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The growth and production of vegetables is restricted by crop loss to infection by pathogenic microbes. The pathogen Peronospora effusa, or spinach downy mildew, is a major threat to spinach production and causes tremendous yield losses. During infection, downy mildew pathogens secrete proteins, called effectors, into the plant tissue and into plant cells. Effectors promote infection by suppressing the plant immune system and by manipulating other plant cell processes. Usually, this occurs by interaction between an pathogen effector and one or multiple plant proteins.

In this thesis the genome of P. effusa strain 1 is uncovered that allowed to identify the genes that encode effector proteins. A selection of effectors was used to identify interacting plant proteins. Eight effectors and their fourteen plant interactors were further studied to understand their role in plant cells. The effector proteins were each found to localize to specific cellular compartments, e.g. the plant cell nucleus, cytoplasm, ER and punctate structures. The majority of effectors co-localized with the identified plant proteins in the same cellular compartments. The cellular localization of six plant interactors and two effector proteins was altered upon co-expression of the effector and interactor proteins.

These results provide possible mechanisms by which  effectors influence and steer disease development. The study of effectors and their interacting plant proteins provides a great opportunity to understand the molecular interaction between plants and pathogens and to translate that knowledge to breed resistant crop varieties.