Plants are sessile organisms well-known to produce a vast array of chemical compounds of which many are used in chemical defence against herbivores and pathogens. The biosynthesis of these plant chemical defence compounds poses a considerable risk of self-toxicity for the plant itself. Several types of adaptations enable plants to avoid the potential lethal effects of their own defence compounds. These adaptations include detoxification and stabilization by glycosylation and the genomic clustering of biosynthetic pathway genes. These two types are the main focus of this PhD thesis on hydroxynitrile glucoside metabolism in the legume model plant Lotus japonicus.

Lotus japonicus produces both cyanogenic and non-cyanogenic hydroxynitrile glucosides as chemical defence compounds. The cyanogenic glucosides linamarin and lotaustralin are stored in the cell vacuole as inactive glycosides and, upon tissue disruption, their hydrolysis by a specific β-glucosidase results in the release of toxic hydrogen cyanide. Hydrolysis of the non-cyanogenic rhodiocyanosides by β-glucosidase activity is shown to produce an anti-fungal furanone. The biosynthetic pathways for these related hydroxynitrile glucosides share the first step, and paralogous enzymes with distinct roles, such as the UDP-glucosyltransferases UGT85K2 and UGT85K3, catalyse subsequent reactions.

The results presented in this PhD thesis provide unique insight in the biosynthesis of hydroxynitrile glucosides in Lotus japonicus in terms of gene function and evolution. Further, it contributes to our understanding of the formation and role of biosynthetic gene clusters in plant chemical defence. The bifurcation in hydroxynitrile glucoside biosynthesis and catabolism observed in Lotus japonicus makes it a very suitable model system to study the emergence of new chemical defence pathways.