biotic stress
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Unit-VPlant Genetic Engineering forProductivity and Performance
Biotic Stress
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Introduction
Plants have to exploit their immediateenvironment to maximum productivity.
They are unable to move from one place toother.
Best way of dealing with many stresses isthrough physiological or morphologicalchanges.
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Introduction
Higher plants are subjected to various adverseenvironmental conditions through out their life timesuch as
-Drought or Low Soil Moisture-Cold
-Temperature
-Prolonged periods of cloudiness
-High wind velocities-Soil salinity
-Air Pollution, Snow etc..
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Introduction
All these types of environmental conditions maycause damage/injury to plants.
Resulting from reduction in growth to death.
Generally stresses are of two types
Biotic stress Abiotic stress
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Biotic stress
Biotic stress is caused by biological organismssuch as
Insects
Herb (Another plant)
Bacteria
Virus
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Insect-Plant Interaction
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HERBS-CROP
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Bacterial stem rot of tomato (Erwinia carotovora subsp. carotovora)can occur on staked or trellised tomatoes when plants are pruned.
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Citrus canker
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Viral Infection on Plants
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Insect Resistance
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Natural pest resistance
Natural pest resistance mechanismsoccurring in higher plants can be classifiedinto
1). Preformed resistance mechanisms
2). Inducible resistance mechanisms.
P f d i
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1. Preformed resistance
mechanisms
Structural, Morphologic, and Chemical factors
In entomology, it has long been known that
innate morphological and anatomical featuressuch as leaf and flower color, presence oftrichomes, and even the texture of cuticle maycause certain insects to avoid a plant, thusconstituting resistance mechanisms.
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Preformed resistance mechanisms
Anatomical features may also discourageinsect feeding.
These include the degree of secondarywall thickening and other aspects of basicplant structure.
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Preformed resistance mechanisms
Plants typically contain significant amountsof preformed chemicals produced viasecondary metabolism.
These include phenolics of varyingstructural sophistication, terpenoids, andsteroids.
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Preformed resistance mechanisms
Some preformed compounds are directlytoxic, while others exist as conjugatessuch as glycosides that are not directlytoxic but become toxic following disruption
of the conjugate.
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Preformed resistance mechanisms
Plant glycosides are often hydrolyzed followinginsect damage and releases vacuolarglycosidases.
The aglycones thus produced may be quite toxicto the invader as well as neighboring plant cells.
Since the toxic response is local, however, onlya small portion of the plant is affected
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1.1 ) INDUCIBLE RESISTANCEMECHANISMS
Inducible resistance mechanisms are active, energy-requiring systems.
Specific recognition of an invader that ultimately leads to
the production of proteins or metabolites that areantagonistic to the invader.
These resistance mechanisms have been most studiedin regard to plant pathogens, but the same or similar
mechanisms clearly function against insect pests.
Such active resistance mechanisms are usually referredto collectively as the hypersensitive response (HR).
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Biotechnological approach forInsect Resistance
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Development of Disease resistant Plants
by Bacillus thuringiensisapproach
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Crop damage
It is estimated that 13% of the crop is lostdue to pests world over.
63%
Weeds
pests
Diseases
37% lost due to Weeds, pests and diseases
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Crop damage
Adult insects in general feed on plants anddamages crops.
Most of the problem are due to insect larvae.
The major classes of insects that cause cropdamage are
1. Lepidoptera-Butterflies and moths2. Diptera-Flies and Mosquitoes
3. Orthoptera-Grasashoppers, crickets
4. Homoptera-Aphids
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Lepidoptera-Butterflies and moths
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Diptera-Flies and Mosquitoes
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Orthoptera- Grasshoppers, crickets
H t A hid
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Homoptera-Aphids
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Crop damage
Insects cause a number of diseases which
cause massive crop damage.
To control these diseases, pesticides,insecticides are enormously used.
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Insect resistance: The Bacillus
thuringiensisapproach
Bacillus thuringiensiswas discovered by
Ishiwaki (1901) in diseased silkworms
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Taxonomy of B. thuringiensis
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Bacillus thuringiensis
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BT Toxins
Bt has two classes of toxins; cytolysins(Cyt) and crystal delta-endotoxins (Cry).
While Cyt proteins are toxic towards theinsect orders Coleoptera (beetles) andDiptera (flies).
Cry proteins selectively targetLepidopterans (moths and butterflies).
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Mode of action
The insecticidal activity of Bacillus is because ofparasporal crystal which is synthesized duringthe sporulation.
No significant role for the bacterium has beenidentified.
The parasporal crystal comprises approximately20-30% of dry weight of the sporulated culture andusually consists of 95% protein and 5%carbohydrate
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Mode of action
During sporulation, it synthesizes a cytoplasmicinclusion containing one or more proteins thatare toxic to insect larvae.
Upon completion of sporulation the parentbacterium lyses to release the spore and theinclusion. In these inclusions, the toxins exist
as inactive protoxins.
When the inclusions are ingested by insect
larvae, the alkaline pH solubilizes the crystal.
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Once it has been solubilised in the insect gut,the protoxin is cleaved by a gut protease toproduce an active toxin of about 60kD.
This toxin is termed delta-endotoxin. It binds
to the midgut epithelial cells, creating pores inthe cell membranes and leading to loss of ions.
As a result, the gut is rapidly immobilized, theepithelial cells lyse, the larva stops feeding.
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Mode of action
It has been indicated that the activatedtoxin binds to insect-specific receptorsexposed on the surface of the plasma
membrane of midgut epithelial cells andthen inserts into the membrane tocreate transmembrane pores that cause
cell swelling and lysis and eventuallydeath of the insect.
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Mode of action
Due to their high specificity for theseunique receptors on the membrane of thegut epithelial cells, these toxins (delta-
endotoxins) are harmless to non-targetinsects and the end-user.
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GeneCrystalshape
Proteinsize(kDa)
Insect activity
cry I [several subgroups:A(a), A(b), A(c), B, C, D, E,F, G]
bipyramidal 130-138 lepidoptera larvae
cry II [subgroups A, B, C] cuboidal 69-71 lepidoptera anddiptera
cry III [subgroups A, B, C] flat/irregular 73-74 coleoptera
cry IV [subgroups A, B, C,D]
bipyramidal 73-134 diptera
cry V-IX various 35-129 various
Bt toxins and their classification
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INSECTICIDE RESISTANCE:How Does it Occur?STEP ONE: Mixed population to start with
Repeated spraying, lack of insecticide rotation
Huge reproductive capacity & short life cycle
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INSECTICIDE RESISTANCE:How Does it Occur?
STEP TWO: susceptible individuals are killed.
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INSECTICIDE RESISTANCE:How Does it Occur?
STEP THREE: resistant individuals reproduce giving rise to a
resistant strain within the species.
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