Q.(a) Write the scientific name of the nematode that infests the tobacco plants and the part that it infests.

Concept: Biological control of plant parasitic nematodes using genetic engineering via Agrobacterium-mediated transformation.

(a) Nematode identification

The nematode is Meloidogyne incognita, commonly called the root-knot nematode. It infests the roots of tobacco plants, forming characteristic galls (knots) that disrupt water and nutrient uptake.

(b) Protection mechanism using Agrobacterium

Agrobacterium tumefaciens acts as a vector to introduce nematode-resistance genes into tobacco plants through the following strategy:

1. Gene selection: The nif gene from soil bacterium Bacillus thuringiensis or genes encoding specific dsRNA are cloned into the Ti plasmid of Agrobacterium.

2. Transformation: Agrobacterium infects tobacco cells and transfers the recombinant DNA into the plant genome, creating transgenic plants. …

The tobacco plant faces a serious threat from parasitic nematodes that burrow into its tissues and drain nutrients, stunting growth and reducing yield. The solution lies in a remarkable piece of genetic engineering that turns the plant into its own pesticide factory, using a natural gene-transfer bacterium as the delivery vehicle.

Understanding the nematode threat

The culprit is Meloidogyne incognita, commonly called the root-knot nematode. This microscopic roundworm doesn't attack leaves or stems—it targets the roots specifically. Once inside, it establishes feeding sites that cause characteristic swellings or "knots," disrupting water and nutrient uptake. The damage is often invisible above ground until the plant shows severe stress.

The Agrobacterium-mediated defense strategy

Agrobacterium tumefaciens is nature's genetic engineer. This soil bacterium naturally infects plants and transfers segments of its DNA into the plant genome—a process scientists have repurposed for introducing beneficial genes.

Here's how it protects tobacco from nematode attack:

1. Design the molecular weapon

   Scientists identify genes essential for the nematode's survival—often those involved in basic metabolism or reproduction. They then construct a DNA sequence that will produce double-stranded RNA (dsRNA) complementary to the nematode's mRNA for these genes.

2. Package the construct into Agrobacterium

   The dsRNA-encoding gene is inserted into a Ti plasmid (tumor-inducing plasmid) of Agrobacterium. The tumor-causing genes are removed, leaving only the transfer machinery intact. The construct also includes a plant promoter so the gene will be expressed in tobacco cells.

3. Infect tobacco cells

   Agrobacterium is used to infect tobacco tissue (often leaf discs or callus cultures). The bacterium transfers the dsRNA gene into the plant's chromosomes. Transformed cells are selected and regenerated into whole plants.

4. RNA interference (RNAi) in action

   The transgenic tobacco plant now produces dsRNA molecules. When the nematode feeds on the roots, it ingests this dsRNA along with plant cell contents. Inside the nematode, the dsRNA triggers RNA interference—a cellular defense mechanism where the dsRNA is chopped into small interfering RNAs (siRNAs) that bind to and destroy the nematode's own mRNA.

5. Silencing vital nematode genes …