To understand the molecular mechanisms involved in the transition from vegetative to reproductive growth, the photoperiodic-controlled induction of flowering in the long-day plant Sinapis alba was used to screen for regulatory genes which are expressed upon induction in the apical meristem. By using the conserved MADS box-encoding region of the organ identity gene AGAMOUS, the genes SaMADS A and SaMADS B were identified which are expressed in transition stages of mustard. RNA blot analysis has confirmed that the transcript levels of both genes are drastically increased shortly after the induction of flowering and that both genes are expressed earlier than the known MADS box genes. In situ hybridization studies have shown that the expression of the genes is restricted to the apical meristem of induced plants during early phases of reproductive development. The expression of SaMADS A is first detectable in the central zone of the meristem, a region where the earliest changes of an evoked meristem could be detected by classical physiological methods, suggesting that SaMADS A may have an important function during the transition to flowering.