Ecotypic variation in the response of Arabidopsis thaliana to Phytophthora cinnamomi

A variety of reactions to inoculation with <i>Phytophthora cinnamomi</i> ranging from high susceptibility to moderate resistance were found in 20 ecotypes of <i>Arabidopsis thaliana. P</i>. cinnamomi zoospores successfully colonised both root and leaf tissue of <i>Arabidopsis</i> and sporulation in the form of chlamydospores and sporangia occurred in leaves and roots of each ecotype but the number varied considerably between ecotypes. In the more susceptible ecotypes, colonisation was characterised by rapid intercellular growth and sporulation of the pathogen from 48 h post inoculation. In less susceptible ecotypes, <i>P. cinnamomi</i> was limited to a defined region within tissues. In response to <i>P. cinnamomi</i> infection, several ecotypes expressed active defence responses in both root and leaf tissue. Callose formation was closely associated with lesion restriction as was the production of the reactive oxygen species, hydrogen peroxide. The oxidative burst was not limited to the site of pathogen ingress but also occurred in distant, uninfected tissues. We have characterised an <i>Arabidopsis–P. cinnamomi </i>system that will be useful for further studies of active resistance mechanisms. <br />