An Overview of Omics for Wheat Grain Quality Improvement - Crop Improvement: New Approaches and Modern Techniques - page 309

Low Molecular Weight Glutenin Subunits
Low molecular weight glutenin subunits (LMW-GS) are also major fraction of glu-
tenins and are also known as prolamine due to high amino acid, glutamines and
pralines. LMW-GS represent about one third of the storage proteins and about 60 %
of total glutenins (Beitz and Wall 1973 ). The genes controlling LMW-GS are pres-
ent at short arm of group 1 chromosomes, Glu-3 loci ( Glu-A3 , Glu-B3 and Glu-
D3 ), which are tightly linked to the Gli-1 loci. Additionally, three new loci Glu-2 ,
Glu-4 (Jackson et al. 1985 ; Liu and Shepherd 1995) and Glu-5 (Sreeramulu and
Singh 1997 ) located on chromosomes 1B, 1D and 7D respectively, with molecular
weights ranging from 30-31,000 Da, have also been reported to encode LMW-GS.
The LMW-GS are more difficult to characterize and study as compared to HMW-
GS, due to their heterogeneity. However, different analytical tools become avail-
able with the advancement of technology which made easier their characterization.
Biochemical classification revealed three classes of LMW-GS i.e. B, C and D types
(Jackson et al. 1983 ). Additionally, B type LMW-GS are further classified into three
classes, LMW-m, LMW-s and LMW-i, based on the first amino acid residue which
may be Methionine, Serine and Ile, respectively. Genes present at Glu-A3 locus
manly encode LMWi type subunits, which is the most recently identified class of
LMW-GS (Zhang et al. 2004 ). Additionally, LMW-i had significant structural dif-
ferences from LMW-m and LMW-s groups due to the lack of an N-terminal region
and localization of cysteine residues in C-terminal region. However, eight number
of cysteine residues are common in all groups. This structural difference is proposed
to encode quality differences by glutenin polymer formation and gluten interac-
tion. The C and D type subunits are composed mainly of proteins related to α/β-,
γ- and ώ-gliadins which have variable numbers of cysteine residues. D'Ovidio and
Masci ( 2004 ) proposed these subunits are incorporated into the polymeric network
by virtue of unpaired cysteines. The dough quality is known to be influenced greatly
by allelic variations at the Glu-3 loci and ranking of different alleles due to their
functional properties have been reported (Juhász and Gianibelli 2006 ).
Gene Expression and Polymorphism
In MacGene (2010) 17, 26 and 11 alleles differentiated by different diagnostic tech-
niques are documented at all Glu-3 loci wheat. However, this does not include the
allelic variants encoded and confirmed in different wild relatives of Triticeae. In
earlier studies, 20 different LMW-GSs were identified in 222 common wheat vari-
eties from 32 countries (Gupta and Shepherd 1990 ). It was revealed that six alleles
were encoded at Glu-A3 , nine at Glu-B3 and five at Glu-D3 locus. Allelic richness
of chromosome 1A encoded subunits was relatively low and even some cultivars
did not express any LMW-GS. While the B-genome encoded LMW-GS, Glu-B3 ,
showed highest polymorphism. Gupta and Shepherd ( 1993 ) provided evidence for
the presence of LMW-GS genes on group 6 chromosomes. There is a close link-
age between gliadins encoded by Gli-1 loci and LMW-GS encoded by Glu-3 loci