Ca ²⁺ signature-dependent control of auxin sensitivity in Arabidopsis

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 auxin sensitivity 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, persistent optogenetic Ca ²⁺ stimulation affects auxin-responsive transcriptional reprogramming. As phenotypic output, reversible inhibition of cell division and elongation leading to root inhibition was observed. Our study thus introduces a new tool 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.