GATE 2020 BT Q.11: solution and answer

Official answer: DSource: Official answer key, GATE 2020 BT (IIT Delhi)

1. The Ti (tumour-inducing) plasmid of Agrobacterium tumefaciens is functionally divided into two regions: T-DNA (transferred into and expressed in the plant genome) and the vir region plus opine-catabolism region (which remain in the bacterium and are never transferred).

2. Option A is false: Vir genes (VirA-VirG etc.) encode the machinery that processes and transfers T-DNA. They act in trans from outside the T-DNA border sequences and are never themselves transferred into the plant genome.

3. Option B is false: T-DNA carries the iaaM/iaaH (auxin) and ipt (cytokinin) genes -- phytohormone biosynthesis genes -- which must be expressed inside the transformed plant cell to cause tumour growth, so they lie WITHIN T-DNA, not outside it.

4. Option C is false: Opine catabolism genes (e.g., the occ and noc operons) let the bacterium -- not the plant -- consume the opines the tumour later secretes. Since they function in the bacterium, these genes remain outside T-DNA, on the bacterial backbone of the Ti plasmid.

5. Option D is true: Opine biosynthesis genes (e.g., nopaline synthase/octopine synthase genes) are carried within T-DNA so they are expressed in the transformed plant cell, driving opine synthesis there for the bacterium's later use via its catabolism genes.

6. This matches Agrobacterium's 'genetic colonization' strategy: T-DNA carries genes useful when expressed in the plant (hormone and opine biosynthesis), while the vir and catabolism genes that function in the bacterium itself stay outside T-DNA. Hence the correct statement is (D).