Abstract

Eukaryotic plant pathogens, such as oomycetes and fungi, cause highly destructive diseases that negatively impact commercial and subsistence agriculture worldwide. These pathogens secrete an arsenal of effector proteins to modulate plant innate immunity and enable parasitic infection. Deciphering the biochemical activities of effectors to understand how pathogens successfully colonize and reproduce on their host plants became a driving paradigm in the field. This presentation will focus on effectors of oomycetes, such as the Irish potato famine pathogen Phytophthora infestans. Tremendous progress has been made recently in understanding the biology of oomycete effectors. Two classes of effectors target distinct sites in the host plant: apoplastic effectors are secreted into the plant extracellular space, while cytoplasmic effectors are translocated inside the plant cell, where they target different subcellular compartments. Of particular interest are the RXLR and Crinkler host-translocated (cytoplasmic) effectors that are characterized by conserved motifs following the signal peptide. The RXLR domain is functionally interchangeable with a malaria host targeting domain and functions in delivery into host cells. The recent completion of six oomycete genome sequences enabled genome-wide cataloguing of the effector secretome revealing hundreds of candidate effectors. Effectors are frequently organized in clusters of paralogous genes, many of which exhibit hallmarks of positive selection probably as a result of a coevolutionary arms race with host factors. We also utilized the discovered RXLR effectors in high-throughput in planta expression assays to screen for avirulence and virulence activities. The perturbations caused by these effectors is helping to elucidate the mechanisms of pathogenicity as well as further illuminate mechanisms of plant defense and innate immunity.