Transgenic Plants and Plant Biotechnology
Eastern New Mexico University
Terms to know
Transgene: It is a gene or genetic material that has been transferred naturally or by
any of a number of genetic engineering techniques from one organism to another.
Transgesis : The process of introducing an exogenous gene called a transgene
into a living organisms so that the organism will exhibit a new property and
transmit that property to its offspring.
Transgenic Plants : The plants which expresses the characters coded by the
transgene are called Transgenic plants.
History of Plant Breeding
Selective Breeding used in the History
Genetics studies started with Mendel
Cross pollination : Pollen from one plant to stigma
of another plant.
Found dominate characteristics in plants
Uses of Traditional Breeding:
Increase crop yield
Increase Resistance to pests and diseases
Disadvantages of Traditional breeding:
Lot of man power
Limited possibility of improved traits.
The Reproductive Organs of a Typical
Plant : Pollen grains are the male
reproductive cells of the plant. They are
made in the anther (orange), the top
portion of the stamen. The female
reproductive cells, the ova, are
sequestered in the ovary. Pollen reaches
the ova via the stigma, which is attached
to the ovary by the pistil.
Treat seeds with mutagens or expose to X rays or gamma rays.
Less predictable results
Lot of man power
Successful in the flower world. Eg; new colours, more petals.
UV Treatment or Mutagens
Tested for Improvements
Found desirable traits
Test for Progeny
Sold to Markets
Transgenic plant : Insertion of a foreign gene into a specific plant.
Difference between Trangenic Technology and traditional Breeding:
Trangenic Technology : Transform gene from any source.
Eg: animals, bacteria, virus etc
Traditional Breeding : Move genes only between members of a particular
genus of plants.
Plant Tissue Culture
Totipotency : Ability of a cell to divide into any type of cell.
Explant : Mass of tissue or cells
Solid medium – Callus culture.
Tissue can be immature embryo, apical meristem, root tip
Liquid medium – suspension culture
Tissue should be protoplast (cells with no cell wall), micro or macrospores.
Nutrients and hormones are used for growth and development.
Eg : 2,4 dichlorophenoxyacetic acid (analogous to auxin)
Callus : Undifferentiated cell which form a crystalline white layer on solid medium.
I. Move callus to other medium with reduced hormones which allows shoot to
II. Move the callus to other medium with no hormone which allows root hairs to
The process of regenerating a plant from a single cell may cause three types
1.Temporary Physiological change
3.True genetic changes
An Entire Plant Can Be Regenerated from a Single Cell
Small samples of tissue, or even single plant cells may
be cultured in vitro. Under appropriate conditions, these
may regenerate into complete plants.
Callus or Liquid Culture of Plant Cells Can Regenerate Entire Plants
In callus culture a mass of undifferentiated cells grows on a solid surface. In liquid culture,
separated single cells are grown. Both types of cultures can develop shoots and roots with
appropriate manipulation of plant hormone levels.
Gene transfer in plants
Why gene transfer?
Reverse genetics - understanding functioning of unknown genes
Biochemistry and metabolism
Gene transfer strategies: Systems and vectors
Direct DNA uptake
Plant transformation with the Ti plasmid of Agrobacterium tumefaciens
A. tumefaciens is a gram-negative soil bacterium which naturally
transforms plant cells, resulting in crown gall (cancer) tumors
Tumor formation is the result of the transfer, integration and expression of
genes on a specific segment of A. tumefaciens plasmid DNA called the TDNA (transferred DNA)
The T-DNA resides on a large plasmid called the Ti (tumor inducing)
plasmid found in A.tumefaciens
Agrobacterium-mediated gene transfer
• To make a segment of DNA that contains a selectable marker and a gene of
interest to look like a T-DNA
• To get this “T-DNA” into an Agrobacterium cell so that it can be
mobilized by the vir genes
• To produce and find transformed plant cells that can be regenerated into
normal, fertile plants
• A transfer cassette bounded by functioning borders
• Ways to get this cassette into Agrobacterium
• Disarmed Ti plasmids that retain functional vir genes
Yields relatively uncomplicated insertion events (low copy number,
Unlimited size of foreign DNA
Efficient (for most plants)
Adaptable to different cell types, culture procedures (protoplasts, tissue
sections, “non-culture” methods)
Transformants are mitotically and meiotically stable
Host range is limited: not all plants may be susceptible to Agrobacterium
With susceptible plants, accessible culture/regeneration systems must be
adaptable to Agrobacterium-mediated gene transfer
The Infection process
Wounded plant cell releases phenolics and nutrients.
Phenolics and nutrients cause chemotaxic response of A. tumefaciens
Attachment of the bacteria to the plant cell.
Certain phenolics (e.g., acetosyringone, hydroxyacetosyringone)
induce vir gene transcription and allow for T-DNA transfer and
integration into plant chromosomal DNA.
Transcription and translation of the T-DNA in the plant cell to
produce opines (food) and tumors (housing) for the bacteria.
The opine permease/catabolism genes on the Ti plasmid allow A.
tumefaciens to use opines as a C, H, O, and N source.
Agrobacterium Transfers Plasmid DNA into Infected Plants
Agrobacterium carrying a Ti plasmid is attracted by acetosyringone to a wounded plant stem. The
Ti plasmid is cut by endonucleases to release single-stranded T-DNA, which is covered with
protective proteins, and transported into the plant cell through a conjugation-like mechanism. The
T-DNA enters the plant nucleus where it integrates into plant chromosomal DNA.
The Ti plasmid of Agrobacterium tumafaciens and the
transfer of its T-DNA to the plant nuclear genome
Crown Gall on
Infection of a plant with
A. tumefaciens and
formation of crown galls
Clone YFG (your favorite gene) or the
target gene in the small T-DNA plasmid
in E. coli, isolate the plasmid and use it
to transform A. tumefaciens containing
the disarmed Ti plasmid
Essential Elements for Carrying a Transgene on Ti Plasmids
The T-DNA segment contains both a transgene and a selective marker or reporter gene. These
have separate promoters and termination signals. The marker or reporter gene must be expressed
all the time, whereas the transgene is often expressed only in certain tissues or under certain
circumstances and usually has a promoter that can be induced by appropriate signals.
structure & function
Transfer of Modified Ti Plasmid into a Plant
Agrobacterium carrying a Ti plasmid is added to plant tissue growing in culture. The T-DNA
carries an antibiotic resistance gene (neomycin in this figure) to allow selection of successfully
transformed plant cells. Both callus cultures (A) and liquid cultures (B) may be used in this
Particle Bombardment Technology
Works with all types of plants.
DNA is carried on microscopic metal particle.
Fired by a gun into plant tissue.
DNA coated on microscopic gold beeds.
Beeds are placed at the end of a plastic bullet.
Blast of helium used to project them.
Plastic meshwork stop is used to stop the bullet.
Alternative method is by strong electrical discharge.
Amount of penetration into tissue can be changed.
Beeds enter the cytoplasm or nucleus of the cell.
DNA is free and recombine with chromosomal DNA.
Leaf transferred to selection media for cell to grow.
Only cells with selectable marker grow other die.
Transformed plants are regenerated using tissue culture
Screened for gene of interest.
DNA Carried on Microscopic Gold Particles Can
Integrate into Plant Chromosomes
After penetrating the cell, the DNA unwinds from around
the gold carrier particle. Some of the DNA enters the
nucleus and is successful in integrating into the plant
Detection of Inserted DNA
Use of selectable marker or reporter gene.
Widely used reporter gene is npt (neomycin phosphotransferase)
In activates Ab neomycin by attaching a phosphate group.
Cells with Ab are not killed but other cells which do not integrate with DNA.
Include a reporter gene coding for luciferase.
Luciferase provides light with its substrate luciferin.
Found in luminous creatures.
Gene coding Eukaryotes is luc and in Prokaryotes is lux
Different chemical nature.
ATP + O2
Use scintillation counter to view light.
Not stable for long.
Active protein is directly proportional to level of gene expression.
Used to test activity of a specific promoter.
Eg : cab promoter controls the expression of the luc gene, and luciferase is only
made when this promoter is turned on in the plant.
Luciferase as a Reporter in Plant Tissues
Plant tissue carrying the luc gene for firefly luciferase emits blue light when provided with the
substrate luciferin. In (A) a leaf disk is viewed by a photocell detector. In (B) the luc gene in a
seedling is expressed under control of an inducible promoter.
C re : Cause recombination
Found in Bacteriophage
Cre protein is a recombinase enzyme.
Recognizes 34 base pair DNA seq (loxP site)
Catalyses recombination between two loxP sites.
Placing loxp on either side of DNA, the enclosed region may be deleted by Cre
Cre gene is also included in the transgenic construct.
This approach allows selected marker genes to be removed from the plant
Cre genes can be added by cross pollination.
Cre/loxP System of Bacteriophage P1
The Cre protein binds to loxP recognition sites in the DNA. Two nearby loxP sites are brought
together, and recombination between them eliminates the intervening DNA. A single loxP site
remains in the target DNA molecule.
Plant Breeding and Testing
Evaluating and testing transformed Transgenic plant is most important.
Event : Each independent case of transgene integration.
The location of the integration affects the expression of transgene.
Transgene with no harmful effects must be moved from exp plant to
Traditional back crossing is used into high yielding varieties.
Back crossing is used for better and higher yield.
This crossing will ensure 98% of genes in the final plant are from the high
yielding variety and 2% are from the original Transgenic plant.
Herbicides : Chemical agents that destroy plants or inhibits their growth.
Eg : Glyphosate
Amino acid phosphate derivative of Glysine
Breaks down into non toxic compounds
Kills plants by blocking the synthetic pathway of Aromatic amino acid
Inhibits the enzyme EPSPS (5-enolpyruvoylshikimate-3-phosphate syntheses)
EPSPS is product of aroA gene in chloroplast.
EPSPS is found in plants, fungi, Bacteria.
Not found in Animals and humans.
Glyphosate Inhibits EPSPS in the Aromatic Pathway
The enzyme 5-enolpyruvoylshikimate-3-phosphate synthase (EPSPS) is the product of the aroA
gene and makes 5-enolpyruvoylshikimate-3-phosphate, a precursor in the pathway to aromatic
amino acids and cofactors. Glyphosate, an analog of phosphoenolpyruvate, inhibits EPSPS.
What scientists did ?
Found EPSPS resistant gene to Glyphosate in Bacteria
Bacterial terminators and sequences were replaced by plant.
Chloroplast Transit peptide was
Transist peptide cleaved off
Expression of the Agrobacterium aroA Gene in Plants
added.The bacterial aroA gene must be placed under control of a
promoter active in plants. Correct localization of the AroA
protein (EPSPS) into the chloroplast requires addition of a
chloroplast transit peptide at the N-terminus of the protein.
Only functional enzyme enters the chloroplast.
This Glyphosate resistant gene transfer were carried out in diff crops.
Canola & cotton plants – Ti plasmid method
Soybean – Gene gun approach.
Insecticides : Chemicals used to kills insects
very costly, hazardous procedures.
More toxic to humans
Similar biochemical pathway in insects and humans.
Naturally occurring insecticides are only harmful to insects.
Eg : Toxin from Bacillus thuringienis (Bt toxin)
These toxin is used to prevent
1. cotton boll worm
---- destroy cotton
2. European corn borer ---- destroy corn
Insect Larvae Are Killed by Bt Toxin
Bacterial spores of Bacillus are found on food eaten by the caterpillar. The crystalline
protein is released by digestion of the spore and its breakdown produces a toxin that
kills the insect larvae.
Bacillus ------ Cry proteins ------ Insects eat ------ Cry realases delta endotoxins
(Bt toxin) ------ Toxin binds to intestinal lining ------ holes generated ------ digestive
system disturbed ------ Death of the insects.
Different Cry proteins produced by Bacillus :
Cry I : kills Butterflies and moths
Cry II : kills Butterflies and flies
Cry III : kills beetles
Cry IV : kills only flies.
Use of Trangenic technology
Toxic gene ----- insert into tomato plant ----- showed partial protection.
Plant made only low levels of toxin. Why ???
The toxin gene is from bacterium and is designed to express well in bacteria and not
So here genes expressed in different host cell.
Use of codon is a problem.
Several different codons encode the same amino acid.
Different orgs favors diff codons of same amino acid.
Have different levels of corresponding t RNA.
Insect toxin gene was altered by changing many bases of the third position of the
20 % of its bases were altered to make it more plantlike in codon usage.
Drought, High salinity are two major problems in growing crop plants.
In drought tolerant plants, fungi, bacteria sugar trehalose protects orgs during
Non reducing storage
Trehalose Synthetic and Degradative Pathways
Two enzymatic reactions make trehalose. First, trehalose phosphate
synthetase converts UDP-glucose plus glucose 6-phosphate into
carbohydrate trehalose 6-phosphate. Next, trehalose-6-phosphate phosphatase
removes the phosphate to make trehalose. Trehalose may be broken
down into to glucose by trehalase.
Absorbs and release water.
Functional genomics in Plants
Functional genomics strategies are followed screening of entire genome
Mostly rely on removal and blockage of gene expression.
Methods to know the function of plant genes are,
3.Fast Neutron Mutagenesis.
4.TILLING (Target Induced Local Lesions in Genomes)
Method to find function of new genes.
Transposon or T DNA insertions are used to generate plant mutants.
T DNA includes only a reporter gene.
Clone the upstream and
downstream of the insertion
Insert into plant chromosome.
Disrupt a plant gene
Phenotype can be screened and assessed
Plant gene identified.
Method to find function of new genes.
Gene silencing is done by RNAi
RNAi is triggered by double stranded RNA, which is cut into short segments
RISC Enzyme complex
Indentify homologous RNA
Prevents expression of mRNA to protein.
Fast Neutron Mutagenesis
Method to generate gene knockouts.
Fast Neutrons are used to induce DNA deletions.
Fast Neutrons are created by nuclear fission.
Hence neutrons of 1 MeV kinetic energy are generated.
These neutrons cause deletions in exposed DNA.
Seeds treated with fast neutron are called M1 seeds.
M1 seeds------Grown into plants (each plant has different deletions)-----M2 seeds------Plants-----Collect DNA into pools of varying sizes.
Identifying Fast Neutron Mutants with PCR
(A) M1 seeds are mutagenized by exposure to fast neutrons. The M2 seeds are grown and DNA harvested from
each plant. The DNA is mixed to form large pools from many M2 seeds and successively smaller pools from
fewer M2 seeds. (B) The seeds are analyzed for deletions using PCR. The primers recognize specific locations
in the plant genome. If the DNA pool contains any deletions, the PCR primer will produce two bands, one from
the wild-type (full-length) gene and one from the plant with the deletion.
Target induced local lesions in Genomes.
First seed are socked in chemical mutagens (EMS) ------ induce G/C and A/T
transitions in DNA.
M1 seeds------Grown into plants------M2 seeds------Plants-----DNA is harvested
and pooled into large megapool and smaller pool.
PCR primers are used to amplify the selected regions of DNA.
PCR products are labeled with two different labels.
Heteroduplexex of mutants and wild type DNA are created.
IF PCR product is cleaved by CEL-1, it will have one fluorescent label.
Uncleaved DNA will have both fluorescent labels.
Identifying Point Mutations with TILLING
TILLING identifies point mutations in a library of plant DNA. (A) EMS, a chemical mutagen, induces point mutations in seeds.
The M1 seeds are grown into plants and the M2 seeds are harvested. Most M2 seeds are stored as a stock, while the remaining M2
seeds are grown into plants. DNA is harvested from each plant and pooled. The larger pools contain DNAs from all the M2 plants,
and the smaller pools contain DNA from one or two different M2 plants.
(B) Point mutations are identified in the DNA pools using PCR to randomly amplify different areas of the plant genomes. Some
PCR products will contain point mutations and others will be normal. These are denatured and annealed so that some of the normal
and mutant strands form hybrids. The reannealed PCR products are labeled at each end with a different fluorescent tag. The PCR
products are then digested with the enzyme CEL-1, which cuts only where the helix has a mismatch. This leaves any mutant:
normal hybrids with a single fluorescent tag.
Food Safety and Starlink Corn
The term transgenic crop is more accurate than genetically modified crop.
Allergenic potential of transgenic crops has caused much controversy.
Starlink an unapproved Transgenic corn was detected in taco shell.
Transgene Cry9C is more resistant to stomach acid.
Transgenic corn was mixed with other corn and sent to markets.
Split approval given by EPA (Environmental Protection Agency)
Sent to markets and allergic reactions were reported by the public.
CDC (Center for Disease Control) examines the blood samples of public and
concluded that allergic reactions was not due to Cry9C protein.
All starlink seed and products were called back from the market.
BT Toxin and Butterflies
Journal Nature suggested that Monarch butterflies were killed by eating
pollen from corn caring Bt gene.
The result of the controversy was a surge of research on transgenic Bt plant
and their effect on the butterflies and non target organisms.
Harmful pollens were Cry1Ab, event # 176
In 2001, this transgene event was no longer approved and was no longer
grown in the United States.
Cry1Ab were only toxic to Monarch caterpillars at density greater than 1000
pollen grain per square centimeter.
Cry1 F and Cry9C showed no toxic effect.
Studies were don in labs where caterpillar had to eat only milkweed
contaminated with pollen.