An Overview of Omics for Wheat Grain Quality Improvement - Crop Improvement: New Approaches and Modern Techniques - page 308

gluten protein result in risen loaf of wheat dough (Shewry and Tatham 1997 ). The
repetitive domains are consisted of short and repeated peptide motifs in both x- and
y-type subunits. These peptide motifs may be tripeptide, hexapeptide and nanopep-
tide, while the presence of the tripeptide motif is unique to the repetitive domain
of x-type subunits (Shewry and Tatham 1997 ). Because the HMW-GS have very
high glutamic acid contents, therefore proline and glycine contents are also very
high, while lysine contents are very low. It is also evident from amino acid com-
position of HMW-GS that the central repetitive domain has hydrophilic nature and
N- and C-terminal domains have hydrophobic characteristics (Shewry et al. 1989 ).
The polypeptide motifs determine the proportion of the different amino acids in
HMW-GS. Shewry and Tatham ( 1997 ) revealed that > 90 % of the repetitive do-
mains are formed from the variations in the consensus repeat sequences (PGQGQQ
and GYYPTSPQQ). Moreover, x-type subunits are characterized by the presence
of unique tri-peptide motif (GQQ). While in y-type subunits, the second proline is
replaced by a leucine in the GYYPTSPQQ repeat motif.
Alleleic identification
The invaluable platform for HMW-GS diagnosis is, no doubt, sodium dodecyl sul-
phate poly-acrylamide gel electrophoresis (SDS-PAGE). However some limitations,
like co-migration of some subunits, difficulty in detecting differences in expression
levels, results in inaccurate identification of alleles differing in functional proper-
ties. Moreover, this technique is only possible from the flour of mature grains. The
advancements in molecular biology has enabled us to overcome these limitations
by using allele specific PCR markers. These markers are developed based on DNA
polymorphism present among the glutenin subunit genes are considered perfect to
study allelic variations for HMW-GS. The major advantage is the high-throughput
analysis of different alleles in breeding materials which is also possible during the
vegetative growth stages (Liu et al. 2008a ). We have discussed in detail the mo-
lecular diagnosis approaches for HMW-GS identification in “Functional markers”
section of this chapter.
Among the other proteomics based technologies used to detect HMW-GS in-
clude reversed-phase high-performance liquid chromatography (RP-HPLC) and
the most recent matrix assisted laser desorption time-of-flight mass spectrometry
(MALDI-TOF). Gao et al. ( 2010 ) analyzed HMW-GS separation and characteriza-
tion of bread wheat and wild accessions on MALDI-TOF, SDS-PAGE and RP-
HPLC. Comparative analysis demonstrated merits and demerits of each method-
ology. Incorrect identification due to low resolution and overestimation has been
the main drawback of SDS-PAGE. Irrespective of its disadvantages, SDS-PAGE
is the simplest and cheaper technique, therefore, suitable for large-scale and high-
throughput HMW-GS screening for wheat genotypes especially when the glutenin
composition is clear in the breeding material. The most recent mass spectroscopy
MALDI-TOF had several technical advantages including high throughput, high
resolution, and accuracy. However, high equipment cost is the main hindrance to
access this technology for many breeding programs.