No evidence for adaptation to local rhizobial mutualists in the legume Medicago lupulina

Local adaptation is a common but not ubiquitous feature of species interactions, and understanding the circumstances under which it evolves illuminates the factors that influence adaptive population divergence. Antagonistic species interactions dominate the local adaptation literature relative to mutualistic ones, preventing an overall assessment of adaptation within interspecific interactions. Here, we tested whether the legume Medicago lupulina is adapted to the locally abundant species of mutualistic nitrogen‐fixing rhizobial bacteria that vary in frequency across its eastern North American range. We reciprocally inoculated northern and southern M. lupulina genotypes with the northern (Ensifer medicae) or southern bacterium (E. meliloti) in a greenhouse experiment. Despite producing different numbers of root nodules (the structures in which the plants house the bacteria), neither northern nor southern plants produced more seeds, flowered earlier, or were more likely to flower when inoculated with their local rhizobia. We then used a pre‐existing dataset to perform a genome scan for loci that showed elevated differentiation between field‐collected plants that hosted different bacteria. None of the loci we identified belonged to the well‐characterized suite of legume–rhizobia symbiosis genes, suggesting that the rhizobia do not drive genetic divergence between M. lupulina populations. Our results demonstrate that symbiont local adaptation has not evolved in this mutualism despite large‐scale geographic variation in the identity of the interacting species.     

Context dependence in the coevolution of plant and rhizobial mutualists

Several mechanisms are expected to rapidly rid mutualisms of genetic variation in partner quality. Variation for mutualist quality, however, appears to be widespread. We used a model legume-rhizobium mutualism to test for evidence that context-dependent selection may maintain variation in partner quality. In a greenhouse experiment using 10 natural populations of Medicago truncatula and two strains of Sinorhizobium medicae, we detected significant genotype x genotype (G x G) interactions for plant fitness, indicating that the most beneficial rhizobium strain depends on the host genotype. In a second experiment using a subset of the plant populations used in the first experiment, we detected significant G x G interactions for both plant and rhizobium fitness. Moreover, the plant population with which rhizobium strains gained the greatest benefit depended on the nitrogen environment. Finally, we found that in a high nitrogen environment, all plant populations had lower fitness when inoculated with a 1:1 mixture of strains than with the worse single strain alone, suggesting that nitrogen shifts the exchange of benefits in favour of rhizobia. Our data suggest that genotype, nitrogen and biotic dependency might contribute to the maintenance of genetic variation in mutualist quality when coupled with spatial or temporal heterogeneity in the environment.     

Symbiosis specificity in the legume: rhizobial mutualism

Legume plants are able to engage in root nodule symbiosis with nitrogen-fixing soil bacteria, collectively called rhizobia. This mutualistic association is highly specific, such that each rhizobial species/strain interacts with only a specific group of legumes, and vice versa. Symbiosis specificity can occur at multiple phases of the interaction, ranging from initial bacterial attachment and infection to late nodule development associated with nitrogen fixation. Genetic control of symbiosis specificity is complex, involving fine-tuned signal communication between the symbiotic partners. Here we review our current understanding of the mechanisms used by the host and bacteria to choose their symbiotic partners, with a special focus on the role that the host immunity plays in controlling the specificity of the legume - rhizobial symbiosis.     

Acacia species and rhizobial interactions: Implications for restoration of native vegetation

Summary For successful restoration of native vegetation on nitrogen-depauperate soils, an understanding of nitrogen-fixing relationships between plant host species and their bacterial symbionts is critical. Each of three geographically restricted Acacia species ( A. fulva , A. nano-dealbata , and A. trachyphloia ) and three widely distributed species ( A. dealbata , A. implexa , and A. melanoxylon ) were inoculated with 20 different rhizobial ( Bradyrhizobium spp.) strains. The strains comprised two obtained from each of 10 different host species, including the six Acacia species listed above plus a further four species, A. cangaiensis , A. cincinnata , A. deanei , A. mearnsii . Neither restricted nor widely distributed species grew more effectively with their own strains than with strains isolated from other species. Thus, host species with restricted geographical ranges did not demonstrate greater specialization in their symbiotic associations with rhizobia than widespread species. Highly significant variation was observed between the strains obtained from each host species with respect to their ability to promote effective plant growth across all host species. In many cases, strains that were highly effective at promoting growth for one host species, were comparatively ineffective in combination with other host species. Strains thus exhibited host specificity in their ability to fix nitrogen. These findings indicate that choosing appropriate rhizobial strains for inoculation prior to revegetation is critical and should be made carefully for both restricted and widespread species.
Key words distribution, nitrogen fixation, revegetation, symbiotic interactions.     

Novel associations between rhizobial populations and legume species within the genera Lathyrus and O xytropis grown in the temperate region of China

Fifty rhizobial isolates of Lathyrus and Oxytropis collected from northern regions of China were studied in their genotypic characterization based upon analyses of ARDRA, 16S-23S IGS PCR-RFLP, TP-RAPD, MLEE, sequences of 16S rDNA gene and housekeeping genes of atpD, recA and glnII. The results demonstrated that most of the Lathyrus rhizobia belonged to Rhizobium and most of the Oxytropis rhizobia belonged to Sinorhizobium. A novel group of Rhizobium sp. I and S. meliloti were identified as the main microsymbionts respectively associated with Lathyrus and Oxytropis species in the collection area, which were new associations between rhizobia and the mentioned hosts. This study also provides new evidence for biogeography of rhizobia. Supported by the National Program for Basic S&T Platform Construction (Grant No. 2005DKA21201-1), the National Natural Science Foundation of China (Grant No. 30670001), and the National Basic Research Program of China (Grant No. 2006CB100206)     

Symbiotic effectiveness of indigenous arctic rhizobia on a temperate forage legume: Sainfoin (Onobrychis viciifolia)

Sainfoin (Onobrychis viciifolia), a temperate perennial forage legume, can be nodulated by rhizobia isolated from 3 arctic legume species:Astragalus alpinus, oxytropis maydelliana andOxytropis arctobia. Arctic rhizobia, which are adapted to growth at low temperatures, may be useful in improving symbiotic nitrogen fixation during cold phases of the growing season, if they are effective on a temperate legume. In this study, we report on the symbiotic effectiveness of arctic rhizobia on sainfoin, as appraised by the total shoot dry matter yield obtained from 2 harvests. Under N-free conditions, 5 arctic strains at the first harvest and 8 at the second harvest were as effective as temperate standard strains. In the presence of 30 mgl⁻¹ NO₃-N, 7 arctic strains gave significantly higher yields than temperate strains at the second harvest. These results indicate that effective arctic rhizobia have a potential for use as inoculants on sainfoin. Contribution no 325 of Agriculture Canada Research Station a Sainte-Foy.     

Unexpectedly diverse Mesorhizobium strains and Rhizobium leguminosarum nodulate native legume genera of New Zealand, while introduced legume weeds are nodulated by Bradyrhizobium species

The New Zealand native legume flora are represented by four genera, Sophora, Carmichaelia, Clianthus, and Montigena. The adventive flora of New Zealand contains several legume species introduced in the 19th century and now established as serious invasive weeds. Until now, nothing has been reported on the identification of the associated rhizobia of native or introduced legumes in New Zealand. The success of the introduced species may be due, at least in part, to the nature of their rhizobial symbioses. This study set out to address this issue by identifying rhizobial strains isolated from species of the four native legume genera and from the introduced weeds: Acacia spp. (wattles), Cytisus scoparius (broom), and Ulex europaeus (gorse). The identities of the isolates and their relationship to known rhizobia were established by comparative analysis of 16S ribosomal DNA, atpD, glnII, and recA gene sequences. Maximum-likelihood analysis of the resultant data partitioned the bacteria into three genera. Most isolates from native legumes aligned with the genus Mesorhizobium, either as members of named species or as putative novel species. The widespread distribution of strains from individual native legume genera across Mesorhizobium spp. contrasts with previous reports implying that bacterial species are specific to limited numbers of legume genera. In addition, four isolates were identified as Rhizobium leguminosarum. In contrast, all sequences from isolates from introduced weeds aligned with Bradyrhizobium species but formed clusters distinct from existing named species. These results show that native legume genera and these introduced legume genera do not have the same rhizobial populations.     

Alfalfa response to elevated atmospheric CO2 varies with the symbiotic rhizobial strain

Increased atmospheric CO₂ was shown to affect a variety of physiological processes in plants, including photosynthesis and growth with repercussions on crop yield and nutritive value. Perennial alfalfa (Medicago sativa L.) is a sustainable crop with a deep root system, living in symbiosis with rhizobium for nitrogen (N) fixation. The objective of the project was to determine the combined effects of elevated CO₂ and rhizobial strains on photosynthesis, growth, N fixation, and nutritive value of alfalfa, and on soil microflora. Alfalfa inoculated with two different strains of rhizobia (Sinorhizobium meliloti strains A2 and NRG34) was grown 2 months at day/night temperatures of 22/17°C under either 400 (near ambient) or 800 (elevated) μmol mol⁻¹ of CO₂. The photosynthetic response of alfalfa to elevated CO₂ differed according to the rhizobial strain. At the end of the experiment, elevated CO₂ stimulated photosynthetic rates by 50% in plants associated with A2 but there was no significant increase in plants nodulated with NRG34. Nitrogenase activity (+38%) and shoot growth (+60%) were stimulated under 800 μmol mol⁻¹ of CO₂ for alfalfa inoculated with both strains. Root dry weight was significantly higher at 800 μmol mol⁻¹ of CO₂ only with strain A2. Fibre concentration decreased in response to elevated atmospheric CO₂ in alfalfa inoculated with strain A2 resulting in plant material with greater nutritive value when inoculated with A2 compared to NRG34. In the soil, elevated CO₂ increased the proportion of fungi in the microbial community while decreasing Gram⁻ bacteria. For alfalfa inoculated with rhizobial strain A2, photosynthetic rates, nitrogenase activity, and growth were all stimulated by increased atmospheric CO₂ compared to less consistently positive responses to elevated CO₂ when inoculated with NRG34. Our results show that it is possible to identify rhizobial strains to improve plant performance under predicted future CO₂ concentrations with no negative effect on nutritive value. The Canadian Government’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

In vitro propagation of Clianthus formosus (Sturt's desert pea) an Australian native legume

Hypocotyl and cotyledon segments of Clianthus formosus were cultured on a modified deFossard medium M supplemented with cytokinins. 62% of cultures on medium with 20 μM BA produced callus which subsequently gave rise to shoots. 40% of shoots excised from callus produced roots after transfer to auxin-rich media (20 μM NAA or 10 μM IBA+10 μM NAA). Root production was enhanced following a 7-day dark treatment. 32% of nodes from mature plants produced multiple shoots on 2 μM BA+2 μM KIN. 30% of these shoots rooted on medium without hormones. 70% of rooted plantlets were successfully transferred to potting medium and glasshouse conditions. A period of cold treatment (10 days at 5°C) reduced vitrification from 68 to 22% of cultures.     

Role of lectins (and rhizobial exopolysaccharides) in legume nodulation.

The lectin recognition hypothesis proposes that plant lectins mediate specificity in the Rhizobium-legume symbiosis. Although the hypothesis was developed eight years before nod genes were identified in rhizobia and sixteen years before Nod factor was shown to be a major determinant of host specificity, experiments performed recently using transgenic lectin plants support its main tenets.     

Contrasting exotic Solenopsis invicta and native Forelius pruinosus ants as mutualists with Catalpa bignonioides, a native plant

Abstract. 1. The suitability of the red imported fire ant Solenopsis invicta Buren and a native ant Forelius pruinosus (Roger) as participants in a food-for-protection mutualism with a native nectaried tree Catalpa bignonioides Walter was compared.
2. The mean mortality of folivore larvae of Ceratomia catalpae Boisduval was similar for S. invicta and F. pruinosus although S. invicta attacked fewer caterpillar aggregations and was a devastating pupal predator. Solenopsis invicta also differed from the native ant in that it attacked the parasitoid Cotesia congregata Say, another plant mutualist, and visited extrafloral nectaries less frequently.
3. Habitats invaded by S. invicta are characterised by a scarcity of both herbivores and of beneficial insects that visit extrafloral nectaries. The plants do not require protection, and extrafloral nectaries are visited rarely. Although plants are defended incidentally by S. invicta , the insect-plant mutualism therein is greatly simplified or defunct.     

Genome conservation among three legume genera detected with DNA markers.

A set of 219 DNA clones derived from mungbean (Vigna radiata), cowpea (V. unguiculata), common bean (Phaseolus vulgaris), and soybean (Glycine max) were used to generate comparative linkage maps among mungbean, common bean, and soybean. The maps allowed an assessment of linkage conservation and collinearity among the three genomes. Mungbean and common bean, both of the subtribe Phaseolinae, exhibited a high degree of linkage conservation and preservation of marker order. Most linkage groups of mungbean consisted of only one or two linkage blocks from common bean (and vice versa). The situation was significantly different with soybean, a member of the subtribe Glycininae. Mungbean and common bean linkage groups were generally mosaics of short soybean linkage blocks, each only a few centimorgans in length. These results suggest that it would be fruitful to join maps of mungbean and common bean, while knowledge of conserved genomic blocks would be useful in increasing marker density in specific genomic regions for all three genera. These comparative maps may also contribute to enhanced understanding of legume evolution.     

Responses of the model legume Medicago truncatula to the rhizobial exopolysaccharide succinoglycan

Many species of rhizobial bacteria can invade their plant hosts and induce development of symbiotic nitrogen-fixing nodules only if they are able to produce an acidic exopolysaccharide (EPS) with certain structural and molecular weight characteristics.^1–3 Sinorhizobium meliloti that produces the functional form of the exopolysaccharide succinoglycan induces formation of invasion structures called infection threads in the root hair cells of its plant hosts alfalfa and Medicago truncatula. However, S. meliloti mutants that cannot produce succinoglycan are not able to induce infection thread formation, resulting in an early arrest of nodule development and in nitrogen starvation of the plant. Mounting evidence has suggested that succinoglycan acts as a signal to these host plants to permit the entry of S. meliloti. Now, our microarray screen and functional category analysis of differentially-expressed genes show that M. truncatula plants inoculated with wild type S. meliloti receive a signal to increase their translation capacity, alter their metabolic activity and prepare for invasion, while those inoculated with a succinoglycan-deficient mutant do not receive this signal, and also more strongly express plant defense genes.Key words: nitrogen fixation, nodule, succinoglycan, microarray, legume, rhizobial bacteria, Sinorhizobium meliloti, Medicago truncatula, infection thread, root hair     

Evaluating the effect of increased rates of rhizobial inoculation on grain legume productivity

Intrinsic promiscuity in cowpea and bean enables plants to nodulate with native rhizobia, though sometimes ineffective rhizobia may occupy nodules, resulting in poor response to inoculation. Field trials were conducted from 2014 to 2017 in Marondera, Natural Region II, Zimbabwe, to determine the effect of increasing inoculation rates on legume growth parameters, nitrogen uptake and grain productivity. Treatments included an un-inoculated control and inoculant rates of ×1 (standard), ×2, ×3, ×4, ×5, ×7 and ×10 for both cowpea (rhizobia - inoculant-strain-MAR 1510) and bean (rhizobia-inoculant-strain-CIAT 899). Biomass productivity ranged from 2.05 (×2) - 2.94 t ha^?1 (×4) and 1.10 (×10) – 1.95 t ha^?1 (×4) for cowpea and bean, respectively. Nitrogen uptake increased with increasing inoculation rates reaching up to 57.56 kg N ha^?1 for bean (×4) and 100.20 kg N ha^?1 for cowpea (×3). The uninoculated control was not significantly different from the standard, {(×1); 1 g inoculant 500 g seed^?1} treatment, for cowpea nodule weight and grain productivity. The highest cowpea and bean nodule weights were recorded from the ×3 and ×4 treatments, respectively, in the first season. Cowpea grain yield significantly varied across treatments, ranging between 0.63 and 1.55 t ha^?1 with the ×3 recording the highest yield. The “×4” treatment recorded the highest bean grain productivity reaching up to 0.88 t ha^?1. It can be concluded that, increasing rhizobia cells concentration per unit seed up to ×3 (cowpea) and ×4 (bean) improves response to inoculation and grain productivity suggesting a need to change product formulation or increase inoculation rate.     

Benefits of silicon-enhanced root nodulation in a model legume are contingent upon rhizobial efficacy

Plant and Soil - Root residues are an important factor influencing soil phosphorus (P) availability for crop uptake, but how the residues from different species combinations in sole cropping or...