CONSTANS (CO). The proteins SPA1 and CO interact in vitro and in vivo, and we therefore propose that SPA1 might be involved in the dark-dependent degradation of the CONSTANS protein. Apart from controlling flowering time and seedling photo- morphogenesis, SPA proteins also regulate elongation growth of adult plants. In these three processes, the four SPA genes have overlapping but distinct functions. An analysis of SPA transcript levels suggests that differences in SPA gene expression patterns contribute to divergence in SPA1–SPA4 function. Thus, the regulation of SPA expression could be crucial in the adjustment of plant growth and development to changes in the light environment. doi:10.1016/j.cbpa.2007.01.504 P2.13 Dissecting and reconstructing the multi-loop mechanism of the circadian clock A. Millar, D. Rand, I. Goryanin, T. Saithong, K Edwards, (University of Edinburgh, United Kingdom) Circadian clocks control 24-hour rhythms, including jet lag and cancer risk in humans, photosynthesis and flowering time in plants. The known clock mechanisms include complex gene circuits with interlocking negative feedback loops among 10– 20 genes and proteins. The protein sequences differ widely among taxonomic groups. Genome-wide, 10–15% of RNAs are targets for circadian regulation (Edwards, Plant Cell 2006). Mis-timing of circadian rhythms under light-dark cycles can cause 50% deficit in the growth of Arabidopsis in the lab (Dodd, Science 2005), so correct temporal regulation sig- nificantly enhances growth rates. The clock is a potential target for crop improvement, and may have been subject to past selection in crop varieties that are adapted to specific locations. My group has identified clock genes in Arabidopsis thaliana, using the luciferase (LUC) reporter to identify clock mutants. We used differential equations constrained by timeseries data to model the plant clock and the photoperiod sensor, allowing us to predict the properties of unidentified clock components. One of the new components was identified by experiment, validating its location in the model (Locke, Mol Syst Biol 2005 and 2006). Comparative analysis of clock models indicates that multi-loop structures allow more flexible evolution, providing a general explanation for the clock complexity (Rand, Interface 2004). We are now studying how the plant clock gene circuit confers flexible timing in the face of seasonal changes in photoperiod. I will discuss new results from joint, theoretical and experimental approaches to study this aspect of the circadian system in plants. doi:10.1016/j.cbpa.2007.01.505 P2.14 Insights into the mechanisms underlying the light control of leaf and chloroplast development E. Lopez-Juez, E Dillon, A. Hills, P. Devlin, L. Bogre, (Royal Holloway, University of London, United States); G. Beemster, (Ghent University, Belgium) Many gymnosperm seedlings green in the dark, due partly to the light-independent expression of photosynthetic nuclear genes. Promoter activity of one such gene (Lhcb) from pine has been shown to be light-independent when expressed in tobacco. Through the use of inhibitors we have observed that such expression requires functional plastids, in the light and the dark, and is therefore under the control of signals of plastid-nuclear communication. Lhcb is also plastid signal- dependent in pine, and plastid biogenesis mutations are epistatic to cop1 and det1 mutations with regards photo- synthetic gene expression in Arabidopsis. We hypothesise that light-regulated photosynthetic gene expression evolved subsequent to, and possibly derived from, plastid signal regulation. In an attempt to understand the light de-repression of meristem activity and leaf development, we have monitored cell cycle activity and carried out a whole transcriptome analysis of the deetiolation process in dissected seedling apices, highly- enriched in shoot apical meristems, and separately in cotyledons. Our results show the synchronous activation of a large number of cell cycle structural and regulatory genes, as well as growth- associated genes (involved in global translation processes), and show similarities and differences between meristems and cotyledons. A number of candidate regulatory processes were identified, some of which include hormone homeostasis or function. doi:10.1016/j.cbpa.2007.01.506 P2.15 Early nuclear and cytoplasmic events in phytochrome signaling C. Fankhauser, (University of Lausanne, Switzerland) Growth and development are influenced by both genetic and environmental factors. The effect of the environment is particularly apparent in the sessile plants. Being photoauto- trophic, plants are exquisitely sensitive to changing light conditions. Molecular genetic studies in the model plant Ara- bidopsis thaliana have identified 3 photoreceptor families that are present in all higher plants: the blue light sensing cryptochromes and phototropins, and the phytochromes that maximally absorb red (R) and far-red (FR) light (cry1-cry3, phot1, phot2, phyA-phyE in Arabidopsis). Phytochromes are synthesized as Pr (R light absorbing); upon light excitation they are photo-transformed into Pfr (FR light absorbing), which is the active conformer in most cases. Photon capture by these S229 Abstracts / Comparative Biochemistry and Physiology, Part A 146 (2007) S225–S233