An Overview of Omics for Wheat Grain Quality Improvement - Crop Improvement: New Approaches and Modern Techniques - page 313

Kernel Texture ( Ha Locus )
In wheat, the grain texture is encoded by the friabilin group of proteins having
Mr~13 k Da. These proteins have strong association with starch. In soft grains,
these are frequently associated with water-washed starch. While in hard grains and
durum they show limited and no association with starch, respectively. It was gener-
ally suggested that friabilins are non-sticky proteins minimizing sticking of starch
granules and protein matrix, thus allowing their easier separation. Friabilin give
rise to two major polypeptides upon electrophoresis separation and amino acid se-
quencing. There are three polypeptides viz. puroindoline a, puroindoline b and grain
softness protein-1 which are designated as PINA , PINB and GSP-1 , respectively.
Morris ( 2002 ) reviewed the properties, purification methods and discovery of fria-
bilin. Hard and soft kernel textures are not due to difference in amount of friabilin
because both classes have similar amount of friabilin. Rather difference is due to
association of strach granules with friabilin during aqueous isolation. The major
locus, Ha , is responsible for textural properties (Symes 1965 ), and has been identi-
fied on 5DS chromosome (Mattern et al. 1973 ; Sourdill et al. 1996 ). Puroindoline
proteins encoded by two strongly linked genes at this locus were identified that is
associated with variation for grain hardiness (Gautier et al. 1994 ). A mutation in
Pinb gene results in change in amino acid giving rise to altered protein structure to
bind with membrane polar lipids (Giroux and Morris 1998 ). This results in altera-
tion of binding strength between protein matrix and starch granules. Apart from this
mutation, Giroux and Morris ( 1998 ) also identified a null allele, Pina-Da , on other
puroindoline gene. It was concluded that a variety will have the hard texture hav-
ing mutant alleles ( Pina-D1b or Pinb-D1b ) at both puroindoline genes. Although,
higher allelic variation observed at these loci (Morris 2002 ) but genotypes having
alleles Pina-D1a / Pinb-D1a (soft), Pina-D1a/Pinb-D1b (hard) and Pina-D1b/Pinb-
D1a (extra hard) are predominant (Cane et al. 2004 ). A positive correlation of these
genotypic classes with water absorption was observed. Genotypes with extra hard
texture absorbed 3.5 % more water than varieties with the hard texture and 8.3 %
more than those with the soft texture. Contrastingly, they, did not find any differ-
ence for water absorption in the ''extra hard'' and ''hard'' classes. However a drop
in milling yield in extra hard genotypes was observed by both. Apart from Ha locus,
several QTLs underlying grain hardness characteristics have been discovered.
GenesandPolymorphism
Bhave and Morris ( 2008 ) reviewed the molecular genetics, gene regulation and
structure of puroindolines. MacGene (McIntosh et al. 2010 ) listed 17 alleles at Pina-
D1 , 29 alleles at Pinb-D1 and 9 alleles at Gsp-D1 locus. Recently, Chen et al. ( 2009 )
identified 9 haplotypes in 56 sequences from einkorn wheat. Guzman et al. ( 2012 )