Ion channel with a moderate preference for potassium over sodium and calcium. Involved in perinuclear calcium spiking but not in cytosolic calcium influx. Closed at negative voltages in presence of magnesium. Required for early signal transduction events leading to endosymbiosis. Acts early in a signal transduction chain leading from the perception of Nod factor to the activation of calcium spiking. Also involved in fungal entry into root epidermal cells during the establishment of the arbuscular mycorrhizal symbiosis. Homooligomer. The chloroplastic localization proposed by PubMed:15616514 is probably an overexpression artifact. Expressed in infected and uninfected roots, leaves, seed pods, and flower buds. Slightly repressed during the first 2 days after bacterial or Nod factor treatment. Belongs to the castor/pollux (TC 1.A.1.23) family.