189. VirA and Virg form a two component signaling system during
Agrobacterium infection in plants. In an experiment, VirG was mutated resulting in non-functional
VirG protein. Which one of the following consequences is most likely?
A. Signaling leading to successful Agrobacterium infection will not be affected
B. Phenolic signal will be perceived but not transduced
C. Phenolic signal will not be perceived
D. VirD1 will take over the function of VirG

Explanation:

Agrobacterium tumefaciens uses a two-component signaling system (TCS) to sense plant-derived compounds and initiate the transfer of T-DNA into plant cells. The two main proteins involved are:

• VirA: A sensor histidine kinase located in the inner membrane. It detects plant phenolic compounds (like acetosyringone).

• VirG: A response regulator protein that, upon phosphorylation by VirA, activates the transcription of other virulence (vir) genes, including VirD, VirE, VirB, etc.

If VirG is mutated and non-functional:

• Phenolic signals like acetosyringone will still be detected by VirA → So perception remains intact.

• But VirG cannot be phosphorylated or cannot activate downstream gene expression.

• Therefore, no transcription of essential vir genes → No T-DNA processing or transfer.

Why Other Options Are Incorrect:

• A. Incorrect – infection will be affected because downstream vir gene activation is blocked.

• C. Incorrect – VirA still perceives the phenolic signal.

• D. Incorrect – VirD1 has a different role (in DNA processing), not signal transduction.

Conclusion:

A mutation in VirG results in phenolic signal perception without downstream transcriptional activation of vir genes