Question

Although the Ti plasmid has revolutionised plant genetic engineering, one limitation of its use is that it -
A) Cannot infect broadleaf plants
B) Cannot be used on fruit-bearing plants
C) Cannot transmit prokaryotic genes
D) Does not infect cereal plants such as corn and rice

Answer 1

Hint: The Ti plasmid is found in species of Agrobacterium like A. tumefaciens which are pathogenic. It is a tumour inducing plasmid. The Ti plasmid is used to transform plants which are normally infected by Agrobacterium species.

Complete Answer:
- The main plants infected by the agrobacterium are dicotyledons which includes several broad-leafed species. The Ti plasmid can be used to infect and transfer these plants. Option A is therefore incorrect.
- On infection with a constitutive promoter added to the T plasmid, it has been seen to be expressed in each part of the plant, including its fruits and seed. Therefore it can be used on fruit-bearing plants, and option B is incorrect.
- The Ti plasmid is isolated from bacterial cells. It can be used for the gene transfer of genes between bacteria as well as to plant cells. In fact, it is suitable for studying prokaryotic gene flow in general. Option C is therefore incorrect.
- Agrobacterium does not infect monocotyledons bringing in some limitations. This includes all the various types of grasses including cereal grasses like corn and rice, and the Ti plasmid cannot be used to transform these plants. Option D is the correct answer.

Hence the correct answer is option D.

Note: Agrobacterium is responsible for crown gall disease. The Ti plasmid inserts into the plants DNA and expression of these genes is responsible for the abnormal cell growth in plants producing gall tumours. The actual part of the bacterial genome that inserts is only a small section called the T DNA and this is what is replaced in genetic engineering studies.