The Structure and Evolution of Beta-Rhizobial Symbiotic Genes Deduced from Their Complete Genomes

Rhizobia are soil bacteria able to establish a nitrogen-fixing symbiosis with leguminous plants. Most of them belong to the Alphaproteobacteria based on the sequences of the gene coding for 16S rRNA [1,2]. However, over the last 15 years studies have reported the presence of legume-nodulating bacteria in the genera Burkholderia and Cupriavidus (Burkholderiaceae) in the Betaproteobacteria [3-21]. Nodulation and nitrogen fixation capacity are very important factors in understanding the evolution of Rhizobia. Burkholderia and Cupriavidus strains were previously reported as exclusively non-symbiotic bacteria before these genera were discovered to contain Rhizobia, being isolated from soil, water, plants, rhizosphere and from infected humans [4,2225]. This extreme diversity in habitats and ecological lifestyles illustrates their remarkable capacity for adaptation [26,27]. Beta-rhizobial symbionts have different geographical distributions, with South America and South Africa as their main centers of diversity. Mimosanodulating Burkholderia symbionts have been isolated from native and invasive Mimosa species across Brazil, Uruguay, North America, Taiwan, China and Australia [5-10,28-32], as well as from related legumes in the Mimosoideae that are native and endemic to South America, particularly those in the “Piptadenia Group” [13,33,34,]. Mimosa-nodulating Cupriavidus symbionts were initially found in Taiwan, India, China and other parts of the tropics [4-6,28,29,35-39] and later isolated from the native ranges of their invasive hosts, M. pigra and M. pudica, in Costa Rica and Texas [9,39], and in recent years from various native Mimosoid hosts in French Guyana, Brazil and Uruguay [20,22,32,34]. Parallel studies on strains from South Africa revealed that Burkholderia symbionts were widespread in native and endemic papilionoid legumes in the tribes Podalyriae, Crotalariae, Phaseoleae and Indigoferae [12,33,40-44]. In this context, it should be noted that the Burkholderia strains originated from South Africa are in different species to those so far described for the Mimosoideae-nodulating strains from South America, and that they are largely incapable of nodulating each other’s hosts. The only species so far shown to be in common between the two continents is B. tuberum [45], which exists in two symbiovars, sv. mimosae in South America and sv. papilionoideae in South Africa [32]. Interestingly, nodulating strains isolated from the invasive South African legume, Dipogon lignosus (Phaseoleae) in New *Corresponding author: XiaoYun Liu, Key Laboratory of Microbial Diversity Research and Application of Hebei Province, College of Life Sciences, Hebei University, Baoding 100072, PR China, Tel: 86-03125079696; Fax: 8603125079364; E-mail: [email protected]