We have developed crown-gall resistant transgenic plants capable of suppressing Agrobacterium tumefaciens T-DNA oncogenes. Crown gall tumors result from overproduction of auxin and cytokinin in plant cells transformed by A. tumefaciens. High phytohormone levels result from expression of two auxin biosynthetic genes, tryptophan monooxygenase (iaaM) and indole acetamide hydrolase (iaaH), and isopentenyl transferase (ipt), which mediates cytokinin synthesis. Inactivation of ipt and either one of the two auxin biosynthesis genes prevents crown gall formation. To suppress T-DNA oncogene expression, we created transgenic tobacco that produce the corresponding untranslatable sense-strand RNAs. This phenomenon, called cosuppression, frequently blocks expression of transgenes in plants. Often, expression of an untranslatable sense-strand transgene elicits sequence-specific destruction of both the mutant mRNA and the corresponding wild-type mRNA. Here we show that cosuppression can block expression of A. tumefaciens T-DNA oncogenes, resulting in plants that are resistant to gall induction by certain strains of A. tumefaciens. / Graduation date: 1999