Glucosinolate Breakdown

Abstract

Most known biological roles of the glucosinolate–myrosinase system depend on its functioning as a ‘chemical bomb’. This term describes chemical defences which are based on the accumulation of inert components in separate compartments and rapid release of large amounts of defensive chemicals through mixing of these components and their reaction with each other upon tissue disruption. The essential components of the ‘chemical bomb’ formed by the glucosinolate–myrosinase system are the water-soluble glucosinolates and their hydrolytic enzymes, myrosinases. The defensive chemicals released upon mixing of glucosinolates and myrosinases are the rather lipophilic, irritant and toxic isothiocyanates (mustard oils; reflected in the term ‘mustard oil bomb’) and their derivatives. In addition, so-called specifier proteins may promote the formation of alternative products such as nitriles and epithionitriles. As a consequence, tissue disruption typically leads to the release of complex mixtures of glucosinolate breakdown products. Apart from directly defensive roles (e.g., as toxins), the breakdown products may also be involved in indirect defences (e.g., as signals in tritrophic interactions). Besides glucosinolate breakdown upon tissue disruption, their turnover in intact tissue has been discussed to contribute sulphur and nitrogen for plant growth. In addition, breakdown of glucosinolates in nondisrupted tissue has been observed upon pathogen attack. This chapter summarizes the current knowledge on the protein components of the glucosinolate breakdown machinery and its compartmentation, highlights the relevance of different breakdown product types and their regulated formation, and discusses pathways and roles of glucosinolate breakdown in nondisrupted tissue.