Abstract

Brown algae (Phaeophyta) encompass key primary producers of temperate and cold coastal seas, such as kelps, the cultivation of which is rapidly expanding worldwide. Here, we show that across ten brown algal species, innate resistance against the intracellular oomycete pathogen Eurychasma dicksonii is mediated by local cell death and accompanied by cell-wide deposition of β1-3 glucans and fluorescent metabolites, the accumulation of reactive oxygen species, and the expression of programmed cell death (PCD) markers. This response also occurs in compatible strains for a fraction of the infected algal cells, which makes it a quantitative trait. It is followed by the induction of two inducible autophagy-mediated defense responses already known for the oomycete pathogen Anisolpidium ectocarpii. Overall, we unveil hypersensitive-like cell death as a conserved, core part of brown algal innate defenses, that is intertwined with a second line of inducible, autophagy-mediated defenses. These results open unprecedented avenues to elucidate the molecular mechanisms of pathogen recognition and disease resistance in brown algae. © 2025