Floral meristem identity genes
The flowering-time pathways increase the expression of meristem identity genes (Figure 1.1). These genes confer floral identity on the peripheral zone cells that give rise to primordia, so that they produce flowers rather than vegetative structures.
The major floral meristem identity genes in Arabidopsis are LFY and APETELA1 (AP1). Mutations in either of these genes cause a partial conversion of flowers to shoots (Irish and Sussex, 1990; Schultz and Haughn, 1991; Weigel et al., 1992) . In addition to its role in floral meristem identity, AP1, which encodes a MADS box transcription factor, is also involved in the determination of floral organ identity (Bowman, 1989; Bowman et al., 1993; Weigel and Meyerowitz, 1994) .
LEAFY - LFY
LFY encodes a plant specific protein, which has little similarity to other Arabidopsis proteins (Coen et al., 1990; Weigel et al., 1992) . The LEAFY protein has acidic and proline-rich domains, suggesting it might be involved in transcriptional regulation. LFY is expressed in the SAM and primordia, and is the earliest acting of the floral meristem identity genes (Weigel et al., 1992) . LFY directly upregulates the expression of AP1 in both leaf and floral primordia (Blazquez et al., 1997; Parcy et al., 1998; Liljegren et al., 1999; Wagner et al., 1999) . LFY is nuclear localised and binds to DNA, including sequences in the AP1 promoter (Parcy et al., 1998; Lohmann et al., 2001) . LFY is also able to upregulate CAULIFLOWER, which is described later (Wagner et al., 1999) .
The expression of LFY is under the control of both the photoperiod and the gibberellin pathways (Simon et al., 1996; Blazquez and Weigel, 2000) . In the ga1 mutant expression from the LFY promoter is reduced, and the response to inductive photoperiods is delayed (Blazquez et al., 1998) . The induction of LFY in response to GAs and inductive photoperiods is regulated by different cis elements of the LFY promoter. Therefore, LFY is able to act a point of integration of two different flowering-time pathways (Blazquez and Weigel, 2000) .
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