Beyond a Plant Hormone: Ethylene Receptors and Signaling in Microbes

Ethylene has been recognized as a plant hormone that impacts growth, development, and stress responses for over a century. Hallmarks of hormone receptors are that they have saturable and specific ligand-binding activity and the binding of the ligand regulates cellular functions. Using these characteristics, several reports over the past decade have discovered and characterized plant-like ethylene receptors in bacteria. These studies show ethylene signals via these receptors to have a profound impact on the metabolism and physiology of the bacteria. Several studies also raise the possibility that some fungi contain functional plant-like ethylene receptors. These receptors in non-plant species have ethylene-binding domains similar to plants, but diverge in other domains and downstream signaling pathways. This review summarizes recent advances characterizing plant-like ethylene receptors and signal transduction in microbes and discusses the possibility that there are also noncanonical ethylene receptors that are evolutionarily distinct from those found in plants.