Fig. 11.4 Main steps of the phenylpropanoid pathway leading to formation of benzoic acids, hydroxycinnamates (coumaric, ferulic and sinapic acids) and lignin. The enzymes with an asterisk are known to be influenced by ozone formed by cleavage of a C2 fragment from the phenylpropane structure (Fig. 11.4) (Hahlbrock and Scheel 1989; Iriti and Faoro 2004).
The metabolites till now mentioned may be collectively named as simple phenols to differentiate them from polyphenolic compounds. The latter have an additional benzene ring, arising from the cyclization of three malonic acid (C2) residues via stilbene sinthase (STS) or chalcone synthase (CHS), to give stilbenes (C6-C2-C6) or flavonoids (C6-C3-C6), respectively (Fig. 11.3). Finally, proanthocyanidins (PA), or condensed tannins include oligo- and polymeric flavonoid derivatives with polymerisation degree ranging from 2 to 17 and more. The numerous hydroxyl groups available in these molecules promote the formation of complexes with macromolecules, such as proteins, polysaccharides and metal ions (Hahlbrock and Scheel 1989; Iriti and Faoro 2004).
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