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Plant Growth
Promotinng Rhizobateria
Plant Microbe Interactions – Lab
What is Rhizosphere
• The rhizosphere is the narrow zone of soil
specifically influenced by the root system
• This zone is rich in nutrients when compared with
the bulk soil due to the accumulation of a variety of
plant exudates, such as amino acids and sugars,
providing a rich source of energy and nutrients for
bacteria
• This situation is reflected by the number of bacteria
that are found around the roots of plants, generally
10 to 100 times higher than that in the bulk soil
• The rhizosphere is populated by a diverse range of
microorganisms and the bacteria colonizing this
habitat are called rhizobacteria
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• Plant-associated bacteria can be classified into
beneficial, deleterious and neutral groups on the basis of
their effects on plant growth
• Beneficial free-living soil bacteria are usually referred to
as plant growth-promoting rhizobacteria (PGPR,
• Independent of the mechanisms of vegetal growth
promotion, PGPRs colonize the rhizosphere, the
rhizoplane (root surface), or the root itself (within
radicular tissues)
• It is well established that only 1 to 2% of bacteria
promote plant growth in the rhizosphere
• Bacteria of diverse genera have been identified as PGPR,
of which Bacillus and Pseudomonas spp. are predominant
Mode of Action
• PGPR affect plant growth in two different ways,
indirectly or directly.
• The direct promotion of plant growth by PGPR
entails either providing the plant with a compound
that is synthesized by the bacterium, for example
phytohormones, or facilitating the uptake of certain
nutrients from the environment
• The indirect promotion of plant growth occurs when
PGPR lessen or prevent the deleterious effects of one
or more phytopathogenic organisms. This can
happen by producing antagonistic substances or by
inducing resistance to pathogens
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• A particular PGPR may affect plant growth and
development by using any one, or more, of these
mechanisms.
• PGPR, as biocontrol agents, can act through
various mechanisms, regardless of their role in
direct growth promotion, such as
• by production of auxin phytohormone
• decrease of plant ethylene levels
• nitrogen fixing associated with roots
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• PGPR and their interactions with plants are
exploited commercially and hold great promise
for sustainable agriculture.
• Applications of these associations have been
investigated in maize, wheat, oat, barley, peas,
canola, soy, potatoes, tomatoes, lentils,
radicchio and cucumber
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Psuesomonas flourescence
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• Pseudomonas fluorescens is an aerobic, gram-negative, non-
sporulating rod shaped bacteria in the family
Psuedomonadaceae, order Pseudomonadales of the class
Gammaproteobacteria.
• Rod-shaped cells are approximately 0.5 μm in width and 2.0-2.5
μm in length
• Motility is by a single polar flagellum.
• It is ubiquitous organism present in agricultural soils and is
well adapted to grow in the rhizosphere.
• This rhizobacterium possesses many traits to act as a
biocontrol agent and to promote the plant growth ability.
• It grows rapidly in vitro and can be mass- produced.
• It rapidly utilizes seed and root exudates and colonizes and
multiplies in the rhizosphere environments.
• In the plant rhizosphere, it produces a wide spectrum of
bioactive metabolites, that is, antibiotics, siderophores,
volatiles, and growth-promoting substances;
• It competes aggressively with other microorganisms; and
adapts to environmental stresses.
• In addition, pseudomonads are responsible for the
natural suppressiveness of some soilborne pathogens.
• It suppresses the growth of pathogenic microorganisms
by various mechanisms, namely, production of
antibiotics, bacteriocins, siderophores, hydrolytic
enzymes such as β-1,3-glucanase and chitinases, and
other metabolites such as phytoalexins and induction of
systemic resistance.
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Bacillus
• Bacillus is rod-shaped and Gram-positive bacterium
• When cultured on ordinary nutrient agar, the morphology circular colony of
this bacteria is rough, opaque, fuzzy white or slightly yellow with jagged
edges
• Several species of Bacillus occur in soil, most well studied Bacillus subtilis
• Bacillus species are capable of forming long-lived, stress tolerant spores
and secreting metabolites that stimulate plant growth and prevent pathogen
• Their spore forming ability makes them an ideal candidate for developing
efficient biopesticide products.
• Some of these species directly stimulate plant growth either through
enhancement in acquisition of nutrients or through stimulation of host
plant’s defense mechanisms prior to infection;
• Bacillus spp. secrete exopolysaccharides and siderophores that inhibit the
movement of toxic ions and help to maintain the ionic balance, promote the
movement of water in plant tissues, and inhibit the growth of pathogenic
microbes
• Bacillus subtilis also plays a significant role in improving tolerance to biotic
stresses.
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Bacillus subtilis gram stain
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Bacillus subtilis
Bacillus thuringensis
• Insecticidal BCA
• Bt genes
• ability to live in the
environment free and
independent from other
Gram-positive spore-
forming bacilli
• production of entomocidal
parasporal crystal
proteins
• survival in a unique
environmental niche in
the midgut and hemocoel
of insects.
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Azotobacter
• Azotobacter genus are highly diverse, nonsymbiotic
N2-fixing, gram-negative bacteria
• They are oval or spherical in shape and form thick-
walled cysts (dormant cells resistant to deleterious
conditions) under unfavorable environmental
conditions.
• They produce pigments that are yellow-green, red-
violet, or brownish-black in color.
• Around six species in the genus Azotobacter have
been reported, some of which are motile by means of
peritrichous flagella while others are immotile
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• They are typically polymorphic having size ranging
from 2 to 10 µm long and 1 to 2 µm wide
• Azotobacter was isolated in 1901by Beijerinck and
his co-workers as the first aerobic free-living
nitrogen fixer.
• These bacteria are known to exploit atmospheric
nitrogen for their cellular protein synthesis which is
mineralized in the soil, imparting the crop plants a
considerable part of nitrogen available from the soil
source.
• Azotobacter spp. is sensitive to acidic pH, high salt
concentration and temperature
Azotobacter
• They pose advantageous impacts on the crop
growth and yield through
• the biosynthesis of biologically active
substances,
• instigation of rhizospheric microbes,
• production of phytopathogenic inhibitors
• alteration of nutrient uptake and
• magnifying the biological nitrogen fixation
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Rhizobium
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Rhizobium
• It is a Gram-negative,
• rod-shaped (0.6–1.0 µm × 1.5–3.0 µm) bacterium,
• aerobic, non-spore forming and motile by 1–6 peritrichous flagella
• Colony morphology shows convex, circular and smooth characteristics, while its
colour becomes non-pigmented to light beige on a nutrient agar plate
• They assume a different shape when inside their host, being irregularly shaped or
often ‘Y’ -shaped.
• When Rhizobia are living outside of a plant they are typical heterotrophs feeding
on dead organic material and use the material obtained both as ‘building material’
for growth and to provide substrates that are oxidized in cellular respiration to
provide energy.
• The legume-rhizobia symbiosis has a large impact on productivity of
legumes and plays a significant role in fixation of atmospheric nitrogen
• The root nodule rhizobia approximately reduce 20 million tons of
atmospheric nitrogen to ammonia which is 50–70% of the world
biological nitrogen fixation
• The ability to fix atmospheric nitrogen is through the activity of enzyme
nitrogenase
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