Evidence for independent evolution of resistance to AvrPto and AvrPtoB from the wild tomato species Solanum chmielewskii
C. M. KRAUS (1), K. R. Munkvold (2), G. B. Martin (3). (1) Department of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, NY, U.S.A.; (2) Boyce Thompson Institute for Plant Research, Ithaca, NY, U.S.A.; (3) Boyce Thompson Institute for Plant Research, Department of Plant Pathology and Pl
Resistance in tomato to <i>Pseudomonas syringae </i>pv. <i>tomato </i>(<i>Pst</i>) is activated upon recognition of either of two bacterial effectors, AvrPto and AvrPtoB, by a member of the Pto kinase family and the NB-LRR protein Prf. Pto recognizes both AvrPto and AvrPtoB, whereas another family member, Fen, recognizes only forms of AvrPtoB lacking the C-terminal E3 ubiquitin ligase. This ancient recognition is present in diverse tomato accessions and other <i>Solanaceous </i>plants. We screened wild relatives of tomato for resistance to <i>Pst </i>delivering either AvrPto or AvrPtoB and found a novel resistance phenotype in <i>S. chmielewskii</i>. These plants recognize AvrPtoB with E3 ligase activity, but not AvrPto and subsequent experiments demonstrated that AvrPtoB is being recognized by Pto and not Fen. Sequence comparisons between Pto proteins from resistant Rio Grande-PtoR and <i>S. chmielewskii</i> revealed that two residues at positions 49 and 51 required for AvrPto recognition are polymorphic. Site-directed mutagenesis of these two residues was not enough to recapitulate the recognition of AvrPto. A third residue at position 193, which has not been previously implicated in AvrPto recognition, was needed to activate signaling in response to AvrPto by Pto from <i>S. chmielewskii</i>. We hypothesize that this represents an intermediate evolutionary step in the arms race between <i>Pst </i>and tomato, in which a <i>Solanum </i>species has evolved the ability to recognize full-length AvrPtoB, but not yet AvrPto.
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