An Overview of Omics for Wheat Grain Quality Improvement - Crop Improvement: New Approaches and Modern Techniques - page 310

(Gianibelli et al. 2001 ). This linkage between alleles of two loci helped in diagnosis
of several Glu-B3 and Glu-D3 alleles in wheat genotypes. Several glaidins were
found as reliable markers LMW-GS allele diagnosis, due to their easy detection
(Jackson et al. 1996 ).
Amino Acid Composition and Structure
The N-termninal sequence is most important for identification of LMW-GS, there-
fore seven main types of LMW-GS have been identified on the basis of first ami-
no acid in the N-terminal sequences of the proteins. These include, seven LMW-s
with starting with sequence SHIPGL-, three LMW-m with N-terminal sequences
of METSHIPGL-, METSRIPGL and METSCIPGL respectively. In three LMW-
GS, the N-terminal sequences resembles to the α-, β- and γ-type gliadins (Cloutier
et al. 2001 ). The further classification of LMW-GS is based on deduced amino
acid sequences and cysteine residue position facilitating inter-molecule disulfide
linkage (Ikeda et al. 2002 ). Twelve such LMW-GS groups have been identified in
wheat. Collectively, more than 100 genes of LMW-GS have been characterized and
sequenced from common wheat including several partial and pseudogenes (Cloutier
et al. 2001 ; Zhang et al. 2004 ). There was an effort by Long et al. ( 2005 ) to develop
LMW-GS group specific primers, which they developed from the analysis of 69
known gene sequences from GenBank and classified them into nine groups by the
deduced amino acid sequence of the highly conserved N-terminal domain. Later on,
Ikeda et al. ( 2006 ) also developed 10 primers, based on the available sequences in
the GenBank, which were group specific. In wheat varieties from Australia, Zhao
et al. ( 2006 , 2007) identified 6 different gene sequences and 12 gene haplotypes at
the Glu-D3 locus.
There is high similarity between the secondary structures of LMW-GS and struc-
ture of the S rich gliadins and the only exception of D type LMW-GS (D'Ovidio
et al. 1995 ). There are about 250-300 residues are reported in polypeptides. Sev-
eral workers have reported the further modification in the two domian structure
of LMW-GS (Kasarda et al. 1984 ; Wieser 1995 ). In both domains, the N-terminal
repetitive domains is rich in β-turns while short nonrepetitive domain is rich in
α-helix and is more compact (Thomson et al. 1992 ).
Allelic identification
SDS-PAGE is considered one of the simplest techniques to identify LMW-GS with
some restrictions. At Glu-A3 locus, SDS-PAGE could identify five out of seven al-
leles, while 2-D gel electrophoresis and PCR based markers identified all the allelic
variation at this locus in bread wheat (Liu et al. 2010 ). The Glu-B3 alleles are easier
to be identified by SDS-PAGE, MALDI-TOF and PCR based markers but some ad-
ditional validation is more reliable by 2-DE method. Liu et al. ( 2010 ) compared four
techniques (SDS-PAGE, 2-DE, MALDI-TOF, PCR based markers) as a conduit to