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.
Ti plasmid
Ti plasmid

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.
Agrobacterium-Mediated Transformation
Agrobacterium-Mediated Transformation

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

  1. Gordon, J. E., & Christie, P. J. (2014). The Agrobacterium Ti Plasmids. Microbiology spectrum2(6), 10.1128/microbiolspec.PLAS-0010-2013. https://doi.org/10.1128/microbiolspec.PLAS-0010-2013
  2. Todd R Steck, Ti plasmid type affects T-DNA processing in Agrobacterium tumefaciensFEMS Microbiology Letters, Volume 147, Issue 1, February 1997, Pages 121–125, https://doi.org/10.1111/j.1574-6968.1997.tb10230.x 
  3. 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 sciences9, 159–179. https://doi.org/10.1007/978-1-4684-0880-5_12 
  4. 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 
  5. 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.

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