The role of Constans in the photoperiod response
CO mRNA is expressed at low levels in Arabidopsis, however in situ hybridisation performed on 8 day-old seedlings showed that CO is expressed in the shoot apical meristem and young leaves (Simon et al., 1996) , whilst in older plants it is expressed in the inflorescence and young floral buds (Coupland, 1997) . A fusion of the CO promoter to a GUS reporter gene was generated to further analyse the spatial expression (Putterill et al., 1995; An et al., 2004) . CO:GUS was expressed in the vascular tissue of the hypocotyl, the cotyledons and the leaves, and was also present in the shoot apex. Cross sections demonstrated that CO:GUS was expressed predominantly in the phloem (An et al., 2004) .
Flowering time genes
The quantity of CONSTANS expression has a rate-limiting effect upon flowering time, with greater amounts of the protein leading to earlier flowering (Samach et al., 2000) . Overexpression of CO from the 35S promoter causes an early-flowering phenotype and increased expression of the flowering-time genes FT and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1). As the addition of dexamethasone and cycloheximide results in a rapid increase of FT and SOC1 mRNA levels in transgenic plants that carry the inducible 35S::CO:GR construct, it is thought that both genes are direct targets of CO (Samach et al., 2000) . Overexpression of CO in ft or soc1 mutants causes a partial suppression of the early flowering phenotype. However 35S::CO ft soc1 plants still flower earlier than wild-type plants, suggesting that CO is able to regulate the expression of additional flowering-time genes.
Overexpression of FT and SOC1 results in extreme early flowering and complements the co mutation, confirming that they act downstream of CO (Kardailsky et al., 1999; Kobayashi et al., 1999; Borner et al., 2000; Lee et al., 2000; Onouchi et al., 2000; Samach et al., 2000; Hepworth et al., 2002) . It follows that the main route of activation in the long-day pathway is from GI to CO to FT and SOC1. A class of genes that form part of the photoperiod pathway are also components of the circadian clock (Figure 1.1), these include CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), LATE ELONGATED HYPOCOTYL (LHY), EARLY FLOWERING 3 (ELF3), TOC1 and GI. Single mutants of any of these genes cause early flowering. (Schaffer et al., 1998; Wang and Tobin, 1998; Fowler et al., 1999; Park et al., 1999; McWatters et al., 2000; Strayer et al., 2000; Hicks et al., 2001) . Photoreceptors mediate input signals to the circadian clock entraining it to oscillate with a 24Hr period. Although the photoreceptors have a strong influence upon the circadian clock, more important is the coordination of clock rhythms and light, which has a dramatic impact upon flowering time. (Roden et al., 2002; Yanovsky and Kay, 2002, 2003) . The external coincidence model proposes that there is a light sensitive phase in which flowering is promoted if a plant is exposed to light during a certain phase of the circadian clock. Arabidopsis plants are able to distinguish between photoperiod lengths, by integration of the circadian clock at the CONSTANS level.
When grown under long days there is a broad peak of CO expression starting approximately 12 hours after dawn, this peak remains high until close to dawn. The expression pattern is different in short-day growth conditions. There is a minor peak in expression after 12 hours, which rapidly disappears, followed by a larger peak in expression 20 hours after dawn (Suarez-Lopez et al., 2001; Yanovsky and Kay, 2002) . The clock regulates CO expression so that it is expressed in the dark period in short-days, yet expressed in the light period in long-day conditions (Suarez-Lopez et al., 2001; Yanovsky and Kay, 2003; Valverde et al., 2004) .
Forward to Photoperiod 3