High level expression of the Magnaporthe grisea mitochondrial small sub-unit rRNA during rice leaf colonization and rapid down-regulation prior to the onset of disease symptoms

Differential cDNA screening identi®ed a Magnaporthe grisea gene which is highly expressed during rice leaf colonization by the fungus. The transcript accumulatd 72 h after plant inoculation, just before symptom expression by the pathogen, but rapidly decreased in abundance after this time. DNA sequence analysis showed that the cDNA has homology to mitochondrial small sub-unit rRNAs. Consistent with this, the cDNA hybridized to puri®ed mitochondrial DNA but did not hybridize to nuclear genomic DNA or chromosomal-sized DNA molecules. The orresponding gene also failed to segregate as a nuclear-encoded marker. To investigate expression of the M. grisea small sub-unit mitochondrial rRNA during growth of M. grisea in axenic culture, experiments were carried out which showed that the transcript is constitutively expressed during active fungal growth but down-regulated by oxidative stress or prolonged nutrient starvation. The mitochondrial rRNA was also down-regulated evelopmentally during conidiation. The data are consistent with very active mitochondrial protein synthesis during colonization of rice tissue byM. grisea, followed by rapid down-regulation of mitochondrial activity prior to disease symptom outbreak and the onset of conidiogenesis.