More than 60 dwarf mutants have been reported in rice (Futsuhara and Kikuchi 1993). Of these, the mutant cultivar Fukei 71 carrying dwarf gene d50, shows a strong dwarf phenotype in response to an increase in temperature (Fig.1) (Kitano and Futsuhara 1981). Murai et al. (1990) demonstrated that two independently induced mutants, d50 and d12, showed the allelism. Tamura et al. (2001) showed that d50 was also allelic to d32. Therefore, d50, d12 and d32 are allelic. Furthermore, morphological and chemical analyses of Fukei 71 revealed that irregularly shaped cells and the ectopic accumulation of cell wall components peculiar to grasses were found in the parenchymal regions of elongated internodes (Kitano and Futsuhara 1982, Nishikubo et al. 2000). Although these observations suggest that D50 locus plays an important role in internode elongation by regulating cell wall establishment, the detailed function of the gene remains unclear. So, we tried to isolate D50 gene by map-based cloning. Here, we report the chromosome location of D50 identified using several molecular markers.

To generate mapping population, Fukei 71 was crossed with the indica variety Kasalath. To clear the discrimination of dwarf plants (d50 homozygotes) for mapping population, F₂ dwarf plants were backcrossed to Kasalath three times, and their BC₃F₂ and BC₃F₃ dwarf plants were used for mapping.

Using 118 BC₃F₂ dwarf plants, D50 locus was mapped near RFLP markers R1989, C196

and R1736 on the long arm of chromosome 2 (Tamura et al. 2001). For the successive high resolution map of D50, we designed four kinds of CAPS markers (E30292SB, E61832S, R1736 and R2975) on both sides of C196, based on the sequence information of RFLP markers. For linkage analysis with these CAPS markers, 203 BC₃F₂ and 850 BC₃F₃ dwarf plants were used. One recombinant plant between E61832S and d50, and two recombinant plants between R1736 and d50 were obtained (Fig. 2). According to RGP (Rice Genome Program) map, the genetic distance between E61832S and R1736 is 2.2 cM, whereas the genetic distance calculated from a recombination value is 0.14 cM. This difference may be induced by the decline of recombination frequency. Hence, D50 was mapped at 0.73 cM on E61832S, and 1.47 cM on R1736. For further analysis, about four thousand BC₃F₂ plants were cultivated. We are now determining the precise location of D50 between E61832S and R1736 using several new markers.

References

Futsuhara, Y. and F. Kikuchi, 1993. In:Science of the rice plant, Volume 3, Nobunkyo, Tokyo, P. 300-317.

Harushima, Y., M. Yano, A. Shomura, M. Sato, T. Shimano, Y. Kuboki, T. Yamamoto, S.Y. Lin, B.A. Antonio, A. Parco, H. Kajiya, N. Huang, K. Yamamoto, Y. Nagamura, N. Kurata, G.S. Khush and T. Sasaki, 1998. A high-density rice genetic linkage map with 2275 markers using a single F2 population. Genetics 148: 479-494.

Kitano, H. and Y. Futsuhara, 1981. Character expression of induced dwarf mutant in rice. Effects of temperature on culm elongation in the dwarf mutant line, Fukei No. 71. Japan J. Breed. 31: 9-18.

Kitano, H. and Y. Futsuhara, 1982. Character expression of induced dwarf mutant in rice. Morphological and histological observations on the effects of temperature on culm elongation in the dwarf mutant line, Fukei No. 71. Japan J. Breed. 32: 146-154.

Murai, M., N. Shinbashi, A. Kasutani, S. Hirose, I. Takamure and T. Kinoshita, 1990. Identification of dwarf genes in rice lines, Yukara dwarf and Fukei 71, and its response to environmental factors. Japan J. Breed. 40: 33-45.

Nishikubo, N., T. Araki, S. Kajita, K. Kuroda, H. Kitano and Y. Katayama, 2000. Specific accumulation of polysaccharide-linked hydroxycinnamoyl esters in the cell walls of irregularly shaped and collapsed internode parenchyma cells of the dwarf rice mutant Fukei 71. Plant Cell Physiol. 41: 776-784.

Tamura, K., I. Ashikawa and H. Kitano 2001. RFLP mapping of the dwarf gene in rice line Fukei 71. Breed. Res. 3 (Suppl. 2): 11. (in Japanese).