Sulfated oligosaccharides mediate the interaction between a marine red alga and its green algal pathogenic endophyte

The endophytic green alga Acrochaete operculata completely colonizes the sp orophytes of the red alga Chondrus crispus; however, it does not penetrate beyond the outer cell layers of the gametophytes. Given that the life cycle phases of C. crispus differ in the sulfation pattern of their extracellula r matrix carrageenans, we investigated whether carrageenan fragments could modulate parasite virulence. lambda-Carrageenan oligosaccharides induced re lease of H2O2, stimulated protein synthesis, increased carrageenolytic acti vity, and induced specific polypeptides in the pathogen, resulting in a mar ked increase in pathogenicity. In contrast, kappa-carrageenan oligosacchari des did not induce a marked release of H2O2 from A. operculata but hindered amino acid uptake and enhanced their recognition by the host, resulting in a reduced virulence. Moreover, C. crispus life cycle phases were shown to behave differently in their response to challenge with cell-free extracts o f A, operculata. Gametophytes exhibited a large burst of H2O2, whereas only low levels were released from the sporophytes.