Impact of physiological age and plant organ on transcription and function of foliar blight resistance transgene RB

Millett, B.P., D.S. Mollov, and J.M. Bradeen

Previous phenotypic studies in potato have documented changes in foliar late blight resistance throughout plant development with pre-flowering plants exhibiting greater resistance than older plants. Previous phenotypic studies have also documented that foliar late blight resistance is not well correlated with tuber blight resistance, leading researchers to assume foliar and tuber blight resistance are conditioned by different genes. Recently, foliar late blight resistance R gene RB was cloned from the wild potato species Solanum bulbocastanum, providing an opportunity to explore the effects of plant age and organ type on specific R gene function and transcription. We have developed a transgene-specific RT-PCR assay which allows discrimination between the RB transgene and both the endogenous RB gene and numerous RB paralogues. This was accomplished by taking advantage of (1) an 18 bp indel present in the RB transgene but absent in rb susceptibility alleles and (2) transgene specific long range-PCR-introduced SNPs . Here we report on studies using the RT-PCR assay in conjunction with phenotypic assays to investigate the effects of physiological age and plant organ on transgene RB transcription and function. In replicated studies, foliage from plants of three different physiological ages were challenged with Phytopthora infestans US8. Younger plants exhibited greater resistance than their older counterparts. Transcriptional assays are ongoing. Additionally, tubers from +RB transgenic potato stored for varying amounts of time showed variable levels of infection, despite RB transcription. On-going foliar and tuber experiments continue to provide a more complete picture of the behavior of the R gene RB.