Step 1: Understanding the Concept:
Long before anyone engineered plants in a lab, a soil bacterium had already found a way to insert its own genes into a plant's chromosome as part of causing a disease. Genetic engineers later copied and redirected that natural ability to make transgenic plants.
Step 2: Key Approach:
The clue to trace is the disease this bacterium causes naturally, since the very same DNA-transfer machinery used for that disease is what scientists repurpose in the lab.
Step 3: Detailed Explanation:
Agrobacterium tumefaciens infects wounded plant tissue and causes crown gall disease, a tumor-like swelling at the infection site. It causes this swelling by transferring a piece of its own plasmid, called T-DNA, out of its Ti (tumor-inducing) plasmid and into the plant cell's chromosome, where the bacterial genes make the plant cell produce growth hormones and nutrients the bacterium then feeds on. Genetic engineers remove the disease-causing genes from this T-DNA region and replace them with a gene of interest, so the same natural transfer machinery now delivers a useful gene instead of a tumor. Pseudomonas and Xanthomonas are both plant pathogens too, but they do not carry a Ti plasmid or transfer DNA into the host genome this way. Salmonella is an animal and human pathogen with no role in plant transformation at all.
Step 4: Final Answer:
The Ti plasmid used to make transgenic plants comes from Agrobacterium tumefaciens, so option 3 is correct.