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of AvrPto lacking this motif lack avirulence function (X. Tang, personal communication). Mutant forms of Pto lacking this motif are still functional when overexpressed, but the motif might be required when Pto is present only at endogenous levels (33). Interestingly, Pto appeared to interact weakly with Api4 (Table 2).

Discussion

We have not established whether any of the Adi and Api proteins actually are involved in speck disease resistance or play a role in the manifestation of AvrPto virulence function. Further characterization of the Adi and Api proteins, including in vitro binding and phosphorylation assays, in planta transient expression assays, localization, and other studies, will be required to confirm the functional significance of the interactions we have observed in yeast. Based on the stringency of the screens, the specificity of the interactions, and the potential functions of the proteins isolated in our screens, however, we view the Adi and Api proteins as strong candidates for plant components involved in the responses of tomato to AvrPto. A model for disease resistance mediated by the interaction of AvrPto and Pto and for a role of AvrPto in disease is shown in Fig. 5, and discussed below. We have incorporated the Adi and Api proteins in the model, but their placement should be viewed as tentative.

Upon delivery into the tomato cell via the type III secretion system of Pseudomonas, AvrPto binds to Pto. This event likely activates Pto, stimulating its phosphorylation of substrates such as the transcription factors Pti4/5/6 and the serine/threonine protein kinase Pti1. Phosphorylation of Pti4/5/6 may target these factors to the nucleus or activate them otherwise, leading to the expression of pathogenesis-related proteins involved in plant defense via interaction with the GCC box promoter element (18). Phosphorylation of Pti1 may constitute an early event in a phosphorylation cascade leading to the oxidative burst and hypersensitive response (16). The interaction of AvrPto and Pto may also recruit other putative components of the defense response pathway, the Adi proteins. The interactions with these proteins may be involved in the generation of the oxidative burst (Adi1?), the hypersensitive response (Adi2?), or other processes leading to plant defense. AvrPto may also interact with Api proteins other than Pto in the tomato cell. These interactions might lead to modification and consequent membrane localization of AvrPto (Api4?) or to the disruption of plant stress responses (Api1?) or other metabolic processes such as protein transport (Api2?). In the absence of Pto, some or all of these could constitute a virulence effect of AvrPto. The role of the Prf protein, although required for disease resistance, is currently unknown.

We did not isolate a Prf cDNA in either of our screens, but we do not discount the possibility that Prf interacts with Pto, AvrPto, or an AvrPto/Pto complex. Low transcript abundance or the large transcript size of Prf could have resulted in poor representation in our oligo(dT)-primed cDNA library. Even if present, the large size of the protein could have precluded the entry of the prey fusion into the nucleus necessary for reporter gene activation. Elucidation of the role of Prf awaits a more directed set of experiments.

Ultimately, we hope, further characterization of the Adi and Api proteins will yield a yet more detailed understanding of the molecular mechanisms involved in bacterial speck disease and disease resistance, and contribute toward the development of means to control this disease, and others, through genetic manipulation.

We thank Elizabeth Suryaatmaja and Philip Pusey for assistance conducting the yeast screens and Yuko Nakajima for specificity screening of Adi5 and helpful discussion. This research was supported by U.S. Department of Agriculture grant NRI-9735303-5299 (A.J.B.) and a David and Lucile Packard Foundation Fellowship (G.B.M.).

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