Figure 3:

Pathogen Effector Recognition and the Activation of Effector-Triggered Immunity.

A) The largest class of resistance proteins in Arabidopsis is the CC-NB-LRR class, whose members include the R-proteins RPM1, RPS2 and RPS5. The TIR-class of R-proteins are represented by the well-characterized R-protein RPS4. A recently identified variant of this class, RRS1-R, contains a WRKY domain believed to impart transcriptional regulation as part of its function following pathogen effector recognition.

B) Similar to the activation of PTI, the elicitation of ETI results in the activation of defense signaling via the specific recognition of pathogen-derived elicitors. As a second layer of defense signaling, ETI is the culmination in the recognition of pathogen effector proteins. As shown, delivery of an effector protein via the type III secretion system (T3SS) and subsequent recognition by cognate host R-proteins, leads to the activation of an amplified defense response. The general role of pathogen effector proteins is thought to be the inactivation of PTI, while the role of ETI is to block all mechanisms of pathogen virulence.