This study investigated the complex phenotypic and genetic response of Monterey pine (Pinus radiata) seedlings to co-infections by F. circinatum, the causal agent of pine pitch canker disease, and the oomycetes Phytophthora xcambivora and P. parvispora. Monterey pine seedlings were wound-inoculated with each single pathogen and with the combinations F. circinatum/P. xcambivora and F. circinatum/P. parvispora. Initially, seedlings inoculated only with F. circinatum showed less severe symptoms than seedlings co-inoculated or inoculated only with P. xcambivora or P. parvispora. However, 30 days post-inoculation (dpi), all inoculated seedlings, including those inoculated only with F. circinatum, showed severe symptoms with no significant differences among treatments. The transcriptomic profiles of three genes encoding pathogenesis-related proteins, i.e., chitinase (PR3), thaumatin-like protein (PR5), phenylalanine ammonia-lyase (PAL), and the pyruvate decarboxylase (PDC)-encoding gene were analyzed at various time intervals after inoculation. In seedlings inoculated with single pathogens, F. circinatum stimulated the up-regulation of all genes, while between the two oomycetes, only P. xcambivora induced significant up-regulations. In seedlings co-inoculated with F. circinatum and P.xcambivora or P. parvispora none of the genes showed a significant over-expression 4 dpi. In contrast, at 11 dpi, significant up-regulation was observed for PR5 in the combination F. circinatum/P.xcambivora and PDC in the combination F. circinatum/P. parvispora, thus suggesting a possible synergism of multiple infections in triggering this plant defense mechanism.
Keywords: Monterey pine; P. parvispora; PAL; PR3; PR5; Phytophthora xcambivora; gene expression; housekeeping genes; pitch canker disease; plant- oomycetes- fungal interactions; plant-defense molecular mechanisms.