Photosynthesis Research

Photoprotection by non-photochemical quenching is important for optimal growth and development, especially during dynamic changes of the light intensity. The main component responsible for energy dissipation is called qE. It has been proposed that qE involves the reorganization of the photosynthetic complexes and especially of Photosystem II. However, despite a number of studies, there are still contradictory results concerning the structural changes in PSII during qE induction. The main limitation in addressing this point is the very fast nature of the off switch of qE, since the illumination is usually performed in folio and the preparation of the thylakoids requires a dark period. To avoid qE relaxation during thylakoid isolation, in this work quenching was induced directly on isolated and functional thylakoids that were then solubilized in the light. The analysis of the quenched thylakoids in native gel showed only a small decrease in the large PSII supercomplexes (C2S2M2/C2S2M) which is most likely due to photoinhibition/light acclimation since it does not recover in the dark. This result indicates that qE rise is not accompanied by a structural disassembly of the PSII supercomplexes.

To study the significance of Photosystem (PS) II phosphorylation for the turnover of the D1 protein, phosphorylation was compared with the synthesis and content of the D1 protein in intact chloroplasts. As shown by radioactive labelling with [32Pi] phosphorylation of PS II polypeptides was saturated at light intensities of 125 mol m-2 s-1. Under steady state conditions, in intact chloroplasts D1 protein, once it was phosphorylated, was neither dephosphorylated nor degraded in the light. D1 protein-synthesis was measured as incorporation of [14C] leucine. As shown by non-denaturing gel-electrophoresis followed by SDS-PAGE newly synthesised D1 protein was assembled to intact PS II-centres and no free D1 protein could be detected. D1 protein-synthesis was saturated at light intensities of 500 mol m-2 s-1. The content of D1 protein stayed stable even after illumination with 5000 μmol m-2 s-1 showing that D1 protein-degradation was saturated at the same light intensities. The difference in the light saturation points of phosphorylation and of D1 protein-turnover indicates a complex regulation of D1 protein-turnover by phosphorylation. Separation of the phosphorylated and dephosphorylated D1 protein by LiDS-gelelectrophoresis combined with radioactive pulse-labelling with [14C] leucine and [32Pi] revealed that D1 protein, synthesised under steady state conditions in the light, did not become phosphorylated but instead was rapidly degraded whereas the phosphorylated form of the D1 protein was not a good substrate for degradation. According to these observations phosphorylation of the D1 protein creates a pool of PS II centres which is not involved in D1 to these observations phosphorylation of the D1 protein creates a pool of PS II centres which is not involved in D1 protein-turnover. Fractionation of thylakoid membranes confirms that the phosphorylated, non-turning over pool of PS II-centres was located in the central regions of the grana, whereas PS II-centres involved in D1 protein-turnover were found exclusively in the stroma-lamellae and in the grana-margins.

The enzymes responsible for chlorophyll biosynthesis in plants, algae and cyanobacteria are identified and described, with emphasis on their protein composition and structure, required cofactors, physical and catalytic properties, protein-protein interactions and allosteric modulation of activity. Properties and features of the pathway that enable it to operate in a coordinated way while using unstable and light-sensitive intermediates in potentially hostile biochemical environments are discussed. The evolutionary relationships and possible origins of the chlorophyll biosynthetic enzymes are also discussed.

In the field of photosynthesis research, Otto Warburg (1883–1970) is predominantly known for the role he played in the controversy that began in the late 1930s regarding the maximum quantum yield of photosynthesis, even though by that time he had already been working on the topic for more than a decade. One of Warburg’s first contributions on the subject, which dates from around 1920, is his proposal for a detailed model of photosynthesis, which he never completely abandoned, despite later overwhelming evidence in favor of alternatives. This paper presents a textual and graphical reconstruction of Warburg’s model and of his argument for its validity. Neither has received much attention in the history of science, even though the model was certainly one of the most plausible explanations of the period and therefore could not be so easily discredited.

Many herbicides inhibit the photosynthetic electron transfer in photosystem II by binding to the polypeptide D1. A point mutation in the chloroplast gene psbA, which leads to a change of the amino acid residue 264 of D1 from serine to glycine, is responsible for atrazine resistance in higher plants. We have changed serine 264 to glycine in Synechococcus PCC7942 and compared its phenotype to a mutant with a serine to alanine shift in the same position. The results show that glycine at position 264 in D1 gives rise to a similar phenotype in cyanobacteria and in higher plants, indicating a similar structure of the binding site for herbicides and for the quinone QB in the two systems. A possible mode of binding of phenyl-urea herbicides to D1 is predicted from the difference in herbicidal cross-resistance between glycine and alanine substitutions of serine 264.

An effect of desiccation (a decrease of relative water content from 97% to 10% within 35 h) on Photosystem II was studied in barley leaf segments (Hordeum vulgare L. cv. Akcent) using chlorophyll a fluorescence and thermoluminescence (TL). The O-J-I-P fluorescence induction curve revealed a decrease of FP and a slight shift of the J step to a shorter time with no change in its height. The analysis of the fluorescence decline after a saturating light flash revealed an increased portion of slow exponential components with increasing desiccation. The TL bands obtained after excitation by continuous light were situated at about –27°C (Zv band – recombination of P680+QA −), –14 °C (A band – S3QA −), +12 °C (B band – S2/3QB −) and +45 °C (C band – TyrD+QA −). The bands related to the S-states of oxygen evolving complex (A and B) were reduced by desiccation and shifted to higher and lower temperatures, respectively. In accordance with this, the band observed at about +27 °C (S2QB −) after excitation by 1 flash fired at –10 °C and band at about +20 °C (S2/3QB −) after 2 flashes decreased with increasing water deficit and shifted to lower temperatures. A new band around 5 °C appeared in both regimes of TL excitation for a relative water content of under 42% and was attributed to the Q band (S2QA −). It is suggested that under desiccation, an inhibition of the formation of S2- and S3-states in OEC occurred simultaneously with a lowering of electron transport on the acceptor side of PS II. The temperature down-shift of the TL bands obtained after the flash excitation was induced at the initial phases of water stress, indicating a decrease of the activation energy for the S2/3QB −recombination.