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

The synthetic elicitor 3,5-dichloroanthranilic acid induces NPR1-dependent and NPR1-independent mechanisms of disease resistance in Arabidopsis

Immune responses of Arabidopsis (Arabidopsis thaliana) are at least partially mediated by coordinated transcriptional up-regulation of plant defense genes, such as the Late/sustained Up-regulation in Response to Hyaloperonospora parasitica (LURP) cluster. We found a defined region in the promoter of the LURP member CaBP22 to be important for this response. Using a CaBP22 promoter-reporter fusion, we have established a robust and specific high-throughput screening system for synthetic defense elicitors that can be used to trigger defined subsets of plant immune responses. Screening a collection of 42,000 diversity-oriented molecules, we identified 114 candidate LURP inducers. One representative, 3,5-dichloroanthranilic acid (DCA), efficiently induced defense reactions to the phytopathogens H. parasitica and Pseudomonas syringae. In contrast to known salicylic acid analogs, such as 2,6-dichloroisonicotinic acid (INA), which exhibit a long-lasting defense-inducing activity and are fully dependent on the transcriptional cofactor NPR1 (for Nonexpresser of Pathogenesis-Related genes1), DCA acts transiently and is only partially dependent on NPR1. Microarray analyses revealed a cluster of 142 DCA- and INA-responsive genes that show a pattern of differential expression coinciding with the kinetics of DCA-mediated disease resistance. These ACID genes (for Associated with Chemically Induced Defense) constitute a core gene set associated with chemically induced disease resistance, many of which appear to encode components of the natural immune system of Arabidopsis.

Genetic analysis of acd6-1 reveals complex defense networks and leads to identification of novel defense genes in Arabidopsis

Pathogen infection leads to the activation of defense signaling networks in plants. To study these networks and the relationships between their components, we introduced various defense mutations into acd6-1, a constitutive gain-of-function Arabidopsis mutant that is highly disease resistant. acd6-1 plants show spontaneous cell death, reduced stature, and accumulate high levels of camalexin (an anti-fungal compound) and salicylic acid (SA; a signaling molecule). Disruption of several defense genes revealed that in acd6-1, SA levels/signaling were positively correlated with the degree of disease resistance and defense gene expression. Salicylic acid also modulates the severity of cell death. However, accumulation of camalexin in acd6-1 is largely unaffected by reducing the level of SA. In addition, acd6-1 shows ethylene- and jasmonic acid-mediated signaling that is antagonized and therefore masked by the presence of SA. Mutant analysis revealed a new relationship between the signaling components NPR1 and PAD4 and also indicated that multiple defense pathways were required for phenotypes conferred by acd6-1. In addition, our data confirmed that the size of acd6-1 was inversely correlated with SA levels/signaling. We exploited this unique feature of acd6-1 to identify two genes disrupted in acd6-1 suppressor (sup) mutants: one encodes a known SA biosynthetic component (SID2) and the other encodes an uncharacterized putative metalloprotease (At5g20660). Taken together, acd6-1 is a powerful tool not only for dissecting defense regulatory networks but also for discovering novel defense genes.

Arabidopsis extra large G-protein 2 (XLG2) interacts with the Gbeta subunit of heterotrimeric G protein and functions in disease resistance

Heterotrimeric GTP-binding proteins, which consist of Galpha, Gbeta, and Ggamma subunits, play important roles in transducing extracellular signals perceived by cell surface receptors into intracellular physiological responses. In addition to a single prototypical Galpha protein (GPA1), Arabidopsis has three unique Galpha-like proteins, known as XLG1, XLG2, and XLG3, that have been found to be localized in nuclei, although their functions and mode of action remain largely unknown. Through a transcriptomic analysis, we found that XLG2 and XLG3 were rapidly induced by infection with the bacterial pathogen Pseudomonas syringae, whereas the XLG1 transcript level was not affected by pathogen infection. A reverse genetic screen revealed that the xlg2 loss-of-function mutation causes enhanced susceptibility to P. syringae. Transcriptome profiling revealed that the xlg2 mutation affects pathogen-triggered induction of a small set of defense-related genes. However, xlg1 and xlg3 mutants showed no difference from wild-type plants in resistance to P. syringae. In addition, the xlg2 xlg3 double mutant and the xlg1 xlg2 xlg3 triple mutant were not significantly different from the xlg2 single mutant in the disease resistance phenotype, suggesting that the roles of XLG1 and XLG3 in defense, if any, are less significant than for XLG2. Constitutive overexpression of XLG2 leads to the accumulation of abnormal transcripts from multiple defense-related genes. Through co-immunoprecipitation assays, XLG2 was found to interact with AGB1, the sole Gbeta subunit in Arabidopsis, which has previously been found to be a positive regulator in resistance to necrotrophic fungal pathogens. However, no significant difference was found between three xlg single mutants, the xlg2 xlg3 double mutant, the xlg triple mutant, and wild-type plants in resistance to the necrotrophic fungal pathogens Botrytis cinerea or Alternaria brassicicola. These results suggest that XLG2 and AGB1 are components of a G-protein complex different from the prototypical heterotrimeric G-protein and may have distinct functions in modulating defense responses.

Heterotrimeric G proteins-mediated resistance to necrotrophic pathogens includes mechanisms independent of salicylic acid-, jasmonic acid/ethylene- and abscisic acid-mediated defense signaling

Heterotrimeric G proteins are involved in the defense response against necrotrophic fungi in Arabidopsis. In order to elucidate the resistance mechanisms involving heterotrimeric G proteins, we analyzed the effects of the Gβ (subunit deficiency in the mutant agb1-2 on pathogenesis-related gene expression, as well as the genetic interaction between agb1-2 and a number of mutants of established defense pathways. Gβ-mediated signaling suppresses the induction of salicylic acid (SA)-, jasmonic acid (JA)-, ethylene (ET)- and abscisic acid (ABA)-dependent genes during the initial phase of the infection with Fusarium oxysporum (up to 48 h after inoculation). However, at a later phase it enhances JA/ET-dependent genes such as PDF1.2 and PR4. Quantification of the Fusarium wilt symptoms revealed that Gβ- and SA-deficient mutants were more susceptible than wild-type plants, whereas JA- and ET-insensitive and ABA-deficient mutants demonstrated various levels of resistance. Analysis of the double mutants showed that the Gβ-mediated resistance to F. oxysporum and Alternaria brassicicola was mostly independent of all of the previously mentioned pathways. However, the progressive decay of agb1-2 mutants was compensated by coi1-21 and jin1-9 mutations, suggesting that at this stage of F. oxysporum infection Gβ acts upstream of COI1 and ATMYC2 in JA signaling.

Ethylene modulates the role of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 in cross talk between salicylate and jasmonate signaling

The plant hormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) play crucial roles in the signaling network that regulates induced defense responses against biotic stresses. Antagonism between SA and JA operates as a mechanism to fine-tune defenses that are activated in response to multiple attackers. In Arabidopsis (Arabidopsis thaliana), NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) was demonstrated to be required for SA-mediated suppression of JA-dependent defenses. Because ET is known to enhance SA/NPR1-dependent defense responses, we investigated the role of ET in the SA-JA signal interaction. Pharmacological experiments with gaseous ET and the ET precursor 1-aminocyclopropane-1-carboxylic acid showed that ET potentiated SA/NPR1-dependent PATHOGENESIS-RELATED1 transcription, while it rendered the antagonistic effect of SA on methyl jasmonate-induced PDF1.2 and VSP2 expression NPR1 independent. This overriding effect of ET on NPR1 function in SA-JA cross talk was absent in the npr1-1/ein2-1 double mutant, demonstrating that it is mediated via ET signaling. Abiotic and biotic induction of the ET response similarly abolished the NPR1 dependency of the SA-JA signal interaction. Furthermore, JA-dependent resistance against biotic attackers was antagonized by SA in an NPR1-dependent fashion only when the plant-attacker combination did not result in the production of high levels of endogenous ET. Hence, the interaction between ET and NPR1 plays an important modulating role in the fine tuning of the defense signaling network that is activated upon pathogen and insect attack. Our results suggest a model in which ET modulates the NPR1 dependency of SA-JA antagonism, possibly to compensate for enhanced allocation of NPR1 to function in SA-dependent activation of PR genes.

BTB and TAZ domain scaffold proteins perform a crucial function in Arabidopsis development

In Arabidopsis, bric-a-brac, tramtrack and broad (BTB) domain scaffold proteins form a family of 80 proteins that have involvement in various signaling pathways. The five members of the subfamily of BTB AND TAZ DOMAIN proteins (BT1-BT5) have a typical domain structure that is only observed in land plants. Here, we present a functional analysis of the BT family, of which at least four members are encoded by auxin-responsive genes. BT1 is a short-lived protein that is characteristically targeted for degradation by the 26S proteasome. Expression pattern, gene structure and sequence analyses indicate that BT1 and BT2 are closely related. They both localize to the nucleus and the cytosol, whereas the remaining BT proteins were determined as cytosolic proteins. Detailed molecular and phenotypic analysis of plants segregating for null mutations in the BT family revealed substantial redundancy among the BT members, and highlighted that BT proteins perform crucial roles in both male and female gametophyte development. BT2 seems to be the predominant gene in this process, in which it is functionally replaced by BT3 and BT1 through reciprocal transcription regulation. Compensational expression alters the steady-state mRNA levels among the remaining BT family members when other BT members are lost, and this contributes towards functional redundancy. Our data provide a surprising example of functional redundancy among genes required during gametophyte development, something that could not be detected in the current screens for gametophyte mutants.

Molecular characterization of rice OsBIANK1, encoding a plasma membrane-anchored ankyrin repeat protein, and its inducible expression in defense responses

A rice gene, OsBIANK1, encoding a protein containing a typical ankyrin repeat domain, was cloned and identified. The OsBIANK1 protein, consisting of 329 amino acids, contains a conserved ankyrin repeat domain with two ankyrin repeats organized in tandem and was showed to be localized on cytoplasmic membrane during transient expression in onion epidermal cells. Expression of OsBIANK1 was induced by treatment with benzothiadiazole (BTH), a chemical inducer capable of inducing disease resistance response in rice. In BTH-treated rice seedlings, expression of OsBIANK1 was further induced by infection with Magnaporthe grisea, the rice blast fungus, as compared with those in water-treated seedlings. Our preliminary results confirm previous evidences that OsBIANK1 may be involved in regulation of disease resistance response in rice.

Purification of low-abundance Arabidopsis plasma-membrane protein complexes and identification of candidate components

Purification of low-abundance plasma-membrane (PM) protein complexes is a challenging task. We devised a tandem affinity purification tag termed the HPB tag, which contains the biotin carboxyl carrier protein domain (BCCD) of Arabidopsis 3-methylcrotonal CoA carboxylase. The BCCD is biotinylated in vivo, and the tagged protein can be captured by streptavidin beads. All five C-terminally tagged Arabidopsis proteins tested, including four PM proteins, were functional and biotinylated with high efficiency in Arabidopsis. Transgenic Arabidopsis plants expressing an HPB-tagged protein, RPS2::HPB, were used to develop a method to purify protein complexes containing the HPB-tagged protein. RPS2 is a membrane-associated disease resistance protein of low abundance. The purification method involves microsomal fractionation, chemical cross-linking, solubilization, and one-step affinity purification using magnetic streptavidin beads, followed by protein identification using LC-MS/MS. We identified RIN4, a known RPS2 interactor, as well as other potential components of the RPS2 complex(es). Thus, the HPB tag method is suitable for the purification of low-abundance PM protein complexes.

An important role of a BAHD acyl transferase-like protein in plant innate immunity

Salicylic acid (SA) is an important regulator of plant resistance to biotrophic and hemi-biotrophic pathogens. The enhanced pseudomonas susceptibility 1 (eps1) mutant in Arabidopsis thaliana is hypersusceptible to both virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae. Through positional cloning, the EPS1 gene was isolated and found to encode a novel member of the BAHD acyltransferase superfamily. Pathogen-induced accumulation of SA and expression of pathogenesis-related (PR) genes were compromised in the eps1 mutant. SA could induce PR1 gene expression and restore disease resistance in the eps1 mutant. These results suggest that EPS1 functions upstream of SA and may be involved directly in synthesis of a precursor or a regulatory molecule for SA biosynthesis. Mutations of EPS1 or other genes important for SA accumulation or signaling conferred enhanced resistance to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola in the Nossen-0 background but had little effect in the Columbia-0 background. These results suggest that there is natural variation among Arabidopsis ecotypes with respect to the antagonistic cross-talk between defense signaling pathways against various types of microbial pathogens.

The Pseudomonas syringae type III effector AvrRpm1 induces significant defenses by activating the Arabidopsis nucleotide-binding leucine-rich repeat protein RPS2

Plant disease resistance (R) proteins recognize potential pathogens expressing corresponding avirulence (Avr) proteins through 'gene-for-gene' interactions. RPM1 is an Arabidopsis R-protein that triggers a robust defense response upon recognizing the Pseudomonas syringae effector AvrRpm1. Avr-proteins of phytopathogenic bacteria include type III effector proteins that are often capable of enhancing virulence when not recognized by an R-protein. In rpm1 plants, AvrRpm1 suppresses basal defenses induced by microbe-associated molecular patterns. Here, we show that expression of AvrRpm1 in rpm1 plants induced PR-1, a classical defense marker, and symptoms including chlorosis and necrosis. PR-1 expression and symptoms were reduced in plants with mutations in defense signaling genes (pad4, sid2, npr1, rar1, and ndr1) and were strongly reduced in rpm1 rps2 plants, indicating that AvrRpm1 elicits defense signaling through the Arabidopsis R-protein, RPS2. Bacteria expressing AvrRpm1 grew more on rpm1 rps2 than on rpm1 plants. Thus, independent of its classical 'gene-for-gene' activation of RPM1, AvrRpm1 also induces functionally relevant defenses that are dependent on RPS2. Finally, AvrRpm1 suppressed host defenses and promoted the growth of type III secretion mutant bacteria equally well in rps2 and RPS2 plants, indicating that virulence activity of over-expressed AvrRpm1 predominates over defenses induced by weak activation of RPS2.

Distribution of an ankyrin-repeat protein on the endoplasmic reticulum in Arabidopsis

There are many ankyrin-repeat proteins in plant cells. However, the distribution and function of these proteins are mostly unclear. By reverse transcription-polymerase chain reaction, a gene encoding an ankyrin-like protein was cloned from Arabidopsis and named AtANK1 (GenBank accession no. NM_120340). The 6-His-tagged AtAnk1-N fusion protein was affinity-purified and its rabbit polyclonal antibody was obtained. Immuno-blotting with the purified anti-AtAnk1-N polyclonal antibody revealed that the relative molecular weight of the AtANK1 protein was about 76 kDa. By immunofluorescence labeling and immuno-gold labeling with the purified anti-AtAnk1-N polyclonal antibody, coupled with confocal and transmission electron microscopy observation, AtANK1 was found to be distributed on the membrane of the endoplasmic reticulum in Arabidopsis cells. Based on these results, we suggested that AtANK1 might be involved in endoplasmic reticulum-related protein localization and sorting in plant cells.

Riboflavin-induced priming for pathogen defense in Arabidopsis thaliana

Riboflavin (vitamin B(2)) participates in a variety of redox processes that affect plant defense responses. Previously we have shown that riboflavin induces pathogen resistance in the absence of hypersensitive cell death (HCD) in plants. Herein, we report that riboflavin induces priming of defense responses in Arabidopsis thaliana toward infection by virulent Pseudomonas syringae pv. tomato DC3000 (Pst). Induced resistance was mechanistically connected with the expression of defense response genes and cellular defense events, including H(2)O(2) burst, HCD, and callose deposition in the plant. Riboflavin treatment and inoculation of plants with Pst were neither active but both synergized to induce defense responses. The priming process needed NPR1 (essential regulator of systemic acquired resistance) and maintenance of H(2)O(2) burst but was independent of salicylic acid, jasmonic acid, ethylene, and abscisic acid. Our results suggest that the role of riboflavin in priming defenses is subject to a signaling process distinct from the known pathways of hormone signal transduction.

Extracellular pyridine nucleotides induce PR gene expression and disease resistance in Arabidopsis

Although it is well known that the pyridine nucleotides NAD and NADP function inside the cell to regulate intracellular signaling processes, recent evidence from animal studies suggests that NAD(P) also functions in the extracellular compartment (ECC). Extracellular NAD(P) [eNAD(P)] can either directly bind to plasma membrane receptors or be metabolized by ecto-enzymes to produce cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate, and/or may ADP-ribosylate cell-surface receptors, resulting in activation of transmembrane signaling. In this study, we report that, in plants, exogenous NAD(P) induces the expression of pathogenesis-related (PR) genes and resistance to the bacterial pathogen Pseudomonas syringae pv. maculicola ES4326. Chelation of Ca(2+) by EGTA significantly inhibits the induction of PR genes by exogenous NAD(P), suggesting that exogenous NAD(P) may induce PR genes through a pathway that involves Ca(2+) signaling. We show that exogenous application of NAD(P) causes accumulation of the defense signal molecule salicylic acid (SA), and induces both SA/NPR1-dependent and -independent PR gene expression and disease resistance. Furthermore, we demonstrate that NAD(P) leaks into the plant ECC after mechanical wounding and pathogen infection, and that the amount of NAD(P) leaking into the ECC after P. syringae pv. tobacco DC3000/avrRpt2 infection is sufficient for induction of both PR gene expression and disease resistance. We propose that NAD(P) leakage from cells losing membrane integrity upon environmental stress may function as an elicitor to activate plant defense responses. Our data provide evidence that eNAD(P) functions in plant signaling, and illustrate the potential importance of eNAD(P) in plant innate immunity.

Rice blast fungus (Magnaporthe oryzae) infects Arabidopsis via a mechanism distinct from that required for the infection of rice

Magnaporthe oryzae is a hemibiotrophic fungal pathogen that causes rice (Oryza sativa) blast. Although M. oryzae as a whole infects a wide variety of monocotyledonous hosts, no dicotyledonous plant has been reported as a host. We found that two rice pathogenic strains of M. oryzae, KJ201 and 70-15, interacted differentially with 16 ecotypes of Arabidopsis (Arabidopsis thaliana). Strain KJ201 infected all ecotypes with varying degrees of virulence, whereas strain 70-15 caused no symptoms in certain ecotypes. In highly susceptible ecotypes, small chlorotic lesions appeared on infected leaves within 3 d after inoculation and subsequently expanded across the affected leaves. The fungus produced spores in susceptible ecotypes but not in resistant ecotypes. Fungal cultures recovered from necrotic lesions caused the same symptoms in healthy plants, satisfying Koch's postulates. Histochemical analyses showed that infection by the fungus caused an accumulation of reactive oxygen species and eventual cell death. Similar to the infection process in rice, the fungus differentiated to form appressorium and directly penetrated the leaf surface in Arabidopsis. However, the pathogenic mechanism in Arabidopsis appears distinct from that in rice; three fungal genes essential for pathogenicity in rice played only limited roles in causing disease symptoms in Arabidopsis, and the fungus seems to colonize Arabidopsis as a necrotroph through the secretion of phytotoxic compounds, including 9,12-octadecadienoic acid. Expression of PR-1 and PDF1.2 was induced in response to infection by the fungus, suggesting the activation of salicylic acid- and jasmonic acid/ethylene-dependent signaling pathways. However, the roles of these signaling pathways in defense against M. oryzae remain unclear. In combination with the wealth of genetic and genomic resources available for M. oryzae, this newly established pathosystem allows comparison of the molecular and cellular mechanisms underlying pathogenesis and host defense in two well-studied model plants.

Arabidopsis thaliana genes encoding defense signaling and recognition proteins exhibit contrasting evolutionary dynamics

The interplay between pathogen effectors, their host targets, and cognate recognition proteins provides various opportunities for antagonistic cycles of selection acting on plant and pathogen to achieve or abrogate resistance, respectively. Selection has previously been shown to maintain diversity in plant proteins involved in pathogen recognition and some of their cognate pathogen effectors. We analyzed the signatures of selection on 10 Arabidopsis thaliana genes encoding defense signal transduction proteins in plants, which are potential targets of pathogen effectors. There was insufficient evidence to reject neutral evolution for 6 genes encoding signaling components consistent with these proteins not being targets of effectors and/or indicative of constraints on their ability to coevolve with pathogen effectors. Functional constraints on effector targets may have provided the driving selective force for the evolution of guard proteins. PBS1, a known target of an effector, showed little variation but is known to be monitored by a variable guard protein. Evidence of selection maintaining diversity was present at NPR1, PAD4, and EDS1. Differences in the signatures of selection observed may reflect the numbers of effectors that target a particular protein, the presence or absence of a cognate guard protein, as well as functional constraints imposed by biochemical activities or interactions with plant proteins.

Expression of callose synthase genes and its connection with Npr1 signaling pathway during pathogen infection

Callose synthesis occurs at specific stages of plant cell wall development in all cell types, and in response to pathogen attack, wounding and physiological stresses. We determined the expression pattern of "upstream regulatory sequence" of 12 Arabidopsis callose synthase genes (CalS1-12) genes and demonstrated that different callose synthases are expressed specifically in different tissues during plant development. That multiple CalS genes are expressed in the same cell type suggests the possibility that CalS complex may be constituted by heteromeric subunits. Five CalS genes were induced by pathogen (Hyaloperonospora arabidopsis, previously known as Peronospora parasitica, the causal agent of downy mildew) or salicylic acid (SA), while the other seven CalS genes were not affected by these treatments. Among the genes that are induced, CalS1 and CalS12 showed the highest responses. In Arabidopsis npr1 mutant, impaired in response of pathogenesis related (PR) genes to SA, the induction of CalS1 and CalS12 genes by the SA or pathogen treatments was significantly reduced. The patterns of expression of the other three CalS genes were not changed significantly in the npr1 mutant. These results suggest that the high induction observed of CalS1 and CalS12 is Npr1 dependent while the weak induction of five CalS genes is Npr1 independent. In a T-DNA knockout mutant of CalS12, callose encasement around the haustoria on the infected leaves was reduced and the mutant was found to be more resistant to downy mildew as compared to the wild type plants.

The Arabidopsis GRAS protein SCL14 interacts with class II TGA transcription factors and is essential for the activation of stress-inducible promoters

The plant signaling molecule salicylic acid (SA) and/or xenobiotic chemicals like the auxin mimic 2,4-D induce transcriptional activation of defense- and stress-related genes that contain activation sequence-1 (as-1)-like cis-elements in their promoters. as-1-like sequences are recognized by basic/leucine zipper transcription factors of the TGA family. Expression of genes related to the SA-dependent defense program systemic acquired resistance requires the TGA-interacting protein NPR1. However, a number of as-1-containing promoters can be activated independently from NPR1. Here, we report the identification of Arabidopsis thaliana SCARECROW-like 14 (SCL14), a member of the GRAS family of regulatory proteins, as a TGA-interacting protein that is required for the activation of TGA-dependent but NPR1-independent SA- and 2,4-D-inducible promoters. Chromatin immunoprecipitation experiments revealed that class II TGA factors TGA2, TGA5, and/or TGA6 are needed to recruit SCL14 to promoters of selected SCL14 target genes identified by whole-genome transcript profiling experiments. The coding regions and the expression profiles of the SCL14-dependent genes imply that they might be involved in the detoxification of xenobiotics and possibly endogenous harmful metabolites. Consistently, plants ectopically expressing SCL14 showed increased tolerance to toxic doses of the chemicals isonicotinic acid and 2,4,6-triiodobenzoic acid, whereas the scl14 and the tga2 tga5 tga6 mutants were more susceptible. Hence, the TGA/SCL14 complex seems to be involved in the activation of a general broad-spectrum detoxification network upon challenge of plants with xenobiotics.

Elicitor and Fusarium-induced expression of NPR1-like genes in banana

The non-expressor of pathogenesis-related genes 1 (NPR1) is an essential positive regulator of salicylic acid (SA)-induced pathogenesis-related (PR) gene expression and systemic acquired resistance (SAR). Two novel full length NPR1-like genes; MNPR1A and MNPR1B, were isolated from banana by application of the PCR and rapid amplification of cDNA ends (RACE) techniques. The two identified MNPR1 sequences differed greatly in their expression profile using quantitative real time (qRT)-PCR following either elicitor or Fusarium oxysporum Schlecht f. sp. cubense (Smith) Snyd (Foc) treatment. MNPR1A was greatly expressed after Foc treatment with higher and earlier expression in the Foc-tolerant cultivar GCTCV-218 than in the sensitive cultivar Grand Naine. In comparison, MNPR1B was highly responsive to SA, but not to methyl jasmonate (MeJA) treatment, in both the tolerant banana cultivar GCTCV-218 and the more sensitive cultivar Grand Naine. Expression of the MNPR1 genes further directly related to PR gene expression known to be involved in fungal resistance. Reduced sensitivity to Foc in GCTCV-218 might be partially attributed to the higher and an earlier expression of both MNPR1A and PR-1 in this cultivar after Foc treatment. Further characterisation of the MNPR1 genes through complementation of Arabidopsis npr1 mutants and overexpression studies in banana cultivars is the subject of ongoing and future work.

Powdery mildew resistance conferred by loss of the ENHANCED DISEASE RESISTANCE1 protein kinase is suppressed by a missense mutation in KEEP ON GOING, a regulator of abscisic acid signaling

Loss-of-function mutations in the Arabidopsis (Arabidopsis thaliana) ENHANCED DISEASE RESISTANCE1 (EDR1) gene confer enhanced resistance to infection by powdery mildew (Golovinomyces cichoracearum). EDR1 encodes a protein kinase, but its substrates and the pathways regulated by EDR1 are unknown. To identify components of the EDR1 signal transduction pathway(s), we conducted a forward genetic screen for mutations that suppressed edr1-mediated disease resistance. Genetic mapping and cloning of one of these suppressor mutations revealed a recessive missense mutation in the KEEP ON GOING gene (KEG; At5g13530), which we designated keg-4. KEG encodes a multidomain protein that includes a RING E3 ligase domain, a kinase domain, ankyrin repeats, and HERC2-like repeats. The KEG protein has previously been shown to have ubiquitin ligase activity and to negatively regulate protein levels of the transcription factor ABCISIC ACID INSENSITIVE5. KEG mRNA levels were found to be 3-fold higher in edr1 mutant plants compared to wild type. Loss-of-function mutations in KEG are seedling lethal and are hypersensitive to glucose and abscisic acid (ABA). The keg-4 mutation, in contrast, conferred resistance to 6% glucose and suppressed edr1-mediated hypersensitivity to ABA, suggesting that the keg-4 mutation suppresses ABA signaling by altering KEG function. Several ABA-responsive genes were found to be further up-regulated in the edr1 mutant following ABA treatment, and this up-regulation was suppressed by the keg-4 mutation. We conclude that edr1-mediated resistance to powdery mildew is mediated, in part, by enhanced ABA signaling.

The AtrbohD-mediated oxidative burst elicited by oligogalacturonides in Arabidopsis is dispensable for the activation of defense responses effective against Botrytis cinerea

Oligogalacturonides (OGs) are endogenous elicitors of defense responses released after partial degradation of pectin in the plant cell wall. We have previously shown that, in Arabidopsis (Arabidopsis thaliana), OGs induce the expression of PHYTOALEXIN DEFICIENT3 (PAD3) and increase resistance to the necrotrophic fungal pathogen Botrytis cinerea independently of signaling pathways mediated by jasmonate, salicylic acid, and ethylene. Here, we illustrate that the rapid induction of the expression of a variety of genes by OGs is also independent of salicylic acid, ethylene, and jasmonate. OGs elicit a robust extracellular oxidative burst that is generated by the NADPH oxidase AtrbohD. This burst is not required for the expression of OG-responsive genes or for OG-induced resistance to B. cinerea, whereas callose accumulation requires a functional AtrbohD. OG-induced resistance to B. cinerea is also unaffected in powdery mildew resistant4, despite the fact that callose accumulation was almost abolished in this mutant. These results indicate that the OG-induced oxidative burst is not required for the activation of defense responses effective against B. cinerea, leaving open the question of the role of reactive oxygen species in elicitor-mediated defense.

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.

Signaling pathways that regulate the enhanced disease resistance of Arabidopsis "defense, no death" mutants

Arabidopsis dnd1 and dnd2 mutants lack cyclic nucleotide-gated ion channel proteins and carry out avirulence or resistance gene-mediated defense with a greatly reduced hypersensitive response (HR). They also exhibit elevated broad-spectrum disease resistance and constitutively elevated salicylic acid (SA) levels. We examined the contributions of NPR1, SID2 (EDS16), NDR1, and EIN2 to dnd phenotypes. Mutations that affect SA accumulation or signaling (sid2, npr1, and ndr1) abolished the enhanced resistance of dnd mutants against Pseudomonas syringae pv. tomato and Hyaloperonospora parasitica but not Botrytis cinerea. When SA-associated pathways were disrupted, the constitutive activation of NPR1-dependent and NPR1-independent and SA-dependent pathways was redirected toward PDF1.2-associated pathways. This PDF1.2 overexpression was downregulated after infection by P. syringae. Disruption of ethylene signaling abolished the enhanced resistance to B. cinerea but not P. syringae or H. parasitica. However, loss of NPR1, SID2, NDR1, or EIN2 did not detectably alter the reduced HR in dnd mutants. The susceptibility of dnd ein2 plants to B. cinerea despite their reduced-HR phenotype suggests that cell death repression is not the primary cause of dnd resistance to necrotrophic pathogens. The partial restoration of resistance to B. cinerea in dnd1 npr1 ein2 triple mutants indicated that this resistance is not entirely EIN2 dependent. The above findings indicate that the broad-spectrum resistance of dnd mutants occurs due to activation or sensitization of multiple defense pathways, yet none of the investigated pathways are required for the reduced-HR phenotype.

Early leaf senescence is associated with an altered cellular redox balance in Arabidopsis cpr5/old1 mutants

Reactive oxygen species (ROS) are the inevitable by-products of essential cellular metabolic and physiological activities. Plants have developed sophisticated gene networks of ROS generation and scavenging systems. However, ROS regulation is still poorly understood. Here, we report that mutations in the Arabidopsis CPR5/OLD1 gene may cause early senescence through deregulation of the cellular redox balance. Genetic analysis showed that blocking stress-related hormonal signalling pathways, such as ethylene, salicylic acid, jasmonic acid, abscisic acid and sugar, did not affect premature cell death and leaf senescence. We took a bioinformatics approach and analysed publicly available transcriptome data of presymptomatic cpr5/old1 mutants. The results demonstrate that many genes in the ROS gene network show at least fivefold increases in transcripts in comparison with those of wild-type plants, suggesting that presymptomatic cpr5/old1 mutants are in a state of high-cellular oxidative stress. This was further confirmed by a comparative, relative quantitative proteomics study of Arabidopsis wild-type and cpr5/old1 mutant plants, which demonstrated that several Phi family members of glutathione s-transferases significantly increased in abundance. In summary, our genetic, transcriptomic and relative quantitative proteomics analyses indicate that CPR5 plays a central role in regulating redox balance in Arabidopsis.

The Arabidopsis AtNPR1 inversely modulates defense responses against fungal, bacterial, or viral pathogens while conferring hypersensitivity to abiotic stresses in transgenic rice

The nonexpressor of pathogenesis-related (PR) genes (NPR1) protein plays an important role in mediating defense responses activated by pathogens in Arabidopsis. In rice, a disease-resistance pathway similar to the Arabidopsis NPR1-mediated signaling pathway one has been described. Here, we show that constitutive expression of the Arabidopsis NPR1 (AtNPR1) gene in rice confers resistance against fungal and bacterial pathogens. AtNPR1 exerts its protective effects against fungal pathogens by priming the expression of salicylic acid (SA)-responsive endogenous genes, such as the PR1b, TLP (PR5), PR10, and PBZ1. However, expression of AtNPR1 in rice has negative effects on viral infections. The AtNPR1-expressing rice plants showed a higher susceptibility to infection by the Rice yellow mottle virus (RYMV) which correlated well with a misregulation of RYMV-responsive genes, including expression of the SA-regulated RNA-dependent RNA polymerase 1 gene (OsRDR1). Moreover, AtNPR1 negatively regulates the expression of genes playing a role in the plant response to salt and drought stress (rab21, salT, and dip1), which results in a higher sensitivity of AtNPR1 rice to the two types of abiotic stress. These observations suggest that AtNPR1 has both positive and negative regulatory roles in mediating defense responses against biotic and abiotic stresses.

Arabidopsis WRKY38 and WRKY62 transcription factors interact with histone deacetylase 19 in basal defense

Arabidopsis thaliana WRKY38 and WRKY62, encoding two structurally similar type III WRKY transcription factors, are induced in a Nonexpressor of PR Gene1 (NPR1)-dependent manner by salicylic acid (SA) or by virulent Pseudomonas syringae. Disease resistance and SA-regulated Pathogenesis-Related1 (PR1) gene expression are enhanced in the wrky38 and wrky62 single mutants and, to a greater extent, in the double mutants. Overexpression of WRKY38 or WRKY62 reduces disease resistance and PR1 expression. Thus, WRKY38 and WRKY62 function additively as negative regulators of plant basal defense. WRKY38 and WRKY62 interact with Histone Deacetylase 19 (HDA19). Expression of HDA19 is also induced by P. syringae, and the stability of its induced transcripts depends on SA and NPR1 in infected plants. Disruption of HDA19 leads to compromised resistance, whereas its overexpression results in enhanced resistance to P. syringae. Thus, HDA19 has a role opposite from those of WRKY38 and WRKY62 in basal resistance to the bacterial pathogen. Both WRKY38 and WRKY62 are transcriptional activators in plant cells, but their activation activities are abolished by overexpressed HDA19. Interaction of WRKY38 and WRKY62 with HDA19 may act to fine-tune plant basal defense responses.

Mammalian pro-apoptotic bax gene enhances tobacco resistance to pathogens

Emerging evidence suggests that plants and animals may share certain biochemical commonalities in apoptosis, or programmed cell death (PCD) pathways, though plants lack key animal apoptosis related genes. In plants, PCD has many important functions including a role in immunity and resistance to pathogen infection. In this study, a rice phenylalanine ammonia-lyase promoter is used to regulate the expression of the mouse pro-apoptotic bax gene in transgenic tobacco plants. Ectopic expression of the bax negatively affects the growth of transgenic plants. Nonetheless, results show that the bax transgene is induced upon infection by plant pathogens and accumulation of Bax is observed by Western blot analysis. By estimating and measuring the extent of cell death, release of active oxygen species, and accumulation defense-associated gene transcripts, it is shown that bax transgenic plants mount a more robust cell death response compared to control plants. The bax transgenic tobacco plants are also more resistant to infection by Phytophthora parasitica and Ralstonia solanacearum, but have no obvious resistance to tobacco mosaic virus. These results substantiate past studies and illustrate the powerful effects mammalian bax genes may have on plant development and disease resistance.

Arabidopsis GH3.5 regulates salicylic acid-dependent and both NPR1-dependent and independent defense responses

The cross-talk between plant disease resistance and development is fundamental to understanding systemic physiological processes during pathogen attack. Our previous study showed that the Arabidopsis GH3.5 gene acts as a bifunctional modulator of the salicylic acid (SA)-mediated resistance and the auxin-mediated susceptibility during the Arabidopsis-Pseudomonas syringae interaction as well as development. Here, we further study the role and mechanism of GH3.5 involved in the SA-dependent defense pathway. Transcript and histochemical analysis of the GH3.5 promoter::GUS reporter expression indicate that GH3.5 is expressed with a strong temporal and spatial manner with predominant expression in the divisional tissues. Upon bacterial challenge, GUS activity is induced in the junction tissue around the infiltrated zone with higher levels in the vasculature with a pattern different between the incompatible and compatible interactions. Exogenous SA application enhances disease resistance in the activation-tagged mutant gh3.5-1D, while the GH3.5-mediated defense enhancement is depleted in the SA deficient gh3.5-1D/NahG double mutant, indicating that GH3.5 modulates defense response through the SA-dependent pathway. Furthermore, bacterial growth in the gh3.5-1D/npr1 double mutant treated with SA indicates that GH3.5 enhances the SA-mediated defense response through both NPR1-dependent and independent pathways.

Molecular cloning and characterization of GhNPR1, a gene implicated in pathogen responses from cotton (Gossypium hirsutum L.)

A novel gene, designated as GhNPR1 (Gossypium hirsutum non-expressor of pathogenesis-related genes 1), was isolated from G. hirsutum (cotton) by RT-PCR (reverse transcription-PCR) and RACE (rapid amplification of cDNA ends). The full-length cDNA was 2108 bp long and had an ORF (open reading frame) that putatively encoded a polypeptide of 592 amino acids, with a predicted molecular mass of 66 kDa. Comparison of this protein sequence with that of Arabidopsis thaliana, Brassica juncea and Nicotiana tabacum showed that the amino-acid homology was 52.98, 52.32 and 54.98% respectively. Analysis of the exon-intron structure of the GhNPR1 gene showed that GhNPR1 consisted of four exons and three introns. Southern-blot analysis revealed that the GhNPR1 was a single-copy gene in cotton. Northern-blot analysis indicated that GhNPR1 was constitutively expressed in all tested tissues, including roots, stems and leaves, with the high expression in stems and leaves. In addition, GhNPR1 was also found to be induced by signalling molecules for plant defence responses, such as methyl jasmonate, salicylic acid and ethylene, as well as attack by pathogens, such as Fusarium oxysporum and Xanthomonas campestris. These results suggest that GhNPR1 may play an important role in the response to pathogen infections in cotton plants.

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.

Identification and characterisation of barley (Hordeum vulgare) respiratory burst oxidase homologue family members

Respiratory burst oxidase homologues (RBOHs) of the human phagocyte gp91^phox gene have been isolated from several plant species and the proteins that they encode have been shown to play important roles in the cellular response to biotic stress via the production of superoxide. In this study we have identified and preliminarily characterised six RBOHs from barley (Hordeum vulgare L.). Conservation of the genomic structure and conceptual protein sequence was observed between all six barley RBOH genes when compared with Arabidopsis and rice RBOH gene family members. Four of the six barley RBOH transcripts had wide-spread constitutive spatial expression patterns. The inducible expression profiles of HvRBOHF1 and HvRBOHF2 in response to infection by the necrotrophic fungal pathogens Pyrenophora teres f. teres Drechsler and Rhynchosporium secalis (Oudem) J. Davis were further characterised by quantitative real-time PCR (qPCR). Increased expression of both transcripts was observed in leaf epidermal tissue in response to infection, which is in keeping with a suggested role for both transcripts in the early oxidative burst during the plant response to pathogen invasion. This research provides a basis for further analysis and establishment of the roles of this RBOH family in various reactive oxygen species dependent processes in barley.

Identification of specific fragments of HpaG Xooc, a harpin from Xanthomonas oryzae pv. oryzicola, that induce disease resistance and enhance growth in plants

Harpin proteins from gram-negative plant-pathogenic bacteria can stimulate hypersensitive cell death (HCD) and pathogen defense as well as enhance growth in plants. Two of these diverse activities clearly are beneficial and may depend on particular functional regions of the proteins. Identification of beneficial and deleterious regions might facilitate the beneficial use of harpin-related proteins on crops without causing negative effects like cell death. Here, we report the identification and testing of nine functional fragments of HpaG(Xooc), a 137-amino-acid harpin protein from Xanthomonas oryzae pv. oryzicola, the pathogen that causes bacterial leaf streak of rice. Polymerase chain reaction-based mutagenesis generated nine proteinaceous fragments of HpaG(Xooc); these caused different responses following their application to Nicotiana tabacum (tobacco) and Oryza sativa (rice). Fragment HpaG62-137, which spans the indicated amino acid residues of the HpaG, induced more intense HCD; in contrast, HpaG10-42 did not cause evident cell death in tobacco. However, both fragments stimulated stronger defense responses and enhanced more growth in rice than the full-length parent protein, HpaG(Xooc). Of the nine fragments, the parent protein and one deletion mutant of HpaG(Xooc) tested, HpaG10-42, stimulated higher levels of rice growth and resulted in greater levels of resistance to X. oryzae pv. oryzae and Magnaporthe grisea. These pathogens cause bacterial leaf blight and rice blast, respectively, the two most important diseases of rice world-wide. HpaG10-42 was more active than HpaG(Xooc) in inducing expression of several genes that regulate rice defense and growth processes and activating certain signaling pathways, which may explain the greater beneficial effects observed from treatment with that fragment. Overall, our results suggest that HpaG10-42 holds promise for practical agricultural use to induce disease resistance and enhance growth of rice.

Investigations into plant biochemical wound-response pathways involved in the production of aphid-induced plant volatiles

Feeding damage to plants by insect herbivores induces the production of plant volatiles, which are attractive to the herbivores natural enemies. Little is understood about the plant biochemical pathways involved in aphid-induced plant volatile production. The aphid parasitoid Diaeretiella rapae can detect and respond to aphid-induced volatiles produced by Arabidopsis thaliana. When given experience of those volatiles, it can learn those cues and can therefore be used as a novel biosensor to detect them. The pathways involved in aphid-induced volatile production were investigated by comparing the responses of D. rapae to volatiles from a number of different transgenic mutants of A. thaliana, mutated in their octadecanoid, ethylene or salicylic acid wound-response pathways and also from wild-type plants. Plants were either undamaged or infested by the peach-potato aphid, Myzus persicae. It is demonstrated that the octadecanoid pathway and specifically the COI1 gene are required for aphid-induced volatile production. The presence of salicylic acid is also involved in volatile production. Using this model system, in combination with A. thaliana plants with single point gene mutations, has potential for the precise dissection of biochemical pathways involved in the production of aphid-induced volatiles.

Pepper pectin methylesterase inhibitor protein CaPMEI1 is required for antifungal activity, basal disease resistance and abiotic stress tolerance

Pectin is one of the main components of the plant cell wall that functions as the primary barrier against pathogens. Among the extracellular pectinolytic enzymes, pectin methylesterase (PME) demethylesterifies pectin, which is secreted into the cell wall in a highly methylesterified form. Here, we isolated and functionally characterized the pepper (Capsicum annuum L.) gene CaPMEI1, which encodes a pectin methylesterase inhibitor protein (PMEI), in pepper leaves infected by Xanthomonas campestris pv. vesicatoria (Xcv). CaPMEI1 transcripts are localized in the xylem of vascular bundles in leaf tissues, and pathogens and abiotic stresses can induce differential expression of this gene. Purified recombinant CaPMEI1 protein not only inhibits PME, but also exhibits antifungal activity against some plant pathogenic fungi. Virus-induced gene silencing of CaPMEI1 in pepper confers enhanced susceptibility to Xcv, accompanied by suppressed expression of some defense-related genes. Transgenic Arabidopsis CaPMEI1-overexpression lines exhibit enhanced resistance to Pseudomonas syringae pv. tomato, mannitol and methyl viologen, but not to the biotrophic pathogen Hyaloperonospora parasitica. Together, these results suggest that CaPMEI1, an antifungal protein, may be involved in basal disease resistance, as well as in drought and oxidative stress tolerance in plants.

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.

Salicylic acid induces H2O2 production and endochitinase gene expression but not ethylene biosynthesis in Castanea sativa in vitro model system

Salicylic acid (SA), ethylene (ET), and wounding are all known to influence plant defense response. Experiments attempting to determine SA's relation to ET biosynthesis and defense gene expression have shown conflicting results. To confront this, we developed an in vitro model system to investigate how SA affects ET biosynthesis, hydrogen peroxide (H(2)O(2)) production and endochitinase gene expression in the European chestnut. ET measurements of in vitro shoots indicated a critical time point for SA exogenous application, enabling us to study its effects independent of ET. In addition, ET measurements demonstrated that its own increased biosynthesis was a response to wounding but not to SA treatment. Application of the ET biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), on wounded and SA-treated shoots blocked wounding-induced ET production. Interestingly, SA inhibited ET production, but to a lesser extent than AVG. Additionally, SA also induced the accumulation of endochitinase transcript level. Likewise, a sensitive tissue-print assay showed that SA further increased the level of H(2)O(2). Yet, SA-induced endochitinase gene expression and SA-enhanced H(2)O(2) production levels were independent of ET. The cumulative results indicate that SA acts as an inducer of endochitinase PR gene expression and of H(2)O(2) oxidative burst. This suggests that SA is a component of the signal transduction pathway leading to defense against pathogens in chestnut. Further, the model system developed for this experiment should facilitate the deciphering of defense signaling pathways and their cross-talk. Moreover, it should also benefit the study of trees of long generation time that are known to be recalcitrant to in vitro studies.

Activation of defense response pathways by OGs and Flg22 elicitors in Arabidopsis seedlings

We carried out transcriptional profiling analysis in 10-d-old Arabidopsis thaliana seedlings treated with oligogalacturonides (OGs), oligosaccharides derived from the plant cell wall, or the bacterial flagellin peptide Flg22, general elicitors of the basal defense response in plants. Although detected by different receptors, both OGs and Flg22 trigger a fast and transient response that is both similar and comprehensive, and characterized by activation of early stages of multiple defense signaling pathways, particularly JA-associated processes. However, the response to Flg22 is stronger in both the number of genes differentially expressed and the amplitude of change. The magnitude of induction of individual genes is in both cases dose-dependent, but, even at very high concentrations, OGs do not induce a response that is as comprehensive as that seen with Flg22. While high doses of either microbe-associated molecular pattern (MAMP) elicit a late response that includes activation of senescence processes, SA-dependent secretory pathway genes and PR1 expression are substantially induced only by Flg22. These results suggest a lower threshold for activation of early responses than for sustained or SA-mediated late defenses. Expression patterns of amino-cyclopropane-carboxylate synthase genes also implicate ethylene biosynthesis in regulation of the late innate immune response.

Transcription factor WRKY70 displays important but no indispensable roles in jasmonate and salicylic acid signaling

The transcription factor WRKY70 was previously reported to be a common component in salicylic acid (SA) and jasmonate (JA) mediated signal pathways in Arabidopsis. Here, we present that the inactivation of the WRKY70 gene in wrky70-1 mutant does not alter the responses of both JA and SA, and that wrky70 mutation is unable to restore the coi1 mutant in JA responses. However, overexpression of WRKY70 reduces JA responses such as expression of JA-induced genes and JA-inhibitory root growth, and activates expression of SA-inducible PR1. These data indicate that the WRKY70 is important but not indispensable for JA and SA signaling, and that other regulators may display the redundant role with WRKY70 in modulation of JA and SA responses in Arabidopsis. Furthermore, we showed that JA inhibits expression of WRKY70 and PR1 by both COI1-dependent and COI1-independent pathways.

Strong suppression of systemic acquired resistance in Arabidopsis by NRR is dependent on its ability to interact with NPR1 and its putative repression domain

Systemic Acquired Resistance (SAR) in plants confers lasting broad-spectrum resistance to pathogens and requires the phytohormone salicylic acid (SA). Arabidopsis NPR1/NIM1 is a key regulator of the SAR response. Studies attempting to reveal the function of NPR1 and how it mediates SA signaling have led to isolation of two classes of proteins that interact with NPR1: the first class includes rice NRR, Arabidopsis NIMIN1, NIMIN2, and NIMIN3, and tobacco NIMIN2-like proteins; the second class belongs to TGA transcription factors. We have previously shown that overexpression of NRR in rice suppresses both basal and Xa21-mediated resistance. In order to test whether NRR affects SA-induced, NPR1-mediated SAR, we have transformed Arabidopsis with the rice NRR gene and tested its effects on the defense response. Expression of NRR in Arabidopsis results in suppression of PR gene induction by SAR inducer and resistance to pathogens. These phenotypes are even more severe than those of the npr1-1 mutant. The ability of NRR to suppress PR gene induction and disease resistance is correlated with its ability to bind to NPR1 because two point mutations in NRR, which reduce NPR1 binding, fail to suppress NPR1. In contrast, wild-type and a mutant NRR, which still binds to NPR1 strongly, retain the ability to suppress the SAR response. Replacing the C-terminal 79 amino acids of NRR with the VP16 activation domain turns the fusion protein into a transcriptional co-activator. These results indicate that NRR binds to NPR1 in vivo in a protein complex to inhibit transcriptional activation of PR genes and that NRR contains a transcription repression domain for active repression.

Stress- and pathogen-induced Arabidopsis WRKY48 is a transcriptional activator that represses plant basal defense

Plant WRKY transcription factors can function as either positive or negative regulators of plant basal disease resistance. Arabidopsis WRKY48 is induced by mechanical and/or osmotic stress due to infiltration and pathogen infection and, therefore, may play a role in plant defense responses. WRKY48 is localized to the nucleus, recognizes the TTGACC W-box sequence with a high affinity in vitro and functions in plant cells as a strong transcriptional activator. To determine the biological functions directly, we have isolated loss-of-function T-DNA insertion mutants and generated gain-of-function transgenic overexpression plants for WRKY48 in Arabidopsis. Growth of a virulent strain of the bacterial pathogen Pseudomonas syringae was decreased in the wrky48 T-DNA insertion mutants. The enhanced resistance of the loss-of-function mutants was associated with increased induction of salicylic acid-regulated PR1 by the bacterial pathogen. By contrast, transgenic WRKY48-overexpressing plants support enhanced growth of P. syringae and the enhanced susceptibility was associated with reduced expression of defense-related PR genes. These results suggest that WRKY48 is a negative regulator of PR gene expression and basal resistance to the bacterial pathogen P. syringae.

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.

Wall associated kinases from plants - an overview

Wall Associated kinases (WAKs) represent a unique class of receptor-like kinase genes that span the plasma membrane and allow cells to recognize and respond to their extracellular environment 26 WAK/WAK-like genes were identified from the Arabidopsis genome. Functional studies of the different WAK members in Arabidopsis demonstrated that they are involved in various functions in plants, including pathogen resistance, heavy-metal tolerance and plant development. 125 genes from rice (subsp. Japonica) belonging to wall associated kinase gene family were identified by reiterative database searches. We isolated a new member of WAKs in rice, designated as OsiWAK1, the silencing of which led to impaired root development and sterility due to anther indehiscence. In the current review, we discuss about the isolation and identification of WAK members from various plant species, different domains found in the WAK proteins that make them unique and the various roles played by WAKs in the plant growth and development.

Cyst nematode parasitism of Arabidopsis thaliana is inhibited by salicylic acid (SA) and elicits uncoupled SA-independent pathogenesis-related gene expression in roots

Compatible plant-nematode interactions involve the formation of an elaborate feeding site within the host root that requires the evasion of plant defense mechanisms by the parasite. Little is known regarding plant defense signaling pathways that limit nematode parasitism during a compatible interaction. Therefore, we utilized Arabidopsis thaliana mutants perturbed in salicylic acid (SA) biosynthesis or signal transduction to investigate the role of SA in inhibiting parasitism by the beet cyst nematode Heterodera schachtii. We determined that SA-deficient mutants (sid2-1, pad4-1, and NahG) exhibited increased susceptibility to H. schachtii. In contrast, SA-treated wild-type plants showed decreased H. schachtii susceptibility. The npr1-2 and npr1-3 mutants, which are impaired in SA signaling, also showed increased susceptibility to H. schachtii, whereas the npr1-suppressor mutation sni1 showed decreased susceptibility. Constitutive pathogenesis-related (PR) gene-expressing mutants (cpr1 and cpr6) did not show altered susceptibility to H. schachtii; however, constitutive PR gene expression was restricted to cpr1 shoots with wild-type levels of PR-1 transcript present in cpr1 roots. Furthermore, we determined that H. schachtii infection elicits SA-independent PR-2 and PR-5 induction in wild-type roots, while PR-1 transcript and total SA levels remained unaltered. This was in contrast to shoots of infected plants where PR-1 transcript abundance and total SA levels were elevated. We conclude that SA acts via NPR1 to inhibit nematode parasitism which, in turn, is negatively regulated by SNI1. Our results show an inverse correlation between root basal PR-1 expression and plant susceptibility to H. schachtii and suggest that successful cyst nematode parasitism may involve a local suppression of SA signaling in roots.

Cooperative ethylene and jasmonic acid signaling regulates selenite resistance in Arabidopsis

Selenium (Se) is an essential element for many organisms, but excess Se is toxic. To better understand plant Se toxicity and resistance mechanisms, we compared the physiological and molecular responses of two Arabidopsis (Arabidopsis thaliana) accessions, Columbia (Col)-0 and Wassilewskija (Ws)-2, to selenite treatment. Measurement of root length Se tolerance index demonstrated a clear difference between selenite-resistant Col-0 and selenite-sensitive Ws-2. Macroarray analysis showed more pronounced selenite-induced increases in mRNA levels of ethylene- or jasmonic acid (JA)-biosynthesis and -inducible genes in Col-0 than in Ws-2. Indeed, Col-0 exhibited higher levels of ethylene and JA. The selenite-sensitive phenotype of Ws-2 was attenuated by treatment with ethylene precursor or methyl jasmonate (MeJA). Conversely, the selenite resistance of Col-0 was reduced in mutants impaired in ethylene or JA biosynthesis or signaling. Genes encoding sulfur (S) transporters and S assimilation enzymes were up-regulated by selenite in Col-0 but not Ws-2. Accordingly, Col-0 contained higher levels of total S and Se and of nonprotein thiols than Ws-2. Glutathione redox status was reduced by selenite in Ws-2 but not in Col-0. Furthermore, the generation of reactive oxygen species by selenite was higher in Col-0 than in Ws-2. Together, these results indicate that JA and ethylene play important roles in Se resistance in Arabidopsis. Reactive oxygen species may also have a signaling role, and the resistance mechanism appears to involve enhanced S uptake and reduction.

Genome-wide expression profiling Arabidopsis at the stage of Golovinomyces cichoracearum haustorium formation

Compatibility between plants and obligate biotrophic fungi requires fungal mechanisms for efficiently obtaining nutrients and counteracting plant defenses under conditions that are expected to induce changes in the host transcriptome. A key step in the proliferation of biotrophic fungi is haustorium differentiation. Here we analyzed global gene expression patterns in Arabidopsis thaliana leaves during the formation of haustoria by Golovinomyces cichoracearum. At this time, the endogenous levels of salicylic acid (SA) and jasmonic acid (JA) were found to be enhanced. The responses of wild-type, npr1-1, and jar1-1 plants were used to categorize the sensitivity of gene expression changes to NPR1 and JAR1, which are components of the SA and JA signaling pathways, respectively. We found that the infection process was the major source of variation, with 70 genes identified as having similarly altered expression patterns regardless of plant genotype. In addition, principal component analysis (PCA) identified genes responding both to infection and to lack of functional JAR1 (17 genes) or NPR1 (18 genes), indicating that the JA and SA signaling pathways function as secondary sources of variation. Participation of these genes in the SA or JA pathways had not been described previously. We found that some of these genes may be sensitive to the balance between the SA and JA pathways, representing novel markers for the elucidation of cross-talk points between these signaling cascades. Conserved putative regulatory motifs were found in the promoter regions of each subset of genes. Collectively, our results indicate that gene expression changes in response to infection by obligate biotrophic fungi may support fungal nutrition by promoting alterations in host metabolism. In addition, these studies provide novel markers for the characterization of defense pathways and susceptibility features under this infection condition.

Characterization of a stress-responsive ankyrin repeat-containing zinc finger protein of Capsicum annuum (CaKR1)

We isolated many genes induced from pepper cDNA microarray data following their infection with the soybean pustule pathogen Xanthomonas axonopodis pv. glycines 8ra. A full-length cDNA clone of the Capsicum annuum ankyrin-repeat domain C(3)H(1) zinc finger protein (CaKR1) was identified in a chili pepper using the expressed sequence tag (EST) database. The deduced amino acid sequence of CaKR1 showed a significant sequence similarity (46%) to the ankyrin-repeat protein in very diverse family of proteins of Arabidopsis. The gene was induced in response to various biotic and abiotic stresses in the pepper leaves, as well as by an incompatible pathogen, such as salicylic acid (SA) and ethephon. CaKR1 expression was highest in the root and flower, and its expression was induced by treatment with agents such as NaCl and methyl viologen, as well as by cold stresses. These results showed that CaKR1 fusion with soluble, modified green fluorescent protein (smGFP) was localized to the cytosol in Arabidopsis protoplasts, suggesting that CaKR1 might be involved in responses to both biotic and abiotic stresses in pepper plants.

Salicylic acid and systemic acquired resistance play a role in attenuating crown gall disease caused by Agrobacterium tumefaciens

We investigated the effects of salicylic acid (SA) and systemic acquired resistance (SAR) on crown gall disease caused by Agrobacterium tumefaciens. Nicotiana benthamiana plants treated with SA showed decreased susceptibility to Agrobacterium infection. Exogenous application of SA to Agrobacterium cultures decreased its growth, virulence, and attachment to plant cells. Using Agrobacterium whole-genome microarrays, we characterized the direct effects of SA on bacterial gene expression and showed that SA inhibits induction of virulence (vir) genes and the repABC operon, and differentially regulates the expression of many other sets of genes. Using virus-induced gene silencing, we further demonstrate that plant genes involved in SA biosynthesis and signaling are important determinants for Agrobacterium infectivity on plants. Silencing of ICS (isochorismate synthase), NPR1 (nonexpresser of pathogenesis-related gene 1), and SABP2 (SA-binding protein 2) in N. benthamiana enhanced Agrobacterium infection. Moreover, plants treated with benzo-(1,2,3)-thiadiazole-7-carbothioic acid, a potent inducer of SAR, showed reduced disease symptoms. Our data suggest that SA and SAR both play a major role in retarding Agrobacterium infectivity.

Hydrogen peroxide in plants: a versatile molecule of the reactive oxygen species network

Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses that exert adverse effects on plant growth and development. During evolution, plants have evolved complex regulatory mechanisms to adapt to various environmental stressors. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species (ROS), which are subsequently converted to hydrogen peroxide (H(2)O(2)). Even under normal conditions, higher plants produce ROS during metabolic processes. Excess concentrations of ROS result in oxidative damage to or the apoptotic death of cells. Development of an antioxidant defense system in plants protects them against oxidative stress damage. These ROS and, more particularly, H(2)O(2,) play versatile roles in normal plant physiological processes and in resistance to stresses. Recently, H(2)O(2) has been regarded as a signaling molecule and regulator of the expression of some genes in cells. This review describes various aspects of H(2)O(2) function, generation and scavenging, gene regulation and cross-links with other physiological molecules during plant growth, development and resistance responses.

The fou2 gain-of-function allele and the wild-type allele of Two Pore Channel 1 contribute to different extents or by different mechanisms to defense gene expression in Arabidopsis

The fatty acid oxygenation up-regulated 2 (fou2) mutant in Arabidopsis thaliana creates a gain-of-function allele in a non-selective cation channel encoded by the Two Pore Channel 1 (TPC1) gene. This mutant genetically implicates cation fluxes in the control of the positive feedback loop whereby jasmonic acid (JA) stimulates its own synthesis. In this study we observed extensive transcriptome reprogramming in healthy fou2 leaves closely resembling that induced by treatment with methyl jasmonate, biotic stresses and the potassium starvation response. Proteomic analysis of fou2 leaves identified increased levels of seven biotic stress- and JA-inducible proteins. In agreement with these analyses, epistasis studies performed by crossing fou2 with aos indicated that elevated levels of JA in fou2 are the major determinant of the mutant phenotype. In addition, generation of fou2 aba1-5, fou2 etr1-1 and fou2 npr1-1 double mutants showed that the fou2 phenotype was only weakly affected by ABA levels and unaffected by mutations in NPR1 and ETR1. The results now suggest possible mechanisms whereby fou2 could induce JA synthesis/signaling early in the wound response. In contrast to fou2, transcriptome analysis of a loss-of-function allele of TPC1, tpc1-2, revealed no differential expression of JA biosynthesis genes in resting leaves. However, the analysis disclosed reduced mRNA levels of the pathogenesis-related genes PDF1.2a and THI2.1 in healthy and diseased tpc1-2 leaves. The results suggest that wild-type TPC1 contributes to their expression by mechanisms somewhat different from those affecting their expression in fou2.

The plant growth-promoting fungus Penicillium simplicissimum GP17-2 induces resistance in Arabidopsis thaliana by activation of multiple defense signals

Arabidopsis thaliana grown in soil amended with barley grain inocula of Penicillium simplicissimum GP17-2 or receiving root treatment with its culture filtrate (CF) exhibited clear resistance to Pseudomonas syringae pv. tomato DC3000 (Pst). To assess the contribution of different defense pathways, Arabidopsis genotypes implicated in salicylic acid (SA) signaling expressing the NahG transgene or carrying disruption in NPR1 (npr1), jasmonic acid (JA) signaling (jar1) and ethylene (ET) signaling (ein2) were tested. All genotypes screened were protected by GP17-2 or its CF. However, the level of protection was significantly lower in NahG and npr1 plants than it was in similarly treated wild-type plants, indicating that the SA signaling pathway makes a minor contribution to the GP17-2-mediated resistance and is insufficient for a full response. Examination of local and systemic gene expression revealed that GP17-2 and its CF modulate the expression of genes involved in both the SA and JA/ET signaling pathways. Subsequent challenge of GP17-2-colonized plants with Pst was accompanied by direct activation of SA-inducible PR-2 and PR-5 genes as well as potentiated expression of the JA-inducible Vsp gene. In contrast, CF-treated plants infected with Pst exhibited elevated expression of most defense-related genes (PR-1, PR-2, PR-5, PDF1.2 and Hel) studied. Moreover, an initial elevation of SA responses was followed by late induction of JA responses during Pst infection of induced systemic resistance (ISR)-expressing plants. In conclusion, we hypothesize the involvement of multiple defense mechanisms leading to an ISR of Arabidopsis by GP17-2.