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Distribution, variation and function of the AVR-Pita gene family among clonal lineages of Magnaporthe oryzae in the United States

Sook-Young Park: Sunchon National University

<div>New pathogen races emerge through various change in avirulence (<em>AVR</em>) genes. <em>AVR-Pita1</em> and <em>AVR-Pita2</em>, members of the <em>AVR-Pita</em> gene family, but not <em>AVR-Pita3</em>, prevent the rice blast fungus <em>Magnaporthe oryzae</em> from infecting rice cultivars carrying resistance (<em>R</em>) gene <em>Pi-ta</em>. To understand the mechanism underpinning race variation caused by changes in this gene family, we characterized its members among 23 isolates that represent eight clonal lineages of <em>M. oryzae</em> in the United States. Screening of their genomic DNA using three <em>AVR-Pita</em> genes as probes revealed multiple haplotypes, but little variation was observed within individual clonal lineages. All isolates seem to lack <em>AVR-Pita2</em> but carried a single copy of <em>AVR-Pita</em>3. The copy number of <em>AVR-Pita1 </em>varied from 0 to 3. Novel members of the family may be present too. At least six distinct <em>AVR-Pita1</em> products are encoded. Three of them confer AVR function, making the strains expressing them avirulent to rice containing <em>Pi-ta</em>. Comparison of the AVR-Pita1 protein sequences, in light of virulence of the strains carrying them, and site-directed mutagenesis showed that tyrosine residue at the position 192 is essential for AVR function. Unlike the previously characterized <em>AVR-Pita1</em> genes, which are located very close to the telomere, most <em>AVR-Pita1</em> genes in the US isolates might not be telomeric.</div>