Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of legumes and Parasponia

2. Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Order: Rhizobiales
Family: Rhizobiaceae
Genus: Rhizobium

3.  Rods 0.5–1.0 × 1.2–3.0 𝛍m.
 Non- spore forming.
 Gram negative.
 Motile by 1–6 peritrichous flagella.
 Fimbriae have been described on some strains.
 Aerobic, possessing a respiratory type of metabolism with oxygen as the terminal electron acceptor.
 Optimal temperature for growth, 25–30∘C; some species can grow at temperatures >40∘C.
 Optimal pH for growth, 6–7; range pH 4–10.
 Generation times of Rhizobium strains are 1.5–5.0 h.
 Colonies are usually white or beige, circular, convex, semi-translucent or opaque, raised and
mucilaginous, usually 2–4 mm in diameter within 3–5 days.

4. Rhizobiacea family have 5 important genera
1.Rhizobium-slow-growing Rhizobia, produce acid.
2. Bradyrhizobium- Fast-growing Rhizobia, produce alkali.
3. Azorhizobium- it inducing both stem and root nodules.
4. Sinorhizobium.
5.Photorhizobium.

5. legume
rhizobia
Fixed nitrogen
(ammonia)
Fixed carbon
(malate, sucrose)

6. There are also two types of nodule that can be formed:
determinate
and
indeterminate
This outcome is controlled by the plant host
Fast-growing Rhizobium spp. whose nodulation functions (nif, fix) are
encoded on their symbiotic megaplasmids (pSym)
Slow-growing Bradyrhizobium spp. whose N-fixation and nodulation
functions are encoded on their chromosome.

7. Formed on tropical legumes by
Rhizobium and Bradyrhizobium
Meristematic activity not persistent - present only
during early stage of nodule formation;
after that, cells simply expand rather than divide, to
form globose nodules.
Nodules arise just below epidermis;
largely internal vascular system
Uninfected cells dispersed throughout
nodule; equipped to assimilate NH4
+
as ureides (allantoin and allantoic acid)

8. Formed on temperate legumes
(pea, clover, alfalfa); typically by Rhizobium spp.
Cylindrical nodules with a persistent meristem;
nodule growth creates zones of different developmental stages.
Nodule arises near endodermis, and nodule vasculature
clearly connected with root vascular system
Uninfected cells of indeterminate nodules
assimilate NH4
+ as amides (asparagine, glutamine)

9. Critical steps in Root Nodule Formation
Step 1: Recognition and attachment of bacterium to root hairs.
Step 2: Excretion of nod factors by the bacterium
Step 3: Bacterial invasion of the root hair
Step 4: Travel to the main root via the infection thread
Step 5: Formation of bacteroid state within plant cells
Step 6: Continued plant and bacterial division, forming the
mature root nodule

11. 1. The root excretes substances
2. These substances attract
rhizobia and stimulate them
to produce cell-division
factors
3. Cells in the root cortex divide
to form the primary nodule
meristem.

12. 1. Bacteria attach to the root hair.
2. Cells in the pericycle near the xylem
poles are stimulated to divide.
3. The infection thread forms and extends
inward as the primary nodule meristem
and the pericylce continue to divide.
4. The two masses of dividing cells fuse into
a single clump while the infection thread
continues to grow.
5. The nodule elongates and differentiates,
including the vascular connection to the
root stele. Bacteroids are released into
the cells in the centre.

13. Plant cytoplasm Photosynthesis
Symbiosome
membrane
Bacteroid
membrane
Sugars
Organic acids
Bacteroid Succinate
Malate
Fumarate
Pyruvate
e
e
Nitrogenase
Citric
acid
cycle
Proton
motive
force
Electron transport
chain
Lb  Leghemoglobin
Glutamine
Asparagine
The Legume–Root
Nodule Symbiosis
The legume–bacteria symbiosis
is characterized by several
metabolic reactions and
nutrient exchange.

15. Host plant Bacterial symbiont
Alfalfa Rhizobium meliloti
Clover Rhizobium trifolii
Soybean Bradyrhizobium japonicum
Beans Rhizobium phaseoli
Pea Rhizobium leguminosarum
Sesbania Azorhizobium caulinodans
Complete listing can be found at at: http://cmgm.stanford.edu/~mbarnett/rhiz.htm
Both plant and bacterial factors determine specificity

17. SOURCE-
 Lincoln Taiz, Eduardo Zeiger, Ian M. Møller, and Angus Murphy Plant Physiology
and Development Sixth Edition.
 Renu Verma, ... Senthilkumar Murugesan, in Beneficial Microbes in Agro-Ecology,
2020
 F.B. Dazzo, S. Ganter, in Encyclopedia of Microbiology (Third Edition), 2009