This document discusses nitrogen fixation in plants, specifically alfalfa. It describes how nitrogen exists in the atmosphere, mostly as N2, which plants cannot use directly. It then discusses how nitrogen is essential for plant growth and metabolism. The document outlines two types of nitrogen fixation - biological and non-biological. Biological fixation is further divided into symbiotic and non-symbiotic types. Much of the document focuses on symbiotic nitrogen fixation between leguminous plants and rhizobia bacteria, including the formation of root nodules and nodulation factors.

1. NITROGEN FIXATION IN
PLANTS
Alafalfa (Medicago sativa)

2. • Nitrogen is present in atmosphere in organic
and inorganic form.
• 90% of N2 present in atmosphere is in organic
form and plants take only nitrogen in form of
ammonia and nitrates.
• Plants itself can’t fix the atmospheric nitrogen
because N2 atom having triple bond which
require 945kJ energy to break this bond.
NITROGEN IN ATMOSPHERE

3. • Major substance in plants next to water.
• Building blocks
• Constituent elements of chlorophyll,
cytochromes, alkaloids and many vitamins.
• Play important role in metabolism , growth
reproduction.
NITROGEN IS ESSENTIAL FOR PLANT
GROWTH

4. NITROGEN FIXATION
• Conversion of free nitrogen into nitrogenous
salts to make it available for absorption of
plants.

5. TYPES OF NITROGEN FIXATION
NITROGEN FIXATION
NON -
BIOLOGICAL
BIOLOGICAL
NON-
SYMBIOTIC
SYMBIOTIC

6. NITROGEN FIXATION
NON-BIOLOGICAL (ATMOSPHERIC) NITROGEN
FIXATION
N2 lightining and photochemical rxn NO3-
10% of nitrogen is converted into nitrate.
BIOLOGICAL NITROGEN FIXATION
N2 nitrifying bacteria NH4+
90% of nitrogen is converted into ammonia.

7. TWO TYPES OF BIOLOGICAL FIXATION
• NON-SYMBIOTIC
• SYMBIOTIC

8. NON-SYMBIOTIC
• Fixation is carried out by free living
microorganism.
• And by some blue green algae: Oscillatoria,
Nostoc, Anabaena etc.
Aerobic bacteria
(eg: Azotobacter ,
Azospirrilum)
Anaerobic bacteria
(eg:Clostridium,
Chlorobium)

9. SYMBIOTIC
• Fixation of free nitrogen by microorganisms in
soil living symbiotically inside the plant.
Nodule formed
(root nodules eg:
bacterial-Rhizobium
forms nodules
Stem nodules eg:
Sesbania)
Nodule not formed
(eg: Azolla-Anabaena
association
Cycas –Nostoc and
Anabaena association)

10. SYMBIOTIC NITROGEN FIXATION
(in non-leguminous plant)
• Some other plant also produces root nodules
Alnus - Frankia
Parasponia – Rhizobium
• Gymnosperm root -Podocarpus

11. SYMBIOTIC NITROGEN FIXATION
(in leguminous plant)
• Association with roots of plant provides –
- food and shelter to bacteria
- bacteria supply fixed nitrogen to plant.
• And these nitrogen fixing bacteria is called
Diazotrophs.
• 2500 species of family leguminosae (pea,
beans, soybean,alfa-alfa) produce root
nodules with Rhizobium species.

12. NITRATES ARE RECYCLED VIA
MICROBES
Nitrification
Nitrification
Ammonium NH4
+
Ammonification
Nitrite NO2
-
Soil organic nitrogen
Animal
protein
Root
uptake
Nitrate
NO3
-
Plant
protein

13. NITROGEN METABOLISM
• AMMONIFICATION
Organic nitrogen ammonification NH4+ion
• This process carried through ammonifying
bacteria.
• Ex: Bacillus vulgaris , Bacillus romosus.

14. Nitrification
• Inorganic NH4+ nitrosomonas genera NO2-(nitrite)
nitrobacter genera
NO3-(nitrate)
These nitrifying bacteria are called
chemoautotrophs .
It is oxidative and energic process.

15. Liebig’s law of minimum
Growth is controlled not by the total of
resources available ,but by the resources
present in minimum amount . Only by
increasing the amount of the limiting nutrient
growth of a plant can be improved

16. DENITRIFICATION
• Denitrifying bacteria uses nitrates as final e-
acceptor rather than O2 during repiration.
• Ex: Thiobacillus denitrificans , Micrococcus
denitrificans.
• NO3- NO2- NO N2O N2
• This process occur in anerobic condition.

17. Plant utilize nitrogen in form of NH4+.So NO3- and
NO2- must be changed into NH4+.
NO3- NO2- NH4+ amino acid
amino acid
NH4+ & NO3-(absorb most)
NO2-(less absorb)
r
o
o
t
Plant

18. Conversion of Nitrate into Nitrite
• Reductive process.
• Nitrate reductase enzyme is involved.
• Electron donar is NADPH or NADH and
released e- is accepted by NO2.
• Process occur in cytosol.
NADPH NAD(P)+
NO3- nitrate reductase NO2-
• Nitrate reductase is homodimer with 3
prosthetic group(FAD,heme,Mo).

19. Conversion of nitrite into ammonia
• Reductive process.
• Enzyme is nitrite reductase.
• This process occur in plastids of shoot and
proplastids of root.
• NADPH/reduced Fd are electron donor.
• Fe4-S4, sirroheme, are two prosthetic group for
this enzyme.
• NO2- nitrite reductase NH4+

20. • Now the conversion of ammonia into ammino
acid includes two pathways:
• Minor pathway
• Major pathway

21. Minor pathway
-ketogluterate + NH4+ NADH glutamate
This pathway is present in all organism including
human beings also. This is the process of reductive
amination.
NH4+,NADH

22. Major pathway
NH4+ + glutamate ATP dependent/ Mg+2,Mn glutamine
This process occur in all organism

23. • Glutamine plant bacteria glutamate
• glutamine + -ketogluterate reduction NADH
glutamate synthase
2molecule
of glutamate
This process occur in only plants and bacteria.
Most of the NH4+ is incorporated into glutamate.

24. Transamination
• Amino acid (glutamate/ glutamine)
transaminase
another amino acid
So different type of amino acid can be formed
due to this process.

25. NODULE FORMATION IN ROOTS OF
LEGUMINOUS PLANTS
• RHIZOBIUM
• Gram negative
• Non spore forming
• Micro – aerobic
• Belongs to proteobacteria
• Types of rhizobium
Bradyrhizobium-slow growing sp.
Rhizobium- fast growing sp.

26. • Roots of leguminous plants secrete flavinoid ,
isoflavinoid , homoserine compound.
• These compounds are chemotactic so attract
rhizobium bacteria & induces the expression
of nodD gene
• nodD gene nodD protein
activates
nod D factor(nod A,
nod B,nodC)

27. Cont…….
• Protein rhicadhesin is Ca+ binding protein
responsible for attachment of bacteria to
surface roots of plant.

28. CHEMICAL NATURE OF nod D FACTOR
• It is a Lipochitin oilgosaccharide.
• Basic unit is n-acetyl d-glucosamine having
B - 1,4 glycosidic bond.
nodD factor

29. ACTION OF nodD FACTOR
• Curling of root hair
• Mitogenic agent so enhance the cell division
in cortical cells which produces an outgrowth
called nodule.

30. CURLING OF ROOT HAIR
• Bacteria attaches to root
hair
• Plasma membrane
invaginates
• Invaginate memberane
elongates and forms
infection thread (produce
golgi derived membrane
vesicles)
• Membrane continues to
elongate

31. FORMATION OF ROOT NODULE
• Bacterial cell reaches upto
cortical cells
• Bacterial cell divide in extra-
ordinary manner
• Membrane bound structures are
formed called bacteriods
• Covered by peribacterium
membrane
• Bacteriod stops to divide.