Characterizations and fine mapping of a mutant gene for high tillering and dwarf in rice (Oryza sativa L.).

A rice htd-1 mutant, related to tillering and dwarfing, was characterized. We show that the htd-1 mutant increases its tiller number by releasing axillary buds from dormant stage rather than by initiating more axillary buds. The dwarf is caused by averagely reducing each internode and panicle. Based on this dwarfing pattern, the htd-1 mutant could be grouped into dn-type dwarf defined by Takeda (Gamma Field Symp 16:1, 1977). In addition, the dwarfing of the htd-1 mutant was found independent of GA based on the analyses of two GA-mediated processes. Based on the quantitative determination of IAA and ABA and application of the two hormones exogenously to the seedlings, we inferred that the high tillering capacity of the htd-1 mutant should not be attributed to a defect in the synthesis of IAA or ABA. The genetic analysis of the htd-1 mutant indicated that the phenotypes of high tillering and dwarf were controlled by a recessive gene, termed htd1. By map-based cloning, the htd1 gene was fine mapped in a 30-kb DNA region on chromosome 4. Sequencing the target DNA region and comparing the counterpart DNA sequences between the htd-1 mutant and other rice varieties revealed a nucleotide substitution corresponding to an amino acid substitution from prolin to leucine in a predicted rice gene, OsCCD7, the rice orthologous gene of AtMAX3/CCD7. With the evidence of the association between the presence of one amino acid change in OsCCD7 and the abnormal phenotypes of the htd-1 mutant, OsCCD7 was identified as the candidate of the HTD1 gene.