B. Brunel et al., COMPARISON OF NUCLEOTIDE DIVERSITY AND SYMBIOTIC PROPERTIES OF RHIZOBIUM-MELILOTI POPULATIONS FROM ANNUAL MEDICAGO SPECIES, FEMS microbiology, ecology, 19(2), 1996, pp. 71-82
Forty-three isolates of Rhizobium meliloti were trapped from soil with
five annual species of Medicago (M. polymorpha, M. runcatula, M. rigi
dula, M. orbicularis and M. minima) and one perennial species of Medic
ago (M. sativa). The annual species were growing naturally near the so
il sampling site, and the commonly studied perennial species was used
for comparison. Each R. meliloti was characterized by PCR-RFLP methods
applied to two DNA regions nested between 16S rRNA and 23S rRNA genes
and between nifD and nifK genes. They fell into two highly divergent
groups (groups I and II), separated at a genetic distance of 0.024 by
rDNA-amplified pattern analysis (profiles R1 and R2) and at 0.029 by n
if-amplified pattern analysis (profiles N1-N2 and N3). These two group
s were consistent with some cross-nodulation and fixation results: rhi
zobia with the R1 genetic background elicited rudimentary nodules and
could not fix nitrogen on M. polymorpha, while they were able to nodul
ate the five other species of Medicago. In contrast, rhizobia with an
R2 profile were highly effective on M. polymorpha and poorly nodulated
M. rigidula species, but were able to nodulate efficiently the other
species. The striking phenotypic traits on M. polymorpha were also sha
red by reference strains: strains genetically closed to R2 type trigge
red typical and efficient nodules on M. polymorpha while those close t
o R1 type elicited rudimentary and non-efficient ones. Our results sug
gest that the presence of R. meliloti with R2 genetic backgrounds coul
d be favoured by the distribution of M. polymorpha species.