Ethylene biosynthesis: The role of 1-aminocyclopropane-1-carboxylate (ACC) oxidase, and its possible evolutionary origin

Recent developments in our knowledge of the biochemistry of 1-aminocyclopropane-1-carboxylate (ACC) oxidase, the enzyme responsible for the final stage in the biosynthesis of ethylene, are reviewed. Particular reference is made to the role of carbon dioxide as an essential cofactor, the activity of ACC oxidase in the plant cell, the enzyme catalytic centre, and the role of ACC oxidase in the evolutionary development of ethylene biosynthesis in plants. Evidence is marshalled to support a proposal that the membrane requirement for ACC oxidase that is observed in vivo is attributable to a need for a charged plasma membrane to maintain ascorbate in the reduced state for an ACC oxidase located in the apoplast. It is argued on biochemical grounds that the acquisition of the ACC oxidase was the crucial evolutionary step in the development by seed plants of an ethylene biosynthesis pathway that could easily be regulated, and that signalled the plant's response to stress and pathogen attack.