Methionine Metabolism and Ethylene Formation in Etiolated Pea Stem Sections

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Abstract
Stem sections of etiolated pea seedlings (Pisum sativum L. cv. Alaska) were incubated overnight on tracer amounts of L-[U-14C]methionine and, on the following morning, on 0.1 mM IAA to induce ethylene formation. Following the overnight incubation, over 70% of the radioactivity in the soluble fraction was associated with S-methylmethionine (SMM). The specific radioactivity of the ethylene evolved closely paralleled that of C atoms 3 and 4 of methionine extracted from the tissue and was always higher than that determined for C atoms 3 and 4 of extracted SMM. Overnight incubation of pea stem sections on 1 mM methionine enhanced IAA-induced ethylene formation by 5-10%. Under the same conditions, 1 mM homocysteine thiolactone increased ethylene synthesis by 20-25%, while SMM within a concentration range of 0.1-10 mM did not influence ethylene production. When unlabeled methionine or homocysteine thiolactone was applied to stem sections incubated overnight in L-[U-14C]methionine, the specific radioactivity of the ethylene evolved was considerably lowered. Application of unlabeled SMM reduced the specific radioactivity of ethylene only slightly.