Martinus Beijerinck (1851–1931) was one of the great general
made fundamental contributions to microbial ecology and
other fields. He isolated the aerobic nitrogen-fixing bacterium
Bacillus radicicola a root nodule bacterium also capable of
fixing nitrogen (later named Rhizobium)
Rhizobium can be
found in Root
nodules of legumes.
Legumes are herbaceous woody
plants that produce seeds in
pods; examples of legumes include
peas, beans, alfalfa, vetches,
clovers, lupines, trefoils, locust,
and mimosa. (fabaceae)
It is difficult to isolate rhizobium
from the soil..
Rhizobium are predominantly
aerobic chemoorganotrophs and
grow well in the presence of
oxygen and utilize a wide range of
relatively simple carbohydrates
and amino compounds.
The rhizobia doesn’t
grow wellwell on the
peptone media used
Yeast mannitol (YM) is
the most generally
suitable for their growth.
Optimal growth of most
strains occurs at a
temperature range of 25-
30° C and at a pH of 6.0-
Colonies are white pigmented.
Colonies are circular, convex, semi-
translucent, raised and mucilaginous.
Despite their usual aerobic
metabolism, many strains are able
to grow well under
microaerophillic conditions at
oxygen tensions of less than 0.1
Mobile by a single polar flagellum
or two to six peritrichous
•Rhizobium is surrounded by
a slimy capsule made of
protects it from drying out.
And also helps the
bacterium stick to root
hairs during various stages
of its life cycle.
•Rhizobium contains nitrogenase,
which is irreversibly damaged by
exposure to oxygen.
But Rhizobium is aerobic and
requires large amount of energy
for Nitrogen fixation.
It solves this “oxygen dilemma” by
protecting the nitrogenase
through Oxygen Scavenging
Leghaemoglobin a transport
protein which effectively removes
oxygen from module.
It also has exceptionally fast and
efficient aerobic metabolism so
very little Oxygen is stockpiled.
Host plant Bacterial symbiont
Alfalfa Rhizobium meliloti
Clover Rhizobium trifolii
Soybean Bradyrhizobium japonicum
Beans Rhizobium phaseoli
Pea Rhizobium leguminosarum
Sesbania Azorhizobium caulinodans
Some other Rhizobium sp. From Rhizobiaceae-
R. Cellulosilyticum R. daejeonense R. etli R. galegae R. gallicum R. giardinii R. hainanense
R. huautlenseR. Indigoferae R. leguminosarum R. loessense R. lupini R. lusitanum R.
mongolense R. miluonense R. sullae R. tropici R. undicola R. yanglingense
Both plant and bacterial factors determine specificity
The Nodulation Process
1. Chemical recognition of roots and
2. Root hair curling
3. Formation of infection thread
4. Invasion of roots by Rhizobia
5. cortical cell divisions and formation of
6. Bacteria fix nitrogen which is
transferred to plant cells in exchange
for fixed carbon
Chemical recognition of roots and rhizobium.
Very early event.
Tip of a
The rhizobia shown here are clustered on the surface
of the root. Soon they will start to invade the roots
and begin a symbiotic partnership that will benefit
2. Root hair curling.
Root hair curls to trap the colony of rhizobium.
3. Formation of infection thread to penetrate root cortex and
reach vascular tissue.
Pea plants inoculated with R.
leguminosarum carrying the
plasmid pHC60 that constitutively
expresses green fluorescenst
Curled root hairs (arrows) of pea plant.
Infection thread (green
fluorescence) reaching the base
of the root hair in plants.
3. Invasion of roots by
4. cortical cell divisions and
formation of nodule
Bacteria converts to
bacteroids and begin to
Rhizobium fixes nitrogen
which is transferred to
plant cells in exchange
for fixed carbon