Plant Secondary Metabolism

Abstract

Plants synthesise an extraordinary array of natural products that usually do not play a role in their growth and development, and thus are traditionally referred to as secondary metabolites. However, recent advances in plant sciences have revealed that these compounds not only function in response to environmental stimuli, but also play more basic roles in plant growth. Secondary metabolites have several important roles in plants: they protect against herbivores and microbial infection and act as signals for symbiotic bacteria and mycorrhiza, attractants for pollinators and seed‚Äźdispersing animals, allelopathic agents in natural habitats, physical and chemical barriers to abiotic stressors such as UV and evaporation and endogenous regulators of plant growth hormones. Many secondary metabolites are also useful for mankind as dyes, essential oils, flavouring agents, pesticides, pharmaceuticals, tanning agents and so on. Rapid advances in the metabolic engineering and synthetic biology of secondary metabolites have revealed novel physiological roles of these secondary metabolites in plants.

Key Concepts:

  • Plants synthesise a vast array of natural products (secondary metabolites) in response to environmental stimuli.

  • Secondary metabolites have several important roles: they protect against herbivores and microbial infection and act as attractants for pollinators and seed‚Äźdispersing animals, allelopathic agents in natural environments and endogeneous regulators of plant growth.

  • Secondary metabolites are synthesised from primary metabolites such as acetate, pyruvate, and amino acids, and are mainly classified into three major groups; the terpenoids, the phenolics (mainly phenylpropanoids) and alkaloids.

Keywords: secondary metabolites; natural products; biosynthesis; accumulation; taxonomy; raison d'etre; metabolic engineering; environmental response

Figure 1.

Some terpenoid compounds from plants.

Figure 2.

Biosynthesis of isopentenyl pyrophosphate and dimethylallyl pyrophosphate; the biological equivalents of ‘isoprene’.

Figure 3.

The cinnamyl alcohols: precursors of lignins.

Figure 4.

Some simple plant phenolics derived from the ‘hydroxycinnamate pool’. Glc, glucose.

Figure 5.

Tetrahydrocannabinol is synthesised from two building units.

Figure 6.

Plant phenolics derived from ‘acetate’ and ‘hydroxycinnamate pool’ precursors. Glc, glucose.

Figure 7.

Some plant alkaloids; biosynthetic origins shown in parentheses.

Figure 8.

Miscellaneous classes of plant secondary metabolites – polyacetylenes, polyenes, cyanogenic glycosides, glucosinolates, nonprotein amino acids – and their biosynthetic precursors. Glc, glucose.

Figure 9.

Taxol biosynthesis: the role of oxygenation reactions (broken arrows). Ac, acetyl; Bz, benzoyl; Ph, phenyl.

Figure 10.

Plant secondary metabolites that act as hormones and signalling agents.

Figure 11.

Plant secondary metabolites in defence and allelopathy.

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Further Reading

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Sato, Fumihiko(Aug 2014) Plant Secondary Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001812.pub2]