Identification of Genes Expressed in the Tobacco Shoot Apex during the Floral Transition

The shoot apex of higher plants contains undifferentiated meristematic cells that serve as the origin of postembryonic organs. The transition from vegetative to reproductive growth results in the commitment of the apical meristem to floral organ formation. To identify the molecular signals that initiate floral development, we have pursued the isolation of genes that are transcriptionally active in the shoot apex of tobacco during the transition from vegetative to floral growth. The small size of the apex led us to utilize polymerase chain reaction technology for the differential screening of an unamplified, subtracted cDNA library made from transition shoot apices. This approach enabled the isolation of the apex-specific and floral apex-specific cDNA clones described in this paper. One clone, A3, detected an equivalent level of transcript in the shoot apex during all developmental stages observed. The second clone, FA2, detected a unique transcript that increased in abundance in the shoot apex during the transition to flowering and showed high levels of expression in developing petals, stamens, and pistils.