TOMATO MUTANTS ALTERED IN BACTERIAL DISEASE RESISTANCE PROVIDE EVIDENCE FOR A NEW LOCUS CONTROLLING PATHOGEN RECOGNITION

We have employed a genetic approach to study the resistance of tomato to the phytopathogenic bacterium Pseudomonas syringae pv tomato. Resis tance to P.s. tomato depends upon expression of the Pto locus in tomat o, which encodes a protein with similarity to serine/threonine protein kinases and recognizes pathogen strains expressing the avirulence gen e avrPto. Eleven tomato mutants were isolated with altered resistance to P.s. tomato strains expressing avrPto. We identified mutations both in the Pto resistance locus and in a new locus designated Prf (for Ps eudomonas resistance and fenthion sensitivity). The genetic approach a llowed us to dissect the roles of these loci in signal transduction in response to pathogen attack. Lines carrying mutations in the Pto locu s vary 200-fold in the degree to which they are susceptible to P.s. to mato strains expressing avrPto. The pto mutants retain sensitivity to the organophosphate insecticide fenthion; this trait segregates with P to in genetic crosses. This result suggested that contrary to previous hypotheses, the Pto locus controls pathogen recognition but not fenth ion sensitivity. Interestingly, mutations in the prf locus result in b oth complete susceptibility to P.s. tomato and insensitivity to fenthi on, suggesting that Prf plays a role in tomato signaling in response t o both pathogen elicitors and fenthion. Because pto and prf mutations do not alter recognition of Xanthomonas campestris strains expressing avrBsP, an avirulence gene recognized by all tested tomato cultivars, Prf does not play a general role in disease resistance but possibly fu nctions specifically in resistance against P.s. tomato. Genetic analys is of F-2 populations from crosses of pto and prf homozygotes indicate d that the Pto and Prf loci are tightly linked.