The Integration of flowering-time pathways

Several genes play key roles in more than one flowering-time pathway, and therefore seem to integrate the activities of these pathways (Figure 1.1). Two flowering-time genes in this group are SOC1 and FT.

CO promotes the floral transition in response to daylength, through the upregulation of downstream target genes, which include SOC1 and FT (Onouchi et al., 2000; Samach et al., 2000; Hepworth et al., 2002) . The SOC1 gene encodes a MADS box protein that promotes flowering (Borner et al., 2000; Lee et al., 2000; Samach et al., 2000) . In addition to being regulated by the photoperiod pathway through CO, SOC1 is regulated by the autonomous and vernalisation pathways through FLC. FLC binds directly to a CArG box located in the SOC1 promoter, and represses SOC1 transcription (Hepworth et al., 2002 ; Chapter3). The promoter region of SOC1 that CO acts upon is different to that which FLC acts through, so that the photoperiod and vernalisation flowering-time pathways influence SOC1 independently (Samach and Coupland, 2000; Hepworth et al., 2002) ; Chapter3). The gibberellin pathway also has a positive effect on SOC1 transcription. The promoter region that GA acts through is presently unknown (Moon et al., 2003).

Like SOC1, the FT gene is strongly upregulated by CO, and is a strong promoter of flowering (Onouchi et al., 2000; Samach and Coupland, 2000) . FT is also repressed by the overexpression of FLC, although this repression appears to be indirect (Hepworth et al., 2002) . In addition to the autonomous and photoperiod pathways, the GA pathway is also likely to influence FT expression.

LFY integrates signals from both the photoperiod and GA pathways, with separate promoter regions mediating the different stimuli. (Blazquez et al., 1998; Nilsson et al., 1998; Blazquez and Weigel, 2000) . MYB33, a transcription factor that recognises an 8bp sequence of the LFY promoter is upregulated by GA. It is not yet known if MYB33 is essential for upregulation of LFY by GA (Blazquez and Weigel, 2000; Gocal et al., 2001) .

Another protein that may be a floral-pathway integrator is the MADS box transcription factor AGAMOUS-LIKE 24 (AGL24) (Yu et al., 2002; Michaels et al., 2003) . AGL24 is active in the SAM during the floral transition. Overexpression of AGL24 leads to early flowering, whilst mutation of the gene by RNA interference delays the transition to flowering. AGL24 is upregulated by the photoperiod pathway through SOC1, and then increases LFY expression (Yu et al., 2002) . In addition, the constitutive expression of AGL24 causes upregulation of SOC1, suggesting that cross talk occurs between SOC1 and AGL24 (Michaels et al., 2003) . AGL24 appears to respond to vernalisation independently of FLC (Michaels et al., 2003) . AGL24 has recently been shown to have a role in the regulation of floral meristem identity and is repressed by LFY, and AP1 (Yu et al., 2004) . This suggests that AGL24 is able to both activate and be repressed by LFY. The action of AGL24 probably depends upon the timing and spatial expression of the gene. It is also possible that the subcellular localisation of AGL24 plays a role, and this is regulated by phosphorylation.

Forward to Arabidopsis flowering-time genes in detail

©2005-2015 Plant Biology Advice - Dean Ravenscroft