Antimicrobial Compounds and Their Role in Plant Defense

Abstract

Plants are important nutrient source for several organisms like microbes, heterotrophic plants, insects as well as vertebrates. Even though they lack a proper defense mechanism like animals, still they have developed a mixture of chemicals which are mainly protein based and are used as a means of defense by detecting attacking organisms and preventing them from causing major damage. In order to protect themselves from these microbes like fungi, bacteria, etc., plant cells have developed the capability to identify attacking pathogens and use inducible defense mechanism by producing toxic chemicals or antimicrobial compounds in the form of pathogen-degrading enzymes and secondary metabolites involved with plant defense. Secondary metabolites generally are grouped into three major classes of chemicals, i.e. terpenoids, phenolic and alkaloids. Some of these antimicrobial compounds are constitutive in nature, i.e. they occur in biologically active forms in healthy plants, whereas other metabolites are inductive in nature. Glucosinolates and cyanogenic glycosides exist in inactive form and are activated as a response to attack by pathogen or tissue damage. These compounds are activated by release of plant enzymes at the time of breakdown of cells. Preformed antimicrobial compounds are termed as “phytoanticipin”, while “phytoalexins” are those antimicrobial compounds which are synthesized (as a result of synthesis of enzymes) from precursors as a response to attack by pathogen. Preformed inhibitors are usually tissue specific and are mainly present in the outer layers of the cells of plant organs. These inhibitors are mostly successful against comprehensive range of probable pathogens, and specific virulent pathogens might circumvent the effect of these secondary metabolites by eluding them or by enduring or by detoxification. Most of these constitutive plant compounds show antifungal activity, e.g. phenols, phenolic glycoside, unsaturated lactones, sulphur compounds, saponins, etc. “Phytoalexins” are the most considered antimicrobial plant defense compounds. These compounds are pathogen specific and therefore more effective in plant defense mechanism. Transcriptional and translational activities in a plant are prerequisite for the production of phytoalexins. Examples of these antimicrobial phytoalexins are scopoletin, camalexin, glucosinolates, etc. This chapter will mainly discuss the role of both phytoanticipins and phytoalexins as plant defense antimicrobial compounds and also their use as “antibiotic potentiators” and virulence attenuators along with their role in crop protection/phytoprotection.