Photosynthesis Research (v.119, #1-2)

Photosynthesis and the environment by Asaph B. Cousins; Matt Johnson; Andrew D. B. Leakey (1-2).

Regional and global models of the terrestrial biosphere depend critically on models of photosynthesis when predicting impacts of global change. This paper focuses on identifying the primary data needs of these models, what scales drive uncertainty, and how to improve measurements. Overall, there is a need for an open, cross-discipline database on leaf-level photosynthesis in general, and response curves in particular. The parameters in photosynthetic models are not constant through time, space, or canopy position but there is a need for a better understanding of whether relationships with drivers, such as leaf nitrogen, are themselves scale dependent. Across time scales, as ecosystem models become more sophisticated in their representations of succession they needs to be able to approximate sunfleck responses to capture understory growth and survival. At both high and low latitudes, photosynthetic data are inadequate in general and there is a particular need to better understand thermal acclimation. Simple models of acclimation suggest that shifts in optimal temperature are important. However, there is little advantage to synoptic-scale responses and circadian rhythms may be more beneficial than acclimation over shorter timescales. At high latitudes, there is a need for a better understanding of low-temperature photosynthetic limits, while at low latitudes the need is for a better understanding of phosphorus limitations on photosynthesis. In terms of sampling, measuring multivariate photosynthetic response surfaces are potentially more efficient and more accurate than traditional univariate response curves. Finally, there is a need for greater community involvement in model validation and model-data synthesis.
Keywords: Acclimation; Database; Ecosystem model; Statistical design

Earth System Models (ESMs) aim to project global change. Central to this aim is the need to accurately model global carbon fluxes. Photosynthetic carbon dioxide assimilation by the terrestrial biosphere is the largest of these fluxes, and in many ESMs is represented by the Farquhar, von Caemmerer and Berry (FvCB) model of photosynthesis. The maximum rate of carboxylation by the enzyme Rubisco, commonly termed V c,max, is a key parameter in the FvCB model. This study investigated the derivation of the values of V c,max used to represent different plant functional types (PFTs) in ESMs. Four methods for estimating V c,max were identified; (1) an empirical or (2) mechanistic relationship was used to relate V c,max to leaf N content, (3) V c,max was estimated using an approach based on the optimization of photosynthesis and respiration or (4) calibration of a user-defined V c,max to obtain a target model output. Despite representing the same PFTs, the land model components of ESMs were parameterized with a wide range of values for V c,max (−46 to +77 % of the PFT mean). In many cases, parameterization was based on limited data sets and poorly defined coefficients that were used to adjust model parameters and set PFT-specific values for V c,max. Examination of the models that linked leaf N mechanistically to V c,max identified potential changes to fixed parameters that collectively would decrease V c,max by 31 % in C3 plants and 11 % in C4 plants. Plant trait data bases are now available that offer an excellent opportunity for models to update PFT-specific parameters used to estimate V c,max. However, data for parameterizing some PFTs, particularly those in the Tropics and the Arctic are either highly variable or largely absent.
Keywords: Rubisco; V c,max ; Leaf nitrogen; Earth System Models

Significant advances have been made over the past decades in capabilities to simulate diurnal and seasonal variation of leaf-level and canopy-scale photosynthesis in temperate and boreal forests. However, long-term prediction of future forest productivity in a changing climate may be more dependent on how climate and biological anomalies influence extremes in interannual to decadal variability of canopy ecosystem carbon exchanges. These exchanges can differ markedly from leaf level responses, especially owing to the prevalence of long lags in nutrient and water cycling. Until recently, multiple long-term (10+ year) high temporal frequency (daily) observations of canopy exchange were not available to reliably assess this claim. An analysis of one of the longest running North American eddy covariance flux towers reveals that single climate variables do not adequately explain carbon exchange anomalies beyond the seasonal timescale. Daily to weekly lagged anomalies of photosynthesis positively autocorrelate with daily photosynthesis. This effect suggests a negative feedback in photosynthetic response to climate extremes, such as anomalies in evapotranspiration and maximum temperature. Moisture stress in the prior season did inhibit photosynthesis, but mechanisms are difficult to assess. A complex interplay of integrated and lagged productivity and moisture-limiting factors indicate a critical role of seasonal thresholds that limit growing season length and peak productivity. These results lead toward a new conceptual framework for improving earth system models with long-term flux tower observations.
Keywords: Eddy covariance; Canopy photosynthesis; Spectral analysis; Carbon cycle

Most models of photosynthetic activity assume that temperature is the dominant control over physiological processes. Recent studies have found, however, that photoperiod is a better descriptor than temperature of the seasonal variability of photosynthetic physiology at the leaf scale. Incorporating photoperiodic control into global models consequently improves their representation of the seasonality and magnitude of atmospheric CO2 concentration. The role of photoperiod versus that of temperature in controlling the seasonal variability of photosynthetic function at the canopy scale remains unexplored. We quantified the seasonal variability of ecosystem-level light response curves using nearly 400 site years of eddy covariance data from over eighty Free Fair-Use sites in the FLUXNET database. Model parameters describing maximum canopy CO2 uptake and the initial slope of the light response curve peaked after peak temperature in about 2/3 of site years examined, emphasizing the important role of temperature in controlling seasonal photosynthetic function. Akaike’s Information Criterion analyses indicated that photoperiod should be included in models of seasonal parameter variability in over 90 % of the site years investigated here, demonstrating that photoperiod also plays an important role in controlling seasonal photosynthetic function. We also performed a Granger causality analysis on both gross ecosystem productivity (GEP) and GEP normalized by photosynthetic photon flux density (GEP n ). While photoperiod Granger-caused GEP and GEP n in 99 and 92 % of all site years, respectively, air temperature Granger-caused GEP in a mere 32 % of site years but Granger-caused GEP n in 81 % of all site years. Results demonstrate that incorporating photoperiod may be a logical step toward improving models of ecosystem carbon uptake, but not at the expense of including enzyme kinetic-based temperature constraints on canopy-scale photosynthesis.
Keywords: Eddy covariance; Granger causality; Gross ecosystem productivity; Light response curve; Net ecosystem exchange; Seasonal variability

Using leaf optical properties to detect ozone effects on foliar biochemistry by Elizabeth A. Ainsworth; Shawn P. Serbin; Jeffrey A. Skoneczka; Philip A. Townsend (65-76).
Efficient methods for accurate and meaningful high-throughput plant phenotyping are limiting the development and breeding of stress-tolerant crops. A number of emerging techniques, specifically remote sensing methods, have been identified as promising tools for plant phenotyping. These remote sensing methods can be used to accurately and rapidly relate variations in leaf optical properties with important plant characteristics, such as chemistry, morphology, and photosynthetic properties at the leaf and canopy scales. In this study, we explored the potential to utilize optical (λ = 500–2,400 nm) near-surface remote sensing reflectance spectroscopy to evaluate the effects of ozone pollution on photosynthetic capacity of soybean (Glycine max Merr.). The research was conducted at the Soybean Free Air Concentration Enrichment (SoyFACE) facility where we subjected plants to ambient (44 nL L−1) and elevated ozone (79–82 nL L−1 target) concentrations throughout the growing season. Exposure to elevated ozone resulted in a significant loss of productivity, with the ozone-treated plants displaying a ~30 % average decrease in seed yield. From leaf reflectance data, it was also clear that elevated ozone decreased leaf nitrogen and chlorophyll content as well as the photochemical reflectance index (PRI), an optical indicator of the epoxidation state of xanthophyll cycle pigments and thus physiological status. We assessed the potential to use leaf reflectance properties and partial least-squares regression (PLSR) modeling as an alternative, rapid approach to standard gas exchange for the estimation of the maximum rates of RuBP carboxylation (V c,max), an important parameter describing plant photosynthetic capacity. While we did not find a significant impact of ozone fumigation on V c,max, standardized to a reference temperature of 25 °C, the PLSR approach provided accurate and precise estimates of V c,max across ambient plots and ozone treatments (r 2 = 0.88 and RMSE = 13.4 μmol m−2 s−1) based only on the variation in leaf optical properties and despite significant variability in leaf nutritional status. The results of this study illustrate the potential for combining the phenotyping methods used here with high-throughput genotyping methods as a promising approach for elucidating the basis for ozone tolerance in sensitive crops.
Keywords: Air pollution; Photochemical reflectance index; Photosynthesis; Remote sensing; Rubisco; Spectroscopy

A recent resurgence of interest in formal optimisation theory has begun to improve our understanding of how variations in stomatal conductance and photosynthetic capacity control the response of whole plant photosynthesis and growth to the environment. However, mesophyll conductance exhibits similar variation and has similar impact on photosynthesis as stomatal conductance; yet, the role of mesophyll conductance in the economics of photosynthetic resource use has not been thoroughly explored. In this article, we first briefly summarise the knowledge of how mesophyll conductance varies in relation to environmental factors that also affect stomatal conductance and photosynthetic capacity, and then we use a simple analytical approach to begin to explore how these important controls on photosynthesis should mutually co-vary in a plant canopy in the optimum. Our analysis predicts that when either stomatal or mesophyll conductance is limited by fundamental biophysical constraints in some areas of a canopy, e.g. reduced stomatal conductance in upper canopy leaves due to reduced water potential, the other of the two conductances should increase in those leaves, while photosynthetic capacity should decrease. Our analysis also predicts that if mesophyll conductance depends on nitrogen investment in one or more proteins, then nitrogen investment should shift away from Rubisco and towards mesophyll conductance if hydraulic or other constraints cause chloroplastic CO2 concentration to decline. Thorough exploration of these issues awaits better knowledge of whether and how mesophyll conductance is itself limited by nitrogen investment, and about how these determinants of photosynthetic CO2 supply and demand co-vary among leaves in real plant canopies.
Keywords: Mesophyll conductance; Internal conductance; Water use efficiency; Nitrogen use efficiency; Optimisation

While interest in photosynthetic thermal acclimation has been stimulated by climate warming, comparing results across studies requires consistent terminology. We identify five types of photosynthetic adjustments in warming experiments: photosynthesis as measured at the high growth temperature, the growth temperature, and the thermal optimum; the photosynthetic thermal optimum; and leaf-level photosynthetic capacity. Adjustments of any one of these variables need not mean a concurrent adjustment in others, which may resolve apparently contradictory results in papers using different indicators of photosynthetic acclimation. We argue that photosynthetic thermal acclimation (i.e., that benefits a plant in its new growth environment) should include adjustments of both the photosynthetic thermal optimum (T opt) and photosynthetic rates at the growth temperature (A growth), a combination termed constructive adjustment. However, many species show reduced photosynthesis when grown at elevated temperatures, despite adjustment of some photosynthetic variables, a phenomenon we term detractive adjustment. An analysis of 70 studies on 103 species shows that adjustment of T opt and A growth are more common than adjustment of other photosynthetic variables, but only half of the data demonstrate constructive adjustment. No systematic differences in these patterns were found between different plant functional groups. We also discuss the importance of thermal acclimation of respiration for net photosynthesis measurements, as respiratory temperature acclimation can generate apparent acclimation of photosynthetic processes, even if photosynthesis is unaltered. We show that while dark respiration is often used to estimate light respiration, the ratio of light to dark respiration shifts in a non-predictable manner with a change in leaf temperature.
Keywords: Temperature acclimation; Carbon balance; Global change biology; Meta-analysis; Day respiration

Most plants show considerable capacity to adjust their photosynthetic characteristics to their growth temperatures (temperature acclimation). The most typical case is a shift in the optimum temperature for photosynthesis, which can maximize the photosynthetic rate at the growth temperature. These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures. In this review article, we summarize the basic differences in photosynthetic reactions in C3, C4, and CAM plants. We review the current understanding of the temperature responses of C3, C4, and CAM photosynthesis, and then discuss the underlying physiological and biochemical mechanisms for temperature acclimation of photosynthesis in each photosynthetic type. Finally, we use the published data to evaluate the extent of photosynthetic temperature acclimation in higher plants, and analyze which plant groups (i.e., photosynthetic types and functional types) have a greater inherent ability for photosynthetic acclimation to temperature than others, since there have been reported interspecific variations in this ability. We found that the inherent ability for temperature acclimation of photosynthesis was different: (1) among C3, C4, and CAM species; and (2) among functional types within C3 plants. C3 plants generally had a greater ability for temperature acclimation of photosynthesis across a broad temperature range, CAM plants acclimated day and night photosynthetic process differentially to temperature, and C4 plants was adapted to warm environments. Moreover, within C3 species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis (i.e., the photosynthetic rate at high-growth temperature divided by that at low-growth temperature was close to 1.0) than deciduous woody plants and annual herbaceous plants, indicating that photosynthetic acclimation would be particularly important in perennial, long-lived species that would experience a rise in growing season temperatures over their lifespan. Interestingly, across growth temperatures, the extent of temperature homeostasis of photosynthesis was maintained irrespective of the extent of the change in the optimum temperature for photosynthesis (T opt), indicating that some plants achieve greater photosynthesis at the growth temperature by shifting T opt, whereas others can also achieve greater photosynthesis at the growth temperature by changing the shape of the photosynthesis–temperature curve without shifting T opt. It is considered that these differences in the inherent stability of temperature acclimation of photosynthesis would be reflected by differences in the limiting steps of photosynthetic rate.
Keywords: C3 photosynthesis; C4 photosynthesis; CAM photosynthesis; Phenotypic plasticity; Temperature acclimation; Temperature adaptation

The physiological basis for genetic variation in water use efficiency and carbon isotope composition in Arabidopsis thaliana by Hsien Ming Easlon; Krishna S. Nemali; James H. Richards; David T. Hanson; Thomas E. Juenger; John K. McKay (119-129).
Ecologists and physiologists have documented extensive variation in water use efficiency (WUE) in Arabidopsis thaliana, as well as association of WUE with climatic variation. Here, we demonstrate correlations of whole-plant transpiration efficiency and carbon isotope composition (δ13C) among life history classes of A. thaliana. We also use a whole-plant cuvette to examine patterns of co-variation in component traits of WUE and δ13C. We find that stomatal conductance (g s) explains more variation in WUE than does A. Overall, there was a strong genetic correlation between A and g s, consistent with selection acting on the ratio of these traits. At a more detailed level, genetic variation in A was due to underlying variation in both maximal rate of carboxylation (V cmax) and maximum electron transport rate (Jmax). We also found strong effects of leaf anatomy, where lines with lower WUE had higher leaf water content (LWC) and specific leaf area (SLA), suggesting a role for mesophyll conductance (g m) in variation of WUE. We hypothesize that this is due to an effect through g m, and test this hypothesis using the abi4 mutant. We show that mutants of ABI4 have higher SLA, LWC, and g m than wild-type, consistent with variation in leaf anatomy causing variation in g m and δ13C. These functional data also add further support to the central, integrative role of ABI4 in simultaneously altering ABA sensitivity, sugar signaling, and CO2 assimilation. Together our results highlight the need for a more holistic approach in functional studies, both for more accurate annotation of gene function and to understand co-limitations to plant growth and productivity.
Keywords: ABI4; Carbon isotope composition; Mesophyll conductance; Photosynthetic capacity; Stomatal conductance

Guard cells regulate CO2 uptake and water loss of a leaf by controlling stomatal movement in response to environmental factors such as CO2, humidity, and light. The mechanisms by which stomata respond to red light are actively debated in the literature, and even after decades of research it is still controversial whether stomatal movement is related to photosynthesis or not. This review summarizes the current knowledge of the red-light response of stomata. A comparison of published evidence suggests that stomatal movement is controlled by the redox state of photosynthetic electron transport chain components, in particular the redox state of plastoquinone. Potential consequences for the modeling of stomatal conductance are discussed.
Keywords: Guard cell; Photosynthesis; Stomatal conductance; Redox state of plastoquinone

Given that the pathways of photosynthesis and respiration catalyze partially opposing processes, it follows that their relative activities must be carefully regulated within plant cells. Recent evidence has shown that the components of the mitochondrial electron transport chain are essential for the proper maintenance of intracellular redox gradients, to allow considerable rates of photorespiration and in turn efficient photosynthesis. Thus considerable advances have been made in understanding the interaction between respiration and photosynthesis during the last decades and the potential mechanisms linking mitochondrial function and photosynthetic efficiency will be reviewed. Despite the fact that manipulation of various steps of mitochondrial metabolism has been demonstrated to alter photosynthesis under optimal growth conditions, it is likely that these changes will, by and large, not be maintained under sub-optimal situations. Therefore producing plants to meet this aim remains a critical challenge. It is clear, however, that although there have been a range of studies analysing changes in respiratory and photosynthetic rates in response to light, temperature and CO2, our knowledge of the environmental impact on these processes and its linkage still remains fragmented. We will also discuss the metabolic changes associated to plant respiration and photosynthesis as important components of the survival strategy as they considerably extend the period that a plant can withstand to a stress situation.
Keywords: Abiotic and biotic stress; Carbon nitrogen balance; Optimal and sub-optimal growth conditions; Plant respiration; Photorespiration; Photosynthesis; Stress tolerance

Getting the most out of natural variation in C4 photosynthesis by Sarah Covshoff; Steven J. Burgess; Jana Kneřová; Britta M. C. Kümpers (157-167).
C4 photosynthesis is a complex trait that has a high degree of natural variation, involving anatomical and biochemical changes relative to the ancestral C3 state. It has evolved at least 66 times across a variety of lineages and the evolutionary route from C3 to C4 is likely conserved but not necessarily genetically identical. As such, a variety of C4 species are needed to identify what is fundamental to the C4 evolutionary process in a global context. In order to identify the genetic components of C4 form and function, a number of species are used as genetic models. These include Zea mays (maize), Sorghum bicolor (sorghum), Setaria viridis (Setaria), Flaveria bidentis, and Cleome gynandra. Each of these species has different benefits and challenges associated with its use as a model organism. Here, we propose that RNA profiling of a large sampling of C4, C3–C4, and C3 species, from as many lineages as possible, will allow identification of candidate genes necessary and sufficient to confer C4 anatomy and/or biochemistry. Furthermore, C4 model species will play a critical role in the functional characterization of these candidate genes and identification of their regulatory elements, by providing a platform for transformation and through the use of gene expression profiles in mesophyll and bundle sheath cells and along the leaf developmental gradient. Efforts should be made to sequence the genomes of F. bidentis and C. gynandra and to develop congeneric C3 species as genetic models for comparative studies. In combination, such resources would facilitate discovery of common and unique C4 regulatory mechanisms across genera.
Keywords: C4 photosynthesis; Maize; Sorghum; Flaveria ; Setaria viridis ; Cleome gynandra

Until about 10 years ago the general accepted textbook knowledge was that terrestrial C4 photosynthesis requires separation of photosynthetic functions into two specialized cell types, the mesophyll and bundle sheath cells forming the distinctive Kranz anatomy typical for C4 plants. This paradigm has been broken with the discovery of Suaeda aralocaspica, a chenopod from central Asia, performing C4 photosynthesis within individual chlorenchyma cells. Since then, three more single-cell C4 (SCC4) species have been discovered in the genus Bienertia. They are interesting not only because of their unusual mode of photosynthesis but also present a puzzle for cell biologists. In these species, two morphological and biochemical specialized types of chloroplasts develop within individual chlorenchyma cells, a situation that has never been observed in plants before. Here we review recent literature concerning the biochemistry, physiology, and molecular biology of SCC4 photosynthesis. Particularly, we focus on what has been learned in relation to the following questions: How does the specialized morphology required for the operation of SCC4 develop and is there a C3 intermediate type of photosynthesis during development? What is the degree of specialization between the two chloroplast types and how does this compare to the chloroplasts of Kranz C4 species? How do nucleus-encoded proteins that are targeted to chloroplasts accumulate differentially in the two chloroplast types and how efficient is the CO2 concentrating mechanism in SCC4 species compared to the Kranz C4 forms?
Keywords: Photorespiration; Single-cell C4 photosynthesis; Kranz; Mesophyll; Bundle sheath; Chloroplast differentiation

The circadian regulation of photosynthesis by Antony N. Dodd; Jelena Kusakina; Anthony Hall; Peter D. Gould; Mitsumasa Hanaoka (181-190).
Correct circadian regulation increases plant productivity, and photosynthesis is circadian-regulated. Here, we discuss the regulatory basis for the circadian control of photosynthesis. We discuss candidate mechanisms underpinning circadian oscillations of light harvesting and consider how the circadian clock modulates CO2 fixation by Rubisco. We show that new techniques may provide a platform to better understand the signalling pathways that couple the circadian clock with the photosynthetic apparatus. Finally, we discuss how understanding circadian regulation in model systems is underpinning research into the impact of circadian regulation in crop species.
Keywords: Circadian rhythms; Chlorophyll fluorescence; Signal transduction; Chloroplast transcription

Maintaining photosynthetic CO2 fixation via protein remodelling: the Rubisco activases by Oliver Mueller-Cajar; Mathias Stotz; Andreas Bracher (191-201).
The key photosynthetic, CO2-fixing enzyme Rubisco forms inactivated complexes with its substrate ribulose 1,5-bisphosphate (RuBP) and other sugar phosphate inhibitors. The independently evolved AAA+ proteins Rubisco activase and CbbX harness energy from ATP hydrolysis to remodel Rubisco complexes, facilitating release of these inhibitors. Here, we discuss recent structural and mechanistic advances towards the understanding of protein-mediated Rubisco activation. Both activating proteins appear to form ring-shaped hexameric arrangements typical for AAA+ ATPases in their functional form, but display very different regulatory and biochemical properties. Considering the thermolability of the plant enzyme, an improved understanding of the mechanism for Rubisco activation may help in developing heat-resistant plants adapted to the challenge of global warming.
Keywords: Rubisco; Activase; AAA+ proteins; CbbX; CO2-assimilation

Photosynthetic acclimation varies among species, which likely reveals variations at the biochemical level in the pathways that constitute carbon assimilation and energy transfer. Local adaptation and phenotypic plasticity affect the environmental response of photosynthesis. Phenotypic plasticity allows for a wide array of responses from a single individual, encouraging fitness in a broad variety of environments. Rubisco catalyses the first enzymatic step of photosynthesis, and is thus central to life on Earth. The enzyme is well conserved, but there is habitat-dependent variation in kinetic parameters, indicating that local adaptation may occur. Here, we review evidence suggesting that land plants can adjust Rubisco’s intrinsic biochemical characteristics during acclimation. We show that this plasticity can theoretically improve CO2 assimilation; the effect is non-trivial, but small relative to other acclimation responses. We conclude by discussing possible mechanisms that could account for biochemical plasticity in land plant Rubisco.
Keywords: Rubisco; Photosynthesis; Acclimation; rbcL ; rbcS

Many aspects in the regulation of photosynthetic light-harvesting of plants are still quite poorly understood. For example, it is still a matter of debate which physical mechanism(s) results in the regulation and dissipation of excess energy in high light. Many researchers agree that electronic interactions between chlorophylls (Chl) and certain states of carotenoids are involved in these mechanisms. However, in particular, the role of the first excited state of carotenoids (Car S1) is not easily revealed, because of its optical forbidden character. The use of two-photon excitation is an elegant approach to address directly this state and to investigate the energy transfer in the direction Car S1 → Chl. Meanwhile, it has been applied to a large variety of systems starting from simple carotenoid–tetrapyrrole model compounds up to entire plants. Here, we present a systematic summary of the observations obtained by two-photon excitation about Car S1 → Chl energy transfer in systems with increasing complexity and the correlation to fluorescence quenching. We compare these observations directly with the energy transfer in the opposite direction, Chl → Car S1, for the same systems as obtained in pump-probe studies. We discuss what surprising aspects of this comparison led us to the suggestion that quenching excitonic Car–Chl interactions could contribute to the regulation of light harvesting, and how this suggestion can be connected to other models proposed.
Keywords: Light-harvesting; Carotenoids; Chlorophylls; Carotenoid–phthalocyanine dyads; Energy transfer; Excitonic interactions

Chlorophyll (Chl) is an essential component of the photosynthetic apparatus. Embedded into Chl-binding proteins, Chl molecules play a central role in light harvesting and charge separation within the photosystems. It is critical for the photosynthetic cell to not only ensure the synthesis of a sufficient amount of new Chl-binding proteins but also avoids any misbalance between apoprotein synthesis and the formation of potentially phototoxic Chl molecules. According to the available data, Chl-binding proteins are translated on membrane bound ribosomes and their integration into the membrane is provided by the SecYEG/Alb3 translocon machinery. It appears that the insertion of Chl molecules into growing polypeptide is a prerequisite for the correct folding and finishing of Chl-binding protein synthesis. Although the Chl biosynthetic pathway is fairly well-described on the level of enzymatic steps, a link between Chl biosynthesis and the synthesis of apoproteins remains elusive. In this review, I summarize the current knowledge about this issue putting emphasis on protein–protein interactions. I present a model of the Chl biosynthetic pathway organized into a multi-enzymatic complex and physically attached to the SecYEG/Alb3 translocon. Localization of this hypothetical large biosynthetic centre in the cyanobacterial cell is also discussed as well as regulatory mechanisms coordinating the rate of Chl and apoprotein synthesis.
Keywords: Chlorophyll-binding proteins; Chlorophyll biosynthesis; Cyanobacteria; Tetrapyrroles; Cofactors

Light-driven photosynthetic electron transport is coupled to the movement of protons from the chloroplast stroma to the thylakoid lumen. The resulting proton motive force that is generated is used to drive the conformational rotation of the transmembrane thylakoid ATPase enzyme which converts ADP (adenosine diphosphate) and Pi (inorganic phosphate) into ATP (adenosine triphosphate), the energy currency of the plant cell required for carbon fixation and other metabolic processes. According to Mitchell’s chemiosmotic hypothesis, the proton motive force can be parsed into the transmembrane proton gradient (ΔpH) and the electric field gradient (Δψ), which are thermodynamically equivalent. In chloroplasts, the proton motive force has been suggested to be split almost equally between Δψ and ΔpH (Kramer et al., Photosynth Res 60:151–163, 1999). One of the central pieces of evidence for this theory is the existence of a steady-state electrochromic shift (ECS) absorption signal detected ~515 nm in plant leaves during illumination. The interpretation of this signal is complicated, however, by a heavily overlapping absorption change ~535 nm associated with the formation of photoprotective energy dissipation (qE) during illumination. In this study, we present new evidence that dissects the overlapping contributions of the ECS and qE-related absorption changes in wild-type Arabidopsis leaves using specific inhibitors of the ΔpH (nigericin) and Δψ (valinomycin) and separately using leaves of the Arabidopsis lut2npq1 mutant that lacks qE. In both cases, our data show that no steady-state ECS signal persists in the light longer than ~60 s. The consequences of our observations for the suggesting parsing of steady-state thylakoid proton motive force between (ΔpH) and the electric field gradient (Δψ) are discussed.
Keywords: Proton motive force; Electron transport; ATP synthesis; Electrochromic shift; Non-photochemical quenching

Variation in supramolecular organisation of the photosynthetic membrane of Rhodobacter sphaeroides induced by alteration of PufX by Kinga Sznee; Lucy I. Crouch; Michael R. Jones; Jan P. Dekker; Raoul N. Frese (243-256).
In purple bacteria of the genus Rhodobacter (Rba.), an LH1 antenna complex surrounds the photochemical reaction centre (RC) with a PufX protein preventing the LH1 complex from completely encircling the RC. In membranes of Rba. sphaeroides, RC–LH1 complexes associate as dimers which in turn assemble into longer range ordered arrays. The present work uses linear dichroism (LD) and dark-minus-light difference LD (ΔLD) to probe the organisation of genetically altered RC–LH1 complexes in intact membranes. The data support previous proposals that Rba. capsulatus, and Rba. sphaeroides heterologously expressing the PufX protein from Rba. capsulatus, produce monomeric core complexes in membranes that lack long-range order. Similarly, Rba. sphaeroides with a point mutation in the Gly 51 residue of PufX, which is located on the membrane-periplasm interface, assembles mainly non-ordered RC–LH1 complexes that are most likely monomeric. All the Rba. sphaeroides membranes in their ΔLD spectra exhibited a spectral fingerprint of small degree of organisation implying the possibility of ordering influence of LH1, and leading to an important conclusion that PufX itself has no influence on ordering RC–LH1 complexes, as long-range order appears to be induced only through its role of configuring RC–LH1 complexes into dimers.
Keywords: Photosynthesis; Rhodobacter sphaeroides ; RC–LH1 complex; PufX; Linear dichroism; Supramolecular organisation