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Triterpene glycosides are a large, diverse class of amphipathic plant-derived natural products that have high potential as medicinal, agricultural and industrial products. Currently their use is limited as they are typically produced in small amounts in plants and they are difficult to extract or chemically synthesise. In this thesis, two glucosyltransferases from oat (Avena strigosa) required for the biosynthesis of the trisaccharide sugar chain of the antifungal triterpene glycoside, avenacin A-1, were identified. A candidate UDP-glucose dependent glycosyltransferase, AsUGT91, was identified by mining an oat root transcriptome database, phylogenetic analysis and expression profiling. In vitro assays and transient expression in Nicotiana benthamiana confirmed triterpene 3-O-arabinoside b-1,2- glucosyltransferase activity. The triterpene 3-O-arabinoside b-1,4-glucosyltransferase, AsTG, was identified by physical linkage to the avenacin genes, which are clustered in the oat genome. AsTG is a vacuolar transglucosidase in glycosyl hydrolase family 1 (GH1) and is the first of this class of enzyme to be involved in triterpene biosynthesis. Subsequent analysis of oat mutants revealed that AsUGT91 and AsTG correspond to loci required for avenacin glycosylation. Both sets of mutants have root developmental defects, are deficient in avenacin production and show increased susceptibility to the take-all fungal pathogen, Gaeumannomyces graminis var. tritici. This work increases the range of glycoside glycosyltransferase activities available to create novel triterpene glycosides by synthetic biology; contributes towards engineering resistance in other crop species to the agriculturally important root disease, ‘take-all’; and expands the knowledge of triterpene glycoside biosynthesis to include an unusual class of plant specialised metabolite glycosyltransferases.