RNAi-mediated oncogene silencing confers resistance to crown gall tumorigenesis

Crown gall disease, caused by the soil bacterium Agrobacterium turnefaciens , results in significant economic losses in perennial crops worldwide. A. t umefaciens is one of the few organisms with a well characterized horizontal gene transfer system, possessing a suite of oncogenes that, when integrate d into the plant genome, orchestrate de novo auxin and cytokinin biosynthes is to generate tumors. Specifically, the iaaM and ipt oncogenes, which show approximate to 90% DNA sequence identity across studied A. turnefaciens st rains, are required for tumor formation. By expressing two self-complementa ry RNA constructions designed to initiate RNA interference (RNA!) of iaaM a nd ipt, we generated transgenic Arabidopsis thaliana and Lycopersicon escul entum plants that are highly resistant to crown gall disease development. I n in vitro root inoculation bioassays with two biovar I strains of A. tumef aciens, transgenic Arabidopsis lines averaged 0.0-1.5% tumorigenesis, where as wild-type controls averaged 97.5% tumorigenesis. Similarly, several tran sformed tomato lines that were challenged by stem inoculation with three bi ovar I strains, one biovar II strain, and one biovar III strain of A. tumef aciens displayed between 0.0% and 24.2% tumorigenesis, whereas controls ave raged 100% tumorigenesis. This mechanism of resistance, which is based on m RNA sequence homology rather than the highly specific receptor-ligand bindi ng interactions characteristic of traditional plant resistance genes, shoul d be highly durable. If successful and durable under field conditions, RNAi -mediated oncogene silencing may find broad applicability in the improvemen t of tree crop and ornamental rootstocks.