Plant Secondary Metabolites—The Key Drivers of Plant's Defence Mechanisms

Plants can be regarded as the ‘hoard’ of structurally diverse high-value phytochemicals which contributes to the different ethnopharmacological potential of plants. The chemical constituents of plants generally come under two classes, namely primary and secondary metabolites. Primary metabolites are mainly responsible for basic life functions including growth, development and reproduction, whereas secondary metabolites, the products of subsidiary pathways like the shikimic acid pathway, are for defence mechanisms in plants. Many studies revealed the beneficiary effect of secondary metabolites as flavouring agents, medicinal and recreational drugs. Secondary metabolites from the plants can be categorized mainly as terpenes, alkaloids and phenolics depending on their chemical structures. Terpenoids, ubiquitous in plants, are the largest secondary metabolite class. Alkaloids represent another vast group of secondary metabolites which contains nitrogen. Phenolics are also ubiquitous secondary metabolites in plants and they exist as free phenols or their glycosides. In this chapter, an attempt has been made to discuss various classes of plant secondary metabolites, their synthesis, classification, biochemical and physiological role in plants, important applications including medicinal and pharmacological uses, method of extraction, identification, quantification and overview of the current biotechnological approaches for the enhanced production of secondary metabolites in plants.