Agriculture Reference
In-Depth Information
Grain Protein Content (
GPC
) and Nutritional Aspects
Improving the grain protein content has been area of main focus for wheat breed-
ers due to its major contribution in bread and pasta-making quality and has a major
contribution in improving nutritional status of masses. Despite of its importance,
constrains prevail to increase protein contents due to its quantitative inheritence
and high influence of the environment (Simmonds,
1995
). Several reports are avail-
able on QTLs controlling GPC and linked markers are available for MAS. An au-
thentic source of high protein content has been identified in accession of
Triticum
turgidum
L. ssp. dicoccoides (referred to as DIC) during a survey (Avivi (
1978
).
Cantrell and Joppa (
1991
) substituted each chromosome of DIC with the durum
cultivar 'Langdon (LDN)' and later it was found that 6B substitution line of DIC
into LDN (DIC-6B) had highest protein contents (Joppa et al.
1997
). The DIC-6B
substitution line and LDN were used a parents and a RILs mapping population was
developed to map QTL conferring high GPC, which was found on the chromo-
some 6BS. Another secondary mapping population (RILs) was developed to further
shorten this QTL and it was mapped between RFLP probes
Xcdo365
and
Xucw65
as a single Mendelian locus (
Gpc-B1
) within a 2.7 cm region (Olmos et al.
2003
).
Some new markers were developed in this region for high density mapping through
Rice-Wheat micro-colinearity studies. Some additional recombination was initiated
by developing more RILs and
Gpc-B1
locus was reduced with a 0.3 cm segment us-
ing newly developed markers (Distelfeld et al.
2004
). The
Gpc-B1
gene within the
0.3 cm segment was physically mapped which spanned about 250 kb region (Dis-
telfeld et al.
2006
). The Gpc-B1 allele in DIC accelerates leaf senescence and Uauy
et al. (
2006a
) suggested the differences in GPC are actually pelotropic effects of the
in senescence. Kade et al. (
2005
) discovered the effect of DIC
Gpc-B1
allele during
senescence explained the higher levels of soluble proteins and amino acids in flag
leaves after anthesis relative to those with the LDN allele. Higher mineral contents
in DIC were found to be associated with chromosome 6B (Cakmak et al.
2004
), but
its association with 250 kb region including
Gpc-B1
was validated later (Distelfeld
et al.
2007
). The major discovery was reported when map based cloning identified
the
Gpc-B1
as a NAC transcription factor (
TtNAM-B1
) and it was established that
wild emmer wheat has a functional allele whereas modern wheat varieties carry
a nonfunctional allele originated by a frame shift mutation (Uauy et al.
2006b
).
The functional NAM-B1 orthologous has been found on chromosome 6A and 6D
(
TtNAM-A1
and
TaNAM-A1
) and 6D (
TaNAM-D1
), and closely related paralogues
on chromosomes 2B (
TtNAM-B2
and
TaNAM-B2
) and 2D (
TaNAM-D2
). In
RNAi
studies, RNA levels of these NAM homologs was reduced which in turn delayed
senescence for more than 3 months and reduced grain protein and mineral contents
by more than 30 % as compared to control lines (Uauy et al.
2006b
). Most Recently,
Cantu et al. (
2011
) employed mRNA-seq approach to detect small differences in
transcript levels and identified the monocarpic senescence as an active process lead-
ing to large-scale changes in gene expression which begins considerably before the
appearance of visual symptoms of senescence. As a result several GPC-regulated
genes including transporters, hormone regulated genes, and transcription factors
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