image: Heat-tolerant mutant lines of rhizobia obtained by experimental evolution combined with repeated mutagenesis with gamma rays
Credit: Dr. Yoshihiro Hase from the National Institutes for Quantum Science and Technology, Japan
Takasaki, Japan — Heat‑resilient biofertilizers could help crops cope with rising temperatures but engineering them has been slow and uncertain. A new study at the National Institutes for Quantum Science and Technology (QST) shows that pairing experimental evolution with controlled gamma‑ray mutagenesis can accelerate the path to heat‑tolerant nitrogen‑fixing bacteria, shortening development timelines and opening practical routes to more reliable, climate‑ready microbial products for agriculture, food processing, pharmaceuticals, and biofuel production. The study was made available online on November 19, 2025, and published in Volume 831 on July, 01, 2025, in the Mutation Research - Fundamental and Molecular Mechanisms of Mutagenesis journal.
The team focused on Bradyrhizobium diazoefficiens USDA110, a workhorse bacterium used to help soybean and other legumes capture nitrogen. While the wild-type grows best at around 32–34 °C and stalls at ~36 °C, QST researchers raised culture temperatures stepwise from 34 °C to 37 °C over 76–83 days and irradiated populations ten times at specific doses, then selected the lines that continued to form robust colonies at 36 °C.
A clear “sweet spot” emerged: around 40 Gy produced the greatest number of stable, heat‑tolerant lines, whereas higher doses (80–120 Gy) initially yielded more tolerant lines but with smaller colonies and traits that faded when selection relaxed, consistent with an excess of deleterious mutations. In practical terms, the method lets researchers tune the mutation load to favor beneficial changes while preserving overall fitness.
Genomic analyses of the top performers revealed changes in two core genes across independently evolved lines: the 16S rRNA gene, central to the protein‑making machinery, and rpoC, which encodes the β subunit of RNA polymerase. Convergent mutations in such essential systems point to mechanisms that help bacterial transcription and translation continue smoothly under heat stress—precisely the behaviors industry needs in high‑temperature processes.
“By combining adaptive laboratory evolution with precisely repeated doses of gamma rays, we shortened the path to robust, heat‑tolerant bacteria from months or years to just weeks,” said Dr. Yoshihiro Hase, project leader at the Takasaki Institute for Advanced Quantum Science (TIAQ), QST. “It’s a practical lever for making biofertilizers more reliable in hotter fields and bioreactors.”
“This controllable mutagenesis avoids transgenic modifications and can be tuned to maximize beneficial changes while limiting genetic load,” added Dr. Katsuya Satoh, senior principal researcher at TIAQ. “We see a route that industry can adopt safely to boost resilience and productivity.”
Beyond agriculture, the approach could be generalized to yeasts, bacteria, and microalgae used in food processing, therapeutic manufacturing, and biofuel production—helping deliver high‑quality products at lower environmental cost. In the long term, QST anticipates ultra‑low‑cost microalgal cultivation and other heat‑tolerant platforms that contribute to food and energy security.
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Reference
DOI: 10.1016/j.mrfmmm.2025.111919
About National Institutes for Quantum Science and Technology, Japan
The National Institutes for Quantum Science and Technology (QST) was established in April 2016 to promote quantum science and technology in a comprehensive and integrated manner. The new organization was formed from the merger of the National Institute of Radiological Sciences (NIRS) with certain operations that were previously undertaken by the Japan Atomic Energy Agency (JAEA).
QST is committed to advancing quantum science and technology, creating world-leading research and development platforms, and exploring new fields, thereby achieving significant academic, social, and economic impacts.
Website: https://www.qst.go.jp/site/qst-english/
About Dr. Yoshihiro Hase
Dr. Yoshihiro Hase works at the Takasaki Institute for Advanced Quantum Science, National Institutes for Quantum Science and Technology, Japan. His research focuses on mutagenesis by quantum beams in plants and bacteria and he has published more than 70 papers on these topics, which have received more than 1,600 citations.
Funding information
This study was partially supported by The Canon Foundation.
Journal
Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Repeated artificial mutagenesis of Bradyrhizobium diazoefficiens by gamma irradiation accelerates the acquisition of high-temperature tolerance
Article Publication Date
19-Nov-2025
COI Statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.