Genetic and molecular analyses of avirulence in the phytopathogenic fungus Magnaporthe grisea

Magnaporthe grisea is a filamentous ascomycete fungus that causes blast disease on rice and other grasses. Blast is a serious deterrent to rice production and negatively affects production of other cereals, forage crops and economically important grasses. The primary means of blast disease management involves the development and implementation of genetically resistant plants. Understanding the molecular basis of plant resistance is the foundation for the development of unique and durable plant protection. The results presented here focus on genes in the rice blast fungus called avirulence genes that encode molecules acting as effectors of host resistance. Until recently, two avirulence loci had been shown to induce resistance in rice cultivar Maratelli. This study gives an update on the current status of one, the AVR1-MARA locus, and describes a new Maratelli avirulence locus that is not allelic to AVR1-MARA or AVR2-MARA. Additionally, evidence is given that indicates a genome rearrangement is responsible for generation of the newly described avirulence locus. Genetic data, hybridization results and DNA sequence analysis demonstrate the translocation of a large AT-rich fragment from one chromosomal location to another. Molecular detection of the translocation is demonstrated by hybridization of certain AVR1-MARA markers that only follow the avirulent phenotype in strains after the rearrangement. The rearrangement is detectable genetically, as the avirulent phenotype controlled at this locus segregates independently from progenitor strains that also contain a single Maratelli-specific avirulence gene. CHEF electrophoretic separation of chromosome-sized DNA shows that the AT rich sequences are located on one of the larger M. grisea chromosomes both before and after cross 4134. Hybridization of CHEF blots indicates that two chromosomes may have been involved in a translocation, however a reorganization of chromosome 2 cannot be ruled out. A homing enzyme strategy for determining the size of the translocated fragment is described. These results demonstrate an example of genomic plasticity leading to a translocation and creation of a new avirulence locus in the rice blast fungus M. grisea.