5.16
Transgenic Plants
Recombinant DNA technology called transgenesis is often used to add a foreign gene or remove a detrimental gene from an organism. Such genetically mod…
Transgenic plants are produced by introducing a foreign gene into a plant’s genome. These plants can be developed for a variety of reasons, including virus or pest resistance, higher yield, or therapeutic protein production.
To produce a transgenic plant, a gene of interest and its promoter are introduced into the plant protoplast, a plant cell without a cell wall, by incubating both in a medium containing polyethylene glycol.
The recombinant protoplast is cultivated on a sterile medium in the presence of growth regulators and nutrients.
The cells of the protoplast grow indefinitely, producing a mass of undifferentiated cells called a callus. Some cells of the callus, called totipotent cells, give rise to new shoots and roots, which can develop into a whole new plant.
Transgenic plants can also be created using the bacteria Agrobacterium tumefaciens, a gram-negative bacterium found in soil. This bacteria is a pathogen that normally causes disease in plants and contains a Ti -plasmid that can integrate into a plant cell genome.
The Ti plasmid has two regions of interest- T-DNA or transfer DNA region which can be inserted into the host cell genome and the virulence region to aid the transfer of T-DNA into the host cell.
This plasmid is modified by inserting a desired gene into the T-DNA region and a gene for antibiotic resistance to enable selective growth of plasmid-containing organisms. The recombinant plasmid is then inserted into the bacteria by applying an electric pulse to increase cell permeability and enable the uptake of the plasmid into the bacteria.
The Agrobacterium tumefaciens carrying recombinant plasmid is grown in a culture medium containing antibiotics.
Plant parts such as the cotyledonary nodes from a tomato plant are cut and incubated in the medium containing bacteria.
The wounded plant cells at the edge of the stem node release compounds that attract the bacteria. The bacteria infects the plant and releases the recombinant plasmid, which then transfers the foreign gene into the plant cell genome.
Only the transformed cells with recombinant DNA and antibiotic-resistant genes survive in the medium containing antibiotics.
The transformed cells now form a callus and develop into a new transgenic plant carrying a transgene.
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Q1: How is a foreign gene introduced into a plant cell to create a transgenic plant?
A foreign gene and its promoter are introduced into a plant protoplast—a plant cell without a cell wall—using polyethylene glycol in a medium. The recombinant protoplast is then cultured on sterile medium with growth regulators and nutrients, allowing cells to divide and form a callus. Totipotent cells within the callus develop into shoots and roots, eventually forming a complete transgenic plant.
Q2: What role does Agrobacterium tumefaciens play in creating transgenic plants?
Agrobacterium tumefaciens is a soil bacterium containing a Ti plasmid that naturally integrates into plant genomes. Scientists modify this plasmid by inserting a desired gene into the T-DNA region and adding an antibiotic resistance gene. When the recombinant bacteria infects wounded plant cells, it transfers the foreign gene into the plant genome, creating transformed cells that develop into transgenic plants.
Q3: What is the purpose of the Ti plasmid's T-DNA and virulence regions?
The T-DNA region of the Ti plasmid is the segment that integrates into the host plant cell genome and carries the desired foreign gene. The virulence region facilitates the transfer of T-DNA into the plant cell. Together, these regions enable Agrobacterium to successfully deliver and incorporate recombinant DNA into the plant's genetic material.
Q4: How does electroporation transfer DNA into plant protoplasts?
Electroporation uses high voltage pulses delivered to a protoplast suspension to temporarily increase cell membrane permeability. This allows foreign DNA to enter the cell and integrate into the plant genome. The method is effective for introducing genes into protoplasts and is one of several physical methods used in plant genetic engineering.
Q5: What are practical applications of transgenic plants in agriculture and nutrition?
Transgenic plants are developed for pest and virus resistance, improved yield, and enhanced nutritional content. Golden rice, for example, produces β-carotene—a vitamin A precursor—to address nutritional deficiencies. Bt cotton produces crystal protein that kills harmful insects. These modifications demonstrate how synthetic biology artificially synthesizing designing organisms improves crop performance and human health.
Q6: How does antibiotic resistance selection ensure only transformed plant cells survive?
An antibiotic resistance gene is inserted into the recombinant plasmid alongside the desired foreign gene. When the transformed plant cells are cultured in medium containing antibiotics, only cells that have successfully incorporated the plasmid survive. Non-transformed cells lack the resistance gene and die, allowing researchers to isolate and propagate only the transgenic cells.
Q7: What is a callus and why is it important in transgenic plant development?
A callus is a mass of undifferentiated cells that forms when recombinant protoplasts are cultured on sterile medium with growth regulators and nutrients. Within the callus, totipotent cells—cells capable of developing into any plant tissue—differentiate to form shoots and roots. This regenerative capacity allows the callus to develop into a complete, functional transgenic plant.