Cyanogenic glucosides are amino acid derived natural products. Upon cellular disruption, they are degraded by -glucosidases and -hydroxynitrilases with a resulting release of hydrogen cyanide. This provides plants with an immediate chemical defence response to herbivores and pathogens that cause tissue damage. Using genetic engineering, the entire pathway for the cyanogenic glucoside dhurrin has been transferred from sorghum to Arabidopsis thaliana. Dhurrin is only produced after coordinated expression of all three sorghum genes, in which case dhurrin level reaches 7% of dry weight. Dhurrin synthesis proceeds in a metabolon as demonstrated by CLSM analyses of epidermal sorghum cells transiently expressing fusion proteins between the biosynthetic enzymes and spectral variants of green fluorescent protein. The dhurrin metabolon is preferentially located in distinct domains of the ER membrane system at the outer sphere of biosynthetically active cells. Metabolon formation provides a mechanism to explain why insertion of the entire biosynthetic pathway for dhurrin biosynthesis in A. thaliana causes no inadvertent effects on the accumulation of side products as monitored by LC-MS and no significant changes in gene expression as studied by DNA array analyses. In contrast, insertion of the incomplete pathway induces major changes as observed by accumulation of new glucosides (detoxification products) and changes in the gene expression pattern. The transgenic dhurrin-producing A. thaliana plants showed improved resistance against the flea beetle Phyllotreta nemorum which is a crucifer specialist but no effects on the feeding patterns of a number of other tested insects including aphids, the Diamond back moth and a parasitic whasp.
Future goals are to identify the peptide domains in the dhurrin metabolon that mediate metabolon formation. This knowledge is then used to genetically engineer new biosynthetic pathways in micro organisms and plants organized as metabolons to achieve efficient “conveyor belt” formation of new desired compounds like pharmaceuticals and slow release aroma compounds without inadvertent side-effects.