The immediate activation of defense responses in Arabidopsis roots is not sufficient to prevent Phytophthora parasitica infection

P>The outcome of plant-microbe interactions is determined by a fine-tuned molecular interplay between the two partners. Little is currently known about the molecular dialogue between plant roots and filamentous pathogens. We describe here a new pathosystem for the analysis of molecular mechanisms occurring during the establishment of a compatible interaction between Arabidopsis thaliana roots and a root-infecting oomycete. We performed cytological and genetic analyses of root infection during the compatible interaction between A. thaliana and Phytophthora parasitica. Phytophthora parasitica uses appressoria to penetrate A. thaliana roots. Initial biotrophic growth is accompanied by the formation of haustoria, and is followed by a necrotrophic lifestyle. Arabidopsis thaliana mutants with impaired salicylic acid (SA), jasmonic acid (JA) or ethylene (ET) signaling pathways are more susceptible than the wild-type to infection. The salicylate- and jasmonate-dependent signaling pathways are concertedly activated when P. parasitica penetrates the roots, but are downregulated during invasive growth, when ethylene-mediated signaling predominates. Thus, defense responses in A. thaliana roots are triggered immediately on contact with P. parasitica. Our findings suggest that the pattern of early defense mechanism activation differs between roots and leaves.