Filamentous brown algae infected by the marine,holocarpic oomycete Eurychasma dicksonii: first results on the organization and the role of cytoskeleton in both host and parasite

The important role of the cytoskeletal scaffold is increasingly recognized in host-pathogen interactions. The cytoskeleton potentially functions as a weapon for both the plants defending themselves against fungal or oomycete parasites, and for the pathogens trying to overcome the resisting barrier of the plants. This concept, however, had not been investigated in marine algae so far. We are opening this scientific chapter with our study on the functional implications of the cytoskeleton in 3 filamentous brown algal species infected by the marine oomycete Eurychasma dicksonii. Our observations suggest that the cytoskeleton is involved in host defense responses and in fundamental developmental stages of E. dicksonii in its algal host.Oomycetes are important plant and animal pathogens and are the cause of significant crop losses every year. Hence, a plethora of studies with different cultivated and model plant species investigate the diversity of parasite infection pathways and host defense responses.¹ However, little information is available on the interactions between algae and marine oomycetes, despite the epidemic outbreaks reported² and the huge impact on intensive algal aquaculture.³Eurychasma dicksonii is a biotrophic, intracellular marine oomycete, capable to infect at least 45 species of brown seaweeds in laboratory cultures.⁴ Molecular data reveal that E. dicksonii has a basal phylogenetic position in the oomycete lineage.^5,6 The basic stages of the infection are known: the attachment of the parasite spore to the host cell wall, the penetration of its cytoplasm into the host cell, the formation of a multinucleated, unwalled thallus, and zoosporogenesis.⁶ Hitherto, though, there was no knowledge about the role of cytoskeleton in the context of infection, which stimulated our research.In land plants, reorganization of the cytoskeleton is part of the reaction to infection by fungal pathogens. The rearrangement of the cytoplasm and the relocation of the nuclei and other organelles are accompanied by rapid rearrangements of the cytoskeletal elements.⁷ The plant cytoskeleton shows an extreme plasticity in order to serve the intracellular realignment.At the same time, this indicates that the plant cytoskeleton could be the parasite’s target by producing anti-cytoskeletal compounds in an effort to overcome plant resistance, a mechanism known in several fungal and oomycete pathogens of higher plants.^8,9Consequently, the changes in microtubule (MT) organization are associated with both the plant defense and/or susceptibility toward oomycetes, respectively.¹⁰ Therefore, our research on the organization and role of cytoskeleton in the host and the parasite sheds some light into the enormous variability in the specificity of the recognition, defense, and infection mechanisms.