Co-Infections by <i>Fusarium circinatum</i> and <i>Phytophthora</i> spp. on <i>Pinus radiata</i>: Complex Phenotypic and Molecular Interactions

This study investigated the complex phenotypic and genetic response of Monterey pine (<i>Pinus radiata</i>) seedlings to co-infections by <i>F. circinatum</i>, the causal agent of pine pitch canker disease, and the oomycetes <i>Phytophthora xcambivora</i> and <i>P. parvispora</i>. Monterey pine seedlings were wound-inoculated with each single pathogen and with the combinations <i>F. circinatum</i>/<i>P. xcambivora</i> and <i>F. circinatum</i>/<i>P. parvispora</i>. Initially, seedlings inoculated only with <i>F. circinatum</i> showed less severe symptoms than seedlings co-inoculated or inoculated only with <i>P. xcambivora</i> or <i>P. parvispora</i>. However, 30 days post-inoculation (dpi), all inoculated seedlings, including those inoculated only with <i>F. circinatum</i>, 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, <i>F. circinatum</i> stimulated the up-regulation of all genes, while between the two oomycetes, only <i>P. xcambivora</i> induced significant up-regulations. In seedlings co-inoculated with <i>F. circinatum</i> and <i>P.</i><i>xcambivora</i> or <i>P. parvispora</i> 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 <i>F. circinatum</i>/<i>P.</i><i>xcambivora</i> and PDC in the combination <i>F. circinatum/P. parvispora</i>, thus suggesting a possible synergism of multiple infections in triggering this plant defense mechanism.