Rubisco Content Research Articles

Metabolic Processes and Biological Macromolecules Defined the Positive Effects of Protein-Rich Biostimulants on Sugar Beet Plant Development.

Protein-based biostimulants (PBBs) have a positive effect on plant development, although the biological background for this effect is not well understood. Here, hydrolyzed wheat gluten (HWG) and potato protein film (PF) in two levels (1 and 2 g/kg soil) and in two different soils (low and high nutrient; LNC and HNC) were used as PBBs. The effect of these PBBs on agronomic traits, sugars, protein, and peptides, as well as metabolic processes, were evaluated on sugar beet in comparison with no treatment (control) and treatment with nutrient solution (NS). The results showed a significant growth enhancement of the plants using HWG and PF across the two soils. Sucrose and total sugar content in the roots were high in NS-treated plants and correlated to root growth in HNC soil. Traits related to protein composition, including nitrogen, peptide, and RuBisCO contents, were enhanced in PBB-treated plants (mostly for HWG and PF at 2 g/kg soil) by 100% and >250% in HNC and LNC, respectively, compared to control. The transcriptomic analysis revealed that genes associated with ribosomes and photosynthesis were upregulated in the leaf samples of plants treated with either HWG or PP compared to the control. Furthermore, genes associated with the biosynthesis of secondary metabolites were largely down-regulated in root samples of HWG or PF-treated plants. Thus, the PBBs enhanced protein-related traits in the plants through a higher transcription rate of genes related to protein- and photosynthesis, which resulted in increased plant growth, especially when added in certain amounts (2 g/kg soil). However, sucrose accumulation in the roots of sugar beet seemed to be related to the easy availability of nitrogen.

Evaluation of morpho-physiological responses and genotoxicity in Eruca sativa (Mill.) grown in hydroponics from seeds exposed to X-rays.

Due to its potential applications in cultivated plants, ionizing radiation (IR) and its effect on organisms is increasingly studied. Here we measured the effects of ionizing radiation on Eruca sativa by analyzing plants from irradiated seeds (1 and 10 Gy) grown in hydroponics. We measured several morpho-physiological traits and genotoxicity. Radiation stress induced a noticeable variability of the morpho-physiological traits highlighting decreased plant vigor. Shoot length and leaf number were significantly higher in 1 Gy-treated samples, whereas root length was significantly higher in 10 Gy treated plants. Stomata number significantly increased with IR dose, whereas both pigment and Rubisco content decreased under radiation stress. Phenol content significantly increased in 1 Gy treated samples, otherwise from total antioxidants, which were not different from control. Most results could find a feasible explanation in a hormesis-like pattern and in a decreased plant vigor under radiation stress. IR induced genotoxic damage, evaluated by ISSR markers, in 15 day old leaves; specifically, a severe decrease in the genome template stability was observed. However, a partial recovery occurred after 2 weeks, especially under the lowest dose (i.e., 1 Gy), suggesting that DNA damage detection and repair mechanisms are active. Pigment content and genotoxic damage may serve as proxies for evaluating plant responses to IR stress, since they show univocal dose-dependent trends. The use of more checkpoints for analyses and more doses over a wider range, as well as the focus on different metabolites, could help elucidate plant response in terms of morpho-physiological changes.

Ambiguous changes in photosynthetic parameters of Lemna minor L. after short-term exposure to naproxen and paracetamol: Can the risk be ignored?

Non-steroidal anti-inflammatory drugs (NSAID) are recently monitored in the aquatic environment. Naproxen (NPX), paracetamol (PCT) and their transformation products can influence the biochemical and physiological processes at the sub-cellular and cellular levels taking part in the growth and development of plants. This study aimed to compare the effects of NPX and PCT, drugs with different physico-chemical properties, on the growth and photosynthetic processes in Lemna minor during a short-term (7 days) exposure. Although duckweed took up more than five times higher amount of PCT as compared to NPX (275.88 µg/g dry weight to 43.22 µg/g when treated with 10 mg/L), only NPX limited the number of new plants by 9% and 26% under 1 and 10 mg/L, respectively, and increased their dry weight (by 18% under 10 mg/L) and leaf area per plant. A considerable (by 30%) drop in the content of photosynthetic pigments under 10 mg/L treatment by both drugs did not significantly affect the efficiency of the primary processes of photosynthesis. Values of induced chlorophyll fluorescence parameters (F0, FV/FM, ΦII, and NPQ) showed just a mild stimulation by PCT and a negative effect by NPX (by up to 10%), especially on the function of photosystem II and electron transport in both intact duckweed plants and isolated chloroplasts. Lowered efficiency of Hill reaction activity (by more than 10% under 0.1 – 10 mg/L treatments) in isolated chloroplasts suspension proved the only inhibition effect of PCT to primary photosynthetic processes. In intact plants, higher treatments (0.5 - 10 mg/L) by both NPX and PCT induced an increase in RuBisCO content. The results prove that the potential effect of various drugs on plants is hard to generalise.

Preparation of plant-based meat analogs using emulsion gels: Lipid-filled RuBisCo protein hydrogels

RuBisCo from duckweed is a sustainable source of plant proteins with a high water-solubility and good gelling properties. In this study, we examined the impact of RuBisCo concentration (9–33 wt %) and oil droplet concentration (0 to 14 wt %) on the properties of emulsion gels designed to simulate the properties of chicken breast. The color (L*a*b*), water holding capacity (WHC), textural profile analysis, shear modulus, and microstructure of the emulsion gels were measured. The gel hardness and WHC increased significantly with increasing protein concentration, reaching values equivalent to chicken breast. The lightness of the emulsion gels was less than that of chicken breast, due to the presence of pigments (such as polyphenols) in the protein. Shear modulus versus temperature measurements showed that gelation began when the protein solutions were heated to around 40 °C and then the gels hardened appreciably when the temperature was further raised to 90 °C. The shear modulus of the gels then increased during cooling, which was attributed to the strengthening of hydrogen bonds at lower temperatures. The hardness of the gels increased slightly but then decreased when the oil droplet concentration was raised from 0 to 14 %. The lightness of the protein gels increased after adding the oil droplets, which was attributed to increased light scattering. Microstructure analysis showed that the RuBisCo proteins formed a particulate gel after heating, with the oil droplets being in the interstices between the particulates. In summary, RuBisCo proteins can be dissolved at high concentrations and can form strong emulsion gels. Consequently, they may be able to mimic the composition and textural attributes of real chicken.

Plant-Growth Promoting Microbes Change the Photosynthetic Response to Light Quality in Spinach.

In this study, the combined effect of plant growth under different light quality and the application of plant-growth-promoting microbes (PGPM) was considered on spinach (Spinacia oleracea L.) to assess the influence of these factors on the photosynthetic performance. To pursue this goal, spinach plants were grown in a growth chamber at two different light quality regimes, full-spectrum white light (W) and red-blue light (RB), with (I) or without (NI) PGPM-based inoculants. Photosynthesis-light response curves (LRC) and photosynthesis-CO2 response curves (CRC) were performed for the four growth conditions (W-NI, RB-NI, W-I, and RB-I). At each step of LRC and CRC, net photosynthesis (PN), stomatal conductance (gs), Ci/Ca ratio, water use efficiency (WUEi), and fluorescence indexes were calculated. Moreover, parameters derived from the fitting of LRC, such as light-saturated net photosynthesis (PNmax), apparent light efficiency (Qpp), and dark respiration (Rd), as well as the Rubisco large subunit amount, were also determined. In not-inoculated plants, the growth under RB- regime improved PN compared to W-light because it increased stomatal conductance and favored the Rubisco synthesis. Furthermore, the RB regime also stimulates the processes of light conversion into chemical energy through chloroplasts, as indicated by the higher values of Qpp and PNmax in RB compared to W plants. On the contrary, in inoculated plants, the PN enhancement was significantly higher in W (30%) than in RB plants (17%), which showed the highest Rubisco content among all treatments. Our results indicate that the plant-growth-promoting microbes alter the photosynthetic response to light quality. This issue must be considered when PGPMs are used to improve plant growth performance in a controlled environment using artificial lighting.

Nitrogen supply alleviates cold stress by increasing photosynthesis and nitrogen assimilation in maize seedlings.

Cold stress inhibits the early growth of maize, leading to reduced productivity. Nitrogen (N) is an essential nutrient that stimulates maize growth and productivity, but the relationship between N availability and cold tolerance is poorly characterized. Therefore, we studied the acclimation of maize under combined cold stress and N treatments. Exposure to cold stress caused a decline in growth and N assimilation, but increased abscisic acid (ABA) and carbohydrate accumulation. The application of different N concentrations from the priming stage to the recovery period resulted in the following observations: (i) high N supply alleviated cold stress-dependent growth inhibition, as shown by increased biomass, chlorophyll and Rubisco content and PSII efficiency; (ii) cold stress-induced ABA accumulation was repressed under high N, presumably due to enhanced stomatal conductance; (iii) the mitigating effects of high N on cold stress could be due to the increased activities of N assimilation enzymes and improved redox homeostasis. After cold stress, the ability of maize seedlings to recover increased under high N treatment, indicating the potential role of high N in the cold stress tolerance of maize seedlings.

Global photosynthetic capacity of C3 biomes retrieved from solar-induced chlorophyll fluorescence and leaf chlorophyll content

The maximum Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (Vcmax) has been widely used to help determine the photosynthetic capacity for carbon cycle modelling. Field measurements of Vcmax are very limited, so fixed Vcmax values over growing seasons are often used, causing considerable uncertainties in modelled results. Also, the lack of a global Vcmax dataset with seasonal variations limits our understanding of the spatial and temporal patterns of Vcmax, and hence photosynthesis. This study proposes a refined data assimilation scheme to retrieve daily Vcmax using remotely sensed solar-induced chlorophyll fluorescence (SIF) and leaf chlorophyll content (LCC) data. Using the refined method, we produced a global Vcmax dataset with daily 0.1° × 0.1° resolutions. The seasonal variations in Vcmax have been successfully retrieved. Vcmax retrieved in this study was compared with field-measured Vcmax from past literature. Good agreements were achieved in relation to Vcmax measured at 201 sites (R2 is 0.78, and relative RMSE is 34%) and the seasonal variation in Vcmax measured at 9 sites (R2 ranges from 0.57 to 0.99, and the relative RMSE ranged from 6% to 28%). Our method effectively utilizes the information from both SIF and LCC, showing advantages over previous approaches that estimate Vcmax based solely on using SIF or LCC. Our results suggest that the spatial distribution of Vcmax depends on the spatial distribution of biomes, along with air temperature and absorbed photosynthetically active radiation (APAR). Higher seasonal mean air temperature and APAR tend to lead to higher seasonal mean Vcmax. The seasonal variation in Vcmax is driven by the seasonal variations in air temperature and APAR, rather than their seasonal means. We also derived and analyzed Vcmax25, which is Vcmax normalized to 25 °C (Vcmax25) and has been found to be more closely related to the Rubisco content. Vcmax25 also has considerable spatiotemporal variations, and the variations differ to that in Vcmax. Different biomes have different ratios of Vcmax25 to LCC. Air temperature and APAR exert weak but positive effects on the seasonal variation in Vcmax25. Vcmax retrieved in this study enhances our understanding of the spatiotemporal pattern of leaf photosynthetic capacity on the global scale and is potentially useful to terrestrial carbon cycle modelling.

Sufficient potassium improves inorganic phosphate-limited photosynthesis in Brassica napus by enhancing metabolic P fractions and Rubisco activity.

Crop photosynthesis (A) and productivity are often limited by a combination of nutrient stresses, such that changes in the availability of one nutrient may affect that of another nutrient, in turn influencing A. In this study, we examined the synergistic effects of phosphorus (P) and potassium (K) on leaf A in a nutrient amendment experiment, in which P and K were added individually or in combination to Brassica napus grown under P and K co-limitation. The data revealed that P addition gradually removed the dominant limiting factor (i.e., P) and improved leaf A. Strikingly, the addition of K synergistically improved gross P uptake, mainly by boosting plant growth, and compensated the physiological demand for P by prioritising investment in metabolic P pools (P-containing metabolites and inorganic phosphate, Pi). The enlarged pool of metabolically active P was partially associated with upregulation of Pi regeneration through release from triose phosphates rather than replacement of P-containing lipids. This process mitigated P restrictions on A by maintaining the ATP/NADPH and NADPH/NADP+ ratios and increasing the content and activity of Rubisco. Our findings demonstrate that sufficient K increased Pi-limited A by enhancing metabolic P fractions and Rubisco activity. Thus, ionic synergism may be exploited to mitigate nutrient-limiting factors to improve crop productivity.

Analysis of Photosynthetic Differences of Rice Germplasm in Southeast Asia Based on Leaf-Tissue Structure, Physiology, and iTRAQ

Photosynthesis is responsible for 90–95% of organic matter in crop yield. The light energy utilization rate of high-yielding rice varieties is 1.0–1.5%, but the ideal value is about 3–5%. Yield can be further improved by improving photosynthetic function. Through the initial screening and re-screening of 220 Southeast Asian germplasm resources, we found that the net photosynthetic rate of Southeast Asian germplasm resource C1 was 36.96 μmol m−2 S−1, which is close to that of C4 plant maize and 3.26 times higher than that of Southeast Asian germplasm resource G164 at 11.26 μmol m−2 S−1. Using C1 and G164 as materials, we compared the tissue structure, chloroplast ultrastructure, photosynthetic physiological indicators, and proteomics of sword leaves to determine the factors affecting photosynthetic function. Compared with G164, C1 exhibited increased number of vascular bundles, increased stomatal size and density, more abundant and neatly arranged chloroplasts and thylakoid grana, and higher chlorophyll fluorescence parameters. The activities and contents of the key photosynthetic enzyme Rubisco were higher in C1 than in G164. The two germplasm resources were subjected to iTRAQ analysis, and the results showed that compared with C1, nine proteins were down-regulated and one protein was up-regulated and associated with photosynthetic electron transport in G164; a total of 17 differential proteins were associated with CO2 fixation, and nine were up-regulated and eight differential proteins were down-regulated in G164. The identified genes encode proteins in the photosynthesis and carbon fixation pathways, and the changes in gene expression were verified by real-time qPCR. The gene expression patterns were consistent with the protein expression patterns. The results suggest that most differential proteins are involved in electron transfer from PSII to PSI and in the CO2 fixation pathway, and increasing the levels of such proteins can effectively enhance the photosynthetic efficiency. C1 can be used as a donor material for selection of high light efficiency varieties and in-depth photosynthesis studies.

Producing fast and active Rubisco in tobacco to enhance photosynthesis.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) performs most of the carbon fixation on Earth. However, plant Rubisco is an intrinsically inefficient enzyme given its low carboxylation rate, representing a major limitation to photosynthesis. Replacing endogenous plant Rubisco with a faster Rubisco is anticipated to enhance crop photosynthesis and productivity. However, the requirement of chaperones for Rubisco expression and assembly has obstructed the efficient production of functional foreign Rubisco in chloroplasts. Here, we report the engineering of a Form 1A Rubisco from the proteobacterium Halothiobacillus neapolitanus in Escherichia coli and tobacco (Nicotiana tabacum) chloroplasts without any cognate chaperones. The native tobacco gene encoding Rubisco large subunit was genetically replaced with H. neapolitanus Rubisco (HnRubisco) large and small subunit genes. We show that HnRubisco subunits can form functional L8S8 hexadecamers in tobacco chloroplasts at high efficiency, accounting for ∼40% of the wild-type tobacco Rubisco content. The chloroplast-expressed HnRubisco displayed a ∼2-fold greater carboxylation rate and supported a similar autotrophic growth rate of transgenic plants to that of wild-type in air supplemented with 1% CO2. This study represents a step toward the engineering of a fast and highly active Rubisco in chloroplasts to improve crop photosynthesis and growth.

Proteomics and photosynthetic apparatus response to vermicompost attenuation of salinity stress Vicia faba leaves

Crop production and growth are severely affected by salt stress. Nevertheless, the bio-fertilizer vermicompost (VC) can be participated as a potent inhibitor of salinity on plant growth and crop production by regulating photosynthetic efficiency. We investigated the effect of VC on photosynthetic performance of salt-stressed broad bean (Vicia faba L. Aspani cultivar). Seeds were grown in soil mixture; clay and sand in ratio 1:2 by volume with five different volumetric ratios of VC; 0, 2.5, 5, 10 and 15% irrigated with either water and/or 200 mM NaCl. Leaf area, Na and K contents, chlorophylls, photosystem II efficiency, Rubisco content, soluble sugars, chloroplasts’ organization and proteomics were analyzed. The imposed stress decrease leaf area, chlorophyll contents, maximum quantum efficiency (Fv/Fm), Rubisco content, increase soluble sugars and damage chloroplasts organization. Salinity upregulated glucose-1-phosphate adenylyl transferase, ribulose bisphosphate carboxylase large subunit and chloroplastic peptidyl-prolyl cis–trans isomerase. The increased leaf area, chlorophyll a, b and carotenoids, maximum quantum efficiency of photosystem II, Rubisco content, improving the degeneration of thylakoid lamellae and lessening plastoglobuli number in thylakoid membranes are the major benefits attained with vermicompost treatments under salt stress.Analysis of proteomic revealed that VC upregulated chloroplastic ferredoxin–NADP reductase, plastocyanin, polyphenol oxidase, peptidyl-prolyl cis–trans isomerase, alpha-glucan phosphorylase H isozyme and maturase expression under salt stress. The results suggest that VC controls protein expression at the level of transcriptional and translational which may conserve photosynthetic components and prevent salt-induced harmful effects in broad bean plants.

The trajectory in catalytic evolution of Rubisco in Posidonia seagrass species differs from terrestrial plants.

The CO2-fixing enzyme Ribulose bisphosphate carboxylase-oxygenase (Rubisco) links the inorganic and organic phases of the global carbon cycle. In aquatic systems, the catalytic adaptation of algae Rubiscos has been more expansive and followed an evolutionary pathway that appears distinct to terrestrial plant Rubisco. Here, we extend this survey to differing seagrass species of the genus Posidonia to reveal how their disjunctive geographical distribution and diverged phylogeny, along with their CO2 concentrating mechanisms (CCMs) effectiveness, have impacted their Rubisco kinetic properties. The Rubisco from Posidonia species showed lower carboxylation efficiencies and lower sensitivity to O2 inhibition than those measured for terrestrial C3 and C4-plant Rubiscos. Compared with the Australian Posidonia species, Rubisco from the Mediterranean Posidonia oceanica had 1.5-2-fold lower carboxylation and oxygenation efficiencies, coinciding with effective CCMs and five Rubisco large subunit amino acid substitutions. Among the Australian Posidonia species, CCM effectiveness was higher in Posidonia sinuosa and lower in the deep-living Posidonia angustifolia, likely related to the 20%-35% lower Rubisco carboxylation efficiency in P. sinuosa and the two-fold higher Rubisco content in P. angustifolia. Our results suggest that the catalytic evolution of Posidonia Rubisco has been impacted by the low CO2 availability and gas exchange properties of marine environments, but with contrasting Rubisco kinetics according to the time of diversification among the species. As a result, the relationships between maximum carboxylation rate and CO2- and O2-affinities of Posidonia Rubiscos follow an alternative path to that characteristic of terrestrial angiosperm Rubiscos.

Photoacclimation of the polar diatom Chaetoceros neogracilis at low temperature.

Polar microalgae face two major challenges: 1- growing at temperatures (-1.7 to 5°C) that limit enzyme kinetics; and 2- surviving and exploiting a wide range of irradiance. The objective of this study is to understand the adaptation of an Arctic diatom to its environment by studying its ability to acclimate to changes in light and temperature. We acclimated the polar diatom Chaetoceros neogracilis to various light levels at two different temperatures and studied its growth and photosynthetic properties using semi-continuous cultures. Rubisco content was high, to compensate for low catalytic rates, but did not change detectably with growth temperature. Contrary to what is observed in temperate species, in C. neogracilis, carbon fixation rate (20 min 14C incorporation) equaled net growth rate (μ) suggesting very low or very rapid (<20 min) re-oxidation of the newly fixed carbon. The comparison of saturation irradiances for electron transport, oxygen net production and carbon fixation revealed alternative electron pathways that could provide energy and reducing power to the cell without consuming organic carbon which is a very limiting product at low temperatures. High protein contents, low re-oxidation of newly fixed carbon and the use of electron pathways alternative to carbon fixation may be important characteristics allowing efficient growth under those extreme environmental conditions.

Temperature-induced changes in Arabidopsis Rubisco activity and isoform expression.

In many plant species, expression of the nuclear encoded Rubisco small subunit (SSu) varies with environmental changes, but the functional role of any changes in expression remains unclear. In this study, we investigated the impact of differential expression of Rubisco SSu isoforms on carbon assimilation in Arabidopsis. Using plants grown at contrasting temperatures (10 °C and 30 °C), we confirm the previously reported temperature response of the four RbcS genes and extend this to protein expression, finding that warm-grown plants produce Rubisco containing ~65% SSu-B and cold-grown plants produce Rubisco with ~65% SSu-A as a proportion of the total pool of subunits. We find that these changes in isoform concentration are associated with kinetic changes to Rubisco in vitro: warm-grown plants produce a Rubisco having greater CO2 affinity (i.e. higher SC/O and lower KC) but lower kcatCO2 at warm measurement temperatures. Although warm-grown plants produce 38% less Rubisco than cold-grown plants on a leaf area basis, warm-grown plants can maintain similar rates of photosynthesis to cold-grown plants at ambient CO2 and 30 °C, indicating that the carboxylation capacity of warm-grown Rubisco is enhanced at warmer measurement temperatures, and is able to compensate for the lower Rubisco content in warm-grown plants. This association between SSu isoform expression and maintenance of Rubisco activity at high temperature suggests that SSu isoform expression could impact the temperature response of C3 photosynthesis.

Rubisco forms a lattice inside alpha-carboxysomes

Despite the importance of microcompartments in prokaryotic biology and bioengineering, structural heterogeneity has prevented a complete understanding of their architecture, ultrastructure, and spatial organization. Here, we employ cryo-electron tomography to image α-carboxysomes, a pseudo-icosahedral microcompartment responsible for carbon fixation. We have solved a high-resolution subtomogram average of the Rubisco cargo inside the carboxysome, and determined the arrangement of the enzyme. We find that the H. neapolitanus Rubisco polymerizes in vivo, mediated by the small Rubisco subunit. These fibrils can further pack to form a lattice with six-fold pseudo-symmetry. This arrangement preserves freedom of motion and accessibility around the Rubisco active site and the binding sites for two other carboxysome proteins, CsoSCA (a carbonic anhydrase) and the disordered CsoS2, even at Rubisco concentrations exceeding 800 μM. This characterization of Rubisco cargo inside the α-carboxysome provides insight into the balance between order and disorder in microcompartment organization.

Physiological Factors Limiting Leaf Net Photosynthetic Rate in C3 Crops like Rice and Approaches for Improving It

Improving leaf photosynthetic capacity is one of the most promising approaches to further boost crop yield. Clarifying factors limiting leaf photosynthetic capacity, especially in C3 crops, is meaningful for designing strategies to improve it. Leaf net photosynthetic rate (A) is one of the parameters describing leaf photosynthetic capacity. In the present study, physiological factors limiting A in C3 crops such as rice were discussed and different approaches for A improvement were summarized to provide theoretical guidance for increasing leaf photosynthetic capacity. A will be limited by both CO2 availability and light intensity over periods from a few hours to several days, and by one of them over shorter intervals. Under current ambient atmospheric conditions, A of C3 crops is mainly limited by Rubisco activity and the CO2 concentration in chloroplasts. Leaf nitrogen content affects A by regulating Rubisco content and leaf anatomy; leaf morphological and anatomical traits limit A by impacting stomatal and mesophyll CO2 diffusion. Further improvements of A in C3 crops can be achieved by designing or introducing high-activity Rubisco; adjusting leaf nitrogen allocation to optimize leaf anatomy and leaf chemical composition; modifying leaf morphology and anatomy for greater CO2 diffusion; improving the activity of proteins and enzymes associated with sugar transportation and utilization; introducing C4 photosynthetic mechanisms and combining high photosynthetic traits by conventional breeding.

Elevated ozone decreases the activity of Rubisco in poplar but not its activation under fluctuating light.

Increasing tropospheric ozone (O3) is well-known to decrease leaf photosynthesis under steady-state light through reductions in biochemical capacity. However, the effects of O3 on photosynthetic induction and its biochemical limitations in response to fluctuating light remain unclear, despite the rapid fluctuations of light intensity occurring under field conditions. In this study, two hybrid poplar clones with different O3 sensitivities were exposed to elevated O3. Dynamic photosynthetic CO2 response measurements were conducted to quantify the impact of elevated O3 and exposure duration on biochemical limitations during photosynthetic induction. We found that elevated O3 significantly reduced the steady-state light-saturated photosynthetic rate, the maximum rate of carboxylation (Vcmax) and Rubisco content. In addition, elevated O3 significantly decreased the time constants for slow phases and weighting of the fast phase of the Vcmax induction in poplar clone '546' but not in clone '107'. However, elevated O3 did not affect the time, it took to reach a given percentage of full Vcmax activation or photosynthetic induction in either clone. Overall, photosynthetic induction was primarily limited by the activity of Rubisco rather than the regeneration of ribulose-1,5-biphosphate regardless of O3 concentration and exposure duration. The lack of O3-induced effects on the activation of Rubisco observed here would simplify the simulation of impacts of O3 on nonsteady-state photosynthesis in dynamic photosynthetic models.

When time is not of the essence: constraints to the carbon balance of bryophytes

The data available so far indicate that the photosynthetic and relative growth rates of bryophytes are 10% of those reported for tracheophytes. By examining the existing literature and reanalysing data published in over 100 studies, this review examines the ecophysiological, biochemical, and structural reasons behind this phenomenon. The limiting Rubisco content and surface for gas exchange are the internal factors that can explain the low photosynthetic and growth rates of bryophytes. The role of the thicker cell walls of bryophytes in limiting CO2 diffusion is unclear, due to the current uncertainties regarding their porosity and permeability to CO2. From this review, it is also evident that, despite bryophytes having low photosynthetic rates, their positive carbon balance is tightly related to their capacity to deal with extreme conditions. Contributing factors include their capacity to deal with large daily temperature oscillations, and their capacity to delay the cessation of photosynthesis under water deficit (or to tolerate desiccation in extreme situations). Although further studies on bryophytes are needed before more solid conclusions can be drawn, it seems that their success relies on their remarkable tolerance to a highly variable environment, possibly at the expense of their maximum photosynthetic rate.

Melatonin reduces photoinhibition in cucumber during chilling by regulating the Calvin-Benson Cycle

To obtain new insights into the mechanisms through which melatonin reduces photoinhibition in cucumber seedlings during chilling, we investigated its effects on the Calvin-Benson cycle. Photoinhibition due to chilling was significantly reduced by prior irrigation with a 200 μM melatonin solution. This was evidenced by reduced declines in the quantum yield of photochemical energy conversion in PS II [Φ(PSⅡ)], in the rate of electron transport through PSII [Je(PSII)] and in the electron flux in the PCR cycle [Je(PCR)], and an increase in the PSII excitation pressure (1-qP). The reasons for this can be explained as: (1) melatonin upregulates the expressions of CsrbcL and CsrbcS, and lessens the decrease in Rubisco content induced by chilling; (2) melatonin upregulates the expression of CsRCA, increases the content of Rubisco activase (RCA), and promotes the transformation of Rubisco from the inactive state to the active state, thereby increasing the carboxylation efficiency of Rubp and helping maintain normal rates of CO2 fixation; and (3) melatonin upregulates the expressions of key genes in the Rubp regeneration process to help maintain the regeneration of Rubp required for CO2 fixation, and thus the normal operation of the Calvin-Benson cycle. This study provides new evidence that suggests melatonin reduces photoinhibition during chilling by regulating the Calvin-Benson cycle.

Photosynthetic Enhancement, Lifespan Extension, and Leaf Area Enlargement in Flag Leaves Increased the Yield of Transgenic Rice Plants Overproducing Rubisco Under Sufficient N Fertilization

BackgroundImprovement in photosynthesis is one of the most promising approaches to increase grain yields. Transgenic rice plants overproducing Rubisco by 30% (RBCS-sense rice plants) showed up to 28% increase in grain yields under sufficient nitrogen (N) fertilization using an isolated experimental paddy field (Yoon et al. in Nat Food 1:134–139, 2020). The plant N contents above-ground sections and Rubisco contents of the flag leaves were higher in the RBCS-sense plants than in the wild-type rice plants during the ripening period, which may be reasons for the increased yields. However, some imprecise points were left in the previous research, such as contributions of photosynthesis of leaves below the flag leaves to the yield, and maintenance duration of high photosynthesis of RBCS-sense rice plants during ripening periods.ResultIn this research, the photosynthetic capacity and canopy architecture were analyzed to explore factors for the increased yields of RBCS-sense rice plants. It was found that N had already been preferentially distributed into the flag leaves at the early ripening stage, contributing to maintaining higher Rubisco content levels in the enlarged flag leaves and extending the lifespan of the flag leaves of RBCS-sense rice plants throughout ripening periods under sufficient N fertilization. The higher amounts of Rubisco also improved the photosynthetic activity in the flag leaves throughout the ripening period. Although the enlarged flag leaves of the RBCS-sense rice plants occupied large spatial areas of the uppermost layer in the canopy, no significant prevention of light penetration to leaves below the flag leaves was observed. Additionally, since the CO2 assimilation rates of lower leaves between wild-type and RBCS-sense rice plants were the same at the early ripening stage, the lower leaves did not contribute to an increase in yields of the RBCS-sense rice plants.ConclusionWe concluded that improvements in the photosynthetic capacity by higher leaf N and Rubisco contents, enlarged leaf area and extended lifespan of flag leaves led to an increase in grain yields of RBCS-sense rice plants grown under sufficient N fertilization.