article epub_ahead yes Adrijana Smoljan author cced8a85-223e-11ed-af04-b0596c55053b Sarah Koutnik‐Abele author Dmitrii Vladimirtsev author 60466724-5355-11ee-ae5a-fa55e8f99c3d Petr Klíma author Anita Bírošíková author Yuzhou Zhang author 3B6137F2-F248-11E8-B48F-1D18A9856A870000-0003-2627-6956 Jack Merrin author 4515C308-F248-11E8-B48F-1D18A9856A870000-0001-5145-4609 Maximilian Schuster author 37e65def-d415-11eb-ae59-a7b67be103db Katarina Kurtović author Ulrich Z. Hammes author Jan Petrášek author Jiří Friml author 4159519E-F248-11E8-B48F-1D18A9856A870000-0002-8302-7596 JiFr department GradSch department NanoFab department Bio department Cyclic nucleotides as second messengers in plants project Peptide receptors for auxin canalization in Arabidopsis project Guanylate cyclase activity of TIR1/AFBs auxin receptors project Auxin, primarily indole-3-acetic acid (IAA), is a central regulator of growth and development in land plants, but its physiological role in chlorophyte algae remains unclear. Here, we show that exogenous IAA modulates growth in Chlorella sorokiniana, Chlorella variabilis, and Chlamydomonas reinhardtii in a concentration-dependent manner. Low IAA concentrations promoted growth by accelerating the onset of cell division without affecting cell size, whereas higher concentrations inhibited proliferation. Radiotracer assays showed that all three species take up and release IAA across the plasma membrane through a combination of passive diffusion and energy-dependent, saturable processes. Competition by excess unlabeled natural and synthetic auxins further supported the presence of carrier-mediated transport with broad substrate recognition. Phylogenetic analyses identified potential PIN-like auxin exporters in chlorophytes and other non-plant eukaryotes, and structural modeling supported conservation of the overall PIN fold and predicted auxin-binding residues. However, functional assays in Xenopus laevis oocytes, tobacco BY-2 cultured cells, and Arabidopsis thaliana did not support a role for these proteins in directional auxin export. Instead, non-plant PIN homologs localized predominantly to the endoplasmic reticulum and showed limited or no transport activity in heterologous systems. Together, these findings indicate that auxin responsiveness and basic cellular auxin transport predate canonical PIN-mediated directional auxin export, which appears to be a later innovation of the streptophyte lineage. Wiley2026 eng 1672-9072 1744-7909 4227160710.1111/jipb.70309 Smoljan, Adrijana, Sarah Koutnik‐Abele, Dmitrii Vladimirtsev, Petr Klíma, Anita Bírošíková, Yuzhou Zhang, Jack Merrin, et al. “Auxin Response and PIN‐mediated Transport in Chlorophyte Algae.” <i>Journal of Integrative Plant Biology</i>. Wiley, 2026. <a href="https://doi.org/10.1111/jipb.70309">https://doi.org/10.1111/jipb.70309</a>. A. Smoljan <i>et al.</i>, “Auxin response and PIN‐mediated transport in chlorophyte algae,” <i>Journal of Integrative Plant Biology</i>. Wiley, 2026. Smoljan, A., Koutnik‐Abele, S., Vladimirtsev, D., Klíma, P., Bírošíková, A., Zhang, Y., … Friml, J. (2026). Auxin response and PIN‐mediated transport in chlorophyte algae. <i>Journal of Integrative Plant Biology</i>. Wiley. <a href="https://doi.org/10.1111/jipb.70309">https://doi.org/10.1111/jipb.70309</a> A. Smoljan, S. Koutnik‐Abele, D. Vladimirtsev, P. Klíma, A. Bírošíková, Y. Zhang, J. Merrin, M. Schuster, K. Kurtović, U.Z. Hammes, J. Petrášek, J. Friml, Journal of Integrative Plant Biology (2026). Smoljan A, Koutnik‐Abele S, Vladimirtsev D, Klíma P, Bírošíková A, Zhang Y, Merrin J, Schuster M, Kurtović K, Hammes UZ, Petrášek J, Friml J. 2026. Auxin response and PIN‐mediated transport in chlorophyte algae. Journal of Integrative Plant Biology., jipb. 70309. Smoljan A, Koutnik‐Abele S, Vladimirtsev D, et al. Auxin response and PIN‐mediated transport in chlorophyte algae. <i>Journal of Integrative Plant Biology</i>. 2026. doi:<a href="https://doi.org/10.1111/jipb.70309">10.1111/jipb.70309</a> Smoljan, Adrijana, et al. “Auxin Response and PIN‐mediated Transport in Chlorophyte Algae.” <i>Journal of Integrative Plant Biology</i>, jipb. 70309, Wiley, 2026, doi:<a href="https://doi.org/10.1111/jipb.70309">10.1111/jipb.70309</a>. 223012026-07-13T10:44:55Z2026-07-13T14:26:31Z