Agriculture Reference
In-Depth Information
ALTERED STARCH
differences in functional properties do exist
(Epstein et al., 2002; Kim et al., 2003). Waxy
wheat also demonstrates different milling proper-
ties, specifi cally producing higher levels of
damaged starch (Bettge et al., 2000; Kim et al.,
2003). Little is known, however, of possible agro-
nomic effects of various GBSS genotypes, espe-
cially with regard to grain yield.
Breeding waxy wheat cultivars presents the
same challenge as breeding HW wheat. Popula-
tions derived from crosses between waxy and
wild-type wheat will contain only 1/64 waxy
progeny. This frequency is low, especially when
one is attempting to introgress a trait from thor-
oughly unadapted sources. However, automated
seed sorting technology (Dowell et al., 2006),
coupled with near-infrared refl ective spectros-
copy (NIRS), can be used to produce populations
enriched in waxy progeny. Delwiche and Gray-
bosch (2002) demonstrated facility of NIRS to
identify wheat grain based on amylose content.
Subsequently, Delwiche et al. (2006) combined
NIRS with automated seed sorting technology to
extract waxy seed from mixed populations of
waxy, partial waxy, and wild-type durum wheat.
Application of NIRS in commercial settings will
facilitate segregation of waxy wheat from wild-
type wheat.
Waxy cultivars have been developed in Europe
and used to produce a commercial fl our (Westhove
Wheat, Wisconsin) described as an “instant waxy
wheat fl our” (Anonymous 2006). A soft white
waxy wheat cultivar, Waxy-Pen, was released in
the Pacifi c Northwest region of the US (Morris
and King 2007). Spring waxy wheat, with grain
yield equal to that of check cultivars, has been
produced (Graybosch et al., 2004). Waxy winter
wheat grain yield continues to approach that of
commercial wild-type cultivars (Graybosch
2005), and release of waxy winter wheat cultivars
adapted to the Great Plains likely will occur by
2010.
Altered starch breeding
Waxy (amylose-free) wheat
Until the 1990s, little was known of the inherent
genetic variation in wheat starch composition,
and essentially no breeding work to exploit
such variation had been accomplished. Wild-type
wheat starch is composed of approximately 75%
amylopectin and 25% amylose. It long was
thought that genetic variation did not exist for the
ratio of these two polymers. However, in the early
1990s, Nakamura et al. (1992, 1993) devised strat-
egies to separate gene products of the three loci
encoding granule bound starch synthase (GBSS)
in wheat. The enzyme GBSS is primarily respon-
sible for amylose synthesis in endosperm.
Electrophoretic techniques demonstrated exis-
tence of nonfunctional (null) alleles in many wheat
accessions and three distinct GBSS isoforms in
wild-type wheat. Further, Nakamura et al. (1993)
demonstrated that null alleles at one or more waxy
loci led to production of wheat starch with reduced
amylose content. Reduced amylose wheat is
termed “partial waxy.” Finally, by traditional
intermatings of lines carrying the three null
alleles, Nakamura et al. (1995) developed lines
with no functional GBSS and no amylose.
Amylose-free wheat, following the convention
established in other cereal crops, is termed
“waxy.” Null alleles at two of the three waxy loci
are common and are found in adapted US wheat
cultivars (Graybosch et al., 1998). Null mutations
at the third locus (
wx-D1
) are rare (Nakamura
et al., 1995). Development of waxy wheat adapted
to the US wheat belt has been slow, as the only
available donors of the necessary
wx-D1
null allele
were two poorly adapted lines, 'BaiHuo' and
'BaiHuoMai', both from China.
Eight possible GBSS genotypes exist in hexa-
ploid wheat, and four possible combinations exist
in durum wheat. It has yet to be established
whether any one genotype would be preferred for
production of modifi ed food starch. In common
wheat, genotypes do not differ in their gross
starch granule morphology (Kim et al., 2003), but
High-amylose wheat
In maize, mutations at the
ae
(amylose extender)
locus result in production of endosperm starch
