Agriculture Reference
In-Depth Information
erties and are called x-type and y-type subunits (Payne et al.
1980
). These loci are
named
Glu-A1
,
Glu-B1
and
Glu-D1
, respectively. The x- and y-type subunits have
a comparatively high and low molecular weight, respectively. In earlier studies on
the allelic variation of
Glu-1
loci, the number of alleles at all three loci differed
greatly (Payne and Lawrence
1983
).
Glu-A1
had three allelic forms, eleven alleles
for
Glu-1B
, and six alleles for
Glu-1D
were found. However, in the subsequent
studies several alleles were found and the latest information has been documented
by McIntosh et al. (
2010
).
Gene Expression
All bread wheat cultivars express 1Bx, 1Dx, and 1Dy subunits while some culti-
vars also express
1By
and
1Ax
subunit as well. The gene encoding the
1Ay
subunit
usually remains silent. Nevertheless, many accessions of A-genome related spe-
cies
T. monococcum
and
T. urartu
express
1Ay
subunits (Waines and Payne
1987
;
Rasheed et al. Unpublished; Alvarez et al.
2009
; Caballero et al.
2008
; Gutierrez
et al.
2011
). Some bread wheat with six HMW-GS have also been reported (Mar-
giotta et al.
1996
). The extensive studies on the electrophoretic mobility of glute-
nin subunits revealed that durm and bread wheat genotypes lack certain subunits
(Lafiandra et al.
1988
). Beitz et al. (
1975
) reported some mutants lack 1D encoded
HMW-GS in landraces from Nepal. Contrastingly, some tetraploid and hexaploid
genotypes with four and six subunits, respectively, were developed by replacing the
silenced subunit of
Glu-A1
by the expressed ones. These genotypes showed an in-
crement in polymeric glutenin quantity, hence better flour characteristics (Lafiandra
et al.
1998
). Allelic variation at
Glu-D1
has most profound effect on bread-making
quality, although limited numbers of alleles have been reported at this locus. In
addition to hexaploid wheat,
Aegilops tauschii
Cosson, the diploid ancestor of the
D-genome, conserves many unique
Glu-D1
alleles. So far, 14 x-types and 10 y-type
subunits in
Ae. tauschii
have been identified resulting into combination of 85 differ-
ent
Glu-D1
alleles (Rehman et al.
2008
).
Amino Acid Composition and Structure
HMW-GS is a highly complex mixture of proteins and high level of polymorphism
is the main limiting factor to study the structure of gluten proteins. However in the
last 30 years, much of the work has focused on HMW-GSs of wheat (Shewry et al.
2002
). There is a high resemblance in primary structure of x- and y-type subunits.
Both subunits consist of a signal peptide, N- and C-terminal domains and a repeti-
tive central domain (Shewry and Halford
2002
). The significant difference is in the
number of conserved cysteine residues which are four in majority of x-type sub-
units and usually seven in the y-type subunits. These cysteine residues had a major
role in the formation of disulphide bonds within and between subunits. Therefore
these are primarily important in structure and function of the elastic properties of
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