Acquisition of additional photosynthetic pigments enables photosynthetic organisms to survive in particular niches. To reveal the ecological significance of chlorophyll (Chl) f, we investigated the distribution of Chl and cyanobacteria within two microbial mats. In a 7-mm-thick microbial mat beneath the running water of the Nakabusa hot spring, Japan, Chl f was only distributed 4.0–6.5 mm below the surface, where the intensity of far-red light (FR) was higher than that of photosynthetically active radiation (PAR). In the same mat, two ecotypes of Synechococcus and two ecotypes of Chl f-producing Leptolyngbya were detected in the upper and deeper layers, respectively. Only the Leptolyngbya strains could grow when FR was the sole light source. These results suggest that the deeper layer of the microbial mat was a habitat for Chl f-producing cyanobacteria, and Chl f enabled them to survive in a habitat with little PAR.

Symbiosis between organisms influences their evolution via adaptive changes in genome architectures. Immunity of soybean carrying the Rj2 allele is triggered by NopP (type III secretion system [T3SS]-dependent effector), encoded by symbiosis island A (SymA) in B. diazoefficiens USDA122. This immunity was overcome by many mutants with large SymA deletions that encompassed T3SS (rhc) and N₂ fixation (nif) genes and were bounded by insertion sequence (IS) copies in direct orientation, indicating homologous recombination between ISs. Similar deletion events were observed in B. diazoefficiens USDA110 and B. japonicum J5. When we cultured a USDA122 strain with a marker gene sacB inserted into the rhc gene cluster, most sucrose-resistant mutants had deletions in nif/rhc gene clusters, similar to the mutants above. Some deletion mutants were unique to the sacB system and showed lower competitive nodulation capability, indicating that IS-mediated deletions occurred during free-living growth and the host plants selected the mutants. Among 63 natural bradyrhizobial isolates, 2 possessed long duplications (261–357 kb) harboring nif/rhc gene clusters between IS copies in direct orientation via homologous recombination. Therefore, the structures of symbiosis islands are in a state of flux via IS-mediated duplications and deletions during rhizobial saprophytic growth, and host plants select mutualistic variants from the resultant pools of rhizobial populations. Our results demonstrate that homologous recombination between direct IS copies provides a natural mechanism generating deletions and duplications on symbiosis islands.

Soybeans fix atmospheric N₂ through symbiosis with rhizobia. The relationship between rhizobia and soybeans, particularly those with high nitrous oxide (N₂O)-reducing (N₂OR) activities, can be leveraged to reduce N₂O emissions from agricultural soils. However, inoculating soybeans with these rhizobia under field conditions often fails because of the competition from indigenous rhizobia that possess low or no N₂OR activity. In this work, we utilize natural incompatibility systems between soybean and rhizobia to address this challenge. Specifically, Rj2 and GmNNL1 inhibit certain rhizobial infections in response to NopP, an effector protein. By combining a soybean line with a hybrid accumulation of the Rj2 and GmNNL1 genes and bradyrhizobia lacking the nopP gene, we develop a soybean-bradyrhizobial symbiosis system in which strains with high N₂OR activity predominantly infect. Our optimize symbiotic system substantially reduces N₂O emissions in field and laboratory tests, presenting a promising approach for sustainable agricultural practices.

In rice (Oryza sativa L.), leaf sheaths enhance stem strength and lodging resistance. However, little is known about the factors that affect varietal differences and changes over time regarding leaf sheath reinforcement. In the present study, the morphological and physiological traits that are presumed to be related to leaf sheath reinforcement were examined in ‘Chugoku 117’ and ‘Koshihikari’, rice varieties with high and low degrees of leaf sheath reinforcement, respectively. The results showed that Chugoku 117 had thicker leaf sheaths and delayed leaf sheath senescence than Koshihikari, indicating that there were clear varietal differences in these traits. The bending moment at breaking with leaf sheath was correlated with senescence-associated traits, especially strongly correlated with the number of living leaf sheaths in both varieties. Among the components examined in the present study, only starch content was significantly positively correlated with both varieties. On the other hand, the starch in the leaf sheath disappeared in the latter stage of ripening due to translocation to sinks, suggesting that it contributes to stem strength only up to the early stages of ripening. The findings suggest that rice stem strength can be increased by thickening the leaf sheaths and delaying leaf sheath senescence. Thus, in addition to the physical properties of the culm, such as shortening (semi-dwarfing) and strengthening, the findings of the present study provide a new breeding strategy for improving breaking-type lodging resistance in rice. Graphical abstract

Microbial community structure reflects the surrounding natural environment and changes to that environment. Although the subsurface at 5–100 m depth is important for human activities and there are potential risks of environmental pollution in this region, there have been only a few reports of subsurface microbial community structures in terrestrial areas. We investigated the diversity and community compositions of Bacteria and Archaea in boring cores collected from various depths at three different sites in the southern Kanto Plain, Japan. The results of 16S rRNA gene amplicon sequencing using MiSeq showed that the microbial community composition varied with the geological unit. Proteobacteria (Alphaproteobacteria and Gammaproteobacteria) were dominant members within sediments accumulated during the Pleistocene in the Musashino Upland. In contrast, Acidobacteria and Chloroflexi characteristically appeared in the Holocene layers of the Arakawa Lowland. These data suggest that the subsurface microbial composition is controlled by the geological features of the sediments.