18. Dirigent proteins predominantly play an important role in biosynthesis of:

(1) lignans                   (2) alkaloids
(3) Terpenoids         (4) amino acids

Correct answer: (1) lignans

Explanation:

• Dirigent proteins play a crucial role in the biosynthesis of lignans, a class of phenolic secondary metabolites formed by the stereoselective coupling of monolignol radicals.

• They do not have catalytic activity themselves but guide the stereochemical outcome of oxidative coupling reactions to produce specific lignan isomers.

• Dirigent proteins are not involved in the biosynthesis of alkaloids, terpenoids, or amino acids.

Options:

• (1) lignans — correct.

• (2) alkaloids — incorrect.

• (3) terpenoids — incorrect.

• (4) amino acids — incorrect.

Dirigent proteins, while lacking catalytic activity, direct the regio- and stereoselective coupling of monolignols to form lignans, essential plant phenolic compounds with roles in defense and development, making them key regulators in lignan biosynthesis pathways.