Genetic Improvement of Grain Quality in Japonica Rice - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 151

might be possible to develop high-quality (i.e.,
high-palatability) cultivars through modification
of starch synthesis genes in japonica cultivars
(Isshiki et al. 2008; Sun et al. 2011).
There is evidence that grain components other
than starch contribute to cooking characteris-
tics and sensory properties in rice. For exam-
ple, the amounts of both proteins and lipids
affect sensory properties (Martin and Fitzger-
ald 2002; Philpot et al. 2006). Protein content
plays an important role in determining past-
ing characteristics, including grain texture and
the surface hardness of cooked rice (Yanase
et al. 1984; Okadome et al. 1999). The pro-
tein content of japonica rice is negatively cor-
related with palatability (Kim et al. 1997). Yang
and Chang (1999) and Fitzgerald and colleagues
(2003) demonstrated that lipids affect the pasting
properties of rice flour: high lipid content raises
gelatinization and most processing and cooking
temperatures.

tests (Kobayashi and Tomita 2008; Wada et al.
2008). Thus, the sensory test remains the most
effective method to determine eating quality.
Little information is currently available on
the genetic control of eating quality in japon-
ica rice cultivars as evaluated by sensory tests
(Yamamoto and Ogawa 1992; Kwon et al. 2011).
In a QTL analysis using populations derived
from crosses between indica and japonica culti-
vars, major-effect QTLs for eating quality were
detected on the short arm of chromosome 6 (Wan
et al. 2004; Ebitani et al. 2005; Takeuchi et al.
2007). This chromosome region includes the Wx
and Alk loci; therefore, allelic differences in these
two genes control some of the main differences in
eating quality between indica and japonica culti-
vars. Recently, some researchers have identified
eating-quality QTLs by using doubled-haploid
lines, recombinant inbred lines, and backcross
inbred lines derived from crosses between japon-
ica cultivars (Suh et al. 2006; Kobayashi and
Tomita 2008; Takeuchi et al. 2008; Wada et al.
2008; Kwon et al. 2011). In these studies,
QTLs for eating quality were also found in
genomic regions other than the short arm of
chromosome 6.
In Japan, QTL studies of eating quality have
used the Japanese japonica cultivar Koshihikari.
Koshihikari has been a top cultivar in Japan since
1979 (Yokoo et al. 2005) and possesses superior
eating quality among Japanese japonica culti-
vars. Several studies have detected eating-quality
QTLs on the short arm of chromosome 3 (Figure
9.2) (Takeuchi et al. 2007, 2008; Kobayashi and
Tomita 2008; Wada et al. 2008). At each of the
QTLs, the Koshihikari allele was associated with
superior eating quality scores in sensory tests. In
the same region on the short arm of chromo-
some 3, QTLs for other eating-quality-related
traits were also detected, namely for contents
of glutamine and asparagine, hardness/stickiness
ratio, and RVA profiles of hot-paste viscosity,
cool-paste viscosity, and consistency viscosity
(Kobayashi and Tomita 2008; Wada et al. 2008).
No QTLs were detected for amylose or protein
content in this region.

SensoryTest

In practical breeding programs, the eating qual-
ity of cooked rice is usually measured via sen-
sory analysis by well-trained panels of individu-
als (Kobayashi and Tomita 2008; Takeuchi et al.
2008; Wada et al. 2008; Kwon et al. 2011).
Sensory tests evaluate the glossiness, stickiness,
hardness, and taste of cooked rice and provide
a score of overall eating quality (Yamamoto and
Ogawa 1992). Sensory tests are difficult to per-
form with early-generation breeding populations
because of the requirement for large amounts of
grain (e.g., several hundred grams of polished
rice), considerable labor to perform the polish-
ing and cooking, and a well-trained panel of
at least 20 people. Physicochemical properties
such as amylose content, protein content, gela-
tinization temperature, and pasting characteris-
tics have been used to predict the eating qual-
ity of cooked rice. However, these properties
are still supplementary indices of eating qual-
ity, in that they do not completely agree with the
results of eating quality evaluations by sensory