This pathway describes the conversion of L-phenylalanine into a diverse suite of coumarins in Arabidopsis thaliana, tracing a major branch of phenylpropanoid metabolism that supports defense, stress adaptation, and iron acquisition. Beginning with the deamination of L-phenylalanine to trans-cinnamic acid, the pathway proceeds through successive hydroxylation and CoA-activation steps to generate key intermediates such as p-coumarate, caffeate, ferulate, and their corresponding CoA esters. From these central phenylpropanoid nodes, the pathway branches into the biosynthesis of simple and highly oxidized coumarins. Lactonization of caffeoyl-CoA yields esculetin, while hydroxylation and intramolecular cyclization of feruloyl-CoA produce scopoletin, which can be further methylated or oxidized to form scoparone, fraxetin, and finally sideretin, a major coumarin involved in iron mobilization in the rhizosphere. Several alternative or parallel reactions allow interchange between free acids and CoA esters—such as the conversions between p-coumarate, caffeate, p-coumaroyl-CoA, and caffeoyl-CoA—reflecting enzymatic flexibility in the phenylpropanoid network. Together, these reactions form a coherent metabolic route linking amino-acid–derived precursors to specialized coumarin metabolites that play essential biochemical and ecological roles in plants.

Coumarin Biosynthesis References