Cell wall modifications and cell regeneration in brown algae resisting infection by the oomycete pathogen Eurychasma dicksonii

We previously found that that the reaction of clonal brown algal strains to Eurychasma dicksonii infection ranges from extreme susceptibility to complete resistance. Ultrastructural studies on disease-susceptible brown algae revealed a distinct cell wall thickening just beneath the empty pathogen spore, similar to the cell wall papillae reported in plants. Additionally, the algal cell wall deposition was labelled by the fluorochrome aniline blue and the anti-(1,3)-β-D-glucan antibody. According to microscopic observations, although this structure probably hinders pathogen penetration, it seems insufficient to halt the pathogen progression. Here we investigated further the possible occurrence of additional cell wall modifications in resistant algal strains against the oomycete pathogen E.dicksonii. Specifically, we found that algal cells resisting to infection deposit β-1,3 glucans over their entire surface, correlating with the rapid death of the attacked cell. This process is often followed by tissue regeneration according to a defined developmental program. For example, in Ectocarpus fasciculatus upright filaments, the cells adjacent to the originally infected by E. dicksonii cell are asymmetrically divided into two apical cells which grow towards the dead cell, healing the filament. Like other biochemical and histological markers of disease resistance that we previously identified, β-1,3-glucan deposition is conserved in all algal species investigated so far, adding weight to our hypothesis to the existence of core disease resistance mechanisms conserved across the brown algal lineage.