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

¹Department of Biological Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53201 U.S.A.; ²Department of Molecular, Cell, and Developmental Biology and Molecular Biology Institute, University of California, Los Angeles, CA, 90095, U.S.A.; ³IRD, UMR LSTM-Laboratoire des Symbioses Tropicales et Méditerranéennes, F-34398 Montpellier, France; ⁴Department of Seafood Science, National Kaohsiung Marine University, Kaohsiung City 811, Taiwan. ⁵The James Hutton Institute, Craigiebuckler, Aberdeen, AB15 8QH, U.K.; ⁶Génétique et Microbiologie, UMR UHP-INRA 1128, IFR 110 EFABA, Université de Lorraine, Faculté des Sciences et Technologies, BP 239, 54506, Vandoeuvre-lès-Nancy; ⁷Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Ap Postal 565-A, Cuernavaca, Morelos, México; ⁸Depto. de Ciências Agrárias e Ambientais, Universidade Estadual de Santa Cruz, km 16, Ilhéus 45662-900 BA, Brazil; ⁹Embrapa Cerrados, Planaltina, 73301-970, DF, Brazil; ¹⁰Division of Plant Sciences, University of Dundee at JHI, Invergowrie, Dundee DD2 5DA, U.K.; ¹¹Department of Biology, University of York, York YO10 5DD, U.K.; ¹²The James Hutton Institute, Invergowrie, Dundee DD2 5DA, U.K.

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.