Rhizobium yantingense sp. nov., a mineral-weathering bacterium

Silicon: A valuable soil element for improving plant growth and CO2 sequestration

BACKGROUND

Silicon (Si), the second most abundant and quasi-essential soil element, is locked as a recalcitrant silicate mineral in the Earth's crust. The physical abundance of silicates can play an essential role in increasing plant productivity. Plants store Si as biogenic silica (phytoliths), which is mobilized through a chemical weathering process in the soil.

AIM OF REVIEW

Although Si is a critical element for plant growth, there is still a considerable need to understand its dissolution, uptake, and translocation in agroecosystems. Here, we show recent progress in understanding the interactome of Si, CO2, the microbiome, and soil chemistry, which can sustainably govern silicate dissolution and cycling in agriculture.

KEY SCIENTIFIC CONCEPTS OF THIS REVIEW

Si cycling is directly related to carbon cycling, and the resulting climate stability can be enhanced by negative feedback between atmospheric CO2 and the silicate uptake process. Improved Si mobilization in the rhizosphere by the presence of reactive elements (for example, Ca, Na, Al, Zn, and Fe) and Si uptake through genetic transporters in plants are crucial to achieving the dual objectives of (i) enhancing crop productivity and (ii) abiotic stress tolerance. Furthermore, the microbiome is a symbiotic partner of plants. Bacterial and fungal microbiomes can solubilize silicate minerals through intriguingly complex bioweathering mechanisms by producing beneficial metabolites and enzymes. However, the interaction of Si with CO2 and the microbiome's function in mobilization have been understudied. This review shows that enhancing our understanding of Si, CO2, the microbiome, and soil chemistry can help in sustainable crop production during climatic stress events.

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

Rhizobium quercicola sp. nov., isolated from the leaf of Quercus variablis in China

Strain DKSPLA3^T, a novel Gram-negative, catalase-positive, oxidase-positive, non-spore-forming, aerobic, non-nitrogen-fixing, non-motile bacterium was isolated from Quercus variablis leaf, in Zunyi, Guizhou, China. Growth occurred at 4-37 °C (optimum 28 °C), pH 4.0-9.0 (optimum pH 7.0) and up to 4.0% (w/v) NaCl (optimum under 2.0%, w/v). Phylogeny based on 16S rRNA gene indicated that strain DKSPLA3^T was a novel species in the genus Rhizobium, which was supported by average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values. The predominant fatty acids of strain DKSPLA3^T were C_16:0, C_18:1 ω7c and/or C_18:1 ω6c and C_18:1 ω7c 11-methyl. The major respiratory quinone was Q-10. Major polar lipids were diphosphatidyl glycerol (DPG), phosphatidyl glycerol (PG), phosphatidylethanolamine (PE), phosphatidylmonomethylethanolamine (PME), phosphatidylcholine (PC), two unidentified phospholipids (PL) and nine unidentified lipids (L). The genomic G + C content was 64.47 mol%. Based on the phenotypic, phylogenetic and genotypic data, DKSPLA3^T should be classified as a novel species in the genus Rhizobium, for which the name Rhizobium quercicola sp. nov. (KCTC 82843^T = CFCC 16,707T) is proposed.

Obtaining Osmo-resistant Mutants in Nitrogen-Fixing Bacteria Isolated from Saline Soils

Annually, about, more than 7% of the Earth's land area becomes inappropriate for agriculture subsequently of salinization and desertification. Biofertilizers based on halophilic nitrogen-fixing bacteria can restore saline soils and stimulate plant growth, having a positive effect on germination, development of stems and roots, and fruiting. The aim of this work was to obtain osmo-resistant (Osm-r) nitrogen-fixing mutants isolated from saline soils of Armenia and selection of the best ones. To achieve this goal, we have obtained a collection of Osm-r strains based on soil nitrogen-fixing bacteria without the use of genetically modified technologies, which is an innovation in sphere of soil microbiology, and, especially, in nitrogen-fixing microorganisms. These mutants were obtained on the basis of Agrobacterium sp. Y-2 and Agrobacterium sp. M-1 nitrogen-fixing strains, both spontaneously and induced. Four strains with the higher nitrogen-fixing ability, which kept their vital activity in an environment with a high concentration of salts, were selected from collection of mutants. Selected strains in the future can become the basis for creating a new, effective, environmentally friendly biofertilizer for saline soils because they are plasmidless and have the highest priority for intensive use in agriculture.

Taxonomy of Rhizobiaceae revisited: proposal of a new framework for genus delimitation

The alphaproteobacterial family Rhizobiaceae is highly diverse, with 168 species with validly published names classified into 17 genera with validly published names. Most named genera in this family are delineated based on genomic relatedness and phylogenetic relationships, but some historically named genera show inconsistent distribution and phylogenetic breadth. The most problematic is Rhizobium , which is notorious for being highly paraphyletic, as most newly described species in the family are assigned to this genus without consideration of their proximity to existing genera, or the need to create novel genera. Moreover, many Rhizobiaceae genera lack synapomorphic traits that would give them biological and ecological significance. We propose a common framework for genus delimitation within the family Rhizobiaceae , wherein genera are defined as monophyletic groups in a core-genome gene phylogeny, that are separated from related species using a pairwise core-proteome average amino acid identity (cpAAI) threshold of approximately 86 %. We further propose that additional genomic or phenotypic evidence can justify division of species into separate genera even if they share greater than 86 % cpAAI. Applying this framework, we propose to reclassify Rhizobium rhizosphaerae and Rhizobium oryzae into Xaviernesmea gen. nov. Data is also provided to support the formation of Peteryoungia aggregata comb. nov., Endobacterium yantingense comb. nov., Neorhizobium petrolearium comb. nov., Pararhizobium arenae comb. nov., Pseudorhizobium tarimense comb. nov. and Mycoplana azooxidifex comb. nov. Lastly, we present arguments that the unification of the genera Ensifer and Sinorhizobium in Opinion 84 of the Judicial Commission is no longer justified by current genomic and phenotypic data. Despite pairwise cpAAI values for all Ensifer species and all Sinorhizobium species being >86 %, additional genomic and phenotypic data suggest that they significantly differ in their biology and ecology. We therefore propose emended descriptions of Ensifer and Sinorhizobium , which we argue should be considered as separate genera.

Contribution of Arbuscular Mycorrhizal Fungi, Phosphate-Solubilizing Bacteria, and Silicon to P Uptake by Plant

Phosphorus (P) availability is usually low in soils around the globe. Most soils have a deficiency of available P; if they are not fertilized, they will not be able to satisfy the P requirement of plants. P fertilization is generally recommended to manage soil P deficiency; however, the low efficacy of P fertilizers in acidic and in calcareous soils restricts P availability. Moreover, the overuse of P fertilizers is a cause of significant environmental concerns. However, the use of arbuscular mycorrhizal fungi (AMF), phosphate-solubilizing bacteria (PSB), and the addition of silicon (Si) are effective and economical ways to improve the availability and efficacy of P. In this review the contributions of Si, PSB, and AMF in improving the P availability is discussed. Based on what is known about them, the combined strategy of using Si along with AMF and PSB may be highly useful in improving the P availability and as a result, its uptake by plants compared to using either of them alone. A better understanding how the two microorganism groups and Si interact is crucial to preserving soil fertility and improving the economic and environmental sustainability of crop production in P deficient soils. This review summarizes and discusses the current knowledge concerning the interactions among AMF, PSB, and Si in enhancing P availability and its uptake by plants in sustainable agriculture.

Analysis of 1,000+ Type-Strain Genomes Substantially Improves Taxonomic Classification of Alphaproteobacteria

The class Alphaproteobacteria is comprised of a diverse assemblage of Gram-negative bacteria that includes organisms of varying morphologies, physiologies and habitat preferences many of which are of clinical and ecological importance. Alphaproteobacteria classification has proved to be difficult, not least when taxonomic decisions rested heavily on a limited number of phenotypic features and interpretation of poorly resolved 16S rRNA gene trees. Despite progress in recent years regarding the classification of bacteria assigned to the class, there remains a need to further clarify taxonomic relationships. Here, draft genome sequences of a collection of genomes of more than 1000 Alphaproteobacteria and outgroup type strains were used to infer phylogenetic trees from genome-scale data using the principles drawn from phylogenetic systematics. The majority of taxa were found to be monophyletic but several orders, families and genera, including taxa recognized as problematic long ago but also quite recent taxa, as well as a few species were shown to be in need of revision. According proposals are made for the recognition of new orders, families and genera, as well as the transfer of a variety of species to other genera and of a variety of genera to other families. In addition, emended descriptions are given for many species mainly involving information on DNA G+C content and (approximate) genome size, both of which are confirmed as valuable taxonomic markers. Similarly, analysis of the gene content was shown to provide valuable taxonomic insights in the class. Significant incongruities between 16S rRNA gene and whole genome trees were not found in the class. The incongruities that became obvious when comparing the results of the present study with existing classifications appeared to be caused mainly by insufficiently resolved 16S rRNA gene trees or incomplete taxon sampling. Another probable cause of misclassifications in the past is the partially low overall fit of phenotypic characters to the sequence-based tree. Even though a significant degree of phylogenetic conservation was detected in all characters investigated, the overall fit to the tree varied considerably.

Rhizobium wenxiniae sp. nov., an endophytic bacterium isolated from maize root

A novel Gram-stain-negative, aerobic, rod-shaped strain designated 166T was isolated from surface-sterilized root tissue of maize planted in the Fangshan District of Beijing, PR China. The 16S rRNA gene sequence analysis indicated that strain 166T belongs to the genus Rhizobium and is closely related to Rhizobium cellulosilyticum ALA10B2T and Rhizobium yantingense H66T with sequence similarities of 98.8 and 98.3 %, respectively. According to atpD and recA sequence analysis, the highest sequence similarity between strain 166T and R. cellulosilyticum ALA10B2T is 93.8 and 84.7 %, respectively. However, the new isolate exhibited relatively low levels of DNA-DNA relatedness with respect to R. cellulosilyticum DSM 18291T (20.8±2.3 %) and Rhizobium yantingense CCTCC AB 2014007T (47.2±1.4 %). The DNA G+C content of strain 166T was 59.8 mol%. The main polar lipids consisted of phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, diphosphatidylglycerol, an unidentified aminophospholipid and an unidentified aminolipid. The major fatty acids of strain 166T were summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c). The results of the physiological and biochemical tests and minor differences in the fatty acid profiles allowed a clear phenotypic differentiation of strain 166T from the type strains of closely related species, R. cellulosilyticum DSM 18291T and R. yantingense CCTCC AB 2014007T. Strain 166T represents a novel species within the genus Rhizobium, for which the name Rhizobium wenxiniae sp. nov. is proposed, with the type strain 166T (=CGMCC 1.15279T=DSM 100734T).

Rhizobium arenae sp. nov., isolated from the sand of Desert Mu Us, China

A Gram-strain-negative, rod-shaped, motile bacterium, designated MIM27T, was isolated from the sand of the Mu Us Desert, PR China. The strain could grow at 4-45 °C (optimum, 37 °C), at pH 6.6-9.0 (optimum, 8.0) and in the presence of 0-3 % (w/v) NaCl (optimum, 0 % in RNA liquid medium). The results of phylogenetic analysis of 16S rRNA gene sequences indicated that the strain represented a member of the genus Rhizobium, with the highest similarity (96.5 %) to Rhizobium pakistanense BN-19T. The results of analysis of the sequences of the nitrogen fixation gene nifH and three housekeeping genes, recA, atpD and glnII, also indicated that MIM27T was most closely related to the species of the genus Rhizobiumwith validly published names but the similarities were low (≤90.7 %). MIM27T did not form nodules on Pisum sativum, Vicia faba, Astragalus sinicus and Phaseolus vulgaris. The major respiratory quinone of MIM27T was Q-10. The genomic DNA G+C content was 59.8 mol%. Major fatty acids of MIM27T were summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c), C18 : 1ω7c 11-methyl, C16 : 0, summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c) and summed feature 2 (C12 : 0 aldehyde and/or unknown ECL 10.9525). On the basis of the physiological, chemotaxonomic and phenotypic data, MIM27T is suggested to represent a novel species of the genus Rhizobium, for which the name Rhizobium arenae sp. nov. is proposed. The type strain is MIM27T (=KCTC 52299T=MCCC 1K03215T).

Rhizobium oryziradicis sp. nov., isolated from rice roots

Two Gram-stain-negative, aerobic, rod-shaped endophytic bacterial strains, N19T and N11-2, were isolated from fresh rice (Oryza sativa) roots during investigation of the rice endophytic bacterial diversity. The 16S rRNA gene sequence results indicated that the similarity between strains N19T and N11-2 was 100 %. Both of them belong to the genus Rhizobium, with close similarity to Rhizobium taibaishanense CCNWSX 0483T (97.7 %), followed by Rhizobium vitis NCPPB 3554T (97.5 %). The sequence similarities of the housekeeping genes recA, gyrB and glnA between the novel isolates and members of the established species of the genus Rhizobium were less than 87 %. The DNA-DNA hybridization rates between strains N19T and N11-2 were 87.9 % using the initial renaturation rate method. Based on draft genome sequences, strain N19T showed 18.2 % and 19.6 % DNA-DNA hybridization values to R. taibaishanense CCNWSX 0483T and R. vitis S4, which demonstrated that these new isolates represent a novel species in the genus Rhizobium. The main cellular fatty acids were summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c). The DNA G+C content of strain N19T was 58.7 mol% (Tm). The polar lipid profile of N19T consisted of phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine, an unknown lipid, two unknown aminolipids and an unidentified aminophospholipid. According to physiological and biochemical characteristics and genotypic data, strains N19T and N11-2 are considered to represent a novel species of the genus Rhizobium, for which the name Rhizobium oryziradicis sp. nov. is proposed, with N19T (=ACCC 19962T=KCTC 52413T) as the type strain.

Distinct Mineral Weathering Behaviors of the Novel Mineral-Weathering Strains Rhizobium yantingense H66 and Rhizobium etli CFN42

Bacteria play important roles in mineral weathering, soil formation, and element cycling. However, little is known about the interaction between silicate minerals and rhizobia. In this study, Rhizobium yantingense H66 (a novel mineral-weathering rhizobium) and Rhizobium etli CFN42 were compared with respect to potash feldspar weathering, mineral surface adsorption, and metabolic activity during the mineral weathering process. Strain H66 showed significantly higher Si, Al, and K mobilization from the mineral and higher ratios of cell numbers on the mineral surface to total cell numbers than strain CFN42. Although the two strains produced gluconic acid, strain H66 also produced acetic, malic, and succinic acids during mineral weathering in low- and high-glucose media. Notably, higher Si, Al, and K releases, higher ratios of cell numbers on the mineral surface to total cell numbers, and a higher production of organic acids by strain H66 were observed in the low-glucose medium than in the high-glucose medium. Scanning electron microscope analyses of the mineral surfaces and redundancy analysis showed stronger positive correlations between the mineral surface cell adsorption and mineral weathering, indicated by the dissolved Al and K concentrations. The results showed that the two rhizobia behaved differently with respect to mineral weathering. The results suggested that Rhizobium yantingense H66 promoted potash feldspar weathering through increased adsorption of cells to the mineral surface and through differences in glucose metabolism at low and high nutrient concentrations, especially at low nutrient concentrations.

IMPORTANCE

This study reported the potash feldspar weathering, the cell adsorption capacity of the mineral surfaces, and the metabolic differences between the novel mineral-weathering Rhizobium yantingense H66 and Rhizobium etli CFN42 under different nutritional conditions. The results showed that Rhizobium yantingense H66 had a greater ability to weather the mineral in low- and high-glucose media, especially in the low-glucose medium. Furthermore, Rhizobium yantingense H66 promoted mineral weathering through the increased adsorption of cells to the mineral surface and through increased organic acid production. Our results allow us to better comprehend the roles of different rhizobia in silicate mineral weathering, element cycling, and soil formation in various soil environments, providing more insight into the geomicrobial contributions of rhizobia to these processes.