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. 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 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. 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. Plant genomes can be engineered by use of Agrobacterium for the delivery of sequences hosted in T-DNA binary vectors.