Signatures of adaptation to obligate biotrophy in the Hyaloperonospora arabidopsidis genome

Many oomycete and fungal plant pathogens are obligate biotrophs, which extract nutrients only from living plant tissue and cannot grow apart from their hosts. Although these mildews and rusts cause significant crop losses, little is known about the molecular basis or evolution of obligate biotrophy. Here, we report the genome sequence of the oomycete Hyaloperonospora arabidopsidis (Hpa), a natural pathogen of Arabidopsis thaliana. In comparison to genomes of related, hemi-biotrophic Phytophthora species, the Hpa genome exhibits dramatic reductions in genes encoding: 1) RXLR effectors and other secreted pathogenicity proteins; 2) enzymes for assimilation of inorganic nitrogen and sulphur; 3) proteins associated with zoospore formation and motility. Some of these aspects are mirrored in the genomes of biotrophic fungi that have independently evolved to colonize plants, demonstrating strikingly convergent adaptation to a biotrophic existence inside plant hosts.