米のエチレン受容体OsERS1/2はCa2+透過性チャネルとして機能し、エチレン誘発性根成長阻害のカルシウム依存性拮抗作用を媒介する

Rice (Oryza sativa), a staple food for over half the global population, has evolved unique physiological mechanisms to adapt to its semi-aquatic environment, among which root development is critical for nutrient acquisition, stress tolerance, and grain yield. Ethylene, a gaseous phytohormone, regulates rice root elongation. Calcium (Ca²⁺), an essential nutrient and universal second messenger, mediates various plant physiological pathways. However, the molecular link between ethylene signaling and Ca²⁺ dynamics in rice, especially in root growth regulation that impacts agricultural productivity, remains unclear. Here, we identified a calcium-dependent antagonism to ethylene-induced response (CAER) that specifically modulates root elongation in the model cereal rice. Interestingly, we demonstrate that rice ethylene receptors OsERS1/2 function as Ca²⁺-permeable channels. In particular, OsERS1 exhibits permeability to both monovalent and divalent cations. Further mutagenesis analysis reveals that OsERS1 channel activity relies on homomeric assembly sites (Cys4 and Cys6) rather than its ethylene-binding site (Cys65), indicating a clear decoupling of the molecular modules governing receptor signaling and ion channel function. Loss-of-function mutant Osers1 and Osers2 failed to exhibit the CAER phenotype observed in the wild type (WT), confirming that this calcium-dependent regulatory mechanism is dependent on OsERS1/2. Collectively, these findings uncover an unexpected ion-channel function of ethylene receptors, redefining their molecular identity beyond canonical signaling receptors. Moreover, our work introduced the concept of "hormone receptor-type ion channel (HRIC)" as a new functional category, which enriches our understanding of how plant hormones transduce signals at the molecular level.