Proton beam irradiation is a next-generation technique to develop mutant crop varieties. The mutagenic effects and molecular mechanisms of radiation are important multi-disciplinary research subjects. This study was conducted to investigate the types of mutations induced in the soybean genome by proton beam irradiation. In total, 22 plants, including 10 M plants treated with proton beam irradiation at 118 and 239 Gy, each, and two wild-type plants (Daepung) were sequenced by genotyping-by-sequencing (GBS). In total, 7453 single nucleotide polymorphisms (SNPs) were detected in the 20 M plants, compared with the two wild-type controls. The SNP frequency was 1/36,976 bp with proton beam irradiation at 118 Gy, and 1/32,945 bp at 239 Gy. Of these, 3569 SNPs were detected in genic regions. We observed that proton beam irradiation induced more substitutions than small insertion-deletions (INDELs). Based on the mutagenic effect of proton beam irradiation, the frequency of transition mutations was shown to be higher than that of transversions. The proton beam-induced SNPs were distributed uniformly in most of the chromosomes. Gene ontology (GO) analysis showed that there were many genes involved in protein metabolic process under biological process, intracellular membrane-bounded organelle under cellular component, and nucleic acid binding under molecular function. This study could provide valuable information for investigating the potential mechanisms of mutation, and guidance for developing soybeans cultivars using mutation breeding.

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This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT The present study evaluated the biochemical effects of proton beam irradiation in soybean. Seeds of two Korean elite cultivars (Kwangan and Pungsannamul) were irradiated by a 57-MeV proton beam in the range of 50-400 Gy. We measured the contents of malondialdehyde (MDA), antioxidant enzymes, and chlorophyll. MDA contents in proton beam-irradiated plants were higher than those in control plants. The activity of antioxidant enzymes differed between the two cultivars. In Kwangan, ascorbate peroxidase (APX) activity increased by 29% relative to the control at 55 Gy and decreased by 58% at 308 Gy. Superoxide dismutase (SOD) and peroxidase (POD) activities decreased by 47 and 25% relative to the control at 55 Gy and recovered to 87 and 56% of that at 55 Gy with 172 Gy and 117 Gy, respectively. In Pungsannamul, APX and SOD decreased by 32 and 35% relative to the control at 62 Gy, with the highest value observed at 243 Gy. In terms of the chlorophyll content, the two varieties responded similarly to proton beam irradiation, whereas in Kwangan, no significant reduction was observed above 100 Gy when compared with the control. Proton beam irradiation affected chlorophyll b more than chlorophyll a. These results show that the activity of antioxidant enzymes decreased in response to irradiation with approximately 50 Gy proton beams, then increased gradually with increasing doses, followed by a moderate decrease at higher doses. According to correlation with MDA contents and antioxidant enzyme activities, defense system of Pungsannamul was less activated by proton-beam irradiation than that of Kwangan.