Ion beam mutagenesis - an innovative and effective method for plant breeding and gene discovery.

Abstract

We have developed a unique technology for mutation induction of plants using energetic ion beams at the RI Beam Factory (RIBF) of Rikagaku Kenkyūjo (RIKEN) (Institute of Physical and Chemical Research). Ion beams effectively induce mutations at relatively low doses without severely inhibiting growth. The irradiation treatment can be given to various plant materials and mutation can be induced in a short time, between seconds and a few minutes. The linear energy transfer (LET) of ions depends on the nuclide and velocity. Since LET value affects the mutation frequency, it is an important parameter to determine the most effective irradiation condition in mutagenesis. We determined the most effective dose in each LET for mutation induction in imbibed rice seeds. Subsequently, we analysed the mutated DNA responsible for the phenotype in morphological mutants. Most of the mutations were small deletions of less than 100 bp. Irradiations of C-ions and Ne-ions are effective for plant breeding because of the very high mutation rate and sufficient energy to disrupt a single gene. On the other hand, all mutations induced by Ar-ion (290 keV/μm) irradiation were large deletions ranging from 176 bp to approximately 620 kb. The average number of mutations in the target exon regions was 7.3, 8.5 and 4.3 per M₃ mutant plant in C-ions, Ne-ions and Ar-ions, respectively. The number of mutations induced by heavy-ion irradiation was relatively small. We could identify six responsible genes for eight mutants induced by C-ion and Ne-ion irradiations and two responsible genes for four mutants induced by Ar-ion irradiation. Three of these were genes not previously described.