2. The bacterium was studied by Dreyfus B ,J. L. Garica, and M. Gillis
Intoduction
Twenty stem- and root-nodulating
bacterial strains isolated from nodules of
Sesbania rostrata then they were
compared by numerical analysis of 221
phenotypic features with nine strains
which effectively nodulate only the roots
of this plant and with representative
strains from the genera Rhizobium and
Bradyrhizobium
The following conclusions were drawn:
(i) the Sesbania root-nodulating bacterial strains are genuine
rhizobia;
(ii) the Sesbania stem- and root-nodulating strains are quite
different from Rhizobium and Bradyrhizobium, and thus they
constitute a separate rRNA subbranch on the
Rhodopseudomonas palusferis
(iii) the closest relative of these organisms is Xanthobacter,
but they are phenotypically and genotypically different from
the later genus to deserve a separate generic rank.
Because the feature of free-living nitrogen fixation is quite
discriminative, a new genus, Azorhizobium, is proposed,
with one species, Azorhizobium caulinodans
4. Ecology
Its host range for effective
nodulation is very narrow:
although nodulation of
several Sesbania sp. has been
reported, nitrogen-fixing
nodules are formed only on
S. rostrata and S. punctata .
S.rostrata
S.punctata
5. Azhorhizobium caulinodans also induces nodules on
Phaseolus vulgaris and Leucaena leucocephala
Phaseolus vulgaris Leucaena leucocephala
6. Biology
Azorhizobium caulinodans is a member of
the α-proteobacteria,
A. caulinodans is a motile, nitrogen-fixing,
hydrogen-oxidizing, aerobic bacterium with
a preference for organic acids as carbon
source. This lifestyle is reflected in the
metabolic pathways and in clusters for
flagellum synthesis
A high number of genes are dedicated to
transport and regulation, indicating that a
wide range of substrates can be taken up,
but that the pathways are tightly regulated
to limit the metabolic burden.
The genome of A. caulinodansen codes
functions that might be involved in biofilm
formation, possibly facilitating the
interaction with a host.
7. Morphology
Colonies of A.caulinodans
It’s a Gram negative rod shaped bacteria
size
(0.5-0.6 x 1.5-2.5 um)
Mobility
Peritrichous flagella on solid media and
one lateral flagellum in liquid medium
Colonies
colonies are circular and have a creamy
colour
8. Growth parameters
It is obligate aerobe requires oxygen for its survival
Grow well on N2 rich media .if vitamins present even in
a nitrogen free medium,
only glucose is used as a substrae among the sugars.
Organic acids such as lactate, or succinate support
growth.
9. The tropical legume Sesbania rosatrta Is nodulated by Azorhizobium caulinodans
on both its stem and its root system.
Intercellular infection leads to the formation of infection pockets, which then give rise to
infection threads. Concomitantly with infection, cortical cells of the secondary roots
dedifferentiate, forming a meristem which has an "open-basket" configuration and which
surrounds the initial infection site Bacteria are released from the tips of infection threads
into plant cells via "infection droplets," each containing several bacteria. Initially,
nodule differentiation is comparable to that of indeterminate nodules, with the youngest
meristematic cells being located at the periphery and the nitrogen-fixing cells
being located at the nodule center. Because of the peculiar form of the meristem, Sesbania
root nodules develop uniformly around a central axis. Nitrogen fixation
is detected early as 3 days following inoculation, while the nodule meristem is still active.
Two weeks after inoculation, meristematic activity ceases, and nodules then show the
typical histology of determinate nodules. Thus, root nodule organogenesis in S. rostrate
appears to be intermediate between indeterminate and determinate types
Nodulation and Nitrogen fixation
10. Role as a Biofertilizer
Wheat
Inoculation with Azhorhizobium + Azospirillum biofertilizer
leads to considerable improvement in wheat grain and straw
yield as compared with their respective control.
The superiority related to Azorhizobium inoculation was clear
on grain yield followed by inoculum containing a mixture of
Azorhizobium and Azosperillum, although the use of
Azosperillum gave the lowest response, but it was still higher
than the uninoculated plants (control). Increasing levels of
nitrogen caused increase in the wheat grain and straw yields.
