ABA-activated nanomolar Ca2+-CPK signalling controls root cap cycle plasticity and stress adaptation

Abstract Abscisic acid (ABA) regulates plant stress adaptation, growth, and reproduction. Despite extensive ABA-Ca²⁺ signaling links, imaging ABA-induced Ca²⁺ concentration increase was challenging, except in guard cells. Here, we visualize ABA-triggered [Ca²⁺] dynamics in diverse organs and cell types using a genetically-encoded Ca²⁺ ratiometric sensor (CRS) with nanomolar affinity and large dynamic range. Subcellular-targeted CRS reveals time-resolved and unique spatiotemporal Ca²⁺ signatures from the initial plasma-membrane nanodomain, cytosol, to nuclear oscillation. Via receptors and sucrose-non-fermenting1-related protein kinases (SNRK2.2,2.3,2.6), ABA activates nanomolar Ca²⁺ waves and Ca²⁺-sensor protein kinase (CPK10,30,32) signalling in the root cap cycle from stem cells to cell detachment. Surprisingly, unlike the prevailing NaCl-stimulated micromolar Ca²⁺ spike, salt stress induces a nanomolar Ca²⁺ wave through ABA signaling, repressing key transcription factors dictating cell fate and enzymes crucial to root cap maturation and slough. Our findings uncover ultrasensitive ABA-Ca²⁺-CPK signalling in modulating root cap cycle plasticity in adaptation to adverse environments.