Cytosolic Ca2+ as a universal signal for rapid root growth regulation Randuch, Marek Kulich, Ivan Vladimirtsev, Dmitrii Huang, Shouguang Hedrich, Rainer Friml, Jiří ; https://orcid.org/0000-0002-8302-7596 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 info:eu-repo/semantics/preprint doc-type:Preprint Preprint http://purl.org/coar/resource_type/c_816b https://research-explorer.ista.ac.at/record/23174 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> eng info:eu-repo/semantics/altIdentifier/doi/10.1101/2025.10.17.683082 https://creativecommons.org/licenses/by/4.0/ info:eu-repo/semantics/openAccess