Ethylene was the first gaseous growth regulator discovered due to its pronounced effects on plant growth and development. Besides plants, many bacteria also have ethylene-binding proteins, indicating that the ability to bind and respond to ethylene is an evolutionarily ancient sensory mechanism. The recent characterization of an ethylene receptor from cyanobacteria and the finding that it plays a role in phototaxis confirms a prokaryotic role for the ethylene receptors and is consistent with the hypothesis that plants acquired ethylene receptors from the endosymbiont that gave rise to the chloroplast. The signalling pathway acting downstream of the plant ethylene receptors is considerably diverged from that found in bacteria, pointing to adaptations that can occur in transitioning from a prokaryotic to a eukaryotic cellular environment. Interestingly, although pathways for ethylene biosynthesis and signalling are conserved in plant lineages extending back to the green algae, there are examples of plants where these pathways have been lost, with ethylene no longer playing a regulatory role.

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