The termparticle bombardmentcan be used interchangeably with the similar terms
microprojectile bombardment,Biolisticsw,particle acceleration, andgene gun technology.
The term that is currently most often used isparticle bombardment.
As opposed toAgrobacterium, which is a biological vector for DNA introduction, par-
ticle bombardment is a purely physical method for DNA delivery. The DNA is physically
precipitated onto metal particles, and those particles are then rapidly accelerated toward the
target tissue. The particles penetrate through the cell wall by punching holes in that rigid
structure and continue to do so until being stopped by the density of the target tissue. To
visualize what is occurring, imagine bullets penetrating a thin piece of wood to enter
water beneath. The wood is the cell wall and slows down the particles abruptly while the
water gradually slows them down further until they stop. The analogy to bullets above is
no coincidence.
Particle bombardment was invented by John Sanford and colleagues in the mid-1980s.
The approach was further developed and optimized by Ted Klein (see Life Box 10.2), a
postdoc at Cornell University in John Sanford’s laboratory. Conceptually, a 22-caliber
rifle, loaded with blanks, was first used to evaluate the damage to plant tissue from the
shockwave resulting from an ignited powder load. The “gun” with “bullets” concept was
further perpetuated with the introduction of the first commercial device, which used a
22-caliber powder load to generate a controlled explosion to accelerate small tungsten par-
ticles down the barrel of a modified gun. Between “shots,” the particle bombardment device
had to be cleaned with gun-cleaning swabs and brushes. Later versions of particle guns
used other types of forces to generate the energy required to accelerate the small particles
(Finer et al. 1999). The required violent forces, needed to accelerate the particles, could be
created by generating high-voltage arcs across a gap or by using high-pressure air or CO 2 .It
was not unusual at the time to perform bombardments with muffling headsets, to dampen
the sound from the early devices. Today, in most laboratories, high-pressure helium is used
to generate the force needed to accelerate small gold particles (Fig. 10.6) toward the target
tissue. DNA is first precipitated onto the particles, which are then placed as a monolayer on
a Mylar carrier sheet, called amacrocarrier(this term refers to the structure that carries the
Figure 10.6.Gold particles used for particle bombardment, prior to DNA precipitation. Gold
particles are more uniform and spherical than tungsten particles.
256 TRANSGENIC PLANT PRODUCTION