Modern plant genetic engineering involves transferring desired genes into the DNA of some plant cells and regenerating a whole plant from the transformed tissue. New DNA may be introduced into the cell via biological or physical means.
The most widely used biological method for transferring genes into plants capitalizes on a trait of a naturally occurring soil bacterium, Agrobacterium tumefaciens, which causes crown gall disease. This bacterium, in the course of its natural interaction with plants, has the ability to infect a plant cell and transfer a portion of its DNA into a plant's genome. This leads to an abnormal growth on the plant called a gall. Scientists take advantage of this natural transfer mechanism by first removing the disease-causing genes and then inserting a new beneficial gene into A. tumefaciens. The bacteria then transfer the new gene into the plant.
Another gene transfer technique involves using a "gene gun" to literally shoot DNA through plant cell walls and membranes to the cell nucleus, where the DNA can combine with the plant's own genome. In this technique, the DNA is made to adhere to microscopic gold or tungsten particles and is then propelled by a blast of pressurized helium.