Differential changes in copy numbers of rice mitochondrial plasmid-like DNAs and main mitochondrial genomic DNAs that depend on temperature

The mitochondria of rice contain four kinds of circular plasmid-like DNAs, namely, B1, B2, B3 and B4, in addition to the main mitochondrial genomic DNAs. In order to examine the genetic stability of mitochondrial plasmid-like DNAs, changes in the amounts of plasmid-like DNAs and main mitochondrial genomic DNAs were analyzed in calli that had been cultured at various temperatures. The observed effect of temperature on the levels of plasmid-like DNAs was larger than that on the main mitochondrial genomic DNAs. A significant reduction in the copy number of plasmid-like DNAs was detected in calli cultured at 35 °C, as compared to 20 °C, 25 °C and 30 °C. The effect of temperature on DNA synthesis in isolated mitochondria was also analyzed. Synthesis of the main mitochondrial genomic DNAs occurred at all the temperatures examined, whereas synthesis of plasmid-like DNAs occurred only over a limited range of temperatures. The results of both in vivo and in vitro analyses suggest that plasmid-like DNAs are less stably maintained than the main mitochondrial genomic DNAs, which is consistent with the notion that the transmission of mitochondrial plasmid-like DNAs from one generation to the next may be unstable under unusual conditions. Received: 17 February 1997 / 24 April 1998