Ca2+ signature gates responsiveness to exogenous auxin in Arabidopsis roots

Abstract 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, Ca 2+ -permeable Channelrhodopsin 2 variant XXM2.0 to optogenetically impose defined Ca 2+ signatures on Arabidopsis root cells. Repetitive light activation triggered cytosolic Ca 2+ signals that in turn suppressed auxin-induced membrane depolarization and Ca 2+ transients. Moreover, prolonged optogenetic Ca 2+ 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 Ca 2+ -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 Ca 2+ signals act as dynamic regulators of auxin susceptibility in roots.