A Rice Semi-Dwarf Gene, Tan-Ginbozu (D35), Encodes the Gibberellin Biosynthesis Enzyme, ent-Kaurene Oxidase

A rice (Oryza sativa L.) semi-dwarf cultivar, Tan-Ginbozu (d35 Tan-Ginbozu ), contributed to the increase in crop productivity in Japan in the 1950s. Previous studies suggested that the semi-dwarf stature of d35 Tan-Ginbozu is caused by a defective early step of gibberellin biosynthesis, which is catalyzed by ent-kaurene oxidase (KO). To study the molecular characteristics of d35 Tan-Ginbozu , we isolated 5 KO-like(KOL) genes from the rice genome, which encoded proteins highly homologous to Arabidopsis and pumpkin KOs. The genes (OsKOL1to5) were arranged as tandem repeats in the same direction within a 120 kb sequence. Expression analysis revealed that OsKOL2 and OsKOL4 were actively transcribed in various organs, while OsKOL1 and OsKOL5 were expressed only at low levels; OsKOL3 may be a pseudogene. Sequence analysis and complementation experiments demonstrated that OsKOL2corresponds to D35. Homozygote with null alleles of D35showed a severe dwarf phenotype; therefore, d35 Tan-Ginbozu is a weak allele of D35. Introduction of OsKOL4 into d35 Tan-Ginbozu did not rescue its dwarf phenotype, indicating that OsKOL4 is not involved in GA biosynthesis. OsKOL4 and OsKOL5 are likely to take part in phytoalexin biosynthesis, because their expression was promoted by UV irradiation and/or elicitor treatment. Comparing d35 Tan-Ginbozu with other high yielding cultivars, we discuss strategies to produce culm architectures suitable for high crop yield by decreasing GA levels.