Characterization of an Arabidopsis-Phytophthora pathosystem: resistance requires a functional PAD2 gene and is independent of salicylic acid, ethylene and jasmonic acid signalling

Arabidopsis accessions were screened with isolates of Phytophthora porri or iginally isolated from other crucifer species. The described Arabidopsis-Ph ytophthora pathosystem shows the characteristics of a facultative biotrophi c interaction similar to that seen in agronomically important diseases caus ed by Phytophthora species. In susceptible accessions, extensive colonizati on of the host tissue occurred and sexual and asexual spores were formed. I n incompatible combinations, the plants reacted with a hypersensitive respo nse (HR) and the formation of papillae at the sites of attempted penetratio n. Defence pathway mutants such as jar1 (jasmonic acid-insensitive), etr1 ( ethylene receptor mutant) and ein2 (ethylene-insensitive) remained resistan t towards P. porri. However, pad2, a mutant with reduced production of the phytoalexin camalexin, was hyper-susceptible. The accumulation of salicylic acid (SA) and PR1 protein was strongly reduced in pad2. Surprisingly, this lack of SA accumulation does not appear to be the cause of the hyper-susce ptibility because interference with SA signalling in nahG plants or sid2 or npr1 mutants had only a minor effect on resistance. In addition, the funct ional SA analogue benzothiadiazol (BTH) did not induce resistance in suscep tible plants including pad2. Similarly, the complete blockage of camalexin biosynthesis in pad3 did not cause susceptibility. Resistance of Arabidopsi s against A porri appears to depend on unknown defence mechanisms that are under the control of PAD2.