The rice brassinosteroid-deficient dwarf2 mutant, defective in the rice homolog of Arabidopsis DIMINUTO/DWARF1, is rescued by the endogenously accumulated alternative bioactive brassinosteroid, dolichosterone

We have identified a rice ( Oryza sativa) brassinosteroid ( BR)- deficient mutant, BR- deficient dwarf2 ( brd2). The brd2 locus contains a single base deletion in the coding region of Dim/ dwf1, a homolog of Arabidopsis thaliana DIMINUTO/ DWARF1 ( DIM/ DWF1). Introduction of the wild- type Dim/ dwf1 gene into brd2 restored the normal phenotype. Overproduction and repression of Dim/ dwf1 resulted in contrasting phenotypes, with repressors mimicking the brd2 phenotype and overproducers having large stature with increased numbers of flowers and seeds. Although brd2 contains low levels of common 6- oxo- type BRs, the severity of the brd2 phenotype is much milder than brd1 mutants and most similar to d2 and d11, which show a semidwarf phenotype at the young seedling stage. Quantitative analysis suggested that in brd2, the 24- methylene BR biosynthesis pathway is activated and the uncommon BR, dolichosterone ( DS), is produced. DS enhances the rice lamina joint bending angle, rescues the brd1 dwarf phenotype, and inhibits root elongation, indicating that DS is a bioactive BR in rice. Based on these observations, we discuss an alternative BR biosynthetic pathway that produces DS when Dim/ dwf1 is defective.