Arabinogalactan proteins (AGPs) are highly glycosylated, hydroxyproline-rich proteins found at the cell surface of plants, where they play key roles in developmental processes. Brown algae are marine, multicellular, photosynthetic eukaryotes. They belong to the phylum Stramenopiles, which is unrelated to land plants and green algae (Chloroplastida). Brown algae share common evolutionary features with other multicellular organisms, including a carbohydrate-rich cell wall. They differ markedly from plants in their cell wall composition, and AGPs have not been reported in brown algae. Here we investigated the presence of chimeric AGP-like core proteins in this lineage. We report that the genome sequence of the brown algal model Ectocarpus siliculosus encodes AGP protein backbone motifs, in a gene context that differs considerably from what is known in land plants. We showed the occurrence of AGP glycan epitopes in a range of brown algal cell wall extracts. We demonstrated that these chimeric AGP-like core proteins are developmentally regulated in embryos of the order Fucales and showed that AGP loss of function seriously impairs the course of early embryogenesis. Our findings shine a new light on the role of AGPs in cell wall sensing and raise questions about the origin and evolution of AGPs in eukaryotes.

HighlightNew monoclonal antibodies to brown algal cell wall polymers allow tracing of in situ dynamics of alginates and sulfated fucans in Fucus zygotes and in the context of growth disruption

35Brown algae are multicellular photosynthetic stramenopiles that colonize marine rocky shores 36 worldwide. Ectocarpus sp. Ec32 has been established as a genomic model for brown algae. Here we 37 present the genome and metabolic network of the closely related species, Ectocarpus subulatus 38Kützing, which is characterized by high abiotic stress tolerance. Since their separation, both strains 39show new traces of viral sequences and the activity of large retrotransposons, which may also be 40 related to the expansion of a family of chlorophyll-binding proteins. Further features suspected to 41 contribute to stress tolerance include an expanded family of heat shock proteins, the reduction of 42 genes involved in the production of halogenated defence compounds, and the presence of fewer cell 43 wall polysaccharide-modifying enzymes. Overall, E. subulatus has mainly lost members of gene 44 families down-regulated in low salinities, and conserved those that were up-regulated in the same 45 condition. However, 96% of genes that differed between the two examined Ectocarpus species, as 46 well as all genes under positive selection, were found to encode proteins of unknown function. This 47 Organellar genomes 131 Plastid and mitochondrial genomes from E. subulatus have 95.5% and 91.5% sequence identity with 132 their Ectocarpus sp. Ec32 counterparts in the conserved regions respectively. Only minor structural 133 differences were observed between organellar genomes of both Ectocarpus genomes, as detailed in 134Supporting Information Text S1. 135

Brown algae (Phaeophyceae) are multicellular photoautrophic organisms and the largest biomass producers in coastal regions. A variety of observations indicate that their extracellular matrix (ECM) is involved with screening of salts, development, cell fate selection, and defense responses. It is likely that these functionalities are related to its constitutive structures. The major components of the ECM of brown algae are b-glucans, alginates, and fucose-containing sulfated polysaccharides. The genus Ectocarpus comprises a wide range of species that have adapted to different environments, including isolates of Ectocarpus subulatus, a species highly resistant to low salinity. Previous studies on a freshwater strain of E. subulatus indicated that the sulfate remodeling of fucans is related to the external salt concentration. Here we show that the sulfate content of the surrounding medium is a key parameter influencing both the patterning of the alga and the occurrence of the BAM4 sulfated fucan epitope in walls of apical cells. These results indicate that sulfate uptake and incorporation in the sulfated fucans from apical cells is an essential parameter to sustain tip growth, and we discuss its influence on the architectural plasticity of Ectocarpus.