50. Heterogeneity of small plasmid-like DNAs found in rice mitochondria

1) Atomic Energy Research Center, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo, 113 Japan

2) National Institute of Aerobiological Resources, Kannondai, Tsukuba City, 305 Japan

In various plants, small plasmid-like DNAs are found in mitochondria by agarose gel electrophoresis (AGE) and electron microscopy, which are different from mitochondrial DNA (mtDNA) with a high molecular weight. In rice (Oryza sativa L.), four plasmid-like DNAs (B1, B2, B3 and B4) have been identified in the mitochondria of male-sterile cytoplasms. Recently, Kadowaki et al. (1988) have examined mtDNAs from 102 rice cultivars comprising 69 Indicas and 33 Japonicas. Plasmid-like DNAs of different sizes were detected in 65 of them, of which 63 belonged to the Indica type. Based on the electrophoretic banding patterns, the 102 cultivars were classified into three types, i.e., Type I having plasmid-like DNAs of 1.60, 1.25, 1.09 and 0.96 kb, Type II with 1.60 and 1.25 kb, and Type III having no plasmid-like DNA. Although the function of these molecules is not known yet, the data suggest that the distribution of these plasmid-like DNAs differs between the Indica and Japonica varietal groups.

We examined whether or not the plasmid-like DNAs in different varieties have homogeneous sequences. We carried out Southern blot analysis using B1 (2.1 kb) and B2 (1.5 kb) DNAs as probes, which were cloned previously by Sakamoto et al. (1989). The sizes of their original circular forms were estimated to be 1.6 and 1.25 kb in AGE, respectively. Fig. 1 shows an example of the results, in which B1 and B2 hybridized to several bands, some of which could be their own supercoiled forms. We then assumed that the 1.60 kb and 1.25 kb molecules have homologous sequences to B1 and B2 DNAS, respectively.

Fig. 1. Agarose gel electrophoresis (0.7%; A and C) and Southern hybridization (B and D) of mtDNA from various rice cultivars.

A. Sathi (lane 1); Jhangi 34 (2); IRI 183 (3); Thurunga (4); Kelam (5); IRI 183 (6); Hong mi (7); Dourado precoce (8); Tai zhong xian 3 (9); Huang ke (10); Kinan- dang puti (11), Duan bai hua luo (12); Gui zhao 2 (13); Niao ko hua luo (14).

C. Masumikir (lane 1); Xuan Chang mi (2); Qi dan zhong (3); Tong yi (4); Guang lu ai 4 (5); Xi li ke (6); Surjamkhi (7); Padi kenikir putih (8); R.M. (9); Li zi hong (1O); IR2061-214-3 (11); Doa ren qiao (i 2); Ai jiao nan te (1 3); Nabai dhan (14); Cs-s (15).

D. Southern blot analysis of C using B2 as probe. Lane M: lambda DNA digested with EcoRI and HindIII, shown as molecular weight markers

suggests that the electrophoretic migration of closed circular form of B1 is the same as that of open circular form of B2 in AGE, and that some cultivars lack B1 DNA. It is also possible that there are heterogeneous plasmid-like DNAs having the same molecular weight in rice mitochondria.

Thus, our data suggest that most of rice cultivars having plasmid-like DNAs in mitochondria have B2 DNA, but they are divergent as to whether or not they possess B1 DNA. As suggested by Kadowaki et al. (1988), these plasmid-like DNAs may represent a molecular aspect of variation in mitochondria.

Kadowaki, K., K. Yazaki, T. Osumi, K. Harada, M. Katsuta and M. Nakagahra, 1988. Distribution of mitochondrial plasmid-like DNA in cultivated rice (Oryza sativa L.) and its relationship with varietal groups. Theor. Appl. Genet. 76: 809-814.

Sakamoto, W., M. Momose, N. Tsutumi, S. Tano and H. Yamaguchi, 1989. Analysis of homology of small plasmid-like mitochondrial DNAs in the different cytoplasmic male-sterile strains in rice. Jpn. J. Genet. 64: 49-56.