Effects of light on cyanide‐resistant respiration and alternative oxidase function in Arabidopsis seedlings

Mitochondrial alternative oxidase (AOX), the unique respiratory terminal oxidase in plants, catalyzes the energy wasteful cyanide (CN)‐resistant respiration and plays a role in optimizing photosynthesis. Although it has been demonstrated that leaf AOX is upregulated after illumination, the in vivo mechanism of AOX upregulation by light and its physiological significance are still unknown. In this report, red light and blue light‐induced AOX (especially AOX1a) expressions were characterized. Phytochromes, phototropins and cryptochromes, all these photoreceptors mediate the light‐response of AOX1a gene. When aox1a mutant seedlings were grown under a high‐light (HL) condition, photobleaching was more evident in the mutant than the wild‐type plants. More reactive oxygen species (ROS) accumulation and inefficient dissipation of chloroplast reducing‐equivalents in aox1a mutant may account for its worse adaptation to HL stress. When etiolated seedlings were exposed to illumination for 4 h, chlorophyll accumulation was largely delayed in aox1a plants. We first suggest that more reduction of the photosynthetic electron transport chain and more accumulation of reducing‐equivalents in the mutant during de‐etiolation might be the main reasons.     

Physiological impact of mitochondrial alternative oxidase on photosynthesis and growth in Arabidopsis thaliana

The mitochondrial alternative oxidase (AOX) has been suggested to have a beneficial role in illuminated leaves, but its function has not yet been fully elucidated. In this study, we investigated the effects of a knockout of the AOX1a gene on photosynthesis and growth under several light conditions in Arabidopsis thaliana. The AOX-deficient aox1a mutant showed a lowered operating efficiency of photosystem II and an enhanced activity of cyclic electron transport around photosystem I (CET-PSI) at high irradiance. To further address the physiological association of AOX with CET-PSI, we crossed aox1a with the pgr5 mutant, which is impaired in CET-PSI activity. In the pgr5 mutant background, AOX deficiency did not affect the apparent photosynthetic efficiency, indicating that the direct contribution of AOX to photosynthesis is not so large compared with CET-PSI. Nevertheless, the growth of the aox1a pgr5 double mutant was significantly impaired depending on the light intensity under growth conditions. The possibility of a synergistic function of AOX with CET-PSI in supporting plant growth is discussed.     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

Modelling photosynthetic responses to temperature of grapevine (Vitis vinifera cv. Semillon) leaves on vines grown in a hot climate

Field measurements of photosynthesis of Vitis vinifera cv. Semillon leaves in relation to a hot climate, and responses to photon flux densities (PFDs) and internal CO(2) concentrations (c(i) ) at leaf temperatures from 20 to 40 °C were undertaken. Average rates of photosynthesis measured in situ decreased with increasing temperature and were 60% inhibited at 45 °C compared with 25 °C. This reduction in photosynthesis was attributed to 15-30% stomatal closure. Light response curves at different temperatures revealed light-saturated photosynthesis optimal at 30 °C but also PFDs saturating photosynthesis increased from 550 to 1200 μmol (photons) m(-2)s(-1) as temperatures increased. Photosynthesis under saturating CO(2) concentrations was optimal at 36 °C while maximum rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (V(cmax)) and potential maximum electron transport rates (J(max)) were also optimal at 39 and 36 °C, respectively. Furthermore, the high temperature-induced reduction in photosynthesis at ambient CO(2) was largely eliminated. The chloroplast CO(2) concentration at the transition from RuBP regeneration to RuBP carboxylation-limited assimilation increased steeply with an increase in leaf temperature. Semillon assimilation in situ was limited by RuBP regeneration below 30 °C and above limited by RuBP carboxylation, suggesting high temperatures are detrimental to carbon fixation in this species.     

Interactions between ribulose-1,5-bisphosphate carboxylase and stromal metabolites. II. Corroboration of the role of this enzyme as a metabolite buffer

The capacity of ribulose-1,5-bisphosphate carboxylase to bind reversibly chloroplast metabolites which are the substrates for both thylakoid and stromal enzymes was assessed using spinach chloroplasts and chloroplast extracts and with pure wheat ribulose-1,5-bisphosphate carboxylase. Measurements of the rate of coupled electron flow to methyl viologen in ‘leaky’ chloroplasts (which retained the chloroplast envelope and stromal enzymes but which were permeable to metabolites) and also with broken chloroplasts and washed thylakoids were used to study the effects of binding ADP and inorganic phopshate to ribulose-1,5-bisphosphate carboxylase. The presence of ribulose-1,5-bisphosphate carboxylase significantly altered the values obtained for apparent K_m for inorganic phosphate and ADP of coupled electron transport. The K_m (P_i) in washed thylakoids was 60–80 μM, in ‘leaky’ chloroplasts it was increased to 180–200 μM, while in ‘leaky’ chloroplasts preincubated with KCN and ribulose 1,5-bisphosphate the value was decreased to 40–50 μM. Similarly, the K_m (ADP) of coupled electron transport in washed thylakoids was 60–70 μM, in ‘leaky’ chloroplasts it was 130–150 μM and with ‘leaky’ chloroplasts incubated in the presence of KCN and ribulose 1,5-bisphosphate a value of 45–50 μM was obtained. The ability of ribulose 1,5-bisphosphate carboxylase to reduce the levels of free glycerate 3-phosphate in the absence of ribulose 1,5-bisphosphate was examined using a chloroplast extract system by varying the concentrations of stromal protein or purified ribulose 1,5-bisphosphate carboxylase. The effect of binding glycerate 3-phosphate to ribulose-1,5-bisphosphate carboxylase on glycerate 3-phosphate reduction was to reduce both the rate an the amount of NADPH oxidation for a given amount of glycerate 3-phosphate added. The addition of ribulose 1,5-bisphosphate reinitiated NADPH oxidation but ATP or NADPH did not. Incubation of purified ribulose-1,5-bisphosphate carboxylase with carboxyarabinitolbisphosphate completely inhibited the catalytic activity of the enzyme and decreased inhibition of glycerate-3-phosphate reduction. Two binding sites with different affinities for glycerate 3-phosphate were observed with pure ribulose-1,5-bisphosphate carboxylase.     

Inhibition of photosynthesis and energy dissipation induced by water and high light stresses in rice

Photoprotection mechanisms of rice plants were studied when its seedlings were subjected to the combined stress of water and high light. The imposition of water stress, induced by PEG 6000 which was applied to roots, resulted in substantial inhibition of stomatal conductance and net photosynthesis under all irradiance treatments. Under high light stress, the rapid decline of photosynthesis with the development of water stress was accompanied by decreases in the maximum velocity of RuBP carboxylation by Rubisco (V(cmax)), the capacity for ribulose-1,5-bisphosphate regeneration (J(max)), Rubisco and stromal FBPase activities, and the quantum efficiency of photosystem II, in the absence of any stomatal limitation of CO(2) supply. Water stress significantly reduced the energy flux via linear electron transport (J(PSII)), but increased light-dependent and DeltapH- and xanthophyll-mediated thermal dissipation (J(NPQ)). It is concluded that the drought-induced inhibition of photosynthesis under different irradiances in the rice was due to both diffusive and metabolic limitations. Metabolic limitation of photosynthesis may be related to the adverse effects of some metabolic processes and the oxidative damage to the chloroplast. Meanwhile, an enhanced thermal dissipation is an important process to minimize the adverse effects of drought and high irradiance when CO(2) assimilation is suppressed.     

Photosynthesis and ribulose 1,5-bisphosphate levels in intact chloroplasts

The response of ribulose 1,5-bisphosphate levels and CO(2) fixation rates in isolated, intact spinach chloroplasts to pyrophosphate, triose phosphates, dl-glyceraldehyde, O(2), catalase, and irradiance during photosynthesis has been studied. Within 1 minute in the light, a rapid accumulation of ribulose bisphosphate was measured in most preparations of intact chloroplasts, and this subsequently dropped as CO(2) fixation increased. Pyrophosphate, triose phosphates, and catalase increased CO(2) fixation and also the levels of ribulose bisphosphate. CO(2) fixation was inhibited by dl-glyceraldehyde and O(2) with corresponding decreases in ribulose bisphosphate. When the rate of photosynthesis decreased at limiting irradiances (low light), the level of ribulose bisphosphate in the chloroplast did not always decrease, suggesting that ribulose bisphosphate was not limiting CO(2) fixation under these conditions. When triose phosphates (fructose bisphosphate plus aldolase) were added to suspensions of chloroplasts at low irradiances, ribulose bisphosphate increased while CO(2) fixation decreased. These observations provide considerable evidence that high ribulose bisphosphate levels clearly are not solely sufficient to permit rapid rates of CO(2) fixation, but that factors other than ribulose bisphosphate concentration are overriding the control of photosynthesis.Isolated chloroplasts are capable of using carbon reserves to produce considerable ribulose bisphosphate. Upon illumination in the absence of CO(2) and O(2), intact chloroplasts produced up to 13 millimolar ribulose bisphosphate.     

两个品种烟草叶片发育过程中几种光合参数变化的比较

比较烟草2个品种‘NC89’和‘JYH’叶片发育过程中几个光合参数变化的结果表明,烟草叶片发育过程中光合速率变化表现为上升期、高值持续期（APD）和速降期,叶绿素含量变化经历上升期、相对稳定期（RSP）和速降期。光合功能衰退过程中,核酮糖．1,5-二磷酸羧化酶（RuBPCase）活性比电子传递活性下降快。可逆衰退阶段的2个品种类囊体膜多肽组分和‘NC89’的核酮糖-1,5-二磷酸羧化酶加氧酶（Rubisco）大亚基基本上无变化;不可逆衰退阶段的2个品种类囊体膜多肽组分、Rubisco大小亚基均快速降解,尤其是光系统Ⅱ（PSⅡ）复合体和Rubisco小亚基。‘JYH’的叶龄为10-40d的叶中各光合参数与‘NC89’的差别不大,但‘JYH’的光合功能期短,光合功能衰退过程中光合电子传递与碳同化失衡较严重,光合功能衰退比‘NC89’早而迅速。     

Photosynthesis, Ribulose-1,5-bisphosphate Carboxylase, Electron Transport, and Ribulose 1,5-Bisphosphate of Virescent and Normal Green Wheat Leaves

CO₂ gas exchange, ribulose-1,5-bisphosphate, and electron transport have been measured in leaves of a yellow-green mutant of wheat (Triticum durum var Cappelli) and its wild type strain grown in the field. All these parameters, expressed on leaf area basis, were similar in both genotypes except electron transport which was more than double in the wild type. These results, treated according to a recent photosynthesis model for C₃ plants, seem to indicate that the electron transport rate of mutant leaves is not sufficient to support the carboxylation derived through both the assimilation rate and the in vitro ribulose-1,5-bisphosphate carboxylase activity. It is suggested that under our experimental conditions photosynthetic electron transport is not the sole energy-dependent determinant of ribulose-1,5-bisphosphate regeneration in the mutant.     

Variation in photosynthetic electron transport capacity in vivo and its effects on the light modulation of ribulose bisphosphate carboxylase

Photosynthetic electron transport capacity was varied in vivo in sugar beets using iron deficiency, and its effects on the light modulation of ribulose bisphosphate carboxylase (RuBPCase) studied. Three treatment groups corresponding to decreasing amounts of thylakoids per leaf area were examined: iron sufficient (control), moderately iron-stressed, and severely iron-stressed. Reduction in electron transport capacity in vivo was correlated with a substantial decrease in the level of RuBPCase activation, even at saturating irradiances. These results indicate a direct relationship between RuBPCase activation and photosynthetic electron transport. In addition, our data suggest that the activation of RuBPCase could not solely account for the increases in the photosynthetic rate at high irradiances; RuBPCase reached maximal activation at irradiances well below light saturation for net photosynthesis.Abbreviations Chl chlorophyll - FeCN ferricyanide - FBPase fructose 1,6-bisphosphatase - RuBP ribulose 1,5-bisphosphate - RuBPCase ribulose 1,5-bisphosphate carboxylase - SBPase sedoheptulose 1,7-bisphosphatase     

Rate-limiting factors in leaf photosynthesis. I. Carbon fluxes in the calvin cycle

Rates of photosynthesis of spinach leaves were varied by varying light intensity and CO₂ concentration. Metabolism of the leaves was then arrested by freezing them in liquid nitrogen. Chloroplasts were isolated by a nonaqueous procedure. In the chloroplast fractions, levels of intermediates of the carbon reduction cycle were determined and considered in relation to the photosynthetic flux situation of the leaves at the time before freezing. During induction of photosynthesis, ribulose 1,5-bisphosphate levels increased in parallel with CO₂ fixation. In the steady state, a similar relation between ribulose 1,5-bisphosphate levels and CO₂ uptake was observed at light intensities between 0 and 50 W·m⁻². A further increase in light intensity increased CO₂ fixation rates but not ribulose 1,5-bisphosphate levels. Increasing the CO₂ concentration resulted in increased CO₂ uptake, whereas ribulose 1,5-bisphosphate levels decreased. Even under CO₂ saturation, ribulose 1,5-bisphosphate levels were about 100 nmol/mg chlorophyll corresponding to about 3.5 mM ribulose 1,5-bisphosphate in the chloroplast stroma. This suggests that even under CO₂ saturation, ribulose-1,5-bisphosphate carboxylase limits photosynhetic CO₂ uptake. Mass action ratios calculated from measured metabolite levels demonstrated that the thermodynamic gradient required for the regeneration of ribulose 1,5-bisphosphate from hexosephosphate and triosephosphate increased considerably as photosynthetic flux increased. Similar calculations revealed that the enzymatic apparatus responsible for the reduction of 3-phosphoglycerate to dihydroxyacetone phosphate is not displaced much from equilibrium even under maximum rates of photosynthesis at saturating CO₂. The same is true for aldolase. Fructose-1,6-bisphosphatase also did not limit Calvin cycle turnover. Only at very low light intensities and during the first minutes of the induction period was the ratio of fructose 1,6-bisphosphate to fructose 6-phosphate high. This observation was more readily explained in terms of fructose 1,6-bisphosphate binding to ribulose-1,5-bisphosphate carboxylase than by a rate limitation imposed by insufficient activation of fructose-1,6-bisphosphatase.     

The role of chloroplast electron transport and metabolites in modulating Rubisco activity in tobacco. Insights from transgenic plants with reduced amounts of cytochrome b/f complex or glyceraldehyde 3-phosphate dehydrogenase

Leaf metabolites, adenylates, and Rubisco activation were studied in two transgenic tobacco (Nicotiana tabacum L. cv W38) types. Plants with reduced amounts of cytochrome b/f complex (anti-b/f) have impaired electron transport and a low transthylakoid pH gradient that restrict ATP and NADPH synthesis. Plants with reduced glyceraldehyde 3-phosphate dehydrogenase (anti-GAPDH) have a decreased capacity to use ATP and NADPH in carbon assimilation. The activation of the chloroplast NADP-malate dehydrogenase decreased in anti-b/f plants, indicating a low NADPH/NADP(+) ratio. The whole-leaf ATP/ADP in anti-b/f plants was similar to wild type, while it increased in anti-GAPDH plants. In both plant types, the CO(2) assimilation rates decreased with decreasing ribulose 1, 5-bisphosphate concentrations. In anti-b/f plants, CO(2) assimilation was further compromised by reduced carbamylation of Rubisco, whereas in anti-GAPDH plants the carbamylation remained high even at subsaturating ribulose 1,5-bisphosphate concentrations. We propose that the low carbamylation in anti-b/f plants is due to reduced activity of Rubisco activase. The results suggest that light modulation of activase is not directly mediated via the electron transport rate or stromal ATP/ADP, but some other manifestation of the balance between electron transport and the consumption of its products. Possibilities include the transthylakoid pH gradient and the reduction state of the acceptor side of photosystem I and/or the degree of reduction of the thioredoxin pathway.     

A thirty percent increase in UV-B has no impact on photosynthesis in well-watered and droughted pea plants in the field

It has been suggested that field experiments which increase UV-B irradiation by a fixed amount irrespective of ambient light conditions (‘square-wave’), may overestimate the response of photosynthesis to UV-B irradiation. In this study, pea (Pisum sativum L.) plants were grown in the field and subjected to a modulated 30% increase in ambient UK summer UV-B radiation (weighted with an erythemal action spectrum) and a mild drought treatment. UV-A and ambient UV control treatments were also studied. There were no significant effects of the UV-B treatment on the in situ CO₂ assimilation rate throughout the day or on the light-saturated steady-state photosynthesis. This was confirmed by an absence of UV-B effects on the major components contributing to CO₂ assimilation; photosystem II electron transport, ribulose 1,5-bisphosphate regeneration, ribulose 1,5-bisphosphate carboxylase/oxygenase carboxylation, and stomatal conductance. In addition to the absence of an effect on photosynthetic activities, UV-B had no significant impact on plant biomass, leaf area or partitioning. UV-B exposure increased leaf flavonoid content. The UV-A treatment had no observable effect on photosynthesis or productivity. Mild drought resulted in reduced biomass, a change in partitioning away from shoots to roots whilst maintaining leaf area, but had no observable effect on photosynthetic competence. No UV-B and drought treatment interactions were observed on photosynthesis or plant biomass. In conclusion, a 30% increase in UV-B had no effects on photosynthetic performance or productivity in well-watered or droughted pea plants in the field.     

Feedback-limited photosynthesis and regulation of sucrose-starch accumulation during cold acclimation and low-temperature stress in a spring and winter wheat

Regulation of sucrose-starch accumulation and its effect on CO₂ gas exchange and electron transport were studied in low-temperature-stressed and cold-acclimated spring (Katepwa) and winter (Monopol) cultivars of wheat (Triticum aestivum L.). Low-temperature stress of either the spring or winter cultivar was associated with feedback-limited photosynthesis as indicated by a 50–60% reduction in CO₂ assimilation rates, twofold lower ATP/ADP ratio, and threefold lower electron transport rate than 20°C-grown control plants. However, no limitations were evident at the level of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) in low-temperature-stressed plants. Cold acclimation of the spring cultivar resulted in similar feedback-limited photosynthesis observed during low-temperature stress. In contrast, cold acclimation of the winter cultivar resulted in an adjustment of CO₂ assimilation rates to that of control plants. However, we show, for the first time, that this capacity to adjust CO₂ assimilation still appeared to be associated with limited triose phosphate utilisation, a twofold lower ATP/ADP ratio, a reduction in electron transport rates but no restriction at the level of Rubisco compared to controls grown at 20°C. Thus, contrary to previous suggestions, we conclude that cold-acclimated Monopol appears to exhibit feedback limitations at the level of electron transport characteristic of cold-stressed plants despite the maintenance of high rates of CO₂ assimilation. Furthermore, the differential capacity of the winter cultivar to adjust CO₂ assimilation rates was associated with higher levels of sucrose accumulation and a threefold higher sucrose-phosphate synthase activity despite an apparent limitation in triose phosphate utilisation.Abbreviations AGPase ADP-glucose pyrophosphorylase - FBPase fructose-1,6-bisphosphatase - Fru 6-P fructose 6-phosphate - Fru 1,6-BP fructose 1,6-bisphosphate - Glc 6-P glucose 6-phosphate - PGA 3-phosphoglyceric acid - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose 1,5-bisphosphate - SPS sucrose-phosphate synthase - Triose-P triose phosphate     

Acceleration of plastoquinone pool reduction by alternative pathways precedes a decrease in photosynthetic CO2 assimilation in preheated barley leaves

Heat stress causes inhibition of photosynthetic CO₂ assimilation, affects light photosynthetic reactions and accelerates alternative pathways of plastoquinone pool reduction (APPR). We have studied all these heat-sensitive processes after preheating to a broad range of physiological temperatures (24–46°C) to explore a role of these alternative pathways during heat stress. Primarily, the effective quantum yield of PSII photochemistry was reduced (at 40°C). This PSII downregulation was accompanied by the stimulation of APPR and preceded reduction of photosynthetic CO₂ assimilation by 2°; it occurred after preheating at 42°C because of inhibition in Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) activation process. Thus, we suggest that the heat-induced stimulation of APPR is not associated with the heat-induced inhibition of Calvin cycle as it was reported for other types of stresses. A possible role of APPR in the compensation of PSII downregulation is briefly discussed.     

Photosynthesis by isolated chloroplasts. Inhibition by dl-glyceraldehyde of carbon dioxide assimilation

Photosynthetic carbon assimilation and associated CO(2)-dependent O(2) evolution by chloroplasts isolated from pea shoots and spinach leaves is almost completely inhibited by 10mm-dl-glyceraldehyde. The inhibitor is without appreciable effect on photosynthetic electron transport, photophosphorylation, the carboxylation of ribulose 1,5-diphosphate or the reduction of 3-phosphoglycerate, but apparently blocks the conversion of triose phosphate into ribulose 1,5-diphosphate.     

Determining Photosynthetic Parameters from Leaf CO2 Exchange and Chlorophyll Fluorescence (Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Specificity Factor,Dark Respiration in the Light,Excitation Distribution between Photosystems,Alternative Electron Transport Rate,and Mesophyll Diffusion Resistance

Using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, we determined the excitation partitioning to photosystem II (PSII), the CO2/O2 specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase, the dark respiration in the light, and the alternative electron transport rate to acceptors other than bisphosphoglycerate, and the transport resistance for CO2 in the mesophyll cells for individual leaves of herbaceous and tree species. The specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase for CO2 was determined from the slope of the O2 dependence of the CO2 compensation point between 1.5 and 21% O2. Its value, on the basis of dissolved CO2 and O2 concentrations at 25.5[deg]C, varied between 86 and 89. Dark respiration in the light, estimated from the difference between the CO2 compensation point and the CO2 photocompensation point, was about 20 to 50% of the respiration rate in the dark. The excitation distribution to PSII was estimated from the extrapolation of the dependence of the PSII quantum yield on F/Fm to F = 0, where F is steady-state and Fm is pulse-satuarated fluorescence, and varied between 0.45 and 0.6. The alternative electron transport rate was found as the difference between the electron transport rates calculated from fluorescence and from gas exchange, and at low CO2 concentrations and 10 to 21% O2, it was 25 to 30% of the maximum electron transport. The calculated mesophyll diffusion resistance accounted for about 20 to 30% of the total mesophyll resistance, which also includes carboxylation resistance. Whole-leaf photosynthesis is limited by gas phase, mesophyll diffusion, and carboxylation resistances in nearly the same proportion in both herbaceous species and trees.     

The role of enzyme activation state in limiting carbon assimilation under variable light conditions

The mechanisms regulating transient photosynthesis by soybean (Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO₂ assimilation attributable to a lightfleck increased. This enhanced lightfleck CO₂ assimilation was accounted for by a greater postillumination CO₂ fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase fructose 1,6-bisphosphatase (EC 3.1.3.11) - LUE lightfleck use efficiency - P-glycerate 3-phosphoglycerate - PICF post-illumination CO₂ fixation - Ru5P kinase ribulose 5-phosphate kinase (EC 2.7.1.19) - RuBP ribulose 1,5-bisphosphate - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - SBpase sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37)     

Distinct roles of alternative oxidase pathway during the greening process of etiolated algae

The vital function of mitochondrial alternative oxidase(AOX) pathway in optimizing photosynthesis during plant de-etiolation has been well recognized. However, whether and how AOX impacts the chloroplast biogenesis in algal cells remains unclear. In the present study, the role of AOX in regulating the reassembly of chloroplast in algal cells was investigated by treating Auxenochlorella protothecoides with salicylhydroxamic acid(SHAM), the specific inhibitor to AOX, in the heterotrophy to autotrophy transition process. Several lines of evidences including delayed chlorophyll accumulation, lagged reorganization of chloroplast structure, altered PSI/PSII stoichiometry, and declined photosynthetic activities in SHAM treated cells indicated that the impairment in AOX activity dramatically hindered the development of functioning chloroplast in algal cells. Besides, the cellular ROS levels and activities of antioxidant enzymes were increased by SHAM treatment, and the perturbation on the balance of NAD~+/NADH and NADP~+/NADPH ratios was also observed in A. protothecoides lacking AOX activity, indicating that AOX was essential in promoting ROS scavenging and keeping the redox homeostasis for algal chloroplast development during greening. Overall, our study revealed the essentiality of mitochondrial AOX pathway in sustaining algal photosynthetic performance and provided novel insights into the physiological roles of AOX on the biogenesis of photosynthetic organelle in algae.