Diversity of symbiotic rhizobia resident in Canadian soils

Abstract

. 2003. Diversity of symbiotic rhizobia resident in Canadian soils. Can. J. Soil Sci. 83: 311-319. The dependency of agriculture on nitrogen fertilizer inputs is associated with adverse effects on the environment and human health. The importance of biological nitrogen fixation by rhizobia in symbiotic association with legumes is underscored by its potential to reduce or replace chemical fertilizer inputs. This paper reviews research on the diversity of the symbiotic rhizobia resident in Canadian soils. Research has focussed on phenotypic and genotypic variation (e.g., nitrogen fixing efficacy, nodulating competitiveness, host range, adaptation to cool climate) within rhizobial species with the objective of selecting efficient strains for use in inoculants for legume crops. The genetic diversity of rhizobia resident in Canadian soils has been reported only for Sinorhizobium meliloti, Rhizobium leguminosarum and Mesorhizobium spp. There is a need for further studies on populations of other rhizobial species, particularly those associated with native legumes. Exploiting the diversity present in natural soil populations via selection and genetic manipulation should permit the development of superior strains for use in legume inoculants. Other rhizobial traits that may be exploited include plant growth-promoting activity and ability to degrade pollutants. 311-319. La dépendance de l'agriculture sur les engrais azotés est souvent associée à des répercussions néfastes au niveau de la santé humaine et de la qualité de l'environnement. Parallèlement, on sous-estime l'importance de la fixation de l'azote biologique par les rhizobiums en symbiose avec les légumineuses, phénomène qui pourrait nous aider à réduire l'emploi des engrais chimiques ou à remplacer ceux-ci. Cet article passe en revue la recherche sur les rhizobiums symbiotiques présents dans les sols canadiens. Les travaux se concentrent sur la variation du phénotype et du génotype (à savoir, efficacité de la fixation de l'azote, compétitivité au niveau des nodules, gamme d'hôtes, adaptation au climat frais) des diverses espèces, cela dans le but de sélec-tionner les meilleures souches, dont on se servira comme inoculant pour les légumineuses. Dans les sols canadiens, on ne s'est attardé qu'à la diversité génétique de Sinorhizobium meliloti, de Rhizobium leguminosarum et des espèces du genre Mesorhizobium. Il conviendrait d'étudier la population d'autres rhizobiums, en particulier ceux associés aux légumineuses indigènes. En tirant parti de la diversité observée dans les populations naturelles, par sélection ou par manipulation génétique, on devrait obtenir des souches de meilleure qualité pour l'inoculation. D'autres caractères des rhizobiums pourraient être mis à contribution, y compris l'accélération de la croissance des plantes et l'aptitude à dégrader les polluants.

Standing genetic variation in host preference for mutualist microbial symbionts

“…It is largely selfing and typically forms facultative symbiotic associations with rhizobia in the Ensifer genus ( previously Sinorhizobium [46]), especially Ensifer medicae and Ensifer meliloti in southern Ontario [47]. We collected plant lines from 11 sub-populations at the Koffler Scientific Reserve (www.ksr.utoronto.ca) from a range of habitats (old fields, recently disturbed fields, along paths and roads).…”

“…Medicago lupulina is a common exotic that grows in roadsides, fields and disturbed habitats in North America. It is largely selfing and typically forms facultative symbiotic associations with rhizobia in the Ensifer genus ( previously Sinorhizobium [46]), especially Ensifer medicae and Ensifer meliloti in southern Ontario [47].…”

“…However, under rhizobial densities exceeding available host infection sites, genotypes with the lowest investment will generally be favoured because of the costs of nodule production causing negative linear selection. As reported cell densities vary in field soil (more than 10-10 7 cells g 21 soil [47]), follow-up experiments measuring other aspects of symbiotic association (e.g. nodule size, energy storage in rhizobia cells) and manipulating symbiotic bacteria densities, such that rhizobia availability is lower than the expected total number of infection sites on the host, could be used to test this hypothesis.…”

“…As reported cell densities vary in field soil (more than 10-10 7 cells g 21 soil [47]), follow-up experiments measuring other aspects of symbiotic association (e.g. nodule size, energy storage in rhizobia cells) and manipulating symbiotic bacteria densities, such that rhizobia availability is lower than the expected total number of infection sites on the host, could be used to test this hypothesis.…”

Selection of rhizobial strains differing in their nodulation kinetics under low temperature in four temperate legume species

“…Nitrogenase activity was measured by the acetylene reduction test using pots enclosed in air-tight plastic bags (volume of 10 L) fitted with a rubber septum (Prévost & Bromfield, 2003). Plastic bags containing the potted plants were hermetically sealed and 10% of the volume (1 L) was replaced by acetylene (C 2 H 2 ) by injecting pure MS-grade acetylene through the septum using air-tight 60-mL syringes (14 injections were used for each bag).…”