Figure 11.

Model of PexRD2 Function during Infection.

Schematic diagram illustrating P. infestans delivering PexRD2 into a haustoriated plant cell. PexRD2 interacts with MAPKKKε in the plant cytosol and inhibits signaling by this protein. The cell death triggered by overexpression of MAPKKKε is inhibited by the presence of PexRD2, but not the cell death triggered by MAPKKKα or MEK2. PexRD2 also suppresses the phosphorylation of MAPKs (orange circle), which occurs following overexpression of MAPKKKε. The HR following recognition of the C. fulvum effector Avr4 by Cf4 and the P. syringe effector AvrPto mediated by Pto/Prf, which are known to be MAPKKKε dependent (red, solid white border), are suppressed by the presence of PexRD2. The HR resulting from the recognition of AVR3a^KI by R3a, the cell death following recognition of the PAMP-like INF1, and the cell death mediated by CRN8 are all MAPKKKε independent (dark gray) and are not suppressed by PexRD2. We propose that, in planta, MAPKKKε confers enhanced resistance against P. infestans following recognition of an unidentified PAMP (red triangle, dashed outline), by a PRR or recognized effector/avirulence protein (AVR, red oval, dashed outline) by an R protein. The pathogen delivers PexRD2 inside the host cell to specifically inhibit this MAPKKKε-dependent resistance and promote proliferation.