Ben Millett and James Bradeen

University of Minnesota, Department of Plant Pathology

495 Borlaug Hall/1991 Upper Buford Circle, St. Paul, MN USA

Late blight of potato caused by Phytophthora infestans is an important disease resulting in multibillion dollar losses annually worldwide.  Recent efforts have led to the identification and cloning of multiple late blight resistance (R) genes.  One of the most promising genes, RB, identified from wild potato Solanum bulbocastanum, imparts broad spectrum foliar blight resistance.  Previous studies have noted that foliar and tuber resistance to late blight are not necessarily controlled by the same R genes.  The cloning of RB has provided an opportunity to directly examine this phenomenon.  Tubers of potato cultivars transformed with RB were examined using phenotypic and molecular assays for resistance differences.  The development of PCR- and RT-PCR-based assays specific to the transgene RB, which differs from the endogenous RB by three SNPs, has been reported previously.  These assays established that RB is transcribed in tubers at multiple time points post-harvest.  Whole tuber assays were conducted to observe resistance differences between transformed and untransformed lines of cultivated potato.  Because RB is transcribed at all time points, refinements to the PCR assay were required to understand transcriptional regulation of RB.  To this end, SYBR® Green Real-Time qRT-PCR was employed, resulting in assays that were quantitative in nature.  When paired with whole tuber phenotypic assays, these quantitative data provided a clearer view of the effect of RB transcription on tuber blight resistance. On-going foliar and tuber blight experiments continue to provide a more complete picture of the behavior of the foliar blight R gene RB.