What makes Agrobacterium tumefaciens so special?
The bacterium A. tumefaciens lives in the soil and on plants. Like every other bacteria and living organism, it needs nutrients to survive and reproduce. This bacteria found a very inventive way to use plants in its quest to gain nutrients. Agrobacterium can transfer a specific piece of its DNA into a (wounded) plant cell. The transferred DNA (T-DNA) can be integrated at random in the genomic DNA of the plant cell. The T-DNA contains a gene that translates for opines synthesis and genes for auxin and cytokine production
Role of opines synthesis: opines are small molecules which are normally not made by plants. When a T-DNA is integrated in the plant genome, the plant cell will produce these molecules. The molecules have no use for the plant cell, but it does for the Agrobacterium. The Agrobacterium contains genes that translate for opine catabolism (=break down). This means that the Agrobacterium can use the opines as a nutrient source.
Role of the auxin and cytokine production: auxins and cytokines are plant hormones that stimulate plant cell division and cell growth. When the T-DNA is inserted, those plant cells which contain the T-DNA will start to divide and grow. This will result in a plant tumor, called the crown gall tumor. This is advantageous for the bacterium because more plant cells that produce the opines equals more food for the bacteria.
How does the T-DNA transfer works?
Bacteria can contain plasmids. A plasmid is a small piece of circular double stranded DNA that exists next to the genomic DNA of the bacteria. The plasmid in Agrobacteria tumefaciens contains the following genes:
- virulence genes (vir genes)
- genes for opine catabolism
- origin of replication
The T-DNA is the piece of single stranded DNA that will be transferred into plant cells. It begins and ends with a specific DNA sequence (T-DNA border sequence) so that the bacteria knows what piece of DNA has to be transferred.
The vir genes (virA, virB, virC, virD, virE, virF and virG) are essential for the transfer process.
- VirA translates for a receptor. When a plant is wounded, phenolic components will be secreted. These components can bind to the receptor. This way, the Agrobacterium can “sense” the wounded plant and activate the transfer process. The transfer process is activated by starting the transcription of the other vir genes.
- VirD will translate for proteins that can recognize the T-DNA border sequences and will cut a single strand of the T-DNA out of the plasmid.
- VirE will translate for proteins that will form a complex with the single stranded T-DNA. This is needed to protect the T-DNA and to guide it into the nucleus of the plant cell.
- VirB will translate for proteins that will form a “needle” that connect the Agrobacterium with the plant cell. The T-DNA can move through the needle into the plant cell.
Importance of Agrobacterium tumefaciens in plant research
Researchers found out how Agrobacterium tumefaciens works when they were investigating the crown gall tumors in plants. They thought they were going to find a solution against tumor growth, and discovered “by accident” the wonder of gene transfer from bacteria to plants. Nowadays, it is possible to replace the T-DNA in Agrobacterium by another piece of DNA. This can be achieved by DNA recombination. Only the original T-DNA border sequences are needed for recognition. The new T-DNA can translate for genes involved with plant stress resistance, pathogen resistance, herbicide resistance, etc …