Genetic Effects of X-ray in Combination with Streptomycin on Arabidopsis thaliana I

Soaking seeds of Arabidopsis in 0.005% streptomycin (SM) produced no second generation chlorophyll mutants. But when seeds were soaked in 0.005% SM for 20 hours before irradiation with X-ray, the percentage of chlorophyll mutants was lower than in plants obtained from seeds that were soaked in distilled water before exposure to X-ray. This protective action of SM was antagonized by 0.01 M Mncl,. It is suggested that this change in the radiosensitivity of Arabdopsis by SM is caused by chelation of Mn++ with SM, thus causing a deficiency of the Mn+• ion, which is essential for the integrity of the chromosomes. Lethal mutants, although frequently appearing in flowering plants, have been regarded with little interest; and with the exception of chlorophyll mutants no systematic study of their frequency or physiology has been made. The reason for this is that precise work on the organic nutrition of intact flowering plants is technically difficult. In recent years, Arabidopsis thaliana has been increasingly used for the study of the induction of mutations in higher plants because it has a short life cycle, produces abundant seeds, and is selfpollinated. Until 1927, all mutations, whether genie or chromosomal, were supposed to have been of spontaneous origin. In that year, H. J. Muller demonstrated that X-rays produce mutations in the sperm cells of Drosophilia. Various factors have been known to influence radiosensitivity of organisms (Nilan, 1956, Konzak, 1957). Working with Antirrhinum, Sparrow et al ( 1961) found the larger the nuclear volume, the mote sensitive the cell to X-ray. McKelvie ( 1962) found that Synkavit alone does not act as a mutagen, but when used with X-ray the percentage of mutation is much higher than with X-ray alone. Moutschen-Dahmen (1963) observed that the presence of copper, zinc or both during treatment with EMS (Ethyl Methane Sulfonate) increased the rate of chromosome breakage. Bhatia and Narayanan (1965) found a higher percentage of cholorophyll mutants when EMS was used along with copper or zinc. Alexander (1960) noted that incorporation of 5bromodeoxyuracil into the DNA of cells renders the cells radiosensitive. Hazama et al (1963) reported protective action of metallic ions such as Mn++, Co++, Ni++, and Cu++ on radiation sensitivity of seeds. Bhatia and Narayanan (1965) found 7% chlorophyll mutants in the F 2 generation when EMS was buffered at pH5, but at pH9, they got 4% mutation. Alexander (1965) reported that 1 Ellsworth College, Iowa Falls.