Identity and taxonomy of indigenous South African rhizobia

Abstract

A phenotypic-molecular method (analysis of SDS-PAGE of whole-cell proteins) was used to investigate the taxonomic position of 346 new rhizobia isolates from 14 7 legumes, mainly indigenous species, from geographically separate localities in South Africa. No comprehensive study of the indigenous South African rhizobia has been done before, despite the richness of indigenous legume species and their possible practical importance in intensive as well as extensive, sustainable agriculture. Our isolates were compared with 45 authentic rhizobial cultures. The resulting protein profiles were analyzed by the GelCompar 3.0 programme and a dendrogram constructed. These organisms aggregated into 24 distinct clusters, 10 of which do not contain any reference strains. The results in this study show that the genus Rhizobium and related genera are very heterogeneous. A large group of the isolates clustered with members of the recognised genera Rhizobium, Bradyrhizobium and Sinorhizobium. However, we also found differences from published taxonomic data, as well as possibly new taxa. Whereas R. tropici and R. etli are reportedly nodulating only species of Phaseolus and Leucaena, and Phaseolus respectively, we found that R. tropici revealed a strong resemblance to isolates from Bolusanthus and Spartium, and R. etli to strains from Desmodium, Melolobium, Indigofera and two tree legumes Acacia melanoxylon and Chamaecrista stricta. Members isolated from the tree legumes formed an assemblage of heterogeneous isolates and a network of lines and groups of various relationships. It seems obvious from the results that any comprehensive taxonomic exploration will result in a proliferation of new taxa. Genetic studies are needed to further clarify the taxonomic and phylogenetic position of the new isolates and recognised genera of rhizobia. Rhizobium were isolated from hitherto unconfirmed nodulated legume genera Cassia, Bauhinia and Schizolobium. Indigenous counterparts have been found for each of the 9 rhizobia strains used for commercial legume inoculant production in South Africa.