Structural and expressional variations of the mitochondrial genome conferring the wild abortive type of cytoplasmic male sterility in rice

The so-called "wild abortive" (WA) type of cytoplasmic male sterility (CMS) derived from a wild rice species Oryza rufipogon has been extensively used for hybrid rice breeding. However, extensive analysis of the structure of the related mitochondrial genome has not been reported, and the CMS-associated gene(s) remain unknown. In this study, we exploited a mitochondrial genome-wide strategy to examine the structural and expressional variations in the mitochondrial genome conferring the CMS. The entire mitochondrial genomes of a CMS-WA line and two normal fertile rice lines were amplified by Long-polymerase chain reaction into tilling fragments of up to 15.2 kb. Restriction and DNA blotting analyses of these fragments revealed that structural variations occurred in several regions in the WA mitochondrial genome, as compared to those of the fertile lines. All of the amplified fragments covering the entire mitochondrial genome were used as RNA blot probes to examine the mitochondrial expression profile among the CMS-WA and fertile lines. As a result, only two mRNAs were found to be differentially expressed between the CMS-WA and the fertile lines, which were detected by a probe containing the nad5 and orf153 genes and the other having the ribosomal protein gene rp15, respectively. These mRNAs are proposed to be the candidates for further identification and functional studies of the CMS gene.