Novel genera of root nodule bacteria demonstrate the microsymbiont diversity of the legume/rhizobia symbiosis

Abstract

Root nodule bacteria isolated from native legumes in various biogeographical areas have demonstrated that rhizobia are more phylogenetically diverse than originally supposed. We present here an overview of our studies on novel species, isolated from nodules of legume hosts in Australia and Africa, which are affiliated to Burkholderia, Methylobacterium and Microvirga. The microsymbionts’ physiological adaptations to their environment, host specificity and phylogeny of nodulation and nitrogen fixation genes were examined, along with the modes of plant infection and nodule formation. Important findings include the apparent adaptation of Burkholderia spp. to infertile soils of wide pH range, the confirmation of specificity in the non-root hair-mediated Lotononis/Methylobacterium symbiosis and the potential of legume nodule morphology as a taxonomic aid. These species’ inclusion in the Genomic Encyclopedia for Bacteria and Archaea sequencing project will aid elucidation of the diverse rhizobial genomic architecture that underlies symbiotic ability and specificity