Insights into the Phylogeny, Nodule Function, and Biogeographic Distribution of Microsymbionts Nodulating the Orphan Kersting's Groundnut Macrotyloma geocarpum (Harms) Marechal Baudet in African Soils

摘要

Kersting's groundnut Macrotyloma geocarpum (Harms) Marechal Baudet is a neglected indigenous African legume adapted to growth in N-deficient soils due to its ability to fix atmospheric N-2 via symbiosis with rhizobia. Despite its nutritional and medicinal uses, to date there is little information on the phylogeny and functional traits of its microsymbionts, aspects that are much needed for its conservation and improvement. This study explored the morphogenetic diversity, phylogenetic relationships, and N-2-fixing efficiency of Kersting's groundnut rhizobial isolates from contrasting environments in Ghana, South Africa, and Mozambique. BOX-PCR fingerprinting revealed high diversity among the rhizobial populations, which was influenced by geographic origin. Of the 164 isolates evaluated, 130 BOX-PCR types were identified at a 70 similarity coefficient, indicating that they were not clones. Soil pH and mineral concentrations were found to influence the distribution of bradyrhizobial populations in African soils. Phylogenetic analysis of 16S rRNA genes and multilocus sequence analysis of protein-coding genes (atpD, gyrB, and rpoB) and symbiotic genes (nifty and nodC) showed that Kersting's groundnut is primarily nodulated by members of the genus Bradyrhizobium, which are closely related to Bradyrhizobium vignae 7-2(T), Bradyrhizobium kavangense 14-3(T), Bradyrhizobium subterraneum 58-2-1(T), Bradyrhizobium pachyrhizi PAC48(T), the type strain of Bradyrhizobium elkanii, and novel groups of Bradyrhizobium species. The bradyrhizobial populations identified exhibited high N-2 fixation and induced greater nodulation, leaf chlorophyll concentration, and photosynthetic rates in their homologous host than did the 5 mM KNO3-fed plants and/or the commercial Bradyrhizobium sp. strain CB756, suggesting that they could be good candidates for inoculant formulations upon field testing.