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Analysis of Transgenic Plants: Methods and Techniques

This lecture discusses the established methods to determine if a plant is transgenic, including Southern and northern blotting, agarose and SDS-PAGE gel electrophoresis, and gene expression measurement. It also examines the relevance of phenotypic data and the importance of designing a Southern blot experiment. The lecture explores different molecular characterization techniques such as PCR, Southern blot, Northern blot, Western blot, and qRT-PCR. The use of PCR and gel electrophoresis for analysis and the potential issue of false positives are also discussed.

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Analysis of Transgenic Plants: Methods and Techniques

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  1. Lecture 18, Chapter 11Analysis of transgenic plantspart I Mat Halter 3/27/12

  2. Discussion questions 1. What are the established methods to determine if a plant is transgenic and whether the transgene(s) is expressed? 2. In a Southern or northern blot, through what type of chemical bond does the complementary probe bind to nucleic acid? 3. Nucleic acids and proteins are separated according to size in agarose and sodium dodecyl sulfate–polyacrylamide gel electrophoreisis (SDS-PAGE) gels, respectively. Why do both types of macromolecules migrate toward the anode in an electrical current? 4. What is gene expression, and how can you measure it? 5. Explain why phenotypic data provide evidence of transformation but not proof of a transformation event. 6. What factors are most important when designing a Southern blot experiment to test for transgenic status? Is my plant transgenic?

  3. LB RB P Visual Selection T P Antibiotic Selection T P Gene of Interest T Transformation Visual Selection (ex. OFP) Antibiotic Selection (ex. Hygromycin)

  4. Fluorescent Proteins http://en.wikipedia.org/wiki/File:FPbeachTsien.jpg

  5. Visual selection using FPs T+ T- T- T+

  6. Antibiotic Selection • When a mixture of transformed and untransformed callus is placed on antibiotic selection media, only the transformed callus carrying the antibiotic selection cassette is able to survive and grow. • In most cases, the untransformed callus dies, making it “easy” to select for callus carrying the T-DNA.

  7. Figure 9.3 Sometimes “escapes” occur– for kanamycin resistance markers tissue is red—very stressed

  8. Is my plant transgenic? • Surviving selection—but remember that there can be escapes. Need more proof besides surviving selection. • Reporter genes—better. But there must be a reporter gene in the vector. • All around easy test—PCR. But what if Agrobacterium survives in low amounts in the T0 plants? Could give a false positive band. PCR is ok for biolistics. • Can do PCR on T1 plants or look at segregation of the transgene.

  9. GFP+Bt segregation Using GFP screening to “see” Bt when the transgenes are linked. Nat Biotechnol 17:1125

  10. Stable integration of transgene • Transgene is permanently integrated into the genome of the host plant. • Transmitted to progeny (Tn plants) in Mendelian fashion • Need convincing proof of stable integration • Multiple assays are possible—but most researchers are best convinced by Southern blot data. Why all the mystique and skepticism?

  11. Good reasons for doubt • New methods don’t always work, but wishful thinking takes over (see Chapter 10 section—the Rush to Publish) • Resilient Agrobacterium can linger • The unexpected can be tricky. • Others?

  12. Molecular characterization of transgenic plants • PCR- Simplest and fastest method. Prone to false positives. • Southern Blot- Confirms insertion of the tDNA into the genomic DNA of the target organism, as well as provides insertion copy number. • Northern Blot- Confirms the presence of RNA transcript accumulation from the transgene of interest. • Western Blot- Confirms presence of the PROTEIN produced from the inserted transgene of interest. • qRT-PCR- Provides a relative expression level for the gene of interest—transcript—like Northern blot.

  13. PCR and DNA Gel Electrophoresis PCR- Polymerase chain reaction, uses DNA primers to amplify a target sequence of DNA, producing billions of copies of identical DNA. Gene cloning Molecular analysis (Confirmation of the presence of a particular fragment of DNA in a pool of DNA)

  14. PCR analysis by gel electrophoresis - - - - + + + + - - - -

  15. Gel electrophoresis • The migration of DNA through an Agarose matrix using the application of an electric field. • Agarose, when solidified in a gelatin form, produces a thick netting that allows small particles to move through it quickly, while larger particles move more slowly. • By moving particles of different size through the agarose gel, they can be separated, with the small particles moving quickly away from the slower moving large particles. • This method is used to separate DNA fragments by size.

  16. PCR analysis by gel electrophoresis - Ladder Sample 1500 bp 1000 bp 750 bp 500 bp +

  17. PCR and False Positives Genomic DNA Transgenic plant produced from Agrobacterium-mediated transformation • In T0 plants, Agrobacterium left over from the initial transformation is still present in all tissues. • Contamination of the genomic DNA with the initial transformation vector that is still present in the agrobacterium can produce a PCR band.

  18. Southern Blot • Southern blotting confirms the presence of the gene of interest in the genomic DNA of the target plant and avoids the pitfalls of potential false positives. • Steps • Genomic DNA isolation • Restriction enzyme digestion of genomic DNA • Running digested DNA on agarose gel to separate fragmented DNA by size. • Transfer of separated DNA to nylon membrane • Hybridization with radioactive DNA probe

  19. Restriction Digestion of Genomic DNA • Restriction digestion of genomic DNA produces a streak on an agarose gel rather than a single band. Why?

  20. Example: EcoRI • What is the probability of a sequence of DNA in a plant genome having the sequence of bases corresponding to an EcoRI cut site? • Each site can be 4 possible bases (A, T, C, or G), and the EcoRI enzyme requires 6 sites (GAATTC) • The probability of finding a random site in a genome that happens to have the sequence GAATTC can be calculated: 1⁄4 x 1⁄4 x 1⁄4 x 1⁄4 x 1⁄4 x 1⁄4 = 1⁄4096 • Probability states that there will be an EcoRI cut site once every 4096 bases, purely by chance.

  21. EcoRI example, cont. • The Arabidopsis thaliana genome is roughly 157,000,000 base pairs in size. 157,000,000⁄4096 = 38,330 • Though this value is only based on probability, and therefore may not be the TRUE number of EcoRI cut sites in this genome, it can still accurately be assumed that there are A LOT of cut sites. • If restriction digested with EcoRI, the arabidopsis genome would be cut into tens of thousands of pieces, all of unique size. • This is why when you run a sample of digested genomic DNA, you see a streak, rather than a band. The streak shows all sizes of DNA produced by the random assortment of cut sites within the genome.

  22. Essentially, every known restriction enzyme will have cut sites in a plant genome. • How can enzyme selection be used to detect copies of an inserted transgene? Digested Genomic EcoRI Site DNA Probe LB RB • Single cutting enzymes can be designed into the T-DNA before transformation that will enable proper digestion of the genome as well as a single cut within the T-DNA.

  23. Southern Blotting Isolated genomic DNA from transgenic plant Gel electrophoresis - Restriction enzyme digest +

  24. Southern Blotting Transfer separated DNA from agarose gel to nylon membrane Agarose Gel Nylon Membrane

  25. Southern Blotting Hybridizing the DNA probe • The DNA probe is designed to be complimentary to your gene of interest. • It is synthesized using radioactive phosphorus, which emits a detectable signal. • The complimentary probe will bind (by hydrogen bonding) only to your gene of interest because of the high sequence specificity.

  26. Southern Blotting The Final product Lane 1- Ladder Lane 2- Negative Control Lanes 3-8-Experimental Events • Bands at different places from event to event indicate insertion at different places in the genome. • The number of bands in each well indicates how many insertions there were in each event.

  27. Why is a single cut within the T-DNA necessary? EcoRI Site EcoRI Site LB RB LB RB RB If there is no EcoRI site within the tDNA, after digestion with EcoRI these two insertion sites will be indistinguishable from one another after electrophoresis and probing. Cutting within the T-DNA is necessary to distinguish each and every insertion event. This is VERY important.

  28. Restriction enzyme selection for Southern blotting

  29. Southern analysis of mOrange plants genomic DNA. A. Genomic DNA digested with BamHI, electrophoresed on 1% agaroseArabidopsisLane 1. 21-12-3-3 Lane 2. 21-12-6-1 Lane 3. 21-12-6-6 Lane 4. 21-12-7 Lane 5. 21-12-29 Lane 6. 21-12-34 Lane 7. ‘Columbia’ Tobacco Lane 1. 21-12-2-7 Lane 2. 21-12-5-5 Lane 3. 21-12-5-8 Lane 4. 21-12-6-6 Lane 5. 21-12-8-7 Lane 6. ‘Xanthi’ plasmid pMDC32-mOrange (21-12) BamHI digest B. Transferred to nylon membrane. Hybridization to 32-P labeled probe of mOrange coding sequence. Exposed to phosphor screen 3 weeks. plasmid plasmid Arabidopsis Arabidopsis Tobacco Tobacco 1 1 2 2 3 3 4 4 5 5 6 6 (Kb) 1 1 2 2 3 3 4 4 5 5 6 6 7 7 10 8 6 4 3 2 1.55 1.4 1 Southern blot analysis of transgenic plants with an orange fluorescent protein llg 2007-05-02 Sgel3 mOrange.tif llg 2007-06-26 S mOrange 3wk.tif What’s wrong?

  30. Northern Blot • Confirms the presence of mRNA transcripts transcribed from the gene of interest in the target organism. • Extremely similar to the Southern blot, but detects RNA instead of DNA. • Steps: • Isolation of RNA • Running RNA on agarose gel to separate by size • Transfer separated RNA from gel to membrane • Hybridize a radioactive DNA probe to the RNA on the membrane Sound familiar?

  31. Northern Blot No digestion necessary… why is this? RNA loading controls are necessary to ensure an equal amount of RNA is loaded in each well.

  32. Discussion questions 1. What are the established methods to determine if a plant is transgenic and whether the transgene(s) is expressed? 2. In a Southern or northern blot, through what type of chemical bond does the complementary probe bind to nucleic acid? 3. Nucleic acids and proteins are separated according to size in agarose and sodium dodecyl sulfate–polyacrylamide gel electrophoreisis (SDS-PAGE) gels, respectively. Why do both types of macromolecules migrate toward the anode in an electrical current? 4. What is gene expression, and how can you measure it? 5. Explain why phenotypic data provide evidence of transformation but not proof of a transformation event. 6. What factors are most important when designing a Southern blot experiment to test for transgenic status?

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