History of plant transformation: the first transgenic plants expressing engineered foreign genes were produced in tobacco using which biological system?

Introduction / Context:
Agrobacterium tumefaciens naturally transfers T-DNA into plant genomes, causing crown gall disease. This ability was retooled to deliver engineered genes, establishing the foundation of plant genetic engineering.

Given Data / Assumptions:

• Early successful demonstrations used tobacco as a model dicot.
• Engineered Ti plasmids replace tumor genes with selectable markers or traits.
• Binary vector systems separate virulence and T-DNA regions for flexibility.

Concept / Approach:
Researchers harnessed the T-DNA transfer mechanism to integrate transgenes stably. Regenerated plants expressed the introduced genes and transmitted them to progeny, confirming true transformation.

Step-by-Step Solution:

Verification / Alternative check:
Primary literature and reviews consistently cite Agrobacterium as the first reliable tool for producing transgenic plants.

Why Other Options Are Wrong:

• B: Bt provides insecticidal genes but is not a DNA delivery system.
• C: Arabidopsis became a model later; the earliest landmark demonstrations were in tobacco using Agrobacterium.
• D/E: Not the classical plant DNA delivery systems for the first transgenics.

Common Pitfalls:
Confusing the source of a transgene (e.g., Bt) with the transformation vector (Agrobacterium).

Final Answer:
Agrobacterium tumefaciens (Ti plasmid–mediated transfer)