Plant pathogenic oomycetes: counterbalancing resistance, susceptibility and adaptation

The genetic basis for the interaction(s) between plants and pathogens has been classically illustrated as a gene-for-gene relationship, through which a single gene product from the pathogen interacts within a single gene product found within the plant. In the simplest terms, it is the outcome of these interactions that underpin resistance and/or susceptibility signalling processes. While this basic concept shapes our understanding of many of the molecular-genetic mechanisms controlling immune signalling in plants, current research has revealed that the timing and ultimate outcome of these interactions are far more complex, and are typically regulated in a genome-by-genome manner. As a central theme for this review, we will focus on recent discoveries in the field of plant-oomycete interactions, primarily on the downy mildews, an important group of obligate oomycete pathogens of plants. Recent transcriptome-based studies have shown that survival, adaptation and virulence rely on complex, bi-directional interactions between the host and pathogen genomes. Taking advantage of the obligate nature of the downy mildews, insight into the transcriptional plasticity of these genomes has revealed a remarkable ability to adapt to host and environmental stressors. Herein, we will highlight a recent body of research using the Pseudoperonospora cubensis - Cucumis sativus interaction, which has identified a suite of alternative splicing and sRNA-based regulatory signals that are induced in a temporal and host-specific manner. In combination with recent studies in other plant-oomycete pathosystems, a comprehensive transcriptional profile of resistance and susceptibility within the host and pathogen illustrates the remarkable ability of this group of pathogens to adapt to host and environment to infect and cause disease in a diverse array of agriculturally important crops.