1. The effectiveness of hairpin RNA (inverted repeat) constructs that target iaaM will be compared to the opposing-promoter constructs which conferred crown gall resistance to 12% of the transgenic apple trees produced during phase I studies. Although not critical, a higher success rate will aid in the introduction of the gall-resistance trait into a commercially useful apple rootstock. 2. To test whether protection is broad enough to be commercially useful, a variety of pathogenic Agrobacterium strains will be used to challenge apple lines that exhibit resistance to octopine-type A. tumefaciens. 3. Apple lines that silence A. tumefaciens oncogenes and prevent gall formation will be examined for transgene expression, copy number, and structure. 4. Intron-containing constructs that encode hairpin iaaH will also be tested for their ability to confer crown gall resistance. Transgenic plants that silence both iaaM and iaaH may have more durable gall resistance than lines that silence only one of these oncogenes.
