A soil-inhabiting, plant pathogenic
bacterium, Agro bacterium tumefaciens, infects broad-leaved crops including
tomato, soyabean, sunflower and cotton, but not the cereals. It causes tumours
called crown galls.
Tumour formation is induced by its
plasmid, which is, therefore called Ti-plasmid
(Ti for tumour inducing). The
Ti-plasmid integrates a segment of its DNA, termed T-DNA, into the chromosomal
DNA of its host plant cells. The T-DNA causes tumours. As gene transfer occurs
without human effort, the bacterium is known as natural genetic engineer of
Plant molecular biologists have
started using Ti-plasmids as vectors to transfer foreign genes of interest into
the target plant cells. They use a version of the plasmid from which tumour
forming gene has been eliminated. The transformed bacteria do not cause
disease, but still deliver genes of interest into a variety of plants.
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