| The tomato resistance genes Pto and Prf confer resistance to the bacterial pathogen Pseudomonas syringae pv tomato carrying the avirulence gene avrPto. Pto also interacts with several putative down-stream proteins called Ptis. Among them, Pti1 is a serine/threonine kinase and possibly serves as a direct downstream component. Three others, Pti4/5/6, resemble putative transcription factors. Overexpressing Pto under the control of the Cauliflower Mosaic Virus 35S promoter constitutively activates defense responses in the absence of pathogen infection and nonspecifically enhances disease resistance. This is referred to as ligand-independent resistance. We have taken two approaches to elucidate the ligand-dependent and ligand-independent resistance mediated by Pto. In the first approach, we sought to isolate cDNAs corresponding to transcripts that accumulated in 35S::Pto plants. By using suppression subtractive hybridization (SSH), we isolated 82 unique cDNA clones, most of which corresponded to differentially expressed transcripts. Most of the genes examined were also induced by pathogen-inoculation. A large number of genes encode defense-related proteins. The cDNAs also include those with a putative role in the oxidative burst, proteolysis, hypersensitive response, signal transduction, and a number of genes with unknown functions. In the second approach, we sought to isolate Pto mutants that were unable to interact with one or more Pti proteins, but retained normal interaction with AvrPto. These mutants were then tested for both ligand-independent and ligand-dependent resistance. Overexpression of two mutants, Pto G50S and PtoR150S, indicated that they no longer activate defense responses in the absence of avrPto. In plants overexpressing the wild-type Pto, 35S::PtoG50S dominant suppressed the ligand-independent resistance. 35S::PtoG50S also blocked the induction of a number of defense genes by the wild-type 35S::Pto. However, 35S::PtoG50s and 35S::Pto R150s plants are completely resistant to P. s. tomato (avrPto), indicating a normal gene-for-gene resistance. Furthermore, 35S::PtoG50s plants exhibited normal incompatible induction of defense genes upon recognition of avrPto. Interestingly, a Prf mutation dominantly suppressed spontaneous lesions in 35S::Pto plants, indicating a dual role of Prf in gene-for-gene resistance and ligand-independent defense. Together, the results suggest that the ligand-independent defense activation and gene for-gene resistance are regulated by two pathways bifurcating downstream of Pto and Prf. |
