Effect of Chlamydomonas plastid terminal oxidase 1 expressed in tobacco on photosynthetic electron transfer

The plastid terminal oxidase PTOX is a plastohydroquinone:oxygen oxidoreductase that is important for carotenoid biosynthesis and plastid development. Its role in photosynthesis is controversially discussed. Under a number of abiotic stress conditions, the protein level of PTOX increases. PTOX is thought to act as a safety valve under high light protecting the photosynthetic apparatus against photodamage. However, transformants with high PTOX level were reported to suffer from photoinhibition. To analyze the effect of PTOX on the photosynthetic electron transport, tobacco expressing PTOX‐1 from Chlamydomonas reinhardtii (Cr‐PTOX1) was studied by chlorophyll fluorescence, thermoluminescence, P700 absorption kinetics and CO₂ assimilation. Cr‐PTOX1 was shown to compete very efficiently with the photosynthetic electron transport for PQH₂. High pressure liquid chromatography (HPLC) analysis confirmed that the PQ pool was highly oxidized in the transformant. Immunoblots showed that, in the wild‐type, PTOX was associated with the thylakoid membrane only at a relatively alkaline pH value while it was detached from the membrane at neutral pH. We present a model proposing that PTOX associates with the membrane and oxidizes PQH₂ only when the oxidation of PQH₂ by the cytochrome b₆f complex is limiting forward electron transport due to a high proton gradient across the thylakoid membrane.     

Photosynthesis, leaf conductance and water relations of in vitro cultured grapevine rootstock in relation to acclimatisation

Leaf net CO₂ uptake and leaf photosynthetic capacity were investigated in micropropagated 41B grapevine rootstock (Vitis vinifera‘Chasselas’×Vitis berlandieri, Mill. De Gr.) plants grown in the presence of four sucrose concentrations (6.25, 12.5, 25.0 or 37.5 g l^?1). Sucrose concentration in the medium during growth in vitro did not affect the leaf photosynthetic performance of plants neither before nor after transplantation. The maximum photosynthetic rate, measured as CO₂-dependent O₂ evolution, was 7.3 µmol m^?2 s^?1 before transplanting and 15.4 µmol m^?2 s^?1 one month after transplantation. The maximum quantum yield of O₂ evolution (on the basis of incident light) was about 0.07 for all sucrose treatments both before and after transplantation. Dry biomass before transplanting was highest in plants grown with 25.0 or 37.5 g l^?1 sucrose in the medium. One month after transplantation the highest dry biomass was also observed for the same treatments. Survival of plants was 100% for all treatments. Leaf conductance to water vapour was always higher in plants before than after transplantation. Both before and after transplanting it increased with increasing light intensity and decreased slightly with increasing CO₂ molar ratio in in vitro plants. Stomata of plants before transplantation were unresponsive to vapour pressure deficit. In vitro plants experience an acute water stress when they are maintained with the whole root system in water and exposed to ambient controlled conditions in a growth chamber. However, there was no wilting of the leaves when similar plants with roots cut off were left in the same conditions. Hydraulic conductivity was low at both root and shoot-root connection levels. It is likely that water supply could be limiting during transplantation because of the low root and root-stem connection conductivity. Water uptake by roots rather than water loss from the shoots would be of primary importance for the maintenance of water balance during acclimatisation.     

Spatial,Temporal, and Habitat-Related Variation in Abundance of Pelagic Fishes in the Gulf of Mexico: Potential Implications of the Deepwater Horizon Oil Spill

Time-series data collected over a four-year period were used to characterize patterns of abundance for pelagic fishes in the northern Gulf of Mexico (GoM) before (2007–2009) and after (2010) the Deepwater Horizon oil spill. Four numerically dominant pelagic species (blackfin tuna, blue marlin, dolphinfish, and sailfish) were included in our assessment, and larval density of each species was lower in 2010 than any of the three years prior to the oil spill, although larval abundance in 2010 was often statistically similar to other years surveyed. To assess potential overlap between suitable habitat of pelagic fish larvae and surface oil, generalized additive models (GAMs) were developed to evaluate the influence of ocean conditions on the abundance of larvae from 2007–2009. Explanatory variables from GAMs were then linked to environmental data from 2010 to predict the probability of occurrence for each species. The spatial extent of surface oil overlapped with early life habitat of each species, possibly indicating that the availability of high quality habitat was affected by the DH oil spill. Shifts in the distribution of spawning adults is another factor known to influence the abundance of larvae, and the spatial occurrence of a model pelagic predator (blue marlin) was characterized over the same four-year period using electronic tags. The spatial extent of oil coincided with areas used by adult blue marlin from 2007–2009, and the occurrence of blue marlin in areas impacted by the DH oil spill was lower in 2010 relative to pre-spill years.     

Flexible coupling between light-dependent electron and vectorial proton transport in illuminated leaves of C3 plants. Role of photosystem I-dependent proton pumping

The role of cyclic electron transport has been re-examined in leaves of C₃ plants because the bioenergetics of chloroplasts (H⁺/e = 3 in the presence of a Q-cycle; H⁺/ATP = 4 of ATP synthesis) had suggested that cyclic electron flow has no function in C₃ photosynthesis. After light activation of pea leaves, the dark reduction of P700 (the donor pigment of PSI) following far-red oxidation was much accelerated. This corresponded to loss of sensitivity of P700 to oxidation by far-red light and a large increase in the number of electrons available to reduce P700⁺ in the dark. At low CO₂ and O₂ molar ratios, far-red light was capable of decreasing the activity of photosystem II (measured as the ratio of variable to maximal chlorophyll fluorescence, F_v/F_m) and of increasing light scattering at 535 nm and zeaxanthin synthesis, indicating formation of a transthylakoid pH gradient. Both the light-induced increase in the number of electrons capable of reducing far-red-oxidised P700 and the decline in F_v/F_m brought about by far-red in leaves were prevented by methyl viologen. Antimycin A inhibited CO₂-dependent O₂ evolution of pea leaves at saturating but not under limiting light; in its presence, far-red light failed to decrease F_v/F_m. The results indicate that cyclic electron flow regulates the quantum yield of photosystem II by decreasing the intrathylakoid pH when there is a reduction in the availability of electron acceptors at the PSI level (e.g. during drought or cold stresses). It also provides ATP for the carbon-reduction cycle under high light. Under these conditions, the Q-cycle is not able to maintain a H⁺/e ratio of 3 for ATP synthesis: we suggest that the ratio is flexible, not obligatory. Received: 23 February 1999 / Accepted: 19 August 1999     

Regulatory T Lymphocytes Are Associated with Less Aggressive Histologic Features in Microsatellite-Unstable Colorectal Cancers

Background

Colorectal cancers (CRCs) with microsatellite instability (MSI) are associated with a good prognosis and a high density of tumor-infiltrating lymphocytes (TILs). We have undertaken to determine the link between TIL densities and MSI CRC histologic features.

Patients and Methods

Using tissue microarrays, T-cell sub-population infiltration, including T cells (CD3), cytotoxic T cells (CD8) and regulatory T cells (FoxP3) were studied in 86 MSI CRCs. We separately analyzed TILs of the stromal and epithelial compartments in the tumor center, the tumoral invasion margin and associated normal tissue.

Results

For FoxP3+ TIL density in the tumor center stromal compartment, we found a strong negative correlation with T4 stage (p = 0.01), node invasion (p<0.001) and VELIPI (vascular emboli, lymphatic invasion and perinervous invasion) criteria (p = 0.002).

Conclusion

The strong correlation between regulatory T cell density and the absence of VELIPI criteria suggests that this sub-group of T cells is preferentially associated with less invasive tumors.     

Photoinhibition of Photosynthesis in Broken Chloroplasts as a Function of Electron Transfer Rates during Light Treatment

Photoinhibition was studied in osmotically broken chloroplasts isolated from spinach leaves (Spinacia oleracea L.). Both whole chain electron transport (measured as ferricyanide-dependent O₂ evolution in the presence of NH₄Cl) and photosystem II activity (measured as O₂ evolution in the presence of either silicomolybdate plus 3-(3,4-diphenyl)-1,1 dimethylurea or parabenzoquinone) showed similar decreases in activity in response to a photoinhibitory treatment (8 minutes of high light given in the absence of an electron acceptor other than O₂). Photosystem I activity was less affected. Photoinhibition of silicomolybdate reduction was largely reversible by an 8 minute dark incubation following the light treatment. Decreasing the O₂ concentration during photoinhibition below 2% increased photoinhibition of whole chain electron transport. Addition of superoxide dismutase to the reaction medium did not affect photoinhibition. Photoinhibition of both photosystem I and photosystem II activity increased as the rate of electron transfer during the treatment increased, and was largely prevented when 3-(3,4-diphenyl)-1,1-dimethylurea was present during the photoinhibition period. Noncyclic photophosphorylation was decreased as a consequence of whole chain electron transfer photoinhibition. Since diphenyl carbazide added after light treatment did not relieve photoinhibition of dichlorophenol indophenol reduction, we conclude that the site of inhibition is located within or near the photosystem II reaction center.     

Photochemical efficiency of Photosystem II and xanthophyll cycle components in Zea mays leaves exposed to water stress and high light

The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 µmol m^–2 s^–1) on the photochemical efficiency of Photosystem II (PS II) (measured as variable to maximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters were followed under the same light conditions in well-hydrated leaves maintained either in normal or CO₂-free air. The net CO₂ assimilation of dehydrated leaves declined rapidly as their relative water content (RWC) decreased from 100 to 60% while the PS II efficiency measured after a prolonged dark period of 16 h declined only when RWC leaves was lower than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained elevated level of zeaxanthin and antheraxanthin at the end of the long dark period. The leaf water deficit enhanced the sensitivity of PS II efficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to control leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previous PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxidation led to an accumulation of antheraxanthin in dehydrated leaves. All these drought effects on PS II efficiency and xanthophyll cycle components were also obtained in well-hydrated leaves by short-term CO₂ deprivation during illumination. We conclude that the increased susceptibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO₂ availability and the resulting low net CO₂ assimilation.