Question 2: Describe the role of Agrobacterium tumefaciens in transforming a plant cell.
Answer:
Ti (tumour- inducing) plasmid which is present in Agrobacterium tumefaciens is used as vector for plants that has been considered as natural genetic engineers for transforming plants. It causes a crown gall disease in dicot plants by transferring a segment of Transfer- DNA from Ti- plasmid into nuclear genome of plant cells. When the plant molecular biologists have started using Ti plasmid as vectors when it is disarmed, then it is no more harmful for the plants and they don’t cause any disease.
Structure of Ti Plasmid:-
This Ti plasmid of Agrobacterium tumefaciens contains ori (origin of replication), bundle or cluster of virulence genes (helps to encode proteins that are required for T- DNA processing) and the T- DNA. And, T- DNA is flanked by improper direct repeats which are known as left and right border (for transfer and integration to the plant genome) sequences. T- DNA carries genes for the production of phytohormones like auxin, cytokinin and opine; and these genes are transcribed and translated in the plant cells. Their overproduction at the site of infection leads to multiplication of wound cells into the gall tumour and the opines are used as the carbon and nitrogen source which are being used by the Agrobacterium.
Diagram:-
Agrobacterium tumefaciens genetic transformation is a complex process which involves the Agrobacterium and the host. It involves T- DNA, virulence genes and T4SS (for macromolecules exportation). This process occurs by various virulence genes and every gene has their specific role in this process. In short, Firstly bacteria will attach to the plant cell which facilitates the interaction between the plant inducers. And this activates the transcription activator virulence then induce the rest of the virulence genes that are present on Ti- plasmid. T- DNA is transferred into the nucleus and integrates into the plant cell chromosome through the T4SS (type IV secretion system) which is also known as Mpf (Mating pair formation) apparatus that forms a pore or channel through which the DNA or protein get transferred from donor to the recipient cell.
Diagram:-
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