FOR 1261: Specific Light Driven Reactions in Unicellular Model Algae
Final Report Abstract
Cyanobacteria and eukaryotic unicellular phototrophs (microalgae) contribute roughly to 50% of CO2 fixation on Earth. Light represents their primary source of energy used for photosynthesis, delivers information to trigger behavioral and developmental responses and entrains their circadian clocks under the control of sensory photoreceptors. Within research group 1261, we resolved light-driven reactions including the photoreceptors mainly in two representative algae, the green biflagellate Chlamydomonas reinhardtii, as well as the diatom, Phaeodactylum tricornutum. To this end, specialists from different scientific areas worked together to integrate biophysical, physiological, biochemical, molecular biological and -omics-driven approaches. Moreover, the complete genome sequences as well as numerous molecular tools were available for both algae. Interestingly, the algae bear a large (> 20) and diverse repertoire of photoreceptors including new types and going far beyond those found in higher plants. This finding may reflect their natural habitats (freshwater and moist soil in case of C. reinhardtii and marine environment for P. tricornutum), exhibiting frequent changes in light conditions. But it may also point to phylogenetic selection processes. Both are being exposed to different light qualities and intensities throughout the water column as well as throughout the day-night cycles of the different seasons. In C. reinhardtii, the genome does not only encode a classical plant cryptochrome (pCRY, formerly CPH1), but also an animal-like CRY (aCRY) beside two DASH-type CRYs. Variances in the presence or absence of the different CRYs (plant, plant-like, animal-like and CRY-DASH) depend indeed on the species in the green and red lineages of algae. In C. reinhardtii, a phototropin (PHOT) is also present, as well as a UV-receptor and several channelrhodopsins. Some of the latter belong to the mostly unexplored group of histidin-kinase rhodopsins (HKRs), where the light sensing domain is covalently linked to signal-transduction modules. However, the red light receptor typical for higher plants, phytochrome (PHY) is not encoded by C. reinhardtii. In P. tricornutum, PHY is present, but PHOT is missing. Instead, other receptors, the aureochromes (AUREO1-4), are present bearing the same sensory unit, a LOV domain, but containing a transcription factor bZIP domain. A typical plant CRY is missing, but these algae have a plant-like CRY (CryP) beside other CRYs including a member closely related to C. reinhardtii aCRY, the so called CPF1. Major achievements towards the structures, functions and properties of the investigated photoreceptors include an allosteric regulation between LOV and bZIP in AUREOs, the red light sensitivity of aCRY in addition to blue-light as well as the underlying relevant amino acid(s) relevant for this spectral response. The broad spectral response has been also found for CryP, based in both cases on the neutral radical form of flavin. Moreover, the novel HKR1 covalently linked to a C-terminal guanylyl-cyclase effector was shown to exhibit a UVA-blue light switch. Functional analyses on the basis of knock-down/out mutants generated by specialized methods such as broad scale insertional mutagenesis followed by selective PCR, TALEN or Zinc Finger Nuclease approaches revealed central roles of specific AUREOs in the cell cycle and high light acclimation in the diatom, key roles of aCRY and pCRY in the sexual cycle of C. reinhardtii as positive (together with PHOT) regulators of germination or as negative regulators (in contrast to PHOT) for gametogenesis. Moreover, pCRY is linked not only to circadian input but seems also connected to the oscillatory system. Intriguingly, it has been also found that a photoreceptor, namely PHOT, is directly linked to photosynthesis and photoprotection.
Publications
- 2017. An update on aureochromes: phylogeny – mechanism – function. J. Plant Physiol.
Kroth, P.G., Wilhelm, C., Kottke, T.
(See online at https://doi.org/10.1016/j.jplph.2017.06.010) - 2017. Cryptochrome photoreceptors in green algae: Unexpected versatility of mechanisms and functions. J. Plant Physiol.
Kottke, T., Oldemeyer, S., Wenzel, S., Zou, Y., Mittag, M.
(See online at https://doi.org/10.1016/j.jplph.2017.05.021) - 2017. Functional proteomics of light-harvesting complex proteins under varying light-conditions in diatoms. J. Plant Physiol.
Büchel, C., Wilhelm, C., Wagner, V., Mittag, M.
(See online at https://doi.org/10.1016/j.jplph.2017.06.007) - 2017. Light driven reactions in model algae. J. Plant Physiol.
Mittag, M., Wilhelm, C.
(See online at https://doi.org/10.1016/j.jplph.2017.07.010) - 2017. PtAUREO1a and PtAUREO1b knockout mutants of the diatom Phaeodactylum tricornutum are blocked in photoacclimation to blue light. J. Plant Physiol.
Mann, M., Serif, S., Jakob, T., Kroth, P.G., Wilhelm, C.
(See online at https://doi.org/10.1016/j.jplph.2017.05.020) - 2017. Structural and evolutionary aspects of algal blue light receptors. J. Plant Physiol.
Essen, L.-O., Franz, S., Banerjee, A.
(See online at https://doi.org/10.1016/j.jplph.2017.07.005) - 2017. The cryptochrome – photolyase protein family in diatoms. J. Plant Physiol.
König, S., Juhas. M., Jäger, S., Kottke, T., Büchel, C.
(See online at https://doi.org/10.1016/j.jplph.2017.06.015) - 2017. The two parallel photocycles of the Chlamydomonas sensory photoreceptor histidine kinase rhodopsin 1. J. Plant Physiol.
Luck, M., Hegemann, P.
(See online at https://doi.org/10.1016/j.jplph.2017.07.008) - 2017. Transcriptional response of the extremophile red alga Cyanidioschyzon merolae to changes in CO2 concentrations. J. Plant Physiol.
Rademacher, N., Wrobel, T.J., Rossoni, A.W., Kurz, S., Bräutigam, A., Weber, A.P.M., Eisenhut, M.
(See online at https://doi.org/10.1016/j.jplph.2017.06.014)
