Abstract: Bestatin, an inhibitor of some aminopeptidases, potently activates jasmonic acid-induced defense genes expression in the model systems of tomato and Arabidopsis. We have employed bestatin as a small molecular chemical tool to dissect jasmonic acid signaling in Arabidopsis and identified a collection of bestatin response (ber) mutants. ber15 is one of these mutants that are resistant to the inhibitory effect of bestatin on root elongation. Subsequent investigation revealed that ber15 is less sensitive to applied jasmonic acid in term of root growth inhibition, suggesting that the BER15 gene play a role in JA signaling. Map-based cloning of BER15 demonstrated BER15 encodes a cytochrome P450 monooxygenase which acts as a key enzyme in brassinolide synthesis. Further functional analysis of BER15 will broaden our knowledge of the interaction mechanism between brassinolide biosynthesis and JA signaling.