Genetic Improvement of Grain Quality in Japonica Rice - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 146

Medium- and short-grain rice cultivars are gen-
erally preferred by people from Japan, northern
China, and North and South Korea, where japon-
ica rice is typically grown. Slender grains are pre-
ferred in many areas where indica rice is grown
(Sundaram et al. 2008).
Many genetic studies of grain shape in rice
have confirmed that seed size, shape, and weight
are all under polygenic control. Numerous QTLs
associated with each aspect of grain size have
been identified in diverse populations derived
from crosses between indica and japonica culti-
vars over the past 20 years (Yonemaru et al. 2010;
Youens-Clark et al. 2011). Recently, five grain-
shape genes ( GS3 , GW2 , qSW5 , GIF1 , and GS5 )
have been cloned and characterized (Fan et al.
2006; Song et al. 2007; Shomura et al. 2008;
Wang et al. 2008; Li et al. 2011). Cultivars car-
rying the recessive alleles of GS3 , qSW5 , GW2 ,
and GIF1 have longer, wider, and heavier seeds
than those carrying the wild-type alleles. In con-
trast, GS5 functions as a positive and dominant
regulator of grain size by increasing cell number
and cell size and leading to enhanced length of
the grain. Endosperm size in rice is largely deter-
mined by the dimensions of the lemma and palea
(Li et al. 2004). Each of the five subpopulations
of rice cultivars ( indica , aus , tropical japonica ,
temperate japonica , and basmati ) carry differ-
ent combinations of alleles of grain-size genes,
and the indica rice population contains signif-
icantly greater genetic variation for grain size
than the others (Takano-Kai et al. 2009). The
large number of possible allele combinations
makes it possible to obtain a range of grain
sizes to satisfy the diverse quality requirements
of consumers. By creating novel allele combi-
nations or introducing indica rice alleles into
japonica rice cultivars, we could make changes
and improvements to rice grain shape within the
range acceptable to japonica rice consumers.
To this end, Japanese breeders have developed
the large-grain cultivars Oochikara (Figure 9.1b)
and BG1 from crosses between indica and
japonica cultivars (Kobayashi et al. 1990; Song
et al. 2007).

HuskingandMillingQuality

Assessing the appearance of husked and milled
(polished) rice grains is necessary to determine
the grade and price of harvested grain (Bergman
et al. 2004). The grade is reduced by the pres-
ence of cracked, discolored, or immature grains.
The majority of consumers prefer well-milled
rice that has little to no bran remaining, despite
the fact that brown rice contains more pro-
tein, lipids, vitamins, minerals, and phytochem-
icals with potential health benefits than milled
rice (Bergman et al. 2004). Previous studies
have suggested that grain shape, grain chalki-
ness, hull and bran diffusivity, and endosperm
chemical components may affect the frequency
of grain breakage during husking and milling
(Bhattacharya 1980; Juliano et al. 1993; Sarker
et al. 1996). Environmental factors are known
to have a considerable impact on the frequency
of grain breakage during husking and milling
(Jodari and Linscombe 1996). For example, rain
just before harvest increases the moisture con-
tent of grain and promotes grain fissuring (Lan
and Kunze 1996). Takita (1992) reported varietal
differences in cracked grain ratio. Rice breed-
ers have tried to select cultivars showing a low
frequency of cracked, discolored, and immature
grains as breeding materials to develop novel
rice cultivars. However, little is known about the
genetic contribution to the number of cracked
grains produced during the husking and milling
processes.

Grain Chalkiness

Chalkiness is an important aspect of grain
appearance that affects consumer acceptance.
Chalky grains have opaque spots in various
regions of the endosperm (Lisle et al. 2000;
Fitzgerald et al. 2009). Chalkiness is classi-
fied into five types based on the positions
of the opaque spots: white-belly, white-back,
white-based, white-core, and milky-white (Fig-
ure 9.1c). In many cases, the presence of grain
chalkiness leads to a downgrade in grain quality