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 alpha- and beta-rhizobia, which could be a driving force behind the ecological success of this tree species.
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