Question

Crown gall disease of plants is caused by
A. Ti-plasmid
B. Pi-plasmid
C. Virus
D. Protozoan

Answer 1

Hint: Crown gall disease is the disease caused by the genetically modified plasmid of the bacteria and responsible for causing tumors like growth in the infected plants. It mainly causes tumorous growth at the junction of roots and shoots of the plant.

Complete Answer:
- Agrobacterium tumefaciens is the gram negative bacteria which is responsible for causing crown gall disease in plants.
- It takes place by the help of conjugative transfer of DNA segment from the bacterial tumor inducing Ti-plasmid. It mainly causes tumorous growth at the junction of roots and shoots of the plant.
- The plasmid T-DNA is integrated semi randomly into the genome of the host cell and the tumor morphology genes on the T-DNA are expressed causing the formation of tumor.
- These T-DNA genes are responsible for producing the unusual amino acids like octopine and nopaline.
- Apart from producing unusual amino acids, it also causes alteration in the plant hormones balance. These two factors thereby cause tumor growth in the plant.

The correct option is A i.e. Ti-plasmid.

Additional information: There are three biovars of Agrobacterium. These are
- Agrobacterium tumefaciens (Ti-plasmid)
- Agrobacterium rhizogenes (Ri-plasmid)
- Agrobacterium vitis (Ti-plasmid in grapevines mainly)
Due to its ability to transfer DNA between itself and plants, these are used as an important tool in genetic engineering.

Note: Agrobacterium is used as a vector for the production of genetically modified crops and plants like soybean, cotton, maize, sugar beet, alfalfa, wheat, golden rice and many more. These genetically modified crops are well known for their resistance to disease, shelf- life, nutrients, climate and parasites.