Heterotrimeric G proteins facilitate Arabidopsis resistance to necrotrophic pathogens and are involved in jasmonate signaling

Heterotrimeric G proteins have been previously linked to plant defense; however a role for the Gbetagamma dimer in defense signaling has not been described to date. Using available Arabidopsis (Arabidopsis thaliana) mutants lacking functional Galpha or Gbeta subunits, we show that defense against the necrotrophic pathogens Alternaria brassicicola and Fusarium oxysporum is impaired in Gbeta-deficient mutants while Galpha-deficient mutants show slightly increased resistance compared to wild-type Columbia ecotype plants. In contrast, responses to virulent (DC3000) and avirulent (JL1065) strains of Pseudomonas syringae appear to be independent of heterotrimeric G proteins. The induction of a number of defense-related genes in Gbeta-deficient mutants were severely reduced in response to A. brassicicola infection. In addition, Gbeta-deficient mutants exhibit decreased sensitivity to a number of methyl jasmonate-induced responses such as induction of the plant defensin gene PDF1.2, inhibition of root elongation, seed germination, and growth of plants in sublethal concentrations of methyl jasmonate. In all cases, the behavior of the Galpha-deficient mutants is coherent with the classic heterotrimeric mechanism of action, indicating that jasmonic acid signaling is influenced by the Gbetagamma functional subunit but not by Galpha. We hypothesize that Gbetagamma acts as a direct or indirect enhancer of the jasmonate signaling pathway in plants.     

The BOS loci of Arabidopsis are required for resistance to Botrytis cinerea infection

Three Botrytis-susceptible mutants bos2, bos3, and bos4 which define independent and novel genetic loci required for Arabidopsis resistance to Botrytis cinerea were isolated. The bos2 mutant is susceptible to B. cinerea but retains wild-type levels of resistance to other pathogens tested, indicative of a defect in a response pathway more specific to B. cinerea. The bos3 and bos4 mutants also show increased susceptibility to Alternaria brassicicola, another necrotrophic pathogen, suggesting a broader role for these loci in resistance. bos4 shows the broadest range of effects on resistance, being more susceptible to avirulent strain of Pseudomonas syringae pv. tomato. Interestingly, bos3 is more resistant than wild-type plants to virulent strains of the biotrophic pathogen Peronospora parasitica and the bacterial pathogen P. syringae pv. tomato. The Pathogenesis Related gene 1 (PR-1), a molecular marker of the salicylic acid (SA)-dependent resistance pathway, shows a wild-type pattern of expression in bos2, while in bos3 this gene was expressed at elevated levels, both constitutively and in response to pathogen challenge. In bos4 plants, PR-1 expression was reduced compared with wild type in response to B. cinerea and SA. In bos3, the mutant most susceptible to B. cinerea and with the highest expression of PR-1, removal of SA resulted in reduced PR-1 expression but no change to the B. cinerea response. Expression of the plant defensin gene PDF1-2 was generally lower in bos mutants compared with wild-type plants, with a particularly strong reduction in bos3. Production of the phytoalexin camalexin is another well-characterized plant defense response. The bos2 and bos4 mutants accumulate reduced levels of camalexin whereas bos3 accumulates significantly higher levels of camalexin than wild-type plants in response to B. cinerea. The BOS2, BOS3, and BOS4 loci may affect camalexin levels and responsiveness to ethylene and jasmonate. The three new mutants appear to mediate disease responses through mechanisms independent of the previously described BOS1 gene. Based on the differences in the phenotypes of the bos mutants, it appears that they affect different points in defense response pathways.     

beta-Aminobutyric acid-induced protection of Arabidopsis against the necrotrophic fungus Botrytis cinerea

The non-protein amino acid beta-aminobutyric acid (BABA) protects numerous plants against various pathogens. Protection of Arabidopsis plants against virulent pathogens involves the potentiation of pathogen-specific defense responses. To extend the analysis of the mode of action of BABA to necrotrophs we evaluated the effect of this chemical on Arabidopsis plants infected with the gray mold fungus Botrytis cinerea. BABA-treated Arabidopsis were found to be less sensitive to two different strains of this pathogen. BABA protected mutants defective in the jasmonate and ethylene pathways, but was inactive in plants impaired in the systemic acquired resistance transduction pathway. Treatments with benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester, a functional analog of salicylic acid (SA), also markedly reduced the level of infection. Moreover, BABA potentiated mRNA accumulation of the SA-associated PR-1, but not the jasmonate/ethylene-dependent PDF1.2 gene. Thus, besides jasmonate/ethylene-dependent defense responses, SA-dependent signaling also contributes to restrict B. cinerea infection in Arabidopsis. Our results also suggest that SA-dependent signaling is down-regulated after infection by B. cinerea. The observed up-regulation of the PDF1.2 gene in mutants defective in the SA-dependent signaling pathway points to a cross-talk between SA- and jasmonate/ethylene-dependent signaling pathways during pathogen ingress.     

Resistance to Pseudomonas syringae conferred by an Arabidopsis thaliana coronatine-insensitive (coi1) mutation occurs through two distinct mechanisms

A new allele of the coronatine-insensitive locus (COI1) was isolated in a screen for Arabidopsis thaliana mutants with enhanced resistance to the bacterial pathogen Pseudomonas syringae. This mutant, designated coi1-20, exhibits robust resistance to several P. syringae isolates but remains susceptible to the virulent pathogens Erisyphe and cauliflower mosaic virus. Resistance to P. syringae strain PstDC3000 in coi1-20 plants is correlated with hyperactivation of PR-1 expression and accumulation of elevated levels of salicylic acid (SA) following infection, suggesting that the SA-mediated defense response pathway is sensitized in this mutant. Restriction of growth of PstDC3000 in coi1-20 leaves is partially dependent on NPR1 and fully dependent on SA, indicating that SA-mediated defenses are required for restriction of PstDC3000 growth in coi1-20 plants. Surprisingly, despite high levels of PstDC3000 growth in coi1-20 plants carrying the salicylate hydroxylase (nahG) transgene, these plants do not exhibit disease symptoms. Thus resistance to P. syringae in coi1-20 plants is conferred by two different mechanisms: (i) restriction of pathogen growth via activation of the SA-dependent defense pathway; and (ii) an SA-independent inability to develop disease symptoms. These findings are consistent with the hypotheses that the P. syringae phytotoxin coronatine acts to promote virulence by inhibiting host defense responses and by promoting lesion formation.     

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

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

Arabidopsis DND2, a second cyclic nucleotide-gated ion channel gene for which mutation causes the "defense, no death" phenotype

A previous mutant screen identified Arabidopsis dnd1 and dnd2 "defense, no death" mutants, which exhibit loss of hypersensitive response (HR) cell death without loss of gene-for-gene resistance. The dnd1 phenotype is caused by mutation of the gene encoding cyclic nucleotide-gated (CNG) ion channel AtCNGC2. This study characterizes dnd2 plants. Even in the presence of high titers of Pseudomonas syringae expressing avrRpt2, most leaf mesophyll cells in the dnd2 mutant exhibited no HR. These plants retained strong RPS2-, RPM1-, or RPS4-mediated restriction of P. syringae pathogen growth. Mutant dnd2 plants also exhibited enhanced broad-spectrum resistance against virulent P. syringae and constitutively elevated levels of salicylic acid, and pathogenesis-related (PR) gene expression. Unlike the wild type, dnd2 plants responding to virulent and avirulent P. syringae exhibited elevated expression of both salicylate-dependent PR-1 and jasmonate and ethylene-dependent PDF1.2. Introduction of nahG+ (salicylate hydroxylase) into the dnd2 background, which removes salicylic acid and causes other defense alterations, eliminated constitutive disease resistance and PR gene expression but only weakly impacted the HR- phenotype. Map-based cloning revealed that dnd2 phenotypes are caused by mutation of a second CNG ion channel gene, AtCNGC4. Hence, loss of either of two functionally nonredundant CNG ion channels can cause dnd phenotypes. The dnd mutants provide a unique genetic background for dissection of defense signaling.     

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.     

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

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

Cross-talk between jasmonate and salicylate plant defense pathways: effects on several plant parasites

Plants are often attacked by many herbivorous insects and pathogens at the same time. Two important suites of responses to attack are mediated by plant hormones, jasmonate and salicylate, which independently provide resistance to herbivorous insects and pathogens, respectively. Several lines of evidence suggest that there is negative cross-talk between the jasmonate and salicylate response pathways. This biochemical link between general plant defense strategies means that deploying defenses against one attacker can positively or negatively affect other attackers. In this study, we tested for cross-talk in the jasmonate and salicylate signaling pathways in a wild tomato and examined the effects of cross-talk on an array of herbivores of cultivated tomato plants. In the wild cultivar, induction of defenses signaled by salicylate reduced biochemical expression of the jasmonate pathway but did not influence performance of S. exigua caterpillars. This indicates that the signal interaction is not a result of agricultural selection. In cultivated tomato, biochemical attenuation of the activity of a defense protein (polyphenol oxidase) in dual-elicited plants resulted in increased of performance of cabbage looper caterpillars, but not thrips, spider mites, hornworm caterpillars or the bacteria Pseudomonas syringae pv. tomato. In addition, we tested the effects of jasmonate-induced resistance on the ability of thrips to vector tomato spotted wilt virus. Although thrips fed less on induced plants, this did not affect the level of disease. Thus, the negative interaction between jasmonate and salicylate signaling had biological consequences for two lepidopteran larvae but not for several other herbivores tested or on the spread of a disease.     

Probenazole induces systemic acquired resistance in Arabidopsis with a novel type of action

Probenazole (PBZ; 3-allyloxy-1,2-benzisothiazole-1,1-dioxide), which is the active ingredient in Oryzemate, has been used widely in Asia to protect rice plants against the rice blast fungus Magnaporthe grisea. To study PBZ's mode of action, we analyzed its ability, as well as that of its active metabolite 1, 2-benzisothiazol-3 (2H)-one 1,1-dioxide (BIT) to induce defense gene expression and resistance in Arabidopsis mutants that are defective in various defense signaling pathways. Wild-type Arabidopsis treated with PBZ or BIT exhibited increased expression of several pathogenesis-related genes, increased levels of total salicylic acid (SA), and enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC 3000 and the oomycete pathogen Peronospora parasitica Emco5. The role of several defense signaling hormones, such as SA, ethylene and jasmonic acid (JA), in activating resistance following PBZ or BIT treatment was analyzed using NahG transgenic plants and etr1-1 and coi1-1 mutant plants, respectively. In addition, the involvement of NPR1, a key component in the SA signaling pathway leading to defense responses, was assessed. PBZ or BIT treatment did not induce disease resistance or PR-1 expression in NahG transgenic or npr1 mutant plants, but it did activate these phenomena in etr1-1 and coi 1-1 mutant plants. Thus SA and NPR1 appear to be required for PBZ- and BIT-mediated activation of defense responses, while ethylene and JA are not. Furthermore, our data suggest that PBZ and BIT comprise a novel class of defense activators that stimulate the SA/NPR1-mediated defense signaling pathway upstream of SA.     

A two-strain mixture of rhizobacteria elicits induction of systemic resistance against Pseudomonas syringae and Cucumber mosaic virus coupled to promotion of plant growth on Arabidopsis thaliana

We evaluated a commercial biopreparation of plant growth-promoting rhizobacteria (PGPR) strains Bacillus subtilis GB03 and B. amyloliquefaciens IN937a formulated with the carrier chitosan (BioYield) for its capacity to elicit growth promotion and induced systemic resistance against infection by Cucumber Mosaic Virus (CMV) and Pseudomonas syringae pv. tomato DC3000 in Arabidopsis thaliana. The biopreparation promoted plant growth of Arabidopsis hormonal mutants, which included auxin, gibberellic acid, ethylene, jasmonate, salicylic acid, and brassinosteroid insensitive lines as well as each wild-type. The biopreparation protected plants against CMV based on disease severity in wild-type plants. However, virus titre was not lower in control plants and those treated with biopreparation, suggesting that the biopreparation induced tolerance rather than resistance against CMV. Interestingly, the biopreparation induced resistance against CMV in NahG plants, as evidenced by both reduced disease severity and virus titer. The biopreparation also elicited induced resistance against P. syringae pv. tomato in the wild-type but not in NahG transgenic plants, which degrade endogenous salicylic acid, indicating the involvement of salicylic acid signaling. Our results indicate that some PGPR strains can elicit plant growth promotion by mechanisms that are different from known hormonal signaling pathways. In addition, the mechanism for elicitation of induced resistance by PGPR may be pathogen-dependent. Collectively, the two-Bacilli strain mixture can be utilized as a biological inoculant for both protection of plant against bacterial and viral pathogens and enhancement of plant growth.     

Jasmonate-dependent induction of indole glucosinolates in Arabidopsis by culture filtrates of the nonspecific pathogen Erwinia carotovora

Elicitors from the plant pathogen Erwinia carotovora trigger coordinate induction of the tryptophan (Trp) biosynthesis pathway and Trp oxidizing genes in Arabidopsis. To elucidate the biological role of such pathogen-induced activation we characterized the production of secondary defense metabolites such as camalexin and indole glucosinolates derived from precursors of this pathway. Elicitor induction was followed by a specific increase in 3-indolylmethylglucosinolate (IGS) content, but only a barely detectable accumulation of the indole-derived phytoalexin camalexin. The response is mediated by jasmonic acid as shown by lack of IGS induction in the jasmonate-insensitive mutant coi1-1. In accordance with this, methyl jasmonate was able to trigger IGS accumulation in Arabidopsis. In contrast, ethylene and salicylic acid seem to play a minor role in the response. They did not trigger alterations in IGS levels, and methyl jasmonate- or elicitor-induced IGS accumulation in NahG and ethylene-insensitive ein2-1 mutant plants was similar as in the wild type. The breakdown products of IGS and other glucosinolates were able to inhibit growth of E. carotovora. The results suggest that IGS is of importance in the defense against bacterial pathogens.     

Salicylic acid induction-deficient mutants of Arabidopsis express PR-2 and PR-5 and accumulate high levels of camalexin after pathogen inoculation.

In Arabidopsis, systemic acquired resistance against pathogens has been associated with the accumulation of salicylic acid (SA) and the expression of the pathogenesis-related proteins PR-1, PR-2, and PR-5. We report here the isolation of two nonallelic mutants impaired in the pathway leading to SA biosynthesis. These SA induction-deficient (sid) mutants do not accumulate SA after pathogen inoculation and are more susceptible to both virulent and avirulent forms of Pseudomonas syringae and Peronospora parasitica. However, sid mutants are not as susceptible to these pathogens as are transgenic plants expressing the nahG gene encoding an SA hydroxylase that degrades SA to catechol. In contrast to NahG plants, only the expression of PR-1 is strongly reduced in sid mutants, whereas PR-2 and PR-5 are still expressed after pathogen attack. Furthermore, the accumulation of the phytoalexin camalexin is normal. These results indicate that SA-independent compensation pathways that do not operate in NahG plants are active in sid mutants. One of the mutants is allelic to eds5 (for enhanced disease susceptibility), whereas the other mutant has not been described previously.     

PAD4 functions upstream from salicylic acid to control defense responses in Arabidopsis.

The Arabidopsis PAD4 gene was previously shown to be required for synthesis of camalexin in response to infection by the virulent bacterial pathogen Pseudomonas syringae pv maculicola ES4326 but not in response to challenge by the non-host fungal pathogen Cochliobolus carbonum. In this study, we show that pad4 mutants exhibit defects in defense responses, including camalexin synthesis and pathogenesis-related PR-1 gene expression, when infected by P. s. maculicola ES4 326. No such defects were observed in response to infection by an isogenic avirulent strain carrying the avirulence gene avrRpt2. In P. s. maculicola ES4 326-infected pad4 plants, synthesis of salicylic acid (SA) was found to be reduced and delayed when compared with SA synthesis in wild-type plants. Moreover, treatment of pad4 plants with SA partially reversed the camalexin deficiency and PR-1 gene expression phenotypes of P. s. maculicola ES4 326-infected pad4 plants. These findings support the hypothesis that PAD4 acts upstream from SA accumulation in regulating defense response expression in plants infected with P. s. maculicola ES4 326. A working model of the role of PAD4 in governing expression of defense responses is presented.     

Mining the plant-herbivore interface with a leafmining Drosophila of Arabidopsis

Experimental infections of Arabidopsis thaliana (Arabidopsis) with genomically characterized plant pathogens such as Pseudomonas syringae have facilitated the dissection of canonical eukaryotic defence pathways and parasite virulence factors. Plants are also attacked by herbivorous insects, and the development of an ecologically relevant genetic model herbivore that feeds on Arabidopsis will enable the parallel dissection of host defence and reciprocal resistance pathways such as those involved in xenobiotic metabolism. An ideal candidate is Scaptomyza flava, a drosophilid fly whose leafmining larvae are true herbivores that can be found in nature feeding on Arabidopsis and other crucifers. Here, we describe the life cycle of S.?flava on Arabidopsis and use multiple approaches to characterize the response of Arabidopsis to S.?flava attack. Oviposition choice tests and growth performance assays on different Arabidopsis ecotypes, defence-related mutants, and hormone and chitin-treated plants revealed significant differences in host preference and variation in larval performance across Arabidopsis accessions. The jasmonate and glucosinolate pathways in Arabidopsis are important in mediating quantitative resistance against S.?flava, and priming with jasmonate or chitin resulted in increased resistance. Expression of xenobiotic detoxification genes was reduced in S.?flava larvae reared on Arabidopsis jasmonate signalling mutants and increased in plants pretreated with chitin. These results and future research directions are discussed in the context of developing a genetic model system to analyse insect-plant interactions.     

Pseudomonas syringae elicits emission of the terpenoid (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene in Arabidopsis leaves via jasmonate signaling and expression of the terpene synthase TPS4

Volatile, low-molecular weight terpenoids have been implicated in plant defenses, but their direct role in resistance against microbial pathogens is not clearly defined. We have examined a possible role of terpenoid metabolism in the induced defense of Arabidopsis thaliana plants against leaf infection with the bacterial pathogen Pseudomonas syringae. Inoculation of plants with virulent or avirulent P. syringae strains induces the emission of the terpenoids (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT), beta-ionone and alpha-farnesene. While the most abundant volatile, the C16-homoterpene TMTT, is produced relatively early in compatible and incompatible interactions, emission of both beta-ionone and alpha-farnesene only increases in later stages of the compatible interaction. Pathogen-induced synthesis of TMTT is controlled through jasmonic acid (JA)-dependent signaling but is independent of a functional salicylic acid (SA) pathway. We have identified Arabidopsis T-DNA insertion lines with defects in the terpene synthase gene TPS4, which is expressed in response to P. syringae inoculation. The tps4 knockout mutant completely lacks induced emission of TMTT but is capable of beta-ionone and alpha-farnesene production, demonstrating that TPS4 is specifically involved in TMTT formation. The tps4 plants display at least wild type-like resistance against P. syringae, indicating that TMTT per se does not protect against the bacterial pathogen in Arabidopsis leaves. Similarly, the ability to mount SA-dependent defenses and systemic acquired resistance (SAR) is barely affected in tps4, which excludes a signaling function of TMTT during SAR. Besides P. syringae challenge, intoxication of Arabidopsis leaves with copper sulfate, a treatment that strongly activates JA biosynthesis, triggers production of TMTT, beta-ionone, and alpha-farnesene. Taken together, our data suggest that induced TMTT production in Arabidopsis is a by-product of activated JA signaling, rather than an effective defense response that contributes to resistance against P. syringae.     

Harpin induces disease resistance in Arabidopsis through the systemic acquired resistance pathway mediated by salicylic acid and the NIM1 gene

Harpin, the product of the hrpN gene of Erwinia amylovora, elicits the hypersensitive response and disease resistance in many plants. Harpin and known inducers of systemic acquired resistance (SAR) were tested on five genotypes of Arabidopsis thaliana to assess the role of SAR in harpin-induced resistance. In wild-type plants, harpin elicited systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. tomato, accompanied by induction of the SAR genes PR-1 and PR-2. However, in experiments with transgenic Arabidopsis plants containing the nahG gene which prevents accumulation of salicylic acid (SA), harpin neither elicited resistance nor activated SAR gene expression. Harpin also failed to activate SAR when applied to nim1 (non-inducible immunity) mutants, which are defective in responding to SA and regulation of SAR. In contrast, mutants compromised in responsiveness to methyl jasmonate and ethylene developed the same resistance as did wild-type plants. Thus, harpin elicits disease resistance through the NIM1-mediated SAR signal transduction pathway in an SA-dependent fashion. The site of action of harpin in the SAR regulatory pathway is upstream of SA.     

Dissecting the beta-aminobutyric acid-induced priming phenomenon in Arabidopsis

Plants treated with the nonprotein amino acid beta-aminobutyric acid (BABA) develop an enhanced capacity to resist biotic and abiotic stresses. This BABA-induced resistance (BABA-IR) is associated with an augmented capacity to express basal defense responses, a phenomenon known as priming. Based on the observation that high amounts of BABA induce sterility in Arabidopsis thaliana, a mutagenesis screen was performed to select mutants impaired in BABA-induced sterility (ibs). Here, we report the isolation and subsequent characterization of three T-DNA-tagged ibs mutants. Mutant ibs1 is affected in a cyclin-dependent kinase-like protein, and ibs2 is defective in AtSAC1b encoding a polyphosphoinositide phosphatase. Mutant ibs3 is affected in the regulation of the ABA1 gene encoding the abscisic acid (ABA) biosynthetic enzyme zeaxanthin epoxidase. To elucidate the function of the three IBS genes in plant resistance, the mutants were tested for BABA-IR against the bacterium Pseudomonas syringae pv tomato, the oomycete Hyaloperonospora parasitica, and BABA-induced tolerance to salt. All three ibs mutants were compromised in BABA-IR against H. parasitica, although to a different extent. Whereas ibs1 was reduced in priming for salicylate (SA)-dependent trailing necrosis, mutants ibs2 and ibs3 were affected in the priming for callose deposition. Only ibs1 failed to express BABA-IR against P. syringae, which coincided with a defect in priming for SA-inducible PR-1 gene expression. By contrast, ibs2 and ibs3 showed reduced BABA-induced tolerance to salt, which correlated with an affected priming for ABA-inducible gene expression. For all three ibs alleles, the defects in BABA-induced sterility and BABA-induced protection against P. syringae, H. parasitica, and salt could be confirmed in independent mutants. The data presented here introduce three novel regulatory genes involved in priming for different defense responses.