Plants mount defense responses by recognizing indicators of pathogen invasion, including microbe-associated molecular patterns (MAMPs). Flagellin, from the bacterial pathogen Pseudomonas syringae pv. tomato (Pst), contains two MAMPs, flg22 and flgII-28, that are recognized by tomato (Solanum lycopersicum) receptors Flagellin sensing2 (Fls2) and Fls3, respectively, but to what degree each receptor contributes to immunity and whether they promote immune responses using the same molecular mechanisms are unknown. Here, we characterized CRISPR/Cas9-generated Fls2 and Fls3 tomato mutants and found that the two receptors contribute equally to disease resistance both on the leaf surface and in the apoplast. However, we observed striking differences in certain host responses mediated by the two receptors. Compared to Fls2, Fls3 mediated a more sustained production of reactive oxygen species and an increase in transcript abundance of 44 tomato genes, with two genes serving as specific reporters for the Fls3 pathway. Fls3 had greater in vitro kinase activity than Fls2 and could transphosphorylate a substrate. Using chimeric Fls2/Fls3 proteins, we found no evidence that a single receptor domain is responsible for the Fls3sustained reactive oxygen species, suggesting involvement of multiple structural features or a nullified function of the chimeric construct. This work reveals differences in certain immunity outputs between Fls2 and Fls3, suggesting that they might use distinct molecular mechanisms to activate pattern-triggered immunity in response to flagellin-derived MAMPs.

The interaction between tomato and Pseudomonas syringae pv. tomato (Pst) is a well-developed model for investigating the molecular basis of the plant immune system. There is extensive natural variation in Solanum lycopersicum (tomato) but it has not been fully leveraged to enhance our understanding of the tomato-Pst pathosystem. We screened 216 genetically diverse accessions of cultivated tomato and a wild tomato species for natural variation in their response to three strains of Pst. The screen uncovered a broad range of previously unseen host symptoms in response to Pst, and one of these, stem galls, was found to be simply inherited. The screen also identified tomato accessions that showed enhanced responses to flagellin in bacterial population assays and in reactive oxygen species assays upon exposure to flagellin-derived peptides, flg22 and flgII-28. Reporter genes confirmed that the host responses were due primarily to pattern recognition receptor-immunity. This study revealed extensive natural variation in tomato for susceptibility and resistance to Pst and will enable elucidation of the molecular mechanisms underlying these host responses.Key Words: Bacterial speck disease, flagellin, natural variation, NLR-triggered immunity, patterntriggered immunity, plant immunity, PRR-triggered immunity Abramovitch RB, Janjusevic R, Stebbins CE, Martin GB. 2006. Type III effector AvrPtoB requires intrinsic E3 ubiquitin ligase activity to suppress plant cell death and immunity. Proc Natl Acad Sci USA 103: 2851-2856. Abramovitch RB, Kim Y-J, Chen S, Dickman MB, Martin GB. 2003. Pseudomonas type III effector AvrPtoB induces plant disease susceptibility by inhibition of host programmed cell death. EMBO J 22: 60-69. Arredondo CR, Davis RM. 2000. First report of Pseudomonas syringae pv. tomato race 1 on tomato in California. Plant Disease 84: 371. Bao Z, Meng F, Strickler SR, Dunham DM, Munkvold KR, Martin GB. 2015. Identification of a candidate gene in Solanum habrochaites for resistance to a race 1 strain of Pseudomonas syringae pv. tomato. The Plant Genome 8: 10.3835/plantgenome2015.3802.0006. Buell CR, Joardar V, Lindeberg M, Selengut J, Paulsen IT, Gwinn ML, Dodson RJ, Deboy RT, Durkin AS, Kolonay JF, et al. 2003. The complete genome sequence of the Arabidopsis and tomato pathogen Pseudomonas syringae pv. tomato DC3000.

Summary The interaction between tomato and Pseudomonas syringae pv tomato (Pst) is a well‐developed model for investigating the molecular basis of the plant immune system. There is extensive natural variation in Solanum lycopersicum (tomato) but it has not been fully leveraged to enhance our understanding of the tomato–Pst pathosystem. We screened 216 genetically diverse accessions of cultivated tomato and a wild tomato species for natural variation in their response to three strains of Pst. The host response to Pst was investigated using multiple Pst strains, tomato accessions with available genome sequences, reactive oxygen species (ROS) assays, reporter genes and bacterial population measurements. The screen uncovered a broad range of previously unseen host symptoms in response to Pst, and one of these, stem galls, was found to be simply inherited. The screen also identified tomato accessions that showed enhanced responses to flagellin in bacterial population assays and in ROS assays upon exposure to flagellin‐derived peptides, flg22 and flgII‐28. Reporter genes confirmed that the host responses were due primarily to pattern recognition receptor‐triggered immunity. This study revealed extensive natural variation in tomato for susceptibility and resistance to Pst and will enable elucidation of the molecular mechanisms underlying these host responses.