Rice Ethylene Receptors Function as Ca²⁺-Permeable Ion Channels to Orchestrate Calcium-Dependent Antagonism of Ethylene Responses in Roots

The gaseous phytohormone ethylene governs essential plant processes spanning development, productivity, and stress resistance. As an essential nutrient and second messenger, calcium (Ca ²⁺ ) is widely implicated in diverse plant physiological activities; however, its role in the ethylene signal transduction pathway remains elusive. Here, we identified a calcium-dependent antagonism of ethylene response (CAER) specifically modulates root elongation in the model cereal rice ( Oryza sativa ). Interestingly, we demonstrate that the ethylene receptors OsERS1 and OsERS2 both function as Ca ²⁺ -permeable channels, with OsERS1 further acting as a non-selective cation channel capable of permeating both monovalent and divalent cations. Mutagenesis analysis reveals that OsERS1 channel activity relies on homomeric assembly sites (C4/C6) rather than its ethylene-binding site (C65), indicating a clear decoupling of the molecular modules governing receptor signaling and ion channel function. The loss-of-function mutant Osers1/2 does not exhibit the CAER phenotype of the wild type (WT), confirm that this calcium-dependent regulatory mechanism is dependent on both OsERS1 and OsERS2. These findings uncover an unexpected ion-channel function of ethylene receptors, redefining their molecular identity beyond canonical signaling proteins and establishing the novel “hormone receptor-type ion channel (HRIC)” paradigm—one that fundamentally expands our understanding of how plant hormones transduce signals at the membrane.