The Ti plasmid contains genes for auxin and cytokinin formation, and for synthesis or utilization of opines. Ti plasmids range in size from 200 to 800 kbp. Of this, about 30% is common to nopaline and octopine plasmids (Genes VIII, Benjamin Lewin, pp. 525). The genes of common region are required for the interaction between Agrobacterium and plant hosts. The part of Ti plasmid which is transferred to the plant nucleus is called T-DNA. It gets incorporated into the plant genome. The T-DNA is referred to as the T-region. T-region is approximately 10 to 30 kbp in size and represents less than 10% of the Ti plasmid. Ti plasmids generally contain one T-region, but some Ti plasmids have multiple T-regions on them. T-region is flanked by T-DNA border repeats. These are 25 bp long and highly homologous in sequence. The T-DNA border sequences flank the T-region in directly repeated orientation. The T-DNA border sequences delimit the T-DNA , and are the target of the border-specific endonucleases.
