36. For which one of the plant hormone biosynthetic pathways, 1-aminocyclo- propane- 1 -carboxylic acid is an intermediate?
(1) Abscisic acid            (2) Brassinosteroid
(3) Ethylene                    (4) Gibberellic acid

Correct answer: (3) Ethylene

Explanation:

• 1-aminocyclopropane-1-carboxylic acid (ACC) is a key intermediate in the ethylene biosynthesis pathway.

• Methionine is first converted into S-adenosyl-L-methionine (SAM), then SAM is converted into ACC by the enzyme ACC synthase. ACC is subsequently oxidized by ACC oxidase to produce ethylene gas.

• This pathway is unique to ethylene biosynthesis and is not shared by abscisic acid, brassinosteroids, or gibberellic acid biosynthetic pathways.

Option details:

• (1) Abscisic acid — synthesized differently, no ACC intermediate.

• (2) Brassinosteroid — unrelated biosynthetic pathway.

• (3) Ethylene — correct pathway involving ACC intermediate.

• (4) Gibberellic acid — diterpene pathway distinct from ethylene biosynthesis.

  ACC is the immediate precursor of the gaseous plant hormone ethylene, synthesized via the Yang cycle. The production of ACC by ACC synthase controls ethylene production rates, influencing processes such as fruit ripening, leaf abscission, and stress responses.