Parasitism by Cuscuta pentagona sequentially induces JA and SA defence pathways in tomato

While plant responses to herbivores and pathogens are well characterized, responses to attack by other plants remain largely unexplored. We measured phytohormones and C₁₈ fatty acids in tomato attacked by the parasitic plant Cuscuta pentagona, and used transgenic and mutant plants to explore the roles of the defence‐related phytohormones salicylic acid (SA) and jasmonic acid (JA). Parasite attachment to 10‐day‐old tomato plants elicited few biochemical changes, but a second attachment 10 d later elicited a 60‐fold increase in JA, a 30‐fold increase in SA and a hypersensitive‐like response (HLR). Host age also influenced the response: neither Cuscuta seedlings nor established vines elicited a HLR in 10‐day‐old hosts, but both did in 20‐day‐old hosts. Parasites grew larger on hosts deficient in SA (NahG) or insensitive to JA [jasmonic acid‐insensitive1 (jai1) ], suggesting that both phytohormones mediate effective defences. Moreover, amounts of JA peaked 12 h before SA, indicating that defences may be coordinated via sequential induction of these hormones. Parasitism also induced increases in free linolenic and linoleic acids and abscisic acid. These findings provide the first documentation of plant hormonal signalling induced by a parasitic plant and show that tomato responses to C. pentagona display characteristics similar to both herbivore‐ and pathogen‐induced responses.     

Functional analysis of endo‐1,4‐β‐glucanases in response to Botrytis cinerea and Pseudomonas syringae reveals their involvement in plant–pathogen interactions

Plant cell wall modification is a critical component in stress responses. Endo‐1,4‐β‐glucanases (EGs) take part in cell wall editing processes, e.g. elongation, ripening and abscission. Here we studied the infection response of Solanum lycopersicum and Arabidopsis thaliana with impaired EGs. Transgenic TomCel1 and TomCel2 tomato antisense plants challenged with Pseudomonas syringae showed higher susceptibility, callose priming and increased jasmonic acid pathway marker gene expression. These two EGs could be resistance factors and may act as negative regulators of callose deposition, probably by interfering with the defence‐signalling network. A study of a set of Arabidopsis EG T‐DNA insertion mutants challenged with P. syringae and Botrytis cinerea revealed that the lack of other EGs interferes with infection phenotype, callose deposition, expression of signalling pathway marker genes and hormonal balance. We conclude that a lack of EGs could alter plant response to pathogens by modifying the properties of the cell wall and/or interfering with signalling pathways, contributing to generate the appropriate signalling outcomes. Analysis of microarray data demonstrates that EGs are differentially expressed upon many different plant–pathogen challenges, hormone treatments and many abiotic stresses. We found some Arabidopsis EG mutants with increased tolerance to osmotic and salt stress. Our results show that impairing EGs can alter plant–pathogen interactions and may contribute to appropriate signalling outcomes in many different biotic and abiotic plant stress responses.     

Auxin promotes susceptibility to Pseudomonas syringae via a mechanism independent of suppression of salicylic acid‐mediated defenses

Auxin is a key plant growth regulator that also impacts plant–pathogen interactions. Several lines of evidence suggest that the bacterial plant pathogen Pseudomonas syringae manipulates auxin physiology in Arabidopsis thaliana to promote pathogenesis. Pseudomonas syringae strategies to alter host auxin biology include synthesis of the auxin indole‐3‐acetic acid (IAA) and production of virulence factors that alter auxin responses in host cells. The application of exogenous auxin enhances disease caused by P. syringae strain DC3000. This is hypothesized to result from antagonism between auxin and salicylic acid (SA), a major regulator of plant defenses, but this hypothesis has not been tested in the context of infected plants. We further investigated the role of auxin during pathogenesis by examining the interaction of auxin and SA in the context of infection in plants with elevated endogenous levels of auxin. We demonstrated that elevated IAA biosynthesis in transgenic plants overexpressing the YUCCA 1 (YUC1) auxin biosynthesis gene led to enhanced susceptibility to DC3000. Elevated IAA levels did not interfere significantly with host defenses, as effector‐triggered immunity was active in YUC1‐overexpressing plants, and we observed only minor effects on SA levels and SA‐mediated responses. Furthermore, a plant line carrying both the YUC1‐overexpression transgene and the salicylic acid induction deficient 2 (sid2) mutation, which impairs SA synthesis, exhibited additive effects of enhanced susceptibility from both elevated auxin levels and impaired SA‐mediated defenses. Thus, in IAA overproducing plants, the promotion of pathogen growth occurs independently of suppression of SA‐mediated defenses.     

Rpa1 mediates an immune response to avrRpm1Psa and confers resistance against Pseudomonas syringae pv. actinidiae

The type three effector AvrRpm1_Pma from Pseudomonas syringae pv. maculicola (Pma) triggers an RPM1‐mediated immune response linked to phosphorylation of RIN4 (RPM1‐interacting protein 4) in Arabidopsis. However, the effector–resistance (R) gene interaction is not well established with different AvrRpm1 effectors from other pathovars. We investigated the AvrRpm1‐triggered immune responses in Nicotiana species and isolated Rpa1 (R esistance to P seudomonas syringae pv. a ctinidiae 1) via a reverse genetic screen in Nicotiana tabacum. Transient expression and gene silencing were performed in combination with co‐immunoprecipitation and growth assays to investigate the specificity of interactions that lead to inhibition of pathogen growth. Two closely related AvrRpm1 effectors derived from Pseudomonas syringae pv. actinidiae biovar 3 (AvrRpm1_Psa) and Pseudomonas syringae pv. syringae strain B728a (AvrRpm1_Psy) trigger immune responses mediated by RPA1, a nucleotide‐binding leucine‐rich repeat protein with an N‐terminal coiled‐coil domain. In a display of contrasting specificities, RPA1 does not respond to AvrRpm1_Pma, and correspondingly AvrRpm1_Psa and AvrRpm1_Psy do not trigger the RPM1‐mediated response, demonstrating that separate R genes mediate specific immune responses to different AvrRpm1 effectors. AvrRpm1_Psa co‐immunoprecipitates with RPA1, and both proteins co‐immunoprecipitate with RIN4. In contrast with RPM1, however, RPA1 was not activated by the phosphomimic RIN4^T166D and silencing of RIN4 did not affect the RPA1 activity. Delivery of AvrRpm1_Psa by Pseudomonas syringae pv. tomato (Pto) in combination with transient expression of Rpa1 resulted in inhibition of the pathogen growth in N. benthamiana. Psa growth was also inhibited by RPA1 in N. tabacum.     

IDL6‐HAE/HSL2 impacts pectin degradation and resistance to Pseudomonas syringae pv tomato DC3000 in Arabidopsis leaves

Plant cell walls undergo dynamic structural and chemical changes during plant development and growth. Floral organ abscission and lateral root emergence are both accompanied by cell‐wall remodeling, which involves the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA)‐derived peptide and its receptors, HAESA (HAE) and HAESA‐LIKE2 (HSL2). Plant cell walls also act as barriers against pathogenic invaders. Thus, the cell‐wall remodeling during plant development could have an influence on plant resistance to phytopathogens. Here, we identified IDA‐like 6 (IDL6), a gene that is prominently expressed in Arabidopsis leaves. IDL6 expression in Arabidopsis leaves is significantly upregulated when the plant is suffering from attacks of the bacterial Pseudomonas syringae pv. tomato (Pst) DC3000. IDL6 overexpression and knockdown lines respectively decrease and increase the Arabidopsis resistance to Pst DC3000, indicating that the gene promotes the Arabidopsis susceptibility to Pst DC3000. Moreover, IDL6 promotes the expression of a polygalacturonase (PG) gene, ADPG2, and increases PG activity in Arabidopsis leaves, which in turn reduces leaf pectin content and leaf robustness. ADPG2 overexpression restrains Arabidopsis resistance to Pst DC3000, whereas ADPG2 loss‐of‐function mutants increase the resistance to the bacterium. Pst DC3000 infection elevates the ADPG2 expression partially through HAE and HSL2. Taken together, our results suggest that IDL6‐HAE/HSL2 facilitates the ingress of Pst DC3000 by promoting pectin degradation in Arabidopsis leaves, and Pst DC3000 might enhance its infection by manipulating the IDL6‐HAE/HSL2‐ADPG2 signaling pathway.     

The Pseudomonas syringae type III effector AvrRpt2 functions downstream or independently of SA to promote virulence on Arabidopsis thaliana

AvrRpt2, a Pseudomonas syringae type III effector protein, functions from inside plant cells to promote the virulence of P. syringae pv. tomato strain DC3000 (PstDC3000) on Arabidopsis thaliana plants lacking a functional copy of the corresponding RPS2 resistance gene. In this study, we extended our understanding of AvrRpt2 virulence activity by exploring the hypothesis that AvrRpt2 promotes PstDC3000 virulence by suppressing plant defenses. When delivered by PstDC3000, AvrRpt2 suppresses pathogen-related (PR) gene expression during infection, suggesting that AvrRpt2 suppresses defenses mediated by salicylic acid (SA). However, AvrRpt2 promotes PstDC3000 growth on transgenic plants expressing the SA-degrading enzyme NahG, indicating that AvrRpt2 does not promote bacterial virulence by modulating SA levels during infection. AvrRpt2 general virulence activity does not depend on the RPM1 resistance gene, as mutations in RPM1 had no effect on AvrRpt2-induced phenotypes. Transgenic plants expressing AvrRpt2 displayed enhanced susceptibility to PstDC3000 strains defective in type III secretion, indicating that enhanced susceptibility of these plants is not because of suppression of defense responses elicited by other type III effectors. Additionally, avrRpt2 transgenic plants did not exhibit increased susceptibility to Peronospora parasitica and Erysiphe cichoracearum, suggesting that AvrRpt2 virulence activity is specific to P. syringae.     

Virulence systems of Pseudomonas syringae pv. tomato promote bacterial speck disease in tomato by targeting the jasmonate signaling pathway

Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000) causes bacterial speck disease on tomato. The pathogenicity of Pst DC3000 depends on both the type III secretion system that delivers virulence effector proteins into host cells and the phytotoxin coronatine (COR), which is thought to mimic the action of the plant hormone jasmonic acid (JA). We found that a JA-insensitive mutant (jai1) of tomato was unresponsive to COR and highly resistant to Pst DC3000, whereas host genotypes that are defective in JA biosynthesis were as susceptible to Pst DC3000 as wild-type (WT) plants. Treatment of WT plants with exogenous methyl-JA (MeJA) complemented the virulence defect of a bacterial mutant deficient in COR production, but not a mutant defective in the type III secretion system. Analysis of host gene expression using cDNA microarrays revealed that COR works through Jai1 to induce the massive expression of JA and wound response genes that have been implicated in defense against herbivores. Concomitant with the induction of JA and wound response genes, the type III secretion system and COR repressed the expression of pathogenesis-related (PR) genes in Pst DC3000-infected WT plants. Resistance of jai1 plants to Pst DC3000 was correlated with a high level of PR gene expression and reduced expression of JA/wound response genes. These results indicate that COR promotes bacterial virulence by activating the host's JA signaling pathway, and further suggest that the type III secretion system might also modify host defense by targeting the JA signaling pathway in susceptible tomato plants.