Abstract:
Vachellia karroo (formerly Acacia karroo) is a wide-spread legume species indigenous
to southern Africa. Little is known regarding the identity or diversity of rhizobia that
associate with this plant in its native range in South Africa. The aims of this study were
therefore: (i) to gather a collection of rhizobia associated with V. karroo from a wide range
of geographic locations and biomes; (ii) to identify the isolates and infer their evolutionary
relationships with known rhizobia; (iii) to confirm their nodulation abilities by using
them in inoculation assays to induce nodules under glasshouse conditions. To achieve
these aims, soil samples were collected from 28 locations in seven biomes throughout
South Africa, which were then used to grow V. karroo seedlings under nitrogen-free
conditions. The resulting 88 bacterial isolates were identified to genus-level using 16S
rRNA sequence analysis and to putative species-level using recA-based phylogenetic
analyses. Our results showed that the rhizobial isolates represented members of
several genera of Alphaproteobacteria (Bradyrhizobium, Ensifer, Mesorhizobium, and
Rhizobium), as well as Paraburkholderia from the Betaproteobacteria. Our study
therefore greatly increases the known number of Paraburkholderia isolates which can
associate with this southern African mimosoid host. We also show for the first time that
members of this genus can associate with legumes, not only in the Fynbos biome, but
also in the Albany Thicket and Succulent Karoo biomes. Twenty-six putative species
were delineated among the 88 isolates, many of which appeared to be new to Science
with other likely being conspecific or closely related to E. alkalisoli, M. abyssinicae,
M. shonense, and P. tropica. We encountered only a single isolate of Bradyrhizobium,
which is in contrast to the dominant association of this genus with Australian Acacia.
V. karroo also associates with diverse genera in the Grassland biome where it is
quite invasive and involved in bush encroachment. Our findings therefore suggest that
V. karroo is a promiscuous host capable of forming effective nodules with both alphaand
beta-rhizobia, which could be a driving force behind the ecological success of this
tree species.
