Agrobacterium virulence determinants In plant biotechnology, on which genetic element is the virulence trait of Agrobacterium tumefaciens primarily borne and transmitted to plants?

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:

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