Design principles of plant photosensory networks : quantitative analysis and modelling of phytochrome dimer dynamics in Arabidopsis thaliana Zusammenfassung: For plants as sessile organisms the acclimatisation to environmental fluctuations is the most essential critical and at the same time challenging process to evolve plasticity (Smith 1982 2000). Such acclimatisation processes require refined sensitive mechanisms including a variety of different light sensing photoreceptors (Smith 1982; Montgomery and Lagarias 2002). Phytochromes are the most important photoreceptors in plants regulating several essential developmental changes like seed germination and de etiolation as well as stem elongation (Figure 1.1). However the two major phytochromes phyA and phyB exhibit dramatic differences in their action spectra for physiological responses with phyB being most active in red light and phyA in far red light (Casal et al. 1998). These differences cannot be explained by the photo physical properties of the phytochromes because these are virtually identical for all phytochromes (Eichenberg et al. 2000)