Arabidopsis dth9 mutation identifies a gene involved in regulating disease susceptibility without affecting salicylic acid-dependent responses

To determine which components of the plant defense response make important contributions to limiting pathogen attack, an M(2) mutagenized population of a transgenic Arabidopsis line was screened for mutants showing constitutive expression of beta-glucuronidase activity driven by the promoter region of the CEVI-1 gene. The CEVI-1 gene originally was isolated from tomato plants and has been shown to be induced in susceptible varieties of tomato plants by virus infection in a salicylic acid-independent manner. We report here the characterization of a recessive mutant, detachment9 (dth9). This mutant is more susceptible to both virulent and avirulent forms of the oomycete Peronospora and also exhibits increased susceptibility to the moderately virulent bacterial pathogen Pseudomonas syringae pv maculicola ES4326. However, this mutant is not affected in salicylic acid metabolism and shows normal expression of pathogenesis-related (PR) genes after pathogen attack. Furthermore, after inoculation with avirulent pathogens, the dth9 mutant shows a compromised systemic acquired resistance response that cannot be complemented by exogenous application of salicylic acid, although this molecule is able to promote normal activation of PR genes. Therefore, the dth9 mutation defines a regulator of disease susceptibility that operates upstream or independently of salicylic acid. Pleiotropy is also evident in the dth9 mutant in the sense that the shoots of dth9 plants are insensitive to the exogenously applied auxin analog 2,4-dichlorophenoxyacetic acid.     

Salicylic acid-mediated innate immunity in Arabidopsis is regulated by SIZ1 SUMO E3 ligase

Reversible modifications of target proteins by small ubiquitin-like modifier (SUMO) proteins are involved in many cellular processes in yeast and animals. Yet little is known about the function of sumoylation in plants. Here, we show that the SIZ1 gene, which encodes an Arabidopsis SUMO E3 ligase, regulates innate immunity. Mutant siz1 plants exhibit constitutive systemic-acquired resistance (SAR) characterized by elevated accumulation of salicylic acid (SA), increased expression of pathogenesis-related (PR) genes, and increased resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Transfer of the NahG gene to siz1 plants results in reversal of these phenotypes back to wild-type. Analyses of the double mutants, npr1 siz1, pad4 siz1 and ndr1 siz1 revealed that SIZ1 controls SA signalling. SIZ1 interacts epistatically with PAD4 to regulate PR expression and disease resistance. Consistent with these observations, siz1 plants exhibited enhanced resistance to Pst DC3000 expressing avrRps4, a bacterial avirulence determinant that responds to the EDS1/PAD4-dependent TIR-NBS-type R gene. In contrast, siz1 plants were not resistant to Pst DC3000 expressing avrRpm1, a bacterial avirulence determinant that responds to the NDR1-dependent CC-NBS-type R gene. Jasmonic acid (JA)-induced PDF1.2 expression and susceptibility to Botrytis cinerea were unaltered in siz1 plants. Taken together, these results demonstrate that SIZ1 is required for SA and PAD4-mediated R gene signalling, which in turn confers innate immunity in Arabidopsis.     

Potential of salicylic acid activity derived from stress-induced (water) Tomato against Meloidogyne incognita

Plants have evolved several types of sophisticated defence mechanisms to protect themselves from enemies in which the signalling molecules such as salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) often play crucial roles. An experiment was conducted to investigate the water induced stress in young tomato plants (Lycopersicon esculentum var. Pusa Ruby) during water deprivation and after relief from stress in association with salicylic acid. Application of SA or certain analogues such as aspirin induces expression of pathogenesis-related proteins (PR) in plants which serve as molecular markers for the systemic acquired response (SAR). In plants, the production of reactive oxygen species (ROS), such as superoxide radical (O₂), hydroxyl radical (OH) and hydrogen peroxide (H₂O₂), is also common in metabolic and physiological processes. Similar to the defence-regulating compounds such as SA and JA, plant-derived ET is also known to be involved in disease resistance. The role of ROS or ET could be investigated which was carried out in the present study to know the osmotic stress mediated resistance by SA against Meloidogyne incognita in tomato plants.     

The H2O2-regulated Ep5C gene encodes a peroxidase required for bacterial speck susceptibility in tomato

Bacterial speck caused by the pathogen Pseudomonas syringae pv. tomato (P. s. tomato) is a devastating disease of tomato plants. Here we show that inhibition of Ep5C gene expression, which encodes a secreted cationic peroxidase, is sufficient to confer resistance against P. s. tomato. The inhibition of Ep5C protein accumulation in antisense tomato plants established resistance that was not accompanied by the pre-activation of known defense pathways. Therefore, Ep5C inhibition represents a novel form of disease resistance based on a loss-of-gene function in the plant required for successful infection by a compatible bacterial pathogen. Ep5C expression is rapidly induced by H2O2, a reactive oxygen intermediate normally generated during the course of a plant-pathogen interaction. This was corroborated by monitoring the expression of an Ep5C-GUS gene in transgenic Arabidopsis plants. Collectively, these results identify a signaling pathway that uses early signals generated during the oxidative burst, such as H2O2, for the selective activation of host factors required for mounting a compatible interaction. Thus, Ep5C provides a new resource for developing bacterial speck disease-resistant varieties.     

Abscisic acid determines basal susceptibility of tomato to Botrytis cinerea and suppresses salicylic acid-dependent signaling mechanisms

Abscisic acid (ABA) is one of the plant hormones involved in the interaction between plants and pathogens. In this work, we show that tomato (Lycopersicon esculentum Mill. cv Moneymaker) mutants with reduced ABA levels (sitiens plants) are much more resistant to the necrotrophic fungus Botrytis cinerea than wild-type (WT) plants. Exogenous application of ABA restored susceptibility to B. cinerea in sitiens plants and increased susceptibility in WT plants. These results indicate that ABA plays a major role in the susceptibility of tomato to B. cinerea. ABA appeared to interact with a functional plant defense response against B. cinerea. Experiments with transgenic NahG tomato plants and benzo(1,2,3)thiadiazole-7-carbothioic acid demonstrated the importance of salicylic acid in the tomato-B. cinerea interaction. In addition, upon infection with B. cinerea, sitiens plants showed a clear increase in phenylalanine ammonia lyase activity, which was not observed in infected WT plants, indicating that the ABA levels in healthy WT tomato plants partly repress phenylalanine ammonia lyase activity. In addition, sitiens plants became more sensitive to benzo(1,2,3)thiadiazole-7-carbothioic acid root treatment. The threshold values for PR1a gene expression declined with a factor 10 to 100 in sitiens compared with WT plants. Thus, ABA appears to negatively modulate the salicylic acid-dependent defense pathway in tomato, which may be one of the mechanisms by which ABA levels determine susceptibility to B. cinerea.     

Suppression of CaCYP1, a novel cytochrome P450 gene, compromises the basal pathogen defense response of pepper plants

A putative cytochrome P450 gene from chili pepper, Capsicum annuum L. Bukang cytochrome P450 (CaCYP1), was identified using cDNA microarray analysis of gene expression following induction of the leaf hypersensitive response by inoculation of pepper plants with the non-host pathogen Xanthomonas axonopodis pv. glycines 8ra. The full-length cDNA of CaCYP1 encoded a protein of 514 amino acid residues, which contained a putative hydrophobic membrane anchoring domain in the N-terminal region, and a heme-binding motif in the C-terminal region. Analysis of the deduced amino acid sequence of CaCYP1 revealed that it has high homology to Arabidopsis CYP89A5, the function of which is unknown. Expression of CaCYP1 was preferentially increased in pepper plants in response to non-host pathogen inoculation and also during the host resistance response. CaCYP1 expression also increased following treatment with salicylic acid and abscisic acid, while treatment with ethylene had a mild effect. Using a virus-induced gene silencing-based reverse genetics approach, we demonstrated that suppression of CaCYP1 results in enhanced susceptibility to bacterial pathogens. Interestingly, gene silencing of CaCYP1 in pepper plants resulted in the reduced expression of the defense-related genes CaLTP1, CaSIG4, and Cadhn. Our results indicated that CaCYP1, a novel cytochrome P450 in pepper plants, may play a role in plant defense response pathways that involve salicylic acid and abscisic acid signaling pathways.     

Salicylic acid is not required for Cf-2- and Cf-9-dependent resistance of tomato to Cladosporium fulvum

Tomato leaves or cotyledons expressing the Cf-2 or Cf-9 Cladosporium fulvum resistance genes induce salicylic acid (SA) synthesis following infiltration with intercellular washing fluid (IF) containing the fungal peptide elicitors Avr2 and Avr9. We investigated whether SA was required for Cf gene-dependent resistance. Tomato plants expressing the bacterial gene nahG, encoding salicylate hydroxylase, did not accumulate SA in response to IF infiltration but remained fully resistant to C. fulvum. NahG Cf0 plants were as susceptible to C. fulvum as wild-type Cf0. Neither free nor conjugated salicylic acid accumulated in IF-infiltrated Cf2 and Cf9 NahG leaves and cotyledons but conjugated catechol did accumulate. The Cf-9-dependent necrotic response to IF was prevented in NahG plants and replaced by a chlorotic Cf-2-like response. SA also potentiated Cf-9-mediated necrosis in IF-infiltrated wild-type leaves. In contrast, the Cf-2-dependent IF response was retained in NahG leaves and chlorosis was more pronounced than in the wild-type. The distribution of cell death between different cell types was altered in both Cf2 and Cf9 NahG leaves after IF injection. IF-induced accumulation of three SA-inducible defence-related genes was delayed and reduced but not abolished in NahG Cf2 and Cf9 leaves and cotyledons. NahG Tm-22 tomato showed increased hypersensitive response (HR) lesion size upon TMV infection, as observed in TMV-inoculated N gene-containing NahG tobacco plants.     

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.     

INF1 Elicitin Activates Jasmonic Acid- and Ethylene-mediated Signalling Pathways and Induces Resistance to Bacterial Wilt Disease in Tomato

Phytophthora infestans , the cause of potato and tomato late blight disease, produces INF1 elicitin, a 10 kDa extracellular protein. INF1 induces a hypersensitive response (HR) and systemic acquired resistance in species of the Nicotiana genus and a few other genera. We analysed the response of tomato to INF1 and INF1 S3 , which has a Cys to Ser substitution at position 3 of the processed protein and therefore lacks HR induction activity in tobacco. No HR cell death was induced in either INF1- or INF1 S3 -treated tomato leaves. The expression of salicylic acid (SA)-responsive PR-1a ( P6 ) and PR-2a genes was not induced by treatment with either INF1 or INF1 S3 . However, the expression of jasmonic acid (JA)-responsive PR-6 encoding proteinase inhibitor II, LeATL6 encoding ubiquitin ligase E3, and LOX-E encoding lipoxygenase, was up-regulated in tomato leaves treated with INF1 but not in those treated with INF1 S3 . Their induction was completely compromised in INF1-treated jai1-1 mutant tomato, in which the JA signalling pathway is impaired. The accumulation of ethylene (ET) and the expression of ET-responsive genes were also induced in tomato by INF1 but not INF1 S3 treatment. The activation of JA and ET-mediated signals but not the SA-mediated signalling in INF1-treated tomato was also demonstrated by global gene expression analysis. INF1-treated tomatoes, but not those treated with INF1 S3 , exhibited resistance to bacterial wilt disease caused by Ralstonia solanacearum . Thus, INF1 seems to induce resistance to bacterial wilt disease in tomato and activate JA- and ET-mediated signalling pathways without development of HR cell death.