Genetic Engineering in Plants


Genetic engineering is an effective way to increase crop yield and ward off diseases or insects that kill crops. Genetically modified plants are already used by farmers in many places, and a reason for their widespread use may be the ease of creating genetically altered plants.

Most plants are totipotent, meaning an entire plant can grow from a single cell (Peters). Scientists can isolate a plant cell and introduce specific enzymes to the plant cell (Peters). These enzymes then digest the cell wall of the plant cell, creating a protoplast (Peters). Without a cell wall, manipulation of cellular DNA becomes much easier (Peters). Scientists can, through various methods, introduce modified DNA into the plant genome (Peters). As the protoplast re-grows and divides, the modified DNA will be transferred to every cell in the new plant (Peters).


The Ti plasmid. Source:  http://openlearn.open.ac.uk/file.php/2808/S250_1_004i.jpg
The Ti plasmid. Source: http://openlearn.open.ac.uk/file.php/2808/S250_1_004i.jpg

The Ti plasmid

One metho d used to genetically engineer plants makes use of the Ti plasmid, found naturally in Agrobacterium tumefaciens. Naturally, Agrobacterium tumefaciens infections are trig gered by the wound response molecules, such as acetosynringone (Thomson). The release of these molecules leads to the virulence of Agrobacterium tumefaciens (Thomson). Agrobacterium tumefaciens injects the Ti plasmid into plant cells’ DNA, and then causes the growth of a crown gall (a tumor) (Thomson). The section of the plasmid that enters plant cells is called T DNA, and is about 30,000 base pairs long. The tumor-inducing region of the plasmid, called the onc gene, codes for plant growth hormones which cause plant cell proliferation and a resulting tumor (Thomson). The onc gene also codes for some derivatives of arginine (nopaline, octopine), which help the bacteria grow (Thomson). The bacterial interaction in this situation is called genetic colonization; by causing the tumor, the bacteria also provide a means of supporting themselves (Thomson).



The Ti plasmid as a vector for gene delivery

The genetic sequence is designed to contain the targeted gene and some complementary bases to the sticky ends of the cut plasmid (Thomson). The onc gene region present in the T DNA is deleted and replaced by the foreign gene and a marker gene like, antibiotic resistance (Thomson).

This plasmid is reintroduced into Agrobacterium tumefaciens, and plant protoplasts are exposed to the bacteria (Thomson). Agrobacterium tumefaciens infect the protoplasts and insert the gene of interest from the Ti plasmid into the cellular DNA (Thomson). This protoplast will then replicate to form a genetically engineered plant (Thomson).

ch7f49.jpg
A diagram of the genetic engineering process. Source: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cell&part=A1360

Using cells with cell walls, DNA from the Ti plasmid can still enter dicotyledonous plants (Thomson). In this technique, small pieces of a leaf will be cut and grown in culture with the Agrobacterium tumefaciens which has the modified Ti plasmid (Thomson). Cuts in leaves trigger the virulence of Agrobacterium tumefaciens, and the bacteria will then infect the plant cells (Thomson). The plant cells, once transformed, will then be placed in media with the toxin that the marker gene codes resistance against, and the surviving cells will be successfully transformed (Thomson).

After transformation, the cells are grown in a media under controlled conditions. The hormone auxin may be added, to initiate and maintain callus. Then the cytokinin hormones will be added to the medium to trigger shoot development. “Withdrawal of the cytokinin promotes root growth,” and the plants will be allowed to grow in culture. When the plants have developed completely, they are planted in soil. In this fashion, genetically engineered plants will be made in a laboratory setting, and then implemented into the real world (Thomson).

The Ti plasmid is a highly effective method for genetic engineering because it infects a large variety of dicotyledonous plants, including carrots, tomatoes, potatoes, petunias, and cabbage (Peters). There are over 170,000 different species of dicotyledonous plants, so the action of Agrobacterium tumefaciens is highly efficient for genetic engineering (Peters). Many crops, however, are monocotyledonous. Staple crops including rice, wheat and corn cannot be infected by Agrobacterium tumefaciens, so other methods of genetic engineering are needed (Peters).


Genetic engineering in monocotyledonous plants- overcoming the limitations of the Ti vector

Techniques for monocot plants include microinjection, electroporation, and the “shotgun method”. Microinjection is the process in which the Ti plasmid is directly injected into the protoplast using a fine needle ("Plant Genetic Engineering: Methodology"). In electroporation, “brief pulses of high voltage electricity…induce the formation of transient pores in the membrane of the host cell” ("Plant Genetic Engineering: Methodology"). These pores allow DNA to enter the plant cell ("Plant Genetic Engineering: Methodology"). In the “shotgun method”, Helium is used to propel small bullets of gold and tungsten coated in DNA into the cell. The bullets are coated with both the targeted DNA, and marker DNA ("Plant Genetic Engineering: Methodology"). Some bullets go through the cell membrane, and into the cell. Some bullets may even enter chloroplasts or mitochondria ("Plant Genetic Engineering: Methodology"). Once the DNA is in the cell, it can join the cellular DNA ("Plant Genetic Engineering: Methodology"). The cells that were bombarded will be grown in a culture, and those expressing the marker gene will be identified as successful examples of this method ("Plant Genetic Engineering: Methodology"). These cells will then be grown into plants ("Genetic Engineering Methods").This method has become prevalent recently, and is applicable to almost all species of crops. It is used frequently in rice, a major grain source in the world ("Plant Genetic Engineering: Methodology").

Overcoming the potential harm in genetic engineering

One potential danger is that these modified genes will be transferred to other plants through cross pollination (Thomson). A solution may be if genes are introduced into chloroplasts or mitochondria, organelles which are maternally inherited (Thomson). In such a case, the modified DNA will not be present in the pollen of plants, do genes added to plants will not spread with cross-pollination to other plants (Thomson).

Works Cited

“Genetic Engineering Methods.” Evolutionary Powerhouses Replicators. Think Quest, 2000. Web. 2 Mar. 2010. <http://library.thinkquest.org/‌C004367/‌be9.shtml>.

Peters, Pamela. “Transforming Plants - Basic Genetic Engineering Techniques.” Access Excellence @ the national health museum Resource Center. National Health Museum, 1993. Web. 2 Mar. 2010. <http://www.accessexcellence.org/‌RC/‌AB/‌BA/‌Transforming_Plants.php>.

“Plant Genetic Engineering: Methodology.” Genetic Engineering and Society. Arizona State University, 16 Sept. 2006. Web. 2 Mar. 2010. <http://bioenergy.asu.edu/‌photosyn/‌courses/‌BIO_343/‌lecture/‌geneng.html>.

Thomson, J.A. “Genetic Engineering of Plants.” Encyclopedia of Life Support SystemsBio. University of Cape Town, South Africa, n.d. <http://www.eolss.net/‌ebooks/‌Sample%20Chapters/‌C17/‌E6-58-03-04.pdf>. Rpt. in Biotechnology. Encyclopedia of Life Support Systems. Web. 3 Mar. 2010. <http://www.eolss.net/‌ebooks/‌Sample%20Chapters/‌C17/‌E6-58-03-04.pdf>.


Works Cited for Pictures


Ti Plasmid: http://openlearn.open.ac.uk/file.php/2808/S250_1_004i.jpg
Diagram of the vector transformation process: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cell&part=A1360