AnArabidopsisF-box protein acts as a transcriptional co-factor to regulate floral development

Abstract

Plants flower in response to both environmental and endogenous signals. 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. 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. 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. 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. Moreover, chemical or genetic disruption of proteasome activity compromises LFYdependent AP3 activation, indicating that protein degradation is required to promote LFY activity. 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. These results suggest a novel mechanism for promoting flower development via protein degradation and concomitant activation of the LFY transcription factor. This mechanism may be widely conserved, as homologs of UFO and LFY have been identified in a wide array of plant species.

Patterning of Inflorescences and Flowers by the F-Box Protein DOUBLE TOP and the LEAFY Homolog ABERRANT LEAF AND FLOWER of Petunia

“…Our yeast twohybrid results differ on several points with those of Chae et al (2008). In our assays, which were dedicated to a library screen, we detected a weak transcription activation activity in the N terminus of ALF and LFY that was not detected by Chae et al (2008), possibly due to a lower sensitivity of their quantitative b-galactosidase assay.…”

“…Our yeast twohybrid results differ on several points with those of Chae et al (2008). In our assays, which were dedicated to a library screen, we detected a weak transcription activation activity in the N terminus of ALF and LFY that was not detected by Chae et al (2008), possibly due to a lower sensitivity of their quantitative b-galactosidase assay. Second, we mapped the ALF domain interacting with full DOT in the N terminus (ALF 1-152 ), whereas Chae et al (2008) mapped the interaction of a truncated UFO protein to a C-terminal part ).…”

“…In our assays, which were dedicated to a library screen, we detected a weak transcription activation activity in the N terminus of ALF and LFY that was not detected by Chae et al (2008), possibly due to a lower sensitivity of their quantitative b-galactosidase assay. Second, we mapped the ALF domain interacting with full DOT in the N terminus (ALF 1-152 ), whereas Chae et al (2008) mapped the interaction of a truncated UFO protein to a C-terminal part ). The reasons for this discrepancy are unknown, but they might be due to the different break points of the ALF and LFY deletions, the opposite orientations of prey and bait, or the use of the entire DOT protein versus a truncated UFO.…”

“…On immunoblots, Chae et al (2008) observed a smear of 150-to 220-kD polyubiquitinated isoforms of LFY and a 155-kD species that does not react with anti-ubiquitin, both of which are reduced in the ufo-2 background. However, we could not detect such isoforms in petunia seedlings expressing 35S:LFY together with either 35S:UFO or 35S:DOT (see Supplemental Figure 11 online).…”

“…Recently, Chae et al (2008) reported largely complementary data, which indicate that UFO is recruited by LFY to the promoter of the B gene AP3 to promote AP3 transcription. Our yeast twohybrid results differ on several points with those of Chae et al (2008).…”