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Life Science Research and Sustainable Development ISBN: 978-98-84663-33-9

(Arabidopsis thaliana) (Yamamuroet al., 2000). The D2 protein represents a novel type of P450
(CYP90D2) that catalyzes the steps from 6-deoxoteaster-one to 3-dehydro-6-deoxoteasterone and
from teasterone to 3-dehydroteasterone in the late BR biosynthesis pathway (Hong et al., 2003).
The D11 P450 (YP724B1) enzyme is required for the supply of 6-deoxotyphasterol and typhasterol
in the BR biosynthesis network (Tanabe et al., 2005).
Gn1a, a major QTL, has been cloned andcharacterized for the grain number per panicle
trait inthe Koshihikari/Habataki-derived advanced backcrosspopulation. Functional analysis
revealed that Gn1aencoded cytokininoxidase/dehydrogenase (OsCKX2),an enzyme that
degrades the phytohormonecytokinin(Ashikariet al., 2005). Grain weight is usually
representedby 1,000-grainweight in breeding techniques andis determined by grain width,
length, and thickness.ManyQTLs associated with rice grainweight have beenidentified in the last
decade.Of these,GS3, a majorQTLfor grain length and weight and also as a minor QTL forgrain
width and thickness, has been mapped onchromosome 3. These cloned genes governing rice yield
componentsnot only elucidate themolecular mechanisms regulatinggrain number per panicle
and grain weight and widthtraits but also directly provide information on sequencevariation.
This will enable us to design gene-taggedmarkers in the application of MAS for yield traits.
To clone Ghd8, the NIL-F2 population (NIL-F2(HR5)) wasobtained from a heterozygous
F7 plant derived from a crossbetween an old version of Zhenshan 97 (ZS) and HR5 (Zhang et al.,
2006) was used. In rice, each HAP subunit is encoded by a gene family;therefore, various
possibilities were available to form differentHAP complexes, which regulate multi-
developmental processes.This explanation may account for the pleiotropic effectsof these
Ghd7/Ghd8 genes.
GRAIN SIZE 3 (GS3) is a major QTL for grain length and weight and a minor QTL for
grain width and thickness and functions as a negative regulator for grain size (Fan et al., 2006).
GS3 was originally detected from the progeny produced by a cross between Minghui 63 and
Chuan7. The GS3 protein contains an organ size regulation (OSR) domain in the N terminus, a
transmembrane domain, a tumor necrosis factor receptor/nerve growth factor receptor
(TNFR/NGFR)-like domain, and a von Will-ebrand factor type C (VWFC) domain in the C
terminus. The OSR domain functions as a negative regulator of grain length and deletion mutants
of this domain result in the formation of long-grain rice. The C-terminal TNFR/NGFR and VWFC
domains act as positive regulators of grain length and loss-of-function mutations of these
domains lead to the development of very short grain (Fanet al., 2009 and Mao et al., 2010). A
molecular maker based on GS3 has been developed for the selection of long-grain lines in rice
breeding programs (Wang et al., 2011). The GS3 ortholog in maize (Zea mays) has also been cloned
and characterized. ZmGS3 has functional domains in common with the rice GS3 protein and
ZmGS3 has been shown to be involved in maize kernel development (Li et al., 2010).
GIF1 was primarily mapped with simple sequence repeat markers using 300 F2 gif1
individuals and was further placed in a 32kb region between the markers CAPS4 and CAPS8
(Wang et al., 2008). GIF1 was also shown to cosegregate with CAPS7 using 900 F2 mutant plants.
The grain incomplete filling 1 (gif1) mutant was isolated from a screen for mutants with grain-
filling defects. The GIF1 mutant also has more grain chalkiness as a result of loosely packed starch
granules. GIF1 encodes a cell-wall invertase required for carbon partitioning during early grain
filling. A frameshift mutation caused by a 1-bp nucleotide deletion in GIF1 results in premature
termination of its open reading frame. The GIF1 gene is expressed in a more restricted pattern in
the flowers of cultivated rice varieties than in the flowers of wild rice, which is apparently a
consequence of accumulated changes in the regulatory sequence of the promoter as a result of

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