Soil characteristics drive contrasting patterns of association between symbiotic rhizobia of endemic and widespread Mimosa species in Brazil

Neotropical Beta-rhizobia have a particular affinity to the large legume (Fabaceae) genus Mimosa and some of its relatives in the tribe Mimosae of the Caesalpinioideae subfamily. However, little is still known about the ecology of this interaction, especially the relationship between the rhizobia of “widespread” pan-tropical Mimosa species like M. pudica and the rhizobia that nodulate endemic Mimosa species that are very restricted in their habitats. The objective of this study was to examine the microsymbionts of Mimosa spp. and other mimosoids in climates ranging from tropical to subtropical, humid to semi-arid, with varied soil characteristics and altitudes, with the aim of testing the hypothesis that widespread species have more cosmopolitan symbiont preferences than endemic ones. Nodules were sampled from >30 Mimosa spp. and related taxa in 13 Brazilian states covering all five national regions; many of the species were endemics or biome-restricted, but particular attention was also paid to sample nodules from the widespread species M. pudica at all locations. The Mimosa symbionts comprised 21 potential 16S rRNA and recA groups at the species level, with 17 belonging to the genus Paraburkholderia, including four lineages that may represent new species. The remaining genotypes consisted of 14 strains in two lineages of Cupriavidus that were mainly isolated from M. pudica growing at low altitudes, and a single lineage of Rhizobium also from M. pudica. In addition, a strain of Trinickia symbiotica was isolated from M. misera. It is concluded that diverse genotypes of Paraburkholderia dominate as symbionts of Mimosa in the acidic soils of its main center of radiation in Central Brazil but that Cupriavidus and Rhizobium comprise a significant minority of symbionts of widespread Mimosa spp., especially M. pudica, in lowland or disturbed areas with less acidic soils. Mimosa symbiont selection is thus driven either by edapho-climatic characteristics for widespread species and/or by co-evolution of the symbiotic partners for endemic species.