RESUMEN
Formation of a photorespiration-based CO2-concentrating mechanism in C3-C4 intermediate plants is seen as a prerequisite for the evolution of C4 photosynthesis, but it is not known how efficient this mechanism is. Here, using in vivo Rubisco carboxylation-to-oxygenation ratios as a proxy to assess relative intraplastidial CO2 levels is suggested. Such ratios were determined for the C3-C4 intermediate species Flaveria pubescens compared with the closely related C3 plant F. cronquistii and the C4 plant F. trinervia. To this end, a model was developed to describe the major carbon fluxes and metabolite pools involved in photosynthetic-photorespiratory carbon metabolism and used quantitatively to evaluate the labelling kinetics during short-term (14)CO2 incorporation. Our data suggest that the photorespiratory CO2 pump elevates the intraplastidial CO2 concentration about 3-fold in leaves of the C3-C4 intermediate species F. pubescens relative to the C3 species F. cronquistii.
Asunto(s)
Ciclo del Carbono , Dióxido de Carbono/metabolismo , Flaveria/fisiología , Fotosíntesis , Ribulosa-Bifosfato Carboxilasa/metabolismo , Evolución Biológica , Dióxido de Carbono/análisis , Radioisótopos de Carbono/análisis , Respiración de la Célula , Flaveria/genética , Flaveria/efectos de la radiación , Luz , Modelos Biológicos , Filogenia , Hojas de la Planta/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Ribulosa-Bifosfato Carboxilasa/genética , Especificidad de la Especie , Fosfatos de Azúcar/metabolismoRESUMEN
The growth rate of triploid European aspen (Populus tremula L.) and hybrid aspen (P. tremulaâ ×â Populus tremuloides Michx.) significantly exceeds that of diploid aspen, but the underlying physiological controls of the superior growth rates of these genotypes are not known. We tested the hypothesis that the superior growth rate of triploid and hybrid aspen reflects their greater net photosynthesis rate. Micropropagated clonal plants varying in age from 2.5 to 19 months were used to investigate the ploidy and plant age interaction. The quantum yield of net CO2 fixation (Φ) in leaves of young 2.5-month-old hybrid aspen was lower than that of diploid and triploid trees. However, Φ in 19-month-old hybrid aspen was equal to that in triploid aspen and higher than that in diploid aspen. Φ and the rate of light-saturated net photosynthesis (ANS) increased with plant age, largely due to higher leaf dry mass per unit area in older plants. ANS in leaves of 19-month-old trees was highest in hybrid, medium in triploid and lowest in diploid aspen. Light-saturated photosynthesis had a broad temperature optimum between 20 and 35â °C. Rate of respiration in the dark (RDS) did not vary among the genotypes in 2.5-month-old plants, and the shape of the temperature response was also similar. RDS increased with plant age, but RDS was still not significantly different among the leaves of 19-month-old diploid and triploid aspen, but it was significantly lower in leaves of 19-month-old hybrid plants. The initial differences in the growth of plants with different ploidy were minor up to the age of 19 months, but during the next 2 years, the growth rate of hybrid aspen exceeded that of triploid plants by 2.7 times and of diploid plants by five times, in line with differences in ANS of 19-month-old plants of these species. It is suggested that differences in photosynthesis and growth became more pronounced with tree aging, indicating that ontogeny plays a key role in the expression of superior traits determining the productivity of given genotypes.
Asunto(s)
Dióxido de Carbono/metabolismo , Fotosíntesis/fisiología , Populus/fisiología , Biomasa , Respiración de la Célula , Quimera , Diploidia , Luz , Hojas de la Planta/crecimiento & desarrollo , Hojas de la Planta/metabolismo , Hojas de la Planta/fisiología , Hojas de la Planta/efectos de la radiación , Ploidias , Populus/crecimiento & desarrollo , Populus/metabolismo , Populus/efectos de la radiación , Árboles , TriploidíaRESUMEN
Changes in leaf sugar concentrations are a possible mechanism of short-term adaptation to temperature changes, with natural fluctuations in sugar concentrations in the field expected to modify the heat sensitivity of respiration. We studied temperature-response curves of leaf dark respiration in the temperate tree Populus tremula (L.) in relation to leaf sugar concentration (1) under natural conditions or (2) leaves with artificially enhanced sugar concentration. Temperature-response curves were obtained by increasing the leaf temperature at a rate of 1°C min⻹. We demonstrate that respiration, similarly to chlorophyll fluorescence, has a break-point at high temperature, where respiration starts to increase with a faster rate. The average break-point temperature (T(RD) ) was 48.6 ± 0.7°C at natural sugar concentration. Pulse-chase experiments with ¹4CO2 demonstrated that substrates of respiration were derived mainly from the products of starch degradation. Starch degradation exhibited a similar temperature-response curve as respiration with a break-point at high temperatures. Acceleration of starch breakdown may be one of the reasons for the observed high-temperature rise in respiration. We also demonstrate that enhanced leaf sugar concentrations or enhanced osmotic potential may protect leaf cells from heat stress, i.e. higher sugar concentrations significantly modify the temperature-response curve of respiration, abolishing the fast increase of respiration. Sugars or enhanced osmotic potential may non-specifically protect respiratory membranes or may block the high-temperature increase in starch degradation and consumption in respiratory processes, thus eliminating the break-points in temperature curves of respiration in sugar-fed leaves.
Asunto(s)
Metabolismo de los Hidratos de Carbono/fisiología , Carbohidratos/análisis , Populus/fisiología , Estrés Fisiológico/fisiología , Dióxido de Carbono/metabolismo , Respiración de la Célula/fisiología , Clorofila , Oscuridad , Fluorescencia , Calor , Presión Osmótica , Fotosíntesis/fisiología , Hojas de la Planta/metabolismo , Hojas de la Planta/fisiología , Transpiración de Plantas , Populus/metabolismo , Almidón/metabolismo , Árboles/metabolismo , Árboles/fisiologíaRESUMEN
Rates of carbon fluxes and pool sizes of photosynthetic metabolites in different cellular compartments of barley protoplasts were calculated from the time curves of their labeling in the medium of ¹4CO2. Using membrane filtration procedure, kinetics of ¹4C incorporation into the products of steady-state photosynthesis was determined separately in chloroplasts, mitochondria and cytosol of barley protoplasts illuminated for different periods in the air containing ¹4CO2. To extract the quantitative information, analytical labeling functions P(t) describing the dependence of ¹4C content in the primary, intermediate and end products of a linear reaction chain upon the duration of tracer feeding have been derived. The parameters of these functions represent pool sizes of metabolites and rates of carbon fluxes. The values of these parameters were determined by fitting the experimental labeling curves to the functions P(t) by means of non-linear regression procedure. To elucidate the possible effects of fractionation on the photosynthetic carbon metabolism, the parameters of protoplasts were compared with corresponding values in intact leaves of barley.
Asunto(s)
Carbono/metabolismo , Hordeum/metabolismo , Fotosíntesis/fisiología , Hojas de la Planta/metabolismo , Protoplastos/metabolismo , Dióxido de Carbono/metabolismo , Radioisótopos de Carbono/metabolismo , Hordeum/fisiología , Cinética , Análisis de RegresiónRESUMEN
The MSC16 cucumber (Cucumis sativus L.) mitochondrial mutant was used to study the effect of mitochondrial dysfunction and disturbed subcellular redox state on leaf day/night carbon and nitrogen metabolism. We have shown that the mitochondrial dysfunction in MSC16 plants had no effect on photosynthetic CO(2) assimilation, but the concentration of soluble carbohydrates and starch was higher in leaves of MSC16 plants. Impaired mitochondrial respiratory chain activity was associated with the perturbation of mitochondrial TCA cycle manifested, e.g., by lowered decarboxylation rate. Mitochondrial dysfunction in MSC16 plants had different influence on leaf cell metabolism under dark or light conditions. In the dark, when the main mitochondrial function is the energy production, the altered activity of TCA cycle in mutated plants was connected with the accumulation of pyruvate and TCA cycle intermediates (citrate and 2-OG). In the light, when TCA activity is needed for synthesis of carbon skeletons required as the acceptors for NH(4) (+) assimilation, the concentration of pyruvate and TCA intermediates was tightly coupled with nitrate metabolism. Enhanced incorporation of ammonium group into amino acids structures in mutated plants has resulted in decreased concentration of organic acids and accumulation of Glu.
Asunto(s)
Carbono/metabolismo , Cucumis sativus/metabolismo , ADN Mitocondrial/genética , Nitrógeno/metabolismo , Hojas de la Planta/metabolismo , Western Blotting , Ciclo del Ácido CítricoRESUMEN
Deletion of any of the core enzymes of the photorespiratory cycle, one of the major pathways of plant primary metabolism, results in severe air-sensitivity of the respective mutants. The peroxisomal enzyme hydroxypyruvate reductase (HPR1) represents the only exception to this rule. This indicates the presence of extraperoxisomal reactions of photorespiratory hydroxypyruvate metabolism. We have identified a second hydroxypyruvate reductase, HPR2, and present genetic and biochemical evidence that the enzyme provides a cytosolic bypass to the photorespiratory core cycle in Arabidopsis thaliana. Deletion of HPR2 results in elevated levels of hydroxypyruvate and other metabolites in leaves. Photosynthetic gas exchange is slightly altered, especially under long-day conditions. Otherwise, the mutant closely resembles wild-type plants. The combined deletion of both HPR1 and HPR2, however, results in distinct air-sensitivity and a dramatic reduction in photosynthetic performance. These results suggest that photorespiratory metabolism is not confined to chloroplasts, peroxisomes, and mitochondria but also extends to the cytosol. The extent to which cytosolic reactions contribute to the operation of the photorespiratory cycle in varying natural environments is not yet known, but it might be dynamically regulated by the availability of NADH in the context of peroxisomal redox homeostasis.
Asunto(s)
Arabidopsis/metabolismo , Citosol/metabolismo , Ácidos Glicéricos/metabolismo , Piruvatos/metabolismo , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/fisiología , Dióxido de Carbono/metabolismo , Cloroplastos/metabolismo , Eliminación de Gen , Hidroxipiruvato Reductasa/genética , Hidroxipiruvato Reductasa/fisiología , Mutación , NADP/metabolismo , Oxígeno/metabolismo , Fotosíntesis , Hojas de la Planta/metabolismoRESUMEN
The mitochondrial multienzyme glycine decarboxylase (GDC) catalyzes the tetrahydrofolate-dependent catabolism of glycine to 5,10-methylene-tetrahydrofolate and the side products NADH, CO(2), and NH(3). This reaction forms part of the photorespiratory cycle and contributes to one-carbon metabolism. While the important role of GDC for these two metabolic pathways is well established, the existence of bypassing reactions has also been suggested. Therefore, it is not clear to what extent GDC is obligatory for these processes. Here, we report on features of individual and combined T-DNA insertion mutants for one of the GDC subunits, P protein, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana). The individual knockout of either of these two genes does not significantly alter metabolism and photosynthetic performance indicating functional redundancy. In contrast, the double mutant does not develop beyond the cotyledon stage in air enriched with 0.9% CO(2). Rosette leaves do not appear and the seedlings do not survive for longer than about 3 to 4 weeks under these nonphotorespiratory conditions. This feature distinguishes the GDC-lacking double mutant from all other known photorespiratory mutants and provides evidence for the nonreplaceable function of GDC in vital metabolic processes other than photorespiration.
Asunto(s)
Arabidopsis/enzimología , Glicina-Deshidrogenasa (Descarboxilante)/metabolismo , Arabidopsis/fisiología , Cotiledón , Eliminación de Gen , Glicina-Deshidrogenasa (Descarboxilante)/genética , Mutagénesis Insercional , Fotosíntesis/fisiología , Plantones/crecimiento & desarrolloRESUMEN
Oxygen may enhance CO2-saturated photosynthesis in intact leaves, which display the Warburg effect when illuminated at the current atmospheric level of CO2 and O2, of about 350 microl l(-1) and 21%, respectively. The magnitude of the stimulation depends on irradiance. The K(M)(O2) of the stimulation is 128 microM (10.6% O2). Maximum enhancement in wheat leaves is 6.1 and 5.3 micromol m(-2) s(-1 )under 27.9 and 18.7 mW cm(-2), respectively, corresponding to a 25-30% increase in the ribulose 1,5-bisphosphate (RuBP) turnover rate if compared with O2-free ambient gas phase. The stimulation appears in 5-10 s after a sharp increase in O2. In response to a decrease in O2, the new stabilized rate is reached in 5-7 min. The stimulation does not involve any increase in the activity of Rubisco. The effect correlates with increased concentration of RuBP. Oxygen enhances CO2-saturated photosynthesis by acting as a terminal electron acceptor in the photosynthetic electron transport. The magnitude of the effect may be adopted as an index of the pseudocyclic photophosphorylation in vivo.
Asunto(s)
Dióxido de Carbono/farmacología , Luz , Oxígeno/farmacología , Fotosíntesis/efectos de los fármacos , Fotosíntesis/efectos de la radiación , Dióxido de Carbono/metabolismo , Oxígeno/metabolismo , Hojas de la Planta/efectos de los fármacos , Hojas de la Planta/metabolismo , Plantago/metabolismo , Triticum/metabolismoRESUMEN
Potato (Solanum tuberosum L. cv. Désirée) plants with an antisense reduction in the P-protein of the glycine decarboxylase complex (GDC) were used to study the interaction between respiration and photorespiration. Mitochondria isolated from transgenic plants had a decreased capacity for glycine oxidation and glycine accumulated in the leaves. Malate consumption increased in leaves of GDC deficient plants and the capacity for malate and NADH oxidation increased in isolated mitochondria. A lower level of alternative oxidase protein and decreased partitioning of electrons to the alternative pathway was found in these plants. The adenylate status was altered in protoplasts from transgenic plants, most notably the chloroplastic ATP/ADP ratio increased. The lower capacity for photorespiration in leaves of GDC deficient plants was compensated for by increased respiratory decarboxylations in the light. This is interpreted as a decreased light suppression of the tricarboxylic acid cycle in GDC deficient plants in comparison to wild-type plants. The results support the view that respiratory decarboxylations in the light are restricted at the level of the pyruvate dehydrogenase complex and/or isocitrate dehydrogenase and that this effect is likely to be mediated by mitochondrial photorespiratory products.
Asunto(s)
Respiración de la Célula/fisiología , Respiración de la Célula/efectos de la radiación , Glicina-Deshidrogenasa (Descarboxilante)/deficiencia , Luz , Oxígeno/metabolismo , Solanum tuberosum/metabolismo , Solanum tuberosum/efectos de la radiación , Adenosina Difosfato/metabolismo , Adenosina Trifosfato/metabolismo , Dióxido de Carbono/metabolismo , Transporte de Electrón , Complejo IV de Transporte de Electrones/metabolismo , Glicina-Deshidrogenasa (Descarboxilante)/genética , Glicina-Deshidrogenasa (Descarboxilante)/metabolismo , Malatos/metabolismo , Mitocondrias/metabolismo , Oligonucleótidos Antisentido/genética , Oligonucleótidos Antisentido/metabolismo , Oxidación-Reducción , Consumo de Oxígeno , Fotosíntesis , Hojas de la Planta/citología , Hojas de la Planta/metabolismo , Plantas Modificadas Genéticamente , Protoplastos/metabolismo , Solanum tuberosum/enzimología , Solanum tuberosum/genéticaRESUMEN
Acclimation of 25 degrees C-grown Arabidopsis thaliana at 5 degrees C resulted in a marked increase of leaf respiration in darkness (Rd) measured at 5 degrees C. Rd was particularly high in leaves developed at 5 degrees C. Leaf respiration (non-photorespiratory intracellular decarboxylation) in the light (Rl) also increased during cold acclimation, but less so than did Rd. The ratio Rd/Pt (Pt - true photosynthesis) was higher in more acclimated or cold-developed leaves, while the ratio Rl/Pt remained unchanged. In cold-acclimated leaves, Rl did not correlate with 3-phosphoglycerate and pyruvate nor with hexose phosphate pools in the cytosol. Rl in A. thaliana leaves was probably not limited by the substrate during cold acclimation. Under the conditions tested, Rd was more sensitive to low temperature stress than Rl.