FIG. 2.

Genetic model of the glucosinolate biosynthetic pathway in Brassicales. (A) A simple genetic system for the production of glucosinolates from methionine. The initial step involves the conversion of methionine to an aldoxime through the activity of gene products of the CYP79 gene family. The aldoxime undergoes conjugation with cysteine, which acts as a sulfur donor, and is then cleaved by a C-S lyase. Glucosinolate products are subsequently formed through detoxification of potentially toxic thiohydroximates by glucosyltransferase-driven glucosylation, and sulfation by sulphotransferase. Side chain modifications occur with a high frequency, and profiles become particularly complex after elongation of amino acids that are dependent on genetic variation. (B) Genetic model of methionine-derived glucosinolate biosynthesis. Total level and nature of the glucosinolate is determined early in the process, and the initial entry of methionine into the pathway is catalyzed by methylthioalkylmalate (MAM) synthase genes at the GS-ELONG loci. MAM synthases catalyze the condensation of acetyl CoA to result in elongated methionine. Different members of this family may catalyze different number of rounds of elongation, with genetic variation at the GS-ELONG and GS-PRO loci enabling selection for different glucosinolate profiles, while allelic variation at the quantitative trait loci (QTL) determines overall amount.