CBSE · Class 12 · Biology · Biotechnology and its ApplicationsWhat is RNA Interference (RNAi)? Describe the mechanism of RNAi and explain how it is utilized in developing pest-resistant plants, specifically against Meloidogyne incognita in tobacco plants.

Introduction to RNA Interference (RNAi)\nRNA interference (RNAi) is a cellular defense mechanism found in all eukaryotic organisms. This process involves the silencing of a specific mRNA due to a complementary double-stranded RNA (dsRNA) molecule that binds to and prevents the translation of the mRNA (silencing). This mechanism acts as a form of genomic defense, protecting cells from viral infections and mobile genetic elements (transposons).

Mechanism of RNAi

1. Introduction of dsRNA: The process starts with the presence of a double-stranded RNA molecule in the cell, either introduced artificially or produced by viral vectors/transposons.
2. Dicer Cleavage: An enzyme known as Dicer (an RNase III-like enzyme) cleaves the long dsRNA molecules into short, double-stranded fragments called small interfering RNAs (siRNAs) of about 21-25 nucleotides.
3. RISC Complex Assembly: These siRNAs are incorporated into an RNA-induced silencing complex (RISC), where the duplex unwinds into single-stranded guide RNA.
4. Target Recognition and Silencing: The single-stranded guide RNA pairs with the complementary sequence on the target messenger RNA (mRNA) of the parasite or virus. Once bound, the RISC complex cleaves the target mRNA or inhibits its translation, effectively preventing the production of the harmful protein.

Application: Pest Resistance in Tobacco Plants\nA specific nematode, Meloidogyne incognita, infects the roots of tobacco plants and causes a great reduction in yield.

• Agrobacterium Vectors: Using Agrobacterium vectors, nematode-specific genes were introduced into the host tobacco plant.
• Sense and Antisense RNA Production: The introduction of DNA was such that it produced both sense and anti-sense RNA in the plant cells.
• Formation of dsRNA: Since these two RNA strands are complementary to each other, they formed a double-stranded RNA (dsRNA) that initiated RNAi.
• Protection Mechanism: When the nematode ingests plant tissues containing this dsRNA, its cellular machinery triggers RNAi against a vital gene of the nematode. The complementary dsRNA silences specific mRNA of the nematode, resulting in the death of the parasite within the transgenic plant, thus protecting the crop completely.