Abstract
Survival assay and mutation analyses have been carried out in higher plants in order to evaluate the biological effects of UV light and ionizing radiations. However, the mutation spectrum analysis, which provides important knowledge about DNA repair and mutation induction mechanisms, is not quite advanced in higher plants. This is partly due to the fact that no experimental system was available to easily detect various types of mutations in higher plants. We constructed a mutation detection system by integrating a linearized plasmid DNA that has Escherichia coli rpsL gene into the chromosomal DNA of Arabidopsis thaliana, a model plant widely used in plant research.
We first analyzed the UV-induced mutation spectrum. The frequency of G to A transitions was increased after the exposure to UV light. The G to A transitions have been thought to be caused by the cyclobutane pyrimidine dimers (CPDs), which are the major DNA lesions induced by UV light. To evaluate the effects of photorepair activity for CPDs in maintaining genomic integrity, we constructed RNAi plants in which the expression of CPD photolyase was suppressed. The mutation spectrum analysis in the RNAi plants showed that the frequency of frameshift mutations was increased together with that of G to A transitions. This suggests that the suppression of CPD photorepair not only increase the typical base substitutions but also other kinds of mutations.
We next analyzed the mutation spectrum induced by gamma rays and carbon-ion beams (LET 122 keV/μm). The results showed that gamma rays tended to induce shorter size of deletions than the carbon ions. This suggests that different types of DNA lesions are induced by different types of ionizing radiations in Arabidopsis thaliana.
