Dirigent proteins predominantly play an important role in biosynthesis of:

The correct answer is Option 1 i.e.Lignans.

Concept:

• Dirigent proteins (DIRs) are thought to be important in secondary metabolism in plants.
• They lack catalytic activity but direct the results of bimolecular coupling reactions toward the formation of regio- and stereospecific products.
•  Functionally defined DIRs impart specificity to the oxidative coupling of coniferyl alcohol, resulting in the preferential production of either (+)- or (-)-pinoresinol, the first intermediates in the enantio complementary lignan biosynthesis pathways.

Explanation:

Option 1: lignans

•  As discussed in the concept this option is true.

Option 2: alkaloids

• The initial precursor of purine alkaloid synthesis, xanthosine, is synthesised from inosine-5′-monophosphate (IMP) via reactions catalysed by IMP dehydrogenase, hence this option is not true.

Option 3: terpenoids

• Mevalonic acid serves as an intermediary in the best-known biosynthetic pathway for terpenoids, which is known as the mevalonate pathway.
• So this option is also not true.

Option 4: amino acids

• amino acids are building blocks for any protein and a very generic option so is not very relevant as pertaining to the question.

 Hence the correct answer is Option 1