Genetic Basis of High-Tillering Dwarf Trait in Rice DUS Test Standard Variety Cong’ai 2

Cong’ai 2 (cl2), one of the standard varieties of rice DUS test in China, shows typical high-tillering dwarf phenotype. Genetic analysis showed that this trait is controlled by a single recessive nucleus gene, which was mapped between two InDel markers C4-CL5 and C4-CL4 on the long arm of chromosome 4. Between these two markers, there is a known gene D17/HTD1, mutations in which caused high-tillering dwarf phenotype. Sequencing analysis of the D17/HTD1 allele in cl2 revealed that the 1796th base was substituted from C to T, changing the 599th amino acid from proline to leucine. Another high-tillering dwarf mutant S1-40 was obtained from japonica variety Nipponbare mutagenized by EMS. The 3' splicing site of the 3rd intron of D17/HTD1 was substituted from AG to AA, which caused the formation of two aberrant transcripts in S1-40. D17/HTD1 encodes the carotenoid cleavage dioxygenase 7 (CCD7), which is a key enzyme involved in the biosynthesis of a new plant hormone strigolactones (SLs). Exogenous application of GR24, a synthetic analogue of SLs, inhibited the tillering phenotype of cl2. Phylogenetic analysis revealed that CCD7 homologs existed in almost all plant species. Real-time RT-PCR showed D17/HTD1 was expressed in all rice tissues examined, with the highest expression in stems.