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内容記述 |
Biofertilizers are promising technologies to achieve 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, we performed a mutant screening 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 three-day heat-shock treatment at 43 °C. In particular, M14 showed superior growth at 36 °C, at which 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 metabo-lism 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. No-tably, transmembrane proteins, including porins, were enriched among the genes upregulated in both M10 and M14. M14 was confirmed to retain symbiotic functions with soybean. These re-sults indicate that high-temperature tolerance can be conferred by random mutagenesis while maintaining the symbiotic functions of rhizobia. |
