An alteration in transcription patterns of mitochondrial genes in alloplasmic lines of Brassica rapa

The accumulation of transcripts of mitochondrial genes in young buds was examined in euplasmic and alloplasmic lines of Brassica rapa for the `mur' system of cytoplasmic male sterility (CMS). Southern blotting analysis revealed the general absence of restriction fragment length polymorphism between Diplotaxis muralis, the donor of cytoplasm, and each of three sterile alloplasmic lines, indicating that the rearrangement of mitochondrial DNA might not be caused by the nucleus of B. rapa. However, different Northern hybridization patterns were detected when coxI and nad3 were used as probes. Production of the 2.5-kb transcript of coxI and a reduction in levels of the transcript of nad3 in the sterile alloplasmic lines were clearly under the control of the nucleus of B. rapa. No such differences in patterns of transcripts were detected in the leaves. This observation suggests that some factor(s) or gene(s) in the nucleus of B. rapa acts in an organ-specific manner to alter the size of transcripts in the mitochondria of the donor cytoplasm without any structural changes in the mitochondrial genome. Further studies are required to clarify whether or not the unique transcripts in the young buds induce a CMS event.