Plants have evolved a sophisticated innate immunity system to protect themselves from the attack of potential pathogens. Plant disease resistance (R) genes are believed to act at the top of this system to recognize pathogens and activate downstream defense responses. Over 30 plant R genes have been characterized, majority of which encode proteins containing a nucleotide-binding site and leucine-rich-repeat motifs (NB-LRR). In the model species Arabidopsis thaliana, apart from NB-LRR genes, RPW8 defines a unique type of R gene that encodes a novel protein containing a transmembrane domain and a coiled-coil and confers broad-spectrum resistance to biotrophic fungal pathogen powdery mildew. The RPW8 locus of A. thaliana accession Ms-0 contains 5 homologous genes, two of which are functional in resistance and named RPW8.1 and RPW8.2, and the remaining three are homologs of RPW8, named HR1, HR2 and HR3. Recent work suggests that RPW8 triggers defense responses against powdery mildew by stimulating a highly conserved pathway that is also recruited by NB-LRR R genes. To understand how RPW8 evolved, we have conducted an evolutionary analysis and our results showed that the RPW8 gene family in Brassicaceae originated from an HR3-like ancestor gene through a series of duplications and that RPW8.1 and RPW8.2 evolved from functional diversification through positive selection before the separation of A. thaliana from its close relative A. lyrata. These data also suggest that RPW8-mediated broad-spectrum resistance may also be durable. Examination of the sequence polymorphism of over 50 A. thaliana accessions at the RPW8 locus and their disease reaction phenotypes revealed that the RPW8 locus defines a major source of resistance to powdery mildew diseases and the sequence polymorphism at the RPW8 locus in A. thaliana appears to be maintained by two opposing selective forces: a benefit-of-resistance vs a cost-of-resistance.
See more of Symposium--Genetics and Breeding of Disease Resistance
See more of C01 Crop Breeding, Genetics & Cytology
See more of The ASA-CSSA-SSSA International Annual Meetings (November 6-10, 2005)