Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: 6/27/2002
Publication Date: 9/11/2002
Citation: Valent, B., Dalby, M., Kankanala, P., Klaahsen, D., Khang, C., Kang, S., Jia, Y., Bryan, G.T. 2002. Analysis of AVR gene function in pathogenicity and host specificity [abstract]. Proceedings of the Third International Rice Blast Conference. p. 12. Interpretive Summary:
Technical Abstract: Understanding the dual functions of avirulence (AVR) genes in triggering resistance and promoting pathogenicity is a first step toward producing durable disease resistance. We continue our characterization of the rice blast AVR-Pita gene and its corresponding resistance gene Pi-ta. AVR-Pita is a member of a gene family, and phylogenetic analysis using the nucleotide sequence of AVR-Pita homologues from diverse strains has identified three distinct clades that are strongly supported by a bootstrap analysis. Sequence analysis suggests that AVR-Pita encodes a secreted zinc metalloprotease of the Deuterolysin metalloprotease (M35) family. Transient expression assays have shown that AVR-Pita is the only fungal protein required to trigger Pi-ta-mediated resistance in rice. Yeast 2-hybrid and membrane hybridization studies indicate that the putative mature AVR-Pita protease binds to the Pi-ta protein. Our current data suggest that the protein is an intracellular receptor that binds directly to the mature AVR-Pita protease inside the plant cell, initiating defense responses. We are analyzing structure-function relationships for Pi-ta and AVR-Pita using in vitro mutagenesis as well as analysis of naturally occurring variation. Virulent M. Grisea field isolates contain members of the AVR-Pita gene family that fail to trigger Pi-ta-mediated resistance. Expression of AVR-Pita is infection-specific, and the gene is highly expressed during the colonization phase of the susceptible interaction. Current research objectives include further characterization of the AVR-Pita family, demonstrating protease activity for AVR-Pita in vitro, defining its natural substrates within the infected plant tissue, and ultimately determining the relationship between protease activity and the molecular recognition event that triggers disease resistance. AVR-Pita has properties characteristic of bacterial effector proteins that are delivered into the cytosol of living plant cells through a specialized type III secretion system. We have initiated studies to define when and how AVR-Pita is delivered into the plant cytosol to interact with Pi-ta. A thorough understanding of AVR-Pita biology will lead to strategies for identification of additional fungal effector molecules and for understanding the mechanism for delivery of critical pathogenicity factors into living plant cells.