The scientific aims of our research projects are to:
- understand how phytochromes and phytochrome signalling evolved
- understand how light signals are transduced from phytochromes to downstream signalling components
- understand how light perception networks modulate light responses (e.g. shift the specificity, increase the sensitivity)
Plants do not only use light for photosynthesis but also as a source of information, which is essential to adapt growth and development to diverse and ever-changing environmental conditions. Phytochromes are the only photoreceptors in plants able to detect red (R) and far-red light (FR). They can exist in two different states, the inactive Pr form with maximal absorption in R and the biologically active Pfr form, which has an absorption peak in FR. By absorption of light the two forms reversibly convert into each other, resulting in an equilibrium with a wavelength-specific Pfr/Ptot ratio (Ptot=Pfr+Pr).
The model plant Arabidopsis thaliana contains five phytochromes, of which phyA and phyB have a general function in R and FR perception. PhyB is the main phytochrome in light grown and adult plants and measures the R:FR ratio in the environment, which is the basis for the shade avoidance response. In addition, it is essential for de-etiolation, i.e. the switch from hetero- to photoautotrophic growth in light conditions resulting in high Pfr/Ptot ratios (e.g. R or white light). In contrast, phyA is highly abundant in dark-grown seedlings and rapidly degraded in light. PhyA mediates germination and de-etiolation in response to light conditions establishing only low Pfr/Ptot ratios, i.e. in FR or very weak light of any wavelength. As such, it is essential for seedling establishment in canopy shade, where the light environment is dominated by FR.
Phytochromes localise to the cytosol in the dark and accumulate in the nucleus after activation by light, which is essential for further downstream signalling. Nuclear transport of phyA depends on the two functional homologs FHY1 and FHL (Figure 1), whereas phyB does not require FHY1/FHL for nuclear accumulation.
PhyA and phyB have virtually identical photophysical properties, based on which they are expected to have an action peak in R. Yet, phyA dependent responses are most efficiently triggered by FR, where the Pfr/Ptot ratio is roughly 40-fold lower than in R. We could show that the fast degradation of phyA in Pfr and its Pfr-specific interaction with FHY1/FHL shift the maximal response from R to FR. Thus, FR perception is a network property and not the property of a the phyA photoreceptor itself (Figure 2).