Cell biology is adding a new dimension to the study of plant pathogen interactions involving the important plant pathogen Phytophthora infestans. The ability to transform P. infestans with fluorescent protein tags and protein fusions and infect plants expressing different fluorescent tags or fusions paves the way for a wide range of cell biological studies.

Biotrophy and the development of haustoria by Phytophthora infestans

The biotrophic phase of growth is a particular focus of the P. infestans research conducted at SCRI as it represents a point of vulnerability and the stage at which the pathogen must exert the most control over the host responses. The formation of haustoria is crucial for the establishment of biotrophy. A recently identified transmembrane protein appears to be essential to haustorial development. We have shown that the protein is specifically localised to the haustoria using confocal imaging of an mRFP fusion form of the protein expressed in transgenic P. infestans.

Phytophthora infestans effector translocation

The P. infestans Avr protein, pAvr3a, contains a conserved motif known to be involved in protein translocation from the malaria parasite into the infected blood cells. The P. infestans pAvr3a is detected by the plant resistance protein within the plant cell cytoplasm and the RXLR-EER motif is required for that interaction to occur. mRFP tagged pAvr3a is localised to the haustoria. Forms of pAvr3a in which the RXLR-EER motif has been mutated accumulate to much higher levels around haustoria (as seen with mRFP fusions) than the wild type, suggesting that the mutant forms can no longer translocate from the extra-haustorial space into the plant cell.

The development of cell biological techniques for observing effector delivery and plant responses is important for EU Bioexploit, BBSRC Crop Science Initiative and Scottish Government systems biology grants and Scottish Government work packages.

Extracellular plant HR activator Cathepsin B

A Cathepsin B homologue was found to be involved in the hypersensitive response to P. infestans. Cathepsin B is involved in apoptosis in animals; it moves from the lysosome to the cytoplasm to execute or regulate cell death responses. To determine where the plant Cathepsin B was localised fluorescent protein fusions to Nicotiana benthamiana Cathepsin B were expressed transiently in leaves. N. benthamiana Cathepsin B appeared to be an extracellular protein. Co-expression with a plasma membrane marker, GFP-LTi, and treatment with the inhibitor of protein secretion, Brefeldin A, in addition to biochemical assays confirmed the extracellular nature of plant Cathepsin B.