Both Alpha- and Beta-Rhizobia Occupy the Root Nodules of Vachellia karroo in South Africa

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

Bacterial endophytes diversity of tree legumes from Argentina

“…Thus, this diversity may contribute to the ecological success of Vachellia sp. as pioneer species, as previously noted, for example, for V. Karoo (70) .…”

“…Vachellia is a genus that includes promiscuous species that might be nodulated by several species of rhizobia (70) . Our results confirmed this since isolate ACS41 was clustered with Sinorhizobim sp.…”

“…(77) , among others. Beukes and others (70) studied Vachellia karroo root nodules that contained the genera Bradyrhizobium, Ensifer (32 isolates), Mesorhizobium (27 isolates), and Rhizobium, as well as Betaproteobacteria; and more recently, Pereira-Gómez and others (22) reported 8 V. caven isolates, affiliated to the genus Mesorhizobium. Thus, this diversity may contribute to the ecological success of Vachellia sp.…”

“…and Senegalia sp. ), which also are predominantly nodulated by isolates of Mesorhizobium, Rhizobium and Ensifer (70) . Also, Cordero and others (14) found that V. macracantha, a legume tree distributed across neotropical regions, spreading from the Southern United States to Northern Argentina, was nodulated by Ensifer numidicus, Ensifer kummerowiae, Ensifer fredii, Ensifer americanus and Ensifer saheli.…”

Revegetation of urban green space rewilds soil microbiotas with implications for human health and urban design

“…Moreover, the higher abundance of the aforementioned nitrifiers in the more floristically diverse green spaces potentially led to more active nitrification, theoretically leading to the observed lower pH and ammonium levels in those soils. Furthermore, the nitrogen‐fixers Mesorhizobium and Ensifer (Dhaoui et al ; Beukes et al ) that were more abundant in lawns and vacant lots were correlated with ammonium, suggesting that ammonification, as well as fertilization, was leading to the higher ammonium and pH levels in the lawn soils.…”