Agriculture Reference
In-Depth Information
& E. Henn.) for 21 and 35 days after application
of glyphosate (Anderson and Kolmer 2005).
Although the candidate transgenic wheat per-
formed as well as or better than its parent in these
various tests, Monsanto elected to withdraw its
petitions for US commercial production of
Roundup Ready wheat in spring 2004. This deci-
sion was attributed to opposition by trading part-
ners in Europe and Asia (Wilson et al., 2003) and
to the relatively low demand for weed control in
US spring wheat (Skostad 2004). Hard red spring
wheat constitutes only 20% of US exports and
plantings have decreased since 1997 due to the
threat of losses to Fusarium head blight and the
northward expansion of growing regions suitable
for maize and soybean production (Skostad
2004).
An interesting aspect of the history of trans-
genic glyphosate-resistant wheat is that less than
1% of the approximately 3,000 lines produced by
Agrobacterium
transformation and direct selection
for glyphosate resistance met all the criteria for
commercial release imposed by Monsanto (Hu
et al., 2003). Even fewer of the transgenic lines
produced in parallel by bombardment met those
criteria (Hu et al., 2003). In the next section, we
describe the limitations of current wheat transfor-
mation methods that result in large numbers of
transgenic plants that are neither informative to
basic researchers nor useful for commercial release
in production agriculture.
for the properties controlled and/or infl uenced
by the transgene protein product. In practice,
there are several limitations of current transfor-
mation methodology that can result in less-
than-ideal outcomes. None of the diffi culties
encountered in recovering useful transformants
are unique to wheat and some problematic out-
comes of transformation have been better studied
in other transgenic crop and model species
(Filipecki and Malepszy 2006).
Genotype
Not all wheat genotypes can be transformed by
current methods. Only some genotypes respond
to
in vitro
culture to produce embryogenic callus.
The calli derived from some genotypes cannot
recover from wounding by particle bombardment
or co-cultivation with
Agrobacterium
. Further-
more, some genotypes of embryogenic calli are
not able to differentiate into shoots and roots in
response to exogenous hormone changes in
culture. A completely genotype-independent
method of wheat transformation is unlikely in the
near future, since none are known even for model
plants such as tobacco and Arabidopsis. As long
as genotype limitations persist, conventional
breeding will be needed to move transgene-
encoded traits into locally adapted cultivars. For
breeders, a useful property of transgenes is that
they almost always include, by their very nature,
new sequences that can serve as molecular markers
for following inheritance.
LIMITATIONS OF WHEAT
TRANSFORMATION TECHNOLOGY
Structures of integrated transgenes
The simplest structures for integrated transgenes
are shown in Fig. 18.1, but transgene insertions
often have higher copy numbers. For biolistic
transformations, transgene copy number can vary
from 1, in a third of transformants, to more than
100 (Rasco-Gaunt et al., 2001). The median copy
number among 25 lines investigated by Rasco-
Gaunt et al. (2001) was in the range of 3 to 5. In
the fi rst report of
Agrobacterium
transformation of
wheat, Cheng et al. (1997) found that one-third
of 26 transformants had a single transgene copy
Ideally, a wheat transformation experiment could
be applied to any genotype and would result in a
plant that contains a predictable number of copies
of a gene or genes of interest integrated at a single
site. The transgene would be expressed at the
required levels when and where its product was
needed. It would not have any other effects on the
plant except those directly affected by the pres-
ence of the gene product in appropriate tissues.
The transformed plant would be identical to its
parent in growth, development, and yield except
