Diatom thylakoids contain much higher amounts of sulfoquinovosyl diacylglycerol (SQDG) than vascular plants and the hypothesis was brought forward that this relates to their special thylakoid structure. To test this hypothesis we created knock-down mutants in Thalassiosira pseudonana that exhibited a decreased SQDG content per cell. Surprisingly, the ratio between the different lipid classes did not change, pointing to strict regulation of thylakoid lipid composition. The antenna proteins, fucoxanthin-chlorophyll proteins (FCP), were reduced and photosystem (PS) I compared to PSII was increased as judged from absorbance spectra. CD spectroscopy indicated a tighter packing of chromophores. The reduction in FCP might help to avoid diametral changes in excitation energy transfer. In contrast, the increase in PSI in the mutants might counteract the diminishment of the usually huge PSI antenna. No changes in thylakoid structure were observed since the stoichiometry between different lipid classes seems to be carefully balanced.
PSBO is an essential subunit of the oxygen-evolving complex and we recently demonstrated that its lifetime depends on environmental conditions in Chlamydomonas reinhardtii. To assess PSBO lifetime with a high time resolution, we employed (1) a microfluidic platform enabling the trapping of single cells and the parallel measurement of photosynthetic activity, and (2) a nitrate-inducible PSBO amiRNA line. Our microfluidic platform allowed the rapid replacement of the nutrient solution necessary for induction. It also enabled the precise monitoring of the decline in the Fv/Fm value, reflecting PSBO loss. We found that in the dark, at medium and high light intensity, the Fv/Fm value decreased with halftimes of about 25, 12.5, and 5 h, respectively. We also observed that photosynthetic activity was better sustained upon carbon limitation. In the absence of acetate, the halftimes of Fv/Fm diminishment doubled to quadrupled compared with the control, acetate-supplied cultures.
The review highlights the relationship between the molecular organization of the light-harvesting complex of photosystem II (LHCII) and sunlight utilization by higher plants. The molecular form of LHCII switches rapidly and reversibly during diurnal changes of light intensity, from low (ca. 10) to high [ca. 1,000 μmol(photon) m-2 s-1], so the sensitivity of LHCII to light may control the balance between light harvesting and photoprotection state. Our understanding and concept of this mechanism are based on the knowledge of the structure and photophysics of different LHCII molecular forms: monomer, dimer, trimer, and aggregate. It is proposed that LHCII monomers, dimers, and lateral aggregates are fundamental blocks of excess light-dissipation machinery. Trimer is exceptionally well suited to play a physiological role of an antenna complex. A correlation between the LHCII molecular form and the presence of xanthophyll cycle pigment violaxanthin and zeaxanthin in the complex structure is also shown. Moreover, the role of LHCII protein phosphorylation in thylakoid membrane architecture is also discussed. The dual function of LHCII has been studied in the natural thylakoid membranes of chloroplasts, in the artificial lipid-LHCII model membranes, and by suspension of LHCII in a detergent solution.
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