The gaseous phytohormone ethylene regulates many developmental processes and responses to environmental conditions in higher plants.In Arabidopsis thaliana,ethylene perception and initiation of signaling are mediated by a family of five receptors which are related to prokaryotic two-component sensor histidine kinases.The transient expression of fluorescence-tagged receptors in tobacco (Nicotiana benthamiana) epidermal leaf cells demonstrated that all ethylene receptors are targeted to the ER endomembrane network and do not localize to the plasmalemma.In support of in planta overlay studies,the ethylene receptors form homomeric and heteromeric protein complexes at the ER in living plant cells,as shown by membrane recruitment assays.A comparable in vivo interaction patte was found in the yeast mating-based split-ubiquitin system.The overlapping but distinct expression patte of the ethylene receptor genes suggests a differential composition of the ethylene receptor complexes in different plant tissues.Our findings may have crucial functional implications on the ethylene receptor-mediated efficiency of hormone perception,induction of signaling,signal attenuation and output.