Particle Bombardment

Many different strategies have been tried to introduce naked DNA into plant cells (including microinjection, DNA pollination, silicon whiskers, electroporation of cells or tissues, electroporation or chemically induced introduction of DNA into protoplasts), but the major alternative to Agrobacterium transformation is particle bombardment.
This approach now normally involves coating 1 mm diameter particles of tungsten or gold with DNA, putting them on a support, accelerating them to high speed using a pulse of high pressure helium into an evacuated chamber containing the target tissues. Particles can penetrate up to about six cell layers and the DNA released from the particles in surviving cells may be expressed transiently and, in a small proportion of cases, it becomes integrated into the nuclear genome of that cell. With appropriate tissue culture and selection, transgenic plants can be regenerated.
Particle bombardment is usually limited to the generation of transgenic cereals and its use has decreased since Agrobacterium transformation now works for cereals such as rice and barley It is still used widely for maize and wheat transformation, although there is one report of wheat transformation using Agrobacterium as a vector.