Plant ethylene biosynthesis, termed Yang cycle, converts sulfur amino acid methionine to gaseous hormone ethylene through two committed enzymatic steps plus recycling. Methionine is first activated by SAM synthetase encoded by MAT genes using ATP to form S-adenosylmethionine, universal methyl donor and precursor for polyamines and ethylene. The rate-limiting step is catalyzed by ACC synthase, a pyridoxal 5'-phosphate dependent aminotransferase encoded by multigene family ACS1-ACS12 differentially regulated by auxin, cytokinin, wounding, flooding and fruit ripening cues. ACS cleaves SAM into 1-aminocyclopropane-1-carboxylic acid plus 5'-methylthioadenosine, which is recycled to methionine via salvage pathway involving MTA nucleosidase, MTI kinase, conserving sulfur. ACC accumulation regulated by conjugation to malonyl and glutamyl derivatives and xylem transport. Transcriptional activation of ACS by ripening transcription factors RIN and NOR, and post-translational phosphorylation by MPK6 stabilizing protein and ETO1 mediated degradation, underlies autocatalytic ethylene burst in climacteric fruits. Inhibition via antisense or aminoethoxyvinylglycine blocks ACC pool and downstream signaling cascade.
Ref:
Taiz & Zeiger Plant Physiology 6th ed Ch 22 Ethylene; Yang Cycle review Annu Rev Plant Physiol 1984.
