The transformation method that is being more widely used in many plant species makes use of Agrobacterium tumefaciens, a soil bacteria that works as a natural genetic engineer. Agrobacterium tumefaciens can invade plants through wounds in the stem or root. These bacteria have an extra piece of DNA called the Ti plasmid.
Once inside the plant, a portion of the Ti plasmid will leave the bacteria, enter the plant cell and insert itself into the plant's chromosomes.
Upon insertion, the genes on the Ti plasmid are turned on. These genes encode enzymes that force the plant cell to make metabolites the bacteria need for growth. The Agrobacterium takes over (genetically engineers) the plant, forcing it to become a good host. This allows the bacteria to proliferate until the plant dies. Agrobacterium then assumes its soil bacterial lifestyle again, waiting until it encounters another wounded plant.
Genetic engineers have taken advantage of Agrobacterium's natural abilities. They have removed the genes from the Ti plasmid that alter metabolism and cause disease and in their place incorporated desired genes. These bacteria with their altered plasmids can be used to infect and transform plant cells growing in tissue culture. Although originally thought to work only in dicot systems, scientists have recently developed methods to effectively use Agrobacterium on some monocot species, such as rice, corn, and oats.