Molecular mechanisms of virulence and avirulence in the Avr3a family of Phytophthora.

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The oomycetes are fungus-like eukaryotic microorganisms. Several species of the oomycetes are devastating pathogens that are of great importance to world agriculture and food security. In essence, this means that they destroy crops that are critical sources of food. The word 'Phytophthora', which is a genus with the oomycete lineage means 'plant destroyer'. Phytophthora species include the organism responsible for the Irish potato famine, Phytophthora infestans, which causes a disease commonly know as 'potato blight' or 'late blight' (it also causes tomato blight), as well as pepper blight (Phytophthora capsici) and soybean root rot (Phytophthora sojae). Recently, Phytophthora ramorum has gained considerable press in the UK as it is threatening iconic trees, such as oak. Phytophthora infestans remains the best-known and, arguably, the most important oomycete pathogen. It continues to cost modern agriculture billions of pounds annually and also impacts subsistence farming in developing countries. With potato now ranked the third most important crop in the world, P. infestans is an important biotic threat to global food security. Damage caused by other Phytophthora species is also severe, on a global scale. Our long-term objective is to understand how oomycetes, particularly Phytophthora, successfully infect plants and dissect the plant processes that are affected by these pathogens. It is now well established that, like other pathogens, Phytophthora species secrete a number of proteins, termed 'effectors', that modulate the immune response of plants and enable host colonization. Deciphering the biochemical activities of effectors is critical to understanding mechanisms of pathogenesis. Among these proteins, RxLR-type effectors, named after an amino acid sequence present in the protein, are targeted to the inside of plant cells. In this proposal, we focus on a particular group of RxLR effectors that is found in the Phytophthora species mentioned above. These are all members of the Avr3a family, as they share amino acid sequence homology to the Phytophthora infestans effector Avr3a. Building on preliminary data we have already obtained, we aim to define what conserved and divergent functions these proteins have in suppressing the plant immune system. Through host factors, members of this protein family can also initiate a programmed cell-death response in plants designed to limit pathogen growth. We also aim to understand some details of how early steps in this response occur, specifically how these effectors are detected by host cell factors. This study will help to establish functional connections between pathogen proteins and plant processes. Detailed knowledge of how the Avr3a family is able to manipulate certain aspects of plant immunity and be recognized by others will improve our understanding of the infection process and enable novel strategies for engineering resistance to Phytophthora pathogens.