Difference Between Ti and Ri Plasmid

Main Difference – Ti vs Ri Plasmid

Agrobacterium species are plant pathogens that induce various diseases in higher plants. Ti (tumor-inducing) and Ri (root-inducing) plasmids are two types of natural plasmids produced by Agrobacterium species. Ti plasmids are produced by Agrobacterium tumefaciens while Ri plasmids are produced by Agrobacterium rhizogenes. Both Ti and Ri plasmids consist of a part of the plasmid DNA known as T-DNA that is transferred into the plant genome with the aid of virulence (vir) genes. The main difference between Ti and Ri plasmid is that Ti Plasmid induce tumor/crown gall in dicots whereas Ri plasmid induces hairy roots. Due to their ability to infect plants, both Ti and Ri plasmids are widely used as vectors to produce transgenic plants.

Key Areas Covered

1. What is Ti Plasmid
– Definition, Structure, Vector Production
2. What is Ri Plasmid
– Definition, Structure, Vector Formation
3. What are the Similarities Between Ti and Ri Plasmid
– Outline of Common Features
4. What is the Difference Between Ti and Ri Plasmid
– Comparison of Key Differences

Key Terms: Agrobacterium Species, Genetic Engineering, Hairy Roots, Ri Plasmid (pRi), T-DNA, Ti Plasmid (pTi), Tumor, vir Genes

What is Ti Plasmid

Ti plasmid refers to a type of plasmid of that allows Agrobacterium tumefaciens to infect plants, producing a tumor/crown gall tumor. Thus, the Ti plasmid consists of both tumor forming and virulence (vir) genes. The vir genes are responsible for the transfer of T-DNA that contains the tumor-forming genes into the plant genome by excision and integration. A typical Ti plasmid consists of four regions: region A (T-DNA responsible for tumor formation), region B (responsible for replication), region C (responsible for conjugation), and region D (responsible for virulence). The genes in the T-DNA region controls the production of plant growth hormones (auxin and agropine), inducing the proliferation of the infected plant cells. Both ends of the T-DNA region are flanked by a border sequence that contains direct sequence repeats of 24 base pairs. Only the right border is indispensable for T-DNA transfer. The structure of Ti plasmid is shown in figure 1.

Figure 1: Ti Plasmid

In the Agrobacterium-mediated transformation of genes, the vector is designed based on the Ti-Plasmid. The vector is composed of T-DNA border sequences, the vir genes, and the modified T-DNA region. During the modification of the T-DNA region, the genes responsible for the tumor formation are removed and replaced by the foreign genes that are to be transformed.

What is Ri Plasmid

Ri plasmid refers to a type of plasmid that allows Agrobacterium rhizogenes to infect pant cells by producing hairy roots. It consists of two T-DNA fragments known as Tr-DNA and T1-DNA. Both fragments are separated by a 15 kb DNA segment. The Tr-DNA is similar to the T-DNA region of the Ti plasmid and contains genes that induce the production of growth hormones. The T1-DNA region consists of four genes: rolA (responsible for the formation of hairy roots), rolB (induces root initiation and callus formation), rolC (promotes root growth), and rolD (suppresses callus growth). The transformation of T-DNA is induced by the vir genes. The formation of hairy roots by Agrobacterium rhizogenes is shown in figure 2.

Figure 2: Hairy Roots

Ti plasmids are mostly replaced by Ri plasmids in genetic engineering.

Similarities Between Ti and Ri Plasmid

Ti and Ri plasmids are naturally produced by Agrobacterium.
Both types of plasmids cause diseases in many dicotyledonous plants.
Both Ti and Ri plasmids transfer a part of the plasmid DNA known as T DNA into the plant genome with the aid of virulence (vir) genes.
The size of both Ti and Ri plasmids are around 200 kb.
The regulatory sequences of some genes found in the T DNA of both plasmids are recognized by the plant cellular mechanisms.
The other genes in the T DNA consist of prokaryotic regulatory sequences.
Due to the ability to infect plant cells, both Ti and Ri plasmids are used in genetic engineering to produce transgenic plants.

Definition

Ti Plasmid: Ti plasmid is a type of plasmid of that allows the bacteria to infect plants, producing a tumor/crown gall tumor.

Ri Plasmid: Ri plasmid is a type of plasmid that allows bacteria to infect plant cells by producing hairy roots.

Type of Agrobacterium species

Ti Plasmid: Ti plasmid is produced by Agrobacterium tumefaciens.

Ri Plasmid: Ri plasmid is produced by Agrobacterium rhizogenes.

Size

Ti Plasmid: The size of the Ti plasmid is 180-205 kb.

Ri Plasmid: The size of the Ri plasmid is 250 kb.

Type of Disease

Ti Plasmid: Ti plasmid induces the formation of crown gall/tumor.

Ri Plasmid: Ri plasmid induces the formation of hairy roots.

Type of Study

Ti Plasmid: Ti plasmids are less studied compared to Ri plasmids.

Ri Plasmid: Ri plasmids are well-studied.

Conclusion

Ti plasmid and Ri plasmid are two types of plasmids found in different Agrobacterium species. Both Ti plasmids and Ri plasmids consist of T-DNA and vir genes. Ti plasmids can be found in Agrobacterium tumefaciens that induces the formation of crown gall tumor. Ri plasmids can be found in Agrobacterium rhizogenes that induces the formation of hairy roots. The main difference between Ti and Ri plasmid is the type of disease caused by each type of plasmids.

Reference:

1.“Lecture 26: Ti and Ri Plasmids.” NPTEL, IIT Guwahati, Available here.
2.Haneef, Deena T Kochunni Jazir. “Agrobacterium rhizhogenes and Ri Plasmid.” Available here.

Image Courtesy:

1. “Ti plasmid” By Mouagip – Own work (CC0) via Commons Wikimedia
2. “Agrobacterium rhizogenes” By FruitDefendu – Own work (CC BY-SA 3.0) via Commons Wikimedia