Choose the mechanism that is used in the tobacco plants that are introduced with nematode - specific using certain vectors.

Options

A. RNAi
B. Insecticidal protein
C. DNAi
D. Cry gene

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the mechanism used in tobacco plants that have been introduced with nematode-specific genes using certain vectors.

2. **Identifying the Mechanism**: The mechanism that is relevant here is RNA interference (RNAi), also known as RNA silencing. This technology is used to silence specific genes in the plant.

3. **How RNA Interference Works**: RNA interference involves the introduction of a complementary strand of RNA into the plant. When this complementary RNA pairs with the target RNA, it forms double-stranded RNA (dsRNA).

4. **Effect of Double-Stranded RNA**: The presence of dsRNA prevents the synthesis of proteins from the target RNA. This is because the cellular machinery that translates RNA into proteins cannot process double-stranded RNA, effectively silencing the gene.

5. **Impact on Nematodes**: The nematode, specifically Meloidogyne incognita, relies on the tobacco plant for proteins. When the plant's genes are silenced and protein synthesis is inhibited, the nematode cannot obtain the necessary proteins and will leave the plant.

6. **Conclusion**: Therefore, the mechanism used in tobacco plants introduced with nematode-specific genes is RNA interference (RNAi). Thus, the correct answer is option number 1: RNA interference.