Ti plasmid- Definition, Structure, Types, Applications

Ti plasmid (Tumor-inducing plasmid) is an extrachromosomal genetic material found in Agrobacterium tumefaciens which causes crown ball disease in certain dicot plant species.
Ti plasmid was developed in the 1980s from various observations.
Ti plasmid can be transferred to plant chromosomes.
It also has the ability to transfer other foreign DNA sequences into the plant chromosome.
It is around 200 kb large.
Ti plasmid has three important regions.

Table of Contents

Structure of Ti plasmid

T-DNA region – this region carries genes responsible for inducing tumors in plants. Other foreign genes of interest can be inserted in this region.

For inducing a tumor, only the T- DNA is transferred from one cell to another.

Vir region – this region consists of virulence genes. It is responsible for the excision, integration, and transfer of T-DNA into the plant chromosome.

Opine catabolism region – this region catabolizes opines, which are specialized amino acids. It is responsible for catabolizing opines produced by the T-DNA region. Opines often act as a source of Nitrogen in the bacteria.

Other regions include the origin of replication, T-DNA border sequence, plant hormone synthesis region, and conjugation region.

Plant hormone synthesis region – this region is responsible for the synthesis of essential plant hormones like auxins and cytokinin.

Ti plasmid Conjugation region – process involving Ti plasmid includes the transfer of whole Ti plasmid from one cell to another.

The T-DNA border sequence is 24bp with direct repeats. It marks the border of the T-DNA and is important for the transfer of plant chromosomes.

Types of Ti plasmid

Based on the differences in the T DNA region, there are two types of Ti plasmids:

1. Nopaline

It has a continuous region of T-DNA, which is approximately 25 kb in length.
It produces an opine known as Nopaline (C₁₁H₂₀N₄O₆).

2. Octopine

It is subdivided into two regions which are 13 kb and 8 kb long.
It produces an opine known as Octopine (C₉H₁₈N₄O₄).
It was first isolated from Octopus octopodia in 1927.

Applications of Ti plasmid

Ti plasmid can be used as a vector for transformation involving Agrobacterium tumefaciens and is used for producing transgenic plants.
With the use of restriction enzymes, a particular gene of interest can be inserted into the plasmid and transformed.
It is used for the development of stress-tolerant plant varieties.
Ti plasmids are also responsible for inducing amino acids and sugar-phosphate derivatives known as opines.

Limitations of Ti plasmid

Ti plasmid is relatively a larger plasmid, and smaller plasmid vectors may be preferred in some cases.
Ti plasmid usually works only in plants that can be infected by Agrobacterium tumefaciens.
It cannot be used for monocot plants.

References

Gordon, J. E., & Christie, P. J. (2014). The Agrobacterium Ti Plasmids. Microbiology spectrum, 2(6), 10.1128/microbiolspec.PLAS-0010-2013. https://doi.org/10.1128/microbiolspec.PLAS-0010-2013
Todd R Steck, Ti plasmid type affects T-DNA processing in Agrobacterium tumefaciens, FEMS Microbiology Letters, Volume 147, Issue 1, February 1997, Pages 121–125, https://doi.org/10.1111/j.1574-6968.1997.tb10230.x
Schell, J., & Van Montagu, M. (1977). The Ti-plasmid of Agrobacterium tumefaciens, a natural vector for the introduction of nif genes in plants?. Basic life sciences, 9, 159–179. https://doi.org/10.1007/978-1-4684-0880-5_12
An G. (1995). Binary Ti plasmid vectors. Methods in molecular biology (Clifton, N.J.), 44, 47–58. https://doi.org/10.1385/0-89603-302-3:47
National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 108172, Octopine. Retrieved July 6, 2022 from https://pubchem.ncbi.nlm.nih.gov/compound/Octopine.