Ethylene differs from other plant hormones because it is:
Ethylene stands unique among classical phytohormones being only gaseous hydrocarbon C2H4 with high diffusivity without need for active transport proteins. Biosynthesis follows methionine to S-adenosyl methionine AdoMet then ACC via ACC synthase ACS multigene family rate limiting, finally oxidation to ethylene by ACC oxidase ACO requiring iron ascorbate oxygen. As volatile it accumulates in sealed culture vessels influencing morphogenesis unless vented. Signaling occurs at endoplasmic reticulum membrane where receptors ETR1, ERS1, ETR2 function as negative regulators; in absence of ethylene they activate CTR1 Raf like kinase suppressing EIN2. Ethylene binding inactivates receptors, deactivating CTR1 allowing EIN2 C terminal cleavage translocating to nucleus stabilizing EIN3 EIL1 transcription factors which induce ERF ethylene response factors activating genes for fruit ripening polygalacturonase cellulase, abscission, triple response exaggerated apical hook shortened hypocotyl in etiolated seedlings. In vitro excess ethylene causes hyperhydricity leaf yellowing abscission, mitigated by inhibitors silver nitrate silver thiosulfate that block receptor or by using vented lids permeable closures.
Ref: Bleecker & Kende Annu Rev Cell Dev Biol 2000 ethylene; NCBI NBK21448 ethylene.