Growth and Development of Horticultural Crops (1+1)

Biosynthesis of ethylene

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• Ethylene is known to be synthesized in plant tissues from the amino acid methionine. A non-protein amino acid, ACC is an important intermediate and also immediate precursor of ethylene biosynthesis. The two carbons of ethylene molecule are derived from carbon no.3 and 4 of methionine. Whole process of ethylene bio-synthesis is a three steps pathway and is aerobic:
First Step

• In the first step, an adenosine group (i.e., adenine+ribose) is transferred to methionine by ATP to form S-adenosylmethionine (SAM), This reaction is catalysed by the enzyme SAM-synthetase (methionine adenosyl transferase).
Second Step

• In the second step, SAM is cleaved to form 1-aminocyclopropane-1-carboxylic acid (ACC) and 5’-methylthioadenosine (MTA) by the enzyme ACC-synthase.
• Synthesis of ACC is rate limiting step in ethylene biosynthesis in plant tissues.
• Exogenously supplied ACC greatly enhance production of ethylene in plant tissues.
Third Step

• In the third and last step of ethylene biosynthesis, ACC is oxidized by the enzyme ACC-oxidase (previously called ethylene forming enzyme i.e., EFE) to form ethylene. Two molecules, in each of HCN and H2O are eliminated.
• ACC oxidase activity can be rate limiting step in ethylene biosynthesis in plant tissues which show high rate of ethylene production such as ripening fruit.The enzyme ACC oxidase requires ferrous iron (Fe²⁺⁾ and ascorbate as cofactors.ACC can be conjugated to give N-malonyl ACC and thus, may play an important role is regulation of ethylene biosynthesis.