Ca2+ signature gates responsiveness to exogenous auxin in Arabidopsis roots.

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 signaling in real time remain unknown. Here, we use the light-gated, Ca2+-permeable Channelrhodopsin 2 variant XXM2.0 to optogenetically impose defined Ca2+ signatures on Arabidopsis root cells. Repetitive light activation triggers cytosolic Ca2+ signals that in turn suppress membrane depolarization and Ca2+ transients triggered by exogenous auxin. Moreover, prolonged optogenetic Ca2+ stimulation affects auxin-responsive transcriptional reprogramming. As phenotypic output, reversible inhibition of root growth by suppressing cell division and elongation is observed. We further identify a candidate CaM7-CNGC14 module that likely mediates Ca2+-dependent gating of responsiveness to exogenous auxin as well as Ca2+-dependent root growth inhibition. 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.