- American Society of Plant Biologists
Fungal pathogens of plants secrete effector proteins that suppress host defenses (reviewed in de Wit and Stergiopoulos, 2009). Some effectors, termed avirulence (AVR) proteins, are recognized by host resistance (R) proteins, triggering a host defense response that can result in resistance to the pathogen. One important model system for studying fungal pathogenesis is rice blast disease caused by Magnaporthe oryzae infection. Rice blast is also a devastating pathogen, resulting in significant crop losses in many rice-growing regions. As a model system, the development of modern genomics tools, including full genome sequence for both the host and the pathogen, has made this a tractable system to study fungal infection (reviewed in Wilson and Talbot, 2009).
At least five M. oryzae effector proteins have been isolated based on their AVR function (de Wit and Stergiopoulos, 2009). However, effectors are difficult to isolate by traditional positional cloning methods, and many likely remain to be characterized. To isolate novel AVR genes from M. oryzae, Yoshida et al. (pages 1573–1591) used a large-scale association approach. Starting with the sequenced type genome of isolate 70-15, they examined 1032 predicted secreted proteins for polymorphisms that could be associated with AVR function on a panel of rice cultivars harboring different R genes. However, no associations were found with this large set of candidate effectors. Realizing that more information might be found beyond the type genome, they then produced deep genomic sequence of another M. oryzae isolate, Ina 168, which carries at least nine AVR genes as determined by its pathogenicity on a panel of rice cultivars (see figure ).
A diagnostic panel of rice cultivars harboring different R genes shows resistance or susceptibility to the rice blast isolate Ina 168. For each cultivar, the absence of disease symptoms indicates that Ina 168 contains the cognate AVR gene; this is a resistant reaction.
The authors found that the Ina 168 genome contained 1.68 Mb of sequence that was not present in the 70-15 type genome and that included 316 putative secreted proteins. The coding regions for three of these proteins showed polymorphisms that associated with AVR function in a panel of M. oryzae isolates. The three genes were transcribed during pathogen infection but were of unknown function. The authors validated their identification as novel AVR genes by complementation of an M. oryzae isolate lacking AVR function. For example, the putative secreted protein named pex22 was identified as AVR-Pia because the pex22 open reading frame was present in all pathogen isolates that gave a resistant reaction on rice cultivars containing the R gene Pia. Also, expression of pex22 in an AVR-Pia minus M. oryzae isolate produced a resistance reaction in the Pia-containing rice cultivar. Moreover, conditional expression of the AVR genes in rice cultivars harboring the cognate R gene resulted in cell death similar to a plant resistance reaction.
This work provides an excellent example of how genome sequence information and genetic association studies can be combined to identify effector genes from plant pathogens.
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