Tomato mutants altered in bacterial disease resistance provide evidence for a new locus controlling pathogen recognition.

Abstract

We have employed a genetic approach to study the resistance of tomato to the phytopathogenic bacterium Pseudomonas syringae pv tomato. Resistance to I? s. tomato depends upon expression of the Pto locus in tomato, which encodes a protein with similarity to serinehhreonine protein kinases and recsgnizes pathogen strains expressing the avirulence gene avrPto. Eleven tomato mutants were isolated with altered resistance to I? s. tomato strains expressing avrPto. We identified mutations both in the Pto resistance locus and in a new locus designated Prf (for Pseudomonas resistance and fenthion sensitivity). The genetic approach allowed us to dissect the roles of these loci in signal transduction in response to pathogen attack. Lines carrying mutations in the Pto locus vary 200-fold in the degree to which they are susceptible to I? s. tomato strains expressing avrPto. The pto mutants retain sensitivity to the organophosphate insecticide fenthion; this trait segregates with Pto in genetic crosses. This result suggested that contrary to previous hypotheses, the Pto locus controls pathogen recognition but not fenthion sensitivity. Interestingly, mutations in the prf locus result in both complete susceptibility to I? s . tomato and insensitivity to fenthion, suggesting that Prf plays a role in tomato signaling in response to both pathogen elicitors and fenthion. Because pto and prf mutations do not alter recognition of Xanthomonas campestris strains expressing avrBsR an avirulence gene recognized by all tested tomato cultivars, Prf does not play a general role in disease resistance but possibly functionsspecifically in resistance against I? s. tomato.Genetic analysis of F2 populations from crosses of pto and prf homozygotes indicated that the Pto and Prf loci are tightly lin ked .

Natural Variation in the Pto Disease Resistance Gene Within Species of Wild Tomato (Lycopersicon). II. Population Genetics of Pto

“…Of the $300 residues, 12 are essentially invariant among kinases, and these 12 sites are also invariant in the collection of 48 Pto alleles described here. Furthermore, functional studies of the Pto protein using site-directed and random mutagenesis have specifically characterized the functional consequences of variation at 66 positions (20%) of the protein (Salmeron et al 1994;Scofield et al 1996;Frederick et al 1998;Rathjen et al 1999;Sessa et al 2000;Xiao et al 2003;Wu et al 2004;Bernal et al 2005;de Vries et al 2006). The vast majority (.86%) of the positions that knock out Avr recognition and/or signaling are invariant among the alleles collected from these natural populations.…”

Plant NBS-LRR proteins in pathogen sensing and host defense

“…The tomato protein Prf is yet another candidate NBS-LRR protein involved in the indirect detection of pathogen effectors, specifically AvrPto and AvrPtoB found in a subset of P. syringae pathovar tomato strains [27][28][29] . Genetic evidence indicates that Prf is required for the activation of defense in response to AvrPto and AvrPtoB, although direct interaction between Prf and AvrPto or AvrPtoB has not been detected.…”

Plant-Pathogen Encounters in Edinburgh

“…Whereas Pfo and fen have different functions, the former being required for the response to avrpto and the latter for fenthion sensitivity, a third gene, Prf, is required for both activities (Salmeron et al, 1994). Staskawicz presented evidence suggesting that Prf to0 lies in the Pfo region.…”