Transfer DNA

The transfer DNA (abbreviated T-DNA) is the transferred DNA of the tumor-inducing (Ti) plasmid of some species of bacteria such as Agrobacterium tumefaciens and Agrobacterium rhizogenes (actually an Ri plasmid). The T-DNA is transferred from bacterium into the host plant's nuclear DNA genome.^[1] The capability of this specialized tumor-inducing (Ti) plasmid is attributed to two essential regions required for DNA transfer to the host cell. 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.

The bacterial T-DNA is about 24,000 base pairs long^[2][3] and contains plant-expressed genes that code for enzymes synthesizing opines and phytohormones. By transferring the T-DNA into the plant genome, the bacterium essentially reprograms the plant cells to grow into a tumor and produce a unique food source for the bacteria. The synthesis of the plant hormones auxin and cytokinin by enzymes encoded in the T-DNA enables the plant cell to overgrow, thus forming the crown gall tumors typically induced by Agrobacterium tumefaciens infection.^[4] Agrobacterium rhizogenes causes a similar infection known as hairy root disease. The opines are amino acid derivatives used by the bacterium as a source of carbon and energy. This natural process of horizontal gene transfer in plants is being utilized as a tool for fundamental and applied research in plant biology through Agrobacterium tumefaciens mediated foreign gene transformation and insertional mutagenesis.^[5][6] Plant genomes can be engineered by use of Agrobacterium for the delivery of sequences hosted in T-DNA binary vectors.