Search

Mesembryanthemum crystallinum, which switches the mode of photosynthesis from C3 to crassulacean acid metabolism (CAM) upon high salt stress, was shown here to exhibit diurnal changes in not only the CO2 fixation pathway but also Chl fluorescence parameters under CAM-induced conditions. We conducted comprehensive time course measurements of M. crystallinum leaf Chl fluorescence using the same leaf throughout the CAM induction period. By doing so, we were able to distinguish the effect of CAM induction from that of photoinhibition and avoid the possible effects of differences in foliar age. We found that the diurnal change in the status of electron transfer could be ascribed to the formation of a proton gradient across thylakoid membranes presumably resulting from diurnal changes in the ATP/ADP ratio reported earlier. The electron transport by actinic illumination thus became limited at the step of plastoquinol oxidation by the Cyt b6/f complex in the ‘night’ period upon CAM induction, resulting in high levels of non-photochemical quenching. The actinically induced non-photochemical quenching in the ‘night’ period correlated well with the degree of CAM induction. Chl fluorescence parameters, such as NPQ or qN, could be used as a simple indexing system for the CAM induction.

In acclimation to changing light environments, photosynthetic organisms modulate the ratio of two photosynthetic reaction centers (photosystem I and photosystem II). Two mutants, which could not modulate photosystem stoichiometry upon the shift to high light, were isolated from mutants created by random transposon mutagenesis. Measurements of chlorophyll fluorescence and analysis of the reaction center subunits of photosystem I through Western blotting in these mutants revealed that the content of photosystem I could not be suppressed under high-light condition. In these mutants, transposon was inserted to sll1961, which is homologous to the gene of gluconate operon transcriptional repressor (GntR) in Bacillus anthracis A2012. DNA microarray analysis revealed that the expression of sll1773 was drastically induced in the sll1961 mutant upon exposure to high light for 3 h. Our results demonstrate that a novel transcriptional regulator, Sll1961, and its possible target protein, Sll1773, may be responsible for the regulation of photosystem stoichiometry in response to high light.

Oxygen-evolving photosystem II (PSII) particles were purified from Chlamydomonas reinhardtii having His-tag extension at the C terminus of the CP47 protein, by a single-step Ni(2+)-affinity column chromatography after solubilization of thylakoid membranes with sucrose monolaurate. The PSII particles consisted of, in addition to intrinsic proteins, three extrinsic proteins of 33, 23 and 17 kDa. The preparation showed a high oxygen-evolving activity of 2,300-2,500 micro mol O(2) (mg Chl)(-1) h(-1) in the presence of Ca(2+) using ferricyanide as the electron acceptor, while its activity was 680-720 micro mol O(2) (mg Chl)(-1) h(-1) in the absence of Ca(2+) and Cl(-) ions. The activity was 710-820 micro mol O(2) (mg Chl)(-1) h(-1) independent of the presence or absence of Ca(2+) and Cl(-) when 2,6-dichloro-p-benzoquinone was used as the acceptor. These activities were scarcely inhibited by DCMU. The kinetics of flash-induced fluorescence decay revealed that the electron transfer from Q(A)(-) to Q(B) was significantly inhibited, and the electron transfer from Q(A)(-) to ferricyanide was largely stimulated in the presence of Ca(2+). These results indicate that the acceptor side, Q(B) site, was altered in the PSII particles but its donor side remained intact. Release-reconstitution experiments revealed that the extrinsic 23 and 17 kDa proteins were released only partially by NaCl-wash, while most of the three extrinsic proteins were removed when treated with urea/NaCl, alkaline Tris or CaCl(2). The 23 and 17 kDa proteins directly bound to PSII independent of the other extrinsic proteins, and the 33 kDa protein functionally re-bound to CaCl(2)-treated PSII which had been reconstituted with the 23 and 17 kDa proteins. These binding properties were largely different from those of the extrinsic proteins in higher plant PSII, and suggest that each of the three extrinsic proteins has their own binding sites independent of the others in the green algal PSII.

We characterized the photosynthetic properties of the pmgA mutant of Synechocystis PCC 6803, which cannot change its photosystem stoichiometry under a high-light condition (200 micromol x m(-2) x s(-1)), in order to clarify the physiological significance of the regulation of photosystem stoichiometry. We found that (1) PSII activity was inhibited more in wild-type cells on the first day under the high-light conditions than in mutant cells. (2) The growth of the mutants following the initial imposition of high light was faster than that of wild-type cells. (3) However, growth was severely inhibited in the mutants after the third day of exposure to high light. (4) The growth inhibition in the mutants under the extended high-light conditions was reversed by the addition of sublethal concentrations of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), which seemed to mimic photoinhibition of PSII. These results suggest that the main role of adjusting the photosystem stoichiometry with respect to light intensity is not to maintain efficient photosynthesis, but to down regulate electron transfer. Failure to down regulate electron flow leads to cell death under prolonged exposure to high light in this cyanobacterium.

Previously, we identified a novel gene, pmgA, as an essential factor to support photomixotrophic growth of Synechocystis species PCC 6803 and reported that a strain in which pmgA was deleted grew better than the wild type under photoautotrophic conditions. To gain insight into the role of pmgA, we investigated the mutant phenotype of pmgA in detail. When low-light-grown (20 μE m−2 s−1) cells were transferred to high light (HL [200μE m−2s−1]), pmgA mutants failed to respond in the manner typically associated with Synechocystis. Specifically, mutants lost their ability to suppress accumulation of chlorophyll and photosystem I and, consequently, could not modulate photosystem stoichiometry. These phenotypes seem to result in enhanced rates of photosynthesis and growth during short-term exposure to HL. Moreover, mixed-culture experiments clearly demonstrated that loss ofpmgA function was selected against during longer-term exposure to HL, suggesting that pmgA is involved in acquisition of resistance to HL stress. Finally, early induction ofpmgA expression detected by reverse transcriptase-PCR upon the shift to HL led us to conclude that pmgA is the first gene identified, to our knowledge, as a specific regulatory factor for HL acclimation.

Continuous wetness of leaves in the light causes a reduction in the carbon exchange rate (CER) in Phaseolus vulgaris L. [Ishibashi and Terashima (1995) Plant Cell Environ. 18: 431]. In this study, we investigated the initial cause of photoinhibition upon application of water, designated rain treatment, and we found a large decrease in the rate of electron transport through the whole chain from water to methyl viologen via PSII and PSI. In spite of the decrease in the rate of electron transport, there was no decrease in the activity of either PSI or PSII when these activities were measured separately. The intactness of PSI was also confirmed by the absence of any change in the photooxidizable amount of P-700, the reaction centre of PSI, and the intactness of PSII was confirmed by measurements of Chi fluorescence. The results suggest that the inhibition by the rain treatment, which occurs at the site between PSI and PSII, might be a novel type of photoinhibition, unlike the conventional types of photoinhibition that involve PSI and PSII.

Photoinhibition was denned originally as the decrease in photosynthetic activity that occurs upon excess illumination. The site of photoinhibiti on has generally been considered to be located in PSII. However, a novel type of photoinhibition has recently been characterized in chillingsensitive plants. This photoinhibition occurs under relatively weak illumination at chilling temperatures and the main site of damage is in PSI. The photoinhibition of PSI is initiated by the inactivation of the acceptor side, with the subsequent destruction of the reaction center and the degradation of the product of the psaB gene, which is one of the two major subunit polypeptides of the PSI reaction center complex. Chilling and oxidative stress (the presence of reactive species of oxygen) are characteristic requirements for the photoinhibition of PSI in vivo.

To clarify the way in which the light available for growth affects respiration in leaves of sun and shade plants, we examined the respiratory properties of mature leaves of Spinacia oleracea L., a sun species, and of Alocasia macrorrhiza (L.) G. Don., a shade species, that had been grown at various irradiances. In leaves of S. oleracea, the respiratory rates, on a dry mass basis, decreased with time during the night, and the higher was the growth irradiance during the day, the higher was the respiratory rate. The marked decreases in the respiratory rate during the night were accompanied by decreases in the concentration of carbohydrates in the leaves. By contrast, the respiratory rates of leaves of A. macrorrhiza were virtually constant throughout the night and the absolute rates were lower than those of S. oleracea even though the absolute value of the concentration of carbohydrates and its decrease at night resembled to those in S. oleracea. The maximum activities of respiratory enzymes were also similar to those in S. oleracea. However, the leaves of A. macrorrhiza contained less soluble protein than those of S. oleracea. These results suggest that, in S. oleracea, the concentration of carbohydrates might determine the respiratory rate while such is not the case in A. macrorrhiza. The lower respiratory rates in A. macrorrhiza might be due to a lower demand for ATP.

Although the photosynthetic reaction center is well conserved among different cyanobacterial species, the modes of metabolism, e.g. respiratory, nitrogen and carbon metabolism and their mutual interaction, are quite diverse. To explore such uniformity and diversity among cyanobacteria, here we compare the influence of the light environment on the condition of photosynthetic electron transport through Chl fluorescence measurement of six cyanobacterial species grown under the same photon flux densities and at the same temperature. In the dark or under weak light, up to growth light, a large difference in the plastoquinone (PQ) redox condition was observed among different cyanobacterial species. The observed difference indicates that the degree of interaction between respiratory electron transfer and photosynthetic electron transfer differs among different cyanobacterial species. The variation could not be ascribed to the phylogenetic differences but possibly to the light environment of the original habitat. On the other hand, changes in the redox condition of PQ were essentially identical among different species at photon flux densities higher than the growth light. We further analyzed the response to high light by using a typical energy allocation model and found that 'non-regulated' thermal dissipation was increased under high-light conditions in all cyanobacterial species tested. We assume that such 'non-regulated' thermal dissipation may be an important 'regulatory' mechanism in the acclimation of cyanobacterial cells to high-light conditions.

Stabilities of iron-sulfur centers and reaction center chlorophyll P-700 in Photosystem I reaction center complex (CP1-a), isolated by sodium dodecyl sulfate treatment from the thermophilic cyanobacterium Synechococcus elongatus, were studied by EPR and optical spectroscopy. The isolated Photosystem I reaction center complexes of S. elongatus are still highly resistant to heat