Induction of defence responses by cinnamomins against Phytophthora cinnamomi in Quercus suber and Quercus ilex subs. rotundifolia

Abstract

The strong association between Phytophthora cinnamomi and the mortality and decline of Quercus suber and Q. ilex subsp. rotundifolia has been known for two decades. The ability of elicitins secreted by this pathogen to trigger defence responses in these Quercus against itself was evaluated in this work. Biomass quantification by quantitative real-time PCR revealed a significant decrease in pathogen colonization of Q. suber roots after 24 h pre-treatment with α- and β-cinnamomin. In Q. suber and Q. ilex roots pre-treated with α-cinnamomin, hyphae were unable to reach and colonize the vascular cylinder and showed cytoplasmic disorganization in all the roots observed as contrasted with non-pre-treated roots. The pathogen was restricted to the intercellular spaces of the cortical parenchyma and the concomitant accumulation of electron dense materials was observed in contact with the hyphae. Furthermore, ROS (reactive oxygen species) production and the enzymatic activities of superoxide dismutase, catalase and peroxidase were compared in infected and non-infected Quercus roots in time course trials. There was a significant increase in the production of hydrogen peroxide (H2O2) and superoxide anion (O2 •-) and an enhanced activity of the enzymes in infected roots was observed at each time point. When comparing with elicitin non-treated roots, the α-cinnamomin-treated roots in interaction with P. cinnamomi showed a decrease in ROS accumulation and an increase of the enzyme activities. The overall results were consistent with an induction by the cinnamomins which initiated defence responses against the pathogen invasion of roots. Finally, elicitins were immunolocalized in the contact zone of P. cinnamomi hyphae with epidermal host cells, plasmalemma outer cytoplasm and around the intracellular hyphae in the vacuoles of invaded epidermal cells.