Biochemical Genetics of Glucosinolate Biosynthesis in Brassica

Abstract

Glucosinolates are the major class of secondary metabolites found in Brassica crops. The molecule comprises a β-thioglucose moiety, a sulphonated oxime moiety and a variable side chain, derived from one of several amino acids (Fig. 1). The economic importance of glucosinolates in Brassica crops is due to the biological activity of their degradation products, which include isothiocyanate, nitriles and a range of indole compounds (Figs. 2 and 3). These compounds are generated by the action of the endogenous plant enzyme myrosinase, or by thioglucosidases from gut microbes following consumption of intact glucosinolates. The nature and biological activity of these products is dependent on the glucosinolate chain structure, reviewed in detail below. The importance of these compounds is threefold. Firstly, certain degradation products reduce the nutritional quality of rape meal. This is largely due to 5-vinyloxazolidine2-thione (Fig. 2), derived from 2-hydroxy-3-butenyl glucosinolate (“progoitrin”), which, as the name suggests, has goitrogenic activity. Isothiocyanates and other glucosinolate-derived compounds can also reduce palatability of the meal. Secondly, isothiocyanates are important flavour compounds in cruciferous vegetable crops. These compounds, along with indolyl glucosinolate degradation products, have also been shown to have anticarcinogenic activity. Thirdly, isothiocyanate and other degradation products mediate plantherbivore and tritrophic interactions. This chapter concentrates on the genetic control of glucosinolate biosynthesis. Factors determining the nature of the degradation products and their biological activity have been reviewed elsewhere (Mithen 2001).