A tumor producer: Ti Plasmid - Erakina

Plasmid:

Plasmids are circular DNA that works as autonomously replicating molecules (Accessory chromosomes) in bacteria and bacteriophages which is a virus that can transport its genetic cargo to bacteria. The only disadvantage is that there is a size limit to the number of base pairs that can be easily incorporated into the plasmid or phage vector that is 1-25 kilobase pairs.
They reside in the living cells and even carry genetic information but are not considered chromosomes.
And even if they carry genetic information that may be expressed but they are not a continual part of the cell’s genetic makeup and are not required for cell growth and division in normal circumstances.
Transmission of the plasmids takes place with the help of the effective mechanism called “Conjugation”.
They play a crucial role as genetic tools or vehicles utilized to manipulate and analyze microorganisms via the introduction, modification, or removal of the target genes.

The important elements of Plasmid:

Presence of Ori (Origin of Replication): The ori is the place from where replication starts.
Selectable Marker: It is a gene that is introduced into the host cell that confers a trait for artificial selection.
Multiple cloning sites: These are recognition sites utilized for inserting the foreign DNA fragment employing restriction enzymes.
Promoter Region: This region promotes the process of transcription of the target gene to obtain desired protein.
Primer Binding site: It is a region containing a nucleotide sequence that is employed for PCR amplification and sequence verification.

There are various types of plasmids but today in this blog I will be discussing “Ti Plasmids”.

Ti Plasmids:

The word “ Ti” stands for “Tumor inducing” which means that it’s a tumor-inducing plasmid. The size of the plasmids ranges up to 250 kb.

It is found in the bacterium “Agrobacterium tumefaciens”. This Agrobacterium species was found pathogenic against plants like Agrobacterium vitis, Agrobacterium rubi, Agrobacterium rhizogenes.

It was found that the pathogenicity of the species was due to the presence of “Ti plasmids” which were responsible for the mutational change in the plants affected by it.

Then via mutational analysis and experimentation of introducing “Ti plasmids” into Rhizobium or Phyllobacterium species switch the non-pathogenic nature of the species into tumor-inducing pathogens.

The disease coming from the “Ti plasmids” is “Crown Gall Disease” which ultimately led to the formation of cognate tumors in the plant crown.

Ti plasmid not only give rise to plant tumors but also led to the generation of various amino acid and sugar-phosphate derivatives that are known as “Opines”. After the transformation of the plant cell, it starts secreting opines, which in long run is utilized by the infecting bacterium as its food source. These opines cannot be utilized by other bacteria as they do not have genes to utilize them.

There are two types of Ti plasmids:

Octapine types (Eg: pTiA6, B6, Ach5, 15955, R10)
Nopaline type (Eg: pTiC58, pTi37)

Ti Plasmids code for functions associated with:

Plasmid replication and maintenance
Virulence
Conjugative transfer
Opine utilization
Sensory perception of the signals released by the host plant and the nearby agrobacterial cell at the site of infection.
Genes associated with functions are:

Vir region for virulence
T-DNA (Transfer DNA) for infecting the plants.
tra and trb regions for conjugative plasmid transfer.

How does Agrobacterium enter and affect the plants?

When the plant gets wounded (simple cut or abrasion) it releases chemicals like acetosyringone, belonging to phenolic compounds which acts as an alluring substance for the bacterium “Agrobacterium”. Then the agrobacterium enters the plants via the wounded site and transfers the Ti plasmid into the plant cell via the process of conjugation. And if the agrobacterium is “Agrobacterium tumefacien” then it will ultimately lead to the disease called “Crown Gall disease” and the plant even become the house of the bacterium.

How do signaling networks control Ti plasmid functioning?

As previously explained that plants release certain chemicals from the wounded site which ultimately lead to the beginning of the regulatory cascade that wind up in the following sequence of events:

T-DNA transfer

Generation and utilization of Opine

Elevation in the Ti plasmid copy number

Dissemination of the Ti plasmid.

Importance of Ti plasmids:

Modified Ti plasmids are utilized in the field of genetic engineering of plants.

Laboratory scale experiments are done by utilizing Ti plasmids engineered Agrobacterium for transferring the gene of interest into the plants.

It is even utilized for the analysis of gene function in plants.

Note:

Agrobacterium tumefaciens is renowned as “nature’s genetic engineer”
Bacterial Conjugation: It is the process of transferring DNA from one bacterium to another via physical contact.
The most utilized model plant is “Arabidopsis thaliana” which is employed to generate a set of gene knockouts by random insertion of T-DNA.
The Agrobacterium has great potential as it can transfer DNA from one species (Bacteria) to another (Plant) accomplishing it as a vital tool for the genetic engineering of the plants.
The bacterium only infects the dicot plants though the one with modified non-virulent plasmid can also bring changes in the monocot plants.
Ti plasmid can be easily modified as per the interest to insert the desired genes to get desired results.

-Priyanka Patra

-25/02/2022

A tumor producer: Ti Plasmid