Agriculture Reference
In-Depth Information
selection gene are resistant to glufosinate-based
herbicides (Gopalakrishnan et al., 2000).
Male-sterile but otherwise normal wheat plants
were produced by expressing the barnase RNAse
under control of maize or rice promoters whose
expression is confi ned to tapetal tissue that nour-
ishes the male gametophyte (De Block et al.,
1997). Single-chain antibodies that could be
useful pharmaceuticals for diagnosis of cancer or
eye diseases were produced by expression of their
coding regions in wheat under the control of seed-
specifi c promoters. Levels of the pharmacological
proteins were reported to be as high as 30 μg g
-1
dry weight (Stöger et al., 2000), and the proteins
were stable for at least one year in dry seeds
(Brereton et al., 2007).
stability of transgene expression and to ascertain
how the presence of selection genes and
HMW-GS transgenes affects yield or other
agronomic properties. In these plants, the
bar
selection gene is under control of the
Ubi1
promoter, and the HMW-GS transgenes are
under control of their native endosperm-specifi c
promoters.
Vasil et al. (2001) observed that fi eld perfor-
mance of six independent lines carrying
HMW-GS
1Ax1
and
Ubi1::bar
transgenes was
equivalent to that of their nontransformed
parent. Expression of the HMW-GS transgenes
was stable through three generations of growth
in the greenhouse and in fourth-generation
plants grown in the fi eld. Barro et al. (2002) grew
four lines carrying
1Ax1
and/or
1Dx5
and selec-
tion transgenes, three lines carrying only the
HMW-GS transgenes, two lines without any
transgenes that had segregated from the trans-
genic plants (null segregants), and two nontrans-
formed parental lines for two years in fi eld
experiments. For traits other than protein com-
position, consistent differences were observed
only for heading date and number of spikelets
per spike. All transgenic lines headed a few days
later and produced more spikelets per spike than
the non-transgenic lines (Barro et al., 2002). No
consistent signifi cant differences were observed
for grain yield. The third through fi fth genera-
tions of a subset of the same lines were grown in
three separate years at two UK sites (Shewry et
al., 2006). Entries in this trial included three
lines carrying
1Ax1
or
1Dx5
transgenes, one null
segregant line, and two nontransformed parental
lines. Transgenic and non-transgenic lines
exhibited the same levels of variability for grain
nitrogen and kernel weight between years and
locations. The expression levels of the trans-
gene-encoded HMW-GS showed no greater
variability between years and locations than
expression of the native HMW-GS genes. The
mixing properties of the HMW-GS transgenics
showed the same differences from their parents,
regardless of their growth environment (Shewry
et al., 2006). The authors concluded that trans-
gene-encoded changes had the same stability
across environments and generations as conven-
Impacts on production agriculture
As of this writing, no transgenic wheat exists in
commercial production in the US or elsewhere.
Applications of wheat transformation to date have
mostly been in the areas of gene discovery and
proof-of-concept experiments. For the latter,
genes were added to wheat with the expectation
of modifying a targeted trait. Some of these trans-
genes were well expressed and, in most of the
published cases, made small but detectable differ-
ences in measurements or symptoms in small-
scale lab or greenhouse tests. Relatively few of
these transgenic lines has been submitted to fi eld
trials to test for expression levels and stability
under standard wheat production conditions in
multiple environments. Thus, in most of the
experiments summarized earlier, it remains
unknown whether such transgenic lines offer a
recognizable benefi t for production agriculture.
There are, however, two types of transgenic wheat
for which the published results of several fi eld
tests show that the modifi ed traits have the stabil-
ity that wheat breeders and producers would
demand before commercial release. These are
Roundup Ready wheat and transgenic wheat with
altered HMW-GS content.
Altered HMW-GS transgenic wheat was
among the fi rst transformation products produced
by several different groups. This material has
since been subjected to fi eld trials to test for
