Chloroplast Respiration : A MEANS OF SUPPLYING OXIDIZED PYRIDINE NUCLEOTIDE FOR DARK CHLOROPLASTIC METABOLISM

Induction of a longer term component of isoprene release in darkened aspen leaves: origin and regulation under different environmental conditions

After darkening, isoprene emission continues for 20 to 30 min following biphasic kinetics. The initial dark release of isoprene (postillumination emission), for 200 to 300 s, occurs mainly at the expense of its immediate substrate, dimethylallyldiphosphate (DMADP), but the origin and controls of the secondary burst of isoprene release (dark-induced emission) between approximately 300 and 1,500 s, are not entirely understood. We used a fast-response gas-exchange system to characterize the controls of dark-induced isoprene emission by light, temperature, and CO(2) and oxygen concentrations preceding leaf darkening and the effects of short light pulses and changing gas concentrations during dark-induced isoprene release in hybrid aspen (Populus tremula × Populus tremuloides). The effect of the 2-C-methyl-D-erythritol-4-phosphate pathway inhibitor fosmidomycin was also investigated. The integral of postillumination isoprene release was considered to constitute the DMADP pool size, while the integral of dark-induced emission was defined as the "dark" pool. Overall, the steady-state emission rate in light and the maximum dark-induced emission rate responded similarly to variations in preceding environmental drivers and atmospheric composition, increasing with increasing light, having maxima at approximately 40 °C and close to the CO(2) compensation point, and were suppressed by lack of oxygen. The DMADP and dark pool sizes were also similar through their environmental dependencies, except for high temperatures, where the dark pool significantly exceeded the DMADP pool. Isoprene release could be enhanced by short lightflecks early during dark-induced isoprene release, but not at later stages. Fosmidomycin strongly suppressed both the isoprene emission rates in light and in the dark, but the dark pool was only moderately affected. These results demonstrate a strong correspondence between the steady-state isoprene emission in light and the dark-induced emission and suggest that the dark pool reflects the total pool size of 2-C-methyl-d-erythritol-4-phosphate pathway metabolites upstream of DMADP. These metabolites are converted to isoprene as soon as ATP and NADPH become available, likely by dark activation of chloroplastic glycolysis and chlororespiration.

Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations

We examined the hypothesis that elevated CO2 concentration would increase NO3- absorption and assimilation using intact wheat canopies (Triticum aestivum cv. Veery 10). Nitrate consumption, the sum of plant absorption and nitrogen loss, was continuously monitored for 23 d following germination under two CO2 concentrations (360 and 1000 micromol mol-1 CO2) and two root zone NO3- concentrations (100 and 1000 mmol m3 NO3-). The plants were grown at high density (1780 m-2) in a 28 m3 controlled environment chamber using solution culture techniques. Wheat responded to 1000 micromol mol-1 CO2 by increasing carbon allocation to root biomass production. Elevated CO2 also increased root zone NO3- consumption, but most of this increase did not result in higher biomass nitrogen. Rather, nitrogen loss accounted for the greatest part of the difference in NO3- consumption between the elevated and ambient [CO2] treatments. The total amount of NO3(-)-N absorbed by roots or the amount of NO3(-)-N assimilated per unit area did not significantly differ between elevated and ambient [CO2] treatments. Instead, specific leaf organic nitrogen content declined, and NO3- accumulated in canopies growing under 1000 micromol mol-1 CO2. Our results indicated that 1000 micromol mol-1 CO2 diminished NO3- assimilation. If NO3- assimilation were impaired by high [CO2], then this offers an explanation for why organic nitrogen contents are often observed to decline in elevated [CO2] environments.

Oxygenic Photoautotrophic Growth Without Photosystem I

Contrary to the prediction of the Z-scheme model of photosynthesis, experiments demonstrated that mutants of Chlamydomonas containing photosystem II (PSII) but lacking photosystem I (PSI) can grow photoautotrophically with O2 evolution, using atmospheric CO2 as the sole carbon source. Autotrophic photosynthesis by PSI-deficient mutants was stable both under anaerobic conditions and in air (21 percent O2) at an actinic intensity of 200 microeinsteins per square meter per second. This PSII photosynthesis, which was sufficient to support cell development and mobility, may also occur in wild-type green algae and higher plants. The mutants can survive under 2000 microeinsteins per square meter per second with air, although they have less resistance to photoinhibition.

Effects of Anaerobiosis on Chlorophyll Fluorescence Yield in Spinach (Spinacia oleracea) Leaf Discs

When spinach (Spinacia oleracea) leaf discs were incubated in a dark anaerobic environment, the chlorophyll fluorescence yield was much increased relative to the aerobic control. Occasionally, the fluorescence yield of the darkened anaerobic samples approached 80% of the maximum fluorescence. The anaerobic incubation period also induced in a leaf disc the capacity to exhibit a low light-mediated chlorophyll fluorescence induction phenomenon. This involved a rapid and slow increase in fluorescence yield, followed by a slow quenching. This could be induced by light levels as low as 400 [mu]W m-2. The anaerobic-dependent increase in chlorophyll fluorescence yield could be relaxed by either far-red light, O2, or a saturating pulse of white light. It was concluded that the anaerobic-dependent increase in chlorophyll fluorescence yield was due to a dark reduction of the plastoquinone pool and its relaxation by reoxidation. Darkened isolated chloroplasts did not exhibit a fluorescence yield increase under anaerobic conditions. Fluorescence slowly increased only when dithiothreitol or dithionite was added.

Characterization of an Electron Transport Pathway Associated with Glucose and Fructose Respiration in the Intact Chloroplasts of Chlamydomonas reinhardtii and Spinach

The role of an electron transport pathway associated with aerobic carbohydrate degradation in isolated, intact chloroplasts was evaluated. This was accomplished by monitoring the evolution of (14)CO(2) from darkened spinach (Spinacia oleracea) and Chlamydomonas reinhardtii chloroplasts externally supplied with [(14)C]fructose and [(14)C]glucose, respectively, in the presence of nitrite, oxaloacetate, and conventional electron transport inhibitors. Addition of nitrite or oxaloacetate increased the release of (14)CO(2), but it was shown that O(2) continued to function as a terminal electron acceptor. (14)CO(2) evolution was inhibited up to 30 and 15% in Chlamydomonas and spinach, respectively, by 50 mum rotenone and by amytal, but at 500- to 1000-fold higher concentrations, indicating the involvement of a reduced nicotinamide adenine dinucleotide phosphate-plastoquinone oxidoreductase. (14)CO(2) release from the spinach chloroplast was inhibited 80% by 25 mum 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone. (14)CO(2) release was sensitive to propylgallate, exhibiting approximately 50% inhibition in Chlamydomonas and in spinach chloroplasts of 100 and 250 mum concentrations, respectively. These concentrations were 20- to 50-fold lower than the concentrations of salicylhydroxamic acid (SHAM) required to produce an equivalent sensitivity. Antimycin A (100 mum) inhibited approximately 80 to 90% of (14)CO(2) release from both types of chloroplast. At 75 mum, sodium azide inhibited (14)CO(2) evolution about 50% in Chlamydomonas and 30% in spinach. Sodium azide (100 mm) combined with antimycin A (100 mum) inhibited (14)CO(2) evolution more than 90%. (14)CO(2) release was unaffected by uncouplers. These results are interpreted as evidence for a respiratory electron transport pathway functioning in the darkened, isolated chloroplast. Chloroplast respiration defined as (14)CO(2) release from externally supplied [1-(14)C]glucose can account for at least 10% of the total respiratory capacity (endogenous release of CO(2)) of the Chlamydomonas reinhardtii cell.

Decrease of Nitrate Reductase Activity in Spinach Leaves during a Light-Dark Transition

In leaves of spinach plants (Spinacia oleracea L.) performing CO(2) and NO(3) (-) assimilation, at the time of sudden darkening, which eliminates photosystem I-dependent nitrite reduction, only a minor temporary increase of the leaf nitrite content is observed. Because nitrate reduction does not depend on redox equivalents generated by photosystem I activity, a continuation of nitrate reduction after darkening would result in a large accumulation of nitrite in the leaves within a very short time, which is not observed. Measurements of the extractable nitrate reductase activity from spinach leaves assayed under standard conditions showed that in these leaves the nitrate reductase activity decreased during darkening to 15% of the control value with a half-time of only 2 minutes. Apparently, in these leaves nitrate reductase is very rapidly inactivated at sudden darkness avoiding an accumulation of the toxic nitrite in the cells.

Aerobic and anaerobic respiration in the intact spinach chloroplast

Aerobic and anaerobic chloroplastic respiration was monitored by measuring (14)CO(2) evolution from [(14)C]glucose in the darkened spinach (Spinacia oleracea) chloroplast and by estimating the conversion of fructose 1,6-bisphosphate to glycerate 3-phosphate in the darkened spinach chloroplast in air with O(2) or in N(2) with nitrite or oxaloacetate as electron acceptors. The pathway of (14)CO(2) evolution from labeled glucose in the absence and presence of the inhibitors iodoacetamide and glycolate 2-phosphate under air or N(2) were those expected from the oxidative pentose phosphate cycle and glycolysis. Of the electron acceptors, O(2) was the best (2.4 nanomoles CO(2) per milligram chlorophyll per hour), followed by nitrite and oxaloacetate. With respect to glycerate 3-phosphate formation from fructose 1,6-bisphosphate, methylene blue increased the aerobic rate from 3.7 to 5.4 micromoles per milligram chlorophyll per hour. A rate of 4.8 micromoles per milligram chlorophyll per hour was observed under N(2) with nitrite and oxaloacetate.

Inhibition of chloroplastic respiration by osmotic dehydration

The respiratory capacity of isolated spinach (Spinacia oleracea L.) chloroplasts, measured as the rate of (14)CO(2) evolved from the oxidative pentose phosphate cycle in darkened chloroplasts exogenously supplied with [(14)C]glucose, was progressively diminished by escalating osmotic dehydration with betaine or sorbitol. Comparing the inhibitions of CO(2) evolution generated by osmotic dehydration in chloroplasts given C-1 and C-6 labeled glucose, 54% and 84% respectively, indicates that osmotic dehydration effects to a greater extent the recycling of the oxidative pentose phosphate intermediates, fructose-6P and glyceraldehyde-3P. Respiratory inhibition in the darkened chloroplast could be alleviated by addition of NH(4)Cl (a stromal alkylating agent), iodoacetamide) an inhibitor of glyceraldehyde-3P dehydrogenase), or glycolate-2P (an inhibitor of phosphofructokinase). It is concluded that the site which primarily mediates respiratory inhibition in the darkened chloroplast occurs at the fructose 1,6-bisphosphatase/phosphofructokinase junction.

Spinach Leaf Chloroplast CO(2) and NO(2) Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations

Potential competition between CO(2) and NO(2) (-) photoassimilation for photogenerated reductant (e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature ;source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO(2) and/or NO(2) (-) to produce the highest demand for reductant by CO(2) and/or NO(2) (-) assimilatory processes (enzymes). Even in the presence of CO(2) fixation, NO(2) (-) reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO(2) assimilation. Regardless of the magnitude of the incident light energy, CO(2) assimilation did not repress NO(2) (-) reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO(2) as well as with inorganic nitrogen photoassimilation.

Activation of fatty acids by non-glyoxysomal peroxisomes

Peroxisomes from mung-bean hypocotyls catalyze, in the presence of fatty acids, CoASH, ATP, and MgCl2, the formation of acyl-CoA, AMP, and pyrophosphate in a 1:1:1 stoichiometry. This observation demonstrates that the peroxisomes of mung-bean hypocotyls possess an acyl-CoA synthetase (EC 6.2.1.3) for fatty-acid activation. Acyl-CoA synthetase activity is associated with the non-glyoxysomal peroxisomes from various tissues. The acyl-CoA synthetase of the peroxisomes of the mung-bean hypocotyl utilizes oleic, linoleic, and linolenic acid most effectively (3 nkat·mg(-1) peroxisomal protein). In contrast to the β-oxidation enzymes of the peroxisomes whith are largely solubilized in the presence of 0.2 mol·l(-1) KCl, the acyl-CoA synthetase remains associated with the membrane fraction of peroxisomes. On the basis of the latency of the enzyme and its resistance to protease treatment of the peroxisomes, it is concluded that the enzyme is located at the matrix face of the peroxisome membrane.

The effect of oxygen on nitrate and nitrite assimilation in leaves of Zea mays L. under dark conditions

The assimilation of nitrate under dark-N2 and dark-O2 conditions in Zea mays leaf tissue was investigated using colourimetric and (15)N techniques for the determination of organic and inorganic nitrogen. Studies using (15)N indicated that nitrate was assimilated under dark conditions. However, the rate of nitrate assimilation in the dark was only 28% of the rate under non-saturating light conditions. No nitrite accumulated under dark aerobiosis, even though nitrate reduction occurred under these conditions. The pattern of nitrite accumulation in leaf tissue in response to dark-N2 conditions consisted of three phases: an initial lag phase, followed by a period of rapid nitrite accumulation and finally a phase during which the rate of nitrite accumulation declined. After a 1-h period of dark-anaerobiosis, both nitrate reduction and nitrite accumulation declined considerably. However, when O2 was supplied, nitrate reduction was stimulated and the accumulated nitrite was rapidly reduced. Anaerobic conditions stimulated nitrate reduction in leaf tissue after a period of dark-aerobic pretreatment.

Carbon dioxide and nitrite photoassimilatory processes do not intercompete for reducing equivalents in spinach and soybean leaf chloroplasts

Previously, C Baysdorfer and JM Robinson (1985 Plant Physiol 77: 318-320) demonstrated that, in a reconstituted spinach chloroplast system, NADP photoreduction functioning at most maximal rate and reductant demand, was the successful competitor with NO(2) (-) photoreduction for reduced ferredoxin. This resulted in a repression of NO(2) (-) reduction until all NADP available had been almost totally reduced. Further experiments, employing isolated, intact spinach leaf plastids and soybean leaf mesophyll cells, were conducted to examine competition for reductant between CO(2) and NO(2) (-) photoassimilation, in situ. In isolated, intact plastid preparations, regardless of whether the demand for reductant by CO(2) photoassimilation was high (5 millimolar ;CO(2)') with rates of CO(2) fixation in the range 40 to 90 micromoles CO(2) fixed per hour per milligram chlorophyll, low (0.5 millimolar ;CO(2)') with rates in the range 5 to 8 micromoles CO(2) per hour per milligram chlorophyll, or zero (no ;CO(2)'), NO(2) (-) photoreduction displayed equal rates in the range of 8 to 22 micromoles per hour per milligram chlorophyll. In the absence of ;CO(2)', but in the presence of saturating white light, 3-phosphoglycerate photoreduction at rates of 82 to 127 micromoles per hour per milligram chlorophyll did not repress, and occasionally stimulated concomitant rates of NO(2) (-) reduction which ranged from 23.4 to 38.5. Conversely, in plastid preparations, NO(2) (-) at levels of 50 to 100 micromolar, stimulated plastid CO(2) fixation when ;CO(2)' was saturating with respect to carboxylation. Further, levels of NO(2) (-) in the range 250 to 2500 micromolar, stimulated soybean leaf mesophyll cell net CO(2) fixation as much as 1.5-fold if ;CO(2)' was saturating with respect to CO(2) fixation. It appeared likely that, in high light in vivo, CO(2) and NO(2) (-) photoassimilatory processes are not forced to intercompete for reduced ferredoxin in the intact chloroplast.

A light-enhanced metabolism of sulfite in cells of Cucumis sativus L. cotyledons

The effects of light and several photosynthetic inhibitors on the rate of sulfite metabolism in cells obtained from Cucumis sativus L. cotyledons was studied. The cells were treated with 200 μM Na2SO3 and the disappearance of sulfite was monitored using either dithiobisnitrobenzoic acid or fuchsin. The rate of sulfite disappearance in light was double the dark rate. Disalicylidene propanediamine at 1 mM increased this light-enhanced metabolism approx. 50%; neither 1 μM 3,4-dichlorophenyl-N,N-dimethylurea nor 0.1 mM cyanazine, which completely inhibited CO2-dependent oxygen evolution, affected the rate of sulfite metabolism. Addition of 200 μM Na2SO3 to the cells partially inhibited (14)CO2 fixation. The rate of sulfite consumption by the cells did not affect this inhibition. We conclude that light-dependent sulfite metabolism is cucumber cells may utilize reduced ferredoxin generated as a result of photosynthetic electron transport. An injurious interaction between CO2 fixation and sulfite appears to occur independently of the sulfite-metabolism process.

Fermentative Metabolism of Chlamydomonas reinhardtii: II. Role of Plastoquinone

Evidence is presented to substantiate a chloroplastic respiratory pathway in the green alga, Chlamydomonas reinhardtii, whereby reducing equivalents generated during the degradation of starch enter the thylakoidal chain at the plastoquinone site catalyzed by NADH-plastoquinone reductase. In this formulation, the reduced plastoquinone is oxidized either by the photoevolution (photosystem I) of H(2) under anaerobic conditions or by O(2) during dark respiration.

Dependency of Nitrate Reduction on Soluble Carbohydrates in Primary Leaves of Barley under Aerobic Conditions

Nitrate reduction was studied as a function of carbohydrate concentration in detached primary leaves of barley (Hordeum vulgare L. cv Numar) seedlings under aerobic conditions in light and darkness. Seedlings were grown either in continuous light for 8 days or under a regimen of 16-hour light and 8-hour dark for 8 to 15 days. Leaves of 8-day-old seedlings grown in continuous light accumulated 4 times more carbohydrates than leaves of plants grown under a light and dark regimen. When detached leaves from these seedlings were supplied with NO(3) (-) in darkness, those with the higher levels of carbohydrates reduced a greater proportion of the NO(3) (-) that was taken up. In darkness, added glucose increased the percentage of NO(3) (-) reduced up to 2.6-fold depending on the endogenous carbohydrate status of the leaves. Both NO(3) (-) reduction and carbohydrate content of the leaves increased with age. Fructose and sucrose also increased NO(3) (-) reduction in darkness to the same extent as glucose. Krebs cycle intermediates, citrate and succinate, did not increase NO(3) (-) reduction, whereas malate slightly stimulated it in darkness.In light, 73 to 90% of the NO(3) (-) taken up was reduced by the detached leaves; therefore, an exogenous supply of glucose had little additional effect on NO(3) (-) reduction. The results indicate that in darkness the rate of NO(3) (-) reduction in primary leaves of barley depends upon the availability of carbohydrates.

Fermentative Metabolism of Chlamydomonas reinhardtii: I. Analysis of Fermentative Products from Starch in Dark and Light

The anaerobic starch breakdown into end-products in the green alga Chlamydomonas reinhardtii F-60 has been investigated in the dark and in the light. The effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and carbonyl cyanide-p-trifluoromethoxyphenyl hydrazone (FCCP) on the fermentation in the light have also been investigated.Anaerobic starch breakdown rate (13.1 +/- 3.5 micromoles C per milligram chlorophyll per hour) is increased 2-fold by FCCP in the dark. Light (100 watts per square meter) decreases up to 4-fold the dark rate, an inhibition reversed by FCCP. Stimulation of starch breakdown by the proton ionophore FCCP points to a pH-controlled rate-limiting step in the dark, while inhibition by light, and its reversal by FCCP, indicates a control by energy charge in the light.In the dark, formate, acetate, and ethanol are formed in the ratios of 2.07:1.07:0.91, and account for roughly 100% of the C from the starch. H(2) production is 0.43 mole per mole glucose in the starch. Glycerol, d-lactate, and CO(2) have been detected in minor amounts.In the light, with DCMU and FCCP present, acetate is produced in a 1:1 ratio to formate, and H(2) evolution is 2.13 moles per mole glucose. When FCCP only is present, acetate production is lower, and CO(2) and H(2) evolution is 1.60 and 4.73 moles per mole glucose, respectively.When DCMU alone is present, CO(2) and H(2) photoevolution is higher than in the dark. Without DCMU, CO(2) and H(2) evolution is about 100% higher than in its presence. In both conditions, acetate is not formed. In all conditions in the light, ethanol is a minor product. Formate production is least affected by light.The stoichiometry in the dark indicates that starch is degraded via the glycolytic pathway, and pyruvate is broken down into acetyl-CoA and formate. Acetyl-CoA is further dissimilated into acetate and ethanol. In the light, acetate is produced only in the presence of FCCP and, when photophosphorylation is possible, it is used in unidentified reactions. Ethanol formation is inhibited by the light in all conditions.

Oxidation of cysteine to cystine by membrane fractions of Chlorella fusca

Isolated membrane fractions of Chlorella fusca 211-8b obtained by french-press treatment and sonication catalyzed the oxidation of L-cysteine to L-cystine. The pH-optimum of this reaction was determined to be around 8-8.5 and a stoichiometry of 4 SH-groups oxidized for one O2 consumed was obtained. This thiol-oxidation system was specific for D-and L-cysteine; DL-homocysteine and cysteamine were oxidized at about half the rate whereas all other thiols tested including glutathione, mercaptoethanol, mercaptopropionic acid and dithioerythritol were not oxidized by these membrane fractions. The apparent Km for L-cysteine was determined as 3.3 mmol l(-1). Rates of 200 μmol cysteine oxidized mg(-1) chlorophyll h(-1) were normally obtained. Extremely high rates of oxygen uptake were measured using L-cysteine methyl ester and L-cysteine ethyl ester. This thioloxidation system was not inhibited by mitochondrial electron-transport inhibitors such as rotenone or antimycin A, nor by the chloroplast electron-transport inhibitors 2,5-dibromothymochinone and 2,4-dinitrophenylether of iodonitrothymol. The cysteine oxidation catalyzed by C. fusca membranes was inhibited, however, by salicylhydroxamic acid, o-phenanthrolin, N,N'-disalicyliden-1,3-diaminopropane 5,5'-disulfonic acid, ethylenediaminetetraacetic acid, high KCN levels and by the buffers, N-[2-hydroxyl-1,1-bis(hydroxymethyl) ethyl] glycine and phosphate. This cysteine-oxidation system seems to function as a counterpart of thioredoxin-mediated light activation of enzymes, allowing reduced thiol groups to be oxidized again by O2 (dark inactivation).

Effect of photosynthetic inhibitors and uncouplers of oxidative phosphorylation on nitrate and nitrite reduction in barley leaves

The effects of several photosynthetic inhibitors and uncouplers of oxidative phosphorylation on NO(3) (-) and NO(2) (-) assimilation were studied using detached barley (Hordeum vulgare L. cv Numar) leaves in which only endogenous NO(3) (-) or NO(2) (-) were available for reduction. Uncouplers of oxidative phosphorylation greatly increased NO(3) (-) reduction in both light and darkness, while photosynthetic inhibitors did not.The NO(2) (-) concentration in the control leaves was very low in both light and darkness; 98% or more of the NO(2) (-) formed from NO(3) (-) was further assimilated in control leaves. More NO(2) (-) accumulated in the leaves in light and darkness in the presence of photosynthetic inhibitors. Of this NO(2) (-), 94% or more was further assimilated. It appears that metabolites, either external or internal to the chloroplast, capable of reducing NADP (which, in turn, could reduce ferredoxin via NADP reductase) might support NO(2) (-) reduction in darkness and light when photosynthetic electron flow is inhibited by photosynthetic inhibitors.NITRITE ASSIMILATION WAS MUCH MORE SENSITIVE TO UNCOUPLERS IN DARKNESS THAN IN LIGHT: in darkness, 74% or more of NO(2) (-) formed from NO(3) (-) was further assimilated, whereas in light, 95% or more of the NO(2) (-) was further assimilated.