In this article we will discuss about:- 1. What Is T-DNA? 2. Features of Ti Plasmid 3. Mechanism of T-DNA to transfer 4. Effcts of T-DNA on The Plant and 5. Application of T-DNA
1. What Is T-DNA?
- T DNA stands for transfer DNA
- It derives its name from the fact that the bacterium transfers this DNA fragment into the host plant's nuclear DNA genome.
- It is a part of Ti plasmid (Tumor inducer plasmid) of the Agrobacterium tumefaciens
- Agrobacterium tumefaciens is a phytopathogenic, gram- negative, non sporing, motile bacillus.
- It is found in the rhizosphere
- It causes crown gall disease in plants, a disease of dicotyledonous plants characterized by a tumorous phenotype.
2. Features of Ti Plasmid:
- It is responsible for crown gall disease
- It has 200 kb and It has 8 genes (vir, shi, roi, nos, noc, ocs, occ and ori V)
- A part of Ti-plasmid, T DNA, integrates into the plant chromosomal DNA
- The T-DNA is bordered by 25-base-pair repeats on each end. Transfer is initiated at the right border and terminated at the left border and requires the vir genes of the Ti plasmid.
3. Mechanism of T-DNA to transfer:
- A.Tumefaciens is attracted towards wound site by chemotaxis The wound induces the production of phenolic compounds such as acetosyringone The lipopolysaccharides aid in attachment of A. Tumefaciens.
- Ti plasmid code for chemotactic receptors. The processing of T-DNA transfer is mediated by the vir (virulence) region. The vir gene encode a set of proteins responsible for the excision, transfer and integration of the T-DNA into the plant nuclear genome: VirB, VirC, VirD, VirE. Vir A and vir G proteins constitute a two component regulatory system that controls vir gene expression. VirA detects the phenolic compounds, acetosyringone, resulting in autophosphorylation.
- The autophosphorylated VirA phosphorylates VirG which leads to the activation of vir gene and this one get expressed. After vir gene activation, T strand, a single stranded copy of T DNA is produced:
- Two VirD proteins, i.e VirD1 &VirD2, recognize the 25 bp border sequence and produce a ss endonucleolytic cleavage in the bottom end of each border These nicks are used as initiation and termination sites for T strand production
- VirC locus then produces 2 proteins, VirC1 and VirC2 which promotes generation of multiple copies per cell of T strand.
- Then VirD1 is detached from T strand whereas VirD2 remains tightly bound on the 5’end to ensure that in subsequent steps, the 5’ end is the leading end. To prevent T strand degradation by endonucleases during its journey to the plant nucleus, VirE locus produce VirE2 ss nucleic acid-binding protein
- It binds tightly on T-strand which results in the unfolding of the ssDNA, i.e T-strand, thus reducing its diameter to 2nm facilitating its transfer through membrane channels. The T-strand along with VirD2 & VirE2 is called as T complex
- The T complex exit the bacterial cell membranes and cross the plant cell membranes and enter the plant cell. Once inside the cell, T complex targets the plant nucleus and cross the nuclear membrane where it integrates into the plant DNA. The transport of T Complex is mediated by VirB-VirD4 transporter complex.
- Once inside the plant, T complex is led by VirD2 and VirE2 interacting proteins along with VirF towards the nucleus.
- In the nucleus, VirF with the interacting proteins unbind from the t complex. Then the right border initiates integration of tDNA region while VirD2 and VirE2 are unbinding from the T strand.
4. Effcts of T-DNA on The Plant
- T-DNA disorganize the proliferation of cells in the plant
- It reprograms the cell to grow into a tumor and produce a unique source food for the bacteria
- The synthesis of the plant hormones auxin and cytokinin enables the plant cell to grow unc ontrollably, thus forming the crown gall tumors typically.
- The morphology of the galls is controlled by the loci in T-DNA:
- Tml causes large tumors Tmr induces tumor in roots
- Tms induces tumor in shoots
5. Application of T-DNA
T-DNA transfer capabilities are used in genetic engineering to introduce foreign genes into desired plants: the tumor-promoting and opine- synthesis genes of T-DNA are removed and replaced by genes of interest. Such plants are then called transgenic plants.
Mechanism of T-DNA to transfer Short Notes | Biotechnology
Reviewed by Rajkumar
on
April 24, 2019
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