44. Virus induced gene silencing (VIGS) is a process that takes advantage of the RNAi mediated antiviral defense mechanism. Which one of the following ultimately guides siRNA to degrade the target transcript (mRNA)?
(1) dsRNA
(2) ssRNA
(3) RNA Induced Silencing (RIS) Complex
(4) dsRNA binding protein

Introduction

Virus-induced gene silencing (VIGS) is a powerful technique that exploits the natural RNA interference (RNAi) antiviral defense mechanism in plants. When a plant is infected with a virus carrying a fragment of a plant gene, the plant’s RNAi machinery is triggered to silence the corresponding endogenous gene. Central to this process is the generation of small interfering RNAs (siRNAs) and their incorporation into the RNA-induced silencing complex (RISC), which mediates targeted mRNA degradation.

How VIGS Works

• Virus Infection and dsRNA Formation:
  Viral infection leads to the production of double-stranded RNA (dsRNA) derived from the viral genome and inserted plant gene fragments.

• Dicer Cleavage:
  The plant enzyme Dicer cleaves dsRNA into siRNAs approximately 21–24 nucleotides long.

• siRNA Incorporation into RISC:
  These siRNAs are loaded into the RNA-induced silencing complex (RISC), a multiprotein complex that uses the siRNA as a guide.

• Target mRNA Degradation:
  Guided by siRNA, RISC binds complementary mRNA transcripts and cleaves them, leading to their degradation and subsequent gene silencing.

Which Component Guides siRNA to Degrade Target mRNA?

Among the options:

1. dsRNA: The precursor molecule cleaved by Dicer, but it does not directly guide mRNA degradation.

2. ssRNA: Single-stranded RNA is not the guiding molecule in RNAi.

3. RNA-Induced Silencing Complex (RISC):
   The active effector complex that, guided by siRNA, binds and degrades target mRNA.

4. dsRNA Binding Protein: Proteins that may assist in processing but do not guide degradation.

Conclusion

The RNA-Induced Silencing Complex (RISC) is the key molecular machine that, guided by siRNAs, targets and degrades complementary mRNAs during virus-induced gene silencing. This process is fundamental to the RNAi pathway and the plant’s antiviral defense mechanism.

Answer:
The correct option is (3) RNA Induced Silencing (RIS) Complex.