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The hydroxylation of gibberellin (GA) by GA 2-oxidase (GA2ox), a 2-oxoglutarate dependent dioxygenase, is known to be one of its major inactivation reactions. The regulation of this catabolic step is very important for a fluctuation of endogenous GA in plants, and therefore is a good chemical target to control the gibberellin action. At present, prohexadione is known as a commercially available inhibitor to GA2oxs, however prohexadione has a characteristic that it inhibits other 2-oxoglutarate dependent dioxygenases (2ODD) like GA 3-oxidases. Last year we reported in this meeting that we performed in vitro random screening to obtain novel inhibitors more specific to GA2oxs, and we discovered an inhibitory effect of CBTC to the enzymatic activity of GA2ox2. As a result CBTC promoted both the germination of Arabidopsis seeds and their growth. So in this report, we examined if CBTC inhibits other 2ODD, AtGA2ox1・AtGA2ox6・ATGA2ox8・AtGA3ox1・AtGA3ox4・AtGA20ox3, by using recombinant proteins, and how CBTC and prohexadione inhibit GA2ox2 by binding assay. These results strongly suggest that CBTC specifically inhibits GA2oxs, not GA3oxs, in Arabidopsis. These data well conform with the results obtained in planta assay.