Agrobacterium virulence determinants In plant biotechnology, on which genetic element is the virulence trait of Agrobacterium tumefaciens primarily borne and transmitted to plants?
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AChromosomal DNA
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BTumor-inducing (Ti) plasmid DNA
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CBoth chromosomal DNA and plasmid DNA equally
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DA cryptic (nonfunctional) plasmid DNA
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EA lytic bacteriophage genome
Answer
Correct Answer: Tumor-inducing (Ti) plasmid DNA
Explanation
Introduction / Context:Agrobacterium tumefaciens is renowned for its natural ability to transfer DNA into plant cells, causing crown gall disease. The core question tests whether you know where the virulence (disease-causing and DNA-transfer) functions reside and how they underpin modern plant genetic engineering.
Given Data / Assumptions:
- The bacterium possesses a large tumor-inducing (Ti) plasmid.
- Plant tumors (galls) result after a specific DNA segment integrates into the plant genome.
- We are asked about the location of virulence determinants that enable DNA processing and transfer.
Concept / Approach:Virulence is primarily encoded on the Ti plasmid. Two key functional parts are: (1) T-DNA, the segment that is transferred and integrated into the plant genome, and (2) the vir region, a cluster of virulence genes that sense plant signals, process T-DNA at the borders, and export it through a type IV secretion system into plant cells. While some chromosomal genes help bacterial physiology, the essential transfer machinery and the oncogenic/opine traits are Ti plasmid-encoded.
Step-by-Step Solution:
Identify the DNA element that carries T-DNA and vir genes: the Ti plasmid.Recall that vir genes (virA, virG, virB, virD, etc.) direct sensing, nicking, and export of T-DNA.Remember that the T-DNA portion integrates into plant chromosomes and expresses oncogenes/opine synthase.Conclude that the virulence trait is borne on the Ti plasmid DNA.Verification / Alternative check:Disarmed binary vector systems separate vir functions (on a helper plasmid) from engineered T-DNA, yet transformation still occurs. This proves vir resides in plasmid-borne modules and does not require bacterial chromosomal relocation.
Why Other Options Are Wrong:
- Chromosomal DNA: contributes to general metabolism; not the main virulence locus.
- Both equally: Ti plasmid specifically encodes transfer/onco traits.
- Cryptic plasmid: by definition lacks known function.
- Bacteriophage genome: not involved in Agrobacterium virulence.
Common Pitfalls:Confusing the transferred T-DNA (plant-integrated) with the bacterial vir genes (remain in Agrobacterium). Only T-DNA moves; vir genes act in trans from the bacterium.
Final Answer:Tumor-inducing (Ti) plasmid DNA