Legume-Nodulating Betaproteobacteria: Diversity, Host Range, and Future Prospects

Abstract

Rhizobia form specialized nodules on the roots of legumes (family Fabaceae) and fix nitrogen in exchange for carbon from the host plant. Although the majority of legumes form symbioses with members of genus Rhizobium and its relatives in class Alphaproteobacteria, some legumes, such as those in the large genus Mimosa, are nodulated predominantly by betaproteobacteria in the genera Burkholderia and Cupriavidus. The principal centers of diversity of these bacteria are in central Brazil and South Africa. Molecular phylogenetic studies have shown that betaproteobacteria have existed as legume symbionts for approximately 50 million years, and that, although they have a common origin, the symbiosis genes in both subclasses have evolved separately since then. Additionally, some species of genus Burkholderia, such as B. phymatum, are highly promiscuous, effectively nodulating several important legumes, including common bean (Phaseolus vulgaris). In contrast to genus Burkholderia, only one species of genus Cupriavidus (C. taiwanensis) has so far been shown to nodulate legumes. The recent availability of the genome sequences of C. taiwanensis, B. phymatum, and B. tuberum has paved the way for a more detailed analysis of the evolutionary and mechanistic differences between nodulating strains of alpha- and betaproteobacteria. Initial analyses of genome sequences have suggested that plant-associated Burkholderia spp. have lower G+C contents than Burkholderia spp. that are opportunistic human pathogens, thus supporting previous suggestions that the plant- and human-associated groups of Burkholderia actually belong in separate genera.

Novel Burkholderia bacteria isolated from Lebeckia ambigua – A perennial suffrutescent legume of the fynbos

“…If the ability to grow on acid agar reflects acid soil tolerance, then these new species of Burkholderia are amongst the most acid tolerant nodule bacteria so far described (Table 2). Nodulating Burkholderia strains have previously been noted as micro-organisms adapted to acid and infertile soils (Garau et al, 2009;Gyaneshwar et al, 2011). However, growth on acidified media is not always indicative of success in acid soil, and edaphic evaluation must be conducted in situ (Howieson et al, 1988).…”

Biological nitrogen fixation: rates, patterns and ecological controls in terrestrial ecosystems

“…Even the bestknown system, the nodulated legume/rhizobial symbiosis, has been found to be much more diverse than previously understood-with new discoveries ranging from additional microbial partners (e.g. Burkholderia) that nodulate legumes and fix N 2 effectively [15] to a diversity of pathways by which nodules themselves are initiated and infected [16]. Our expanded ability to identify organisms capable of BNF has outpaced increases in our understanding of how BNF is regulated in these different organisms.…”

“…A number of studies have used 15 N natural abundance to determine whether late-successional legumes actively fix N-an approach made feasible by the fact that non-fixing plants in many tropical forests are substantially enriched in 15 N as a consequence of their open N cycle [64]. In most sites, 15 N natural abundance studies suggest that only a small fraction of canopy legumes are actively fixing N-even though many of them belong to genera that are known to nodulate and fix N under appropriate conditions [64][65][66]. An independent approach showed that very little symbiotic BNF was needed to balance N losses in an Amazonian forest [67].…”

Transfer of eleven species of the genus Burkholderia to the genus Paraburkholderia and proposal of Caballeronia gen. nov. to accommodate twelve species of the genera Burkholderia and Paraburkholderia

“…planned to describe the second group as a new genus, and even the genus name, Caballeronia, was proposed (Gyaneshwar et al, 2011). But eventually the authors of this multilocus sequence analysis study only declared in their publication that these two large groups might represent different genera and indicated that there were two other distinct lineages within the genus tree, consisting of one (Burkholderia andropogonis) and two (Burkholderia rhizoxinica and Burkholderia endofungorum) species, respectively, which might be considered as separate genera as well (Estrada-de los Santos et al, 2013).…”

“…Evolutionary distances were calculated according to Kimura's two-parameter model (Kimura, 1980), and the determination of the robustness of the tree by bootstrap analysis was based on 1000 random replicates (Felsenstein, 1985). Gyaneshwar et al (2011) mentioned an essential difference in the DNA G+C contents between species from the first and second Burkholderia groups. Expanded analysis of the DNA G+C values of species of the genus Burkholderia confirmed that the DNA G+C content of the first group (67.1 ±1.0 mol%) is higher than that of the second group (62.9 ±1.3 mol%), and the independent lineages of B. andropogonis (59.0 mol%) and B. rhizoxinica/B.…”