The homologous genes FLORlCAULA (FLO) of Antirrhinum and LEAFY (LFY) of Arabidopsis regulate the formation of determinate floral meristems. Transcripts of these single-copy genes are confined to floral meristems and some floral organs as well as to the leaflike bracts that subtend Antirrhinum flowers. Based on these observations, we hypothesized that the transcription of genes homologous to FLO and LFY in tobacco, a determinate plant in which the primary shoot apex is consumed in the production of a terminal flower, would serve as a molecular marker for floral commitment. Surprisingly, transcripts of the tobacco homologs NFLl and NFLP (Bicotiana FLOILFY ) were found not only in floral meristems, but also in indeterminate vegetative meristems. This implies that the transcriptional expression of the FLOlLFY homologous genes in the apical meristem is not sufficient for the initiation of floral meristem development. In addition, the transcript patterns of the NFL genes identified a previously undescribed subset of cells within the shoot apical meristem that may indicate unique functional compartmentalization. This suggests that, unlike FLO and LFY, which specify determinacy only during floral development, the NFL genes act to specify determinacy in the progenitor cells for both flowers and leaves. INTRODUCTIONThe rigorous characterization of mutations that affect floral organ number and identity has provided a detailed account of the genetic regulation of flower development (for reviews, see Schwarz-Sommer et al., 1990;Coen, 1991;Coen and Meyerowitz, 1991;Okamuro et al., 1993;Ma, 1994). These studies have led to models describing the epistatic interactions among several genes that control floral organ identity and the effects of the genes on the determinacy of floral meristems. The cloning of several of the genes that participate in flower development has enabled the elucidation of some molecular aspects of floral organ specification and has allowed the direct testing of many facets of the models.Early steps in floral ontogeny involve the initial establishment of floral meristems produced from inflorescence meristems. Mutations in FLORlCAULA (FLO) of Antirrhinum Coen et al., 1990) and the cognate homolog LEAFY (LFY) of Arabidopsis (Schultz and Haughn, 1991;Huala and Sussex, 1992;Weigel et al., 1992) result in the replacement of flowers with leafy shoots. Secondary, tertiary, and higher order inflorescence shoots can arise from the initial transformed shoot, resulting in the reiteration of the inflorescence phase of development. The wild-type FLO and LFY products are thus required for the specification of determinate floral meristems.The racemose inflorescences of wild-type Antirrhinum and Arabidopsis plants are characterized by a spiral phyllotaxy of ' To whom correspondence should be addressed flowers that are initiated on the flanks of an indeterminate inflorescence meristem. In Antirrhinum, the flowers are subtended by leaflike bracts; in Arabidopsis, flowers are not subtended by any leaflike organs, but lateral ...

The homologous genes FLORICAULA (FLO) of Antirrhinum and LEAFY (LFY) of Arabidopsis regulate the formation of determinate floral meristems. Transcripts of these single-copy genes are confined to floral meristems and some floral organs as well as to the leaflike bracts that subtend Antirrhinum flowers. Based on these observations, we hypothesized that the transcription of genes homologous to FLO and LFY in tobacco, a determinate plant in which the primary shoot apex is consumed in the production of a terminal flower, would serve as a molecular marker for floral commitment. Surprisingly, transcripts of the tobacco homologs NFL1 and NFL2 (Nicotiana FLO/LFY) were found not only in floral meristems, but also in indeterminate vegetative meristems. This implies that the transcriptional expression of the FLO/LFY homologous genes in the apical meristem is not sufficient for the initiation of floral meristem development. In addition, the transcript patterns of the NFL genes identified a previously undescribed subset of cells within the shoot apical meristem that may indicate unique functional compartmentalization. This suggests that, unlike FLO and LFY, which specify determinacy only during floral development, the NFL genes act to specify determinacy in the progenitor cells for both flowers and leaves.