Crown gall is a multimillion dollar worldwide problem in fruit and nut orchards, vineyards, and nurseries. Tumors result from overproduction of auxin and cytokinin in plant cells transformed by Agrobacterium tumefaciens. These high phytohormone levels result from expression of three genes transferred stably into the plant genome from A. tumefaciens: iaaM, iaaH, and ipt. Inactivation of iptand either iaaM or iaaH abolishes gall formation. Adequate means do not exist to control crown gall on grapes, fruit and nut trees, and other nursery crops. Our collaborators have constructed a transgene that elicits post transcriptional gene silencing of iaaM and ipt, and they have used this approach to prevent tumorigenesis in transgenic tobacco. Post-transcriptional gene silencing is triggered by production of double-stranded RNA from a transgene, which causes systemic, sequence-specific destruction of transgene-encoded mRNA and other mRNAs that have sufficient sequence identity. We have introduced this construct into apple. The goal of our proposed work is to test the effectiveness of this transgene in apple and assess the level of crown gall resistance in the 50 apple lines we have generated. ANTICIPATED RESULTS & POTENTIAL COMMERCIAL APPLICATIONS OF RESEARCH Transgenic fruit, nut, and grape rootstocks that suppress crown gall may result from this research. After Phase II and Phase III field studies of the durability of crown gall resistance and the agronomic properties of the rootstock, crown gall resistant rootstock will be ready for commercialization. This will be facilitated by the close association of Dry Creek Laboratory with Duarte Nursery, the largest grapevine nursery in the U.S., with over 15 million potted grapevines and 500,000 potted fruit and nut trees. Monsanto, a leading plant biotechnology firm, supported the patent application covering this invention, and they have expressed interest in commercialization by signing a licensing option agreement.
