# DBT JRF 2022 — Question 77

Exam: DBT JRF 2022 · Section: B · Topic: Plant Biology › Agrobacterium & T-DNA Transfer · Difficulty: Medium

## Question

VirA and VirG form a two component signaling system during Agrobacterium infection in plants. In an experiment, VirG was mutated resulting in nonfunctional VirG protein. Which one of the following consequences is most likely?

## Options

- **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

## Answer

Option B

## Explanation

VirA-VirG two-component system in Agrobacterium: Agrobacterium tumefaciens virulence signal transduction uses a two-component system: 1. VirA: the sensor histidine kinase (located in the bacterial inner membrane). • Perceives phenolic signals (e.g., acetosyringone) released from wounded plant cells. • Upon sensing, VirA AUTOPHOSPHORYLATES on a conserved histidine residue. • Transfers the phosphate to VirG. 2. VirG: the response regulator (transcription factor). • When phosphorylated (by VirA), VirG activates transcription of all vir genes (virB, virC, virD, virE, virF, virH). • vir gene products mediate T-DNA processing and transfer. If VirG is non-functional: • VirA can still perceive and autophosphorylate normally. • But without functional VirG to receive the phosphate, downstream vir genes are NOT activated. • Signal is PERCEIVED but NOT TRANSDUCED → no virulence. Answer: VirA (sensor) perceives plant signals normally; non-functional VirG cannot transduce the signal → virulence blocked.

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