In situ lateral transfer of symbiosis islands results in rapid evolution of diverse competitive strains of mesorhizobia suboptimal in symbiotic nitrogen fixation on the pasture legume Biserrula pelecinus L

The multi-billion dollar asset attributed to symbiotic nitrogen fixation is often threatened by the nodulation of legumes by rhizobia that are ineffective or poorly effective in N(2) fixation. This study investigated the development of rhizobial diversity for the pasture legume Biserrula pelecinus L., 6 years after its introduction, and inoculation with Mesorhizobium ciceri bv. biserrulae strain WSM1271, to Western Australia. Molecular fingerprinting of 88 nodule isolates indicated seven were distinctive. Two of these were ineffective while five were poorly effective in N(2) fixation on B. pelecinus. Three novel isolates had wider host ranges for nodulation than WSM1271, and four had distinct carbon utilization patterns. Novel isolates were identified as Mesorhizobium sp. using 16S rRNA, dnaK and GSII phylogenies. In a second study, a large number of nodules were collected from commercially grown B. pelecinus from a broader geographical area. These plants were originally inoculated with M. c bv. biserrulae WSM1497 5-6 years prior to isolation of strains for this study. Nearly 50% of isolates from these nodules had distinct molecular fingerprints. At two sites diverse strains dominated nodule occupancy indicating recently evolved strains are highly competitive. All isolates tested were less effective and six were ineffective in N(2) fixation. Twelve randomly selected diverse isolates clustered together, based on dnaK sequences, within Mesorhizobium and distantly to M. c bv. biserrulae. All 12 had identical sequences for the symbiosis island insertion region with WSM1497. This study shows the rapid evolution of competitive, yet suboptimal strains for N(2) fixation on B. pelecinus following the lateral transfer of a symbiosis island from inoculants to other soil bacteria.

Rapid in situ evolution of nodulating strains for Biserrula pelecinus L. through lateral transfer of a symbiosis island from the original mesorhizobial inoculant

Diverse rhizobia able to nodulate Biserrula pelecinus evolved following in situ transfer of nodA and nifH from an inoculant to soil bacteria. Transfer of these chromosomal genes and the presence of an identical integrase gene adjacent to a Phe tRNA gene in both the inoculant and recipients indicate that there was lateral transfer of a symbiosis island.

Mesorhizobium ciceri biovar biserrulae, a novel biovar nodulating the pasture legume Biserrula pelecinus L

Biserrula pelecinus L. is a pasture legume species that forms a highly specific nitrogen-fixing symbiotic interaction with a group of bacteria that belong to Mesorhizobium. These mesorhizobia have >98.8 % sequence similarity to Mesorhizobium ciceri and Mesorhizobium loti for the 16S rRNA gene (1440 bp) and >99.3 % sequence similarity to M. ciceri for the dnaK gene (300 bp), and strain WSM1271 has 100 % sequence similarity to M. ciceri for GSII (600 bp). Strain WSM1271 had 85 % relatedness to M. ciceri LMG 14989(T) and 50 % relatedness to M. loti LMG 6125(T) when DNA-DNA hybridization was performed. WSM1271 also had a similar cellular fatty acid profile to M. ciceri. These results are strong evidence that the Biserrula mesorhizobia and M. ciceri belong to the same group of bacteria. Significant differences were revealed between the Biserrula mesorhizobia and M. ciceri in growth conditions, antibiotic resistance and carbon source utilization. The G+C content of the DNA of WSM1271 was 62.7 mol%, compared to 63-64 mol% for M. ciceri. The Biserrula mesorhizobia contained a plasmid ( approximately 500 bp), but the symbiotic genes were detected on a mobile symbiosis island and considerable variation was present in the symbiotic genes of Biserrula mesorhizobia and M. ciceri. There was <78.6 % sequence similarity for nodA and <66.9 % for nifH between Biserrula mesorhizobia and M. ciceri. Moreover, the Biserrula mesorhizobia did not nodulate the legume host of M. ciceri, Cicer arietinum, and M. ciceri did not nodulate B. pelecinus. These significant differences observed between Biserrula mesorhizobia and M. ciceri warrant the proposal of a novel biovar for Biserrula mesorhizobia within M. ciceri. The name Mesorhizobium ciceri biovar biserrulae is proposed, with strain WSM1271 (=LMG 23838=HAMBI 2942) as the reference strain.

Complete genome sequence of Mesorhizobium ciceri bv. biserrulae type strain (WSM1271(T))

Mesorhizobium ciceri bv. biserrulae strain WSM1271(T) was isolated from root nodules of the pasture legume Biserrula pelecinus growing in the Mediterranean basin. Previous studies have shown this aerobic, motile, Gram negative, non-spore-forming rod preferably nodulates B. pelecinus - a legume with many beneficial agronomic attributes for sustainable agriculture in Australia. We describe the genome of Mesorhizobium ciceri bv. biserrulae strain WSM1271(T) consisting of a 6,264,489 bp chromosome and a 425,539 bp plasmid that together encode 6,470 protein-coding genes and 61 RNA-only encoding genes.

Phylogenetic relationships of three bacterial strains isolated from the pasture legume Biserrula pelecinus L

Three bacterial strains (WSM 1283, WSM 1284, WSM 1497) isolated from root nodules of the pasture legume Biserrula pelecinus L. growing in Morocco, Italy and Greece, respectively, were studied in order to determine their phylogenetic relationship to the other members of the family Rhizobiaceae. A polyphasic approach, which included analyses of morphological and physiological characteristics, plasmid profiles, symbiotic performance and 16S rRNA gene sequencing, indicated that these strains belong to the genus Mesorhizobium.

Complete genome sequence of Mesorhizobium australicum type strain (WSM2073(T))

Mesorhizobium australicum strain WSM2073^T was isolated from root nodules on the pasture legume Biserrula pelecinus growing in Australia in 2000. This aerobic, motile, gram negative, non-spore-forming rod is poorly effective in N₂ fixation on B. pelecinus and has gained the ability to nodulate B. pelecinus following in situ lateral transfer of a symbiosis island from the original inoculant strain for this legume, Mesorhizobium ciceri bv. biserrulae WSM1271. We describe that the genome size of M. australicum strain WSM2073^T is 6,200,534 bp encoding 6,013 protein-coding genes and 67 RNA-only encoding genes. This genome does not contain any plasmids but has a 455.7 kb genomic island from Mesorhizobium ciceri bv. biserrulae WSM1271 that has been integrated into a phenylalanine-tRNA gene.

Complete genome sequence of Mesorhizobium opportunistum type strain WSM2075(T.)

Mesorhizobium opportunistum strain WSM2075(T) was isolated in Western Australia in 2000 from root nodules of the pasture legume Biserrula pelecinus that had been inoculated with M. ciceri bv. biserrulae WSM1271. WSM2075(T) is an aerobic, motile, Gram negative, non-spore-forming rod that has gained the ability to nodulate B. pelecinus but is completely ineffective in N2 fixation with this host. This report reveals that the genome of M. opportunistum strain WSM2075(T) contains a chromosome of size 6,884,444 bp, encoding 6,685 protein-coding genes and 62 RNA-only encoding genes. The genome contains no plasmids, but does harbor a 455.7 kb genomic island from Mesorhizobium ciceri bv. biserrulae WSM1271 that has been integrated into a phenylalanine-tRNA gene.