Arabidopsis ͉ LHCII ͉ NPQ ͉ two-photon excitation P lants are exposed to sunlight intensities varying over several orders of magnitude during a typical day (1). Under low light conditions, almost all absorbed sunlight photons are used for the primary photosynthetic reaction steps. However, under high light conditions the photosynthetic apparatus must be protected from excess excitation energy, because it may lead to deleterious side-effects. Balancing between efficient utilization of solar energy under restrictive light conditions and dissipation of excess energy when the absorbed light exceeds the photosynthetic capacity is therefore essential for the survival and fitness of plants (2). It is known that light-induced increase of the pH gradient across the thylakoid membrane (3, 4) and the presence of the protein PsbS (5) are necessary for the down-regulation of the photosynthetic activity under excess light and that Zea is simultaneously formed from violaxanthin (Vio) through the enzymatic xanthophyll cycle (6). However, although many different studies have been undertaken to elucidate the details of this important regulation, a complete picture of its mechanisms is still missing. Several different regulation models have been proposed and indeed it cannot be excluded that different mechanisms contribute more or less to plants adaptation to varying light conditions. However, at present even the regulation site and photophysical mechanisms are unresolved, because the models are at least partly contradicting each other (5, 7-15).The most important measurable signature of plants regulation activity is its varying residual Chl fluorescence intensity (16), which is proportional to the regulated amount of excitation energy in the photosynthetic apparatus. The actual extent of adaptation-dependent quenching of Chl singlet excited state energy, known as nonphotochemical quenching (NPQ), is typically quantified by the parameterwhich reflects the reduction in the residual Chl fluorescence of plants, FЈ m , brought about by the unknown excitation energy dissipation mechanisms, in comparison to the residual Chl fluorescence observable from the completely dark adapted plant, F m , in which no photoprotective energy dissipation is operating. FЈ m and F m are usually measured using short, intense light flashes that saturate the photosynthetic reaction center chemistry. This guarantees that the observed differences in FЈ m and F m reflect only the extent of energy dissipation through photoprotective channels, without affecting the adaptation status of the plant (16). It is long known that carotenoids play an important role in the regulation mechanisms and several different types of electronic interactions between carotenoids and Chls have been proposed to play a key role as dissipation valves for excess excitation energy (9, 10, 12). However, so far it was difficult to quantify the extent of these interactions and to investigate directly their involvement in the flow of excitation energy and its regulation, especially in living...
SummaryAnabaena sp. PCC 7120 is a prototype filamentous nitrogen-fixing cyanobacterium, in which nitrogen fixation and photosynthesis are spatially separated. Recent molecular and cellular studies have established the importance of molecular exchange between cells in the filament, but the routes involved are still under investigation. Two current models propose either a continuous periplasm or direct connections between adjacent cells whose integrity requires the protein SepJ. We used electron tomography to analyze the ultrastructure of the septum between vegetative cells in the Anabaena filament and were able to visualize intercellular connections that we term 'SEPTOSOMES'. We observed that, whereas the existence of the septosome does not depend on the presence of SepJ, the spacing between the two plasma membranes of the septum was significantly decreased in a DsepJ mutant. In addition, we observed that the peptidoglycan layer of each cell enters the septum but the outer membrane does not. Thus, each cell in the filament is individually surrounded by a plasma membrane and a peptidoglycan layer, and physical cell-cell contacts are mediated by the septosome.
The photosystem II (PSII) subunit S (PsbS) plays a key role in nonphotochemical quenching, a photoprotective mechanism for dissipation of excess excitation energy in plants. The precise function of PsbS in nonphotochemical quenching is unknown. By reconstituting PsbS together with the major light-harvesting complex of PSII (LHC-II) and the xanthophyll zeaxanthin (Zea) into proteoliposomes, we have tested the individual contributions of PSII complexes and Zea to chlorophyll (Chl) fluorescence quenching in a membrane environment. We demonstrate that PsbS is stable in the absence of pigments in vitro. Significant Chl fluorescence quenching of reconstituted LHC-II was observed in the presence of PsbS and Zea, although neither Zea nor PsbS alone was sufficient to induce the same quenching. Coreconstitution with PsbS resulted in the formation of LHC-II/PsbS heterodimers, indicating their direct interaction in the lipid bilayer. Two-photon excitation measurements on liposomes containing LHC-II, PsbS, and Zea showed an increase of electronic interactions between carotenoid S 1 and Chl states, Φ CarS1−ChlCoupling , that correlated directly with Chl fluorescence quenching. These findings are in agreement with a carotenoid-dependent Chl fluorescence quenching by direct interactions of LHCs of PSII with PsbS monomers.
Recently, excitonic carotenoid-chlorophyll interactions have been proposed as a simple but effective model for the down-regulation of photosynthesis in plants. The model was proposed on the basis of quenchingcorrelated electronic carotenoid-chlorophyll interactions (Car S 1 f Chl) determined by Car S 1 two-photon excitation and red-shifted absorption bands. However, if excitonic interactions are indeed responsible for this effect, a simultaneous correlation of quenching with increased energy transfer in the opposite direction, Chl Q y f Car S 1 , should be observed. Here we present a systematic study on the correlation of Car S 1 f Chl and Chl f Car S 1 energy transfer with the occurrence of red-shifted bands and quenching in isolated LHCII. We found a direct correlation between all four phenomena, supporting our conclusion that excitonic Car S 1 -Chl interactions provide low-lying states serving as energy traps and dissipative valves for excess excitation energy.
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