9-5

What is the True Character of Ion Beam-Induced Mutation in Plants?



Fig. 9-10 Mutation rates after C ion- and electron-irradiation

The mutations tt (lack of pigment in seed coat) and gl (lack of trichomes on leaves) were induced by C ion- and electron-irradiation. It was found that the incidence of mutations by C ions was 17 times higher than that induced by electrons.



Fig. 9-11 Classification of C ion-induced mutations

Mutations were classified into "a small change", which means a change within a gene, and "a large structural alteration", which refers to strand breaks in a gene, followed by rejoining to other unrelated regions. After C ion irradiation, mutations were categorized into these two types with a 1:1 ratio. This suggests that C ions, when compared to conventional radiation, are more likely to induce "large structural alterations".



Fig. 9-12 An example of a large structural alteration induced by C ions

In this gl1 mutant, the GL1 and Atpk7 genes, which both lie on chromosome 3, were broken, and the generated fragment was rejoined in an inverted orientation. At the same time, 119 bp was deleted from chromosome 2 and most of the deleted sequence (107 bp) was inserted at the inverted region in chromosome 3.


Owing to its nature of depositing a large amount of energy along the beam track, ion beams may induce mutations qualitatively different from those induced by other mutagens. Indeed, we have isolated novel plant resources, such as UV-resistant Arabidopsis and the flower colour mutants of Chrysanthemum by ion beam irradiation. However, it was not clear whether mutations induced by ion beams are actually unique.
Arabidopsis thaliana, which is a model plant for genetic and DNA analysis, was irradiated with C ions (220 MeV, 150 Gy) to investigate the mutation rate and the nature of the induced mutations. We have found that the mutation rate obtained with C ions was 17-fold higher than that with conventional low-LET radiation (i.e., electrons) (Fig. 9-10), and that compared with electrons, C ions are more likely to induce "large structural alterations", such as deletions, inversions, translocations, and insertions (Figs. 9-11,
9-12).
These results imply that ion beams are a useful and effective mutagen. We believe that our work provides a basis for the use of ion beam mutagenesis for plant genetics and crop breeding.


Reference
N. Shikazono et al., Rearrangements of the DNA in Carbon Ion-Induced Mutants of Arabidopsis thaliana., Genetics., 157, 379 (2001).

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