Aliphatic glucosinolates are thioglucosides synthesized by genera within the Capparales including Arabidopsis and Brassica. They have been shown to mediate pest or pathogen interactions and to reduce the feeding quality of rapeseed meal. Their biological activity is largely dependent upon the structure of the side chain which determines the nature of hydrolysis products produced following tissue damage. A generalized model of the biosynthesis of aliphatic glucosinolates is proposed and tested with reference to the genetic regulation of side chain length by analysing recombinant populations of B. napus and A. thaliana developed from parental lines which varied in their glucosinolate content. The results of the B. napus studies were consistent with a model in which alleles at a single locus (Gsl-pro) regulate the presence or absence of propyl glucosinolates, and those at two other loci ( Gsl-elong-C and Gsl-elong-A) (which map onto a pair of homoeologous linkage groups, one from the C genome and one from the A genome) regulate side chain elongation of the amino acid derivative which results in the production of butyl and pentyl glucosinolates. As null alleles do not occur at the Gsl-elong-A locus, the proportion of propyl glucosinolates is limited to below 30 per cent of total aliphatic glucosinolates. Alleles at a single locus in A. thaliana (Gsl-elong-Ar), which maps 1.3 cM from the RFLP locus m291 on chromosome 5, regulate side chain elongation of aliphatic glucosinolates in this species. It is suggested that the Gsl-elong-Ar gene is homologous to the Gsl-elong-A and Gsl-elong-C genes and, if cloned, could be used to down-regulate the endogenous Gsl-elong genes in B. napus.
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