The hemi-biotroph Phytophthora infestans is the causal agent of late blight of potatoes. The mechanisms of the plant-pathogen interaction are being studied at SCRI at the cell and molecular level. The avirulence determinant Avr3a contains a conserved protein motif that is found in many other pathogen effector molecules. The protein motif is believed to be involved in translocation of the pathogen protein into the host’s cytoplasm. CLSM studies have shown that disruption of the motif results in accumulation of Avr3a in the haustoria of the oomycete, but have been unable to detect the translocated protein. TEM-IGL experiments are being carried out to better resolve the location of the mutated and unmodified effector protein.

Fig. 1 (top left) shows immunogold labelling (IGL) of an invading hypha using an antibody targeted against the expressed Avr3a protein: gold label is predominantly located at the periphery of the hypha. Fig. 2 (top right) shows IGL of a hypha and two developing haustoria: gold label is observed in the hypha, haustoria and extra haustorial matrix (EHM). Fig. 3 (bottom left) shows IGL of an infection by a strain that expresses Avr3a with an unmutated targeting motif: gold particles are seen on the hyphal periphery, in the haustorium and in the cytoplasm of the invaded plant cell. Fig. 4 (bottom right) shows IGL of an infection by a strain that expresses Avr3a with a mutated targeting motif: gold particles are found in the hypha and haustorium, but not in the host cell’s cytoplasm. These studies are ongoing and we hope to determine the final destination of the effector protein in the plant cells.