Induction of avirulence by AVR-Pita1 in virulent U.S. field isolates of Magnaporthe oryzae

The AVR-Pita1 gene, from the Chinese isolate O-137 of Magnaporthe oryzae, is an effector that determines the efficacy of the Pi-ta rice blast resistance gene. In the present study, the avirulence function of AVR-Pita1 was induced by transformation of field isolates (TM2, ZN19, B2 and B8) that originally were collected from the U.S. and are virulent on Pi-ta-carrying rice cultivars. The presence of AVR-Pita1 from O-137 in independent transformants was detected by PCR using AVR-Pita1 specific primers and verified by DNA sequencing and Southern blot analysis using the AVR-Pita1 coding region as a probe. The results of pathogenicity assays showed that the AVR-Pita1-transformed isolates were not able to infect rice cultivars Katy and Drew carrying Pi-ta. Control isolates that were transformed with inserts lacking the AVR-Pita1 gene remained virulent. Our findings demonstrate that AVR-Pita1 can be used to induce novel gene-specific blast resistance in nature.