Auxin, indole-3-acetic acid (IAA), is a key phytohormone with diverse morphogenic roles in land plants, but its function and transport mechanisms in algae remain poorly understood. We therefore aimed to explore the role of IAA in a complex, streptophyte algae Chara braunii. Here, we described novel responses of C. braunii to IAA and characterized two homologs of PIN auxin efflux carriers: CbPINa and CbPINc. We determined their localization in C. braunii using epitope-specific antibodies and tested their function in heterologous land plant models. Further, using phosphoproteomic analysis, we identified IAA-induced phosphorylation events. The thallus regeneration assay showed that IAA promotes thallus elongation and side branch development. Immunolocalization of CbPINa and CbPINc confirmed their presence on the plasma membrane of vegetative and generative cells of C. braunii. However, functional assays in tobacco BY-2 cells demonstrated that CbPINa affects auxin transport, whereas CbPINc does not. The IAA is effective in the acceleration of cytoplasmic streaming and the phosphorylation of evolutionary conserved targets such as homolog of RAF-like kinase. These findings suggest that, although canonical PIN-mediated auxin transport mechanisms might not be fully conserved in Chara, IAA is involved in morphogenesis and fast signaling processes.

ARC Centre of Excellence for Plant Success in Nature and Agriculture Monash University Melbourne 3800 Vic Australia

Department of Experimental Plant Biology Faculty of Science Charles University Viničná 5 Prague 2 128 44 Czech Republic

Department of Plant Cell Biology Green Life Sciences Cluster Swammerdam Institute for Life Sciences University of Amsterdam Amsterdam 1098XH the Netherlands

Faculty of Biotechnology University of Wroclaw Joliot Curie 14a Wroclaw 50 383 Poland

Institute of Science and Technology Austria Klosterneuburg 3400 Austria

Laboratory of Biochemistry Wageningen University Stippeneng 4 Wageningen 6708 SP the Netherlands

Laboratory of Hormonal Regulations in Plants Institute of Experimental Botany of the Czech Academy of Sciences Rozvojová 263 Prague 6 165 02 Czech Republic

School of Biological Sciences Monash University Melbourne 3800 Vic Australia

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