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As an important factor of rice production, Grain length also impacts on rice appearance quality and worth of further researches on its molecular level. At the same time, cloning the major grain-length genes will absolutely benefit the better understanding of the mechanisms of grain development, grain-length regulation, as well as determinant programs of grain weight in rice. Moreover, it will also enhance the efficiency of rice quality improvement. In this research, a completely dominant minute-grain gene, designated as Mi3(t), was studied with the approach of classic map-based cloning strategy. The main results were presented as follows:1. Genetic analysis for minute grain: Through investigations and genetic analysis in three genetic populations, as Shuhui 527/Y34, Shuhui 881/Y34 and 9311/Y34, we found that the grain-length trait of Y34 was controlled by a completely dominant minute-grain gene. Further studies indicated that the minute-grain gene mainly controlled the grain length trait by remarkably shortening 35.7%-47.4% of grain length. Meanwhile, we also found that the minute-grain gene also resulted in the improvement of 1000-grain weight, but not in the phenotypes of round grain, low production and later flowering.2. Molecular mapping of the minute grain gene: F2 populations including Shuhui527/Y34, in which there were 112 recessive individuals, and Shuhui 88a/Y34, in which there were 261 recessive individuals, were analyzed with 841 pairs of SSR makers. By Shuhui 527/Y34 F2 population, Mi3(t) was mapped on Chromosome 3 between the SSR makers RM282 and RM411, with the genetic distance of 6.4cM and 4.9cM, respectively. Furthermore, we mapped Mi3(t) by Shuhui881/Y34 F2 population between RM282 and RM6283 on Chromosome 3 with the genetic distance of 5.1cM and 0.9cM, respectively. As two minute grain gene, Mi/Mil and Mik/Mi2, were named in previous studies, minute grain gene studied in present paper was temporarily designed as Mi3(t).3. Fine mapping of Mi3(t): 778 recessive individuals(long grain individuals) from 9311/Y34 F2 population were further analyzed to narrow the Mi3(t) locus. Mi3(t) was first mapped on chromosome 3 between RM6881 and RM6283 in this population, with the genetic distance of 0.5cM and 3.6cM.
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