Sammendrag
Leguminous trees play an important role in agroforestry in Ethiopia, but studies of their rhizobial symbionts
are scarce. In earlier studies, we surveyed natural nodulation of native leguminous trees growing
in different agro-ecological zones in Southern Ethiopia, isolated 400 rhizobia, and characterized them
based on different phenotypic and genotypic methods. In the present study we characterized 18 strains
belonging to the genus Mesorhizobium, isolated from nodules of Acacia abyssinica, A. senegal, A. tortilis
and Sesbania sesban. Phylogenetic analysis of nearly full-length 16S rRNA gene grouped the test strains
into three distinct clades separated from all currently recognized Mesorhizobium species. Three divergent
strains formed separate branches while the other 15 strains formed three distinct groups, genospecies
I–III. Grouping of the isolates under study based on the house-keeping genes recA, gyrB, rpoB and gltA
were consistent and in agreement with that of 16S rRNA. Similarly phylogenetic relationships based
on the symbiosis-related genes nodC, nodA and nifH were generally similar to those shown by the core
genes, suggesting that these Acacia and Sesbania symbionts have a long history of separate evolution
within Mesorhizobium. Cross inoculation experiments demonstrated a large variation in the ability of
the test strains to elicit effective nodules. The Sesbania isolates, occupying a distinct clade in the nodC
phylogenetic tree, formed effective nodules only with this host legume. The study strongly suggests that
this collection of Mesorhizobium strains comprises several new species, and also indicates the role of the
symbiotic genes in determining the host range of these bacteria.
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