Phaseolus vulgaris L. (common bean) is a legume indigenous to America with two major centres of diversity in Mesoamerican and Andean regions (Bitocchia et al., 2012) Currently this legume is cultivated worldwide and it can establish symbiosis with several species and symbiovars of genera Rhizobium and Sinorhizobium (Shamseldin and Velázquez, 2020). Species of this last genus have been found in P. vulgaris nodules in countries of Africa, Europe and America, such as Sinorhizobium fredii and Sinorhizobium meliloti in Tunisia (Mnasri et al., 2007) and Spain (Herrera-Cervera et al., 1999, Zurdo-Piñeiro et al., 2009) and Sinorhizobium americanum in Mexico and Tunisia (Verástegui-Valdés et al., 2014, Mnasri et al., 2012). The Spanish S. fredii strains were isolated in a location at the South of mainland Spain (Herrera-Cervera et al., 1999) and the S. meliloti ones were isolated in Lanzarote island (Canary Islands) in a location at the East of this island (Zurdo-Piñeiro et al., 2009). Therefore, the first aim of this work was to isolate and identify the strains nodulating P. vulgaris across Lanzarote in two coastal areas located at the Northwest and Southwest outside cultivated areas and where there is no data about the microsymbionts of this legume.

On the other hand, the species of genus Sinorhizobium nodulating P. vulgaris harboured different symbiovars from which the symbiovar mediterranense was firstly found in Tunisia (Mnasri et al., 2007). These strains were identifed as S. meliloti and S. fredii and were not able to nodulate the hosts firstly described for these species, Medicago sativa and Glycine max, respectively (Mnasri et al., 2007). In posterior works, the symbiovar mediterranense was found in P. vulgaris nodulating strains of S. meliloti in Canary Islands (Zurdo-Piñeiro et al., 2009) and Sinorhizobium americanum in Tunisia (Mnasri et al., 2012) and Mexico (Verástegui-Valdés et al., 2014). Unlike other Sinorhizobium symbiovars, the symbiovar mediterranense was defined on the basis on the nodC gene analysis, which showed that the strain of S. meliloti isolated in Tunisia was closely related to the strains of S. meliloti isolated in Canary Islands and one strain of S. fredii isolated in mainland Spain, whereas a second Tunisian strain of S. fredii was phylogenetically more distant (Mnasri et al., 2007, Zurdo-Piñeiro et al., 2009). Therefore, the second aim of this work was to compare the nodC gene sequences of strains isolated from P. vulgaris in Lanzarote with those of the remaining Sinorhizobium symbiovars.

Since for the strains used in the original description of some Sinorhizobium symbiovars only the nodA gene is available, these symbiovars are commonly missing in later studies where the nodC gene analysis was used to define new symbiovars. To date, eleven symbiovars have been described within the genus Sinorhizobium and two of them, aegeanense and fredii, share very closely related nodC gene sequences (Tampakaki et al., 2017). Many of the described symbiovars include the type strains of several Sinorhizobium species, however some type strains of other species have not been yet assigned to a symbiovar. Therefore, a third aim of this work was to revise the currently described Sinorhizobium symbiovars through the comparison of the nodA and nodC gene phylogenies in order to select the most resolutive phylogenetic marker and to propose a cut-off similarity value for the delineation of Sinorhizobium symbiovars. Also, we propose four novel symbiovars to accommodate the type strains of Sinorhizobium species still unasigned to a symbiovar.