7. High resolution mapping of a rice gene affecting shoot organization, SHO1.

1) Bioscience Center, Nagoya University, Nagoya, 464-8601 Japan
2) Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan
3) Rice Genome Research Program, National Institute of Agrobiological Resources, Institute of the Society for Techno-Innovation of Agriculture, Forestry, and Fisheries, Tsukuba, 305-8602 Japan
4) Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan

Three rice shoot organization (sho) mutants designated as sho1, sho2, and sho3 have been identified (Itoh et al. 2000). These mutants show random phyllotaxy, short plastochron, abnormal leaves, which are thread-like or short and narrow, and malformed shoot apical meristem (SAM) in the early phase of seedling development (Fig. 1) (Tamura et al. 1992) . The expression domain of a rice homeobox gene,OSH1, in the SAM of sho homozygote shoot meristems is severely reduced, which might cause the abnormal maintenance of SAM. Thus, the isolation and characterization of sho genes is important for elucidating the molecular mechanism of shoot organization during the early vegetative phase through the homeobox genes. As an initial step, we started to isolate one of these

genes, sho1, by map-based cloning.

To identify the chromosomal location of sho1, trisomic analysis was performed. Seven kinds of trisomic plants, KT-1, 3, 5, 7, 9, 15 and 17 (Yasui et al. 1989), were crossed with the plants which were heterozygous for sho1 (SHO1 / sho1). The segregation ratio in the F2 plants derived from the cross between KT-9, whose chromosome 4 was trisomic, and the plants heterozygous for sho1 was about 13:1 (wild type : sho1 mutant), while the segregation ratios in the F2 populations derived from crosses with the other six kinds of trisomic plants agreed with 3:1. The results indicate that the sho1 is located on chromosome 4.

Fifty F2 plants from the cross between the plants which were heterozygous for sho1 (SHO1 / sho1) and an Indica rice, cv. Kasalath, were used for linkage analysis with one RFLP marker, C335, on chromosome 4. The marker C335 cosegregated with sho1. For a high resolution map of sho1, Cleaved Amplified Polymorphic Sequence (CAPS) markers were designed from the sequence information of C377 and E1171 (EST : unpublished data), which are located on both sides of C335 (Fig. 2). According to the RGP (Rice Genome Program) map (Harushima et al. 1998), the genetic distance between C335 and C377 is 2.2cM, and that between C335 and E1171 is about 6.5cM. Six hundred and four homozygous sho1 plants were selected from 2688 F2 plants, and linkage analysis was carried out with these CAPS markers. Thirty eight plants recombinant between C377 and sho1, and 93 plants recombinant between E1171 and sho1, were obtained. Thus, sho1 was mapped at the distance of 3.5cM from C377 and 8.7cM from E1171 (Fig. 2- B). We are determining more precise location of sho1 between C377 and E1171.


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