Armadillo Repeat Only Proteins Are Essential Activators of Plant CNGC Channels.

The versatile Ca2+ signaling system governs plant responses to a wide array of environmental and developmental cues. CYCLIC NUCLEOTIDE-GATED CHANNELS (CNGCs) trigger cellular responses to diverse signals, including phytohormones, biotic and abiotic stresses; and as such their activity is tightly controlled. Unlike their animal paralogs, plant CNGCs does not seem to be gated by cyclic nucleotides, and the mechanism of their activation remains unresolved. Here we report ARMADILLO REPEAT ONLY (ARO) proteins as novel, plant-specific, and essential activators of plant CNGCs. Reciprocal proximity labeling revealed interactions between all sporophytic CNGCs and AROs. Loss-of-function aro mutants fail to induce Ca2+ transients in response to known CNGC-triggering stimuli. Structural modeling, mutational analysis, and electrophysiological data show that AROs assemble into a complex with CNGC tetramers by interaction with the conserved Calmodulin-binding IQ domain. AROs represent CNGC activators, competing with Calmodulins, showcasing an evolutionarily unique solution to regulation of calcium signaling in plants.