Trends in Rhizobial Evolution and Some Taxonomic Remarks
Bacteria that establish nitrogen-fixing symbiosis in specialized plant structures belong to only three of over 100 bacterial phyla. Among these, rhizobial symbioses are the best known and nodulation genes (nod) have been described in many species. nodA ph
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Trends in Rhizobial Evolution and Some Taxonomic Remarks Julio C. Martı´nez-Romero, Ernesto Ormen˜o-Orrillo, Marco A. Rogel, Aline Lo´pez-Lo´pez, and Esperanza Martı´nez-Romero
Abstract Bacteria that establish nitrogen-fixing symbiosis in specialized plant structures belong to only three of over 100 bacterial phyla. Among these, rhizobial symbioses are the best known and nodulation genes (nod) have been described in many species. nodA phylogenies revealed a larger diversity in Bradyrhizobium than in other genera and suggest that bradyrhizobial nod genes are the oldest in agreement to the proposal that nod genes evolved in Bradyrhizobium (Plant Soil 161:11–20, 1994). In many cases, rhizobial symbiotic and housekeeping genes have different evolutionary histories in relation to the lateral transfer of symbiotic genes among bacteria. Misclassified Rhizobium strains were identified, to properly identify rhizobial species we propose the use of fragments of the rpoB and dnaK genes, which according to probability analyses reflect the behavior of whole genes. With these analyses several rhizobial species related to Agrobacterium tumefaciens may be reclassified to a genus other than Rhizobium.
18.1
Introduction
Legume plants are widespread and diverse with a large number of species; they profit from symbiosis with nitrogen-fixing bacteria (collectively designated as rhizobia and comprising different, not closely related genera, such as Bradyrhizobium, Mesorhizobium, Azorhizobium, Sinorhizobium, Rhizobium, and others) that induce the formation of nodules on roots and rarely on stems and provide nitrogen that allows the plants to grow in nitrogen poor soils. Rhizobia are used as inoculants in agriculture, a practice that has been in use for over a hundred years, substituting fertilizers and saving millions of dollars in some cases (Hungria et al. 2000, 2005). J.C. Martı´nez-Romero, E. Ormen˜o-Orrillo, M.A. Rogel, A. Lo´pez-Lo´pez, and E. Martı´nezRomero Centro de Ciencias Geno´micas, UNAM, Av. Universidad, Cuernavaca, Morelos 62210, Me´xico e-mail: [email protected]
P. Pontarotti (ed.), Evolutionary Biology – Concepts, Molecular and Morphological Evolution, DOI 10.1007/978-3-642-12340-5_18, # Springer-Verlag Berlin Heidelberg 2010
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Rhizobial evolution and diversity (reviewed in Terefework et al. 2000; Wang and Martı´nez-Romero 2000; Sprent 2001; Sessitsch et al. 2002; Provorov and Vorobyov 2008; Martinez-Romero 2009) and molecular mechanisms mediating their interaction with legume hosts (Barnett and Fisher 2006; Jones et al. 2007) have been studied for a small proportion of legume-rhizobial symbioses (Lo´pez-Lo´pez et al. 2010). The coevolution of Rhizobium and legumes in symbiosis has been critically analyzed (Sprent 1997; Martinez-Romero 2009).
18.2
Nitrogen-Fixing Symbioses with Plants
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