# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 2 The Four Major Specialized Metabolome Pathways
### Phenolics
Successive hydroxylations, by cytochrome P450 monooxygenases, and methylations, by methyltransferases, introduce hydroxyl and methoxyl substituents on the benzene ring, generating \(p\)-coumaric acid (or 4-coumaric acid), caffe acid, ferulic acid and sinapic acid [55]. Following CoA esterification, the resulting phenylpropanoyl CoA esters can enter several of the above-mentioned specialized metabolic pathways, such as flavonoid and monolignol biosynthesis (see below). Besides their biosynthesis via the shikimate pathway, benzenoids can be produced from phenylpropanoids by shortening their side chains in a CoA-dependent \(\beta\)-oxidative pathway (analogous to the \(\beta\)-oxidation of fatty acids), or in a CoA-dependent or -independent non-oxidative pathway [65]. The benzenoid salicylic acid can thus be synthesized (i) directly from chorismate, (ii) through isochorismate, (iii) by hydroxylation of benzoic acid (iv) or by side-chain cleavage of \(o\)-coumaric acid (or 2-coumaric acid), which is formed by the 2-hydroxylation of cinnamic acid (Figure 5, [66]). Salicin, the precursor for the synthesis of acetylsalicylic acid, a widely used pain killer, is produced via reduction in the acidic function of salicylic acid, yielding salicaldehyde, followed by glycosylation and a subsequent reduction in the aldehyde function to an alcohol [67]. Salicin is the building block of the phenolic glycosides, a major compound class in the Salicaceae [68]. Figure 4: Biosynthesis of the aromatic amino acids via the shikimate pathway. Dashed arrows indicate multiple conversions. The ortho-hydroxylation of phenylpropanoyl CoA esters is essential in the formation of coumarins. The first step involves a cis-trans isomerization of the side-chain, which is then followed by lactonization [55; 69]. As such, cinnamic acid and \(p\)-coumaric acid give rise to coumarin and umbelliferone, respectively (Figure 5). Further hydroxylation and methylation of the benzene ring yield esculetin and scopoletin, in which the position of the benzene substitutions reflects those found in caffeic acid and ferulic acid. Reduction of the various phenylpropanoyl CoA esters to their corresponding aldehydes and further to their alcohols constitutes the monolignol biosynthetic pathway. The above-mentioned benzene ring-substitutions may also (partially) occur at the CoA, aldehyde or alcohol level rather than at the acid level [69; 70]. Monolignols are exported to the cell wall, where they are oxidized by percoalases and/or lacases to relatively stable, electron-delocalized radicals, and such monolignol radicals are also thought to be formed by oxidation within the cytoplasm [4; 71]. Subsequent radical-radical coupling between monolignols, most often guided in a stereospecific way by a dirigent protein, leads to the synthesis of (neo)lignans. Coupling products derived from two monolignols that are linked via an 8-8 bond are referred to as lignans. If two monolignols are coupled via a different carbon-carbon bond, they are referred to as neolignans, and if they are linked via an ether linkage, they are referred to as oxyneoligrans. Sesquineoligrans are coupling products between three monolignols, and dineoligrans are coupling products involving four monolignols [72]. This structural variety of (neo)lignans is further increased by additional cyclizations, oxydations, hydroxylations and other modifications, resulting in a plethora of structures. Certain lignans, such as podophyllotoxin and its derivatives, have pronounced cytotoxic activity and are used as antitumor compounds [73]. Although  
Figure 5: Biosynthesis of coumarins, benzenoids, flavonoids, stilbenoids, lignins and (neo)lignans via the phenylpropanoid pathway. Dashed arrows indicate multiple conversions. the biosynthesis of therapeutic lignans has been studied in depth, the role of these compounds in plants remains obscure today.