Agrobacterium – Nature’s own genetic engineer!!
Agrobacterium is nature’s beautiful creation and it is nature’s own genetic engineer. This microscopic genius has the ability to transfer its gene to plant and this property has made Agrobacterium very popular in the field of genetic engineering. Because utilizing this property scientists are able to obtain improved quality plants.
Agrobacterium is Gram negative soil bacteria and when it infects plant i.e. when it transfers its gene to plant, the plant develops Crown Gall disease. So in this post we will see the important genes involved in the transfer and of course at the end I will share my trick to remember which gene does what function. So let’s get started 🙂
Agrobacterium’s gene transfer property is within its plasmid called Ti plasmid. Ti stands for Tumor Inducing.
Figure a shows the important segments of Ti plasmid which are as followed.
- Ti plasmid has T-DNA region which is transfer region and it is the only part which gets transfer from bacteria to plant. This T-region contains auxin production, cytokinin production and opine synthesis genes.
- Ti plasmid also has virulence region which is require to mediate the gene transfer. This region includes genes such as vir A, vir B, vir G, vir C, vir D and vir E.
Mechanism of Gene Transfer :
- When a plant gets injured (may be by insect bite), it causes secretion of phenolics such as acetosyringone from plant. This acetosyringone acts as attractant for Agrobacterium. As a result, lot of Agrobacterium would get attracted towards the wounded region of the plant.
- Acetosyringone activates the vir A which is a transmembrane protein by autophosphorylating it. Once vir A is activated it activates vir G by phosphorylating vir G. Now protein G is transcription factor which means once it is activated it transcribes remaining genes of vir region i.e. vir B, vir C, vir D and vir E.
- The first vir gene to be transcribed is vir C, which makes a complex with right T-DNA border.
- Second gene in the line is vir D which act as endonuclease. It recognizes the right T-DNA border + protein C complex and it will digest or form a nick in that region.
- Third event is formation multiple copies of vir E gene. Now this multiple copies of protein E would bind to the nicked portion of T-DNA and escorts it from bacteria to plant.
- But wait, how would it take T-DNA from bacteria to plant? Is there any channel or bridge between both? This problem is solved by vir B gene which provides bridge between the bacteria and plant and T-DNA is transferred through this.
- Following the transfer of T-DNA to plant, it gets inserted into the chromosome of plant. Now remember we just saw that T-DNA has auxin production, cytokinin production and opine synthesis genes? So these genes will now start getting expressed. When auxin and cytokinin genes are expressed, it causes uncontrolled growth of plant cell resulting into tumor or Crown Gall disease.
- What about opine synthesis gene? When it is expressed, it secretes opine into the soil which attracts the soil Agrobacterium and they will utilize this opine as their nutrient source.
Wonderful mechanism, isn’t it? Now my favorite part!! How to remember functions of these genes? What I do is, I correlated their function with what a gene is called. Like,
A = Autophophorylation
G = Gene transcription
C = Complex formation with T-DNA
D = Digestion
E = Escorting T-DNA
B = Bridge formation
For more explanation watch this video.
