Explanation of each option

1. Agrobacterium tumefaciens – Correct

• A. tumefaciens is a natural plant pathogen that transfers a segment of its Ti plasmid DNA (T‑DNA) into plant cells, where it integrates into the plant genome.

• Biotechnology has harnessed this system by replacing tumor‑inducing genes with desired transgenes, making Agrobacterium-mediated transformation the standard, widely used method for creating transgenic plants across many species (both dicots and, now, many monocots).

2. Bacillus thuringiensis – Incorrect in this context

• B. thuringiensis (Bt) is a soil bacterium that produces insecticidal Cry proteins.

• The Bt genes are cloned into plants using transformation systems such as Agrobacterium or gene gun; Bt itself is not the main vehicle for DNA delivery into plant genomes.

3. Baculoviruses – Incorrect

• Baculoviruses are viruses that infect insects and are widely used for expressing recombinant proteins in insect cell culture, not for transforming higher plants.

• They are not a standard tool for producing stable transgenic plants.

4. Erwinia carotovora – Incorrect

• E. carotovora is a plant pathogenic bacterium causing soft rot diseases.

• It is not used as a vector for plant genetic transformation; instead, it is typically an organism breeders try to resist through transgenic or conventional approaches.

SEO‑oriented introduction (for article use)

In plant biotechnology, Agrobacterium tumefaciens has become the workhorse organism for generating transgenic plants because its natural T‑DNA transfer system can be engineered to carry any gene of interest into the plant genome. In contrast, Bacillus thuringiensis serves mainly as a source of insecticidal genes, baculoviruses are tools for insect cell expression, and Erwinia carotovora is simply a plant pathogen rather than a transformation vector.