Mutation-breeding technologies are useful tools for the development of new biological resources in plants and microorganisms. In Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) at the National Institutes for Quantum and Radiological Science and Technology, Japan, ion beams were explored as novel mutagens. The mutagenic effects of various ion beams on eukaryotic and prokaryotic microorganisms were described and their application in breeding technology for industrial microorganisms were discussed. Generally, the relative biological effectiveness (RBE) depended on the liner energy transfer (LET) and the highest RBE values were obtained with ¹²C⁵⁺ ion beams. The highest mutation frequencies were obtained at radiation doses that gave 1%–10% of surviving fraction. By using ¹²C⁵⁺ ion beams in this dose range, many microorganisms have been improved successfully at TIARA. Therefore, ion-beam breeding technology for microorganisms will have applications in many industries, including stable food production, sustainable agriculture, environmental conservation, and development of energy resources in the near future. Moreover, genome analyses of the ion-beam-induced mutants are in progress to clear the differences of mutational functions induced by different LET radiations in microorganisms. Further characterizations of mutations induced by different LET radiations will facilitate more effective use of ion beams in microorganisms breeding.

Mutations caused by ion beams have been well-studied in plants, including ornamental flowers, rice, and algae. It has been shown that ion beams have several significantly interesting features, such as a high biological effect and unique mutation spectrum, which is in contrast to low linear energy transfer (LET) radiation such as gamma rays. In this study, the effects of double strand breaks and 8-oxo-2′-deoxyguanosine (8-oxodG) caused by ion-beam irradiation were examined. We irradiated repair-gene-inactive strains rad52, ogg1, and msh2 using carbon ion beams, analyzed the lethality and mutagenicity, and characterized the mutations. High-LET carbon ion-beam radiation was found to cause oxidative base damage, such as 8-oxodG, which can lead to mutations. The present observations suggested that nucleotide incorporation of oxidative damage gave only a limited effect on cell viability and genome fidelity. The ion-beam mutations occurred predominantly in 5′-ACA-3′ sequences, and we detected a hotspot at around +79 to +98 in URA3 in wild-type and mutant lines, suggesting the presence of a mutation-susceptible region.

Cluster ion beams have unique features in energy deposition, but their biological effects are yet to be examined. In this study, we employed bacterial spores as a model organism, established an irradiation method, and examined the lethal effect of 2 MeV C, 4 MeV C₂, and 6 MeV C₃ ion beams. The lethal effect per particle (per number of molecular ions) was not significantly different between cluster and monomer ion beams. The relative biological effectiveness and inactivation cross section as a function of linear energy transfer (LET) suggested that the single atoms of 2 MeV C deposited enough energy to kill the spores, and, therefore, there was no significant difference between the cluster and monomer ion beams in the cell killing effect under this experimental condition. We also considered the behavior of the atoms of cluster ions in the spores after the dissociation of cluster ions into monomer ions by losing bonding electrons through inelastic collisions with atoms on the surface. To the best of our knowledge, this is the first report to provide a basis for examining the biological effect of cluster ions.

The unique energy transfer characteristics of swift cluster ions have attracted the attention of many researchers working on the analysis or processing of material surfaces, but the effects on living organisms remain unclear. We irradiated B. subtilis spores with monomer and cluster proton beams and examined their lethality; the 2 MeV H₂⁺ shows a clearly lower lethality than 340 keV H⁺, even though both have a comparable linear energy transfer. The 2 MeV H₂⁺ dissociates into a pair of 1 MeV H⁺ by losing the bonding electrons at the target surface. The estimated internuclear distance and the radial dose distribution suggest that the spread of deposited total energy over two areas separated by just several nanometers greatly diminishes beam lethality and that the energy density in the very center of the trajectory, possibly within a 1 nm radius, has a great impact on lethality. We also performed a whole genome resequencing of the surviving colonies to compare the molecular nature of mutations but failed to find a clear difference in overall characteristics. Our results suggest that cluster beams may be a useful tool for understanding biological effects of high linear energy transfer radiation.

Deinococcus aetherius ST0316 is a radioresistant bacterium that possess proficient DNA repair capacity. Here, we report the complete genome sequence of D. aetherius , which was obtained by hybrid assembly using short- and long-read sequencing. This sequence will be important information for elucidating the unique DNA repair mechanism of Deinococcus bacteria.

Biofertilizers are promising technologies for achieving sustainable agriculture. However, high-temperature tolerance is a constraint that limits the function of microbial inoculants. To characterize the genetic changes responsible for the high-temperature tolerance of rhizobia, mutant screening was performed using Bradyrhizobium diazoefficiens USDA110. The wild-type cells were mutagenized with carbon-ion irradiation, and two mutant strains, designated M10 and M14, were obtained after a three-day heat-shock treatment at 43 °C. In particular, M14 showed superior growth at 36 °C, at which temperature growth of the wild type was extremely slow, whereas M14 grew more slowly than the wild type at 32 °C. Whole-genome sequencing revealed that M10 had seven point mutations, whereas M14 had eight point mutations together with a 1.27 Mb inversion. RNA sequencing showed that the number of differentially expressed genes greatly exceeded the actual number of induced mutations. In M14, a gene cluster associated with pyruvate metabolism was markedly downregulated, probably because of disjunction with the promoter region after inversion, and was considered to be the cause of the slow growth rate of M14 at 32 °C. Notably, transmembrane proteins, including porins, were enriched among the genes upregulated in both M10 and M14. M14 was confirmed to retain symbiotic functions with soybeans. These results indicate that high-temperature tolerance was conferred by random mutagenesis while the symbiotic functions of rhizobia was maintained.