Oxygenic photosynthesis and the distribution of chloroplasts

The integrated functioning of two photosystems (I and II) whether in cyanobacteria or in chloroplasts is the outstanding sign of a common ancestral origin. Many variations on the basic theme are currently evident in oxygenic photosynthetic organisms whether they are prokaryotes, unicellular, or multicellular. By conservative estimates, oxygenic photosynthesis has been around for at least ca. 2.2–2.7 billions years, consistent with cyanobacteria-type microfossils, biomarkers, and an atmospheric rise in oxygen to less than 1.0% of the present concentration. The presumptions of chloroplast formation by the cyanobacterial uptake into a eukaryote prior to 1.6 BYa ago are confounded by assumptions of host type(s) and potential tolerance of oxygen toxicity. The attempted dating and interrelationships of particular chloroplasts in various plant or animal lineages has relied heavily on phylogenomic analysis and evaluations that have been difficult to confirm separately. Many variations occur in algal groups, involving the type and number of accessory pigments, and the number(s) of membranes (2–4) enclosing a chloroplast, which can both help and complicate inferences made about early or late origins of chloroplasts. Integration of updated phylogenomics with physiological and cytological observations remains a special challenge, but could lead to more accurate assumptions of initial and extant endosymbiotic event(s) leading toward stable chloroplast associations.     

Effect of powdery mildew infection on photosynthesis by leaves and chloroplasts of sugar beets

Chloroplasts isolated from powdery mildew-infected (Erysiphe polygoni DC) sugar beet leaves (Beta vulgaris L) showed a reduction in the rate of electron transport and in the accompanying ATP formation in noncyclic photophosphorylation (water as electron donor, NADP as electron acceptor) and little or no change in the rate of ATP formation in cyclic photophosphorylation catalyzed by phenazine methosulfate. The inhibition of noncyclic photophosphorylation appeared to lead in the parent leaves to a decreased rate of photosynthetic CO₂ assimilation and a shift in products resulting in a relative increase of amino acids. These changes were accompanied by alterations in chloroplast ultrastructure and by a reduction in the activity of enzymes necessary for the formation of organic acids (phosphoenolpyruvate carboxylase and malate dehydrogenase). These results are similar to the findings of Montalbini and Buchanan (1974 Physiol. Plant Pathol. 4: 191-196) with chloroplasts from rust-infected Vicia faba leaves.     

Biochemical differentiation of plastids and other organelles in rye leaves with a high-temperature-induced deficiency of plastid ribosomes

Summary 1. In developing rye (Secale cereale L.) leaves the formation of plastidic ribosomes was selectively prevented in light as well as in darkness, when the seedlings were grown at an elevated temperature of 32° instead of 22° where normal development ocurred. Plastid ribosome deficient parts of lightgrown leaves were chlorotic at 32°. — 2. At both temperatures the leaves contained under all conditions (light or dark, on H₂O or nutrient solution) equal or very similar amounts of total amino nitrogen. In light, the contents of total protein and dry weight were lower at 32° than at 22°, especially when the plants were grown on nutrient solution. — 3. Mitochondrial marker enzymes had normal or even higher activities in 32°-grown leaves. Respiration rates were similar for segments of leaves grown on water in light either at 32° or at 22° but by 20–30% lower for 32°-grown plants when they had been raised in darkness or on nutrient solution. In contrast to 22°-grown tissue, respiration of 32°-grown leaf segments was rather insensitive to KCN. Comparative inhibitor studies indicated the presence of both the cyanide-sensitive and the cyanide-insensitive pathway of respiration in 32°-grown leaves. — 4. Leaf microbody marker enzymes were present in leaves grown at 32°. From chlorotic parts of 32°-light-grown leaves a typical microbody fraction was isolated on sucrose densitygradients. — 5. Leaves of seedlings grown at 32° contained only very low levels of ribulosediphosphate carboxylase activity and of fraction I protein. Photosynthetic ¹⁴CO₂-fixation of such leaves was only a few per cent of that observed in normal leaves, and no photosynthetic oxygen evolution was observed in chlorotic leaf segments. However, ten other soluble enzymes which are exclusively or partially localized in chloroplasts reached high activities under all conditions at 32° (Table 4). — 6. From chlorotic parts of 32°-light-grown leaves as well as from etiolated 32°-grown leaves a fraction of intact plastids was isolated and purified by sucrose gradient centrifugation which contained several soluble chloroplast enzymes. From the results we conclude that cytoplasmic protein synthesis must contribute a functional chloroplast envelope including the mechanism for the recognition and uptake of chloroplast proteins which are synthesized on cytoplasmic ribosomes.     

Effects of pH and other factors on the phosphate dependence of photosynthesis in spinach chloroplasts

Chloroplast stromal volume and pH influenced the phosphate (Pi)-dependence of photosynthesis of spinach (Spinacia oleracea L.) chloroplasts. Decreasing the sorbitol concentration in the reaction mixture from 0.35 to 0.25 M, or decreasing the external pH from 8.3 to 7.3, extended the induction period of photosynthesis and decreased both the optimal [Pi] and the minimal [Pi] required to inhibit O₂ evolution completely. At least part of the effect of external pH was attributable to changes in stromal pH on the basis of effects of NH₄Cl and sodium acetate at a constant external pH. When the external pH was increased from 7.3 to 8.3, the stromal pH changed only about 0.6 pH units. Hence, the pH gradient across the envelope was diminished and the efflux of phosphoglycerate relative to dihydroxyacetone phosphate was enhanced.Calvin-cycle activity, varied with light intesity or electron transport inhibitors, affected the rate of photosynthesis but not the induction period or the Pi optimum for photosynthesis. Relatively low Calvin-cycle activity was apparently sufficient to fill metabolite pools and thus terminate the induction period. The results indicate that pH does not affect the Pi dependence of photosynthesis by reducing Calvin-cycle activity. Rather, it is postulated that at low stromal pH, larger metabolic pools are required to maintain maximum rates of photosynthesis because of changes in substrate affinity of some Calvin-cycle enzymes. Consequently, chloroplast photosynthesis would be more sensitive to exogenous Pi.Abbreviations DHAP dihydroxyacetone phosphate - PGA 3-phosphoglycerate - Pi inorganic phosphate Cooperative investigations of the North Carolina Agricultural Research Service and Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, Raleigh, N.C. Paper No. 6391 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, N.C., USA     

探究非绿叶植物的光合作用

光合作用是高中生物学中重要的一部分,现行各个版本的高中教材均将绿叶中色素的提取作为一个重要的实验。自然界中非绿叶植物虽然只占很小的比例,但是由于叶片的颜色与常见的绿色叶片不同,使其光合作用很容易引起学生的注意,作为学生课外探究实验非常具有可行性。本实验通过提取、分离非绿叶植物叶片中的色素及测定其色素的作用光谱来探究非绿叶植物的光合作用,并以此为契机引入探究实验的一般模式．引导学生掌握科学探究的过程。     

Interaction of cadmium with glutathione and photosynthesis in developing leaves and chloroplasts of Phragmites australis (Cav.) Trin. ex Steudel

We investigated how the presence of cadmium (Cd) at the emergence of Phragmites australis Trin. (Cav.) ex Steudel plants from rhizomes interacted with leaf and chloroplast physiological and biochemical processes. About 8.5 nmol Cd mg-1 chlorophyll was found in leaves, and 0.83 nmol Cd mg-1 chlorophyll was found in chloroplasts of plants treated with 50 microm Cd. As a result, a 30% loss of chlorophyll was measured concomitantly with a comparable percentage reduction in light-saturated photosynthesis. Rubisco content and activity were lowered by 10% and 60%, respectively. Antioxidant activity was stimulated by Cd treatment and was associated with an increase in the glutathione and pyridine pools, and with a larger pool of reduced glutathione. It is suggested that the glutathione pool and its predominance in the reduced state protected the activity of many key photosynthetic enzymes against the thiophilic binding of Cd. Chloroplast ultrastructure was not significantly altered with 50 microm treatment and the efficiency of photosystem II, measured as the fluorescence ratio Fv/Fm, remained high because F0 and Fm were proportionally decreased. In plants treated with 100 microm Cd, all effects were exacerbated, but Fv/Fm remained close to that of control leaves and the glutathione and pyridine nucleotides pools were lowered. The results suggest that glutathione exerted a direct important protective role on photosynthesis in the presence of Cd.     

Characteristics of thermoluminescence bands of intact leaves and isolated chloroplasts in relation to the water-splitting activity in photosynthesis.

Plant materials (intact leaves, chloroplasts or subchloroplast particles) pre-illuminated at a low temperature (e.g. -60 degrees C) were rapidly cooled to -196 degrees C and then the luminescence emitted from the sample on raising the temperature was measured as a function of temperature, by means of a sensitive photo-electron counting technique. Mature spinach leaves showed five luminescence bands at different temperatures which were denoted as ZV, A, B1, B2 and C bands. The A, B1, B2 and C bands appeared at constant temperatures, -10, +25, +40 and +55 degrees C, respectively, being independent of the illumination temperature, but the ZV band appeared at a variable temperature slightly higher than the illumination temperature. The B1 and B2 bands were absent in the thermoluminescence profiles of samples devoid of the oxygen-evolving activity, such as heat-treated spinach leaves, wheat leaves greened under intermittent illumination and photosystem-II particles prepared with Triton X-100. It was deduced that these luminescence bands arise from the energy stored by the electron flow in photosystem II to evolve oxygen, and other bands were ascribed to charge-separation in some other sites not related to the oxygen evolving system.     

Effect of 3-formylchromones substituted with 4-aminosalicylic acid and some other aniline derivatives on photosynthesis inhibition in spinach chloroplasts

The inhibitory effect was investigated of 16 different 3-formylchromone derivatives, the condensation products of 6-R¹-3-formylchromone with 4-aminosalicylic acid and of the adducts of 6-R¹-3-formylchromone withn-alcohols and aminosalicylic acids or some other derivatives of aniline, on the photochemical activity of spinach chloroplasts. The inhibitory activity of the compounds studied correlated with the lipophilicity of the R¹ and R² (alkoxy) substituents. Using fluorescence study it was found that the site of action of the studied effectors is photosystem (PS) 2. By EPR spectroscopy it was confirmed that the studied effectors interact with the intermediates Z⁺/Y⁺ which are situated in D 1 and D 2 proteins on the donor side of PS 2 which is reflected in a partial decrease in the photosynthetic electron flow through PS 2 to PS 1. It was found that the core of PS 2 is not damaged.     

Role of orthophosphate and other factors in the regulation of starch formation in leaves and isolated chloroplasts

Starch synthesis in leaves was increased by phosphate starvation or by treatments which decreased cytoplasmic orthophosphate levels (such as mannose feeding). Usually less than 30% of the total carbon fixed during CO₂ assimilation was incorporated into starch in spinach (Spinacia oleracea L.), spinach beet (Beta vulgaris), and tobacco (Nicotiana tabacum) leaves.     

Effect of linolenate on photosynthesis by intact spinach chloroplasts II. Influence of preillumination of leaves on the inhibition of photosynthesis by linolenate

The inhibitory effect of linolenate on intact spinach chloroplastsdepends on the level of the internal pool of metabolites. Chloroplastsfrom preilluminated leaves or chloroplasts artificially loadedwith 3-phosphoglyceric acid required higher concentrations oforthophosphate for maximal rates of CO₂ dependent O₂ evolutionthan untreated chloroplasts. The loaded chloroplasts were moresensitive to linolenate, and in the presence of linolenate theoptimal phosphate concentration was shifted toward lower values.We propose that the inhibition of photosynthesis by linolenateis due to inhibition of the "phosphate translocator". ¹ Part of this work has been published in the Book of Abstracts,4th International Congress on Photosynthesis, Reading, U.K.,1977, p. 265–266. ² This work is part of a doctoral programme carried out by L.Mv6 Akamba in this laboratory. ³ To whom reprint requests should be adressed. (Received October 14, 1978; )     

Distribution of enzymes in mesophyll and parenchyma-sheath chloroplasts of maize leaves in relation to the C4-dicarboxylic acid pathway of photosynthesis

1. Mesophyll and parenchyma-sheath chloroplasts of maize leaves were separated by density fractionation in non-aqueous media. 2. An investigation of the distribution of photosynthetic enzymes indicated that the mesophyll chloroplasts probably contain the entire leaf complement of pyruvate,P(i) dikinase, NADP-specific malate dehydrogenase, glycerate kinase and nitrite reductase and most of the adenylate kinase and pyrophosphatase. The fractionation pattern of phosphopyruvate carboxylase suggested that this enzyme may be associated with the bounding membrane of mesophyll chloroplasts. 3. Ribulose diphosphate carboxylase, ribose phosphate isomerase, phosphoribulokinase, fructose diphosphate aldolase, alkaline fructose diphosphatase and NADP-specific ;malic' enzyme appear to be wholly localized in the parenchyma-sheath chloroplasts. Phosphoglycerate kinase and NADP-specific glyceraldehyde phosphate dehydrogenase, on the other hand, are distributed approximately equally between the two types of chloroplast. 4. After exposure of illuminated leaves to (14)CO(2) for 25sec., labelled malate, aspartate and 3-phosphoglycerate had similar fractionation patterns, and a large proportion of each was isolated with mesophyll chloroplasts. Labelled fructose phosphates and ribulose phosphates were mainly isolated in fractions containing parenchyma-sheath chloroplasts, and dihydroxyacetone phosphate had a fractionation pattern intermediate between those of C(4) dicarboxylic acids and sugar phosphates. 6. These results indicate that the mesophyll and parenchyma-sheath chloroplasts have a co-operative function in the operation of the C(4)-dicarboxylic acid pathway. Possible routes for the transfer of carbon from C(4) dicarboxylic acids to sugars are discussed.     

Sulfur dioxide inhibition of photosynthesis in isolated spinach chloroplasts

Photosynthetic oxygen evolution by isolated spinach (Spinacia oleracea L.) chloroplasts approached complete inhibition in the presence of a 5 mm concentration of sulfur dioxide. A similar inhibition was observed in the presence of equimolar concentrations of bisulfite ions, suggesting a parallel mode of action. In contrast, an equimolar concentration of sulfite ions was markedly less inhibitory and sulfate ions caused negligible inhibition of apparent photosynthesis. The mode of action of sulfur dioxide and related sulfur anions in inhibiting photosynthesis was found to be essentially independent of direct hydrogen-ion effects. Supplements of inorganic pyrophosphate lessened the inhibition of oxygen evolution caused by sulfur dioxide and the sulfur anions.Sulfur dioxide and the sulfur anions were almost equally effective in inhibiting cyclic and noncyclic photophosphorylation in chloroplast suspensions. However, the extent of the inhibition of these photosynthetic reactions does not appear sufficient to account for the inhibition of photosynthetic oxygen evolution by sulfur dioxide.     

Inhibition of photosynthesis by oxygen in isolated spinach chloroplasts

The inhibition of photosynthetic CO₂ fixation by O₂, commonly referred to as the Warburg effect, was examined in isolated intact spinach (Spinacia oleracea) chloroplasts. The major characteristics of this effect in isolated chloroplasts are rapid reversibility when O₂ is replaced by N₂, an increased inhibition by O₂ at low concentrations of CO₂ and a decreased effect of O₂ with increased concentrations of CO₂.     

叶绿体光合代谢中活性氧的产生与清除

有氧代谢不可避免产生活性氧(ROS),叶绿体的PSI和PSII反应中心均是ROS产生的主要位点。叶绿体产生的ROS主要有超氧阴离子(O2-)、过氧化氢(H2O2)、羟自由基(.OH)和单线氧(1O2),其中在PSI产生的O2-将进一步产生H2O2和.OH,而1O2产生在PSII。正常生理代谢条件下,叶绿体内抗氧化系统和光能吸收利用的调节保持活性氧产生和消灭的平衡,不会影响植物的正常生理功能。     

Effects of glyoxylate on photosynthesis by intact chloroplasts

Because glyoxylate inhibits CO₂ fixation by intact chloroplasts and purified ribulose bisphosphate carboxylase/oxygenase, glyoxylate might be expected to exert some regulatory effect on photosynthesis. However, ribulose bisphosphate carboxylase activity and activation in intact chloroplasts from Spinacia oleracea L. leaves were not substantially inhibited by 10 millimolar glyoxylate. In the light, the ribulose bisphosphate pool decreased to half when 10 millimolar glyoxylate was present, whereas this pool doubled in the control. When 10 millimolar glyoxylate or formate was present during photosynthesis, the fructose bisphosphate pool in the chloroplasts doubled. Thus, glyoxylate appeared to inhibit the regeneration of ribulose bisphosphate, but not its utilization.

The fixation of CO₂ by intact chloroplasts was inhibited by salts of several weak acids, and the inhibition was more severe at pH 6.0 than at pH 8.0. At pH 6.0, glyoxylate inhibited CO₂ fixation by 50% at 50 micromolar, and glycolate caused 50% inhibition at 150 micromolar. This inhibition of CO₂ fixation seems to be a general effect of salts of weak acids.

Radioactive glyoxylate was reduced to glycolate by chloroplasts more rapidly in the light than in the dark. Glyoxylate reductase (NADP⁺) from intact chloroplast preparations had an apparent K_m (glyoxylate) of 140 micromolar and a V_max of 3 micromoles per minute per milligram chlorophyll.

     

Imaging of organelles by electron microscopy reveals protein-protein interactions in mitochondria and chloroplasts

Ongoing progress in electron microscopy (EM) offers now an opening to visualize cells at the nanoscale by cryo-electron tomography (ET). Large protein complexes can be resolved at near-atomic resolution by single particle averaging. Some examples from mitochondria and chloroplasts illustrate the possibilities with an emphasis on the membrane organization. Cryo-ET performed on non-chemically fixed, unstained, ice-embedded material can visualize specific large membrane protein complexes. In combination with averaging methods, 3D structures were calculated of mitochondrial ATP synthase at 6 nm resolution and of chloroplast photosystem II at 3.5 nm.     

Interaction between photosynthesis and respiration in illuminated leaves

Plants are sessile organisms that often receive excessive amounts of light energy. This excess energy can be exported from the chloroplasts and dissipated by the mitochondrial respiratory chain. The inner membrane of plant mitochondria possesses unique non-phosphorylating pathways, involving alternative oxidase and type II NAD(P)H dehydrogenases. There are accumulating amounts of evidence showing that these energy-wasteful pathways are up-regulated under excess light conditions, suggesting that they play key roles in efficient photosynthesis. Based on recent advances in our understanding about the metabolic interaction between chloroplasts and mitochondria, we discuss the importance of the respiratory chain for stabilizing the photosynthetic system.