HIGH-LEVEL EXPRESSION OF THE MAGNAPORTHE-GRISEA MITOCHONDRIAL SMALL-SUBUNIT RIBOSOMAL-RNA DURING RICE LEAF COLONIZATION AND RAPID DOWN-REGULATION PRIOR TO THE ONSET OF DISEASE SYMPTOMS

Differential cDNA screening identified 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 sympto m 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 hy bridized to purified mitochondrial DNA but did not hybridize to nuclea r genomic DNA or chromosomal-sized DNA molecules. The corresponding ge ne also failed to segregate as a nuclear-encoded marker. To investigat e expression of the M. grisea small sub-unit mitochondrial rRNA during growth of M, grisea in axenic culture, experiments were carried out w hich showed that the transcript is constitutively expressed during act ive fungal growth but down-regulated by oxidative stress or prolonged nutrient starvation. The mitochondrial rRNA was also down-regulated de velopmentally during conidiation. The data are consistent with very ac tive mitochondrial protein synthesis during colonization of rice tissu e by M. grisea, followed by rapid down-regulation of mitochondrial act ivity prior to disease symptom outbreak and the onset of conidiogenesi s. (C) 1998 Academic Press.