Key Difference – Ti vs Ri Plasmid
Agrobacterium is a bacterial genus which causes several diseases in dicotyledonous plants including crown gall disease and hairy root disease. These two diseases are encoded by the genes located in plasmids (non chromosomal DNA) of bacteria. Bacterial species Agrobacterium tumerfaciens bears a tumor inducing plasmid (Ti plasmid) which is responsible for crown gall disease in plants. Agrobacterium rhizogenes is another bacterium which harbors root inducing plasmid (Ri plasmid) responsible for hairy root disease in plants. Ti and Ri plasmids are pathogenic plasmids unique to this bacterial genus. The key difference between Ti and Ri plasmid is that Ti plasmid is encoded with genes responsible for causing crown gall disease while Ri plasmid is encoded with genes for hairy root disease in plants. These pathogenic plasmids contain gene clusters responsible for DNA replication, virulence, T-DNA, opine utilization, and conjugation. During the infection, Agrobacterium excises its T-DNA (transfer DNA) region of the plasmid and integrates with plant genome to cause disease. This ability is exploited by molecular biologists to introduce important genes into plants in genetic engineering. Hence, Agrobacterium is considered as an important tool in Biotechnology and Molecular Biology for the introduction of chimeric DNA into different plants species.
CONTENTS
1. Overview and Key Difference
2. What is Ti Plasmid
3. What is Ri Plasmid
4. Side by Side Comparison – Ti vs Ri Plasmid
5. Summary
What is Ti Plasmid?
Tumor inducing plasmid (Ti plasmid) is a larger plasmid which is harbored by Agrobacterium tumerfaciens to cause crown gall disease in a wide range of dicot plants. The name crown gall disease is used due to the formation of large tumor like swellings (galls) at the crown of the plants above the soil surface due to the over production of plant hormones auxin and cytokinins by A. tumerfaciens. T-DNA region contains the tumor inducing genes. Agrobacterium tumerfaciens enters the plants through damaged plant tissues, especially via wounds, and transfers its part of plasmid DNA (T-DNA) together with disease causing genes into plant cells. This T-DNA then integrates into the plant cell genome and transcribes. Expression of genes causes the formation of tumors and associated changes in cell metabolism. Crown gall disease does not cause serious damages to older plants. However, it reduces the quality of nursery plants.
Due to this unique mode of infection, A. tumerfaciens is extensively used as a tool in genetic engineering for the production of transgenic plants. Tumor inducing genes are suppressed, and desired genes such as insecticide resistant genes and herbicide resistant genes are inserted or recombined to Ti plasmid using recombinant DNA technology and used in plant breeding programs. When T-DNA is transferred upon the infection of A. tumerfaciens to plants, plants acquire the effects of desired genes naturally. Therefore, any foreign DNA inserted into the T-DNA can be integrated into plant cell genomes via the help of natural infection process of this bacterium.
Figure 01: Ti plasmid of A. tumerfaciens
What is Ri Plasmid?
Root inducing plasmid (Ri Plasmid) is a plasmid which bears by the bacterium A. rhizogenes. Ri plasmid is responsible for the disease called hairy root disease in dicot plants. The infection of A. rhizogenes causes extensive formation of adventitious roots at or near the infection site. Hairy root inducing genes are located in the T-DNA region of the Ri plasmid. Ri plasmid is a larger plasmid similar to Ti plasmid. A. rhizogenes is also capable of transferring T-DNA region of RI plasmid into plant cells and integrating with plant cell genome to get transcribes using plant cell machinery to cause diseases. Hence, Ri plasmids also serve as important vectors in plant genetic engineering.
What is the difference between Ti and Ri Plasmid?
Ti vs Ri Plasmid |
|
| Ti plasmid is a circular and larger plasmid harbored by A. tumerfaciens | Ri plasmid is a circular and larger plasmid harbored by A.rhizogenes |
| Disease | |
| Ti plasmid encodes genes for crown gall disease in plants | Ri plasmid encodes genes for hairy root disease in dicot plants. |
Summary – Ti vs Ri Plasmids
Ti and Ri plasmids are pathogenic plasmids harboring by A. tumerfaciencs and A. rhizogenes, respectively. Ti plasmid has tumor inducing genes which cause crown gall disease in plants. Ri plasmid has root inducing genes which cause hairy root disease in plants. This is the main difference between Ti and Ri plasmids. These plasmids can be used as vectors in plant genetic engineering due to their natural capability of transferring part of their plasmid DNA into the host genome. They contain gene clusters and are approximately 200 kbp in sizes. Each plasmid contains unique genes.
References
1. Katsunori Suzuki, Katsuyuki Tanaka, Shinji Yamamoto, Kazuya Kiyokawa, Kazuki Moriguchi and Kazuo Yoshida. 2. “Ti and Ri Plasmids.”Springer. Springer Berlin Heidelberg, 01 Jan. 1970. Web. 21 Apr. 2017
3. Zupan, J. R., and P. Zambryski. “Transfer of T-DNA from Agrobacterium to the plant cell.” Plant Physiology. U.S. National Library of Medicine, Apr. 1995. Web. 22 Apr. 2017
Image Courtesy:
1. “Ti plasmid”By MouagipThis vector graphics image was created with Adobe Illustrator. – Own work (CC0) via Commons Wikimedia