The spatially and temporally restricted transcriptional activation of both AP1 and AG depends on the concerted action of LFY in combination with other transcription factors. However, AP1, AP3 and AG are expressed in distinct spatial domains,while LFY is expressed throughout the floral meristem, implying that the mode of action of LFY in activating different classes of floral homeotic genes is distinct ( Parcy et al., 1998). LFY has been shown to bind to a loosely defined CC(A/T)(A/G)(G/T)G(G/T) consensus site in the regulatory regions of the APETALA1 ( AP1), APETALA3 ( AP3) and AGAMOUS ( AG) floral homeotic genes( Busch et al., 1999 Lamb et al., 2002 Lohmann et al., 2001 Parcy et al., 1998 Wagner et al., 1999). LFY encodes a novel plant-specific transcription factor, and its role in activating floral homeotic genes has been shown to be separable from that of conferring floral meristem identity ( Parcy et al., 1998). This mechanism may be widely conserved, as homologs of UFO and LFY have been identified in a wide array of plant species. These results suggest a novel mechanism for promoting flower development via protein degradation and concomitant activation of the LFY transcription factor. These results define an unexpected role for an F-box protein in functioning as a DNA-associated transcriptional co-factor in regulating floral homeotic gene expression. Moreover, chemical or genetic disruption of proteasome activity compromises LFY-dependent AP3 activation,indicating that protein degradation is required to promote LFY activity. Furthermore, a transcriptional repressor domain fused to UFO reduces endogenous LFY activity in plants, supporting the idea that UFO acts as part of a transcriptional complex at the AP3 promoter. Here, we show that UFO physically interacts with LFY both in vitro and in vivo, and this interaction is necessary to recruit UFO to the AP3 promoter. LFY-dependent activation of the homeotic APETALA3 ( AP3) gene requires the activity of UNUSUAL FLORAL ORGANS (UFO), an F-box component of an SCF ubiquitin ligase, yet how this regulation is effected has remained unclear. The Arabidopsis LEAFY (LFY) transcription factor is crucial in integrating these signals, and acts in part by activating the expression of multiple floral homeotic genes. Plants flower in response to both environmental and endogenous signals.
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