Chromosomal and Symbiotic Relationships of Rhizobia Nodulating Medicago truncatula and M. laciniata

Abstract

Multilocus sequence typing (MLST) is a sequence-based method used to characterize bacterial genomes. This method was used to examine the genetic structure of Medicago-nodulating rhizobia at the Amra site, which is located in an arid region of Tunisia. Here the annual medics Medicago laciniata and M. truncatula are part of the natural flora. The goal of this study was to identify whether distinct chromosomal groups of rhizobia nodulate M. laciniata because of its restricted requirement for specific rhizobia. The MLST analysis involved determination of sequence variation in 10 chromosomal loci of 74 isolates each of M. laciniata and M. truncatula. M. truncatula was used as a control trap host, because unlike M. laciniata, it has relatively unrestricted rhizobial requirements. Allelic diversity among the plasmid nodC alleles in the isolates was also determined. The 148 isolates were placed into 26 chromosomal sequence types (STs), only 3 of which had been identified previously. The rhizobia of M. laciniata were shown to be part of the general Medicago-nodulating population in the soil because 99.95% of the isolates had chromosomal genotypes similar to those recovered from M. truncatula. However, the isolates recovered from M. laciniata were less diverse than those recovered from M. truncatula, and they also harbored an unusual nodC allele. This could perhaps be best explained by horizontal transfer of the different nodC alleles among members of the Medicago-nodulating rhizobial population at the field site. Evidence indicating a history of lateral transfer of rhizobial symbiotic genes across distinct chromosomal backgrounds is provided.Traditionally, legumes have been important in a practice referred to as low-input sustainable agriculture. The recognition of their agricultural importance led to the demonstration that legumes bearing nodules inhabited by bacteria assimilated dinitrogen in a process termed biological nitrogen fixation. The bacteria, commonly referred to as rhizobia, harbor the genetic information necessary for establishing the nitrogen fixation process when they reside symbiotically inside the nodules. Plants of the genus Medicago are legumes that benefit from such a symbiosis with rhizobia that are currently divided into two species, Sinorhizobium meliloti (11) and S. medicae (25).The plant genus Medicago L., as currently defined, consists of about 85 species (28), including Medicago sativa (alfalfa), the world's most important cultivated forage crop. Heyn (16) reported that this genus is native to Western Asia and the Mediterranean countries, although many annual species have invaded wide areas in both the Old and New Worlds.Among the 85 species, M. laciniata has symbiotic properties that are unusual. While most Medicago species nodulated and formed nitrogen-fixing symbioses when they were grown in Australian soils with naturalized rhizobial populations, Ballard and Charman (3) observed that there was a lower incidence of nodulation with M. laciniata. They attributed this observation to the spec...

Effects ofMedicago truncatulaGenetic Diversity, Rhizobial Competition, and Strain Effectiveness on the Diversity of a NaturalSinorhizobiumSpecies Community

“…As previously found in several studies by studying the genetic diversity of rhizobia at five loci (11,40), S. medicae…”

“…The genetic diversity of these two bacterial species, especially S. meliloti, has usually been described as quite important (2,7,11,40). This bacterial diversity, assessed by phenotypic or genotypic analyses, has been reported to be influenced by several factors, including geographical location (32), soil factors (14), and plant species (19,37).…”

“…Among the various plant-bacterium couples studied so far, the Medicago truncatula association with Sinorhizobium meliloti is particularly interesting (8) and has often been studied as a model system for genetic description of the molecular pathways involved in the establishment of the symbiosis (16). M. truncatula has the ability to form an efficient symbiosis with two bacterial species, S. meliloti (9) and its sister species, Sinorhizobium medicae (29).The genetic diversity of these two bacterial species, especially S. meliloti, has usually been described as quite important (2,7,11,40). This bacterial diversity, assessed by phenotypic or genotypic analyses, has been reported to be influenced by several factors, including geographical location (32), soil factors (14), and plant species (19,37).…”

Application of Multilocus Sequence Typing To Study the Genetic Structure of Megaplasmids in Medicago -Nodulating Rhizobia

“…The most agriculturally significant species of rhizobia that nodulate these plants are Sinorhizobium meliloti (9) and Sinorhizobium medicae (22). Previously reported population genetic analyses of these bacteria have focused on the study of how allelic variants at multiple loci are distributed within and among natural populations (2,3,10,26,31,32). This was also the focus of the present study, but it was extended by examining more loci in many more strains of both species of Sinorhizobium coupled with an analysis having a range of symbiotic genotypes.…”

Rhizobium tibeticum sp. nov., a symbiotic bacterium isolated from Trigonella archiducis-nicolai (Sirj.) Vassilcz.

“…Symbiotic genes (nif and nod) may have evolutionary histories that are different from those of housekeeping genes in rhizobia, and they have been used to estimate the symbiotic properties of rhizobia (Kalita et al, 2006;Laranjo et al, 2008;van Berkum et al, 2007). Fragments of the nifH (nitrogenase reductase; about 680 bp) and nodA (N-acyltransferase; about 600 bp) genes were amplified from CCBAU 85039 T and sequenced directly, with primers nifH-1 and nifH-2 (Eardly et al, 1992) and nodA-1 and nodA-2 (Haukka et al, 1998), respectively.…”