Poster: Plant Pathogen/Symbiont Interactions

Abs # 723: Molecular genetic dissection of the RPP7 resistance pathway

Presenter:	Hoff, Troy C, thoff@vt.edu
Authors	Hoff, Troy C (A)   Cuzick, Alayne  (A) (B)  Holub, Eric  (B)   McDowell, John M (A)
Affiliations:	(A): Fralin Biotechnology Center, Virginia Tech (B): Horticulture Research International

The RPP7 gene activates race-specific resistance to the downy mildew pathogen Peronospora parasitica. Previously published genetic epistasis tests have established that RPP7 activates defense responses through a signaling mechanism that does not require accumulation of salicylic acid (SA) or components of the ethylene and jasmonic acid response pathways encoded by EIN2, JAR1, or COI1. Furthermore, RPP7 is not suppressed by mutations in a variety of putative signal transducers that are required by various NBS-LRR resistance genes (e.g. pad4-1, ndr1-1, npr1-1, pbs2-1). In an effort to better understand the genetic requirements for signal transduction from RPP7, we have constructed a series of double mutants to test for additive or functionally redundant contributions by known defense signaling components. Most of these combinations display a slightly enhanced level of asexual sporulation, with the ndr1/pad4 combination having the strongest effect. Trypan blue staining revealed that all of the double mutants are capable of inducing the HR, but this response is delayed to varying degrees. These results reveal "cryptic" roles for PAD4, PBS2, NDR1, and NPR1 in RPP7 signaling, and suggest that RPP7 activates resistance through multiple signaling pathways that collectively regulate the kinetics of the HR. We are currently examining the effect of the double mutations on the kinetics of ROI accumulation and defense gene induction. We have also cloned the RPP7 gene, using map-based methods. This gene belongs to a cluster of eight highly related CC-NBS-LRR genes on Chr.1, which share significant similarity with RPP8. The RPP7 gene is 18.6 Kb long. Most of the gene is comprised of long introns within a short 5’ UTR.