Agriculture Reference
In-Depth Information
Unintended effects of transformation,
transgene insertion, or expression
than their nontransformed parents
(Chrimes
et al., 2005).
Only a few experiments have been reported
thus far that detected subtle changes in nontarget
gene expression in transgenic wheat. A survey of
common seed metabolites was conducted in three
transgenic wheat lines expressing added HMW-
GS genes and in three non-transgenic controls
grown in three separate years at two sites (Baker
et al., 2006). While small differences occurred
between one transgenic line and its parent for
maltose or sucrose contents and for some free
amino acids, none of the other differences
exceeded environmental differences between
control genotypes. In addition, no signifi cant dif-
ferences were detected among the same sets of
HMW-GS transgenic, selection-gene transgen-
ics, and nontransformed parental plants when leaf
and endosperm transcripts were surveyed (Baudo
et al., 2006). The authors concluded that, except
for changes in the relative abundances of seed
storage protein types predicted by the expression
of the HMW-GS transgenes, composition of the
transgenic seeds was substantially equivalent to
that of the parent and showed no more variability
than that among traditionally bred cultivars.
Rakszegi et al. (2005), growing the same set of
lines for three years in a different location, also
observed that while the HMW-GS lines had the
same yields as their nontransformed parents,
the transgenics had slight increases in kernel
hardness and decreases in kernel size.
Horváth-Szanics et al. (2006) used proteomics
to examine the albumin and globulin families of
seed proteins in transgenic wheat plants express-
ing
bar
-encoded herbicide resistance transgenes.
When they compared transgenic and control
plants under drought stress, they noted higher
levels of several proteinase inhibitor proteins
(15-27 kDa) in the transgenics (Horváth-Szanics
et al., 2006). They hypothesized that the trans-
genics were more stressed than non-transgenics
by the drought treatment.
Gregersen et al. (2005) used hybridization to
measure expression of 9,000 genes at three
time-points in developing seeds of transgenic
wheat plants expressing an
Aspergillus
phytase
A side effect of transformation protocols is the
sporadic occurrence of mutations and chromo-
some rearrangements that result from tissue
culture rather than the integration of any trans-
genes (somaclonal variation) (Qureshi et al., 1992;
Philips et al., 1994; Ivanov et al., 1998). The phe-
notypic effect of such variation may be partially
masked in wheat because of its polyploid nature.
Somaclonal variants can be easily removed by
crossing or backcrossing, since they are not likely
to be linked to the transgenes.
A potentially serious consequence of the ran-
domness of transgene integration is the possibility
that the insertion results in changes in the expres-
sion of the native chromosomal DNA at the inte-
grated locus (Filipecki and Malepszy 2006). Most
simply, the transgene could physically interrupt
a native gene by virtue of its integration, resulting
in a null mutation. Alternatively, the transgene
promoter could read into adjacent native genes,
changing their expression patterns (Fig. 18.4c).
Such changes could be manifested as reduced
fi tness of the transgenic line or as changes in
expression of genes not expected to be affected by
the transgene itself.
Constitutive transgene expression can result in
inappropriate accumulation of the gene product
in tissues where it has a detrimental effect on
plant development or fertility. When the maize
Ubi1
promoter was used to drive expression of
two different wheat peptidyl prolyl
cis-trans
isom-
erase isozymes, the transgenic plants and/or their
seeds were abnormal (Kurek et al., 2002). Like-
wise, wheat plants transformed with a
Ubi1
::
Lr10
gene construct had signifi cantly lower kernel
weight than their nontransformed parent in the
absence of leaf rust (Romeis et al., 2007). Even
nonconstitutive promoters can result in transgene
expression that interferes with normal plant
growth or seed set. Transgenic wheat plants car-
rying a cell-cycle regulator gene under control of
the wheat ADP-glucose pyrophosphorylase large
subunit gene promoter had abnormal spikes,
decreased seed set, and 50% less viable pollen
