Agriculture Reference
In-Depth Information
bradyrhizobia are reported in Table 4.3 that nodulate the related genera
Medicago
,and
Melilotus
.
B. denitrificans
was originally called
Blastobacter denitrificans
. There are still
many difficulties with this genus and several species are loosely defined as, for exam-
ple,
Bradyrhizobium
sp. (
Lupinus
). This is a particularly interesting species as it comes
from Mediterranean areas, where bradyrhizobia are uncommon, and is both acid and
aluminium tolerant (Howieson et al., 1998). There are also a number of photosynthetic
bradyrhizobia, not yet categorised at the specific level, but with numerous interesting
features. Some of them lack the common nod genes
nodABC
, essential for root hair
infection; these can nodulate some species of
Aeschynomene
, by a crack entry process
(Giraud et al., 2007). The genus
Aeschynomene
has at least 180 species of which 41 are
known to nodulate (Table 1.9). Most of these have been further divided according to
whether or not they form stem nodules and, more recently, whether the latter nodulate
with photosynthetic bradyrhizobia. The three resulting groups are I, those that do not
form stem nodules, their root nodules being initiated by strains of the so-called cow-
pea miscellany of bradyrhizobia; II, stem-nodulating species that can nodulate with
either photosynthetic or non-photosynthetic bradyrhizobia; and III, those nodulated
exclusively by photosynthetic bradyrhizobia. (Giraud & Fleischman, 2004). The stem-
nodulating species have been further sub-divided according to conditions under which
nodules are formed (Boivin et al., 1997; see also Section 5.5.1). In evolutionary terms,
there is the possibility that the primitive state of bradyrhizobia was photosynthetic and
root-nodulating forms have lost this ability. The ability to photosynthesise could be a
particular advantage in the free-living state of the bacteria, when they are found on the
surfaces of leaves and stems. A further interesting feature of at least one of these strains,
Bradyrhizobium
sp. ORS278, is that it appears to have acquired a bacteriophytochrome
gene by lateral transfer, probably from a cyanobacterium (Jaubert et al., 2007). With
this suite of light-sensing genes, strain ORS278 seems well equipped for both free and
symbiotic life. In addition to the defined species there are many other bradyrhizobia
reported to nodulate all three sub-families of legumes (see examples in Chapter 1).
4.1.5
Azorhizobium
and
Devosia
Both
Azorhizobium
and
Devosia
are in family Hyphomicrobiaceae.
A. caulinodans
was
first described in 1988 and aroused great interest for a variety of reasons, one of which
was its ability to fix nitrogen and grow on the products ex planta. It is also the symbiont
of
Sesbania rostrata
(see Chapters 1, 3 and 5), which has been the subject of a great deal
of research. As mentioned earlier, although it forms nodules on the stem of its host,
these are not stemnodules in the strict sense as they are plumbed into adventitious root
initials. More recently a second species,
A. doebereinerae
, has been described, this time
from the root nodules of of
Sesbania virgata
. This species of
Azorhizobium
has only low
nitrogenase activity ex planta (Moreira et al., (2006). Both species show a high degree of
specificity for their hosts. The genome of
A. caulinodans
has now been sequenced. It is
closely related to species of
Xanthobacter
that can fix nitrogen in the free-living state, and
it was suggested that the capacity for
A. caulinodans
to nodulate was probably acquired
by horizontal transfer of a symbiotic island, from an as-yet-unknown ancestor (Lee
et al., 2008).
Devosia
has two species, but only
D. neptuniae
,from
Neptunia natans
,an
aquaticmimosoid legume from India, has been shown to fix nitrogen (Rivas et al., 2003).
