ebisu dwarf ( d 2) is a mutant caused by mutation in a rice brassinosteroid biosynthetic enzyme gene, CYP 90 D 2/ D 2, thereby conferring a brassinosteroid-deficient dwarf phenotype. Three newly isolated d 2 alleles derived from a Nippon- bare mutant library ( d 2-3, d 2-4, and d 2-6) produced more severe dwarf phenotypes than the previously characterized null allele from a Taichung 65 mutant library, d 2-1. Linkage analysis and a complementation test clearly indicated that the mutant phenotypes in d 2-6 were caused by defects in CYP 90 D 2/ D 2, and exogenous treatment with brassinolide, a bioactive brassinosteroid, rescued the dwarf phenotype of three Nipponbare-derived d 2 mutants. However, the content of endogenous bioactive brassinosteroid, castasterone, and the expression of brassinosteroid-response genes indicated that partial suppression of the brassinosteroid response in addition to a brassinosteroid deficiency has occurred in the Nipponbare-derived d 2 mutants. Based on these results, we discuss the possibility that wild-type Nipponbare has some defects in an unknown factor or factors related to the brassinosteroid response in rice.
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