Exploiting the Phytophthora infestans genome: targets for sustainable potato protection

Pathogen attack of crop plants is a key issue affecting agricultural sustainability in terms of both yield loss due to disease and environmental impact due to fungicide application. The oomycete pathogen Phytophthora infestans is the most significant pathogen of potato, the world's fourth largest crop. P. infestans is responsible for large yield losses through late blight disease, and costs associated with chemical control amount to £3M globally per year. Genetic resistance to P. infestans and control chemicals have been deployed with limited success, as both have been readily overcome by variation in pathogen populations. This proposal aims to address the problems faced by existing control measures through exploitation of the P. infestans genome to seek vital and invariant components of its pathogenicity arsenal that can be targeted for sustainable potato protection. Specifically, this information will be used to identify sources of durable potato disease resistance for breeding and to develop novel control strategies that are intrinsically difficult for the pathogen to overcome. The oomycetes include more than 70 Phytophthora species and are arguably the most significant pathogens of dicotyledenous plants. In the last year or so, genes have been identified from oomycete pathogens of the model plant Arabidopsis, of soybean, and from P. infestans itself (by the SCRI group), that encode proteins that trigger resistance. These proteins are very different to each other except from a conserved motif that is similar to a sequence required for delivery of malaria virulence proteins inside human blood cells. Preliminary evidence suggests that this motif is required to deliver the oomycete proteins into the cells of their respective plant hosts. The motif has provided a signature to search for other proteins that are delivered inside host cells, where they may be exposed to defence surveillance systems. In this proposal we aim to identify the entire complement of such proteins from P. infestans. We will characterize these proteins to seek those that are essential for infection (and thus are not easily lost by the pathogen) and those that show little sequence variation in diverse strains of the pathogen (and thus appear to be under selective pressure to remain unchanged). We postulate that such proteins represent potential Achilles' Heels for the pathogen if resistances can be found that recognize them. To this end, we will search in a wild potato biodiversity collection at SCRI (The Commonwealth Potato Collection) for plants that are resistant to these proteins (and thus to most, if not all, strains of P. infestans). These resistances are likely to be highly durable and thus will be prioritized for introduction into cultivated potato in commercially supported breeding programmes at SCRI. The second 'Achilles' Heel' of P. infestans that we intend to exploit is the machinery required for translocation of these virulence proteins inside potato cells. The translocation machinery is potentially a very suitable target for disease control, since inhibition of this delivery process would prevent effector proteins entering host cells and thus inhibit the pathogen's normal infection process. Experiments will be conducted to find the proteins responsible for translocation by identifying proteins that bind to the conserved delivery motif. We will conduct experiments to determine how they work. Mimicks of these proteins which bind to the delivery motif in oomycete virulence proteins will potentially not only prevent P. infestans from causing infection but will have a wider application by inhibiting other oomycete plant pathogens and will possibly extend to unrelated pathogens such as malaria. The biotechnology company Syngenta is the end-user that will exploit our findings in this aspect of the project.

Oomycetes comprise more than 70 species of Phytophthora, the most devastating pathogens of dicotyledonous plants. Chief amongst them is P. infestans, cause of potato late blight. Genetic resistance to P. infestans, in the form of resistance (R) genes, exists in cultivated potato and oomycete active control chemicals have been developed to prevent late blight. However, both have readily been overcome by variation in pathogen populations. Recently, a number of avirulence (Avr) genes encoding proteins recognised by the products of host R genes, have been isolated from oomycete pathogens. These AVR effectors are recognised in the host cytoplasm, implying that they are delivered inside host cells. The AVR proteins contain a motif, RXLR. We, and collaborators, have shown that RXLR is required for translocation of P. infestans effectors inside plant cells. This sequence signature provides a means to identify the entire RXLR effector complement from the P. infestans genome sequence. This project seeks essential and invariant P. infestans RXLR pathogenicity effectors that will be used to seek new sources of durable disease resistance by expressing them in wild, late-blight resistant potato germplasm. The resistant germplasm, and the effector expression vectors (which provide phenotypic markers for accelerated selection in breeding), will be used to introgress new resistances into cultivated potato via commercially funded breeding programmes at SCRI. Moreover, the RXLR effector translocation process will be studied to identify proteins required for the translocation mechanism. These will be used to screen for antibodies and peptides that bind to RXLR and which can act as inhibitors of RXLR effector translocation (and thus prevent P. infestans infection). This strategy, providing an initial mode-of-action with which to seek specific inhibitors, will have wide application in preventing oomycete pathogens of both plants and animals.