Agrobacterium tumefaciens is a rod-shaped aerobic soil bacterium that can infect dicot plants especially apple, pear, peach, cherry, almond, raspberry, rose & grapevines. It’s method of infection is unique, for it transfers some of its DNA to the chromosomes of the plant cell, causing a tumour to grow & causing the plant to make special organic food molecules for the bacterium.
The process begins when a plant is wounded, often near the base of the stem. This causes the release of compounds that attract Agrobacterium to the damaged cells. Transcription of genes on one of the bacterium’s plasmids begins, transferring a 20 kbase piece of single-stranded T-DNA into the plant nucleus. Here the T-DNA randomly integrates into the plant chromosome, translating 2 groups of substances:-
1. One group of chemicals causes an over-production of plant hormones, leading to crown gall, which is an uncontrolled growth of cells, a cancer. Galls can develop any-where on stems & roots but are usually near the soil line & vary from pea-size up to tennis-ball-size.
2. The other group of substances directs the plant to make unusual compounds called opines, which the bacterium can use as a food source.
The genes for all these substances are under the control of plant gene regulators. This infection does not kill the plant, it merely alters the functions within the plant.
The plasmid which controls this infection process is the Ti (tumour-inducing) plasmid. It is a very big, double-stranded, circular piece of DNA, which was sequenced in 2001. It contains a number of important parts:-
1. Oncogene – this is the gene that causes crown
2. VIV – the origin of DNA replication
3. Opine gene – this is the gene that codes for the
making of the opines, the bacterial food source.
4. A Left & a Right border - 25b repeats needed
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