What modification has been made to the Ti plasmid of Agrobacterium tu…

The correct answer is Option (2) → It has been modified into a cloning vector for delivering genes of interest.

Agrobacterium tumefaciens is a pathogen of several dicot plants. Its Ti (tumour-inducing) plasmid delivers a piece of DNA called T-DNA into plant cells and turns them into a tumour. Scientists used this natural skill. The Ti plasmid has been modified into a cloning vector that is no longer pathogenic but can still deliver genes of our interest into many plants.

Option (1) It has been modified to deliver harmful genes. — Incorrect. The change was made to remove the harmful property. The modified plasmid no longer causes disease in plants.

Option (2) It has been modified into a cloning vector for delivering genes of interest. — Correct. The disarmed Ti plasmid is now a cloning vector that carries desired genes into plant cells.

Option (3) It has been modified to produce viral proteins. — Incorrect. The Ti plasmid is a bacterial plasmid. It has not been changed to make viral proteins; its use is to carry chosen genes into plants.

Option (4) It has been modified to only infect animal cells. — Incorrect. Agrobacterium naturally infects plants, and the vector made from its plasmid is used for plants. For animal cells, disarmed retroviruses are used instead.

NCERT Extract:
Cloning Vectors – “… For example, Agrobacterium tumifaciens, a pathogen of several dicot plants is able to deliver a piece of DNA known as ‘T-DNA’ to transform normal plant cells into a tumor and direct these tumor cells to produce the chemicals required by the pathogen. … The tumor inducing (Ti) plasmid of Agrobacterium tumifaciens has now been modified into a cloning vector which is no more pathogenic to the plants but is still able to use the mechanisms to deliver genes of our interest into a variety of plants. Similarly, retroviruses have also been disarmed and are now used to deliver desirable genes into animal cells.”