Hikoyuki Yamaguchi

Arrangement and association of somatic chromosomes induced by chloramphenicol in barley

Chromosome number in the cells of the first division cycle in the root tip of Hordeum vulgare (2n=14) was apparently reduced from 2n to n by the chloramphenicol (CAP) treatments in early S, S late and/or early G2 stages. Morphological observations and measurements of relative DNA content per cell indicated that such reduction was due to tight alignment of chromosomes in pairs. —It was supposed that homologous chromosomes are close together in the interphase nucleus, and their successive association up to mitotic prometaphase was induced by the CAP treatment.

Repair of radiation-induced single-strand breaks in DNA of barley embryo

Repair of single-strand breaks induced by gamma-irradiation was studied in barley embryos. Nuclei were isolated from embryos at specified times after initiation of water absorption under different temperature conditions. From the shift of the peak of DNA in alkaline sucrose gradient, it was observed that clear repair of induced breaks occurred in the irradiated embryo during 2 h after soaking. However, this type of repair was not observed after 5 h up to 16 h, because natural degradation of DNA also occurred in the non-irradiated embryos. The estimated mol. wt. of DNA from the non-irradiated dry embryo was 0.93 X 10(-8) daltons; on the other hand, it was decreased to 0.15 X 10(-8) daltons by 30 kR of gamma-irradiation. This decreased mol. wt. recovered to 0.52 X 10(-8) daltons (64% recovery) after 2 h soaking at 25 degrees C. The repair observed in this experiment was not dependent on the regular synthesis of macromolecules, especially DNA.

The variation of quantitative characters in the irradiated progenies of two rice varieties and their hybrids

Abstract The effects of irradiation with γ-rays on the genetic variability of quantitatively inherited economic characters in pure varieties and hybrid populations of rice were evaluated. Dry seeds of both Kinmaze and Sasashigure and F 1 seeds of Kinmaze × Sasashigure were exposed to γ-rays from a Co 60 source. To study the variability in the X 3 generation, a space-plant row from each X 2 plant was grown in the field, where a randomized block design was laid down with two replications. Data were recorded for several quantitative characters of individual plants. For the X 4 generation, two plants were chosen at random from each of the five highest yielding progenies from each population. The experiment was repeated four times in the field. Frequency distribution of all the characters for the control and each of the X 3 populations were fitted to the expected normal distribution curve. Analyses of variance of the data indicated that the radiation treatment increased variances in most of the characters while the mean value of each treated population was almost the same as that of non-treated populations. It was assumed that mutations for most of the quantitative characters occurred symmetrically in plus and minus directions. The data of the X 4 experiment indicated that one line appeared towards the higher yielding side of the histogram for the irradiated hybrid, while one line appeared towards the lower yielding side of the histogram for control hybrid population. From this study it can be suggested that manifestation of mutated characters was modified by background genotype, and it was apparent that we could select more desirable variants by the irradiation of hybrid seeds in comparison to irradiation of the seeds of pure varieties.

RADIATION-INDUCED SINGLE-STRAND BREAKS IN THE DNA OF DORMANT BARLEY SEEDS.

Abstract The efficiency of γ-ray-induced single-strand breaks in the DNA of dormant barley seeds was studied using a modified alkaline sucrose density gradient technique. The diphenylamine reagent was used for the assessment of the distribution of DNA molecules in the gradient after centrifugation. Reproducible sedimentation patterns were obtained, when 200 or 300 μg of DNA were used, but with higher amounts of DNA non-reproducible patterns were obtained. For the standardization of the sedimentation rates and the molecular weights calculated for the DNA of dormant barley seeds, under the conditions of this experiment, T 4 phage DNA was used as a marker known molecular weight. The number-average molecular weight of the DNA of the un-irradiated dormant barley seeds was found to 0.79·10 8 . The sedimentation patterns of the DNA of dormant barley seeds were studied after γ-irradiation with 10, 20, 30 and 40 kR (80, 70, 40 and 30% relative reduction in seedling height, respectively). The decrease in the sedimentation velocity of the irradiated samples compared with the un-irradiated controls was taken as an indication of the efficiency of radiation-induced single-strand breaks. Furthermore, a linear relationship was found between the number of induced single-strand breaks/g DNA and the applied radiation dose. The energy required for the induction of a single-strand per gram of DNA was 90 eV.

Effects of low-dose irradiation from /sup 131/I on the induction of somatic mutations in Tradescantia

Low doses of 131I (5 to 100 nCi) were applied to the inflorescence of Tradescantia clone 02 and the mutational events were observed in the stamen hair cells. A linear dose-effect relationship was obtained over this very low dose range. However, environmental condition may cause fluctuation in mutation frequency; therefore, it is suggested that great caution has to be paid to elucidate the validity of this monitoring system.

Effects of several nitroso compounds on the induction of somatic mutations in Tradescantia with special regard to the dose response and threshold dose

Abstract Five kinds of chemical mutagens (EMS, NEU, NMU, NEDA and NDMA) were tested for the induction of somatic mutations using teh Tradescantia stamen hair system with special regard to the dose-response relationships and the minimum effective dose for the induction of mutations. Three types of dose-response relationship were obtained depending on the particular chemical. The minimum dose for the induction of mutations in this system was estimated to be 5 pg and 20 pg per flower for NMU and NEW, respectively and 100 pg per flower for EMS, NDEA and NDMA.

Mutational response of rice seeds to ethyl methanesulfonate and busulfan in relation to chromosome duplication

Abstract At various times after soaking of rice seeds they were treated with ethyl methanesulfonate (EMS) and its bifunctional agent 1,4-di(methanesulfonoxy)butane (busulfan). After 60-h soaked seeds had been treated for 2 h with EMS and busulfan, the LD 50 was estimated as 4·10 −4 M for the former, and 1.3·10 −5 M for the latter. In EMS- or busulfan-treated materials field survival was reduced to a marked extent at the S stage, and the maximal lethality was attained at the late S and/or early G 2 phase. After EMS treatment the mutation frequency was very high in late G 1 , decreased suddenly at middle S, increased again markedly at late S and/or early G 2 and subsequently decreased. On the other hand, busulfan produced the mutation only in the cells that were in the process of DNA synthesis. Concerning the mutation spectrum the tigrina mutant appeared only in treatments made in the G 1 phase. In the light of these results, the mutagenesis of EMS and busulfan is discussed.

Effect of gamma-irradiation on cell elongation and auxin level in Avena coleoptiles

To examine the effect of γ-irradiation on cell elongation, which is mediated by auxin, Avena seedlings at various growth stages were irradiated. At a very young stage (2 mm coleoptile stage) where cell division was considered to be taken place, the growth of coleoptiles was inhibited by a low dose of radiation, whereas at an advanced stage (14 mm coleoptile stage) a higher dose was required to inhibit the growth of coleoptiles. The doses required for 50 per cent inhibition were 1 kR and 300 kR for the 2 mm and 14 mm stages, respectively. In the latter stage no cell division occurred and growth inhibition was associated solely with the inhibition of cell elongation. A possible mechanism of inhibition of cell elongation by the high dose was sought. The conversion of tryptophan to indoleacetic acid (IAA) by an enzyme preparation extracted from irradiated coleoptiles was not greatly suppressed (by only 20 per cent at 500 kR). The auxin supply from the coleoptile tips to the growing zones and auxin transport in the coleoptiles were not affected at all by doses up to 680 kR. However, the growth responses to IAA of sections excised from irradiated coleoptiles were greatly reduced as compared to the non-irradiated controls. From these results it was concluded that cell elongation of Avena coleoptiles was inhibited by a high dose of γ-rays through the reduction of reactivity to IAA.

The effects of gamma irradiation on the production and secretion of enzymes, and on enzyme activities in barley seeds

Abstract Gamma-rays at mega rad quantities inhibited de novo synthesis and secretion of some enzymes in barley seeds during germination. The degree of inhibition, when the daily increase of enzyme activities was calculated, increased with the length of the germination period, up to the time of a maximum enzyme activity for each enzyme. Even though there was a decrease in the production and secretion of enzymes, there was no significant delay in the initiation of these processes. Gamma-rays also inhibited enzyme activities in resting and germinated barley seeds. The enzymes studied were divided into two groups—those sensitive to and those resistant to gamma irradiation. The enzymes of the sensitive group (phosphomonoesterase, phosphodiesterase and ATPase), were those which metabolize phosphate compounds. The enzymes of the resistant group (α- and β-amylase, β-glucosidase, β-galactosidase, and β-1, 3-glucanase) were those which metabolize carbohydrate compounds.

Chemical Nature of Radiation-Induced Single-Strand Breaks in the DNA of Dormant Barley Seeds in Vivo

Gamma irradiation of dormant barley seeds has led to the production of DNA single-strand breaks, the liberation of inorganic phosphate, and the formation of phosphomonoester groups. For each single-strnad break produced in DNA, 0.22 molecules of inorganic phosphate were liberated and 1.1 molecules of phosphomonoester groups were formed. Of the phosphomonoester groups formed, 40% were 5′-phosphomonoesters. Only 8.6% of the 5′-termini produced by radiation were 5′-hydroxyl termini. The chemical mechanisms involved in the induction of single-strand breaks in the DNA of dormant barley seeds are discussed.