Lecture 39: Applications of Genetic Engineering & Biotechnology - Genetically Modified Foods

ST 4.1 Genetically Modified Foods

Genetically modified foods (GMOs): Derived from genetically modified organisms.
Genetically modified organisms: Plants and animals of agricultural importance whose genomes have been altered.
Modification occurs via genetic engineering or recombinant DNA technologies.

GM foods address malnutrition.
More than 200 different GM foods have been created.
Golden Rice:
- A variety of rice that contains the vitamin A precursor (beta-carotene).
- Developed on a humanitarian nonprofit basis.
- Purpose: To alleviate vitamin A deficiencies in the developing world.
- Controversy: Protestors argue that Golden Rice is a threat to human health and biodiversity.

Genetic engineering allows genes to be cloned and transferred from one organism to another.
Transgenic organism: Created when genes are transferred between unrelated species (e.g., dog to cat).
Cisgenic: Genes transferred within a species (e.g., mouse to mouse).

GM crops are grown in approximately 30 countries (and continuing to grow each year).
90% are grown in the United States, Brazil, Argentina, Canada, and India.
Significant percentages of major crops are GM:
- 93% of soybeans
- 88% of corn
70% of processed foods in the United States contain GM crop ingredients.

Herbicide-resistant (HR) GM crops.
Weed infestations destroy 10% of crops worldwide.
Herbicides can be so efficient that they kill crop plants.
Herbicide-tolerant (HT) GM crops are the most widely planted (70% of GM crops).
Contain a bacterial gene that confers tolerance to the broad-spectrum herbicide glyphosate.
Glyphosate:
- The active ingredient in Roundup.
- Works by blocking an enzyme essential for plant growth.

Insect-resistant (IR) GM crops.
Insect resistance is the 2nd most prevalent GM modification.
Makes plants resistant to agricultural pests.
Insect damage is a serious threat to food production.
Farmers use insecticides to combat the issue.
Insect-resistant GM crops: Bt crops
Bt (Bacillus thuringiensis) is a group of soil-dwelling bacterial strains.
- Produce Cry proteins, which are toxic to insects.
- Bt crops are engineered by scientists.
- cry genes are introduced into plant cells.
- The GM crop plant now can manufacture its own Bt Cry proteins, which kills the target pest when it eats the plant’s tissue.

Most GM crops are made to increase yield.
Golden Rice was engineered to combat vitamin A deficiency in developing countries.
Golden Rice was genetically engineered to synthesize beta-carotene (precursor to vitamin A).
Controversy regarding Golden Rice:
- Clinical trials show that the beta-carotene in Golden Rice 2 was efficiently converted into vitamin A.
- Golden Rice 2 is undergoing biosafety testing.
- Critics of GM foods suggest that Golden Rice will make farmers too dependent on one type of food.
- There are concerns about long-term health or environmental effects.
Golden Rice 2 contains high levels of beta-carotene, giving the rice endosperm a yellow color.
The intensity of the color reflects the amount of beta-carotene in the endosperm.

Two approaches to creating GM plants:
- Biolistic method
- Agrobacterium tumefaciens-mediated transformation
Both methods target plant cells growing in vitro.
Plant tissue is cultured that is grown in presence of nutrients or hormones.
Cultured cells form clumps that will form roots.
The rooted plant develops into a normal plant.

Biolistic method:
- Physical method of introducing DNA to cells.
- Particles of heavy metals (gold) are coated with the DNA that will transform the cells.
- These are introduced to cells in vitro using a gene gun.
- Plants with desired gene are selected for the desired phenotype.
Agrobacterium-mediated technology:
- Utilizes bacteria Agrobacterium tumefaciens, a soil microbe that infects plant cells and causes tumors.
- These characteristics are conferred by Ti plasmid.
- After Agrobacterium infection, the Ti plasmid integrates a segment of DNA (T-DNA) into plant genome.
- Scientists remove T-DNA and replace with cloned DNA of genes desired.

The rates of successful T-DNA integration and expression are low (1/1000).
Selectable markers allow scientists to distinguish between transformed and non-transformed products (colonies, plants, tissues, etc.).
Positive selection:
- Positive selection markers are used to select for cells that have successfully incorporated a desired gene or DNA fragment.
- The marker gene allows the organism or cell to survive or grow in conditions that would otherwise be lethal or unfavorable without the marker (e.g., antibiotic resistance, reporter genes (color change)).
Negative selection:
- Negative selection markers are used to select against cells that have incorporated undesirable genetic material or to eliminate cells that have undergone certain unwanted transformations.
- These markers work by making the cell susceptible to a lethal condition when the marker gene is expressed. (e.g., suicide genes, antibiotic sensitivity genes).

Ti plasmid was used to create Golden Rice 2.
Plasmid contained several genes:
- crtl gene: Carotene desaturase cloned from bacteria
- psy gene: Phytoene synthase cloned from maize
- pmi gene: Phosphomannose isomerase cloned from E. coli
The glutelin (Glu) gene promoter directs transcription in rice endosperm, and the polyubiquitin (Ubi1) promoter directs transcription in all tissues.
Transcription termination signals were provided by the nopaline synthase (nos) gene 3’ region.

GM food controversies.
GM foods have increased farm productivity.
Critics claim GM foods are unsafe.
Every GM crop or organism has different expression sequences.
Making general statements about all GM foods is not possible.
Health and safety.
- GM food advocates state that there is no evidence of adverse health effects.
- Critics counter that most GM foods are consumed by livestock.
- The processing of foods removes DNA and proteins.
- Testing of food-derived ingredients is not sufficient.
- GM foods have been consumed for over 20 years.
Environmental effects.
- Critics of GM foods state that GMOs released into the environment have potential consequences.
- Advocates argue that potential consequences can be identified and managed.