The biosynthesis of benzoic acid glucosinolate esters in Arabidopsis thaliana

The siliques and seeds of Arabidopsis thaliana accumulate a series of glucosinolates containing an alkyl side chain of varying length with a terminal benzoate ester function. The biosynthesis of these unusual nitrogen- and sulfur-containing natural products was investigated by feeding isotopically-labeled precursors to detached flowering stems. Glucosinolates were extracted, purified and analyzed by tandem mass spectrometry. Phenylalanine and benzoic acid were incorporated into the benzoate ester function, and methionine and acetate were incorporated into the aliphatic portion of the side chain in a position-specific manner. The labeling patterns observed were consistent with the chain extension of methionine by a three-step elongation cycle which begins with the condensation of acetyl-CoA with a 2-oxo acid derived from methionine and ends with an oxidative decarboxylation forming a new 2-oxo acid with an additional methylene group. Incorporation of desulfo-4-methylthiobutyl glucosinolate into 4-benzoyloxybutyl olucosinolate suggested chain-extended methionine derivatives are first converted to their corresponding methylthioalkyl glucosinolates with further side chain modification occurring later. Transformation of the methylthiol function to a hydroxyl group is followed by esterification with benzoic acid. The siliques appear to possess the complete machinery for carrying out all of the reactions in the biosyntheis of these complex glucosinolates.