How Oomycete and Fungal Effectors Enter Host Cells and Promote Infection
The genus Phytophthora contains a large number of species that are known plant pathogens of a variety of important crops. Phytophthora sojae, a hemibiotroph, causes approximately 1-2 billion dollars (US) of lost soybean world-wide each year. P. infestans, the causative agent of the Irish potato famine, is responsible for over 5 billion dollars (US) worth of lost potato each year. These destructive plant pathogens facilitate pathogenesis through the use of small secreted proteins known as effector proteins. A large subset of effector proteins is able to translocate into host cells and target plant defense pathways. P. sojae Avr1b is able to suppress cell death triggered by BAX and hydrogen peroxide. The W-domain of Avr1b is responsible for this functionality, and is recognized by the Rps1b gene product to induce effector triggered immunity.
These oomycete effector proteins translocate into host cells via a highly conserved N-terminal motif known as RXLR-dEER without the use of any pathogen encoded machinery. In fungi an RXLR-like motif exists, [R,K,H] X [L,F,Y,M,~I] X, that is able to facilitate translocation without pathogen encoded machinery. Both functional RXLR and RXLR-like motifs are able to bind phosphatidylinositol-3-phosphate (PtdIns- 3-P) to mediate entry into host cells. The use of novel inhibitory mechanisms has shown effector entry can be blocked either by sequestering PtdIns-3-P on the outer leaflet of plant and animal cells or by competitive inhibition of the binding pocket of the RXLR or RXLR-like motifs.