141
IV
Biosynthesis of
Secondary Metabolites
in Plants
Secondary metabolites are in principle characterized as such because
their presence is not essential to the plants’ life; it rather is an optimization and adaptation to environmental conditions. The primary metabolism provides the carbon backbones for secondary metabolites, usually
via glycolysis or Calvin cycle, whereas the nitrogen groups are mostly
derived from amino acids. Tailoring enzymes are responsible for the
diversity of the compounds. Pathways can take place in different
compartments of a cell, and storage often occurs in specialized cell
types such as in glandular trichomes, resin ducts or laticifers. The
biosynthetic pathways are tightly regulated to balance growth versus
defence.
The major secondary plant products are terpenes or terpenoids,
phenylpropanoids and alkaloids. Terpenoids are produced from isoprene units (C5), either in the cytosol (MVA pathway) or in the plastid
(MEP pathway). Phenylpropanoids are characterized by the phenol ring
derived from phenylalanine. Alkaloids are nitrogen-containing compounds where the nitrogen is usually part of the heterocyclic ring.
Precursors for alkaloids are often amino acids.
Contents
Chapter 9
Secondary Metabolites in Plants:
General Introduction – 143
Chapter 10 Terpenes and Terpenoids – 153