RHIZOSPHERE EFFECTS.pptx

PLANT NUTRITION AND CROP
PRODUCTIVITY
Said H. Marzouk
RHIZOSPHERE EFFECT ON MINERAL
NUTRITION OF PLANTS WITH EMPHASIS ON
BNF, NUTRIENT SOLUBILIZATION AND
UPTAKE
4/6/2022 binmarzouk@gmail.com

Rhizosphere • The zone of soil immediately
adjacent to roots that supports high
levels of microbial activity.
(Rovira and Davey, 1973)
Two major systems
1. Shoot system =
photosynthesis
2. Root system =
water and
minerals uptake

Rhizosphere effects
• Rhizosphere is strongly adhering dense layer and
consists of root hairs, soil particles, and soil microbes.
• The effects of the biological, chemical, and physical
changes in the Rhizosphere soils that occur because of
root exudates and rhizodeposition is known as
Rhizosphere effects (REs)
• The root exudates acts as a signaling messenger that
initiates biological communication between the soil
microbes and plant roots

Microbes in the
Rhizosphere
• Microbial community, including
endophytes, pathogens and
beneficial microbes
• Beneficial microbes are classified
into two broad groups based on
their effects:
I. Plant growth-promoting
microbes, PGPM)
II. Biological control agents (BCA)
Endophytes
• provide
metabolites that
promotes plant
growth and help in
adapt better
environment
• Protect plant
against biotic and
abiotic stress
• Alleviate metal
toxicity

Plant growth-promoting rhizobacteria (PGPM)
• Group of beneficial microbes capable of colonizing the
rhizosphere and able to benefit plants by improving their
productivity and immunity.
(i) Synthesis of substances that can be assimilated directly
by plants
(ii) Mobilization of nutrients
(iii) Prevention of plant diseases

Factors affecting microbes in Rhizosphere
Soil PH/ Rhizosphere PH:
• pH of the rhizosphere decreases due to root
respiration.
• Acidification of rhizosphere decrease number of
microbes in the rhizosphere
• Rhizosphere effect for bacteria and protozoa is more
in slightly alkaline soil and for that of fungi is more
in acidic soils.

• Proximity of root with Soil:
• The number of rhizospheric organisms is greater
near the root and their number continuously
decreases with increase in distance from the root.
• It is because concentration of organic matter
released by root in exudates decreases with
increases in distance from the root.

Temperature and light intensity:
• Low temperature and low light intensity decreases
the rate of exudate secretion from the root so that
number of rhizospheric organisms decreases.
• On the other hand number of microbes in
rhizosphere increases when temperature and light
intensity increases as multiplication rate is high.

Depth of root:
• In general number of rhizosphere microorganisms
decrease with increase in depth of root, which is
mainly due to anaerobic condition.

Plant Species:
• Amounts and types of exudates secretion differ
with plant species, that influence growth of
rhizosphere microbes.
• For example some plant roots releases
antimicrobial chemicals such as glycosides,
hydrocyanic acids, and several antifungal agents
that inhibit rhizospheric microbes

Root exudates :
• Root exudates mostly include sugars, amino acids,
peptides, vitamins, nucleotides, organic acids, enzymes,
fungal stimulants, and also some other compounds which
help in plant water uptake, plant defense, and stimulation
(Pate et al., 2001)
• It is the main factor that influence microbial community
• Different microbial community were influenced by
different root exudates

Mineral solubilization
• Mineral solubilization is ability of M.O to convert
insoluble forms of minerals to soluble form that can be
absorbed by plants
• Different microbial species have mineralization and
solubilization potential for organic and inorganic minerals
(Hilda and Fraga, 2000)
• Solubilizing activity is determined by the ability of
microbes to release metabolites such as organic acids,
which through their hydroxyl and carboxyl groups chelate
the cation bound to metals and converted to soluble forms
(Sagoe et al., 1998).

Phosphorous
• Phosphorous is a major growth limiting nutrient.
• As like nitrogen, there is no large atmospheric source that
can be made biologically available
• Soil P precipitated as orthophosphate and adsorbed by Fe
and Al oxides.
• P also precipitated and adsorbed by Ca in alkaline soils

Phosphate solubilization cont…
• Numerous soil micro-flora were reported to solubilize
insoluble phosphorous complexes into solution and make
it possible for its use by the plant (Tripura et al. 2005).
• Several groups of fungi and bacteria, popularly called as
phosphate-solubilizing micro-organisms (PSMs) assist
the plants in mobilization of insoluble forms of
phosphate.
• Some bacterial species have mineralization and
solubilization potential for organic and inorganic
phosphorus.

• Phosphorus solubilizing activity is determined by the
ability of microbes to release metabolites such as organic
acids
• OA through their hydroxyl and carboxyl groups chelate
the cation bound to phosphate, the latter being converted
to soluble forms (Sagoe et al., 1998).
• Symbiotic relationship between PSB and plants is
synergistic in nature as bacteria provide soluble
phosphate and plants supply sugars that can be
metabolized for bacterial growth (Pérez et al., 2007)

Phosphate solubilization
• A significant increase in the grain yield was observed for
rice, soybean, cowpea and also an increase in the
phosphate uptake in the potato tubers was observed
when Pseudomonas striata, Aspergillus
awamori and Bacillus polymyxa were used (Gaur and
Ostwal, 1972).

Organic acids produced by PSMs that aid in P solubilization

Biological nitrogen fixation
• Nitrogen is one of the most important macronutrient
• N2 gas are found 78.084% in the atmosphere as N≡N
• Biological nitrogen fixation is a specific process in which
the atmospheric nitrogen is converted to ammonia by an
enzyme called nitrogenase
• BNF is mostly accomplished by microorganisms called
diazotrophs or N2 fixers.

Biological nitrogen
• Diazotrophs includes some species of bacteria,
fungi, blue green algae (Cyanobacteria), lichens
etc.
• The enzyme nitrogenase fix atmospheric nitrogen
to NH3
• Only microorganism with nitrogenase enzyme are
able to fix N2

Types of Biological Nitrogen Fixers
• Biological nitrogen fixers are classified based on
fixing microorganisms.
• They are usually two types as follows:
1. Symbiotic
2. Non-symbiotic

Factors affecting N2 fixation
1. Presence of nitrate or ammonium
2. Presence of certain inorganic substances Ca, Co, Mo –
influence N2 fixation along with P.
3. Availability of energy source – addn. of C source
increase N2 fixation.
4. pH : Neutral – favours Azotobacter – Acidic Beijerinkia
5. Soil moisture : Adequate is good for fixation.
6. Temperature: Mesophilic – 30°C.

• There are three ways that nitrogen gets “fixed”
• Industrial N2 fixation
• Non biological N2 fixation
• biological N2 fixation

• Industrial N2 fixation
• Accomplished by Haber -Bosch process
• Developed in Germany 1914 by Fritz haber & Karl bosch
• Process- N2 and H2 react with each other in presence of
• Industrial catalyst( nickel / iron)
• High temperature about 500 ͦ c High pressure – 200 atm
To form NH3
• Source of H2 - is methane (natural gas)
• Industrial production of fertilizers and explosives

• Non biological N₂ fixation
• Non biological / physico chemical N2 fixation involves
the photochemical & electro chemical conversion of atm.
N₂ to soil NO2, NO3, NH3.
• It is brought about by ionizing phenomena such as
cosmic radiations, meteor trails ,lightning, thunderstorms,
volcanic eruptions etc.
• These provides high energy for breaking N≡N & also for
the formation of free N₂ with oxygen or hydrogen of atm
H₂0

• combination of N₂ with o₂ forms nitrous and nitric oxides.
• Combination of N₂ with hydrogen forms NH3 Nitrous and
nitric oxides get hydrated with atm. Water vapour and
forms nitrous and nitric acids
• Rain water bring these acids and NH3 to soil surface
• There ,the acids react with metallic ions and form metallic
nitrates.
• These nitrates and NH3 enrich the surface soil
• Non biological N₂ fixation amounts to only ˂ 10% of the
natural N₂ fixation
• It is common in some tropical regions, where thunder bolt
are frequent

• Biological Nitrogen Fixation
• Biological nitrogen fixation was discovered by the
German agronomist Hermann Hellriegel and Dutch
microbiologist Martinus Beijerinck.
• Biological nitrogen fixation (BNF) occurs when
atmospheric nitrogen is converted to ammonia by an
enzyme called nitrogenase
• The reaction for BNF is:
N2+16ATP+8H++8 e − → 2 NH3+H2+16ADP+16Pi

2) Theory of Burris and Wilson
Hydroxylamine is the central compound of N fixation from
which ammonia is formed through reduction.

Special features of diazotrophs
• Free living –bacteria like Azotobacter, rhodopseudomonas
fix atm. N₂ & also protects nitrogenase enzyme –
sensitive to 0₂
• They produce exopolysaccharides (slime) which retains
water and prevents diffusion of 0₂ inside cell during N₂
fixation
• Azotobacter have exceedingly high rate of respiratory
metabolism thus preventing 0₂ retention inside the cell
• Blue green algae –are of 2 kinds
• One that possess heterocyst
• Other that devoid of it ( non heterocystous)

• Filamentous cyanobacteria contain pale , thick walled , hollow
cells called heterocyst
• Heterocyst are the site of N₂ fixation
• They lack PSII and photosynthetic bile proteins
• Non heterocystous N₂ fixing BGA like (oscillatoria) - the
filaments are arranged clumps and N₂ fixation takes place in
internally organized cell having reduced conditions
•
• Azospirillum peplum survive in microaerophilic conditions
associated with the rhizosphere ( area surrounding the roots )
of paddy plants - fix atm. N₂ in the rhizosphere.

Site and mechanism of nitrogen fixation in nodules
Site
• Bacteroids -site of N-fixation.
Mechanism
1) Theory of Virtanen
• N fixation in roots appear immediately after nodule
formation.
• young plants fix N than the old plants.
• A great part of N- converted to-L-aspartic acid and
Lglutamic acid.
• Apart from this alpha-alanine present in nodule-
produced from L-aspartic acid by decarboxylation.
• small amount of Oxime-N and nitrite-N are also
present.

2) Theory of Burris and Wilson – hydroxylamine is
the central compound of N fixation from which
ammonia is formed through reduction.

Biochemistry of nitrogen fixation
• N₂ fixers utilize atm. N₂ to synthesize NH3
• In this process , N₂ is first split up into free N₂ atoms by
breaking the triple bond , with help of enzyme
nitrogenase.
• This reaction is endergonic (energy consuming), it
requires an input of nearly 160kcal energy.

Free nitrogen combines with hydrogen forming NH3
This reaction is exergonic (energy releasing)
Mediated by enzyme hydrogenase and it releases nearly 13
kcal energy.
BNF requires a net input of 147 kcal energy & an
expenditure of nearly 16 mols of ATP per each molecule of
nitrogen.

Nitrogenase Complex
• Two protein components:
• Nitrogenase reductase and Nitrogenase
• Nitrogenase reductase is a 60 kD homodimer
with a single 4Fe-4S cluster
• Very oxygen-sensitive
• Binds Mg ATP
• 4 ATP required per pair of electrons transferred
• Reduction of N2 to 2NH3 + H2 requires 4 pairs
of electrons, so 16 ATP are consumed per N2

• Nitrogenase
• A 220 kD heterotetramer
• Each molecule of enzyme contains 2 Mo, 32 Fe, 30
equivalents of acid-labile sulfide (Fe8S 7-8clusters, etc)
• Four 4Fe-4S clusters plus two Fe, Mo, Co, an iron-
molybdenum cofactor (Fe 7 S9 Mo homocitrate)
Nitrogenase is slow - 12 e- pairs per second, i.e., only
three molecules of N2 per second

Pathway of nitrogen fixation in root nodules
• Glucose-6-phosphate acts as a electron donor
• Glucose-6-phosphate is converted to phosphogluconic
acid
Glucose-6-phosphate + NADP+ + H2O 6-phosphogluconic acid + NADPH + H+
• NADPH donates electrons to ferredoxin. Protons
released and ferredoxin is reduced.
• Reduced ferredoxin acts as electron carrier.
• Donate electron to Fe-protein to reduce it. Electrons
released from ferredoxin thus oxidized

• Reduced Fe-protein combines with ATP in the presence of
Mg +2
• Second sub-unit is activated and reduced
• It donates electrons to N2 to NH3
• Enzyme set free after complete reduction of N2 to NH3

Genes involved in Nitrogen fixation
• Genes involved in root nodule formation - called
nodulin genes ( nod genes)
• Nodulin genes essential for infection of plant root and
nodule formation by symbiotic N₂ fixing bacteria -
divided into 2 classes
1) include genes that specify biochemical composition of
bacterial cell surface.
• such as gene determining the synthesis of
exopolysaccharides ( exo genes) • Lipopolysaccharides
(lps gene)
• Capsular polysaccharides or K antigen & β 1,2 glucans
(ndu genes)

• exo & lps genes - play a role in determining host
specificity
2) consist of nodulation genes (nod or nol)
• nod genes are involved in nodulation of particular host -
called host specific nod (hsn) genes.
• Fast growing rhizobium sps - nod genes are located on
large sym plasmids
• Slow growing brady rhizobium sps – carry late nod gene
on the bacterial chromosome.

Symbiotic N2 Fixers
• Fixation of free nitrogen by micro-organisms living
symbiotically with plants.
• The term ‘symbiosis’ was first introduced by a biologist
named Debary.
These can be discussed under the following three heads
1. Nodule formation in leguminous plants.
2. Nodule formation in non-leguminous plants.
3. Non nodulation

Nodule formation in Leguminous plants
• More than 2500 species of the family Leguminosae
produce root nodules with Rhizobium spp.
• Examples are Pea, Soya bean, Clover, and alfalfa
• They fix nitrogen only inside the root nodules.
• The host plants provides food and shelter to the bacteria.
• The bacteria supply fixed nitrogen to the plant.

Nodule formation in non-leguminous plants
• Besides leguminous plants, some other plants also found
to form root nodules.
• Actinorhizal plants have the ability to develop an
endosymbiosis with the nitrogen-fixing soil
actinomycetes Frankia.
• The establishment of the symbiotic process results in the
formation of root nodules in which Frankia provides
fixed nitrogen to the host plant in exchange for reduced
carbon.
• Actinorhizal plants are woody shrubs and trees
• Some examples include, Causuarina, Myrica gale etc.

Non-nodulation
• Lichens- Cyanobacteria
• Anthoceros- Nostoc
• Anabaena azollae
• Cycas- Nostoc

Azolla anabaena
• Is a symbiotic relationship between cyanobacteria and
floating fern Azolla
• Anabaena is a genus of filamentous cyanobacteria, or
blue-green algae
• It is known for its nitrogen fixing abilities.
• They form symbiotic relationship with certain plants,
such as the mosquito ferns
• Anabaena has filamentous structure
• The filaments are either straight or circulate or irregular

• Filamentous structure consist two type of cells
• Vegetative cell and heterocyst
• heterocyst is a differentiated Cyanobacterial cell that
carries out nitrogen fixation.
• The heterocyst function as the site for nitrogen fixation
under aerobic conditions
• Vegetative cell have high oxygen affinity and it function
as a site for photosynthesis
• Bacteria provide nitrogen to the fern, and plant provide
habitat for bacteria

Non symbiotic nitrogen fixers
• Non-symbiotic N2 fixers are also known as free living
nitrogen fixers.
• They inhabit both in terrestrial and aquatic conditions.
• Fixation carried out by free living micro-organisms
which are categorized into three different groups such as
Aerobic, Anaerobic and blue green algae.
1. Free living aerobic : Azotobacter
2. Free living anaerobic : Clostridium.
3. Blue green algae

Mycorrhizal
• In mycorrhizal SYMBIOSIS fungus provides to the host
plant with nutrients, such as phosphate and nitrogen.
• In return, it gets photo synthetically fixed carbon from
the host.
• They form two types of mycelium, The mycelium that is
formed within the root, the intraradical mycelium (IRM)
differs morphologically and functionally from the
extraradical mycelium (ERM), the mycelium that grows
into the soil.

• Mycorrhizal plants can take up nutrients from the soil via
two pathways:
• The ‘plant pathway’ that involves the direct uptake of
nutrients from the soil by the root epidermis and its root
hairs.
• The ‘mycorrhizal pathway’ that involves the uptake of
nutrients via the extraradical mycelium of the fungus.

• THANK YOU

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