Systemic resistance in Arabidopsis induced by biocontrol bacteria is independent of salicylic acid accumulation and pathogenesis-related gene expression

Salicylic acid-independent plant defence pathways

Salicylic acid is an important signalling molecule involved in both locally and systemically induced disease resistance responses. Recent advances in our understanding of plant defence signalling have revealed that plants employ a network of signal transduction pathways, some of which are independent of salicylic acid. Evidence is emerging that jasmonic acid and ethylene play key roles in these salicylic acid-independent pathways. Cross-talk between the salicylic acid-dependent and the salicylic acid-independent pathways provides great regulatory potential for activating multiple resistance mechanisms in varying combinations.

The Arabidopsis ssi1 mutation restores pathogenesis-related gene expression in npr1 plants and renders defensin gene expression salicylic acid dependent

The Arabidopsis NPR1 gene was previously shown to be required for the salicylic acid (SA)- and benzothiadiazole (BTH)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance. The dominant ssi1 (for suppressor of SA insensitivity) mutation characterized in this study defines a new component of the SA signal transduction pathway that bypasses the requirement of NPR1 for expression of the PR genes and disease resistance. The ssi1 mutation caused PR (PR-1, BGL2 [PR-2], and PR-5) genes to be constitutively expressed and restored resistance to an avirulent strain of Pseudomonas syringae pv tomato in npr1-5 (previously called sai1) mutant plants. In addition, ssi1 plants were small, spontaneously developed hypersensitive response-like lesions, accumulated elevated levels of SA, and constitutively expressed the antimicrobial defensin gene PDF1.2. The phenotypes of the ssi1 mutant are SA dependent. When SA accumulation was prevented in ssi1 npr1-5 plants by expressing the SA-degrading salicylate hydroxylase (nahG) gene, all of the phenotypes associated with the ssi1 mutation were suppressed. However, lesion formation and expression of the PR genes were restored in these plants by the application of BTH. Interestingly, expression of PDF1.2, which previously has been shown to be SA independent but jasmonic acid and ethylene dependent, was also suppressed in ssi1 npr1-5 plants by the nahG gene. Furthermore, exogenous application of BTH restored PDF1.2 expression in these plants. Our results suggest that SSI1 may function as a switch modulating cross-talk between the SA- and jasmonic acid/ethylene-mediated defense signal transduction pathways.

Overexpression of Pto activates defense responses and confers broad resistance

The tomato disease resistance (R) gene Pto specifies race-specific resistance to the bacterial pathogen Pseudomonas syringae pv tomato carrying the avrPto gene. Pto encodes a serine/threonine protein kinase that is postulated to be activated by a physical interaction with the AvrPto protein. Here, we report that overexpression of Pto in tomato activates defense responses in the absence of the Pto-AvrPto interaction. Leaves of three transgenic tomato lines carrying the cauliflower mosaic virus 35S::Pto transgene exhibited microscopic cell death, salicylic acid accumulation, and increased expression of pathogenesis-related genes. Cell death in these plants was limited to palisade mesophyll cells and required light for induction. Mesophyll cells of 35S::Pto plants showed the accumulation of autofluorescent compounds, callose deposition, and lignification. When inoculated with P. s. tomato without avrPto, all three 35S::Pto lines displayed significant resistance and supported less bacterial growth than did nontransgenic lines. Similarly, the 35S::Pto lines also were more resistant to Xanthomonas campestris pv vesicatoria and Cladosporium fulvum. These results demonstrate that defense responses and general resistance can be activated by the overexpression of an R gene.

Concomitant activation of jasmonate and ethylene response pathways is required for induction of a plant defensin gene in Arabidopsis

Activation of the plant defensin gene PDF1.2 in Arabidopsis by pathogens has been shown previously to be blocked in the ethylene response mutant ein2-1 and the jasmonate response mutant coi1-1. In this work, we have further investigated the interactions between the ethylene and jasmonate signal pathways for the induction of this defense response. Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola led to a marked increase in production of jasmonic acid, and this response was not blocked in the ein2-1 mutant. Likewise, A. brassicicola infection caused stimulated emission of ethylene both in wild-type plants and in coi1-1 mutants. However, treatment of either ein2-1 or coi1-1 mutants with methyl jasmonate or ethylene did not induce PDF1.2, as it did in wild-type plants. We conclude from these experiments that both the ethylene and jasmonate signaling pathways need to be triggered concomitantly, and not sequentially, to activate PDF1.2 upon pathogen infection. In support of this idea, we observed a marked synergy between ethylene and methyl jasmonate for the induction of PDF1.2 in plants grown under sterile conditions. In contrast to the clear interdependence of the ethylene and jasmonate pathways for pathogen-induced activation of PDF1.2, functional ethylene and jasmonate signaling pathways are not required for growth responses induced by jasmonate and ethylene, respectively.

Concomitant activation of jasmonate and ethylene response pathways is required for induction of a plant defensin gene in Arabidopsis

Activation of the plant defensin gene PDF1.2 in Arabidopsis by pathogens has been shown previously to be blocked in the ethylene response mutant ein2-1 and the jasmonate response mutant coi1-1. In this work, we have further investigated the interactions between the ethylene and jasmonate signal pathways for the induction of this defense response. Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola led to a marked increase in production of jasmonic acid, and this response was not blocked in the ein2-1 mutant. Likewise, A. brassicicola infection caused stimulated emission of ethylene both in wild-type plants and in coi1-1 mutants. However, treatment of either ein2-1 or coi1-1 mutants with methyl jasmonate or ethylene did not induce PDF1.2, as it did in wild-type plants. We conclude from these experiments that both the ethylene and jasmonate signaling pathways need to be triggered concomitantly, and not sequentially, to activate PDF1.2 upon pathogen infection. In support of this idea, we observed a marked synergy between ethylene and methyl jasmonate for the induction of PDF1.2 in plants grown under sterile conditions. In contrast to the clear interdependence of the ethylene and jasmonate pathways for pathogen-induced activation of PDF1.2, functional ethylene and jasmonate signaling pathways are not required for growth responses induced by jasmonate and ethylene, respectively.

Biocontrol by Phenazine-1-carboxamide-Producing Pseudomonas chlororaphis PCL1391 of Tomato Root Rot Caused by Fusarium oxysporum f. sp. radicis-lycopersici

Seventy bacterial isolates from the rhizosphere of tomato were screened for antagonistic activity against the tomato foot and root rot-causing fungal pathogen Fusarium oxysporum f. sp. radicis-lycopersici. One isolate, strain PCL1391, appeared to be an efficient colonizer of tomato roots and an excellent biocontrol strain in an F. oxysporum/tomato test system. Strain PCL1391 was identified as Pseudomonas chlororaphis and further characterization showed that it produces a broad spectrum of antifungal factors (AFFs), including a hydrophobic compound, hydrogen cyanide, chitinase(s), and protease(s). Through mass spectrometry and nuclear magnetic resonance, the hydrophobic compound was identified as phenazine-1-carboxamide (PCN). We have studied the production and action of this AFF both in vitro and in vivo. Using a PCL1391 transposon mutant, with a lux reporter gene inserted in the phenazine biosynthetic operon (phz), we showed that this phenazine biosynthetic mutant was substantially decreased in both in vitro antifungal activity and biocontrol activity. Moreover, with the same mutant it was shown that the phz biosynthetic operon is expressed in the tomato rhizosphere. Comparison of the biocontrol activity of the PCN-producing strain PCL1391 with those of phenazine-1-carboxylic acid (PCA)-producing strains P. fluorescens 2-79 and P. aureofaciens 30-84 showed that the PCN-producing strain is able to suppress disease in the tomato/F. oxysporum system, whereas the PCA-producing strains are not. Comparison of in vitro antifungal activity of PCN and PCA showed that the antifungal activity of PCN was at least 10 times higher at neutral pH, suggesting that this may contribute to the superior biocontrol performance of strain PCL1391 in the tomato/F. oxysporum system.

Jasmonate and salicylate as global signals for defense gene expression

Remarkably, only a few low molecular mass signals, including jasmonic acid, ethylene and salicylic acid, upregulate the expression of scores of defense-related genes. Using these regulators, the plant fine-tunes its defense gene expression against aggressors which, in some cases, may be able to disrupt or amplify plant defense signal pathways to their own ends.

Non-toxic concentrations of cadmium inhibit systemic movement of turnip vein clearing virus by a salicylic acid-independent mechanism

Systemic movement of plant viruses is a central event in viral infection. To better understand this process, the heavy metal cadmium was used to inhibit systemic spread of turnip vein clearing virus (TVCV), a tobamovirus, in tobacco plants. Study of the mechanism by which cadmium exerts this inhibitory effect may provide insights into the essential steps of the TVCV systemic movement pathway. Our results demonstrated that cadmium treatment did not affect TVCV transport from the inoculated non-vascular tissue into the plant vasculature but blocked viral exit into uninoculated non-vascular tissues. Thus, TVCV virions may enter and exit the host plant vascular system by two different mechanisms. We also showed that cadmium-treated plants still supported systemic spread of an unrelated tobacco etch virus (TEV), suggesting multiple pathways for systemic infection. Finally, cadmium-induced arrest in TVCV systemic infection was shown to occur by a salicylic acid-independent mechanism.

A novel signaling pathway controlling induced systemic resistance in Arabidopsis

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis, nonpathogenic, root-colonizing Pseudomonas fluorescens bacteria trigger an induced systemic resistance (ISR) response against infection by the bacterial leaf pathogen P. syringae pv tomato. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR response is independent of salicylic acid accumulation and pathogenesis-related gene activation. Using the jasmonate response mutant jar1, the ethylene response mutant etr1, and the SAR regulatory mutant npr1, we demonstrate that signal transduction leading to P. fluorescens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene and is dependent on NPR1. Similar to P. fluorescens WCS417r, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylate were effective in inducing resistance against P. s. tomato in salicylic acid-nonaccumulating NahG plants. Moreover, methyl jasmonate-induced protection was blocked in jar1, etr1, and npr1 plants, whereas 1-aminocyclopropane-1-carboxylate-induced protection was affected in etr1 and npr1 plants but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signaling pathway in which components from the jasmonate and ethylene response are engaged successively to trigger a defense reaction that, like SAR, is regulated by NPR1. We provide evidence that the processes downstream of NPR1 in the ISR pathway are divergent from those in the SAR pathway, indicating that NPR1 differentially regulates defense responses, depending on the signals that are elicited during induction of resistance.

Characterization of a tobacco epoxide hydrolase gene induced during the resistance response to TMV

A clone encoding a putative soluble epoxide hydrolase (EH-1), an enzyme which converts epoxides to diols, was isolated by differential screening of a cDNA library prepared from tobacco mosaic virus (TMV)-infected tobacco leaves. To confirm that EH-1 encodes an epoxide hydrolase, the recombinant EH-1 protein produced in bacteria was shown to have high epoxide hydrolase activity in vitro. Infection of resistant but not susceptible tobacco cultivars induced the accumulation of EH-1 transcripts in both the inoculated and uninoculated, systemic leaves. EH-1 expression was also induced in the inoculated and systemic tissues of TMV-infected NahG plants, which are unable to accumulate salicylic acid (SA). However, EH-1 expression in the inoculated leaves of NahG plants was delayed, whilst in the systemic leaves the induction was both later and weaker, compared to that observed in wild-type plants. Furthermore, exogenously applied SA or its functional analog 2,6-dichloroisonicotinic acid (INA) caused a rapid and transient accumulation of EH-1 transcripts, whereas an inactive SA analog did not. Thus, the induction of EH-1 gene expression appears to be regulated by both SA-independent and SA-dependent pathways. Since EH-1 was expressed only in TMV-resistant tobacco after infection, and the encoded enzyme is thought to help metabolize toxic compounds, we propose that EH-1 may play a role in protection from oxidative damage associated with defense responses. It may also play a role in generating signals for activation of certain defense responses.

A novel signaling pathway controlling induced systemic resistance in Arabidopsis

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis, nonpathogenic, root-colonizing Pseudomonas fluorescens bacteria trigger an induced systemic resistance (ISR) response against infection by the bacterial leaf pathogen P. syringae pv tomato. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR response is independent of salicylic acid accumulation and pathogenesis-related gene activation. Using the jasmonate response mutant jar1, the ethylene response mutant etr1, and the SAR regulatory mutant npr1, we demonstrate that signal transduction leading to P. fluorescens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene and is dependent on NPR1. Similar to P. fluorescens WCS417r, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylate were effective in inducing resistance against P. s. tomato in salicylic acid-nonaccumulating NahG plants. Moreover, methyl jasmonate-induced protection was blocked in jar1, etr1, and npr1 plants, whereas 1-aminocyclopropane-1-carboxylate-induced protection was affected in etr1 and npr1 plants but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signaling pathway in which components from the jasmonate and ethylene response are engaged successively to trigger a defense reaction that, like SAR, is regulated by NPR1. We provide evidence that the processes downstream of NPR1 in the ISR pathway are divergent from those in the SAR pathway, indicating that NPR1 differentially regulates defense responses, depending on the signals that are elicited during induction of resistance.

SA, JA, ethylene, and disease resistance in plants

Exciting advances have been made during the past year: isolating mutants affecting plant disease resistance, cloning genes involved in the regulation of various defense responses, and characterizing novel defense signaling pathways. Recent studies have demonstrated that jasmonic acid and ethylene are important for the induction of nonspecific disease resistance through signaling pathways that are distinct from the classical systemic acquired resistance response pathway regulated by salicylic acid.

Salicylic Acid Biosynthetic Genes Expressed in Pseudomonas fluorescens Strain P3 Improve the Induction of Systemic Resistance in Tobacco Against Tobacco Necrosis Virus

ABSTRACT Application of salicylic acid induces systemic acquired resistance in tobacco. pchA and pchB, which encode for the biosynthesis of salicylic acid in Pseudomonas aeruginosa, were cloned into two expression vectors, and these constructs were introduced into two root-colonizing strains of P. fluorescens. Introduction of pchBA into strain P3, which does not produce salicylic acid, rendered this strain capable of salicylic acid production in vitro and significantly improved its ability to induce systemic resistance in tobacco against tobacco necrosis virus. Strain CHA0 is a well-described biocontrol agent that naturally produces salicylic acid under conditions of iron limitation. Introduction of pchBA into CHA0 increased the production of salicylic acid in vitro and in the rhizosphere of tobacco, but did not improve the ability of CHA0 to induce systemic resistance in tobacco. In addition, these genes did not improve significantly the capacity of strains P3 and CHA0 to suppress black root rot of tobacco in a gnotobiotic system.

Gene-for-gene disease resistance without the hypersensitive response in Arabidopsis dnd1 mutant

The cell death response known as the hypersensitive response (HR) is a central feature of gene-for-gene plant disease resistance. A mutant line of Arabidopsis thaliana was identified in which effective gene-for-gene resistance occurs despite the virtual absence of HR cell death. Plants mutated at the DND1 locus are defective in HR cell death but retain characteristic responses to avirulent Pseudomonas syringae such as induction of pathogenesis-related gene expression and strong restriction of pathogen growth. Mutant dnd1 plants also exhibit enhanced resistance against a broad spectrum of virulent fungal, bacterial, and viral pathogens. The resistance against virulent pathogens in dnd1 plants is quantitatively less strong and is differentiable from the gene-for-gene resistance mediated by resistance genes RPS2 and RPM1. Levels of salicylic acid compounds and mRNAs for pathogenesis-related genes are elevated constitutively in dnd1 plants. This constitutive induction of systemic acquired resistance may substitute for HR cell death in potentiating the stronger gene-for-gene defense response. Although cell death may contribute to defense signal transduction in wild-type plants, the dnd1 mutant demonstrates that strong restriction of pathogen growth can occur in the absence of extensive HR cell death in the gene-for-gene resistance response of Arabidopsis against P. syringae.

An Arabidopsis mutant with enhanced resistance to powdery mildew

We have identified an Arabidopsis mutant that displays enhanced disease resistance to the fungus Erysiphe cichoracearum, causal agent of powdery mildew. The edr1 mutant does not constitutively express the pathogenesis-related genes PR-1, BGL2, or PR-5 and thus differs from previously described disease-resistant mutants of Arabidopsis. E. cichoracearum conidia (asexual spores) germinated normally and formed extensive hyphae on edr1 plants, indicating that the initial stages of infection were not inhibited. Production of conidiophores on edr1 plants, however, was <16% of that observed on wild-type Arabidopsis. Reduction in sporulation correlated with a more rapid induction of defense responses. Autofluorescent compounds and callose accumulated in edr1 leaves 3 days after inoculation with E. cichoracearum, and dead mesophyll cells accumulated in edr1 leaves starting 5 days after inoculation. Macroscopic patches of dead cells appeared 6 days after inoculation. This resistance phenotype is similar to that conferred by "late-acting" powdery mildew resistance genes of wheat and barley. The edr1 mutation is recessive and maps to chromosome 1 between molecular markers ATEAT1 and NCC1. We speculate that the edr1 mutation derepresses multiple defense responses, making them more easily induced by virulent pathogens.

An Arabidopsis mutant with enhanced resistance to powdery mildew

We have identified an Arabidopsis mutant that displays enhanced disease resistance to the fungus Erysiphe cichoracearum, causal agent of powdery mildew. The edr1 mutant does not constitutively express the pathogenesis-related genes PR-1, BGL2, or PR-5 and thus differs from previously described disease-resistant mutants of Arabidopsis. E. cichoracearum conidia (asexual spores) germinated normally and formed extensive hyphae on edr1 plants, indicating that the initial stages of infection were not inhibited. Production of conidiophores on edr1 plants, however, was <16% of that observed on wild-type Arabidopsis. Reduction in sporulation correlated with a more rapid induction of defense responses. Autofluorescent compounds and callose accumulated in edr1 leaves 3 days after inoculation with E. cichoracearum, and dead mesophyll cells accumulated in edr1 leaves starting 5 days after inoculation. Macroscopic patches of dead cells appeared 6 days after inoculation. This resistance phenotype is similar to that conferred by "late-acting" powdery mildew resistance genes of wheat and barley. The edr1 mutation is recessive and maps to chromosome 1 between molecular markers ATEAT1 and NCC1. We speculate that the edr1 mutation derepresses multiple defense responses, making them more easily induced by virulent pathogens.

Compost and compost water extract-induced systemic acquired resistance in cucumber and Arabidopsis

ABSTRACT A biocontrol agent-fortified compost mix, suppressive to several diseases caused by soilborne plant pathogens, induced systemic acquired resistance (SAR) in cucumber against anthracnose caused by Colletotrichum orbiculare and in Arabidopsis against bacterial speck caused by Pseudomonas syringae pv. maculicola KD4326. A peat mix conducive to soilborne diseases did not induce SAR. The population size of P. syringae pv. maculicola KD4326 was significantly lower in leaves of Arabidopsis plants grown in the compost mix compared to those grown in the peat mix. Autoclaving destroyed the SAR-inducing effect of the compost mix, and inoculation of the autoclaved mix with nonautoclaved compost mix or Pantoea agglomerans 278A restored the effect, suggesting the SAR-inducing activity of the compost mix was biological in nature. Topical sprays with water extract prepared from the compost mix reduced symptoms of bacterial speck and the population size of pathogenic KD4326 in Arabidopsis grown in the peat mix but not in the compost mix. The peat mix water extract applied as a spray did not control bacterial speck on plants grown in either mix. Topical sprays with salicylic acid (SA) reduced the severity of bacterial speck on plants in the peat mix but did not further reduce the severity of symptoms on plants in the compost mix. The activity of the compost water extract was heat-stable and passed through a 0.2-mum membrane filter. beta-1,3-Glucanase activity was low in cucumber plants grown in either mix, but when infected with C. orbiculare, this activity was induced to significantly higher levels in plants grown in the compost mix than in plants grown in the peat mix. Similar results were obtained for beta-D-glucuronidase (GUS) activity driven by a PR2 (beta-1,3-glucanase) gene promoter in transgenic Arabidopsis plants grown in the compost or peat mix. GUS activity was induced with topical sprays of the compost water extract or SA in plants not inoculated with the pathogen, suggesting that compost-induced disease suppression more than likely involved the potentiation of resistance responses rather than their activation and that compost-induced SAR differed from SAR induced by pathogens, SA, or compost water extract.

Uncoupling PR gene expression from NPR1 and bacterial resistance: characterization of the dominant Arabidopsis cpr6-1 mutant

In Arabidopsis, NPR1 mediates the salicylic acid (SA)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance (SAR). Here, we report the identification of another component, CPR 6, that may function with NPR1 in regulating PR gene expression. The dominant CPR 6-1 mutant expresses the SA/NPR1-regulated PR genes (PR-1, BGL 2, and PR-5) and displays enhanced resistance to Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2 in the absence of SAR induction. cpr 6-1-induced PR gene expression is not suppressed in the cpr 6-1 npr1-1 double mutant but is suppressed when SA is removed by salicylate hydroxylase. Thus, constitutive PR gene expression in cpr 6-1 requires SA but not NPR1. In addition, resistance to P. s. maculicola ES4326 is suppressed in the cpr 6-1 npr1-1 double mutant, despite expression of PR-1, BGL 2, and PR-5. Resistance to P. s. maculicola ES4326 must therefore be accomplished through unidentified antibacterial gene products that are regulated through NPR1. These results show that CPR 6 is an important regulator of multiple signal transduction pathways involved in plant defense.

Inhibition of plant viral systemic infection by non-toxic concentrations of cadmium

Heavy metal, such as cadmium, have a significant impact on plant physiology. However, their potential effect on plant-pathogen interaction, an important biological process, has not been examined. This study shows that exposure of tobacco plants to non-toxic concentrations of cadmium completely blocked viral disease caused by turnip vein clearing virus. Cadmium-mediated viral protection was due to inhibition of the systemic movement of the virus, i.e. its spread from the inoculated into uninoculated leaves. Exposure of plants to cadmium had no effect on viral replication, assembly and local movement within the inoculated leaf. Analysis of the viral presence in different tissues suggested that cadmium treatment inhibited virus exit from the vascular tissue into uninoculated leaves rather than its entry into the host plant vasculature. Higher, toxic levels of cadmium did not produce this inhibitory effect on viral movement, allowing the systemic spread of the virus and development of the viral disease. These observations suggest that cadmium-induced viral protection requires a relatively healthy, unpoisoned plant in which non-toxic levels of cadmium may trigger the production of cellular factors which interfere with the viral systemic movement.

Antimicrobial proteins in induced plant defense

During the past few years a wide spectrum of plant antimicrobial proteins has been detailed, and enhanced resistance has been obtained by introducing the corresponding genes into crop species to produce transgenic lines. With the aim of manipulating the plant signals that regulate an array of defense responses, the most intense research focuses on the avr-R-mediated recognition events and elucidation of the subsequent signaling pathways that govern the activation of genes encoding antimicrobial proteins.

Systemic resistance induced by rhizosphere bacteria

Nonpathogenic rhizobacteria can induce a systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Rhizobacteria-mediated induced systemic resistance (ISR) has been demonstrated against fungi, bacteria, and viruses in Arabidopsis, bean, carnation, cucumber, radish, tobacco, and tomato under conditions in which the inducing bacteria and the challenging pathogen remained spatially separated. Bacterial strains differ in their ability to induce resistance in different plant species, and plants show variation in the expression of ISR upon induction by specific bacterial strains. Bacterial determinants of ISR include lipopolysaccharides, siderophores, and salicylic acid (SA). Whereas some of the rhizobacteria induce resistance through the SA-dependent SAR pathway, others do not and require jasmonic acid and ethylene perception by the plant for ISR to develop. No consistent host plant alterations are associated with the induced state, but upon challenge inoculation, resistance responses are accelerated and enhanced. ISR is effective under field conditions and offers a natural mechanism for biological control of plant disease.

The cpr5 mutant of Arabidopsis expresses both NPR1-dependent and NPR1-independent resistance

The cpr5 mutant was identified from a screen for constitutive expression of systemic acquired resistance (SAR). This single recessive mutation also leads to spontaneous expression of chlorotic lesions and reduced trichome development. The cpr5 plants were found to be constitutively resistant to two virulent pathogens, Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2; to have endogenous expression of the pathogenesis-related gene 1 (PR-1); and to have an elevated level of salicylic acid (SA). Lines homozygous for cpr5 and either the SA-degrading bacterial gene nahG or the SA-insensitive mutation npr1 do not express PR-1 or exhibit resistance to P. s. maculicola ES4326. Therefore, we conclude that cpr5 acts upstream of SA in inducing SAR. However, the cpr5 npr1 plants retained heightened resistance to P. parasitica Noco2 and elevated expression of the defensin gene PDF1.2, implying that NPR1-independent resistance signaling also occurs. We conclude that the cpr5 mutation leads to constitutive expression of both an NPR1-dependent and an NPR1-independent SAR pathway. Identification of this mutation indicates that these pathways are connected in early signal transduction steps and that they have overlapping functions in providing resistance.

The cpr5 mutant of Arabidopsis expresses both NPR1-dependent and NPR1-independent resistance

The cpr5 mutant was identified from a screen for constitutive expression of systemic acquired resistance (SAR). This single recessive mutation also leads to spontaneous expression of chlorotic lesions and reduced trichome development. The cpr5 plants were found to be constitutively resistant to two virulent pathogens, Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2; to have endogenous expression of the pathogenesis-related gene 1 (PR-1); and to have an elevated level of salicylic acid (SA). Lines homozygous for cpr5 and either the SA-degrading bacterial gene nahG or the SA-insensitive mutation npr1 do not express PR-1 or exhibit resistance to P. s. maculicola ES4326. Therefore, we conclude that cpr5 acts upstream of SA in inducing SAR. However, the cpr5 npr1 plants retained heightened resistance to P. parasitica Noco2 and elevated expression of the defensin gene PDF1.2, implying that NPR1-independent resistance signaling also occurs. We conclude that the cpr5 mutation leads to constitutive expression of both an NPR1-dependent and an NPR1-independent SAR pathway. Identification of this mutation indicates that these pathways are connected in early signal transduction steps and that they have overlapping functions in providing resistance.

Differential induction of systemic resistance in Arabidopsis by biocontrol bacteria

Selected nonpathogenic, root-colonizing bacteria are able to elicit induced systemic resistance (ISR) in plants. To elucidate the molecular mechanisms underlying this type of systemic resistance, an Arabidopsis-based model system was developed in which Pseudomonas syringae pv. tomato and Fusarium oxysporum f. sp. raphani were used as challenging pathogens. In Arabidopsis thaliana ecotypes Columbia and Landsberg erecta, colonization of the rhizosphere by P. fluorescens strain WCS417r induced systemic resistance against both pathogens. In contrast, ecotype RLD did not respond to WCS417r treatment, whereas all three ecotypes expressed systemic acquired resistance upon treatment with salicylic acid (SA). P. fluorescens strain WCS374r, previously shown to induce ISR in radish, did not elicit ISR in Arabidopsis. The opposite was found for P. putida strain WCS358r, which induced ISR in Arabidopsis but not in radish. These results demonstrate that rhizosphere pseudomonads are differentially active in eliciting ISR in related plant species. The outer membrane lipopolysaccharide (LPS) of WCS417r is the main ISR-inducing determinant in radish and carnation, and LPS-containing cell walls also elicit ISR in Arabidopsis. However, mutant WCS417rOA-, lacking the O-antigenic side chain of the LPS, induced levels of protection similar to those induced by wild-type WCS417r. This indicates that ISR-inducing bacteria produce more than a single factor that trigger ISR in Arabidopsis. Furthermore, WCS417r and WCS358r induced protection in both wild-type Arabidopsis and SA-nonaccumulating NahG plants without activating pathogenesis-related gene expression. This suggests that elicitation of an SA-independent signaling pathway is a characteristic feature of ISR-inducing biocontrol bacteria.

Expression of Pokeweed Antiviral Protein in Transgenic Plants Induces Virus Resistance in Grafted Wild-Type Plants Independently of Salicylic Acid Accumulation and Pathogenesis-Related Protein Synthesis

Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytolacca americana, inhibits translation by catalytically removing a specific adenine residue from the large rRNA of the 60S subunit of eukaryotic ribosomes. Transgenic tobacco (Nicotiana tabacum) plants expressing PAP or a variant (PAP-v) were shown to be resistant to a broad spectrum of plant viruses. Expression of PAP-v in transgenic plants induces synthesis of pathogenesis-related proteins and a very weak (<2-fold) increase in salicylic acid levels. Using reciprocal grafting experiments, we demonstrate here that transgenic tobacco rootstocks expressing PAP-v induce resistance to tobacco mosaic virus infection in both N. tabacum NN and nn scions. Increased resistance to potato virus X was also observed in N. tabacum nn scions grafted on transgenic rootstocks. PAP expression was not detected in the wild-type scions or rootstocks that showed virus resistance, nor was there any increase in salicylic acid levels or pathogenesis-related protein synthesis. Grafting experiments with transgenic plants expressing an inactive PAP mutant demonstrated that an intact active site of PAP is necessary for induction of virus resistance in wild-type scions. These results indicate that enzymatic activity of PAP is responsible for generating a signal that renders wild-type scions resistant to virus infection in the absence of increased salicylic acid levels and pathogenesis-related protein synthesis.

Salicylic Acid Produced by the Rhizobacterium Pseudomonas aeruginosa 7NSK2 Induces Resistance to Leaf Infection by Botrytis cinerea on Bean

ABSTRACT Selected strains of nonpathogenic rhizobacteria can induce a systemic resistance in plants that is effective against various pathogens. In an assay with bean plants, we investigated which determinants of the rhizobacterium Pseudomonas aeruginosa 7NSK2 are important for induction of resistance to Botrytis cinerea. By varying the iron nutritional state of the bacterium at inoculation, it was demonstrated that induced resistance by P. aeruginosa 7NSK2 was iron-regulated. As P. aeruginosa 7NSK2 produces three siderophores under iron limitation, pyoverdin, pyochelin, and salicylic acid, we investigated the involvement of these iron-regulated metabolites in induced resistance by using mutants deficient in one or more siderophores. Results demonstrated that salicylic acid production was essential for induction of resistance to B. cinerea by P. aeruginosa 7NSK2 in bean and did not exclude a role for pyochelin. A role for pyoverdin, however, could not be demonstrated. Transcriptional activity of salicylic acid and pyochelin biosynthetic genes was detected during P. aeruginosa 7NSK2 colonization of bean. Moreover, the iron nutritional state at inoculation influenced the transcriptional activity of salicylic acid and pyochelin biosynthetic genes in the same way as it influenced induction of systemic resistance to B. cinerea.

Genetic engineering for fungal and bacterial diseases

Significant new advances at the molecular level in the field of plant-pathogen interactions form the basis for novel transgenic approaches to crop protection. The cloning of disease resistance genes and the dissection of the signal transduction components of the hypersensitive response and systemic acquired resistance pathways have greatly increased the diversity of options available for transgenic disease resistance. These new approaches will supplement our rapidly increasing repertoire of antimicrobial peptides, defense-related proteins and antimicrobial compounds. The combinatorial deployment of these strategies will be exploited for engineering effective and durable resistance to pathogens in the field. The integration of transgenic approaches with classical resistance breeding offers a potentially chemical-free and environmentally friendly solution for controlling pathogens.

Overexpression of an endogenous thionin enhances resistance of Arabidopsis against Fusarium oxysporum

Thionins are antimicrobial proteins that are thought to be involved in plant defense. Concordant with this view, we have recently shown that the Arabidopsis thionin Thi2.1 gene is inducible by phytopathogenic fungi. Here, we demonstrate that constitutive overexpression of this thionin enhances the resistance of the susceptible ecotype Columbia (Col-2) against attack by Fusarium oxysporum f sp matthiolae. Transgenic lines had a reduced loss of chlorophyll after inoculation and supported significantly less fungal growth on the cotyledons, as evaluated by trypan blue staining. Moreover, fungi on cotyledons of transgenic lines had more hyphae with growth anomalies, including hyperbranching, than on cotyledons of the parental line. No transcripts for pathogenesis-related PR-1, PR-5, or the pathogen-inducible plant defensin Pdf1.2 could be detected in uninoculated transgenic seedlings, indicating that all of the observed effects of the overexpressing lines are most likely the result of the toxicity of the THI2.1 thionin. Our findings strongly support the view that thionins are defense proteins.

Overexpression of an endogenous thionin enhances resistance of Arabidopsis against Fusarium oxysporum

Thionins are antimicrobial proteins that are thought to be involved in plant defense. Concordant with this view, we have recently shown that the Arabidopsis thionin Thi2.1 gene is inducible by phytopathogenic fungi. Here, we demonstrate that constitutive overexpression of this thionin enhances the resistance of the susceptible ecotype Columbia (Col-2) against attack by Fusarium oxysporum f sp matthiolae. Transgenic lines had a reduced loss of chlorophyll after inoculation and supported significantly less fungal growth on the cotyledons, as evaluated by trypan blue staining. Moreover, fungi on cotyledons of transgenic lines had more hyphae with growth anomalies, including hyperbranching, than on cotyledons of the parental line. No transcripts for pathogenesis-related PR-1, PR-5, or the pathogen-inducible plant defensin Pdf1.2 could be detected in uninoculated transgenic seedlings, indicating that all of the observed effects of the overexpressing lines are most likely the result of the toxicity of the THI2.1 thionin. Our findings strongly support the view that thionins are defense proteins.

Systemic acquired resistance

This paper examines induced resistance (SAR) in plants against various insect and pathogenic invaders. SAR confers quantitative protection against a broad spectrum of microorganisms in a manner comparable to immunization in mammals, although the underlying mechanisms differ. Discussed here are the molecular events underlying SAR: the mechanisms involved in SAR, including lignification and other structural barriers, pathogenesis-related proteins and their expression, and the signals for SAR including salicylic acid. Recent findings on the biological role of systemin, ethylene, jasmonates, and electrical signals are reviewed. Chemical activators of SAR comprise inorganic compounds, natural compounds, and synthetic compounds. Plants known to exhibit SAR and induced systemic resistance are listed.