An Overview of Gibberellin Biosynthetic Genes, Their Related Mutants and the Specific Phenotype in Sorghum

In history, alleles of a gibberellin (GA) biosynthetic gene (sd1/OsGA20ox2), and a signal transduction gene (Rht), made way for the development of semi-dwarf, high-yielding and lodging-resistant rice and wheat, respectively. In this study, we analyzed the genes controlling culm length in sorghum through a QTL analysis of an F₂ population from the cross between SIL-05 (tall) and bmr-6 (dwarf) parental lines. The results revealed three height-reducing QTLs corresponding to the dwarfing alleles of bmr-6, which surprisingly did not coincide with any known GA-related gene loci. This prompted us to ask why such genes did not even play a crucial role in the history of sorghum dwarf breeding. Thus, we produced a mutant population of SIL-05 by gamma-ray irradiation and screened for GA-related mutants. As a result, four GA biosynthetic mutants were identified, namely, sbcps1, sbks1, sbko1 and sbkao1, all of which showing severe dwarfism. Surprisingly, they also exhibited a bent culm phenotype which became very obvious at the adult stage as a result of the differential growth between two opposite regions in the culm. This suggests that GA deficient conditions trigger the differential growth, resulting in the abnormal bending of the sorghum culm. In conclusion, we propose the possibility that during the dwarf breeding history of sorghum, GA-related genes may have not　been used because of this abnormal phenotype.