The diversity of bradyrhizobium and mesorhizobium strains associated with root nodules of selected indigenous SA legumes

Abstract

This study provides clarity on the evolutionary history and distribution of the Genisteae tribe as well as the associated rhizobia, with a focus on the southern African Genisteae. The diversity and evolutionary histories of Bradyrhizobium and Mesorhizobium strains associated with selected indigenous South African Genisteae (specifically Argyrolobium, Melolobium, and Dichilus) was investigated, in addition to the evolutionary histories of their symbiotic loci (nodA and nifH). Sequence comparisons showed that most strains belong to Mesorhizobium (i.e., 40 strains), while 18 strains represented Bradyrhizobium isolated from nodules of A. rupestre, A. sericeum, A. harveyanum and A. robustum, while a strain of Rhizobium was obtained from a Melolobium sp. Two novel Bradyrhizobium species were described, in addition to several putatively novel Mesorhizobium strains. Taken together, our findings thus show that Genisteae indigenous to South Africa are more likely to be nodulated by Mesorhizobium than Bradyrhizobium, although it would appear that there are novel lineages of Bradyrhizobium capable of interacting with these legumes.