Abstract
Rhizobial symbiosis genes are often carried on symbiotic islands or plasmids that can be transferred (horizontal transfer) between different bacterial species. Symbiosis genes involved in horizontal transfer have different phylogenies with respect to the core genome of their ‘host’. Here, the literature on legume–rhizobium symbioses in field soils was reviewed, and cases of phylogenetic incongruence between rhizobium core and symbiosis genes were collated. The occurrence and importance of horizontal transfer of rhizobial symbiosis genes within and between bacterial genera were assessed. Horizontal transfer of symbiosis genes between rhizobial strains is of common occurrence, is widespread geographically, is not restricted to specific rhizobial genera, and occurs within and between rhizobial genera. The transfer of symbiosis genes to bacteria adapted to local soil conditions can allow these bacteria to become rhizobial symbionts of previously incompatible legumes growing in these soils. This, in turn, will have consequences for the growth, life history, and biogeography of the legume species involved, which provides a critical ecological link connecting the horizontal transfer of symbiosis genes between rhizobial bacteria in the soil to the above-ground floral biodiversity and vegetation community structure.
Highlights
70% of the ca. 19,300 species in the Fabaceae (Leguminosae, the legume family) can fix atmospheric nitrogen (N2 ) via symbiotic bacteria in root nodules [1,2].Rhizobia reduce atmospheric N2 to ammonia (NH3 ) through the enzyme nitrogenase, and this NH3, as ammonium (NH4 + ), is transported to plant cells where it is assimilated into amino acids via the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway [3,4]
In 2000, Mesorhizobium strains, including WSM2073 and WSM2075 with 16S rRNA, dnaK and GS11 phylogenies different from strain WSM1271, were isolated from nodules of B. pelecinus grown in Western Australia and shown to nodulate the legume, the bacteria were largely ineffective with regard to N2 fixation [63,64]
Phylogenetic incongruence occurs between core and symbiosis genes for Azorhizobium, Bradyrhizobium, Burkholderia, Ensifer, Mesorhizobium, Methylobacterium, Microvirga, Neorhizobium, Ochrobactrum, Rhizobium, and Phyllobacterium associated with Papilionoideae legumes with indeterminate nodules excluding the Inverted Repeat-Lacking Clade (IRLC) (Table 3)
Summary
70% of the ca. 19,300 species in the Fabaceae (Leguminosae, the legume family) can fix atmospheric nitrogen (N2 ) via symbiotic bacteria (general term ‘rhizobia’) in root nodules [1,2]. The NodD protein triggers the transcription of a range of genes within the rhizobium, including those required to produce Nod factors, the signal molecules from the rhizobium which induce nodule morphogenesis in the legume [14] These genes include nodABC which encode the enzymes required for the synthesis of the core Nod factor structure of an N-acetyl glucosamine oligosaccharide backbone with a fatty acyl chain at the non-reducing end [12]. Rhizobia are released from the tips of these infection threads into membrane-bound structures within the legume cells, called symbiosomes, where they differentiate into their N2 -fixing form known as bacteroids in root nodules Bacteroids differ in their level of differentiation and viability, and nodules can be indeterminate or determinate in growth, depending on the legume host [2,17,18]. The occurrence and importance of horizontal transfer of rhizobial symbiosis genes within and between bacterial genera were assessed
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