transgenic technology getting dna into a cell getting it stably integrated getting a plant back from...
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TRANSGENIC TECHNOLOGY
getting DNA into a cell
getting it stably integrated
getting a plant back from the cell
Plant transformation
1. a suitable transformation method
2.2. a means of screening for transformantsa means of screening for transformants
3. an efficient regeneration system
4. genes/constructs Vectors
Promoter/terminator
reporter genes
selectable marker genes
‘genes of interest’
Requirement
Transformation methods
DNA must be introduced into plant cells
Indirect - Agrobacterium tumefaciens
Direct - Chemical method
- Electrical method
- Physical methods
Chemical Method
1. Use of PEG (Polyethylene glycol (PEG)-mediated )
2. Protoplasts are incubated with a solution of DNA and PEG
Electrical method
1. Electroporation (electropermeabilization)
2. Cells or protoplast are subjected to short electrical pulse
Physical Methods
1. Particle bombardment
2. Microinjection
3. Silicon Carbide whiskers
AgrobacteriumAgrobacterium-mediated -mediated transformationtransformation
A natural genetic A natural genetic engineerengineer
2 species2 species• A.tumefaciensA.tumefaciens
(produces a gall)(produces a gall)• A. rhizogenesA. rhizogenes
(produces roots)(produces roots) OncOncoogenesgenes (for (for
auxin and cytokinin auxin and cytokinin synthesis) + Opinessynthesis) + Opines
In the presence of In the presence of exudates (e.g. exudates (e.g. acetosyringone) from acetosyringone) from wounded plants, wounded plants, VirVirulence (Vir) genes ulence (Vir) genes are activated and are activated and cause the t-DNA to be cause the t-DNA to be transferred to plants. transferred to plants. Everything between Everything between the left and right the left and right border is transferred.border is transferred.
BACTERIAL GALL DISEASESBACTERIAL GALL DISEASES
Galls:Galls: overgrowth or proliferation of tissue, primarily overgrowth or proliferation of tissue, primarily due to increased cell division (hyperplasia) and due to increased cell division (hyperplasia) and increased cell size (hypertrophy).increased cell size (hypertrophy).
Bacterial Galls:Bacterial Galls:
induced by bacteria in 3 different genera. induced by bacteria in 3 different genera. • AgrobacteriumAgrobacterium• PseudomonasPseudomonas• ClavibacterClavibacter
Genes for plant hormone production found Genes for plant hormone production found on bacterial plasmids!on bacterial plasmids!
Crown Gall Disease: Crown Gall Disease: Agrobacterium tumefaciensAgrobacterium tumefaciens
Gram -Gram - DicotsDicots WorldwideWorldwide
Disease Cycle
Agrobacterium tumefaciensAgrobacterium tumefaciens
CharacteristicsCharacteristics• Plant parasite that causes Crown Gall Plant parasite that causes Crown Gall
DiseaseDisease• Encodes a large (~250kbp) plasmid called Encodes a large (~250kbp) plasmid called
Tumor-inducing (Ti) plasmidTumor-inducing (Ti) plasmid Portion of the Ti plasmid is transferred between Portion of the Ti plasmid is transferred between
bacterial cells and plant cells bacterial cells and plant cells T-DNA (Tumor T-DNA (Tumor DNADNA))
Agrobacterium tumefaciensAgrobacterium tumefaciens
T-DNA integrates stably into plant genomeT-DNA integrates stably into plant genomeSingle stranded T-DNA fragment is Single stranded T-DNA fragment is converted to dsDNA fragment by plant cellconverted to dsDNA fragment by plant cell Then integrated into plant genomeThen integrated into plant genome 2 x 23bp direct repeats play an important role in 2 x 23bp direct repeats play an important role in
the excision and integration processthe excision and integration process
Agrobacterium tumefaciensAgrobacterium tumefaciens
Tumor formation = hyperplasiaTumor formation = hyperplasia Hormone imbalanceHormone imbalance Caused by Caused by A. tumefaciensA. tumefaciens
• Lives in intercellular spaces of the plantLives in intercellular spaces of the plant• Plasmid contains genes responsible for the Plasmid contains genes responsible for the
diseasedisease Part of plasmid is inserted into plant DNAPart of plasmid is inserted into plant DNA Wound = entry point Wound = entry point 10-14 days later, 10-14 days later,
tumor formstumor forms
Agrobacterium tumefaciensAgrobacterium tumefaciens
What is naturally encoded in T-DNA?What is naturally encoded in T-DNA?• Enzymes for auxin and cytokinin synthesisEnzymes for auxin and cytokinin synthesis
Causing hormone imbalance Causing hormone imbalance tumor tumor formation/undifferentiated callusformation/undifferentiated callus
Mutants in enzymes have been characterizedMutants in enzymes have been characterized
• Opine synthesis genes (e.g. octopine or Opine synthesis genes (e.g. octopine or nopaline)nopaline)
Carbon and nitrogen source for Carbon and nitrogen source for A. tumefaciensA. tumefaciens growth growth Insertion genes Insertion genes
• Virulence (vir) genesVirulence (vir) genes• Allow excision and integration into plant genomeAllow excision and integration into plant genome
Ti plasmid of Ti plasmid of A. tumefaciensA. tumefaciens
1. Auxin, cytokinin, opine synthetic genes transferred to plant
2. Plant makes all 3 compounds
3. Auxins and cytokines cause gall formation
4. Opines provide unique carbon/nitrogen source only A. tumefaciens can use!
Agrobacterium tumefaciensAgrobacterium tumefaciens How is T-DNA modified to allow genes of How is T-DNA modified to allow genes of
interest to be inserted?interest to be inserted?• In vitroIn vitro modification of Ti plasmid modification of Ti plasmid
T-DNA tumor causing genes are deleted and replaced T-DNA tumor causing genes are deleted and replaced with desirable genes (under proper regulatory control)with desirable genes (under proper regulatory control)
Insertion genes are retained (vir genes)Insertion genes are retained (vir genes) Selectable marker gene added to track plant cells Selectable marker gene added to track plant cells
successfully rendered transgenic [antibiotic resistance successfully rendered transgenic [antibiotic resistance gene gene geneticin (G418) or hygromycin] geneticin (G418) or hygromycin]
Ti plasmid is reintroduced into Ti plasmid is reintroduced into A. tumefaciensA. tumefaciens A. tumefaciensA. tumefaciens is co-cultured with plant leaf disks is co-cultured with plant leaf disks
under hormone conditions favoring callus under hormone conditions favoring callus development (undifferentiated)development (undifferentiated)
Antibacterial agents (e.g. chloramphenicol) added to Antibacterial agents (e.g. chloramphenicol) added to kill kill A. tumefaciensA. tumefaciens
G418 or hygromycin added to kill non-transgenic plant G418 or hygromycin added to kill non-transgenic plant cellscells
Surviving cells = transgenic plant cellsSurviving cells = transgenic plant cells
Agrobacterium and genetic engineering:Engineering the Ti plasmid
Co-integrative and binary vectorsCo-integrative and binary vectors
Binary vector
LB RB
Co-integrative
cause ‘Crown gall’ disease
Agrobacterium tumefaciens
Agrobacterium-mediated transformation
Agrobacterium is a ‘natural genetic engineer’
i.e. it transfers some of its DNA to plants
Electroporate T-DNA vector into Agrobacterium and select for tetr
Expose wounded plant cells to transformed agro strain
Induce plant regeneration and select for Kanr cell growth
Explants: cells and protoplasts
Most direct way to introduce foreign DNA into the nucleus
ElectroporationElectroporation
Diagram of one techniqueDiagram of one technique
Microprojectile bombardment
• uses a ‘gene gun’
• DNA is coated onto gold (or tungsten) particles
(inert)
• gold is propelled by helium into plant cells
• if DNA goes into the nucleus it can be integrated into the plant chromosomes
• cells can be regenerated to
whole plants
In the "biolistic" (a cross between biology and ballistics In the "biolistic" (a cross between biology and ballistics )or "gene gun" method, microscopic gold beads are )or "gene gun" method, microscopic gold beads are coated with the gene of interest and shot into the coated with the gene of interest and shot into the plant cell with a pulse of helium.plant cell with a pulse of helium.
Once inside the cell, the gene comes off the bead and Once inside the cell, the gene comes off the bead and
integrates into the cell's genome.integrates into the cell's genome.
Model from BioRad: Model from BioRad: Biorad's Helios Gene Biorad's Helios Gene GunGun
Most direct way to introduce foreign DNA into the nucleus
Achieved by electromechanically operated devices that control the insertion of fine glass needles into the nuclei of individuals cells, culture induced embryo, protoplast
Labour intensive and slow
Transformation frequency is very high, typically up to ca. 30%
MicroinjectionMicroinjection
Silicon carbide forms long, needle like crystals
Cells are vortex mixed in the present of whiskers and DNA
DNA can be introduced in the cells following penetration by the whiskers
Silicon Carbide WhiskersSilicon Carbide Whiskers
Gene constructGene construct
BamHI
gus-intron nptII T 35S P 35S T 35S LB RB
P SAG12 ipt P 35S T nos
Gene constructGene construct
VectorsPromoter/terminator
reporter genesselectable marker genes
‘genes of interest’.
VectorsVectors
Ti-plasmid based vectora. Co-integrative plasmidb. Binary plasmid
Coli-plasmid based vectora. Cloning vectorb. Chimeric Plasmid
Viral vectora. It is normally not stably integrated into the plant cellb. It may be intolerant of changes to the organization of its genomec. Genome may show instability
PromoterPromoter
1.1. A nucleotide sequence within an operonA nucleotide sequence within an operon2.2. Lying in front of the structural gene or genesLying in front of the structural gene or genes3.3. Serves as a recognition site and point of Serves as a recognition site and point of
attachment for the RNA polymeraseattachment for the RNA polymerase4.4. It is starting point for transcription of the It is starting point for transcription of the
structural genesstructural genes5.5. It contains many elements which are involved in It contains many elements which are involved in
producing specific pattern and level of expressionproducing specific pattern and level of expression6.6. It can be derived from pathogen, virus, plants It can be derived from pathogen, virus, plants
themselves, artificial promoterthemselves, artificial promoter
Types of PromoterTypes of Promoter Promoter always expressed in most tissue Promoter always expressed in most tissue
(constitutive)(constitutive)-. 35 s promoter from CaMV Virus-. 35 s promoter from CaMV Virus-. Nos, Ocs and Mas Promoter from bacteria-. Nos, Ocs and Mas Promoter from bacteria-. Actin promoter from monocot-. Actin promoter from monocot-. Ubiquitin promoter from monocot-. Ubiquitin promoter from monocot-. Adh1 promoter from monocot-. Adh1 promoter from monocot-. pEMU promoter from monocot-. pEMU promoter from monocot
Tissue specific promoterTissue specific promoter-. Haesa promoter-. Haesa promoter-. Agl12 promoter-. Agl12 promoter
Inducible promoterInducible promoter-. -. Aux promoterAux promoter
Artificial promoterArtificial promoter-. Mac promoter (Mas and 35 s promoter)-. Mac promoter (Mas and 35 s promoter)
easy to visualise or assay
- ß-glucuronidase (GUS) (E.coli)
-green fluorescent protein (GFP) (jellyfish)
- luciferase (firefly)
Reporter gene
GUS
Cells that are transformed with GUS will form a blue precipitate when tissue is soaked in the GUS substrate and incubated at 37oC
this is a destructive assay (cells die)
The UidA gene encoding activity is commonly The UidA gene encoding activity is commonly used. Gives a blue colour from a colourless used. Gives a blue colour from a colourless substrate (substrate (X-gluX-glu) for a qualitative assay. Also ) for a qualitative assay. Also causes fluorescence from causes fluorescence from MMethyl ethyl UUmbelliferyl mbelliferyl GGlucuronide (lucuronide (MUGMUG) for a quantitative assay.) for a quantitative assay.
GUS
Bombardment of GUS gene
- transient expression
Stable expression of GUS in moss Phloem-limited expression of
GUS
HAESA gene encodes a receptor protein kinase that controls floral organ abscission. (A) transgenic plant expressing a HAESA::GUS fusion. It is expressed in the floral abscission zone at the base of an Arabidopsis flower.
Transgenic plants that harbor the AGL12::GUS fusions show root-specific expression.
Inducible expressionInducible expression
GFP (Green Fluorescent Protein)
GFP glows bright green when irradiated by blue or UV light
This is a nondestructive assay so the same cells can be monitored all the way through
Fluoresces green under UV illuminationFluoresces green under UV illumination Problems with a cryptic intron now resolved.Problems with a cryptic intron now resolved. Has been used for selection on its own.Has been used for selection on its own.
GFP
protoplast colony derived from protoplast
mass of callus
regenerated plant
let you kill cells that haven’t taken up DNA- usually genes that confer resistance to a phytotoxic substance
Most common:
1. antibiotic resistance
kanamycin, hygromycin
2. herbicide resistance
phosphinothricin (bialapos); glyphosate
Selectable Marker Gene
Only those cells that have taken up the DNA can grow on media containing the selection agent
Gene of interestGene of interest
Sequence of DNA which will be inserted Sequence of DNA which will be inserted to the host cell and its product will be to the host cell and its product will be
studied or beneficial for mankindstudied or beneficial for mankind
Origin of gene interest:
1. Non plant genes
2. Plant genes
Exogenous genes (non-plant
genes)
pathogen-derived genes
bacterial genes
any other organism
Endogenous genes (Plant
genes)
Enzymes in biochemical pathway
Natural resistance genes
There are many thousands of cells in a leaf disc or callus clump - only a proportion of these will have taken up the DNA
therefore can get hundreds of plants back - maybe only 1% will be transformed
How do we know which plants have taken up the DNA?
Could test each plant - slow, costly
Or use reporter genes & selectable marker genes
Screening technique
ScreeningScreening
Transformation frequency is low (Max 3% of all Transformation frequency is low (Max 3% of all cells) and unless there is a selective advantage for cells) and unless there is a selective advantage for transformed cells, these will be overgrown by non-transformed cells, these will be overgrown by non-transformed.transformed.
Usual to use a positive selective agent like Usual to use a positive selective agent like antibiotic resistance. The NptII gene encoding antibiotic resistance. The NptII gene encoding Neomycin phospho-transferase II phosphorylates Neomycin phospho-transferase II phosphorylates kanamycin group antibiotics and is commonly kanamycin group antibiotics and is commonly
usedused. .
Screening (selection)Screening (selection)
Select at the level of the intact plantSelect at the level of the intact plant Select in cultureSelect in culture
• single cell is selection unitsingle cell is selection unit• possible to plate up to 1,000,000 cells possible to plate up to 1,000,000 cells
on a Petri-dish.on a Petri-dish.• Progressive selection over a number of Progressive selection over a number of
phasesphases
Selection StrategiesSelection Strategies
PositivePositive NegativeNegative
VisualVisual
Positive selectionPositive selection
Add into medium a toxic compound e.g. Add into medium a toxic compound e.g. antibiotic, herbicideantibiotic, herbicide
Only those cells able to grow in the Only those cells able to grow in the presence of the selective agent give presence of the selective agent give coloniescolonies
Plate out and pick off growing colonies.Plate out and pick off growing colonies. Possible to select one colony from millions Possible to select one colony from millions
of plated cells in a days work.of plated cells in a days work. Need a strong selection pressure - get Need a strong selection pressure - get
escapesescapes
Negative selectionNegative selection
Add in an agent that kills dividing cells Add in an agent that kills dividing cells e.g. chlorate / BUdR.e.g. chlorate / BUdR.
Plate out leave for a suitable time, Plate out leave for a suitable time, wash out agent then put on growth wash out agent then put on growth medium.medium.
All cells growing on selective agent All cells growing on selective agent will die leaving only non-growing cells will die leaving only non-growing cells to now grow.to now grow.
Useful for selecting auxotrophsUseful for selecting auxotrophs..
Visual selectionVisual selection
Only useful for coloured or Only useful for coloured or fluorescent compounds fluorescent compounds
Plate out at about 50,000 cells per Plate out at about 50,000 cells per plate.plate.
Pick off coloured / fluorescent Pick off coloured / fluorescent compoundscompounds
Possible to screen about 1,000,000 Possible to screen about 1,000,000 cells in a days work.cells in a days work.
Positive and Visual SelectionPositive and Visual Selection
How do we get plants back from cells?
We use tissue culture techniques to regenerate whole plants from single cells
getting a plant back from a single cell is important so that every cell has the new DNA
Regeneration System
Regeneration
Regeneration of shoots from leaf protoplasts in Arabidopsis thaliana
Plant tissue culture uses growth regulators and nutrients to regenerate plants in vitro
Somatic embryogenesis in peanut