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Biosynthesis of Ethylene from Methionine in Aminoethoxyvinylglycine-Resistant Avocado Tissue

Postharvest Physiology Laboratory, Agricultural Research, Science and Education Administration, United States Department of Agriculture, Beltsville, Maryland 20705

This study was conducted to determine if aminoethoxyvinylglycine (AVG) insensitivity in avocado (Persea americana Mill., Lula, Haas, and Bacon) tissue was due to an alternate pathway of ethylene biosynthesis from methionine. AVG, at 0.1 millimolar, had little or no inhibitory effect on either total ethylene production or [¹⁴C] ethylene production from [¹⁴C]methionine in avocado tissue at various stages of ripening. However, aminoxyacetic acid (AOA), which inhibits 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (the AVG-sensitive enzyme of ethylene biosynthesis), inhibited ethylene production in avocado tissue. Total ethylene production was stimulated, and [¹⁴C]ethylene production from [¹⁴C]methionine was lowered by treating avocado tissue with 1 millimolar ACC. An inhibitor of methionine adenosyltransferase (EC 2.5.1.6), L-2-amino-4-hexynoic acid (AHA), at 1.5 millimolar, effectively inhibited [¹⁴C]ethylene production from [¹⁴C]methionine in avocado tissue but had no effect on total ethylene production during a 2-hour incubation. Rates of [¹⁴C]AVG uptake by avocado and apple (Malus domestica Borkh., Golden Delicious) tissues were similar, and [¹⁴C]AVG was the only radioactive compound in alcohol-soluble fractions of the tissues. Hence, AVG-insensitivity in avocado tissue does not appear to be due to lack of uptake or to metabolism of AVG by avocado tissue. ACC synthase activity in extracts of avocado tissue was strongly inhibited (about 60%) by 10 micromolar AVG. Insensitivity of ethylene production in avocado tissue to AVG may be due to inaccessibility of ACC synthase to AVG. AVG-resistance in the avocado system is, therefore, different from that of early climacteric apple tissue, in which AVG-insensitivity of total ethylene production appears to be due to a high level of endogenous ACC relative to its rate of conversion to ethylene. However, the sensitivity of the avocado system to AOA and AHA, dilution of labeled ethylene production by ACC, and stimulation of total ethylene production by ACC provide evidence for the methionine SAM ACC ethylene pathway in avocado and do not suggest the operation of an alternate pathway.

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