Ann M. Hirsch,2
Geoffrey N. Elliott,5
Paulina Estrada-de los Santos,7
Fabio Bueno dos Reis, Jr.,9
Janet I. Sprent,10
J. Peter W. Young,11 and
Euan K. James12
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.
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Accepted 31 July 2011.
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.
© 2011 The American Phytopathological Society