During bombardment, the majority of the metal particles do not find their target. Most of
the particles either embed in the cell wall, enter the vacuole, or end up somewhere else in
the cytoplasm; only a few reach the nucleus. After all, thousands of particles are delivered
using literally a “shotgun” approach. Evaluation of those cells that express the introduced
DNA shows that the overwhelming majority of cells (.90%) have particles either adjacent
to or in the nucleus (Fig. 10.8) (Yamashita et al. 1991; Hunold et al. 1994). Unlike
Agrobacterium, where integration of the introduced DNA into the plant chromosomal
DNA is orchestrated by bacterial proteins that are bound to the T-strand, particle bombard-
ment results in the introduction of naked DNA. Clues to the fate of the introduced DNA can
be taken from studying the final arrangement of the integrated DNA within plant chromo-
somal DNA.
In general, the patterns of DNA integration in the plant chromosome are very complex.
To be more specific, it can be a real mess. Usually, the introduced DNA integrates into a
single site (locus) on the chromosomal DNA. However, the introduced DNA can also inte-
grate at multiple sites, which makes analysis more difficult. To complicate the situation
further, it is common to obtain multiple copies of the transgene in each integration site.
And (it gets worse) the copies can be partial copies, with varying orientations. In addition
(last thing), the introduced DNA appears to be mixed or interspersed with plant genomic
DNA (Pawlowski and Somers 1998). Imagine the replication and repair machinery of
the nucleus as an army of overworked, frantic, multiarmed, DNA tailors. The DNA
tailors are supposed to make exact copies of chromosomes and fix any small mistakes,
while they are sewing huge amounts of new DNA strands. They are working fine until
the whoosh of this huge boulder (1-mm particle) overhead that is carrying DNA. It looks
like plant DNA, so they take what they can and use it in their sewing operation. It is not
a perfect fit, but they are frantic and under the time constraints to get all of the chromosomal
DNA replicated before the cell divides. For particle bombardment, it is unclear whether the
particles actually physically break the chromosomal DNA or merely deposit DNA in the
proximity of the replicating parts of chromosomes. It is clear that the introduced DNA
can integrate into chromosomal DNA, with very complex patterns. However, complex
Figure 10.8.For successful transformation, particle bombardment results in the delivery of heavy-
metal particles either next to (white arrows) or directly to the nucleus (black arrow). (Photo provided
by Joseph Chiera, Department of Horticulture and Crop Science, OARDC/The Ohio State
University.)
258 TRANSGENIC PLANT PRODUCTION