Transcriptional regulation of photoprotection in dark-to-light transition - more than just a matter of excess light energy

ABSTRACT In nature, photosynthetic organisms are exposed to different light spectra and intensities depending on the time of day and atmospheric and environmental conditions. When photosynthetic cells absorb excess light, they induce non-photochemical quenching to avoid photo-damage and trigger expression of ‘photoprotective’ genes. In this work, we used the green alga Chlamydomonas reinhardtii to assess the impact of light intensity, light quality, wavelength, photosynthetic electron transport and CO 2 on induction of the ‘photoprotective’ genes ( LHCSR1 , LHCSR3 and PSBS ) during dark-to-light transitions. Induction (mRNA accumulation) occurred at very low light intensity, was independently modulated by blue and UV-B radiation through specific photoreceptors, and only LHCSR3 was strongly controlled by CO 2 levels through a putative enhancer function of CIA5, a transcription factor that controls genes of the carbon concentrating mechanism. We propose a model that integrates inputs of independent signaling pathways and how they may help the cells anticipate diel conditions and survive in a dynamic light environment.