Attachment, penetration and early host defense mechanisms during the infection of filamentous brown algae by Eurychasma dicksonii

Abstract

Eurychasma dicksonii is one of the most common and widespread marine pathogens and attacks a broad spectrum of more than 45 brown algal species. The present study focuses on the mechanism used by the pathogen to attach on the host cell wall and force its way into algal cells. Ultrastructural examination revealed a needle-like structure which develops within the attached spore and extends along its main axis. Particular cell wall modifications are present at the basal part of the spore (adhesorium pad) and guide the needle-like tool to penetrate perpendicularly the host cell wall. The unique injection mechanism is shared with Haptoglossa species which suggests that this is an important characteristic of early diverging oomycetes. Furthermore, the encystment and adhesion mechanism of E. dicksonii shows significant similarities with other oomycetes, some of which are plant pathogens. Staining and immunolabelling techniques showed the deposition of β-1,3-glucans on the host cell wall at the pathogen penetration site, a strategy similar to physical responses previously described only in infected plant cells. It is assumed that the host defense in terms of callose-like deposition is an ancient response to infection.

Insoluble (1 → 3), (1 → 4)-β-D-glucan is a component of cell walls in brown algae (Phaeophyceae) and is masked by alginates in tissues

“…Some rare and old studies already depicted the presence of a mixture of (1 → 3)-β- and (1 → 4)-β-linked glucose residues in alkali-soluble materials of Fucus vesiculosus 33, 34 but the chemical structure of the originated polymer was not established. More recent studies point out that some brown algae are able to build up (1 → 3)-β- d -glucan chains in their wall, transiently upon infection in some Ectocarpales and Laminariales 12 , or more ubiquitously in Fucus vesiculosus cell walls 13 . The presence of the two linkage types on the same glucan chain was already suspected but not conclusively determined 14 .…”

“…The later only represent a minor fraction of the wall (~0–10%) and are believed to be composed mostly, if not exclusively, of cellulose 11 . More recent studies point out that some brown algae are able to build up (1 → 3)-β-glucan chains in their wall, transiently upon infection in some Ectocarpales and Laminariales 12 , or more ubiquitously in Fucus vesiculosus cell walls 13 . To the best of our knowledge no additional neutral and/or water insoluble polysaccharide has been reported so far in brown algal cell walls.…”

Dynamics of cell wall assembly during early embryogenesis in the brown algaFucus

“…A β-1,3-glucan, known as callose in land plants, is also present in brown algal walls (Raimundo et al , 2015), where it can be transiently expressed upon infection (Tsirigoti et al , 2014). Arabinogalactan-proteins (AGPs) are also cell surface components in brown algae (Hervé et al , 2016).…”

Not in your usual Top 10: protists that infect plants and algae

“…On infection, cell wall thickening and increased amounts of b-1,3-glucans at the penetration site may build physical barriers to pathogen invasion. Large amounts of b-1,3-glucan occur at cell surfaces of partially resistant Ectocarpus strains (Tsirigoti et al, 2015).…”

“…Zoospores attach, encyst and build adhesorium-like structures at the host surfaces. The parasite cytoplasm is transferred into the host via a needle-like structure which is associated with the formation of the adhesorium chamber at the host-spore contact point (Tsirigoti et al, 2015), similar to the plasmodiophorid 'Rohr und Stachel'. After penetration, multinucleate non-walled immature thalli, with double membrane envelopes of host and parasite (Sekimoto et al, 2008), develop and expand in the infected host cells, until each cell is almost filled.…”