Induced mutations in plant breeding and biological researches in Japan

Abstract Two hundred and forty two direct-use mutant varieties generated by using irradiation, chemical mutagenesis and somaclonal variations, have been registered in Japan. About 61% of these were induced by Gammaray irradiation, largely due to successful collaboration with the Institute of Radiation Breeding. This high percentage of Gamma-ray irradiated mutants indicates that mutation breeding via Gamma-ray irradiation is an effective and highly successful approach for the generation of commercial cultivars. Some mutant cultivars of Japanese pear exhibiting resistance to diseases induced by Gamma-ray irradiation and development of a unique bioassay by using toxins of fungi was discussed. In addition, 228 indirect-use (hybrid) mutant varieties primarily generated in rice and soybean have found value as parental breeding germplasm resources in Japan. In 2005, two direct-use cultivars and 97 indirect-use cultivars of rice contributed approximately 12.4% of the total area of rice cultivation in Japan. The semi-dwarf gene (sd-1) generated in rice is perhaps one of the most significant contributions. For soybean, similar Gammaray induced mutants comprised nearly 9.4% of the total cultivation area of soybean in Japan. Molecular genetic studies focused on genome sequencing have become an extremely powerful tool for identifying the genes and for selecting mutants exhibiting specific phenotypes. It is anticipated that molecular genetic interaction will complement gains in mutation breeding on a dramatic scale. Chronic irradiation in the Gamma Field is also considered to be a useful tool for generating mutant resources for future molecular studies especially in rice, and expand its use into the other graminaceous crops which have genomic synteny to rice. There are interesting reports concerning mutations in rice, such as low glutelin content, in which the size and location of deletions and the mechanisms and phenotypes of low glutelin content were elucidated. Chronic irradiation in the Gamma Field is useful to generate mutant resources for molecular researches.