Hirokazu Nakai

Dual origin of the cultivated rice based on molecular markers of newly collected annual and perennial strains of wild rice species, Oryza nivara and O. rufipogon

The direct ancestor of rice (Oryza sativa L.) is believed to be AA genome wild relatives of rice in Asia. However, the AA genome wild relatives involve both annual and perennial forms. The distribution of the retrotransposon p-SINE1-r2, a short interspersed nuclear element (SINE) at the waxy locus was analyzed in diverse accessions of the AA genome wild relatives of rice (O. rufipogon sensu lato). Most annual wild rice accessions had this retrotransposon, while most perennial types lacked this element, contradicting results to the previous studies. Results presented here suggest that O. sativa has dual origin that lead to indica-japonica differentiation. Results suggest the indica line of rice varieties evolved from the annual genepool of AA genome and the japonica varieties from the perennial genepool of AA genome wild rice.

Molecular cloning of the cDNA for the catalytic subunit of plant DNA polymerase α and its cell-cycle dependent expression

DNA polymerase α has been studied in considerable detail in yeast and animals. Genetic and biochemical analyses reveal that this enzyme is composed of a heterotetramer and is necessary for replicon initiation and primer synthesis in lagging strand synthesis. In spite of the fact that modes of DNA replication in plants seem to be similar to those in other eukaryotes, very little is known about the biochemical components that participate in DNA replication of plants, including DNA polymerases.

Indica-japonica differentiation in Chinese rice landraces

SummaryNinety Chinese rice landraces were examined with special reference to the indica-japonica differentiation in terms of traditional criteria, isozyme analysis and PCR analysis of the chloroplast DNA (cpDNA). Cultivars were separated into indica and japonica defined by a discriminant function (Z) based on key characters, as well as by isozyme genotypes. Most indica landraces had chloroplast DNAs with a deletion at the Pst-12 fragment, while most japonica landraces had cpDNAs without the deletion. Two traditionally recognized varietal groups in China, keng and hsien, corresponded largely to the respective japonica and indica revealed in our study. The results obtained in this study showed good agreement for classification of indica and japonica types by the three methods: discriminant analysis by Z value, isozyme analysis, and PCR analysis for cpDNA.

The contribution of boron to radiobiological effects of thermal neutrons in rice

Abstract Seeds of rice ( Oryza sativa L.) which had been previously soaked in aqueous solution of boric acid at different concentrations (0 to 4000 ppm) were exposed to thermal neutrons in a swimming pool type reactor. Chemical analysis was made for boron, nitrogen and hydrogen in each organ of seeds to estimate the distribution of thermal neutron capture dose by the elements in boron-enriched and unenriched control seeds. It was found from the results that the captured radiation dose absorbed in embryo was more than twice as much as those in other organs or overall seeds both in boron-enriched seeds and control ones. This suggests that when seeds are irradiated, the RBE values of thermal neutrons which have been calculated hitherto in a number of plant species should be corrected to a lower value. The degree of M 1 damage per unit of thermal neutron fluence was increased by boron-enrichment, but the RBE for boron enriched seeds was estimated to be lower than that for control seeds, even if that was calculated from the absorbed dose in embryo, suggesting that incorporated boron atoms were not distributed uniformly inside the cells. Effects of alpha-particles emitted from the 10 B( n , α ) 7 Li capture reaction during thermal neutron irradiation on damage in M 1 plants and chlorophyll mutation in M 2 plants were also discussed.

RBE of thermal neutrons for M1 damage in rice.

Abstract Dry seeds of rice, Oryza sativa L., with about 13 and 10% moisture were exposed to thermal neutrons and 60Co γ-rays. RBEs of thermal neutrons for M1 damages were calculated on the basis of the absorbed dose in the embryo. The RBEs for M1 seedling height and seed set were found to be about 10 and 13, respectively, when seeds containing 13% moisture were exposed to thermal neutrons. These values in rice are much smaller than those previously reported in a number of plant species. This observation is mainly attributable to the fact that prior seed RBE estimates were based on the assumption of neutron capture energy uniformly deposited throughout the entire seeds, whereas the present values were obtained on the basis of the absorbed dose in the embryo, which may be a target influenced directly by ionizations. The RBEs of thermal neutrons obtained in the present experiments are very similar to those of fast neutrons obtained earlier for rice and oats. This suggests that the radiobiological actions of thermal neutrons in plant tissues are the same as those of fast neutrons. Radiobiological effects of thermal neutrons were not affected by moisture content and post-irradiation storage of seeds, whereas the effects of γ-rays were strongly influenced by modifying factors, especially the moisture content of seeds. The RBEs of thermal neutrons relative to γ-rays were decreased to about 3 on M1 seedling height when seeds with 10% moisture were used. It is suggested from the results that the RBE of thermal neutrons for M1 damage can vary greatly depending on various modifying factors.

Induction and selection of mutations for resistance against bacterial leaf blight in rice

SummaryDry seed lots of a rice cultivar, Harebare, susceptible for bacterial leaf blight (BLB), were exposed to thermal neutrons with and without pre-treatment of seed for boron-enrichment, and to gamma-rays. M1 plants of each of the treatments were grown and their seeds were separately harvested. M2 populations were raised in rice fields of a farmer in a region where BLB is epidemic every year, and M2 and control plants which expressed resistant reactions to BLB were selected. M3 and control lines as plant progenies of the selected M2 plants were raised in the same rice fields as in M2 generation in order to investigate their BLB resistance and other agronomically important traits. Variances for disease severity of M2 populations were significantly larger than those of the control population. whereas their means were not singificantly different from that of the control population, suggesting induction of mutations toward both resistance and susceptibility to BLB. Mean disease severity values of the M3 lines from selected M2 plants were significantly smaller than those of the control lines, indicating gains of selection in M2 for quantitative resistance against BLB. Thermal neutrons, especially with boron-enrichment pre-treatment were effective to induce mutations for resistance against BLB. Some M3 mutant lines with quantitatively enhanced resistance against BLB were not modified in other agronomic traits from those of the original cultivar. Significance of the induced quantitative resistance in breeding programmes for BLB resistance is discussed.

Increasing mutagenic efficiency of gamma-radiation in rice by irradiation of seeds at extremely low temperature

SummaryDry seeds of rice, Oryza sativa L., containing about 13 percent moisture, were exposed to 60Co gamma raysat extremely low temperature (−196°C). M1 damage on survival rate, seedling height and seed set were found to become much smaller in the cooling treatment than in the not-cooled control. Chlorophyll and heading date mutation frequencies in the M2 generation were not affected by the decreased M1 damage caused by the cooling treatment, and the mutation frequencies could be significantly increased by applying higher doses combined with the cooling treatment. Frequency of sterile or semi-sterile mutants in the M2 generation was found to be significantly lower in the cooling treatment than in the not-cooled control.It is assumed from the experimental results that extremely low temperatures during irradiation could reduce the amount of physiological damage and gross chromosome aberrations. This results in an increased efficiency of producing minute deletions and/or point mutations.

A new gene, developed through mutagenesis, for resistance of rice to bacterial leaf blight (Xanthomonas campestris pv. oryzae).

(...) Test crosses revealed that the resistance of M41 to the Japanese races I, II, III and IV is controlled by a single recessive gene, considered to be different from four previously identified dominant resistance genes of japonica-type varieties and from three recessive genes in indica-type varieties for resistance to Philippine races. The gene in M41 was tentatively designated xa-nm(t)

Increase in mutagenic efficiency of thermal neutrons through boron enrichment in rice seed

Abstract Dry seed lots of four rice varieties with different heading dates were exposed to thermal neutrons and to gamma-rays, with and without boron-enrichment pre-treatment of seed. M1 plants were grown and their seeds were separately harvested and examined for frequencies of chlorophyll, heading date, and sterile mutations in the M2 generation. The frequencies of chlorophyll mutations were not significantly different among the treatments. The frequencies of heading date mutations tended to be higher in the thermal neutron treatment with boron-enrichment than that without boron-enrichment. For the gamma-ray treatment, the opposite applied to the frequency of sterile mutations. The practical implication of boron-enrichment pre-treatment of seed in the use of thermal neutrons for mutation breeding of crop plants is discussed.