Transgenic Applications in Wheat Improvement - Wheat Science and Trade - page 396

tional mutant Pinb-D1b allele, resulted in soft
grain texture (Beecher et al., 2002). Similarly, the
addition of Pina-D1a complemented the null
mutation ( Pina-D1b ) in the hard white spring
wheat Bobwhite, resulting in soft grain texture
(Martin et al., 2006). Transformants of HiLine
with Pina-D1a and/or Pinb-D1a controlled by a
HMW-GS promoter showed that both puroin-
doline proteins are needed to achieve maximum
softness (Hogg et al., 2004).
Third-generation selfed progeny of these lines
were grown in the fi eld and their harvested grain
was milled and baked into bread. The lines trans-
formed with either the overexpressed Pina or
Pinb genes had mean single-kernel characteriza-
tion system (SKCS) hardness scores of 29 and
52, respectively, compared with the mean SKCS
hardness score of 84 for the nontransformed
parent HiLine. The transgenic lines also exhib-
ited decreased total fl our yield, but increased
break-fl our yield, compared with their nontrans-
formed parent line. Dough made from the trans-
genic fl our had similar mixing properties to that
of the nontransformed parent fl our, but had
decreased water absorption. The transgenic
fl our produced loaves with smaller volumes and
lower crumb grain scores (Hogg et al., 2005;
Martin et al., 2007). However, the soft transgenic
lines performed better than their nontransformed
parent in cookie-baking tests (Martin et al.,
2007). Differences between the transgenic lines
and their nontransformed parent were similar
to those between nontransformed soft and hard
wheats.
Further confi rmation that the Pin genes under-
lie grain hardness came from overexpression of
the Pina coding sequence under control of the
Ubi1 promoter. In some lines, increases in puro-
indoline a from the added transgenes led to softer
texture, while in other lines cosuppression was
triggered, resulting in undetectable levels of
transgene and endogenous Pina expression and
thus hard grain texture (Xia et al., 2008).
Transformation experiments aimed at improv-
ing the nutritional value of wheat grain have
employed genes from other species. In an attempt
to raise iron levels in cereal seeds, Drakakaki

et al. (2000) transformed wheat and rice with a
soybean ( Glycine max L.) gene that encodes fer-
ritin under control of the maize Ubi1 promoter.
However, the anticipated increase in stored
iron was only detected in vegetative tissues, not
in seeds. To improve phosphate and mineral
availability in wheat seeds, Brinch-Pedersen
et al. (2000) added a gene that encodes phytase,
an enzyme that degrades the antinutrient
phytic acid. Their fi rst attempt used an Aspergil-
lus niger phyA coding sequence under the control
of the maize Ubi1 promoter and resulted in trans-
genic wheat seeds with four times the nontrans-
formed parent's secreted phytase activity
(Brinch-Pedersen et al., 2000). More recently,
the same group employed a secreted synthetic
version of the phyA gene under control of the
endosperm-specifi c wheat HMW-GS promoter
and obtained levels of phytase activity in seeds
that were 6.5 times those of the nontransformed
parent (Gregersen et al., 2005). In this case, the
coding region was designed to contain the codons
that are most optimal for translation in wheat
endosperm.
Another important seed trait is sink strength,
which determines the effi ciency of carbon and
nitrogen translocation from photosynthetic tissue
to the developing seed. Smidansky et al. (2002)
transformed wheat with a maize sequence that
encoded a mutant version of the large subunit of
ADP-glucose pyrophosphorylase that was insen-
sitive to allosteric inhibition. The coding sequence
was under control of either its native promoter
(Smidansky et al., 2002) or one from a wheat
HMW-GS gene (Meyer et al., 2004). Fifth-
generation selfed descendents of one resultant
line had increases of 31% in total biomass and
38% in seed weight per plant (Smidansky et al.,
2002). The rates of photosynthesis in the trans-
genic plants were increased, compared with those
of the nontransformed parent, but only in high
levels of light. Concentrations of glucose, fruc-
tose, and sucrose peaked in the fl ag leaves 7 and
14 days after fl owering, but increases in the trans-
genic seed ADP-glucose and UDP-glucose were
not evident until maturity (Smidansky et al.,
2007). The yield increases of the transgenic plants