Loss of function of a rice brassinosteroid insensitive1 homolog prevents internode elongation and bending of the lamina joint

Brassinosteroids (BRs) are plant growth-promoting natural products required for plant growth and development. Physiological studies have demonstrated that exogenous BR, alone or in combination with auxin, enhance bending of t he lamina joint of rice. However, little is known about the function of end ogenous BR in rice or other grass species. We report here the phenotypical and molecular characterization of a rice dwarf mutant, d61, that is less se nsitive to BR compared to the wild type. We cloned a rice gene, OsBRI1, wit h extensive sequence similarity to that of the Arabidopsis BRI gene, which encodes a putative BR receptor kinase. Linkage analysis showed that the OsB RI1 gene is closely linked to the d61 locus. Single nucleotide substitution s found at different sites of the d61 alleles would give rise to amino acid changes in the corresponding polypeptides. Furthermore, introduction of th e entire OsBRI1 coding region, including the 5' and 3' flanking sequences, into d61 plants complemented the mutation to display the wild-type phenotyp e. Transgenic plants carrying the antisense strand of the OsBRI1 transcript showed similar or even more severe phenotypes than those of the d61 mutant s. Our results show that OsBRI1 functions in various growth and development al processes in rice, including (1) internode elongation, by inducing the f ormation of the intercalary meristem and the longitudinal elongation of int ernode cells; (2) bending of the lamina joint; and (3) skotomorphogenesis.