Abstract
Most species in the Leguminosae (legume family) can fix atmospheric nitrogen (N2) via symbiotic bacteria (rhizobia) in root nodules. Here, the literature on legume-rhizobia symbioses in field soils was reviewed and genotypically characterised rhizobia related to the taxonomy of the legumes from which they were isolated. The Leguminosae was divided into three sub-families, the Caesalpinioideae, Mimosoideae and Papilionoideae. Bradyrhizobium spp. were the exclusive rhizobial symbionts of species in the Caesalpinioideae, but data are limited. Generally, a range of rhizobia genera nodulated legume species across the two Mimosoideae tribes Ingeae and Mimoseae, but Mimosa spp. show specificity towards Burkholderia in central and southern Brazil, Rhizobium/Ensifer in central Mexico and Cupriavidus in southern Uruguay. These specific symbioses are likely to be at least in part related to the relative occurrence of the potential symbionts in soils of the different regions. Generally, Papilionoideae species were promiscuous in relation to rhizobial symbionts, but specificity for rhizobial genus appears to hold at the tribe level for the Fabeae (Rhizobium), the genus level for Cytisus (Bradyrhizobium), Lupinus (Bradyrhizobium) and the New Zealand native Sophora spp. (Mesorhizobium) and species level for Cicer arietinum (Mesorhizobium), Listia bainesii (Methylobacterium) and Listia angolensis (Microvirga). Specificity for rhizobial species/symbiovar appears to hold for Galega officinalis (Neorhizobium galegeae sv. officinalis), Galega orientalis (Neorhizobium galegeae sv. orientalis), Hedysarum coronarium (Rhizobium sullae), Medicago laciniata (Ensifer meliloti sv. medicaginis), Medicago rigiduloides (Ensifer meliloti sv. rigiduloides) and Trifolium ambiguum (Rhizobium leguminosarum sv. trifolii). Lateral gene transfer of specific symbiosis genes within rhizobial genera is an important mechanism allowing legumes to form symbioses with rhizobia adapted to particular soils. Strain-specific legume rhizobia symbioses can develop in particular habitats.
Highlights
The Leguminosae (Fabaceae, the legume family) is comprised of ca. 19,300 species within 750 genera that occur as herbs, shrubs, vines or trees in mainly terrestrial habitats and are components of most of the world’s vegetation types [1,2,3]
These bacterial species had similar nodA gene sequences to Mesorhizobium isolated from the same plants, and it was concluded that the lateral gene transfer of these genes had occurred from the Mesorhizobium
Articles were collected by searching the Institute for Scientic Information (ISI) Web of Science using each legume genus partnered with each of the rhizobia, Bradyrhizobium, Burkholderia, Cupriavidus, Ensifer, Mesorhizobium, Rhizobium and Sinorhizobium as the keywords
Summary
The Leguminosae (Fabaceae, the legume family) is comprised of ca. 19,300 species within 750 genera that occur as herbs, shrubs, vines or trees in mainly terrestrial habitats and are components of most of the world’s vegetation types [1,2,3]. Bacterial species from a range of genera in the α-proteobacteria (most commonly Bradyrhizobium, Ensifer (Sinorhizobium), Mesorhizobium and Rhizobium) and two genera in the β-proteobacteria (Burkholderia (Paraburkholderia) and Cupriavidus) can form functional (N2 fixing) nodules on specific legumes (Tables 1–4). For Lotus corniculatus, Geobacillus (phylum Firmicutes), Paenibacillus (Firmicutes) and Rhodococcus (Actinobacteria) were for the first time reported as rhizobial symbionts [39] These bacterial species had similar nodA gene sequences to Mesorhizobium isolated from the same plants, and it was concluded that the lateral gene transfer of these genes had occurred from the Mesorhizobium. Macroptilium purpureum and the grain legumes Phaseolus vulgaris and Vigna unguiculata in the tribe Phaseoleae are nodulated by rhizobia from different genera across the α- and β-proteobacteria [264,283,300]. The objectives of the work were to collate data on legume rhizobia symbioses and assess to what extent legume specificity for rhizobial symbionts is related to legume taxonomy
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