Gibberellin levels in imbibed Arabidopsis thaliana seeds are regulated by light via phytochrome, presumably through regulation of gibberellin biosynthesis genes, AtGA3ox1 and AtGA3ox2, and a deactivation gene, AtGA2ox2. Here, we show that a loss-of-function ga2ox2 mutation causes an increase in GA(4) levels and partly suppresses the germination inability during dark imbibition after inactivation of phytochrome. Experiments using 2,2-dimethylGA(4), a GA(4) analog resistant to gibberellin 2-oxidase, in combination with ga2ox2 mutant seeds suggest that the efficiency of deactivation of exogenous GA(4) by AtGA2ox2 is dependent on light conditions, which partly explains phytochrome-mediated changes in gibberellin effectiveness (sensitivity) found in previous studies.