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‐dispersing 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 an extraordinary array of natural products, with large structural diversity and complexity: terpenoids, phenolics, alkaloids and so on. Terpenoids and phenolics are ubiquitous in plant kingdom, whereas alkaloids are more specific to taxonomic groups or plant species. Secondary metabolites are produced in environmental responses and development. Secondary metabolites have multiple functions in defence and growth. Secondary metabolites are mainly classified based on the biosynthetic pathways; for example terpenoids from isopentenyl pyrophosphate, phenolics (mainly phenylpropanoids) from shikimate and alkaloids from amino acids. Advances in molecular biology and analytical chemistries including transcriptomics, metabolomics and functional genomics identified many missing biosynthetic enzymes in pathway. Accumulating genome sequencing data revealed that molecular evolution of secondary metabolism is based on the gene duplication and neofunctionalisation. Many secondary metabolites are used for mankind as dyes, essential oils, flavouring agents, pesticides, pharmaceuticals, tanning agents and so on. Metabolic engineering characterises the functions of pathway and also improves the production of secondary metabolites. Reconstruction of biosynthetic pathways enables the production of plant‐derived secondary metabolites in heterologous hosts, such as Escherichia coli and yeast.