A rice heading-date-related mutant was isolated from a ^60Co-y-ray-induced mutation pool of Zhejing 22, a conventional japonica cultivar in Zhejiang Province, China. The mutant was characterized by a delayed heading date of almost 20 d longer than the wild type plant. Genetic analysis revealed that the mutation was controlled by a single nuclear-encoded recessive gene that was designed as HD（t） （heading date tentatively）. To isolate the HD（t） gene, a map-based cloning approach was employed using 479 F2 mutant individual plants derived from the cross between the hd（t） mutant （japonica） x Zhenshan 97 （indica）. Finally, the HD（t） gene was mapped to an approximate 53 kb region between the insertion and deletion （InDel） markers of 10-61W and 10-66W on chromosome 10. According to the genome sequence of Nipponbare, the target region contains 11 annotated genes. It is helpful for future cloning of HD（t） gene based on this fine mapping results.
Grain shape and size both determine grain weight and therefore crop yield. However, the molecular mechanisms controlling grain shape and size are still largely unknown. Here, we isolated a rice mutant, beak-shaped grain1 （bsgl）, which produced beak-shaped grains of decreased width, thickness and weight with a loosely interlocked lemma and palea that were unable to close tightly. Starch granules were also irregularly packaged in the bsgl grains. Consistent with the lemma and palea shapes, the outer parenchyma cell layers of these bsgl tissues developed fewer cells with decreased size. Map-based cloning revealed that BSG1 encoded a DUF640 domain protein, TRIANGULAR HULL 1, of unknown function. Quantitative PCR and GUS fusion reporter assays showed that BSG1 was expressed mainly in the young panicle and elongating stem. The BSG1 mutation affected the expression of genes potentially involved in the cell cycle and GW2, an important regulator of grain size in rice. Our results suggest that BSG1 determines grain shape and size probably by modifying cell division and expansion in the grain hull.
YAN DaWeiZHOU YaYE ShengHaiZENG LongJunZHANG XiaoMingHE ZuHua
