Ethylene, First or Second Messenger?

An answer to the question posed by the title of this article clearly depends on the definition adopted for both alternatives. Like the term ‘hormone’ the concept of a second messenger derives from endocrinology, being an intracellular signal whose formation is controlled by the interaction of a hormone with its receptor and which in turn is involved in the transduction mechanism of that hormone via interaction with another intracellular component. Since the precise applicability of the term hormone has itself been called into question in the case of the endogenous plant growth regulators (see e.g. Wareing, 1977) it would merely be an exercise in semantics to attempt to assess whether ethylene is best classified as such or given some other description. It is nevertheless understandable that, partly for historical reasons and partly because it is a gas, ethylene is seen to be different in kind to the other endogenous growth regulators and it is legitimate to raise the question as to whether that difference is real and fundamental or merely apparent, and if it is real whether some appellation needs to be coined to reflect that difference.