Plant disease resistance genes (R) recognize the invading pathogens and subsequently trigger defense responses. In most cases, plant resistance conferred by R genes is associated with hypersensitive response (HR), which is thought to be a form of programmed cell death (PCD) similar to apoptosis of animal cells. Most characterized R genes encode proteins containing a nucleotide-binding domain and leucine-rich-repeats (NB-LRR) and are believed to be of ancient origin and present in all plant species. We isolated a novel type of R gene RPW8 from Arabidopsis thaliana that confers broad-spectrum resistance in Arabidopsis to powdery mildew (Erysiphe), a very important biotrophic fungal pathogen capable of infecting more than 9000 plant species. Interestingly, although RPW8 differs from other classes of R genes in protein structure and disease spectrum, the defense responses mediated by RPW8 are similar to those mediated by other R genes, and several conserved signaling components such as EDS1, PAD4, EDS5, SGT1 and NPR1, known to be recruited by NBS-LRR genes, are also required for RPW8 function. RPW8 also functions in tobacco as a transgene to confer resistance to powdery mildew, indicating that the RPW8-signaling pathway is highly conserved.

To understand how RPW8 originated, we have recently conducted an evolutionary analysis and found that the Arabidopsis RPW8 gene originated from duplication followed by diversification of a progenitor gene by positive selection after the evolutionary split of Arabidopsis from Brassica. The presumable progenitor gene appears to be highly conserved across plant species and may be involved in regulation of plant PCD. We are in the process of determining the cellular function (that should be distinct from the resistance function of RPW8) of the progenitor gene in Arabidopsis. To understand how RPW8 couples the recognition of powdery mildew to the activation of HR and other defenses in a non race-specific manner, we have used RPW8 as bait for Yeast-2-Hybrid screens and identified a number of potential RPW8-interacting genes. Our preliminary genetic analyses suggested that some of them may be conserved regulators of plant PCD and disease resistance. We are now focusing on further characterization of these interacting genes by using a combinatory genetic and biochemical approach. Understanding the molecular mechanisms of RPW8-mediated broad-spectrum resistance may have a great potential for new strategies in bio-control of diseases in crop species and should shed insight into the signaling transduction networks of plant PCD and disease resistance in general.

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