Isolation and genetic characterization of Aurantimonas and Methylobacterium strains from stems of hypernodulated soybeans

Aerobic Methoxydotrophy: Growth on Methoxylated Aromatic Compounds by Methylobacteriaceae

Pink-pigmented facultative methylotrophs have long been studied for their ability to grow on reduced single-carbon (C₁) compounds. The C₁ groups that support methylotrophic growth may come from a variety of sources. Here, we describe a group of Methylobacterium strains that can engage in methoxydotrophy: they can metabolize the methoxy groups from several aromatic compounds that are commonly the product of lignin depolymerization. Furthermore, these organisms can utilize the full aromatic ring as a growth substrate, a phenotype that has rarely been described in Methylobacterium. We demonstrated growth on p-hydroxybenzoate, protocatechuate, vanillate, and ferulate in laboratory culture conditions. We also used comparative genomics to explore the evolutionary history of this trait, finding that the capacity for aromatic catabolism is likely ancestral to two clades of Methylobacterium, but has also been acquired horizontally by closely related organisms. In addition, we surveyed the published metagenome data to find that the most abundant group of aromatic-degrading Methylobacterium in the environment is likely the group related to Methylobacterium nodulans, and they are especially common in soil and root environments. The demethoxylation of lignin-derived aromatic monomers in aerobic environments releases formaldehyde, a metabolite that is a potent cellular toxin but that is also a growth substrate for methylotrophs. We found that, whereas some known lignin-degrading organisms excrete formaldehyde as a byproduct during growth on vanillate, Methylobacterium do not. This observation is especially relevant to our understanding of the ecology and the bioengineering of lignin degradation.

Metagenomic Analysis Revealed Methylamine and Ureide Utilization of Soybean-Associated Methylobacterium

Methylobacterium inhabits the phyllosphere of a large number of plants. We herein report the results of comparative metagenome analyses on methylobacterial communities of soybean plants grown in an experimental field in Tohoku University (Kashimadai, Miyagi, Japan). Methylobacterium was identified as the most dominant genus (33%) among bacteria inhabiting soybean stems. We classified plant-derived Methylobacterium species into Groups I, II, and III based on 16S rRNA gene sequences, and found that Group I members (phylogenetically close to M. extorquens) were dominant in soybean-associated Methylobacterium. By comparing 29 genomes, we found that all Group I members possessed a complete set of genes for the N-methylglutamate pathway for methylamine utilization, and genes for urea degradation (urea carboxylase, urea amidolyase, and conventional urease). Only Group I members and soybean methylobacterial isolates grew in a culture supplemented with methylamine as the sole carbon source. They utilized urea or allantoin (a urea-related compound in legumes) as the sole nitrogen source; however, group III also utilized these compounds. The utilization of allantoin may be crucial in soybean-bacterial interactions because allantoin is a transported form of fixed nitrogen in legume plants. Soybean-derived Group I strain AMS5 colonized the model legume Lotus japonicus well. A comparison among the 29 genomes of plant-derived and other strains suggested that several candidate genes are involved in plant colonization such as csgG (curli fimbriae). Genes for the N-methylglutamate pathway and curli fimbriae were more abundant in soybean microbiomes than in rice microbiomes in the field. Based on these results, we discuss the lifestyle of Methylobacterium in the legume phyllosphere.

Complete Genome Sequence of Methylobacterium sp. Strain AMS5, an Isolate from a Soybean Stem

Nonrhizobial Methylobacterium spp. inhabit the phyllosphere of a wide variety of plants. We report here the complete genome sequence of Methylobacterium sp. AMS5, which was isolated from a soybean stem. The information is useful for understanding the molecular mechanisms of the interaction between nonrhizobial Methylobacterium spp. and plants.

Are Symbiotic Methanotrophs Key Microbes for N Acquisition in Paddy Rice Root?

The relationships between biogeochemical processes and microbial functions in rice (Oryza sativa) paddies have been the focus of a large number of studies. A mechanistic understanding of methane–nitrogen (CH₄–N) cycle interactions is a key unresolved issue in research on rice paddies. This minireview is an opinion paper for highlighting the mechanisms underlying the interactions between biogeochemical processes and plant-associated microbes based on recent metagenomic, metaproteomic, and isotope analyses. A rice symbiotic gene, relevant to rhizobial nodulation and mycorrhization in plants, likely accommodates diazotrophic methanotrophs or the associated bacterial community in root tissues under low-N fertilizer management, which may permit rice plants to acquire N via N₂ fixation. The amount of N fixed in rice roots was previously estimated to be approximately 12% of plant N based on measurements of ¹⁵N natural abundance in a paddy field experiment. Community analyses also indicate that methanotroph populations in rice roots are susceptible to environmental conditions such as the microclimate of rice paddies. Therefore, CH₄ oxidation by methanotrophs is a driving force in shaping bacterial communities in rice roots grown in CH₄-rich environments. Based on these findings, we propose a hypothesis with unanswered questions to describe the interplay between rice plants, root microbiomes, and their biogeochemical functions (CH₄ oxidation and N₂ fixation).

Bacterial clade with the ribosomal RNA operon on a small plasmid rather than the chromosome

rRNA is essential for life because of its functional importance in protein synthesis. The rRNA (rrn) operon encoding 16S, 23S, and 5S rRNAs is located on the "main" chromosome in all bacteria documented to date and is frequently used as a marker of chromosomes. Here, our genome analysis of a plant-associated alphaproteobacterium, Aureimonas sp. AU20, indicates that this strain has its sole rrn operon on a small (9.4 kb), high-copy-number replicon. We designated this unusual replicon carrying the rrn operon on the background of an rrn-lacking chromosome (RLC) as the rrn-plasmid. Four of 12 strains close to AU20 also had this RLC/rrn-plasmid organization. Phylogenetic analysis showed that those strains having the RLC/rrn-plasmid organization represented one clade within the genus Aureimonas. Our finding introduces a previously unaddressed viewpoint into studies of genetics, genomics, and evolution in microbiology and biology in general.

Genes related to antioxidant metabolism are involved in Methylobacterium mesophilicum-soybean interaction

The genus Methylobacterium is composed of pink-pigmented methylotrophic bacterial species that are widespread in natural environments, such as soils, stream water and plants. When in association with plants, this genus colonizes the host plant epiphytically and/or endophytically. This association is known to promote plant growth, induce plant systemic resistance and inhibit plant infection by phytopathogens. In the present study, we focused on evaluating the colonization of soybean seedling-roots by Methylobacterium mesophilicum strain SR1.6/6. We focused on the identification of the key genes involved in the initial step of soybean colonization by methylotrophic bacteria, which includes the plant exudate recognition and adaptation by planktonic bacteria. Visualization by scanning electron microscopy revealed that M. mesophilicum SR1.6/6 colonizes soybean roots surface effectively at 48 h after inoculation, suggesting a mechanism for root recognition and adaptation before this period. The colonization proceeds by the development of a mature biofilm on roots at 96 h after inoculation. Transcriptomic analysis of the planktonic bacteria (with plant) revealed the expression of several genes involved in membrane transport, thus confirming an initial metabolic activation of bacterial responses when in the presence of plant root exudates. Moreover, antioxidant genes were mostly expressed during the interaction with the plant exudates. Further evaluation of stress- and methylotrophic-related genes expression by qPCR showed that glutathione peroxidase and glutathione synthetase genes were up-regulated during the Methylobacterium-soybean interaction. These findings support that glutathione (GSH) is potentially a key molecule involved in cellular detoxification during plant root colonization. In addition to methylotrophic metabolism, antioxidant genes, mainly glutathione-related genes, play a key role during soybean exudate recognition and adaptation, the first step in bacterial colonization.

Impact of Azospirillum sp. B510 inoculation on rice-associated bacterial communities in a paddy field

Rice seedlings were inoculated with Azospirillum sp. B510 and transplanted into a paddy field. Growth in terms of tiller numbers and shoot length was significantly increased by inoculation. Principal-coordinates analysis of rice bacterial communities using the 16S rRNA gene showed no overall change from B510 inoculation. However, the abundance of Veillonellaceae and Aurantimonas significantly increased in the base and shoots, respectively, of B510-inoculated plants. The abundance of Azospirillum did not differ between B510-inoculated and uninoculated plants (0.02-0.50%). These results indicate that the application of Azospirillum sp. B510 not only enhanced rice growth, but also affected minor rice-associated bacteria.

Aureimonas jatrophae sp. nov. and Aureimonas phyllosphaerae sp. nov., leaf-associated bacteria isolated from Jatropha curcas L

Four orange-pigmented isolates, L7-456, L7-484T, L9-479 and L9-753T, originating from surface-sterilized leaf tissues of Jatropha curcas L. cultivars were characterized using a polyphasic taxonomic approach. Phylogenetic analyses based on 16S rRNA gene sequences indicated that all four isolates belong to the genus Aureimonas . In these analyses, strain L7-484T appeared to be most closely related to Aureimonas ureilytica 5715S-12T (95.7 % sequence identity). The 16S rRNA gene sequences of strains L7-456, L9-479 and L9-753T were found to be identical and also shared the highest similarity with A. ureilytica 5715S-12T (97.5 %). Both L7-484T and L9-753T contained Q-10 and Q-9 as predominant ubiquinones and diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, phosphatidylmonomethylethanolamine, phosphatidylethanolamine, phosphatidyldimethylethanolamine, sulfoquinovosyldiacylglycerol and an aminophospholipid as the major polar lipids. C18 : 1ω7c and C16 : 0 were the major fatty acids. Similar to other species in the genus Aureimonas , hydroxylated fatty acids (e.g. C18 : 1 2-OH) and cyclic fatty acids (C19 : 0 cyclo ω8c) were also present. The DNA G+C contents of L7-484T and L9-753T were 66.1 and 69.4 mol%, respectively. Strains L7-484T and L9-753T exhibited less than 40 % DNA–DNA hybridization both between themselves and to A. ureilytica KACC 11607T. Our results support the proposal that strain L7-484T represents a novel species within the genus Aureimonas , for which the name Aureimonas jatrophae sp. nov. is proposed, and that strains L9-753T, L7-456 ( = KACC 16229  = DSM 25023) and L9-479 ( = KACC 16228  = DSM 25024) represent a second novel species within the genus, for which the name Aureimonas phyllosphaerae sp. nov. is proposed. The type strains of Aureimonas jatrophae sp. nov. and Aureimonas phyllosphaerae sp. nov. are respectively L7-484T ( = KACC 16230T  = DSM 25025T) and L9-753T ( = KACC 16231T  = DSM 25026T).

Rhizosphere bacteria ofCostulariaspp. from ultramafic soils in New Caledonia: diversity, tolerance to extreme edaphic conditions, and role in plant growth and mineral nutrition

Rhizosphere bacteria were isolated from Costularia spp., pioneer sedges from ultramafic soils in New Caledonia, which is a hotspot of biodiversity in the South Pacific. Genus identification, ability to tolerate edaphic constraints, and plant-growth-promoting (PGP) properties were analysed. We found that 105colony-forming units per gram of root were dominated by Proteobacteria (69%) and comprised 21 genera, including Burkholderia (28%), Curtobacterium (15%), Bradyrhizobium (9%), Sphingomonas (8%), Rhizobium (7%), and Bacillus (5%). High proportions of bacteria tolerated many elements of the extreme edaphic conditions: 82% tolerated 100 μmol·L–1chromium, 70% 1 mmol·L–1nickel, 63% 10 mmol·L–1manganese, 24% 1 mmol·L–1cobalt, and 42% an unbalanced calcium/magnesium ratio (1/16). These strains also exhibited multiple PGP properties, including the ability to produce ammonia (65%), indole-3-acetic acid (60%), siderophores (52%), and 1-aminocyclopropane-1-carboxylate (ACC) deaminase (39%); as well as the capacity to solubilize phosphates (19%). The best-performing strains were inoculated with Sorghum sp. grown on ultramafic substrate. Three strains significantly enhanced the shoot biomass by up to 33%. The most successful strains influenced plant nutrition through the mobilization of metals in roots and a reduction of metal transfer to shoots. These results suggest a key role of these bacteria in plant growth, nutrition, and adaptation to the ultramafic constraints.

Evolution of Bradyrhizobium-Aeschynomene mutualism: living testimony of the ancient world or highly evolved state?

Until recently it had been well established that the initial step in legume-rhizobia symbioses was flavonoid and Nod factor (NF) signaling. However, NF-independent symbiosis is now known to occur between Bradyrhizobium and some species of Aeschynomene. Since its discovery, this unusual symbiotic system has attracted attention, and efforts have been devoted to revealing the NF-independent symbiotic mechanism, although the molecular mechanisms of nodule initiation still remain to be elucidated. NF-independent symbiosis is also interesting from the perspective of the evolution of legume-rhizobia symbiosis. In this mini-review, we discuss the current literature on the NF-independent symbiotic system in terms of phylogeny of the partners, infection, bacteroid differentiation, nodule structure, photosynthesis, endophytic features and model host plant. We also discuss NF-independent symbiosis, which is generally regarded to be more primitive than NF-dependent symbiosis, because the bacteria invade host plants via 'crack entry'. We propose three possible scenarios concerning the evolution of NF-independent symbiosis, which do not exclude the possibility that the NF-independent system evolved from NF-dependent interactions. Finally, we examine an interesting question on Bradyrhizobium-Aeschynomene mutualism, which is how do they initiate symbiosis without NF. Phylogenetic and genomic analyses of symbiotic and non-symbiotic bradyrhizobia with A. indica may be crucial to address the question, because of the very narrow phylogeny of natural endosymbionts without nod genes compared with other legume-rhizobia symbioses.