XVI International Botanical Congess

The endogenous brassinosteroids (BRs) were examined from 5-week-old plants of a null allele of the BR-insensitive dwarf mutant, bri1-4, and wild type. [2H6]BRs were used as internal standards to determine the endogenous levels of BRs by GC-SIM. The levels of 6-deoxoBRs in bri1-4 were comparable to wild type. However, bri1-4 was shown to accumulate very high levels of brassinolide and castasterone. The results suggest that the biosynthesis of BRs might be normal or might be activated, but utilization of the biologically active BRs is blocked by the mutation in the BRI1 gene, resulting in the accumulation of end products in the BR biosynthetic pathway. These results are consistent with BRI1 acting as a receptor or in a downstream step.