Agriculture Reference
In-Depth Information
Grain hardness
Starch pasting properties
Table 21.5 also lists allele recommendations for
the waxy locus (
Wx-B1
) for granule-bound starch
synthase. White salted noodles (Table 21.1), in
particular, benefi t from the “Null-4A” gene (
Wx-
B1b
null allele) as further discussed later in the
section on starch granules and in Chapter 22.
The quality specifi cations for processing wheat-
based foods (Table 21.1) include many factors in
addition to dough quality, so allelic recommenda-
tions are also provided in Table 21.5 for appropri-
ate puroindoline genes for grain hardness. The
puroindolines are basic lipid-binding proteins,
rich in cysteine, of about 13,000 Da (Gautier
et al., 1994; Morris 2002; Jones et al., 2006).
They belong to the 2S albumin superfamily of
proteins.
The puroindolines are encoded by genes at two
loci on the short arm of chromosome 5D:
Pina-D1
(two alleles,
a
and
b
) and
Pinb-D1
(many alleles,
a
-
g
,
l
,
p
,
q
) (Morris 2002; Jones et al., 2006;
Wanjugi et al., 2008). Bread wheat generally con-
tains two types of puroindolines differing slightly
in size. Soft wheat possesses both puroindoline a
and puroindoline b, due to the allele combination
Pina-D1a
and
Pinb-D1a
(Table 21.5). As their
starch granules are loosely attached to the protein
matrix, soft wheat crushes easily, producing
largely intact starch granules and fi ne fl our. On
the other hand, the starch granules of hard wheat
are tightly bound to the protein matrix, requiring
greater milling energy and producing coarser
fl our with higher levels of starch damage.
The resulting distinctions between fl ours from
hard and soft wheats suit them respectively for
distinct products (Table 21.1). In hard wheat, a
common combination of
Pin
alleles is the
Pina-
D1b
null allele with
Pinb-D1a
. Other hard wheat
genotypes have the combination of
Pina-D1a
with
Pinb-D1b
down to
Pinb-D1g
, and beyond
alphabetically. Durum wheat lacks the
Pin
loci
completely, with the result that durum grain is
very hard (Jones et al., 2006).
The puroindoline alleles are not evenly distrib-
uted among germplasms across the world, so for
any specifi c regional set of wheat genotypes, all
alleles may not be represented. There is thus
motivation for breeders to expand the range of
puroindoline genes in use, thus to extend the
range of endosperm textures available. The spe-
cifi c alleles recommended in Table 21.5 relate to
hardness specifi cations for the products listed in
Table 21.1.
Protein composition and genotype
identifi cation
Finally, protein composition has long served the
valuable purpose of indicating genotype (cultivar)
identity. In 1965, Zuckerkandl and Pauling (1965)
classed proteins as “episemantic molecules,” that
is, their “meaning” (semantic signifi cance) in
relation to genotype lies a few steps away from the
genome (Fig. 21.1). DNA was classed as a
“primary semantide,” and RNA as a “secondary
semantide.” This hierarchy draws attention to the
possibility that environmental infl uences may
corrupt the information about genetic origins
when analyzing protein composition.
Nevertheless, grain protein composition has
served well as a basis for determining the genetic
identity of wheat samples for many years, based
on the composition of gliadins and/or of glutenin
subunits, using acidic or SDS-gel electrophore-
sis, reversed-phase HPLC, or capillary electro-
phoresis (Uthayakumaran et al., 2006). In this
general application, the protein composition pro-
vides a “fi ngerprint,” not necessarily providing
any indication of relationship to grain quality. In
the many years of routine use, the possible con-
fusing factor of growth conditions has not posed
problems, with the occasional exception of severe
sulfur defi ciency, which causes the proportion of
ω
-gliadins to become unusually high.
Application of principles:
Defects explained
An understanding of composition-function rela-
tionships leads to the elucidation of ways in which
grain defects reduce processing effi ciencies. For
example, knowledge of the reliance of dough
strength on the very large glutenin polymers
