The plant hormone auxin (indole-3-acetic acid (IAA) ) regulates plant morphology and development in land plants. Moreover, it was regently shown that auxin can trigger rapid changes in phospho-proteomes of land plants and unicellular algae, suggesting an deeply conserved ancient role of auxin as an environmental signal. Here, we profiled auxin-triggered phospho-proteome responses in morphologically complex the algae Chara braunii.