Decoding dialog: the auxin–calcium signaling crosstalk in plants

Auxin and calcium (Ca 2+ ) are central plant signals that coordinate growth, development, and responses to environmental cues. Typically studied as largely parallel pathways, they are now increasingly recognized as interconnected networks whose activities converge at multiple regulatory levels. Recent advances in live imaging, biosensors, optogenetics, and structural approaches have revealed that auxin can rapidly trigger Ca 2+ influx from the apoplast via TRANSPORT INHIBITOR RESPONSE1 (TIR1)/AUXIN-SIGNALING F-BOX (AFB)-dependent signaling mechanisms and Ca 2+ channels, presumably involving cyclic nucleotides produced by TIR1/AFB auxin receptors. Conversely, Ca 2+ signals arising from mechanosensing, wounding, and environmental stress can modulate auxin transport, signaling, and transcriptional outputs. Together, these discoveries highlight a dynamic, reciprocal signaling axis where Ca 2+ acts both downstream and upstream of auxin to shape growth, development, tropisms, and regeneration.