31. Organization of rice mitochondrial DNA

Mitochondrial DNA (mtDNA) of higher plants has been studied extensively. However, there are only a few reports concerning rice mtDNA (Yamaguchi and Kakiuchi 1983; Yamaguchi et al. 1986; Li Mei-Hua et al. 1985). We isolated mtDNA from 1O-day old etiolated shoots of rice grown in darkness at 25 deg.C. On 1% agarose gel, the mtDNA gave one major band and several low molecular weight bands which differed with species.

Rice mtDNA was digested with several restriction endonucleases such as EcoRI, HindIII, PstI, BamHI, and SalI, respectively. In the case of EcoRI digestion, mtDNAs from different lines yielded 25-27 fragments. There were different intensities among those fragments. This suggests that some fragments are present in different copy numbers. In combination with the data of electron microscopic analysis, the molecular weight of rice mtDNA was estimated about 200 kbp.

Electron microscopy showed that rice mtDNA was heterogeneous. Most of the DNA molecules were of linear form. They were different in size. Most of the linear DNA molecules were of a size about 100 kbp. But they spanned in the range from several kbp to 150 kbp. Only a small proportion of the mtDNA molecules were found to be of a circular form. Quantitative analysis from 1000 scope fields in more than 100 slides showed that more than 98% of the mtDNA molecules were linear form, and less than 2% were circular form. In one strain, Jin Nan-Te (Gambia type), 9 different circular molecules were found. Their molecular weights were calculated as follows: 90, 40, 26, 13.7, 8, 4.7, 2.5, 2 and 0.8 kbp.

In the BT type, two linear plasmid-like mtDNA molecules, PLMR 1 and pLMR 4, were found specifically associated with cytoplasmic male sterility. pLMR 1 was about 19 kbp; PLMR 4 was about 1.6 kbp in size. PLMR 1 and pLMR 4 were present in CMS lines, but absent in normal fertile lines.

For the necessity of further studies, a 15-20 kbp long EcoRI fragment library of mtDNA from Qlu Guang (BT type) was constructed with Lambda phage EMBL 4, and a 7-8 kbp long HindIII fragment library of mtDNA from Qiu Guang was constructed with phage lambda gt 11.

Li, M. H., L. F. Yang and Y. X. Cai, 1985. Plasmid-like small circular DNA molecules in rice mitochondria. RGN 2: 96-97.

Yamaguchi, H. and H. Kakiuchi, 1983. Electrophoretic analysis of mitochondrial DNA from normal and male sterile cytoplasms in rice. Jpn. J. Genet. 58: 607-611.

_____, W. Sakamoto and N. Tsutsumi, 1986. Cloning of mitochondrial DNA genome in rice. RGN 3: 120-121.