Host-pathogen interactions in root infecting oomycete species

The oomycetes include some of the most devastating pathogens on both cultivated crops and wild plants. In the genus Phytophthora some closely related species have a broad host range, while others are very host specific. The aim of this project was to gain an understanding of the mechanisms underlying the differentiation of a subgroup of root-infecting Phytophthora species and to gain knowledge about the plant immune responses triggered by distantly related oomycetes that adapted to the same legume host. We investigated the zoospore chemotaxis of legume-root infecting Phytophthora species to different isoflavonoid compounds and explored a possible connection to host preference. Our results showed that specific chemotaxis towards host isoflavones is of limited importance in Phytophthora sojae and Phytophthora vignae, while, specific chemotaxis of Phytophthora pisi and Phytophthora niederhauserii indicated an adaptation to their pathogenicity on the host and lack of pathogenicity on non-host plants. The comparative proteomic study of P. pisi and P. sojae in germinating cysts and hyphae, using tandem mass spectrometry, revealed specific and common pathogenicity factors involved in initiation of infection and host specificity such as serine proteases, membrane transporters and ricin-B lectin in these closely related species. Furthermore, the data suggested that germinating cysts catabolize lipid reserves through the β-oxidation pathway and the glyoxylate cycle to initiate infection. The transcriptomic response of pea plants towards Aphanomyces euteiches and P. pisi, two distantly related oomycetes, was studied during early phase of infection, using a microarray approach. The results deciphered common and specific immune mechanisms towards these pathogens. Activation of cell wall modification, regulation of jasmonic acid biosynthesis and induction of the ethylene signaling pathway were among the common transcriptional responses to both pathogens. However, induction of chalcone synthesis and auxin signaling were specific transcriptional changes against A. euteiches.