Submitted to: Molecular and General Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/1996
Publication Date: N/A
Interpretive Summary: Rice blast disease is the most important fungal disease of rice worldwide. Rice is the staple food for two thirds of the human population. The most effective method of disease control is to grow disease resistant plants. Unfortunately, the causal fungus is able to overcome this resistance within 2-3 years after these plants are cultivated widely. We are trying to understand the details of how the rice blast fungus is recognized by rice plants that are resistant to blast and how the fungus changes in order to overcome this recognition. In order to improve our understanding of this host-parasite interaction, we have characterized a DNA element which is present in many copies in the DNA of the rice blast fungus. We found that this DNA is part of a transposable element, a DNA that can move in the genome. We also found that this transposable element can cause changes in the genome that allow the fungus to overcome its recogition by disease resistant varieties of rice. Thus transposable elements likely play a majo role in the pathogenic variation of this important plant pathogen.
Technical Abstract: The Magnaporthe grisea repeat (MGR) sequence MGR586 has been widely used for population studies of the rice blast fungus and has enabled characterization of the fungal population into hundreds of genetic lineages. While studying the distribution of MGR586 sequences in strains of M. grisea, we discovered that pCB586 plasmid probe contains a significant amount of single copy DNA. To precisely define the boundary o the repetitive DNA in pCB586, this plasmid and four cosmid clones containing MGR586 were sequenced. Only 740bp of one end of the 2.6 kb insert in the pCB586 plasmid was common to all clones. DNA sequence analysis of cosmid DNA revealed that all the cosmids contained common sequences beyond the cloning site in pCB586, indicating that the repetitive DNA in the fingerprinting clone is only part of a larger element. The entire repetitive element was sequenced and found to resemble an inverted repeat transposon. This putative transposon is 1,860 kb in length and has perfect terminal repeats of 42 bp which themselves contain direct repeats of 16 bp. The internal region of the transposon possesses one open reading frame which shows similarity at the peptide level to the Pot2 transposon from M.-grisea and Fot1 from Fusarium oxysporum. Hybridization studies using the entire element as a probe revealed that some strains of M.- grisea, whose DNA hybridized to the pCB586 probe, entirely lacked MGR586 transposon sequences.