Q.Nematode specific genes were introduced into the tobacco host plant by using the vector (A) Plasmid (B) Bacteriophage (C) pBR 322 (D) Agrobacterium

Concept: Genetic engineering in plants, specifically the use of Agrobacterium tumefaciens as a natural genetic engineer.

Agrobacterium tumefaciens is a bacterium that naturally transfers a segment of its tumor-inducing (Ti) plasmid, known as T-DNA, into the genome of dicotyledonous plants. Scientists exploit this natural mechanism by modifying the Ti plasmid: the tumor-inducing genes are removed, and desired genes (such as nematode-specific genes for pest resistance) are inserted into the T-DNA region. The modified Agrobacterium then serves as a biolog …

Concept and Intuition

In genetic engineering, a "vector" is essentially a vehicle used to carry foreign genetic material (like a gene) into another cell, where it can be replicated and/or expressed. Think of it like a delivery truck for DNA. The choice of vector depends heavily on the host organism into which the genes are being introduced.

For plants, introducing foreign DNA is particularly challenging because plant cells have rigid cell walls and lack natural mechanisms to readily take up external DNA. This is where Agrobacterium tumefaciens comes into play. This bacterium is unique because it naturally infects plants and has evolved a sophisticated mechanism to transfer a specific segment of its own DNA, called T-DNA (Transfer DNA), from its Ti (Tumor-inducing) plasmid into the plant cell's nucleus. Once inside, the T-DNA integrates into the plant's genome, causing the plant to produce compounds that the bacterium can use.

Scientists have cleverly exploited this natural ability. By removing the tumor-inducing genes from the T-DNA and replacing them with desired genes (like those conferring resistance to nematodes), Agrobacterium tumefaciens can be reprogrammed to deliver these beneficial genes into plant cells without causing disease. The bacterium then acts as a biological vector, carrying the engineered Ti plasmid into the plant.

Step-by-Step Solution

1. Identify the Host Organism and Goal: The problem states that nematode-specific genes are being introduced into a "tobacco host plant." This immediately tells us we are dealing with plant genetic engineering. The goal is to transfer genes into the plant's genome.

2. Evaluate Option (A) Plasmid: A plasmid is a small, circular DNA molecule found in bacteria, separate from the bacterial chromosome. While plasmids are widely used as vectors in genetic engineering, the term "Plasmid" alone is too general. Many types of plasmids exist, and not all are suitable for plant transformation. For instance, the Ti plasmid is a plasmid, but it's the Agrobacterium that delivers it.

3. Evaluate Option (B) Bacteriophage: Bacteriophages are viruses that specifically infect bacteria. They are used as vectors for introducing genes into bacterial cells, not plant cells. Therefore, a bacteriophage cannot be used to introduce genes into a tobacco plant.

4. Evaluate Option (C) pBR322: pBR322 is a specific, well-known plasmid vector commonly used in E. coli for cloning and gene manipulation in bacterial systems. It is not designed for or capable of directly transferring genes into plant cells. …