Seminar with guests from Department of Biochemistry, University of Turku – University of Copenhagen

Copenhagen Plant Science Centre > Event calendar > 2014 > Seminar with guests fr...

Seminar with guests from Department of Biochemistry, University of Turku

Eva-Mari Aro, Marjaana Suorsa and Sari Järvi from Department of Biochemistry, University of Turku visit CPSC and give talks at at seminar on 25 November 2014.

Sari JarviSari Järvi: Photosystem I assembly is assisted by FtsH in Arabidopsis. Thylakoid membrane-bound FtsH proteases have a well-characterized role in degradation of the photosystem (PS)II reaction center protein D1 under high intensity light. Here we used FtsH5 (var1) and FtsH2 (var2) mutants to reveal how the deficiency of FtsH alters the photosynthetic electron transfer chain under standard growth conditions. Most drastic consequence of the deficiency of FtsH5 or FtsH2 was a decrease in the amount and activity of PSI centers as compared to wild type. Such a defect can originate either from the insufficient biosynthesis or from the photoinhibition and degradation of PSI. In-depth characterization revealed that PSI biosynthesis was defective in var as compared to the wild type plants. Our results provide evidence that the primary role of FtsH2 and FtsH5 proteins under optimal growth conditions is linked to the de novo assembly of PSI. 

Marjaana SuorsaMarjaana Suorsa: Dynamics of Photosystem I in light acclimation - role of the pigment-protein megacomplexes and the LHCII docking site regulation. Redox regulated thylakoid protein phosphorylation with concomitant rearrangements of the thylakoid pigment-protein complexes constitute an important light acclimation mechanism of the photosynthetic apparatus. Particularly the interphases of the grana thylakoids and stroma-exposed thylakoids (grana margins) are highly important for the dynamic light acclimation. Dynamics of thylakoid pigment-protein complexes was addressed by exposure of Arabidopsis wild type (WT) as well as the stn7 and tap38/pph1 mutants, deficient in phosphorylation and dephosphorylation of LHCII, respectively, to varying light intensities.

Subsequent isolation of thylakoid non-appressed regions with mild anionic detergent digitonin revealed distinct differences in the pattern of the pigment-protein complexes between WT and stn7. Particularly the high molecular mass thylakoid megacomplexes, composed of both PSII and PSI, showed distinct light intensity dependent dynamics in WT, being dependent on the level of LHCII phosphorylation. The PsaL protein that forms a docking site in PSI for the attachment of phosphorylated LHCII, appeared highly dynamic in quantity: it was constitutively up-regulated in stn7 and down-regulated in tap38/pph1 and in WT rapidly responded to a shift of plants from darkness to low light.

It is concluded that besides reversible phosphorylation of LHCII, also dynamic regulation of the PsaL protein is crucial for the formation of the stable PSI-LHCII complexes, which fine-tune balanced excitation energy distribution between the two photosystems upon changing light conditions. The essential role of the proper formation of PSI-LHCII complexes for the balanced function of the photosynthetic machinery was verified by determining the PSI / PSII ratios with EPR spectroscopy. All mutants with defects in formation of the PSI-LHCII complexes (stn7, tap38/pph1, psal and amilhcb2) showed a clearly lower PSI / PSII ratio as compared to WT, both under constant growth light and under fluctuating light conditions.

Eva-Mari AroEva-Mari Aro: Cyanobacteria photosynthesis for solarfuel production. The beauty of oxygenic photosynthetic organisms relies in their capacity to oxidize water molecules by using solar energy and in storing the energy of released electrons in high energy bonds of organic molecules. Photosynthetic light-harvesting and electron transfer reactions are the key factors that specify the photon conversion efficiency (PCE) of oxygenic photosynthetic organisms. Light, under aerobic environments, is an elusive substrate of photosynthesis and thus not only an efficient electron transfer but also efficient photoprotection mechanisms are of crucial importance for photosynthetic organisms.

Electron transfer reactions via the two photosystems have remained very similar during evolution from cyanobacteria to higher plants. On the contrary, the regulation of light harvesting and particularly the electron flow from water to terminal acceptors have experienced remarkable changes during evolution of photosynthetic organisms. In order to understand how the PCE can be enhanced in cyanobacteria for production of sustainable target solar fuels, the basic electron transfer pathways and their photoprotection mechanisms have to be mapped upon varying environmental cues and scenarios have to be developed for maximization of PCE in future bioreactor conditions.