Comparative Genomics across Three Ensifer Species Using a New Complete Genome Sequence of the Medicago Symbiont Sinorhizobium (Ensifer) meliloti WSM1022

Here, we report an improved and complete genome sequence of Sinorhizobium (Ensifer) meliloti strain WSM1022, a microsymbiont of Medicago species, revealing its tripartite structure. This improved genome sequence was generated combining Illumina and Oxford nanopore sequencing technologies to better understand the symbiotic properties of the bacterium. The 6.75 Mb WSM1022 genome consists of three scaffolds, corresponding to a chromosome (3.70 Mb) and the pSymA (1.38 Mb) and pSymB (1.66 Mb) megaplasmids. The assembly has an average GC content of 62.2% and a mean coverage of 77X. Genome annotation of WSM1022 predicted 6058 protein coding sequences (CDSs), 202 pseudogenes, 9 rRNAs (3 each of 5S, 16S, and 23S), 55 tRNAs, and 4 ncRNAs. We compared the genome of WSM1022 to two other rhizobial strains, closely related Sinorhizobium (Ensifer) meliloti Sm1021 and Sinorhizobium (Ensifer) medicae WSM419. Both WSM1022 and WSM419 species are high-efficiency rhizobial strains when in symbiosis with Medicago truncatula, whereas Sm1021 is ineffective. Our findings report significant genomic differences across the three strains with some similarities between the meliloti strains and some others between the high efficiency strains WSM1022 and WSM419. The addition of this high-quality rhizobial genome sequence in conjunction with comparative analyses will help to unravel the features that make a rhizobial symbiont highly efficient for nitrogen fixation.

Evidence that nano-TiO2 induces acute cytotoxicity to the agronomically beneficial nitrogen fixing bacteria Sinorhizobium meliloti

When nano-sized titanium dioxide (nano-TiO2) absorbs ultra-violet (UV-A) radiation, it produces reactive oxygen species that can be toxic to bacteria. We used the agronomically beneficial nitrogen-fixing bacterium Sinorhizobium meliloti strain 1021 as a model microorganism to detect nano-TiO2 toxicity. S. meliloti was exposed to aqueous dispersions of micrometer-sized TiO2 (micron-TiO2, 44 μm) or nanometer-sized TiO2 (nano-TiO2, 21 nm) at nominal concentrations of 0, 100, 300, 600, 900 and 1800 mg TiO2/L. There were fewer viable S. meliloti after exposure to nano-TiO2 under dark and UV-A light conditions. Nano-TiO2 was more toxic to S. meliloti with UV-A irradiation (100% mortality at 100 mg TiO2/L) than under dark conditions (100% mortality at 900 mg TiO2/L). Micron-TiO2 concentrations less than 300 mg TiO2/L had no effect on the S. meliloti viability under dark or UV-A light conditions. Exposure to 600 mg/L or more of micron-TiO2 under UV-A light could also photo-kill S. meliloti cells (100% mortality). Further study is needed to ascertain whether nano-TiO2 interferes with the growth of N2-fixing microorganisms in realistic agricultural environments.

Complete Genome Sequence of Sinorhizobium meliloti Strain AK21, a Salt-Tolerant Isolate from the Aral Sea Region

We report here the complete genome sequence of the salt-tolerant Sinorhizobium meliloti strain AK21, isolated from nodules of Medicago sativa L. subsp. ambigua inhabiting the northern Aral Sea Region. This genome (7.36 Mb) consists of a chromosome and four accessory plasmids, two of which are the symbiotic megaplasmids pSymA and pSymB.

Draft Genome Sequence of Sinorhizobium meliloti Strain CXM1-105

Sinorhizobium meliloti is a Gram-negative bacterium which fixes atmospheric nitrogen in symbiosis with Medicago spp. We report the draft genome sequence of S. meliloti strain CXM1-105, associated with nodules of Medicago sativa subsp.

Draft Genome Sequence of Sinorhizobium meliloti Strain AK170

Root nodule bacteria of Sinorhizobium meliloti species live in a symbiotic relationship with alfalfa plants. We report here the draft genome sequence of S. meliloti strain AK170, recovered from nodules of Medicago orthoceras (Kar.

Priming Is a Suitable Strategy to Enhance Resistance Towards Leaf Rust in Barley

Priming allows plants to respond faster and stronger to abiotic or biotic stresses. Leaf rust (Puccinia hordei) is an important pathogen of barley (Hordeum vulgare), for which resistance genes are known, but mostly overcome. Therefore, the aims of this study were (i) to establish a priming system in barley, based on bacterial N-acyl homoserine lactone (AHL), and (ii) to get information on the effect of priming on the reaction to leaf rust. Plants were inoculated with bacteria, i.e., Ensifer meliloti with repaired expR copy, producing the oxo-C14-homoserine lactone (AHL) and an E. meliloti strain carrying the attM lactonase gene from Agrobacterium tumefaciens, which cleaves the AHL and acts here as negative control. After three bacterial inoculations, plants were challenged with P. hordei strain I-80 at the three leaves stage. Twelve days after infection, scoring of the leaf area diseased and the infection type was conducted followed by the calculation of the relative susceptibility. First results indicate a significantly (P < 0.001) higher resistance level to P. hordei after inoculation with E. meliloti. Furthermore, significant (P < 0.001) differences were detected between the accessions tested for priming efficiency, which can be the basis to screen a larger set of barley accessions to detect quantitative trait loci or candidate genes involved in priming. [Formula: see text] Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Nodule-Specific Cysteine-Rich Peptides Negatively Regulate Nitrogen-Fixing Symbiosis in a Strain-Specific Manner in Medicago truncatula

Medicago truncatula shows a high level of specificity when interacting with its symbiotic partner Sinorhizobium meliloti. This specificity is mainly manifested at the nitrogen-fixing stage of nodule development, such that a particular bacterial strain forms nitrogen-fixing nodules (Nod+/Fix+) on one plant genotype but ineffective nodules (Nod+/Fix−) on another. Recent studies have just begun to reveal the underlying molecular mechanisms that control this specificity. The S. meliloti strain A145 induces the formation of Fix+ nodules on the accession DZA315.16 but Fix− nodules on Jemalong A17. A previous study reported that the formation of Fix− nodules on Jemalong A17 by S. meliloti A145 was conditioned by a single recessive allele named Mtsym6. Here we demonstrate that the specificity associated with S. meliloti A145 is controlled by multiple genes in M. truncatula, including NFS1 and NFS2 that encode nodule-specific cysteine-rich (NCR) peptides. The two NCR peptides acted dominantly to block rather than promote nitrogen fixation by S. meliloti A145. These two NCR peptides are the same ones that negatively regulate nitrogen-fixing symbiosis associated with S. meliloti Rm41.