Biochemical characteristics of a diffusible factor that induces gametophyte to sporophyte switching in the brown alga Ectocarpus

The haploid‐diploid life cycle of the filamentous brown alga Ectocarpus involves alternation between two independent and morphologically distinct multicellular generations, the sporophyte and the gametophyte. Deployment of the sporophyte developmental program requires two TALE homeodomain transcription factors OUROBOROS and SAMSARA. In addition, the sporophyte generation has been shown to secrete a diffusible factor that can induce uni‐spores to switch from the gametophyte to the sporophyte developmental program. Here, we determine optimal conditions for production, storage, and detection of this diffusible factor and show that it is a heat‐resistant, high molecular weight molecule. Based on a combined approach involving proteomic analysis of sporophyte‐conditioned medium and the use of biochemical tools to characterize arabinogalactan proteins, we present evidence that sporophyte‐conditioned medium contains AGP epitopes and suggest that the diffusible factor may belong to this family of glycoproteins.