Rubisco in planta kcat is regulated in balance with photosynthetic electron transport

Bio-Inspired Microreactors Continuously Synthesize Glucose Precursor from CO2 with an Energy Conversion Efficiency 3.3 Times of Rice

Excessive CO2 and food shortage are two grand challenges of human society. Directly converting CO2 into food materials can simultaneously alleviate both, like what green crops do in nature. Nevertheless, natural photosynthesis has a limited energy efficiency due to low activity and specificity of key enzyme D-ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). To enhance the efficiency, many prior studies focused on engineering the enzymes, but this study chooses to learn from the nature to design more efficient reactors. This work is original in mimicking the stacked structure of thylakoids in chloroplasts to immobilize RuBisCO in a microreactor using the layer-by-layer strategy, obtaining the continuous conversion of CO2 into glucose precursor at 1.9 nmol min-1 with enhanced activity (1.5 times), stability (≈8 times), and reusability (96% after 10 reuses) relative to the free RuBisCO. The microreactors are further scaled out from one to six in parallel and achieve the production at 15.8 nmol min-1 with an energy conversion efficiency of 3.3 times of rice, showing better performance of this artificial synthesis than NPS in terms of energy conversion efficiency. The exploration of the potential of mass production would benefit both food supply and carbon neutralization.

Prying into the green black-box

Life-long efforts of the Tartu photosynthesis research group have been summarized. The measurements were facilitated by self-designed instruments, distinct in multifunctionality and fastresponse time. The black-box type kinetical analysis on intact leaves has revealed several physiologically significant features of leaf photosynthesis. Rubisco studies reflected competition for the active site between the substrates and products, linearizing in vivo kinetics compared with the low-K_m in vitro responses. Rubisco Activase usually activates only a small part of the Rubisco, making the rest of it a storage protein. Precisely quantifying absorbed photons and the responding transmittance changes, electron flow rates through cytochrome b₆f, plastocyanin and photosystem I were measured, revealing competition between the proton-uncoupled cyclic electron flow from PSI to Cyt b₆f to P700⁺ and the proton-coupled linear flow from PSII to Cyt b₆f to P700⁺. Analyzing responses of O₂ evolution and Chl fluorescence to ms-length light pulses we concluded that explanation of the sigmoidal fluorescence induction by excitonic connectivity between PSII units is a misconception. Each PSII processes excitation from its own antenna, but the sigmoidicity is caused by rise of the fluorescence yield of the Q_A-reduced PSII units after their Q_B site becomes occupied by reduced plastoquinone (or diuron). Unlike respiration, photosynthetic electrons must prepare their acceptor by coupled synthesis of 3ATP/4e^-. Feedback regulation of this ratio leads to oscillations under saturating light and CO₂, when the rate is P_i-limited. The slow oscillations (period 60s) indicate that the magnitudes of the deflections in the 3ATP/4e^- ratio, corrected by regulating cyclic and alternative electron flow (including the Mehler type O₂ reduction), are only a fraction of a per cent. The P_i limitation causes slip in the ATP synthase, slightly increasing the basic 12H⁺/3ATP requirement.

Post-silking 15N labelling reveals an enhanced nitrogen allocation to leaves in modern maize (Zea mays) genotypes

Nitrogen (N) metabolism is a major research target for increasing productivity in crop plants. In maize (Zea mays L.), yield gain over the last few decades has been associated with increased N absorption and utilization efficiency (i.e. grain biomass per unit of N absorbed). However, a dynamical framework is still needed to unravel the role of internal processes such as uptake, allocation, and translocation of N in these adaptations. This study aimed to 1) characterize how genetic enhancement in N efficiency conceals changes in allocation and translocation of N, and 2) quantify internal fluxes behind grain N sources in two historical genotypes under high and low N supply. The genotypes 3394 and P1197, landmark hybrids representing key eras of genetic improvement (1990s and 2010s), were grown under high and low N supply in a two-year field study. Using stable isotope ¹⁵N labelling, post-silking nitrogen fluxes were modeled through Bayesian estimation by considering the external N (exogenous-N) and the pre-existing N (endogenous-N) supply across plant organs. Regardless of N availability, P1197 exhibited greater exogenous-N accumulated in leaves and cob-husks. This response was translated to a larger amount of N mobilized to grains (as endogenous-N) during grain-filling in this genotype. Furthermore, the enhanced N supply to leaves in P1197 was associated with increased post-silking carbon accumulation. The overall findings suggest that increased N utilization efficiency over time in maize genotypes was associated with an increased allocation of N to leaves and subsequent translocation to the grains.

Assessment of Various Toxicity Endpoints in Duckweed (Lemna minor) at the Physiological, Biochemical, and Molecular Levels as a Measure of Diuron Stress

The common, broad-spectrum herbicide diuron poses some risks to the environment due to its long persistence and high toxicity. Therefore, the effective monitoring of diuron residues will inform efforts to assess its impacts on ecosystems. In this study, we evaluated the toxicity targets of diuron in the model aquatic macrophyte Lemna minor at the physiological (growth and photosynthetic efficiency), biochemical (pigment biosynthesis and reactive oxygen species (ROS) levels), and molecular (rbcL transcript) levels. The toxicity of diuron was detectable after 48 h of exposure and the order of sensitivity of toxicity endpoints was gene transcription > maximum electron transport rate (ETR_max) > non-photochemical quenching (NPQ) > maximum quantum yield (F_v/F_m) > ROS > fresh weight > chlorophyll b > chlorophyll a > total frond area > carotenoids. Under diuron stress, pigment, ROS, and gene transcript levels increased while frond area, fresh weight, and photosynthesis (F_v/F_m and ETR_max) gradually decreased with the increasing duration of exposure. Notably, ROS levels, F_v/F_m, frond area, and fresh weight were highly correlated with diuron concentration. The growth endpoints (frond area and fresh weight) showed a strong negative correlation with ROS levels and a positive correlation with F_v/F_m and ETR_max. These findings shed light on the relative sensitivity of different endpoints for the assessment of diuron toxicity.

The role of Cytochrome b6f in the control of steady-state photosynthesis: a conceptual and quantitative model

Here, we present a conceptual and quantitative model to describe the role of the Cytochrome [Formula: see text] complex in controlling steady-state electron transport in [Formula: see text] leaves. The model is based on new experimental methods to diagnose the maximum activity of Cyt [Formula: see text] in vivo, and to identify conditions under which photosynthetic control of Cyt [Formula: see text] is active or relaxed. With these approaches, we demonstrate that Cyt [Formula: see text] controls the trade-off between the speed and efficiency of electron transport under limiting light, and functions as a metabolic switch that transfers control to carbon metabolism under saturating light. We also present evidence that the onset of photosynthetic control of Cyt [Formula: see text] occurs within milliseconds of exposure to saturating light, much more quickly than the induction of non-photochemical quenching. We propose that photosynthetic control is the primary means of photoprotection and functions to manage excitation pressure, whereas non-photochemical quenching functions to manage excitation balance. We use these findings to extend the Farquhar et al. (Planta 149:78-90, 1980) model of [Formula: see text] photosynthesis to include a mechanistic description of the electron transport system. This framework relates the light captured by PS I and PS II to the energy and mass fluxes linking the photoacts with Cyt [Formula: see text], the ATP synthase, and Rubisco. It enables quantitative interpretation of pulse-amplitude modulated fluorometry and gas-exchange measurements, providing a new basis for analyzing how the electron transport system coordinates the supply of Fd, NADPH, and ATP with the dynamic demands of carbon metabolism, how efficient use of light is achieved under limiting light, and how photoprotection is achieved under saturating light. The model is designed to support forward as well as inverse applications. It can either be used in a stand-alone mode at the leaf-level or coupled to other models that resolve finer-scale or coarser-scale phenomena.

Sucrose Utilization for Improved Crop Yields: A Review Article

Photosynthetic carbon converted to sucrose is vital for plant growth. Sucrose acts as a signaling molecule and a primary energy source that coordinates the source and sink development. Alteration in source-sink balance halts the physiological and developmental processes of plants, since plant growth is mostly triggered when the primary assimilates in the source leaf balance with the metabolic needs of the heterotrophic sinks. To measure up with the sink organ's metabolic needs, the improvement of photosynthetic carbon to synthesis sucrose, its remobilization, and utilization at the sink level becomes imperative. However, environmental cues that influence sucrose balance within these plant organs, limiting positive yield prospects, have also been a rising issue over the past few decades. Thus, this review discusses strategies to improve photosynthetic carbon assimilation, the pathways actively involved in the transport of sucrose from source to sink organs, and their utilization at the sink organ. We further emphasize the impact of various environmental cues on sucrose transport and utilization, and the strategic yield improvement approaches under such conditions.

Elevated CO2 concentration induces photosynthetic down-regulation with changes in leaf structure, non-structural carbohydrates and nitrogen content of soybean

BACKGROUND

Understanding the mechanisms of crops in response to elevated CO2 concentrations is pivotal to estimating the impacts of climate change on the global agricultural production. Based on earlier results of the "doubling-CO2 concentration" experiments, many current climate models may overestimate the CO2 fertilization effect on crops, and meanwhile, underestimate the potential impacts of future climate change on global agriculture ecosystem when the atmospheric CO2 concentration goes beyond the optimal levels for crop growth.

RESULTS

This study examined the photosynthetic response of soybean (Glycine max (L.) Merr.) to elevated CO2 concentration associated with changes in leaf structure, non-structural carbohydrates and nitrogen content with environmental growth chambers where the CO2 concentration was controlled at 400, 600, 800, 1000, 1200, 1400, 1600 ppm. We found CO2-induced down-regulation of leaf photosynthesis as evidenced by the consistently declined leaf net photosynthetic rate (An) with elevated CO2 concentrations. This down-regulation of leaf photosynthesis was evident in biochemical and photochemical processes since the maximum carboxylation rate (Vcmax) and the maximum electron transport rate (Jmax) were dramatically decreased at higher CO2 concentrations exceeding their optimal values of about 600 ppm and 400 ppm, respectively. Moreover, the down-regulation of leaf photosynthesis at high CO2 concentration was partially attributed to the reduced stomatal conductance (Gs) as demonstrated by the declines in stomatal density and stomatal area as well as the changes in the spatial distribution pattern of stomata. In addition, the smaller total mesophyll size (palisade and spongy tissues) and the lower nitrogen availability may also contribute to the down-regulation of leaf photosynthesis when soybean subjected to high CO2 concentration environment.

CONCLUSIONS

Down-regulation of leaf photosynthesis associated with the changes in stomatal traits, mesophyll tissue size, non-structural carbohydrates, and nitrogen availability of soybean in response to future high atmospheric CO2 concentration and climate change.

Leaf Amino Acid Supply Affects Photosynthetic and Plant Nitrogen Use Efficiency under Nitrogen Stress

The coordinated distribution of nitrogen to source leaves and sinks is essential for supporting leaf metabolism while also supplying sufficient nitrogen to seeds for development. This study aimed to understand how regulated amino acid allocation to leaves affects photosynthesis and overall plant nitrogen use efficiency in Arabidopsis (Arabidopsis thaliana) and how soil nitrogen availability influences these processes. Arabidopsis plants with a knockout of AAP2, encoding an amino acid permease involved in xylem-to-phloem transfer of root-derived amino acids, were grown in low-, moderate-, and high-nitrogen environments. We analyzed nitrogen allocation to shoot tissues, photosynthesis, and photosynthetic and plant nitrogen use efficiency in these knockout plants. Our results demonstrate that, independent of nitrogen conditions, aap2 plants allocate more nitrogen to leaves than wild-type plants. Increased leaf nitrogen supply positively affected chlorophyll and Rubisco levels, photosynthetic nitrogen use efficiency, and carbon assimilation and transport to sinks. The aap2 plants outperformed wild-type plants with respect to growth, seed yield and carbon storage pools, and nitrogen use efficiency in both high and deficient nitrogen environments. Overall, this study demonstrates that increasing nitrogen allocation to leaves represents an effective strategy for improving carbon fixation and photosynthetic nitrogen use efficiency. The results indicate that an optimized coordination of nitrogen and carbon partitioning processes is critical for high oilseed production in Arabidopsis, including in plants exposed to limiting nitrogen conditions.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Silicate application increases the photosynthesis and its associated metabolic activities in Kentucky bluegrass under drought stress and post-drought recovery

Drought stress is the most pervasive threat to plant growth, which disrupts the photosynthesis and its associated metabolic activities, while silicate (Si) application may have the potential to alleviate the damaging effects of drought on plant growth. In present study, the role of Si in regulating the photosynthesis and its associated metabolic events in Kentucky bluegrass (cv. Arcadia) were investigated under drought stress. Drought stress and four levels (0, 200, 400, 800 mg L(-1)) of Si (Na2SiO3.9H2O) were imposed on 1-year-old plants removed from field and cultured under glasshouse conditions. After 20 days of drought stress, the plants were re-watered to reach soil field capacity for the examination of recovery on the second and the seventh day. The experiment was arranged in completely randomized design replicated four times. Drought stress severely decreased the photosynthesis, water use efficiency, stomatal conductance, cholorophyll contents, Rubisco activity, and Rubisco activation state in Kentucky bluegrass. Nevertheless, application of Si had a positive influence on all these attributes, particularly under stress conditions. As compared to control, Si application at 400 mg L(-1) recorded 78, 64, and 48 % increase in photosynthesis, Rubisco initial activity, and Rubisco total activity, respectively, at 20 days of drought. Higher photosynthesis and higher Rubisco activity in Si-applied treatments suggest that Si may have possible (direct or indirect) role in maintenance of more active Rubisco enzyme and Rubisco activase and more stable proteins for carbon assimilation under stress conditions, which needs to be elucidated in further studies.

Response and adaptation of photosynthesis, respiration, and antioxidant systems to elevated CO2 with environmental stress in plants

It is well known that plant photosynthesis and respiration are two fundamental and crucial physiological processes, while the critical role of the antioxidant system in response to abiotic factors is still a focus point for investigating physiological stress. Although one key metabolic process and its response to climatic change have already been reported and reviewed, an integrative review, including several biological processes at multiple scales, has not been well reported. The current review will present a synthesis focusing on the underlying mechanisms in the responses to elevated CO2 at multiple scales, including molecular, cellular, biochemical, physiological, and individual aspects, particularly, for these biological processes under elevated CO2 with other key abiotic stresses, such as heat, drought, and ozone pollution, as well as nitrogen limitation. The present comprehensive review may add timely and substantial information about the topic in recent studies, while it presents what has been well established in previous reviews. First, an outline of the critical biological processes, and an overview of their roles in environmental regulation, is presented. Second, the research advances with regard to the individual subtopics are reviewed, including the response and adaptation of the photosynthetic capacity, respiration, and antioxidant system to CO2 enrichment alone, and its combination with other climatic change factors. Finally, the potential applications for plant responses at various levels to climate change are discussed. The above issue is currently of crucial concern worldwide, and this review may help in a better understanding of how plants deal with elevated CO2 using other mainstream abiotic factors, including molecular, cellular, biochemical, physiological, and whole individual processes, and the better management of the ecological environment, climate change, and sustainable development.

Identification of Interactions between Abscisic Acid and Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

Abscisic acid ((+)-ABA) is a phytohormone involved in the modulation of developmental processes and stress responses in plants. A chemical proteomics approach using an ABA mimetic probe was combined with in vitro assays, isothermal titration calorimetry (ITC), x-ray crystallography and in silico modelling to identify putative (+)-ABA binding-proteins in crude extracts of Arabidopsis thaliana. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was identified as a putative ABA-binding protein. Radiolabelled-binding assays yielded a K_d of 47 nM for (+)-ABA binding to spinach Rubisco, which was validated by ITC, and found to be similar to reported and experimentally derived values for the native ribulose-1,5-bisphosphate (RuBP) substrate. Functionally, (+)-ABA caused only weak inhibition of Rubisco catalytic activity (K_i of 2.1 mM), but more potent inhibition of Rubisco activation (K_i of ~ 130 μM). Comparative structural analysis of Rubisco in the presence of (+)-ABA with RuBP in the active site revealed only a putative low occupancy (+)-ABA binding site on the surface of the large subunit at a location distal from the active site. However, subtle distortions in electron density in the binding pocket and in silico docking support the possibility of a higher affinity (+)-ABA binding site in the RuBP binding pocket. Overall we conclude that (+)-ABA interacts with Rubisco. While the low occupancy (+)-ABA binding site and weak non-competitive inhibition of catalysis may not be relevant, the high affinity site may allow ABA to act as a negative effector of Rubisco activation.

Transcriptome analysis of the variations between autotetraploid Paulownia tomentosa and its diploid using high-throughput sequencing

Timber properties of autotetraploid Paulownia tomentosa are heritable with whole genome duplication, but the molecular mechanisms for the predominant characteristics remain unclear. To illuminate the genetic basis, high-throughput sequencing technology was used to identify the related unigenes. 2677 unigenes were found to be significantly differentially expressed in autotetraploid P. tomentosa. In total, 30 photosynthesis-related, 21 transcription factor-related, and 22 lignin-related differentially expressed unigenes were detected, and the roles of the peroxidase in lignin biosynthesis, MYB DNA-binding proteins, and WRKY proteins associated with the regulation of relevant hormones are extensively discussed. The results provide transcriptome data that may bring a new perspective to explain the polyploidy mechanism in the long growth cycle of plants and offer some help to the future Paulownia breeding.

Enhanced chloroplastic generation of H2O2 in stress-resistant Thellungiella salsuginea in comparison to Arabidopsis thaliana

In order to find some basis of salinity resistance in the chloroplastic metabolism, a halophytic Thellungiella salsuginea was compared with glycophytic Arabidopsis thaliana. In control T.s. plants the increased ratios of chlorophyll a/b and of fluorescence emission at 77 K (F730 /F685 ) were documented, in comparison to A.t.. This was accompanied by a higher YII and lower NPQ (non-photochemical quenching) values, and by a more active PSI (photosystem I). Another prominent feature of the photosynthetic electron transport (PET) in T.s. was the intensive production of H2 O2 from PQ (plastoquinone) pool. Salinity treatment (0.15 and 0.30 M NaCl for A.t. and T.s., respectively) led to a decrease in ratios of chl a/b and F730 /F685 . In A.t., a salinity-driven enhancement of YII and NPQ was found, in association with the stimulation of H2 O2 production from PQ pool. In contrast, in salinity-treated T.s., these variables were similar as in controls. The intensive H2 O2 generation was accompanied by a high activity of PTOX (plastid terminal oxidase), whilst inhibition of this enzyme led to an increased H2 O2 formation. It is hypothesized, that the intensive H2 O2 generation from PQ pool might be an important element of stress preparedness in Thellungiella plants. In control T.s. plants, a higher activation state of carboxylase ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was also documented in concert with the attachment of Rubisco activase (RCA) to the thylakoid membranes. It is supposed, that a closer contact of RCA with PSI in T.s. enables a more efficient Rubisco activation than in A.t.

Control of Rubisco function via homeostatic equilibration of CO2 supply

Rubisco is the most abundant protein on Earth that serves as the primary engine of carbon assimilation. It is characterized by a slow rate and low specificity for CO2 leading to photorespiration. We analyze here the challenges of operation of this enzyme as the main carbon fixation engine. The high concentration of Rubisco exceeds that of its substrate CO2 by 2-3 orders of magnitude; however, the total pool of available carbon in chloroplast, i.e., mainly bicarbonate, is comparable to the concentration of Rubisco active sites. This makes the reactant stationary assumption (RSA), which is essential as a condition of satisfying the Michaelis-Menten (MM) kinetics, valid if we assume that the delivery of CO2 from this pool is not limiting. The RSA is supported by active carbonic anhydrases (CA) that quickly equilibrate bicarbonate and CO2 pools and supply CO2 to Rubisco. While the operation of stromal CA is independent of light reactions, the thylakoidal CA associated with PSII and pumping CO2 from the thylakoid lumen is coordinated with the rate of electron transport, water splitting and proton gradient across the thylakoid membrane. At high CO2 concentrations, CA becomes less efficient (the equilibrium becomes unfavorable), so a deviation from the MM kinetics is observed, consistent with Rubisco reaching its Vmax at approximately 50% lower level than expected from the classical MM curve. Previously, this deviation was controversially explained by the limitation of RuBP regeneration. At low ambient CO2 and correspondingly limited capacity of the bicarbonate pool, its depletion at Rubisco sites is relieved in that the enzyme utilizes O2 instead of CO2, i.e., by photorespiration. In this process, CO2 is supplied back to Rubisco, and the chloroplastic redox state and energy level are maintained. It is concluded that the optimal performance of photosynthesis is achieved via the provision of continuous CO2 supply to Rubisco by carbonic anhydrases and photorespiration.

Light-harvesting complex B7 shifts the irradiance response of photosynthetic light-harvesting regulation in leaves of Arabidopsis thaliana

The nuclear LHCB7 gene is common in higher plants, encodes a transcript that is well expressed in a subset of leaf mesophyll cells, and is associated with a protein product that is homologous to pigment-binding components of the photosystem (PS) II peripheral antenna complex. We compared the physiological properties of wild type and LHCB7-deficient leaves [DNA insertion, Arabidopsis thaliana (At) ecotype Columbia] in terms of pigment content, CO2 exchange, in vivo transmittance at 810 nm, and chlorophyll fluorescence. The latter two techniques are functional indicators for PSI and PSII, respectively. Key features of the mutant phenotype were confirmed using antisense technology and a hemizygote of two independent AtLHCB7 DNA insertion lines. Growth, leaf pigment composition, white light absorptance, and levels of AtLHCB1-6 were not significantly different in the mutant compared to wild type. Likewise, neither intrinsic PSII light capture efficiency nor partitioning of absorbed radiation to PSII was affected by the mutation. The absence of AtLHCB7 is associated with lower rates of light-saturated photosynthesis and a diminished irradiance threshold for induction of photoprotective non-photochemical quenching. Overall, the pattern of change in light utilization parameters and plastoquinol level indicated that loss of AtLHCB7 expression led to slower Rubisco turnover characterized by pH-dependent balancing of electron transport to reduced carbon assimilation capacity (photosynthetic control). No effect of AtLHCB7 genotype on xanthophyll de-epoxidation state was detected suggesting that factors in addition to lumenal pH influence zeaxanthin accumulation.

Variation in Rubisco content and activity under variable climatic factors

The main objective of the present review is to provide a compilation of published data of the effects of several climatic conditions on Rubisco, particularly its activity, state of activation, and concentration, and its influence on leaf gas exchange and photosynthesis. The environmental conditions analyzed include drought, salinity, heavy metals, growth temperature, and elevated [O3], [CO2], and ultraviolet-B irradiance. The results show conclusive evidence for a major negative effect on activity of Rubisco with increasing intensity of a range of abiotic stress factors. This decrease in the activity of Rubisco is associated with down-regulation of the activation state of the enzyme (e.g., by de-carbamylation and/or binding of inhibitory sugar phosphates) in response to drought or high temperature. On the contrary, the negative effects of low temperature, heavy metal stress (cadmium), ozone, and UV-B stress on Rubisco activity are associated with changes in the concentration of Rubisco. Notably, in response to all environmental factors, the regulation of in vivo CO2 assimilation rate was related to Rubisco in vitro parameters, either concentration and/or carboxylation, depending on the particular stress. The importance of the loss of Rubisco activity and its repercussion on plant photosynthesis are discussed in the context of climate change. It is suggested that decreased Rubisco activity will be a major effect induced by climate change, which will need to be considered in any prediction model on plant productivity in the near future.

Photosynthetic characterization of Rubisco transplantomic lines reveals alterations on photochemistry and mesophyll conductance

Improving Rubisco catalysis is considered a promising way to enhance C3-photosynthesis and photosynthetic water use efficiency (WUE) provided the introduced changes have little or no impact on other processes affecting photosynthesis such as leaf photochemistry or leaf CO2 diffusion conductances. However, the extent to which the factors affecting photosynthetic capacity are co-regulated is unclear. The aim of the present study was to characterize the photochemistry and CO2 transport processes in the leaves of three transplantomic tobacco genotypes expressing hybrid Rubisco isoforms comprising different Flaveria L-subunits that show variations in catalysis and differing trade-offs between the amount of Rubisco and its activation state. Stomatal conductance (g s) in each transplantomic tobacco line matched wild-type, while their photochemistry showed co-regulation with the variations in Rubisco catalysis. A tight co-regulation was observed between Rubisco activity and mesophyll conductance (g m) that was independent of g s thus producing plants with varying g m/g s ratios. Since the g m/g s ratio has been shown to positively correlate with intrinsic WUE, the present results suggest that altering photosynthesis by modifying Rubisco catalysis may also be useful for targeting WUE.

Chlorophyll a fluorescence responses of temperate Phaeophyceae under submersion and emersion regimes: a comparison of rapid and steady-state light curves

The potential of algae to acclimate to environmental stress is commonly assessed using chlorophyll a fluorescence, with changes in parameters of photosynthesis versus irradiance (P/E) curves measured either as rapid light curves (RLC) or steady-state light curves (LC). Here, effects of emersion on primary photosynthesis of four brown macroalgae (Ascophyllum nodosum, Fucus serratus, Sargassum muticum, Laminaria digitata) were compared by applying both RLC and LC. When LC were used, photosynthetic performance was enhanced during emersion in A. nodosum and F. serratus as shown by increases in q(P), rETR(max) and E(k). By contrast, emersion had no impact on photosynthetic parameters of S. muticum and L. digitata. Relative changes in the NPQ-rETR relationship were reduced in A. nodosum, F. serratus and S. muticum, but remained unaffected in L. digitata. As none of the species developed their potential NPQ(max), corresponding values could not be determined from RLC. Using RLC, observed photosynthetic performance of F. serratus and L. digitata was reduced upon emersion, whilst values for NPQ(max) were enhanced. Only results derived from LC provide evidence for a potential physiological adaptation of brown macroalgae to their natural habitat; it is recommended using the LC protocol to detect environmental impacts on photosynthesis.

Designing biological compartmentalization

Intracellular organization is a key factor in cell metabolism. Cells have evolved various organizational systems to solve the challenges of toxic pathway intermediates, competing metabolic reactions, and slow turnover rates. Inspired by nature, synthetic biologists have utilized proteins, nucleic acids, and lipids to construct synthetic organizational systems that mimic natural systems. Many of these systems have been applied to metabolic pathways and shown to significantly increase the production of industrially and commercially important chemicals. Further engineering and characterization of synthetic organizational systems will allow us to better understand native cellular strategies of spatial organization. Here, we discuss recent advances and ongoing efforts in designing and characterizing synthetic compartmentalization systems to mimic natural strategies and increase metabolic yields of engineered pathways.

Photosynthetic acclimation in relation to nitrogen allocation in cucumber leaves in response to changes in irradiance

Leaves deep in canopies can suddenly be exposed to increased irradiances following e.g. gap formation in forests or pruning in crops. Studies on the acclimation of photosynthesis to increased irradiance have mainly focused on the changes in photosynthetic capacity (A(max)), although actual irradiance often remains below saturating level. We investigated the effect of changes in irradiance on the photosynthesis irradiance response and on nitrogen allocation in fully grown leaves of Cucumis sativus. Leaves that fully developed under low (50 µmol m⁻² s⁻¹) or moderate (200 µmol m⁻² s⁻¹) irradiance were subsequently exposed to, respectively, moderate (LM-leaves) or low (ML-leaves) irradiance or kept at constant irradiance level (LL- and MM-leaves). Acclimation of photosynthesis occurred within 7 days with final A(max) highest in MM-leaves, lowest in LL-leaves and intermediate in ML- and LM-leaves, whereas full acclimation of thylakoid processes underlying photosystem II (PSII) efficiency and non-photochemical quenching occurred in ML- and LM-leaves. Dark respiration correlated with irradiance level, but not with A(max). Light-limited quantum efficiency was similar in all leaves. The increase in photosynthesis at moderate irradiance in LM-leaves was primarily driven by nitrogen import, and nitrogen remained allocated in a similar ratio to Rubisco and bioenergetics, while allocation to light harvesting relatively decreased. A contrary response of nitrogen was associated with the decrease in photosynthesis in ML-leaves. Net assimilation of LM-leaves under moderate irradiance remained lower than in MM-leaves, revealing the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to.

The mechanistic basis of internal conductance: a theoretical analysis of mesophyll cell photosynthesis and CO2 diffusion

Photosynthesis is limited by the conductance of carbon dioxide (CO(2)) from intercellular spaces to the sites of carboxylation. Although the concept of internal conductance (g(i)) has been known for over 50 years, shortcomings in the theoretical description of this process may have resulted in a limited understanding of the underlying mechanisms. To tackle this issue, we developed a three-dimensional reaction-diffusion model of photosynthesis in a typical C(3) mesophyll cell that includes all major components of the CO(2) diffusion pathway and associated reactions. Using this novel systems model, we systematically and quantitatively examined the mechanisms underlying g(i). Our results identify the resistances of the cell wall and chloroplast envelope as the most significant limitations to photosynthesis. In addition, the concentration of carbonic anhydrase in the stroma may also be limiting for the photosynthetic rate. Our analysis demonstrated that higher levels of photorespiration increase the apparent resistance to CO(2) diffusion, an effect that has thus far been ignored when determining g(i). Finally, we show that outward bicarbonate leakage through the chloroplast envelope could contribute to the observed decrease in g(i) under elevated CO(2). Our analysis suggests that physiological and anatomical features associated with g(i) have been evolutionarily fine-tuned to benefit CO(2) diffusion and photosynthesis. The model presented here provides a novel theoretical framework to further analyze the mechanisms underlying diffusion processes in the mesophyll.

The rate of nitrite reduction in leaves as indicated by O₂ and CO₂ exchange during photosynthesis

Light response (at 300 ppm CO(2) and 10-50 ppm O(2) in N(2)) and CO(2) response curves [at absorbed photon fluence rate (PAD) of 550 μmol m(-2) s(-1)] of O(2) evolution and CO(2) uptake were measured in tobacco (Nicotiana tabacum L.) leaves grown on either NO(3)(-) or NH(4)(+) as N source and in potato (Solanum tuberosum L.), sorghum (Sorghum bicolor L. Moench), and amaranth (Amaranthus cruentus L.) leaves grown on NH(4)NO(3). Photosynthetic O(2) evolution in excess of CO(2) uptake was measured with a stabilized zirconia O(2) electrode and an infrared CO(2) analyser, respectively, and the difference assumed to represent the rate of electron flow to acceptors alternative to CO(2), mainly NO(2)(-), SO(4)(2-), and oxaloacetate. In NO(3)(-)-grown tobacco, as well as in sorghum, amaranth, and young potato, the photosynthetic O(2)-CO(2) flux difference rapidly increased to about 1 μmol m(-2) s(-1) at very low PADs and the process was saturated at 50 μmol quanta m(-2) s(-1). At higher PADs the O(2)-CO(2) flux difference continued to increase proportionally with the photosynthetic rate to a maximum of about 2 μmol m(-2) s(-1). In NH(4)(+)-grown tobacco, as well as in potato during tuber filling, the low-PAD component of surplus O(2) evolution was virtually absent. The low-PAD phase was ascribed to photoreduction of NO(2)(-) which successfully competes with CO(2) reduction and saturates at a rate of about 1 μmol O(2) m(-2) s(-1) (9% of the maximum O(2) evolution rate). The high-PAD component of about 1 μmol O(2) m(-2) s(-1), superimposed on NO(2)(-) reduction, may represent oxaloacetate reduction. The roles of NO(2)(-), oxaloacetate, and O(2) reduction in the regulation of ATP/NADPH balance are discussed.

Physiological and morphological adaptations in relation to water use efficiency in Mediterranean accessions of Solanum lycopersicum

The physiological traits underlying the apparent drought resistance of 'Tomàtiga de Ramellet' (TR) cultivars, a population of Mediterranean tomato cultivars with delayed fruit deterioration (DFD) phenotype and typically grown under non-irrigation conditions, are evaluated. Eight different tomato accessions were selected and included six TR accessions, one Mediterranean non-TR accession (NTR(M)) and a processing cultivar (NTR(O)). Among the TR accessions two leaf morphology types, normal divided leaves and potato-leaf, were selected. Plants were field grown under well-watered (WW) and water-stressed (WS) treatments, with 30 and 10% of soil water capacity, respectively. Accessions were clustered according to the leaf type and TR phenotype under WW and WS, respectively. Correlation among parameters under the different water treatments suggested that potential improvements in the intrinsic water-use efficiency (A(N)/g(s)) are possible without negative impacts on yield. Under WS TR accessions displayed higher A(N)/g(s), which was not due to differences in Rubisco-related parameters, but correlated with the ratio between the leaf mesophyll and stomatal conductances (g(m)/g(s)). The results confirm the existence of differential traits in the response to drought stress in Mediterranean accessions of tomato, and demonstrate that increases in the g(m)/g(s) ratio would allow improvements in A(N)/g(s) in horticultural crops.

Rubisco activity in Mediterranean species is regulated by the chloroplastic CO2 concentration under water stress

Water stress decreases the availability of the gaseous substrate for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) by decreasing leaf conductance to CO(2). In spite of limiting photosynthetic carbon assimilation, especially in those environments where drought is the predominant factor affecting plant growth and yield, the effects of water deprivation on the mechanisms that control Rubisco activity are unclear. In the present study, 11 Mediterranean species, representing different growth forms, were subject to increasing levels of drought stress, the most severe one followed by rewatering. The results confirmed species-specific patterns in the decrease in the initial activity and activation state of Rubisco as drought stress and leaf dehydration intensified. Nevertheless, all species followed roughly the same trend when Rubisco activity was related to stomatal conductance (g(s)) and chloroplastic CO(2) concentration (C(c)), suggesting that deactivation of Rubisco sites could be induced by low C(c), as a result of water stress. The threshold level of C(c) that triggered Rubisco deactivation was dependent on leaf characteristics and was related to the maximum attained for each species under non-stressing conditions. Those species adapted to low C(c) were more capable of maintaining active Rubisco as drought stress intensified.