Novel symbiovars ingae, lysilomae and lysilomaefficiens in bradyrhizobia from tree-legume nodules

Rhizobia are capable of forming nitrogen-fixing nodules in legumes that may grow in nitrogen-deficient soils and support sustainable agriculture. Rhizobia have been the subject of numerous studies on diversity, ecology and evolution, molecular genetics of nodulation and nitrogen fixation and agricultural applications that have contributed largely to rhizobial knowledge. Among legumes, trees have the highest records for nitrogen fixation, have deep roots and may have the potential to mitigate climate change (Jensen et al., 2012), however their symbionts have not been as well studied as those from cash crops. Recently we described five novel genomospecies from native Mexican legume trees Inga vera and Lysiloma sp. which both belong to the Ingae tribe but thrive in distinct widely separated habitats (Hernández-Oaxaca et al., 2022).

Rhizobial symbiosis genes are part of the bacterial accessory genes and are located in plasmids and symbiosis islands (SIs) that may be transferred among bacteria (Andrews et al., 2018). The term symbiovar (Rogel et al., 2011) was proposed to account for rhizobial host specificity in relation to the symbiosis plasmids or islands. Notably, the same symbiovar may be found in different non-related species (Rogel et al., 2011). Otherwise, some genetically related species may contain the same symbiovar indicating that plasmids or symbiosis islands may be transferred easily among related species (Ramírez-Puebla et al., 2019). The symbiovar concept is parallel to the term pathovar in pathogens (Riley, 2020), and distinct symbiovars in the same rhizobial species determine their being either generalists or specialists (Rogel et al., 2014).

Recently, novel symbiovars have been identified and the number of symbiovars has increased in Bradyrhizobium from two in the original publication proposing symbiovars (Rogel et al., 2011) to 15 nowadays (Bejarano et al., 2014, Cobo Díaz et al., 2014, Delamuta et al., 2017, Guerrouj et al., 2013, Klepa et al., 2022, Msaddak et al., 2018, Ramírez-Bahena et al., 2016, Ramírez-Bahena et al., 2020, Simbine et al., 2021, Vinuesa et al., 2005) with many more to be described. Here we revisited the symbiosis islands from I. vera and Lysiloma sp. bradyrhizobial symbionts, recognized the novel symbiovars, ingae, lysilomae and lysilomaefficiens and discussed the possible roles of distinct symbiosis-island genes on host specificity.

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