Cytosolic Ca ²⁺ as a universal signal for rapid root growth regulation

Roots must continuously adapt their growth and have evolved the ability to rapidly respond to diverse environmental and hormonal cues, including the phytohormone auxin. Auxin-induced cellular responses involve membrane depolarization, cytosolic calcium (Ca²⁺) elevation, and extracellular pH increase, but how these processes lead to rapid growth regulation remains unclear. Here, we show that cytosolic Ca²⁺ acts as a universal signal integrating diverse cues for rapid regulation of root growth. Using live imaging, microfluidics, and optogenetics in Arabidopsis, we demonstrate that a swift rise in cytosolic Ca²⁺ is the primary target of auxin signaling affecting growth inhibition. Disruption of Ca²⁺ influx abolishes these responses, whereas light-gated Ca²⁺ influx from the apoplast or endoplasmic reticulum stores inhibits growth. Multiple unrelated stimuli—including auxin, extracellular ATP, RALF peptides, and hydrogen peroxide—converge on this Ca²⁺-dependent mechanism. Cytosolic Ca²⁺ elevation thus represents a necessary and sufficient step for rapid growth inhibition, revealing a unifying principle of root signaling.