To date, over 60 dwarf genes have been reported in rice (Futsuhara and
Kikuchi 1993). Introgression of dwarf alleles into elite cultivars, conferring
lodging resistance, is a major task in breeding program since lodging
is a common problem in most cereals and various other crops (Kashiwagi
and Ishimaru 2004). Consequently, the introgressions of dwarf alleles
improve yields, which increase the economic value. Two best famous semi-dwarf
genes, sd1 in rice and Rht1 in wheat, dramatically increase
rice and wheat production, known as the "Green Revolution" (Sasaki
et al. 2002, Peng et al. 1999). Many mutant genes had been
cloned and characterized at molecular and biochemical levels (Ashikari
et al. 1999, Sasaki et al. 2002, Hong et al. 2003,
Itoh et al. 2004). Exploring novel rice mutant genes can be applied
not only in breeding programs but also in scientific research.
We had screened a tiny rice dwarf mutant (tr1) which shows abnormal
morphology such as narrow and short leaf blade, and dark green phenotypes,
from Tos17 mutant lines. The tr1 was
crossed with Kasalath for linkage analysis. Phenotype of the F1
plants shows wild and a 3 to 1 (337 normal and 91 dwarf plants) segregation
was observed in the F2 population. These results indicated
that tr1 was recessive, and we used 91 recessive homozygous plants
for mapping of tr1. Congruently, tr1 was mapped between
PCR markers E4443 and RM264 on the long arm of chromosome 8 (Fig. 2),
with a 16 cM interval based on the high-density linkage map of RGP (Rice
Genome Project, Harushima et al. 1998). We have not investigated
linkage between this mutant and insertion of Tos17. Fine mapping
of tr1 by using more genetic markers and more recombinants in this
region are under progression. Applying positional cloning of tr1
and learn mechanisms that tr1 affects extreme tiny dwarf is our
next objective.
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