Introduction:
In order to introduce nematode-specific genes into infected tobacco plants, different vectors can be used. These vectors help in delivering the desired genes into the plant cells, allowing for the expression of these genes and potential resistance against nematode infection. Among the given options, the Ti plasmid of Agrobacterium is the vector commonly used for introducing nematode-specific genes in infected tobacco plants.

Explanation:
Agrobacterium and Ti plasmid:
Agrobacterium tumefaciens is a soil bacterium that has the ability to transfer a part of its DNA, known as the Ti (tumor-inducing) plasmid, into plant cells. This natural genetic transformation ability of Agrobacterium has been extensively utilized in plant biotechnology for the introduction of foreign genes into plants.

Steps in Agrobacterium-mediated gene transfer:
1. Agrobacterium containing the desired gene of interest is grown in culture.
2. The Ti plasmid is modified to remove the disease-causing genes and replace them with the desired nematode-specific genes.
3. The Agrobacterium is then mixed with a suspension of tobacco leaf cells or explants.
4. The Agrobacterium attaches to the tobacco cells and transfers a part of its DNA, including the nematode-specific genes, into the plant cells.
5. The transferred genes become incorporated into the plant cell's genome and are expressed, leading to the production of nematode-specific proteins.
6. These proteins provide resistance against nematode infection.

Advantages of using Agrobacterium:
- Agrobacterium-mediated gene transfer is a widely used and well-established method for introducing foreign genes into plants.
- Agrobacterium has a natural ability to infect plant cells and transfer genes, making it a suitable vector for gene delivery.
- The Ti plasmid can accommodate relatively large DNA fragments, making it suitable for introducing nematode-specific genes.

Conclusion:
Among the given options, the Ti plasmid of Agrobacterium is the vector commonly used for introducing nematode-specific genes in infected tobacco plants. Agrobacterium-mediated gene transfer offers advantages such as efficient gene delivery, stable integration into the plant genome, and the ability to introduce large DNA fragments. This technique has been successfully utilized to develop nematode-resistant tobacco plants, contributing to crop protection and improvement.

A tumour-inducing plasmid (Ti plasmid) is a plasmid found in pathogenic species of Agrobacterium. Ti plasmid contains a region called T-DNA which is replaced by our gene of interest (nematode specific genes) and Agrobacterium is then allowed to infect the tobacco plant. When Agrobacterium infects the tobacco plant, our gene of interest (nematode specific genes) also gets inserted in tobacco.