibit PA14 growth was sufficient to significantly affect the ability of the bacteria to attach and form biofilm communities on abiotic surfaces. Furthermore, SA down regulated three known virulence factors of PA14: pyocyanin, protease, and elastase. Interestingly, P. aeruginosa produced more pyocyanin when infiltrated into leaves of the Arabidopsis transgenic line NahG, which accumulates less SA than wild-type plants. This finding suggests that endogenous SA plays a role in down regulating the synthesis and secretion of pyocyanin in vivo. To further test if SA directly affects the virulence of P. aeruginosa, we used the Caenorhabditis elegans-P. aeruginosa infection model. The addition of SA to P. aeruginosa lawns significantly diminished the bacterium's ability to kill the worms, without affecting the accumulation of bacteria inside the nematodes' guts, suggesting that SA negatively affects factors that influence the virulence of P. aeruginosa. We employed microarray technology to identify SA target genes. These analyses showed that SA treatment affected expression of 331 genes. It selectively repressed transcription of exoproteins and other virulence factors, while it had no effect on expression of housekeeping genes. Our results indicate that in addition to its role as a signal molecule in plant defense responses, SA works as an anti-infective compound by affecting the physiology of P. aeruginosa and ultimately attenuating its virulence.

Tobacco bZIP transcription factor TGA2.2 and related factor TGA2.1 have distinct roles in plant defense responses and plant development

Salicylic acid (SA) is a crucial internal signaling molecule needed for the induction of plant defense responses upon attack of a variety of pathogens. Basic leucine zipper transcription factors of the TGA family bind to activating sequence-1 (as-1)-like elements which are SA-responsive cis elements found in promoters of 'immediate early' and 'late' SA-inducible genes. TGA2.2 constitutes the main component of tobacco as-1-binding factor-1 (ASF-1). TGA2.1, which differs from TGA2.2 by being able to activate transcription in yeast, constitutes a minor fraction of the complex. Both proteins interact with NPR1, a protein essential for SA inducibility of 'late' genes. Here we demonstrate using dsRNAi mediated gene silencing that reducing the amount of TGA2.2 and TGA2.1 correlates with a significant decrease in ASF-1 activity and with a decreased inducibility of both 'immediate early' and 'late' genes. In contrast, reducing the amount of TGA2.1 alone had no effect on the expression of these target genes suggesting that TGA2.1 is dispensable for SA-inducible gene expression from the as-1 element. Expression of a TGA2.2 mutant unable to form heterodimers with the endogenous pool of TGA factors led to reduced SA-inducibility of 'immediate early' gene Nt103, indicating that the native leucine zipper is important for the protein to act positively on transcription. Plants with reduced amounts of TGA2.1 developed petal like stamens indicating a regulatory role of TGA2.1 in defining organ identity in tobacco flowers. A model is suggested that unifies conflicting results on the function of tobacco TGA factors with respect to activation of the 'late' PR-1a promoter.

Regulation of plant defense responses in Arabidopsis by EDR2, a PH and START domain-containing protein

We have identified an Arabidopsis mutant that displays enhanced disease resistance (edr2) to the biotrophic powdery mildew pathogen Erysiphe cichoracearum. Inhibition of fungal growth on edr2 mutant leaves occurred at a late stage of the infection process and coincided with formation of necrotic lesions approximately 5 days after inoculation. Double-mutant analysis revealed that edr2-mediated resistance is suppressed by mutations that inhibit salicylic acid (SA)-induced defense signaling, including npr1, pad4 and sid2, demonstrating that edr2-mediated disease resistance is dependent on SA. However, edr2 showed normal responses to the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000. EDR2 appears to be constitutively transcribed in all tissues and organs and encodes a novel protein, consisting of a putative pleckstrin homology (PH) domain and a steroidogenic acute regulatory protein-related lipid-transfer (START) domain, and contains an N-terminal mitochondrial targeting sequence. The PH and START domains are implicated in lipid binding, suggesting that EDR2 may provide a link between lipid signaling and activation of programmed cell death mediated by mitochondria.

The Arabidopsis csb3 mutant reveals a regulatory link between salicylic acid-mediated disease resistance and the methyl-erythritol 4-phosphate pathway

We report on constitutive subtilisin3 (csb3), an Arabidopsis mutant showing strikingly enhanced resistance to biotrophic pathogens. Epistasis analyses with pad4, sid2, eds5, NahG, npr1, dth9 and cpr1 mutants revealed that the enhanced resistance of csb3 plants requires intact salicylic acid (SA) synthesis and perception. CSB3 encodes a 1-hydroxy-2-methyl-2-butenyl 4-diphosphate synthase, the enzyme controlling the penultimate step of the biosynthesis of isopentenyl diphosphate via the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway in the chloroplast. CSB3 is expressed constitutively in healthy plants, and shows repression in response to bacterial infection. We also show the pharmacological complementation of the enhanced-resistance phenotype of csb3 plants with fosmidomycin, an inhibitor of the MEP pathway, and propose that CSB3 represents a point of metabolic convergence modulating the magnitude of SA-mediated disease resistance to biotrophic pathogens.

Potato homologs of Arabidopsis thaliana genes functional in defense signaling--identification, genetic mapping, and molecular cloning

Defense against pests and pathogens is a fundamental process controlled by similar molecular mechanisms in all flowering plants. Using Arabidopsis thaliana as a model, steps of the signal transduction pathways that link pathogen recognition to defense activation have been identified and corresponding genes have been characterized. Defense signaling (DS) genes are functional candidates for controlling natural quantitative variation of resistance to plant pathogens. Nineteen Arabidopsis genes operating in defense signaling cascades were selected. Solanaceae EST (expressed sequence tag) databases were employed to identify the closest homologs of potato (Solanum tuberosum). Sixteen novel DS potato homologs were positioned on the molecular maps. Five DS homologs mapped close to known quantitative resistance loci (QRL) against the oomycete Phytophthora infestans causing late blight and the bacterium Erwinia carotovora subsp. atroseptica causing blackleg of stems and tuber soft rot. The five genes are positional candidates for QRL and are highly sequence related to Arabidopsis genes AtSGT1b, AtPAD4, and AtAOS. Full-length complementary DNA and genomic sequences were obtained for potato genes StSGT1, StPAD4, and StEDS1, the latter being a putative interactor of StPAD4. Our results form the basis for further studies on the contributions of these candidate genes to natural variation of potato disease resistance.

A single amino acid insertion in the WRKY domain of the Arabidopsis TIR-NBS-LRR-WRKY-type disease resistance protein SLH1 (sensitive to low humidity 1) causes activation of defense responses and hypersensitive cell death

In this study we characterized the sensitive to low humidity 1 (slh1) mutant of Arabidopsis ecotype No-0 which exhibits normal growth on agar plate medium but which on transfer to soil shows growth arrest and development of necrotic lesions. cDNA microarray hybridization and RNA gel blot analysis revealed that genes associated with activation of disease resistance were upregulated in the slh1 mutants in response to conditions of low humidity. Furthermore, the slh1 mutants accumulate callose, autofluorescent compounds and salicylic acid (SA). We demonstrate that SA is required for the slh1 phenotype but not PAD4 or NPR1. SLH1 was isolated by map-based cloning and it encodes a resistance (R)-like protein consisting of a domain with Toll and interleukin-1 receptor homology (TIR), a nucleotide-binding domain (NB), leucine-rich repeats (LRR) and a carboxy-terminal WRKY domain. SLH1 is identical to the R gene RRS1-R of the Arabidopsis ecotype Nd-1, a gene which confers resistance to the bacterial pathogen Ralstonia solanacearum GMI1000 and also functions as an R gene to this pathogen in No-0. We identified a 3 bp insertion mutation in slh1 that results in the addition of a single amino acid in the WRKY domain; thereby impairing its DNA-binding activity. Our data suggest that SLH1 disease resistance signaling may be negatively regulated by its WRKY domain in the R protein and that the constitutive defense activation conferred by the slh1 mutation is inhibited by conditions of high humidity.

Rice NRR, a negative regulator of disease resistance, interacts with Arabidopsis NPR1 and rice NH1

Arabidopsis NPR1/NIM1 is a key regulator of systemic acquired resistance (SAR), which confers lasting broad-spectrum resistance. Over-expression of Arabidopsis NPR1 or the NPR1 homolog 1 (NH1) in rice results in enhanced resistance to the pathogen Xanthomonasoryzae pv. oryzae (Xoo), suggesting the presence of a related defense pathway in rice. We investigated this pathway in rice by identifying proteins that interact with NH1. Here we report the isolation and characterization of a rice cDNA encoding a novel protein, named NRR (for negative regulator of resistance). NRR interacts with NPR1 in the NPR1-interacting domain (NI25) consisting of 25 amino acids. NRR also interacts with NH1; however, NI25 was not sufficient for a strong interaction, indicating a difference between the rice and the Arabidopsis proteins. Silencing of NRR in rice had little effect on resistance to Xoo. When constitutively over-expressed in rice, NRR affected basal resistance, age-related resistance and Xa21-mediated resistance, causing enhanced susceptibility to Xoo. This phenotype was correlated with elevated NRR mRNA and protein levels and increased Xoo growth. Over-expression of NRR suppressed the induction of defense-related genes. NRR:GFP (green fluorescent protein) protein was localized to the nucleus, indicating that NRR may act directly to suppress the activation of defense genes. The fact that NRR compromises Xa21-mediated resistance indicates cross-talk or overlap between NH1- and Xa21-mediated pathways.

Induced disease resistance and gene expression in cereals

Disease resistance strategies reduce chemical input into the environment and are therefore powerful approaches to sustainable agriculture. Induced resistance (IR) has emerged as a potential alternative, or a complementary strategy, for crop protection. IR signifies the control of pathogens and pests by prior activation of plant defence pathways. A molecular understanding of IR in cereals, including the most important global crops wheat and rice, has been largely missing. Evidence indicating that central elements of IR pathways are conserved among Di- and Monocotyledoneae has only recently been presented, although their regulation and interaction with other plant pathways may be quite divergent. We present here a synopsis of current molecular knowledge of cereal IR mechanisms.

Identification and characterization of GIP1, an Arabidopsis thaliana protein that enhances the DNA binding affinity and reduces the oligomeric state of G-box binding factors

Environmental control of the alcohol dehydrogenase (Adh) and other stress response genes in plants is in part brought about by transcriptional regulation involving the G-box cis-acting DNA element and bZIP G-box Binding Factors (GBFs). The mechanisms of GBF regulation and requirements for additional factors in this control process are not well understood. In an effort to identify potential GBF binding and control partners, maize GBF1 was used as bait in a yeast two-hybrid screen of an A. thaliana cDNA library. GBF Interacting Protein 1 (GIP1) arose from the screen as a 496 amino acid protein with a predicted molecular weight of 53,748 kDa that strongly interacts with GBFs. Northern analysis of A. thaliana tissue suggests a 1.8-1.9 kb GIP1 transcript, predominantly in roots. Immunolocalization studies indicate that GIP1 protein is mainly localized to the nucleus. In vitro electrophoretic mobility shift assays using an Adh G-box DNA probe and recombinant A. thaliana GBF3 or maize GBF1, showed that the presence of GIP1 resulted in a tenfold increase in GBF DNA binding activity without altering the migration, suggesting a transient association between GIP1 and GBF. Addition of GIP1 to intentionally aggregated GBF converted GBF to lower molecular weight macromolecular complexes and GIP1 also refolded denatured rhodanese in the absence of ATP. These data suggest GIP1 functions to enhance GBF DNA binding activity by acting as a potent nuclear chaperone or crowbar, and potentially regulates the multimeric state of GBFs, thereby contributing to bZIP-mediated gene regulation.

Alteration of TGA factor activity in rice results in enhanced tolerance to Xanthomonas oryzae pv. oryzae

In dicotyledonous plants broad-spectrum resistance to pathogens is established after the induction of the systemic acquired resistance (SAR) response. In Arabidopsis the NPR1 protein can regulate SAR by interacting with members of the TGA class of basic, leucine-zipper transcription factors to alter pathogenesis-related (PR) gene expression. Overexpression of (At)NPR1 in Arabidopsis enhances resistance to multiple pathogens. Similarly, overexpression of (At)NPR1 in rice enhances resistance to the bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). These results suggest that components of the (At)NPR1-mediated SAR defense response may be conserved between monocots and dicots. To determine whether or not rice TGA factors are involved in disease resistance responses, the effect of altering the function of rice TGA2.1 was analyzed in transgenic plants. Transgenic rice overexpressing an rTGA2.1 mutant, that can no longer bind DNA, and transgenic rice that have the endogenous rTGA2.1 silenced by dsRNA-mediated silencing were generated. Both types of transgenic rice displayed increased tolerance to Xoo, were dwarfed, and had altered accumulation of PR genes. The results presented in this study suggest that wild-type rTGA2.1 has primarily a negative role in rice basal defense responses to bacterial pathogens.

Fitness costs of mutations affecting the systemic acquired resistance pathway in Arabidopsis thaliana

This study investigated the fitness effects of four mutations (npr1, cpr1, cpr5, and cpr6) and two transgenic genotypes (NPR1-L and NPR1-H) affecting different points of the systemic acquired resistance (SAR) signaling pathway associated with pathogen defense in Arabidopsis thaliana. The npr1 mutation, which resulted in a failure to express SAR, had no effect on fitness under growth chamber conditions, but decreased fitness in the field. The expression of NPR1 positively correlated with the fitness in the field. Constitutive activation of SAR by cpr1, cpr5, and cpr6 generally decreased fitness in the field and under two nutrient levels in two growth chamber conditions. At low-nutrient levels, fitness differences between wild type and the constitutive mutants were unchanged or reduced (especially in cpr5). The reduced fitness of the constitutive mutants suggests that this pathway is costly, with the precise fitness consequences highly dependent on the environmental context.

Induction of resistance to Verticillium dahliae in Arabidopsis thaliana by the biocontrol agent K-165 and pathogenesis-related proteins gene expression

The biocontrol bacterium Paenibacillus alvei K165 has the ability to protect Arabidopsis thaliana against Verticillium dahliae. A direct antagonistic action of strain K165 against V. dahliae was ruled out, making it likely that K165-mediated protection results from induced systemic resistance (ISR) in the host. K165-mediated protection was tested in various Arabidopsis mutants and transgenic plants impaired in defense signaling pathways, including NahG (transgenic line degrading salicylic acid [SA]), etr1-1 (insensitive to ethylene), jar1-1 (insensitive to jasmonate), npr1-1 (nonexpressing NPR1 protein), pad3-1 (phytoalexin deficient), pad4-1 (phytoalexin deficient), eds5/sid1 (enhanced disease susceptibility), and sid2 (SA-induction deficient). ISR was blocked in Arabidopsis mutants npr1-1, eds5/sid1, and sid2, indicating that components of the pathway from isochorismate and a functional NPR1 play a crucial role in the K165-mediated ISR. Furthermore, the concomitant activation and increased transient accumulation of the PR-1, PR-2, and PR-5 genes were observed in the treatment in which both the inducing bacterial strain and the challenging pathogen were present in the rhizosphere of the A. thaliana plants.

Regulation of plant disease resistance, stress responses, cell death, and ethylene signaling in Arabidopsis by the EDR1 protein kinase

ENHANCED DISEASE RESISTANCE 1 (EDR1) encodes a CTR1-like kinase and was previously reported to function as a negative regulator of disease resistance and ethylene-induced senescence. Here, we report that the edr1 mutant displays enhanced stress responses and spontaneous necrotic lesions under drought conditions in the absence of pathogen, suggesting that EDR1 is also involved in stress response signaling and cell death regulation. Double mutant analysis revealed that these drought-induced phenotypes require salicylic acid but not ethylene signaling pathways. In addition, the edr1-mediated ethylene-induced senescence phenotype was suppressed by mutations in EIN2, but not by mutations in SID2, PAD4, EDS1, or NPR1, suggesting that EDR1 functions at a point of cross talk between ethylene and salicylic acid signaling that impinges on senescence and cell death. Two edr1-associated phenotypes, drought-induced growth inhibition and ethylene-induced senescence, were suppressed by mutations in ORE9, implicating ubiquitin-mediated protein degradation in the regulation of these phenotypes. However, the ore9 mutation did not suppress edr1-mediated enhanced disease resistance to powdery mildew or spontaneous lesions, indicating that these phenotypes are controlled by separate signaling pathways. To investigate the function of the EDR1 kinase domain, we expressed the C-terminal third of EDR1 in wild-type Columbia and edr1 backgrounds under the control of a dexamethasone-inducible promoter. Overexpression of the EDR1 kinase domain in an edr1 background had no obvious effect on edr1-associated phenotypes. However, overexpression of the EDR1 kinase domain in a wild-type Columbia background caused dominant negative phenotypes, including enhanced disease resistance to powdery mildew and enhanced ethylene-induced senescence; thus, the overexpressed EDR1 kinase domain alone does not exert EDR1 function, but rather negatively affects the function of native EDR1 protein.

The Arabidopsis Gene CAD1 Controls Programmed Cell Death in the Plant Immune System and Encodes a Protein Containing a MACPF Domain

To clarify the processes involved in plant immunity, we have isolated and characterized a single recessive Arabidopsis mutant, cad1 (constitutively activated cell death 1), which shows a phenotype that mimics the lesions seen in the hypersensitive response (HR). This mutant shows spontaneously activated expression of pathogenesis-related (PR) genes, and leading to a 32-fold increase in salicylic acid (SA). Inoculation of cad1 mutant plants with Pseudomonas syringae pv tomato DC3000 shows that the cad1 mutation results in the restriction of bacterial growth. Cloning of CAD1 reveals that this gene encodes a protein containing a domain with significant homology to the MACPF (membrane attack complex and perforin) domain of complement components and perforin proteins that are involved in innate immunity in animals. Furthermore, cell death is suppressed in transgenic cad1 plants expressing nahG, which encodes an SA-degrading enzyme. We therefore conclude that the CAD1 protein negatively controls the SA-mediated pathway of programmed cell death in plant immunity.

Overexpression of a rice NPR1 homolog leads to constitutive activation of defense response and hypersensitivity to light

Arabidopsis NPR1/NIM1 is a key regulator of systemic acquired resistance (SAR), which confers lasting broad-spectrum resistance. Previous reports indicate that rice has a disease-resistance pathway similar to the Arabidopsis SAR pathway. Here we report the isolation and characterization of a rice NPR1 homologue (NH1). Transgenic rice plants overexpressing NH1 (NH1ox) acquire high levels of resistance to Xanthomonas oryzae pv. oryzae. The resistance phenotype is heritable and correlates with the presence of the transgene and reduced bacterial growth. Northern analysis shows that NH1ox rice spontaneously activates defense genes, contrasting with NPR1-overexpressing Arabidopsis, where defense genes are not activated until induction. Wild-type NH1, but not a point mutant corresponding to npr1-1, interacts strongly with the rice transcription factor rTGA2.2 in yeast two-hybrid. Greenhouse-grown NH1ox plants develop lesion-mimic spots on leaves at preflowering stage although no other developmental effects are observed. However, when grown in growth chambers (GCs) under low light, NH1ox plants are dwarfed, indicating elevated sensitivity to light. The GC-grown NH1ox plants show much higher salicylic acid (SA) levels than the wild type, whereas greenhouse-grown NH1ox plants contain lower SA. These results indicate that NH1 may be involved in the regulation of SA in response to environmental changes.

Transgenic tomato plants expressing the Arabidopsis NPR1 gene display enhanced resistance to a spectrum of fungal and bacterial diseases

Development of effective disease-resistance to a broad-range of pathogens in crops usually requires tremendous resources and effort when traditional breeding approaches are taken. Genetic engineering of disease-resistance in crops has become popular and valuable in terms of cost and efficacy. Due to long-lasting and broad-spectrum of effectiveness against pathogens, employment of systemic acquired resistance (SAR) for the genetic engineering of crop disease-resistance is of particular interest. In this report, we explored the potential of using SAR-related genes for the genetic engineering of enhanced resistance to multiple diseases in tomato. The Arabidopsis NPR1 (nonexpresser of PR genes) gene was introduced into a tomato cultivar, which possesses heat-tolerance and resistance to tomato mosaic virus (ToMV). The transgenic lines expressing NPR1 were normal as regards overall morphology and horticultural traits for at least four generations. Disease screens against eight important tropical diseases revealed that, in addition to the innate ToMV-resistance, the tested transgenic lines conferred significant level of enhanced resistance to bacterial wilt (BW) and Fusarium wilt (FW), and moderate degree of enhanced resistance to gray leaf spot (GLS) and bacterial spot (BS). Transgenic lines that accumulated higher levels of NPR1 proteins exhibited higher levels and a broader spectrum of enhanced resistance to the diseases, and enhanced disease-resistance was stably inherited. The spectrum and degree of these NPR1-transgenic lines are more significant compared to that of transgenic tomatoes reported to date. These transgenic lines may be further explored as future tomato stocks, aiming at building up resistance to a broader spectrum of diseases.

Transcriptional profiling of sorghum induced by methyl jasmonate, salicylic acid, and aminocyclopropane carboxylic acid reveals cooperative regulation and novel gene responses

We have conducted a large-scale study of gene expression in the C4 monocot sorghum (Sorghum bicolor) L. Moench cv BTx623 in response to the signaling compounds salicylic acid (SA), methyl jasmonate (MeJA), and the ethylene precursor aminocyclopropane carboxylic acid. Expression profiles were generated from seedling root and shoot tissue at 3 and 27 h, using a microarray containing 12,982 nonredundant elements. Data from 102 slides and quantitative reverse transcription-PCR data on mRNA abundance from 171 genes were collected and analyzed and are here made publicly available. Numerous gene clusters were identified in which expression was correlated with particular signaling compound and tissue combinations. Many genes previously implicated in defense responded to the treatments, including numerous pathogenesis-related genes and most members of the phenylpropanoid pathway, and several other genes that may represent novel activities or pathways. Genes of the octadecanoic acid pathway of jasmonic acid (JA) synthesis were induced by SA as well as by MeJA. The resulting hypothesis that increased SA could lead to increased endogenous JA production was confirmed by measurement of JA content. Comparison of responses to SA, MeJA, and combined SA+MeJA revealed patterns of one-way and mutual antagonisms, as well as synergistic effects on regulation of some genes. These experiments thus help further define the transcriptional results of cross talk between the SA and JA pathways and suggest that a subset of genes coregulated by SA and JA may comprise a uniquely evolved sector of plant signaling responsive cascades.

Staphylococcus aureus pathogenicity on Arabidopsis thaliana is mediated either by a direct effect of salicylic acid on the pathogen or by SA-dependent, NPR1-independent host responses

Staphylococcus aureus is a ubiquitous gram-positive bacterium that can cause superficial to serious systemic infections in animals and humans. Here we report the development of a plant infection model to study the pathogenesis of this bacterium. Three global regulatory mutants, RN6911 (agr-), ALC 488 (sarA-) ALC 842 (sarA-/agr-) and an alpha-toxin mutant defective in biofilm formation (DU1090) which are attenuated in animal pathogenesis, were also attenuated in their ability to infect plants, suggesting that these regulators that mediate synthesis of virulence factors essential for animal pathogenesis are also required for plant pathogenesis. Further, using Arabidopsis plants altered in defense responses such as the transgenic lines NahG [defective in salicylic acid (SA) accumulation], and 35S-LOX2- (defective in jasmonic acid production and hyper-accumulator of SA), and mutants ics1 (depleted in SA accumulation), and npr1-1 (non-expressor of pathogenesis-related protein) we show that resistance of Arabidopsis to typical plant pathogens and the animal pathogen S. aureus is conserved and is mediated by SA. The data presented here suggest that Arabidopsis thaliana resistance to S. aureus is mediated either by a direct effect of SA on the pathogen, specifically one that affects the attachment/aggregate formation on the root surface and reduces the pathogen's virulence, or by SA-dependent, NPR1-independent host responses.

Salicylic acid (SA)-dependent gene activation can be uncoupled from cell death-mediated gene activation: the SA-inducible NIMIN-1 and NIMIN-2 promoters, unlike the PR-1a promoter, do not respond to cell death signals in tobacco

SUMMARY Tobacco pathogenesis-related (PR) genes of group 1 are induced during pathogen defence (hypersensitive response, HR, and systemic acquired resistance, SAR), after exogenous application of salicylic acid (SA), and by developmental cues. Likewise, SA enhances transcripts for Arabidopsis NIMIN-1 and NIMIN-2, which interact with NPR1/NIM1, a key regulator of SAR. To further illuminate gene activation during pathogen defence, reporter gene expression from the NIMIN-1 and NIMIN-2 promoters was analysed in transgenic tobacco plants in direct comparison to PR-1 gene expression. NIMIN[GUS] chimeric genes were highly sensitive to SA, whereas NIMIN[GUS], unlike PR1a[GUS], expression was only weak in necrotic tissue exhibiting HR. Furthermore, PR-1a, but not NIMIN, promoter constructs were activated systemically in response to local cell death elicited by expression of the proapoptotic Bax gene. Conversely, NIMIN-1[GUS] expression was completely suppressed during pathogen defence in plants depleted from SA, whereas PR-1 proteins still accumulated in necrotic tissue. These findings demonstrate that SA-dependent gene activation can be uncoupled from cell death-induced gene activation. Whereas PR-1a induction during the HR and SAR responses is mediated by HR-associated signals and SA, activation of the NIMIN-1 and NIMIN-2 promoters in infected tobacco relies on SA, but not on cell death signals.

The atypical resistance gene, RPW8, recruits components of basal defence for powdery mildew resistance in Arabidopsis

Genetic studies have identified a number of components of signal transduction pathways leading to plant disease resistance and the accompanying hypersensitive response (HR) following detection of pathogens by plant resistance (R) genes. In Arabidopsis, the majority of R proteins so far characterized belong to a plant superfamily that have a central nucleotide-binding site and C-terminal leucine-rich-repeats (NB-LRRs). Another much less prevalent class comprises RPW8.1 and RPW8.2, two related proteins that possess a putative N-terminal transmembrane domain and a coiled-coil motif, and confer broad-spectrum resistance to powdery mildew. Here we investigated whether RPW8.1 and RPW8.2 engage known pathway(s) for defence signalling. We show that RPW8.1 and RPW8.2 recruit, in addition to salicylic acid and EDS1, the other NB-LRR gene-signalling components PAD4, EDS5, NPR1 and SGT1b for activation of powdery mildew resistance and HR. In contrast, NDR1, RAR1 and PBS3 that are required for function of certain NB-LRR R genes, and COI1 and EIN2 that operate, respectively, in the jasmonic acid and ethylene signalling pathways, do not contribute to RPW8.1 and RPW8.2-mediated resistance. We further demonstrate that EDR1, a gene encoding a conserved MAPKK kinase, exerts negative regulation on HR cell death and powdery mildew resistance by limiting the transcriptional amplification of RPW8.1 and RPW8.2. Our results suggest that RPW8.1 and RPW8.2 stimulate a conserved basal defence pathway that is negatively regulated by EDR1.

Arabidopsis has two redundant Cullin3 proteins that are essential for embryo development and that interact with RBX1 and BTB proteins to form multisubunit E3 ubiquitin ligase complexes in vivo

Cullin-based E3 ubiquitin ligases play important roles in the regulation of diverse developmental processes and environmental responses in eukaryotic organisms. Recently, it was shown in Schizosaccharomyces pombe, Caenorhabditis elegans, and mammals that Cullin3 (CUL3) directly associates with RBX1 and BTB domain proteins in vivo to form a new family of E3 ligases, with the BTB protein subunit functioning in substrate recognition. Here, we demonstrate that Arabidopsis thaliana has two redundant CUL3 (AtCUL3) genes that are essential for embryo development. Besides supporting anticipated specific AtCUL3 interactions with the RING protein AtRBX1 and representative Arabidopsis proteins containing a BTB domain in vitro, we show that AtCUL3 cofractionates and specifically associates with AtRBX1 and a representative BTB protein in vivo. Similar to the AtCUL1 subunit of the SKP1-CUL1-F-box protein-type E3 ligases, the AtCUL3 subunit of the BTB-containing E3 ligase complexes is subjected to modification and possible regulation by the ubiquitin-like protein Related to Ubiquitin in vivo. Together with the presence of large numbers of BTB proteins with diverse structural features and expression patterns, our data suggest that Arabidopsis has conserved AtCUL3-RBX1-BTB protein E3 ubiquitin ligases to target diverse protein substrates for degradation by the ubiquitin/proteasome pathway.

The BLADE ON PETIOLE genes act redundantly to control the growth and development of lateral organs

Developmental processes in multicellular organisms involve an intricate balance between mechanisms that promote cell division activity and growth, and others that promote cell differentiation. Leaf development in Arabidopsis thaliana is controlled by genes like BLADE ON PETIOLE1 (BOP1), which prevent the development of ectopic meristematic activity that leads to the formation of new organs, and JAGGED (JAG), which control the proximodistal development of the leaf by regulating cell-division activity. We have isolated and characterized the BOP1 gene together with a functionally redundant close homolog that we name BOP2. The BOP genes are members of a gene family containing ankyrin repeats and a BTB/POZ domain, suggesting a role in protein-protein interaction. We show that the BOP genes are expressed in the proximal parts of plant lateral organs where they repress the transcription not only of class 1 knox genes but also of JAG. We also show that the BOP genes are acting together with the flower meristem identity gene LEAFY in the suppression of bract formation. These findings show that the BOP genes are important regulators of the growth and development of lateral organs.

Salicylic acid-inducible Arabidopsis CK2-like activity phosphorylates TGA2

We demonstrate that TGA2, TGA5 and TGA6, and TGA3 to a lesser extent, are phosphorylated by an activity in rabbit reticulocytes. Using deletion and point mutagenesis of TGA2, three amino acid (aa) residues, (11)Ser, (12)Thr and (16)Thr, were found to be critical for efficient phosphorylation by a kinase(s) in rabbit reticulocytes. These three residues also were important for phosphorylation by recombinant human Casein Kinase II (CK2) and by a CK2-like kinase in Arabidopsis leaf extracts. Salicylic acid (SA) treatment enhanced the phosphorylation of recombinant TGA2 in vitro; it also enhanced phosphorylation of a TGA2-GFP fusion protein in vivo. By contrast, in vivo phosphorylation of a TGA2-A-GFP fusion protein, in which the (11)Ser, (12)Thr and (16)Thr residues were mutated to non-phosphorylable alanine, was only poorly if at all stimulated by SA treatment. Mutation of the putative CK2 phosphorylation motif did not affect nuclear localization of TGA2. However, the DNA binding activity of TGA2 was reduced by CK2 treatment, whereas that of TGA2-A was unaffected; TGA2's DNA binding activity after incubation in a rabbit reticulocyte lysate also was substantially lower than that of comparably treated TGA2-A. Taken together, these results suggest that phosphorylation at the putative CK2 phosphorylation site negatively regulates the DNA binding activity of TGA2. Analysis of transgenic Arabidopsis overexpressing TGA2-GFP or TGA2-A-GFP, in the absence of SA treatment, revealed that they accumulated similarly elevated levels of PR-1 gene transcripts. Possible reasons why mutations in the putative CK2 phosphorylation site had little effect on PR-1 induction by TGA2 are discussed.

Constitutively elevated salicylic acid signals glutathione-mediated nickel tolerance in Thlaspi nickel hyperaccumulators

Progress is being made in understanding the biochemical and molecular basis of nickel (Ni)/zinc (Zn) hyperaccumulation in Thlaspi; however, the molecular signaling pathways that control these mechanisms are not understood. We observed that elevated concentrations of salicylic acid (SA), a molecule known to be involved in signaling induced pathogen defense responses in plants, is a strong predictor of Ni hyperaccumulation in the six diverse Thlaspi species investigated, including the hyperaccumulators Thlaspi goesingense, Thlaspi rosulare, Thlaspi oxyceras, and Thlaspi caerulescens and the nonaccumulators Thlaspi arvense and Thlaspi perfoliatum. Furthermore, the SA metabolites phenylalanine, cinnamic acid, salicyloyl-glucose, and catechol are also elevated in the hyperaccumulator T. goesingense when compared to the nonaccumulators Arabidopsis (Arabidopsis thaliana) and T. arvense. Elevation of free SA levels in Arabidopsis, both genetically and by exogenous feeding, enhances the specific activity of serine acetyltransferase, leading to elevated glutathione and increased Ni resistance. Such SA-mediated Ni resistance in Arabidopsis phenocopies the glutathione-based Ni tolerance previously observed in Thlaspi, suggesting a biochemical linkage between SA and Ni tolerance in this genus. Intriguingly, the hyperaccumulator T. goesingense also shows enhanced sensitivity to the pathogen powdery mildew (Erysiphe cruciferarum) and fails to induce SA biosynthesis after infection. Nickel hyperaccumulation reverses this pathogen hypersensitivity, suggesting that the interaction between pathogen resistance and Ni tolerance and hyperaccumulation may have played a critical role in the evolution of metal hyperaccumulation in the Thlaspi genus.

Regulation of the Arabidopsis defense transcriptome

Transcriptional re-programming is a key step of plant defense in response to pathogen recognition. Microarray analyses combined with genetic and biochemical approaches are now enabling us to study basic principles and details of regulatory mechanisms controlling the defense transcriptome in Arabidopsis. Recent results show that signaling pathways used by different defense systems converge and target overlapping gene sets. Furthermore, a quantitative mechanism common to multiple defense systems modulates transcript levels of these defense-associated genes. Most importantly, some transcription factors have been proven to play a pivotal role in disease resistance. Regulatory circuits linking signaling and gene regulation are emerging, suggesting that a complex interplay of transcriptional activators and repressors fine-tunes expression of the defense transcriptome.

Molecular and functional characterization of Arabidopsis Cullin 3A

Cullin proteins, which belong to multigenic families in all eukaryotes, associate with other proteins to form ubiquitin protein ligases (E3s) that target substrates for proteolysis by the 26S proteasome. Here, we present the molecular and genetic characterization of a plant Cullin3. In contrast to fungi and animals, the genome of the model plant Arabidopsis thaliana contains two related CUL3 genes, called CUL3A and CUL3B. We found that CUL3A is ubiquitously expressed in plants and is able to interact with the ring-finger protein RBX1. A genomic search revealed the existence of at least 76 BTB-domain proteins in Arabidopsis belonging to 11 major families. Yeast two-hybrid experiments indicate that representative members of certain families are able to physically interact with both CUL3A and CUL3B, suggesting that Arabidopsis CUL3 forms E3 protein complexes with certain BTB domain proteins. In order to determine the function of CUL3A, we used a reverse genetic approach. The cul3a null mutant flowers slightly later than the control plants. Furthermore, this mutant exhibits a reduced sensitivity of the inhibition of hypocotyl growth in far-red light and miss-expresses COP1. The viability of the mutant plants suggests functional redundancy between the two CUL3 genes in Arabidopsis.

Plant defence responses: what have we learnt from Arabidopsis?

To overcome the attack of invading pathogens, a plant's defence system relies on preformed and induced responses. The induced responses are activated following detection of a pathogen, with the subsequent transmission of signals and orchestrated cellular events aimed at eliminating the pathogen and preventing its spread. Numerous studies are proving that the activated signalling pathways are not simply linear, but rather, form complex networks where considerable cross talk takes place. This review covers the recent application of powerful genetic and genomic approaches to identify key defence signalling pathways in the model plant Arabidopsis thaliana (L.) Heynh. The identification of key regulatory components of these pathways may offer new approaches to increase the defence capabilities of crop plants.

Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.

A constitutive PR-1::luciferase expression screen identifies Arabidopsis mutants with differential disease resistance to both biotrophic and necrotrophic pathogens

SUMMARY A complex signal transduction network involving salicylic acid, jasmonic acid and ethylene underlies disease resistance in Arabidopsis. To understand this defence signalling network further, we identified mutants that expressed the marker gene PR-1::luciferase in the absence of pathogen infection. These cir mutants all display constitutive expression of a suite of defence-related genes but exhibit different disease resistance profiles to two biotrophic pathogens, Pseudomonas syringae pv. tomato and Peronospora parasitica NOCO2, and the necrotrophic pathogen Botrytis cinerea. We further characterized cir3, which displays enhanced resistance only to the necrotrophic pathogen. Cir3-mediated resistance to B. cinerea is dependent on accumulated salicylic acid and a functional EIN2 protein.

An Arabidopsis NPR1-like gene, NPR4, is required for disease resistance

The Arabidopsis genome contains six NPR1-related genes. Given the pivotal role played by NPR1 in controlling salicylic acid (SA)-mediated gene expression and disease resistance, functional characterization of other family members appears to be justified. Reverse genetics was used to analyze the role of one NPR1-like gene, which we called NPR4. The NPR4 protein shares 36% identity with NPR1 and interacts with the same spectrum of TGA transcription factors in yeast two-hybrid assays. Plants with T-DNA insertions in NPR4 are more susceptible to the virulent bacterial pathogen Pseudomonas syringe pv. tomato DC3000. This phenotype is complemented by expression of the wild type NPR4 coding region. As determined by the parasite reproduction, the npr4-1 mutant is more susceptible to the fungal pathogen Erysiphe cichoracearum, but does not differ markedly from wild type in its interaction with virulent and avirulent strains of the oomycete Peronospora parasitica. In leaves of wild-type plants, NPR4 mRNA levels increase following pathogen challenge or SA treatment, and decrease rapidly following methyl jasmonic acid (MeJA) treatment. Transcripts of the pathogenesis-related (PR) genes PR-1, PR-2, and PR-5 are only marginally reduced in the npr4-1 mutant following pathogen challenge or SA treatment. This reduction of PR gene expression is more pronounced when leaves are challenged with the bacterial pathogen following SA treatment. Expression of the jasmonic acid-dependent pathway marker gene PDF1.2 is compromised in npr4-1 leaves following application of MeJA or a combination of SA and MeJA. These results indicate that NPR4 is required for basal defense against pathogens, and that it may be implicated in the cross-talk between the SA- and JA-dependent signaling pathways.

Jasmonate: an oxylipin signal with many roles in plants

Jasmonic acid is an oxylipin signaling molecule derived from linolenic acid. So far, jasmonate (JA) (including the free acid and a number of conjugates) has been shown to regulate or co-regulate a wide range of processes in plants, from responses to biotic and abiotic stresses to the developmental maturation of stamens and pollen in Arabidopsis. This review focuses on discoveries in several of these areas. Most work described is from studies in Arabidopsis. While the results are expected to be broadly applicable to other higher plants, there are cases where related but distinct phenotypes have been observed in other species (e.g., tomato). Investigation of JA action in wound- and insect-defense responses has established that this compound is an essential component of the systemic signal that activates defense genes throughout the plant. It is possible that JA acts indirectly through the production of reactive oxygen species including hydrogen peroxide (H2O2). The availability of Arabidopsis mutants deficient in JA synthesis has been central to the identification of additional roles for JA in defense against microbial pathogens and in reproductive development. Currently, the key issues in JA action are to understand the role of the skip/cullin/F-box ubiquitination complex, SCF(COI1), and to identify additional protein components that act in the early steps of JA signaling.

Ankyrin repeat-containing proteins in Arabidopsis: characterization of a novel and abundant group of genes coding ankyrin-transmembrane proteins

Ankyrin repeats are present in a great variety of proteins of eukaryotes, prokaryotes and some viruses and they function as protein-protein interaction domains. We have search for all the ankyrin repeats present in Arabidopsis proteins and determined their consensus sequence. We identified a total of 509 ankyrin repeats present in 105 proteins. Ankyrin repeat containing proteins can be classified in 16 groups of structurally similar proteins. The most abundant group contains proteins with ankyrin repeats and transmembrane domains (AtANKTM). Sequence similarity analysis indicates that these proteins are divided in six families. Some of the AtAnkTm genes are organized in tandem arrays and others are present in duplicated parts of the Arabidopsis genome. The expression of several AtAnkTm genes was analyzed resulting in a wide variety of expression patterns even within the same family. The likely functions of these proteins are discussed in comparison with the known functions of proteins with similar organization in other species.