preprint draft Marek Randuch author 6ac4636d-15b2-11ec-abd3-fb8df79972ae Ivan Kulich author 57a1567c-8314-11eb-9063-c9ddc3451a54 Dmitrii Vladimirtsev author 60466724-5355-11ee-ae5a-fa55e8f99c3d Shouguang Huang author Rainer Hedrich author Jiří Friml author 4159519E-F248-11E8-B48F-1D18A9856A870000-0002-8302-7596 JiFr department Cyclic nucleotides as second messengers in plants project Guanylate cyclase activity of TIR1/AFBs auxin receptors project Roots must continuously adapt their growth and have evolved the ability to rapidly respond to diverse environmental and hormonal cues, including the phytohormone auxin. Auxin-induced cellular responses involve membrane depolarization, cytosolic calcium (Ca²⁺) elevation, and extracellular pH increase, but how these processes lead to rapid growth regulation remains unclear. Here, we show that cytosolic Ca²⁺ acts as a universal signal integrating diverse cues for rapid regulation of root growth. Using live imaging, microfluidics, and optogenetics in Arabidopsis, we demonstrate that a swift rise in cytosolic Ca²⁺ is the primary target of auxin signaling affecting growth inhibition. Disruption of Ca²⁺ influx abolishes these responses, whereas light-gated Ca²⁺ influx from the apoplast or endoplasmic reticulum stores inhibits growth. Multiple unrelated stimuli—including auxin, extracellular ATP, RALF peptides, and hydrogen peroxide—converge on this Ca²⁺-dependent mechanism. Cytosolic Ca²⁺ elevation thus represents a necessary and sufficient step for rapid growth inhibition, revealing a unifying principle of root signaling. 2025 eng 10.1101/2025.10.17.683082 https://research-explorer.ista.ac.at/record/23145 Randuch M, Kulich I, Vladimirtsev D, Huang S, Hedrich R, Friml J. Cytosolic Ca2+ as a universal signal for rapid root growth regulation. bioRxiv, <a href="https://doi.org/10.1101/2025.10.17.683082">10.1101/2025.10.17.683082</a>. M. Randuch, I. Kulich, D. Vladimirtsev, S. Huang, R. Hedrich, J. Friml, BioRxiv (n.d.). Randuch, Marek, et al. “Cytosolic Ca2+ as a Universal Signal for Rapid Root Growth Regulation.” <i>BioRxiv</i>, doi:<a href="https://doi.org/10.1101/2025.10.17.683082">10.1101/2025.10.17.683082</a>. Randuch, Marek, Ivan Kulich, Dmitrii Vladimirtsev, Shouguang Huang, Rainer Hedrich, and Jiří Friml. “Cytosolic Ca2+ as a Universal Signal for Rapid Root Growth Regulation.” <i>BioRxiv</i>, n.d. <a href="https://doi.org/10.1101/2025.10.17.683082">https://doi.org/10.1101/2025.10.17.683082</a>. Randuch M, Kulich I, Vladimirtsev D, Huang S, Hedrich R, Friml J. Cytosolic Ca2+ as a universal signal for rapid root growth regulation. <i>bioRxiv</i>. doi:<a href="https://doi.org/10.1101/2025.10.17.683082">10.1101/2025.10.17.683082</a> M. Randuch, I. Kulich, D. Vladimirtsev, S. Huang, R. Hedrich, and J. Friml, “Cytosolic Ca2+ as a universal signal for rapid root growth regulation,” <i>bioRxiv</i>. . Randuch, M., Kulich, I., Vladimirtsev, D., Huang, S., Hedrich, R., &#38; Friml, J. (n.d.). Cytosolic Ca2+ as a universal signal for rapid root growth regulation. <i>bioRxiv</i>. <a href="https://doi.org/10.1101/2025.10.17.683082">https://doi.org/10.1101/2025.10.17.683082</a> 231742026-10-19T11:44:48Z2026-10-19T12:49:25Z