A new class of oxidosqualene cyclases directs synthesis of antimicrobial phytoprotectants in monocots

Many plants synthesize antimicrobial secondary metabolites as part of their normal program of growth and development, often sequestering them in tissu es where they may protect against microbial attack. These include glycosyla ted triterpenoids (saponins), natural products that are exploited by man fo r a variety of purposes including use as drugs [Hostettmann, K. & Marston, A. (1995) Saponins (Cambridge Univ. Press, Cambridge, U.K.)]. Very little i s known about the genes required for the synthesis of this important family of secondary metabolites in plants. Here we show the novel oxidosqualene c yclase AsbAS1 catalyzes the first committed step in the synthesis of antifu ngal triterpenoid saponins that accumulate in oat roots. We also demonstrat e that two sodium azide-generated saponin-deficient mutants of oat, which d efine the Sad1 genetic complementation group, are defective in the gene enc oding this enzyme and provide molecular genetic evidence indicating a direc t link between AsbAS1, triterpenoid saponin biosynthesis, and disease resis tance. Orthologs of AsbAS1 are absent from modern cereals and may have been lost during selection, raising the possibility that this gene could be exp loited to enhance disease resistance in crop plants.