Abstract
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 N2 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 N2-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, glnII, gyrB, and rpoB) and symbiotic genes (nifH and nodC) showed that Kersting's groundnut is primarily nodulated by members of the genus Bradyrhizobium, which are closely related to Bradyrhizobium vignae 7-2T, Bradyrhizobium kavangense 14-3T, Bradyrhizobium subterraneum 58-2-1T, Bradyrhizobium pachyrhizi PAC48T, the type strain of Bradyrhizobium elkanii, and novel groups of Bradyrhizobium species. The bradyrhizobial populations identified exhibited high N2 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.IMPORTANCE Rhizobia play important roles in agroecosystems, where they contribute to improving overall soil health through their symbiotic relationship with legumes. This study explored the microsymbionts nodulating Kersting's groundnut, a neglected orphan legume. The results revealed the presence of different bradyrhizobial populations with high N2-fixing efficiencies as the dominant symbionts of this legume across diverse agroecologies in Africa. Our findings represent a useful contribution to the literature in terms of the community of microsymbionts nodulating a neglected cultivated legume and its potential for elevation as a major food crop. The presence of potentially novel bradyrhizobial symbionts of Kersting's groundnut found in this study offers an opportunity for future studies to properly describe, characterize, and delineate these isolates functionally and phylogenetically for use in inoculant production to enhance food/nutritional security.
Highlights
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 N2 via symbiosis with rhizobia
With the exception of isolates TUTMGSA172, TUTMGSA179, and TUTMGSA182 from Klipplaatdrift, South Africa, which were extremely slow in growth (20 to 26 days to first colony appearance), the remaining isolates (98%) were visible on yeast mannitol agar (YMA) plates between 4 and 12 days of incubation at 28°C
The identification of diverse rhizobial populations in soils offers an opportunity for the selection of efficient strains for inoculant formulation [13]
Summary
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 N2 via symbiosis with rhizobia. Strain CB756 under glasshouse conditions [8], there is currently no information on its specific microsymbionts in Africa, its continent of origin This is despite the fact that studies of rhizobial symbionts of underutilized legumes, such as Kersting’s groundnut, in previously unexplored environments could lead to the identification of compatible and effective rhizobial strains that can support the species’ survival in such locations. The list of N2-fixing rhizobia responsible for legume nodulation consists of over 100 species from 14 bacterial genera originating from diverse legumes and contrasting environments across different continents [19] These sets of information have contributed significantly to the current progress in tapping the benefits of the legumerhizobium symbiosis for crop production, especially through the use of inoculant formulations with superior N2-fixing rhizobial strains [20, 21]. Photosynthetic rates and shoot biomass accumulation are often used as measures of N2-fixing efficiency in purely symbiotic systems [14]
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