Variability in symbiotic effectiveness of N2 fixing bacteria in Mimosa scabrella

Bracatinga (Mimosa scabrella Benth.) is a leguminous tree native to southern Brazil with high economic potential as a source of timber and biomass. Little is known about the diversity of bacteria in the root nodules of this species and how this diversity is associated with N2 fixation efficiency. We isolated 288 bacterial colonies from root nodules collected from M. scabrella in 61 locations across its native range. Strains were first characterized according to morphological traits and 16S rRNA sequencing and we then selected twenty-eight belonging to known nodulating genera for testing N2 fixation effectiveness. We found that M. scabrella associated with several N2 fixing and endophytic bacteria. Strains closely related to Burkholderia nodosa were the most common N2 fixing bacteria and were capable of providing all N required by the host. Cupriavidus was found nodulating M. scabrella, however, only forming non-fixing nodules. Endophytic bacteria detected in the nodules corresponded to the genera Pseudomonas, Pantoea, and, for the first time we found Paenibacillus, Brevibacillus, Serratia and Arthrobacter. Our study shows both high bacterial diversity and variability in N2 fixing effectiveness even between closely related Burkholderia strains, indicating that careful consideration of N2 fixing bacterial diversity is important to maximize the use of M. scabrella as a natural resource.