Since the transition from vegetative to floral development in plants is likely to be influenced by gene expression in several plant organs, we have used an in vitro system, the tobacco "thin cell layer" system as a model for investigating gene expression associated with the initiation of flowering in higher plants. cDNA cloning has been used to identify mRNAs abundant during thin cell layer floral initiation. These genes are expressed in thin cell layer explants initiating floral meristems but not in thin cell layer explants initiating vegetative shoot meristems or possessing roots. Two of these genes are expressed transcriptionally in incipient floral apices during normal plant development. Transcripts of these genes, plus a third gene, occur at low levels in several plant organs and at high levels in the roots, with the maximum levels of root expression reached just prior to the formation of floral meristems.