Arabidopsis ethylene receptors of low degrees of functional redundancy enrich heterogeneity in ethylene...

The plant hormone ethylene is perceived by a small family of histidine kinase-like ethylene receptor proteins in Arabidopsis. The five receptors, considered functionally redundant, are structurally categorized into two subfamilies. Little is known about the biological importance of subfamily classification in receptor signaling or the degree of redundancy. By testing the genetic interactions of receptor genes, our results annotated differential emergent signaling of receptor isoforms. With the exception of the ethylene-insensitive ETR1-1 receptor, ethylene-insensitive receptors require other wild-type isoforms to convey receptor signaling. The two subfamilies are mutually required for efficient receptor signaling, and emergent receptor signaling is minimal within a subfamily. The receptor signal output was minimal for the subfamily I member ETR1, barely detectable for ERS1, and relatively strong for ETR1 and differential for ERS1 in combination with a subfamily II member. ETR1 has unique roles in receptor signaling. Together with other lines of evidence, our findings imply a low degree of functional interchangeability among receptor isoforms. The low degree of functional redundancy thus enriches the heterogeneity of receptor complexes, enabling an extended range of ethylene responsiveness. The structural features of plant ethylene receptor-like proteins were analyzed, shedding light on the evolution and emergent properties of ethylene receptor-like proteins.