In Planta Transcriptional and Functional Patterns of an Agriculturally Relevant R Gene.
JAMES M. BRADEEN, B.P. Millett, and D.S. Mollov. University of Minnesota, Department of Plant
Pathology, 495 Borlaug Hall, 1991 Upper Bufrod
Circle, St.
Potato late blight
disease, caused by Phytophthora
infestans, is among the most costly crop diseases worldwide. The disease affects both foliage and tubers. Breeding efforts to improve potato late
blight resistance have led to the hypothesis that foliar and tuber blight
resistance is conditioned by different resistance (R) genes. Previous research also documents changes in foliar
blight resistance throughout plant development, suggesting R gene regulation or
function is dependent on physiological stage.
The cloning of the foliar blight gene RB provides tools to test and explore these phenomena. We have developed a highly sensitive RT-PCR
assay to examine transgene RB expression. RB, like many R genes, is one of a cluster
of sequence similar but functionally disparate gene copies. However, our assay differentiates not only
between RB paralogs
and RB alleles, but even between the RB transgene and
the allele from which it was cloned. We
have also optimized whole plant and whole tuber assays to functionally test for
late blight resistance. The potato-P. infestans pathosystem is an ideal system in which to test
tissue-specific transcriptional and functional regulation of R genes, since P. infestans is a natural pathogen of
two distinct plant tissues, leaves and tubers. We compared transgene
RB transcription in foliage and
tubers with results of our functional assays.
Although the RB transgene is expressed in all plant tissues, leaves of
transgenic plants are late blight resistant while tubers of transgenic plants
are late blight susceptible. In related
studies, we are exploring transcriptional and functional regulation of transgene RB throughout
plant development. We have compared
pre-flowering, flowering, and post-flowering transgenics
using our RT-PCR and foliar blight resistance assays. Our research provides insight into strategies
for the integration of transgene RB into potato disease management schemes. Future experiments include exploration of RB protein levels and comparison the
disease response transcriptomes in various plant
tissues and throughout plant development.