FUNCTIONAL ANALYSIS OF THE MAGNAPORTHE GRISEA AVIRULENCE GENE AVR2-YAMO
G BRYAN, L FARRALL and B VALENT
Dupont Agricultural Products, PO Box 80402, Wilmington DE19880-0402, USA
Background and objectives
The ascomycete Magnaporthe grisea causes disease on a wide variety of graminaceous hosts, although individual strains of the fungus are limited to infecting one or two grass species. Some examples of host-pathogen specificity in this system appear to fit the classical gene-for-gene hypothesis. Genetic analysis has demonstrated a classical gene-for-gene interaction between races of the rice blast fungus and rice variety Yashiro-mochi, which contains resistance gene Pi-ta. Pathogen strains with the avirulence gene AVR2-YAMO fail to infect Yashiro-mochi, presumably due to the production of a signal molecule that interacts with Pi-ta and mediates recognition. This research is intended to identify how the AVR2-YAMO signal molecule is recognized in this system.
Results and conclusions
The cloned gene encodes a protein with a predicted molecular weight of 26 kDa that has some sequence similarities to neutral zinc metalloproteases. Transformation of virulent strains of M. grisea with a functional copy of the cloned AVR2-YAMO gene confers avirulence on Yashiro-mochi. Mutation of specific amino acid residues in the putative zinc-binding region removes the ability to confer avirulence. Sequence comparisons with non-functional avr2-yamo homologues from rice pathogens also indicate that maintenance of the putative zinc-binding region is essential. AVR2YAMO promoter fusions to various reporter genes demonstrate that this avirulence gene, which appears not to be expressed in culture, is highly expressed during infection suggesting a role in pathogenicity. Current efforts to identify the signal molecule responsible for the AVR2-YAMO avirulence gene-induced rice resistance response include: the use of polyclonal anti-AVR2-YAMO antisera to localize expression; the use of the yeast two-hybrid system to find genes encoding proteins from both healthy and infected Yashiro-mochi cDNA libraries which interact with the AVR2-YAMO peptide; and a biochemical approach using recombinant AVR2-YAMO protein.
1. Valent B, Chumley FG, 1991. Annual Review of Phytopathology 29, 111-134.