Microbial Partnerships of Pathogenic Oomycetes

Oomycetes are filamentous eukaryotic microorganisms among which several species are plant and animal pathogens [1,2]. Those that cause plant diseases have had great impacts on human activities such as (i) the 19th century Irish famine triggered by the potato late blight (Phytophthora infestans), (ii) the associated massive North American immigration [3], and (iii) the formulation of the Bordeaux mixture, which was the first fungicide to be used worldwide [4]. Because of their ability to develop resistant against chemical treatments and to bypass plant resistance genes, they still have severe economic repercussions on modern crops. To circumvent these problems, most studies of the last ten years have reported on the coevolutionary mechanisms between the plant host immune system and the oomycete effector repertoire that promotes successful infection [5,6,7,8]. As for all other groups of plant pathogens, one of the current challenges is now to understand what is happening beyond the well-understood plant–oomycete interaction. To accomplish this, it is required to get a much broader picture of how the traits of the host and the pathogenic oomycete interact with the biotic environment to shape the evolution of plant resistance or oomycete pathogenicity. Concerning the host plant, the maintenance of a stable disease-resistance gene polymorphism appears to involve coevolution between the R gene and effector pairs but also complex and diffuse community-wide interactions [9]. The plant-associated microbiota contributes to maximize host adaptation to deal with pathogenic infection [10,11,12,13]. Concerning the pathogen, there is less understanding regarding how the pathogen–microbiota interaction accommodates the emergence of a pathogenic population, how it interferes with the expression of the effector repertoire on the plant surface, and, in fine, how it promotes or suppresses the disease. At the same time, an infectious entity is no longer only considered at the species level but also at the level of a resident microbiota or part thereof [14]. This paradigmatic inflexion helps (i) to unravel the molecular basis of interactions between plants and their pathogens in natural systems and (ii) to delineate the complex network of interactions that determine the spatial and temporal distribution of inocula and the genetic structure of the pathogen population as well as the communal virulence-associated mechanisms. This report highlights studies that establish how different aspects of the infectious process can be regulated by interactions between oomycetes or between oomycetes and other microbial species (Fig 1).