Photoperiod pathway

Flowering in Arabidopsis occurs much earlier in long days than in short days. Mutations that delayed flowering in long days but had little, if any, effect under short days defined genes in the long-day photoperiod pathway (Redei, 1962; Koornneef et al., 1991) . Genes involved in this photoperiod pathway include GI, CO, FD, FE, FHA, FT and FWA (Koornneef et al., 1991; figure 1.1).

The expression of GI has a circadian rhythm and mutants of the gene have altered expression of CO. GI encodes for a nuclear protein (Huq et al., 2000) . The gi mutant has low CO mRNA levels and is late flowering (Fowler et al., 1999; Park et al., 1999) . When CO is overexpressed in a gi mutant background it is able to overcome the late flowering phenotype, and is therefore downstream of GI (Suarez-Lopez et al., 2001) .

Photoperiod control

CO plays a central role in photoperiodic flowering control in Arabidopsis. (Putterill et al.,1995) . The protein contains two major regions required for its function that were identified by sequencing mutant alleles and identifying regions of homology with other proteins (Robson et al., 2001) . The CCT domain, which is approximately 60 aa long, was named because of its similarity to CO-like proteins and TIMING OF CHLOROPHYLL A/B BINDING PROTEIN 1 (TOC1) (Strayer et al., 2000; Ledger et al., 2001) . The second domain contains two zinc fingers related to B-box zinc fingers of animal proteins (Putterill et al., 1995; Robson et al., 2001) . By analogy with their function in animal proteins such as the transcription factor XNF7 and ribonucleoprotein PwA33 (Borden, 1998) , these are probably involved in protein-protein interactions (Robson et al., 2001).

Forward to photoperiod pathway pt 2