Table 2.
Four Cases Supporting the Decoy Model
| Case | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Plant species | Tomato | Pepper | Tomato | Arabidopsis |
| Pathogen | P. syringae pv tomato (bacterium) | X. campestris pv vesicatoria (bacterium) | C. fulvum (fungus) | P. syringae (bacterium) |
| Site of perception | Cytoplasm | Nucleus | Apoplast | Cytoplasm |
| R protein | Prf | Bs3 | Cf-2 | RPS2 |
| Biochemical function of R protein | NB-LRR | Flavin monooxygenase | Receptor-like protein | NB-LRR |
| Decoy | Pto | pBs3 | RCR3 | RIN4 |
| Biochemical function of decoy | Kinase | upa box in promoter of Bs3 gene | Cys protease | Negative regulator of basal defense (Kim et al., 2005) |
| Operative target | Le FLS2? | pUpa20 | PIP1 | Not yet identified |
| Structure and function of operative target | Receptor-like kinase required for basal resistance | upa box in promoter of cell size regulator Upa20 and other genes | Cys protease secreted abundantly during defense | Unknown |
| Effector | AvrPto | AvrBs3 | Avr2 | AvrRpt2 |
| Biochemical function of effector | Kinase inhibitor | Transcription factor | Protease inhibitor | Cys protease |
| Presumed perception mechanism | Pto inhibition by AvrPto activates Prf (Mucyn et al., 2006; Xing et al., 2007) | AvrBs3 binds and activates promoter of Bs3 (Römer et al., 2007) | Avr2 inhibits Rcr3, and Avr2-Rcr3 complex probably activates Cf-2 (Rooney et al., 2005) | AvrRpt2 cleaves RIN4 from the RIN4-RPS2 complex, activating RPS2 (Axtell and Staskawicz, 2003; Mackey et al., 2003) |
| Virulence role of the effector? | Yes: AvrPto contributes to virulence on tomato (Chang et al., 2000) and Arabidopsis (Xiang et al., 2008) | Yes: AvrBs3 contributes to virulence on pepper under field conditions (Wichmann and Bergelson, 2004) | Yes: Avr2 contributes to virulence on tomato (van Esse et al., 2008) | Yes: AvrRpt2 contributes to virulence on Arabidopsis (Guttman and Greenberg, 2001) |
| Does pathogen benefit from manipulating decoy? | No?: no enhanced virulence on pto/Prf compared with Pto/prf tomato lines (Chang et al., 2000) | No: not anticipated | No?: No enhanced virulence on MM-Cf2/rcr3 compared with MM-Cf0 tomato lines (Dixon et al., 2000) | No?: No enhanced virulence on rin4/rps2 compared with RIN4/rps2 Arabidopsis lines (Belkhadir et al., 2004; Lim and Kunkel, 2004) |
| Does pathogen benefit from manipulating operative target? | Yes: AvrPto inhibits FLS2 kinase domain and no longer contributes to virulence on fls2 mutants (Xiang et al., 2008) | Yes: AvrBs3 activates the promoter of Upa20, resulting in enhanced cell size, a phenotype that is thought to be beneficial for the bacteria (Kay et al., 2007) | Yes?: Avr2 inhibits the abundant, defense-related protease PIP1 (Shabab et al., 2008). However, a role of PIP1 in defense has not yet been demonstrated. | Not investigated: operative targets are not yet known. |