Phenylpropanoid Metabolism
Authors: Rock Christopher D 
Published: April 2017

Phenylpropanoid compounds encompass a wide range of structural classes with diverse biological functions in defence, survival and structural support associated with normal plant development (belying the term 'secondary metabolite'). The biosynthesis of phenylpropanoids is regulated by diverse environmental stimuli. Phenylpropanoid metabolism (other than flavonoids, not covered here) occupies a central place in the general aromatic metabolism of plants from shikimate to phenylalanine to lignin polymers and also to coumarins, phenolic volatiles and hydrolysable tannins. In recent years, genetics and biochemistry, along with methodology‐driven, computational, transgenic and comparative transcriptomic approaches, have led to significant advances in the identification of the families of genes encoding enzymes, transporters, regulatory factors involved in phenylpropanoid metabolism and a clearer picture of their functions in biotic and abiotic stress responses, plant development and enzyme/pathway evolution driven by interactions of species with their environment.

Key Concepts

Having one phenyl aromatic ring with one or more hydroxyl groups attached gives phenylpropanoids amphiphilic and reducing properties, underlying their ability to physically interact with other biomolecules.

Plants invest a large percentage of their fixed carbon into synthesising phenylpropanoids, from simple volatile phenolic acids such as the defence hormone salicylic acid to complex polyphenolic flavonoids encompassing thousands of compounds with myriad physiological and adaptive functions.

The shikimate pathway is the starting point for phenylpropanoid biosynthesis from shikimate product phenylalanine (
l
‐Phe), via the intermediate chorismate, which also serves as substrate for the synthesis of quinones and tocopherols important as electron acceptors in photosynthesis and aerobic respiration.

Phenylpropanoid biosynthesis proceeds from deamination of Phe to cinnamate by the rate‐limiting and environmentally regulated enzyme PAL, followed by oxidation of the ring to
p
‐coumarate, its activation with coenzymeA and a series of hydroxylation, methylation and reduction reactions to give cinnamic acids, ‐aldehydes and ‐alcohols that serve as substrates to generate a wide range of complex structures, notably polymers of coumaroyl, conferyl and sinapyl alcohols comprising lignin.

Recent studies have established the major pathway in plants which proceeds from
p
‐coumaroyl‐CoA →
p
‐coumaryl‐shikimate → caffeoyl‐shikimate → caffeoyl‐CoA → feruloyl CoA → coniferaldehyde → 5‐OH coniferaldehyde → sinapaldehyde → sinapate/sinapyl alcohol (oxidation versus reduction, respectively), instead of the original model of a grid/matrix of parallel ring oxidation and side‐chain redox pathways from hydroxycinnamic acids to alcohols.

Synthesis involves three subcellular compartments: chloroplast for Phe, cytosol for sinapoyl‐esters and the vacuole for trans‐esterifications. The identity of specific transporters, temporal‐ or organ‐specific regulatory factors for phenylpropanoid flux to (in)soluble polymers in the apoplast in response to biotic and abiotic stressors and whether lignin formation proceeds via precise channeling of individual precursors through metabolons (temporary structural–functional complexes formed between sequential enzymes) are key questions of practical significance that remain to be answered.