Biosynthesis of cabbage phytoalexins from indole glucosinolate

Brassica crop species are prolific producers of indole-sulfur phytoalexins that are thought to have an important role in plant disease resistance. These molecules are conspicuously absent in the model plant Arabidopsis thaliana, and little is known about the enzymatic steps that assemble the key precursor brassinin. Here, we report the minimum set of biosynthetic genes required to generate cruciferous phytoalexins starting from the well-studied glucosinolate pathway. In vitro biochemical characterization revealed an additional role for the previously described carbon-sulfur lyase SUR1 in processing cysteine- isothiocyanate conjugates, as well as the S-methyltransferase DTCMT thatmethylates the resulting dithiocarbamate, together completing a pathway to brassinin. Additionally, the beta-glucosidase BABG that is present in Brassica rapa but absent in Arabidopsis was shown to act as a myrosinase and may be a determinant of plants that synthesize phytoalexins from indole glucosinolate. Transient expression of the entire pathway in Nicotiana benthamiana yields brassinin, demonstrating that the biosynthesis of indole-sulfur phytoalexins can be engineered into noncruciferous plants. The identification of these biosynthetic enzymes and the heterologous reconstitution of the indole-sulfur phytoalexin pathway sheds light on an important pathway in an edible plant and opens the door to using metabolic engineering to systematically quantify the impact of cruciferous phytoalexins on plant disease resistance and human health.