It appears that bacteria proliferate or multiply at low levels and coexist within the tissues
of the plant. They do not invade the cells of the plant as most other pathogens do, but they
bind to suitable target cells for DNA delivery. IfAgrobacteriumtransforms a somatic or
vegetative cell within the plant, this is probably a terminal event. In the plant, a transformed
leaf or stem cell will not give rise to anything other than another leaf or stem cell. For recov-
ery of whole, transgenic plants, the goal is to target germline cells, cells that will contribute
to the fertilized egg. ForAgrobacterium-mediated transformation ofArabidopsis, the egg is
targeted, leading to the production of transgenic seed. Usually each seed is from a different
transformation event.
Why isArabidopsisso easy to transform? Why don’t corn and soybeans work the same
way? (Soybean transformation has been very inefficient since the mid-1980s.) As we learn
more about the transformation process, it may eventually be possible to recover transgenic
corn and soybeans with the same ease as transgenicArabidopsis. For now, inefficiencies in
transformation remain both a mystery and a reason for transformation scientists to continue
working on improvements.
10.4 Particle Bombardment
10.4.1 History of Particle Bombardment
Particle bombardmentrefers to a method where heavy-metal particles ( 1 mm gold or
tungsten) are coated with DNA, accelerated toward the target tissue, and penetrate the
cell wall to rest either adjacent to or directly in the nucleus. The DNA on the particles
somehow finds its way to the native DNA of the target cell, where it becomes integrated
into the chromosome to become a permanent addition to the genome.
Figure 10.5.AgroinfiltratedNicotiana benthamianaplants showing high levels of GFP expression.
The aerial parts of the tobacco plant were submerged in anAgrobacteriumsuspension, and the plant
was then placed under vacuum for infiltration. (Image provided by John Lindbo, Department of Plant
Pathology, University of California-Davis.) See color insert.
10.4. PARTICLE BOMBARDMENT 255