Abstract
Trifolium rubens L., commonly known as the red feather clover, is capable of symbiotic interactions with rhizobia. Up to now, no specific symbionts of T. rubens and their symbiotic compatibility with Trifolium spp. have been described. We characterized the genomic diversity of T. rubens symbionts by analyses of plasmid profiles and BOX–PCR. The phylogeny of T. rubens isolates was inferred based on the nucleotide sequences of 16S rRNA and two core genes (atpD, recA). The nodC phylogeny allowed classification of rhizobia nodulating T. rubens as Rhizobium leguminosarum symbiovar trifolii (Rlt). The symbiotic efficiency of the Rlt isolates was determined on four clover species: T. rubens, T. pratense, T. repens and T. resupinatum. We determined that Rlt strains formed mostly inefficient symbiosis with their native host plant T. rubens and weakly effective (sub-optimal) symbiosis with T. repens and T. pratense. The same Rlt strains were fully compatible in the symbiosis with T. resupinatum. T. rubens did not exhibit strict selectivity in regard to the symbionts and rhizobia closely related to Rhizobium grahamii, Rhizobium galegae and Agrobacterium radiobacter, which did not nodulate Trifolium spp., were found amongst T. rubens nodule isolates.
Highlights
In their symbiotic association with legume plants, rhizobia have the potential to fix nitrogen in amounts sufficient to reduce the dependence of plants on nitrogen fertilizers (Herridge 2008)
Plants play a significant role in the control of later stages of symbiosis, such as bacteroid differentiation inside nodules of some galegoid (IRLC) plants (Medicago, Trifolium, Vicia, Pisum, Astragalus) and endoreduplication of bacterial genomes forming bacteroids (Mergaert et al 2006; Haag et al 2013)
The dendrogram constructed based on these results showed a high genomic diversity of the isolates, which were divided into two large groups at a low-level similarity (*1%) and further separated into numerous branches (Fig. 1)
Summary
In their symbiotic association with legume plants, rhizobia have the potential to fix nitrogen in amounts sufficient to reduce the dependence of plants on nitrogen fertilizers (Herridge 2008). The Nod proteins synthesize lipochitin oligosaccharides (Nod factors) recognized by host plant receptors and, in response, tubular structures called infection threads are formed where bacteria proliferate and are released into plant nodule cells forming symbiosomes In these structures, bacteria differentiate into nitrogen-fixing bacteroids and turn N2 into ammonia, which is assimilated by the legume host (Heidstra and Bisseling 1996; Gage and Margolin 2000). Plants play a significant role in the control of later stages of symbiosis, such as bacteroid differentiation inside nodules of some galegoid (IRLC) plants (Medicago, Trifolium, Vicia, Pisum, Astragalus) and endoreduplication of bacterial genomes forming bacteroids (Mergaert et al 2006; Haag et al 2013) At this stage, bacteroids exhibit decreased cytoplasmic membrane integrity and undergo terminal differentiation in relation to their free-living form while maintaining the metabolic activity required for nitrogen fixation and nutrient exchange with the host plant.
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