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

Abscisic acid (ABA) regulates plant stress adaptation, growth and reproduction. Despite extensive ABA–Ca 2+ signalling links, imaging ABA-induced increases in Ca 2+ concentration has been challenging, except in guard cells. Here we visualize ABA-triggered [Ca 2+ ] dynamics in diverse organs and cell types of Arabidopsis thaliana using a genetically encoded Ca 2+ ratiometric sensor with a low-nanomolar Ca 2+ -binding affinity and a large dynamic range. The subcellular-targeted Ca 2+ ratiometric sensor reveals time-resolved and unique spatiotemporal Ca 2+ signatures from the initial plasma-membrane nanodomain, to cytosol, to nuclear oscillation. Via receptors and sucrose-non-fermenting1-related protein kinases (SnRK2.2/2.3/2.6), ABA activates low-nanomolar Ca 2+ transient and Ca 2+ -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 Ca 2+ spike, salt stress induces a low-nanomolar Ca 2+ transient through ABA signalling, repressing key transcription factors that dictate cell fate and enzymes that are crucial to root cap maturation and slough. Our findings uncover ABA–Ca 2+ –CPK signalling that modulates root cap cycle plasticity in adaptation to adverse environments.