Genetic Improvement of Grain Quality in Japonica Rice - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 147

and retail price. However, some consumers pre-
fer white-core chalkiness and whole-grain chalk-
iness in certain japonica rice cultivars (Tashiro
and Wardlaw 1991; Zakaria et al. 2002). For
example, white-core chalkiness is preferred in
Arborio-style cultivars and Japanese brewing-
rice cultivars; and glutinous rice, which is very
soft and sticky after cooking, has a completely
opaque endosperm.
A wide range of variation in the level of
grain chalkiness among japonica cultivars has
been observed (Wan et al. 2005; Cheng et al.
2005; Kobayashi et al. 2007; Tabata et al. 2007).
The frequencies of white-belly and milky-white
types are increased by low solar radiation and
large numbers of glumes per panicle. The fre-
quencies of white-back and white-based types
are increased by high humidity and low nutri-
ent availability at the ripening stage (Yamakawa
et al. 2007). Grain chalkiness is also increased by
high temperature and large numbers of glumes
per panicle (Yamakawa et al. 2007; Ishimaru
et al. 2009): it is currently being studied by
many researchers because of the potential for an
increase in chalkiness caused by higher temper-
atures during the ripening stage resulting from
global warming. Zhang et al. (2008) reported that
alternate wetting and severe soil drying during
grain maturity significantly reduced grain chalk-
iness and improved grain quality. Chalkiness is
also negatively correlated with grain size: slen-
der and small rice grains tend to have a lower
frequency of chalkiness (Wan et al. 2005).
Starch granules are smaller and more loosely
packed in the chalky areas of the rice endosperm
than in the translucent areas (Lisle et al.
2000; Fitzgerald et al. 2009). Scanning electron
microscopy has revealed incomplete grain-filling
or progressive decomposition on the surface of
starch granules in chalky endosperm (Zakaria
et al. 2002). Therefore, many researchers have
focused on the processes of starch synthesis
and degradation (Bhattacharya 1979; Tashiro
and Wardlaw 1991; Lisle et al. 2000; Patin-
dol and Wang 2003; Wan et al. 2005; Cheng
et al. 2005; Woo et al. 2008). High tempera-

ture decreases the expression of starch synthesis
genes and the enzymatic activities of the result-
ing proteins (Umemoto and Terashima 2002;
Jiang et al. 2003); consequently, amylose con-
tent is reduced and the branching structure of
amylopectin is changed (Asaoka et al. 1984;
Umemoto et al. 1999; Umemoto and Terashima
2002; Yamakawa et al. 2007). Several mutants
of the starch synthesis genes GBSSI , BEIIb , and
PPDKB show abnormal features such as floury
or opaque endosperm (Satoh and Omura 1981;
Nishi et al. 2001; Kang et al. 2005). Overexpres-
sion of the starch-degrading enzymes encoded by
Amy1A and Amy3D results in increasing milky
endosperm, unless the grains are heat-treated
(Asatsuma et al. 2006). Moreover, japonica cul-
tivars showing low levels of chalkiness contain
high levels of nonstructural carbohydrate (NSC)
in the vegetative tissues of the plants (Morita and
Nakano 2011). High NSC content at the grain-
filling stage would help to minimize grain chalk-
iness. These observations support the hypothesis
that source-sink interactions are important in the
development of grain chalkiness.
To perform comprehensive comparisons of
storage components associated with chalkiness
in grain, several researchers have used proteome
and metabolome analyses (Yamakawa et al.
2007; Lin et al. 2010; Yamakawa and Hakata
2010). In these analyses, the levels of allergen-
related proteins, translation elongation factors,
and heat shock proteins increased under high
temperature, whereas the level of 13-kDa pro-
lamin decreased. High temperature increased the
accumulation of pyruvate/oxaloacetate-derived
amino acids and decreased the levels of sugar
phosphates and organic acids involved in gly-
colysis/gluconeogenesis and the TCA cycle.
She et al. (2010) reported that ATP contents
decreased during the grain-filling stage under
high temperature.
Several QTLs for grain chalkiness have been
detected in japonica rice cultivars and found
to be stably expressed across multiple envi-
ronments (Wan et al. 2004; Wan et al. 2005;
Kobayashi et al. 2007; Tabata et al. 2007). Three