Perception of light
Many of the genes that show a response to day length in Arabidopsis encode proteins that are involved in the perception of light. These include the photoreceptors PhyA, PhyB and CRY2 (Johnson et al., 1994; Guo et al., 1998; Guo et al., 1999; El-Din El-Assal et al., 2001; Valverde et al., 2004) .
Late flowering mutants
cry2 (fha) mutants were first identified as late-flowering mutants in long-day/white light photoperiods (Koornneef et al., 1991; Guo et al., 1999) . PhyB is an inhibitor of floral initiation; phyB loss of function mutants are early flowering (Parks and Quail, 1993) . When grown in red light conditions PhyB acts to repress the function of CO. As co phyB double mutants are not early flowering it would seem that CO is required for the early-flowering phenotype of phyB mutants (Putterill et al., 1995) . cry2 mediates a blue light–dependent inhibition of phyB function (Guo et al., 1998; Lin et al., 1998) . Interestingly CO mRNA levels are similar in wild-type and cry2 backgrounds but FT mRNA levels are greatly reduced in these backgrounds. This suggests that cry2 may regulate CO post transcriptionally (Suarez-Lopez et al., 2001; Yanovsky and Kay, 2002) .
The regulation of CO protein stability by light also plays a major role in daylength perception. When plants are exposed to either blue or far-red light, which are known to induce flowering, an accumulation of CO protein is observed in the nucleus. However, when plants are grown in either non-inductive red-light or total darkness, then there is no accumulation of CO protein in the nucleus, probably due to degradation of the protein by ubiquitination (Valverde et al., 2004) . The photoreceptors CRY1, CRY2 and PhyA promote flowering and stabilise CO protein, and in the cry1 cry2 double mutant there is a reduction in the morning and evening peaks of CO protein abundance. Conversely PhyB both delays flowering and enhances the degradation of CO protein. There is no degradation of CO protein in the early flowering phyB mutant (Valverde et al., 2004) . Therefore, the stability of CO protein is strongly influenced by light. It is rapidly degraded in the dark, and therefore only plants growing in long-day photoperiods have stable CO protein (Valverde et al., 2004) .
Forward to Photoperiod 4