Co-Evolution of Secondary Metabolites

Abstract

Plants synthesize and accumulate an arsenal of antimicrobial secondary metabolites in order to protect themselves from invasion of foreign elements (microbes, pathogens, and predators). A few of these metabolites act as constitutive chemical barriers against the microbial attack (phytoanticipins) while others as inducible antimicrobials (phytoalexins). Their properties show them as promising plant and human disease-controlling agents. In this chapter, we are discussing the role of both types of antimicrobial compounds involved in plant defense mechanism. Phytoanticipins are preformed antimicrobial compounds in plants that are unique in action for their property of being synthesized even before the attack of pathogen or infection, i.e., they exist in healthy plants in their biologically active forms (constitutive). Other forms of phytoanticipins such as cyanogenic glycosides and glucosinolates occur as inactive precursors stored in healthy tissues and get activated only in response to tissue damage. Activation of these compounds involves hydrolases (plant enzymes) which are released only after the breakdown of cells. Still we consider them as constitutive metabolites as they are immediately derived from preexisting constituents. Phytoalexins are LMW antimicrobial compounds produced by plants in response to biotic and abiotic stresses. They are formed from remote precursors only in response to pathogen attack after de novo synthesis of phytoalexin biosynthesizing enzymes. We have discussed the key features of both the types of diverse group of molecules such as chemical structures, biosynthesis, regulatory mechanisms, biological activity against pathogens, and molecular engineering of both the plant secondary metabolites.