In two cases, mutations in the same brassinosteroid-related genes caused different phenotypes in japonica varieties Nipponbare and Taichung 65. The mutant phenotypes were less severe in the Taichung 65 background than in the Nip ponbare background. Three newly isolated brassinosteroid-insensitive mutants ( d 61 - 1 N , d 61 - 11, and d 61 - 12) derived from a Nipponbare mutant library were found to be alleles of d 61 , which represent defects in the OsBRI1 gene. Al though the Nipponbare - derived mutant d 61 - 1 N had the same nucleotide substitution as the previously characterized Taichung 65 - derived mutant d 61 - 1 T , these two mutants showed different phenotypes for plant stature, internode elon gation pattern, and seed shape; in each case, d 61 - 1 N (in the Nipponbare genetic background) had the more severe mu tant phenotype. Similar trends were seen for phenotypes caused by mutants of d 2, a brassinosteroid biosynthesis gene. Consistent with these phenotypes, the expression of brassinosteroid-responsive genes was lower in the Nippon bare - derived mutants. These results can be explained by our findings that feed-forward up-regulation of OsBRI 1 did not occur in the Nipponbare - derived mutants and that an mPing transposon is inserted into the promoter region of Nippon bare OsBRI 1. Based on these results, we conclude that the expression of OsBRI 1 , especially its feed-forward up-regula tion, is misregulated in wild-type Nipponbare and in brassinosteroid-related mutants in a Nipponbare genetic back ground. Although Nipponbare is a model rice genotype, it can be categorized as an OsBRI 1 mutant that has reduced sensitivity to brassinosteroid .
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