References

7 Signal Perception and Transduction in Plant Innate Immunity 101
much more complex and that R proteins are unlikely to act as receptors
for microbe-derived Avr proteins (Dangl and Jones 2001; Jones and Takemoto
2004). In many cases, binding sites for Avr proteins have been found
notonlyinresistantbutalsoinsusceptibleplantcultivars,suggesting
that those sites serve as virulence targets for microbial effectors during
attempted infection (Axtell and Staskawicz 2003; Dangl and Jones 2001;
Mackey et al. 2002). These findings form the basis of the so-called guard
hypothesis, which predicts that AVR proteins act as virulence factors that
contact their cognate pathogenicity targets in host plants or even nonhost
plants, but only function as elicitors of cultivar-specific plant resistance
when the complementary R protein is recruited into a functional signal
perception complex. Thus, R proteins monitor (“guard”) AVR-mediated
perturbance of cellular functions and may thus be considered adapter proteins
that – owing to their presence – initiate signaling cascades that are
suppressed in susceptible plant cultivars that lack the appropriate R protein.
AprimeexampleconstitutestheArabidopsis RPM1 gene that confers resistance
against P. syringae strains expressing the type III effectors, AvrRpm1
and AvrB. RPM1 “guards” the plant against pathogens that manipulate
RIN4 (the pathogenicity target in the plant) via AvrRpm1 or AvrB (bacterial
virulence factors) in order to suppress host defenses (Mackey et al.
2002). RIN4 also appears to be the target for another P. syringae pv. tomato
derived AVR protein, AvrRpt2 (Axtell and Staskawicz 2003). However, in
contrast to the previously described situation AvrRpt2 does not assemble
with RIN4 and RPM1, but with RIN4 and its cognate R protein, RPS2,
which confers resistance against bacterial strains expressing AvrRpt2, but
not AvrRpm1 or AvrB, respectively. RIN4 likely acts as a negative regulator
of RPS2-mediated resistance in the uninfected plant. Upon infection, plant
cyclophilin-assisted autocatalytic activation of the cysteine protease, Avr-
Rpt2, resulted in AvrRpt2-mediated breakdown of RIN4 and subsequent
activation of RPS2-mediated plant immunity (Coaker et al. 2005).
Intracellular plant R proteins fall into one of two major structural classes.
Motifs commonly found in R proteins of one class are coiled-coil (CC,
leucine zipper) domains, LRRs and a nucleotide-binding site (CC-NBS-
LRR) (Jones and Takemoto 2004). A second, widely found subset of plant
R genes comprises a TIR domain in conjunction with a NBS and an LRR
domain (TIR-NBS-LRR) (Jones and Takemoto 2004). Plant plasma membrane
R proteins are either composed of extracellular LRR domains only
or are fused to protein kinase domains (LRR-RLK), such as in the Xa21
gene from rice (Song et al. 1995). Notably, the domain structure of Xa21
resembles that of the plant PAMP receptor, FLS2.
Importantly, similar structures are typical for the architecture of PAMP
perception modules in animal cells as well (McGuinness et al. 2003;
Medzhitov and Janeway 2002; Underhill and Ozinsky 2002). For example,