The Contribution of Phytophthora effectors to host range and non-host resistance

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An increasing world population and impacts of climate change place ever-greater demands on the world food supply. A major constraint to global food security is crop loss due to plant pests and diseases. With the increasing stringency of conditions under which chemicals are approved for agriculture, the choice of effective fungicides and pesticides will become more limited in the near future. Furthermore, as introgressed host resistance genes are rapidly defeated by pathogens and pests in the field, there is an urgent need to explore sources of disease and pest resistance that are durable and will provide long term food security.Plants face a constant barrage of pest and microbial threats and defend themselves by employing at least two layers of inducible defence responses. The first layer involves recognition of Pathogen-Associated-Molecular-Patterns (PAMPs) by cell surface pattern recognition receptors (PRRs) to mount PAMP-triggered immunity (PTI). Successful (adapted) pathogens secrete and deliver molecules, effectors, inside host cells to suppress these defences. The second defence layer involves recognition of effectors by immune receptors, or NB-LRR proteins, to evoke effector-triggered immunity (ETI). This recognition is often based on detection of changes by NB-LRRs in the conformational/biochemical state of host proteins that are targeted by effectors. Most plants are resistant to most pathogens, which is termed non-host resistance (NHR). By definition this is effective against all genetic variants of a pathogen species, and thus durable. Effector recognition by NB-LRR proteins has been proposed to be a major determinant of NHR in plant species that are closely related to the pathogens' natural hosts. In contrast, evidence exists that inducible NHR in plant species that are distantly related to the pathogen's natural host(s) is the result of the inability of effectors to appropriately target plant proteins and suppress immunity. Recent progress in the identification of plant pathogen effectors, and their targets, has opened new avenues to investigate the contributions of these proteins to determining non-host resistance.This proposal aims to generate a key understanding of the role of effectors, their targets, and NB-LRR proteins in non-host resistance in the Solanaceae. The Solanaceae include economically important food crops, such as pepper, tomato, and potato, the latter being the world's fourth largest food crop, and the second largest in Europe after wheat. We will exploit effector sets from 2 important solanaceous pathogens with differing host ranges: the oomycetes Phytophthora infestans (infects potato and tomato but not pepper) and P. capsici (infects pepper and tomato but not potato). We will assess the contribution of Phytophthora effectors to defining host range. Our key objectives are to (i) assess whether effectors trigger NB-LRR-mediated immunity in non-host solanaceous plants, and (ii) whether inducible NHR in Arabidopsis, constitutes a failure of effectors to modify intended targets and thus suppress PTI. We will exploit the genome sequences of potato, tomato, N. benthamiana, N. sylvestris and pepper to identify targets for P. infestans and P. capsici effectors. We will then be able to investigate their role in mediating effector recognition in non-host solanaceous plants.The proposed work will shed light on the contribution of NB-LRRs, and the effector targets they monitor, to NHR within solanaceous crops. We will establish an essential platform that determines the molecular basis of NHR, identifies the critical effectors that activate NHR and paves the way to targeted searches for associated genes. Moreover, we will identify Arabidopsis PTI regulatory components, insensitive to effector activity, that can be deployed in Solanaceous crops. This approach will build a highly durable barrier to infection.