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Professor Brett Tyler and collaborators have sequenced the genomes of two deadly Phytophthora pathogens, constructed detailed maps of the disease-related genetic networks of host and pathogen, and mined large datasets of information to reveal a fundamental mechanism for the way oomycete pathogens launch their disease-inducing molecules into the cells of their plant hosts. The work suggests a common mechanism for entry of pathogen proteins into the host cells of not only plants but also animals and humans, and points the way towards the development of effective countermeasures. The pay-off in stopping these pathogens in their tracks could be large. Over the years, Phytophthora sojae alone has caused billions of dollars of losses for soybean farmers in the United States.

Dr. Brett Tyler

Invaders from the sea

Oomycete plant pathogens are fungal-like organisms that are evolutionarily related to algae in the plant kingdom Stramenopila, a group of plants that also includes golden-brown algae, diatoms, and brown algae such as kelp. 1300 million years ago, an algal relative from this plant kingdom engulfed a red alga and adopted its photosynthetic machinery. Over the course of evolution however, oomycetes abandoned their ability to make food from light choosing instead to profit from a parasitic lifestyle. Oomycetes became destructive pathogens of aquatic plants and animals, and today remain a major headache for fish and crustacean (shrimp and crab) farmers.

More recently oomycetes have extended their hunting range to land plants. Revealing their aquatic origins, oomycetes are at their most dangerous when the air is cool and misty or the soil is water-logged.  Oomycete plant pathogens destroy many species important to agriculture, forestry, gardens and natural ecosystems. Phytophthora ramorum, which causes sudden oak death, has attacked and killed tens of thousands of oak trees in California and Oregon.  Phytophthora sojae causes severe damage in soybean crops. In the nineteenth century, the potato late blight pathogen Phytophthora infestans was responsible for the Irish potato famine. Most of the 90 species of Phytophthora are destructive pathogens and scientists want to know how they bring about wide-scale damage in the hope of finding ways to combat infection.

A systems-wide approach to the study of infection

Most plants have effective defense mechanisms to fight disease and thus they are healthy most of the time. However, evolution has been shaping the attributes of pathogens and plants over many thousands of years in what amounts to an ongoing evolutionary arms race with repeated development of new virulence weaponry in the pathogens and new countermeasures in the hosts.

Sequencing the genomes of pathogens, cataloguing the genes, and analyzing the proteins produced by the host and pathogen are all key steps in understanding how the plant-pathogen system works. The scientists are building genetic networks, integrating data from individual experiments on specific genes, and modeling the way the genes work to help build a picture of how the global network of genes operate. Dr. Tyler's group and collaborators are also developing a common language, a gene ontology, to compare the functions of genes across different species. This is helping to model the impact evolution has had on these genes and provide clues for possible disease intervention.

Says Tyler: "To build genetic regulatory networks from biological data, we are putting in place mathematics- and computer-science-based methods for inferring and modeling biological processes. This sheds light on the interconnected genetic regulatory networks that have arisen due to the ongoing co-evolutionary battle between the plant and pathogen."