Ca2+ signature gates early auxin signaling in Arabidopsis roots Song, Huifang Baudon, Angel Freund, Matthias Randuch, Marek Pěnčík, Aleš Ondřej, Novák He, Zhaohui Kaufmann, Kerstin Li, Nan Gilliham, Matthew Friml, Jiří ; https://orcid.org/0000-0002-8302-7596 Hedrich, Rainer Huang, Shouguang Plants must continually balance growth with arrest, especially under stress. Auxin signaling acts as a central regulatory hub in this process, yet the mechanisms that dynamically tune early auxin signalling in real time remain unknown. Here, we used the light-gated, Ca2+-permeable Channelrhodopsin 2 variant XXM2.0 to optogenetically impose defined Ca2+ signatures on Arabidopsis root cells. Repetitive light activation triggered cytosolic Ca2+ signals that in turn suppressed auxin-induced membrane depolarization and Ca2+ transients. Moreover, prolonged optogenetic Ca2+ stimulation affects auxin-responsive transcriptional reprogramming. As phenotypic output, reversible inhibition of root growth by suppressing cell division and elongation was observed. We further identify a candidate CaM7–CNGC14 module that likely mediates Ca2+-dependent gating of early auxin signalling. Our study thus introduces a synthetic biology approach to decompose calcium–auxin crosstalk in plant cells, and demonstrates that optogenetically imposed cytosolic Ca2+ signals act as dynamic regulators of auxin susceptibility in roots. 2026 info:eu-repo/semantics/preprint doc-type:Preprint Preprint http://purl.org/coar/resource_type/c_816b https://research-explorer.ista.ac.at/record/23175 Song H, Baudon A, Freund M, et al. Ca2+ signature gates early auxin signaling in Arabidopsis roots. <i>bioRxiv</i>. doi:<a href="https://doi.org/10.1101/2025.10.04.680446">10.1101/2025.10.04.680446</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1101/2025.10.04.680446 https://creativecommons.org/licenses/by-nc-nd/4.0/ info:eu-repo/semantics/openAccess