Agriculture Reference
In-Depth Information
Further, most of the records for
Rhizobium
so far are from temperate areas. However,
this situation will certainly change as more studies are carried out on tropical areas
and harsh environments, such as dryland Africa. Also there is now great activity in
parts of China, including the dry mountainous plateau regions, that is yielding much
new information. For example, Kan et al. (2007) found that species of
Oxytropis
and
Medicago archiducis-nicolai
could be nodulated by
R. leguminosarum
. On the other hand,
in subtropical regions of China, Liu et al. (2007) found that
Trifolium pratense
and
T. repens
, which are normally nodulated by
R. leguminosarum
bv
trifolii
, could form
nodules with strains belonging to
Bradyrhizobium, Mesorhizobium
and
Sinorhizobium.
R. multihospitium
was so named because of the large number of species in Xinjiang
province, China, from which it was isolated (Han et al., 2008). So far it has only
been shown to nodulate effectively the two species given in Table 4.2, but Han et al.
(2008) note that the legumes from which the species was isolated grow in alkaline
soils with high salt concentrations, so conditions used in their experiments may not
have been optimal. Minor differences in growth conditions can make the difference
between formation of ineffective or effective nodules, as we have found when growing
Mimosa
species and other tropical legumes in sub-optimal light conditions (E.K. James,
personal communication).
R. cellulosilyticum
is included in Table 4.2, even though it
has only been shown to form ineffective nodules on
Medicago sativa
(Garcia-Fraile
et al., 2007), because of its genetic constitution. However, until it has been shown to
form effective nodules on a legume, its nodulation status should be regarded with
caution. There is a proposal to rename the widely studied
R. leguminosarum
,
R. pisi
(Ramırez-Bahena et al., 2008). The proliferation of taxa has been questioned by some
microbiologists who take a more evolutionary view of rhizobia. For example, Silva
et al. (2005) looked at the genetic structure of
R. gallicum
and related species, studying
both chromosomal and plasmid genes as well as their biogeography. They concluded
that
R. mongolense
and
R. yanglingense
belong to the evolutionary lineage of
R. gallicum
and should be reclassified as biovar
orientale
of that species. Plant taxonomists have
long been divided into 'lumpers' and 'splitters', and rhizobial taxonomists may be
going the same way!
4.1.2
Sinorhizobium
and
Ensifer
Sinorhizobium
and
Ensifer
are grouped together in Table 4.3. As with
Rhizobium
, most
of the hosts are papilionoid, but with a significant number of mimosoids. The model
legume
Medicago truncatula
can be nodulated by several sinorhizobia, but recent work
has shown that the species used in the model system,
Sinorhizobium meliloti
,isnot
very good at fixing nitrogen with this host, preferring to associate with
M. sativa
(see
above). The fast-growing, wide host range NGR 234 may be conspecific with
S. fredii
or
a separate species of
Sinorhizobium
. Either way it is unique in the number of genera and
species with which it can nodulate (Pueppke & Broughton, 1999).
S. adhaerens
is a soil
bacterium that can adhere to (hence the specific name) and lyse other soil bacteria. As
yet there is no really clear evidence that it can nodulate any species of legume (hence
the ?? in Table 4.3), but it has been suggested that it might acquire nodulation genes
from other rhizobia (Willems et al., 2003).
