Purification and properties of aromatic L-amino acid decarboxylase (4.1.1.28) of rat brain]

L-aromatic aminoacid decarboxylase has been purified more than thousand times from homogenates of rat brain, in several steps : centrifugation, DEAE-cellulose, CM cellulose, hydroxylapatite, DEAE sephadex. Its properties have been studied, most of them on an intermediate fraction of the purification, because of the instability of the purified enzyme in spite of the addition of different stabilizing agents : the enzyme decarboxylates 5-hydroxytryptophan (5 HTP) and DOPA in a ratio constant throughout the purification but does not decarboxylate tryptophan, tyrosine, histidine at a measurable rate. Optimum pH, Km, Vm, have been measured with 5 HTP and DOPA as substrates. The enzyme has a molecular weight of 115.000, an apparent isoelectric point of 6,4-6,5. It is inhibited by serotonin, dopamine, some cations : Cu++, Fe++, Ni++ by N-ethylmaleimide, sodium dodecylsulfate. Some pyridoxal-5 phosphate (PLP) remains strongly bound to the enzyme. For relatively weak concentrations of substrate, the enzyme is inhibited by an excess of PLP ; for weak concentrations of PLP, the enzyme in inhibited by an excess of substrate, particularly of DOPA. We also observe a spontaneous decarboxylation of the substrates that reaches a plateau and is enhanced by high concentrations of PLP, by serotonin, dopamine, Cu++ and reduced by mercaptoethanol and the presence of crude or boiled homogenates. Several possible explanations of the spontaneous decarboxylation and of the enzymic inhibitions by an excess of PLP and by the substrates are given.     

MicroRNAs as regulators of mitochondrial function: Role in cancer suppression

Background

Mitochondria, essential to the cell homeostasis maintenance, are central to the intrinsic apoptotic pathway and their dysfunction is associated with multiple diseases. Recent research documents that microRNAs (miRNAs) regulate important signalling pathways in mitochondria, and many of these miRNAs are deregulated in various diseases including cancers.

Scope of review

In this review, we summarise the role of miRNAs in the regulation of the mitochondrial bioenergetics/function, and discuss the role of miRNAs modulating the various metabolic pathways resulting in tumour suppression and their possible therapeutic applications.

Major conclusions

MiRNAs have recently emerged as key regulators of metabolism and can affect mitochondria by modulating mitochondrial proteins coded by nuclear genes. They were also found in mitochondria. Reprogramming of the energy metabolism has been postulated as a major feature of cancer. Modulation of miRNAs levels may provide a new therapeutic approach for the treatment of mitochondria-related pathologies, including neoplastic diseases.

General significance

The elucidation of the role of miRNAs in the regulation of mitochondrial activity/bioenergetics will deepen our understanding of the molecular aspects of various aspects of cell biology associated with the genesis and progression of neoplastic diseases. Eventually, this knowledge may promote the development of innovative pharmacological interventions. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.     

Effects of rhizobia inoculation and nitrogen fertilization on photosynthetic physiology of soybean

Plant growth, contents of photosynthetic pigments, photosynthetic gas exchange, and chlorophyll (Chl) fluorescence in soybean [Glycine max (L.) Merr. cv. Heinong37] were investigated after it was inoculated with Sinorhizobium fredii USDA191 or treated with 5 mM (NH₄)₂SO₄ (N5) and 30 mM (NH₄)₂SO₄ (N30), respectively. In the plants following N5 fertilization, not only plant biomass, leaf area, and Chl content, but also net photosynthetic rate (P _N), stomatal conductance (g _s), carboxylation efficiency (CE), maximum photochemical efficiency (F_v/F_m) of photosystem 2 (PS2), and quantum yield of PS2 (Φ_PS2) were markedly improved as compared with the control plants. There were also positive effects on plant growth and plant photosynthesis after rhizobia inoculation, but the effects were much less than those of N5 fertilization. For N30 plants there were no significant positive effects on plant growth and photosynthetic capacity. Plant biomass, P _N, and g _s were similar to those of N-limited (control) plants. Φ_PS2 and photochemical quenching (q_P) were obviously declined while content of carotenoids and non-photochemical quenching (q_N) were significantly enhanced in N30 treated plants. This indicated that excess N supply may cause some negative effects on soybean plants.     

Response of gas exchange of rape to ozone concentration and exposure regime

Gas exchange characteristics, Pn-PAR and/or Pn-CO₂ response parameters of rape (Brassica napus L) were studied in the field under different O₃ concentrations (CF, 50 nl·L^-1 and 100 nl·L^-1) and fumigation regimes (constant concentration and dynamic varying concentrations) with a portable photosynthetic system (CIRAS-1). Results indicated: (1) Relatively to CF, higher ozone concentration decreased in Tr and increased in WUE under a constant concentration fumigation regime. In addition, an increase in Tr and decreases in Pn, Gs and WUE were found in the dynamic ozone exposure regime; (2) In the constant concentration exposure regimes, AQY, LSP and P_max were markedly reduced and Rp and Γ were enhanced with an increase in ozone concentration (100 nl·L^-1). There were significant differences in the parameters such as Rd, LCP, LSP, P_max and CE between O₃ exposure regimes; (3) Regardless of the exposure regimes which were imposed, the increase of O₃ concentration induced significant decreases in Fv/Fo and Fv/Fm of the eighth leaves from the top canopy, but it had no effect on the fifth full-spread leaves. It can be concluded that the dynamic ozone exposure regime has greater detrimental effects on the photosynthesis of rape in spite of equal exposure dose, and thus is unfavorable for plant growth and the accumulation of dry matter.     

Acclimation of leaf respiration consistent with optimal photosynthetic capacity

Plant respiration is an important contributor to the proposed positive global carbon‐cycle feedback to climate change. However, as a major component, leaf mitochondrial (‘dark’) respiration (R_d) differs among species adapted to contrasting environments and is known to acclimate to sustained changes in temperature. No accepted theory explains these phenomena or predicts its magnitude. Here we propose that the acclimation of R_d follows an optimal behaviour related to the need to maintain long‐term average photosynthetic capacity (V_cmax) so that available environmental resources can be most efficiently used for photosynthesis. To test this hypothesis, we extend photosynthetic co‐ordination theory to predict the acclimation of R_d to growth temperature via a link to V_cmax, and compare predictions to a global set of measurements from 112 sites spanning all terrestrial biomes. This extended co‐ordination theory predicts that field‐measured R_d and V_cmax accessed at growth temperature (R_d,tg and V_cmax,tg) should increase by 3.7% and 5.5% per degree increase in growth temperature. These acclimated responses to growth temperature are less steep than the corresponding instantaneous responses, which increase 8.1% and 9.9% per degree of measurement temperature for R_d and V_cmax respectively. Data‐fitted responses proof indistinguishable from the values predicted by our theory, and smaller than the instantaneous responses. Theory and data are also shown to agree that the basal rates of both R_d and V_cmax assessed at 25°C (R_d,25 and V_cmax,25) decline by ~4.4% per degree increase in growth temperature. These results provide a parsimonious general theory for R_d acclimation to temperature that is simpler—and potentially more reliable—than the plant functional type‐based leaf respiration schemes currently employed in most ecosystem and land‐surface models.     

基于FvCB模型分析盐分胁迫对棉花叶片光合作用的影响

为深入理解叶片光合特性对盐胁迫的响应机理,以棉花为试验材料,设置5个盐分(NaCl)浓度处理:0(CK)、50、100、150和200 mmol·L^-1,利用FvCB模型分析盐胁迫对棉花幼苗叶片光合特性的影响。结果表明:与CK相比,50和100 mmol·L^-1盐分处理增加了棉花叶片的最大羧化速率(V_{c max})和最大电子传递速率(J_max),但150和200 mmol·L^-1盐分处理显著降低了V_{c max}和J_max。叶片净光合速率(P_n)、叶肉导度(g_m)和暗呼吸速率(R_d)随盐分浓度升高而下降;与CK相比,50和100 mmol·L^-1盐分处理对g_m无显著影响,但P_n和R_d显著降低。150和200 mmol·L^-1盐分处理明显降低了P_n、g_m和R_d,且与0、50和100 mmol·L^-1盐分处理间存在显著差异;利用FvCB模型模拟了不同盐分胁迫下叶片净光合速率。与不考虑g_m的模拟结果相比,考虑g_m提高模拟值和实测值间的决定系数,并降低了平均绝对误差。棉花幼苗耐盐阈值为100～150 mmol·L^-1,随盐分浓度的增加,光合限制因素由叶肉因素转变为光合机构受损;引入g_m可以提高FvCB模型的模拟精度。     

In vivo temperature limitations of photosynthesis in Phaseolus vulgaris L

The purpose of this study was to evaluate the temperature response of photosynthesis in two common bean genotypes differing in crop yield when grown under warm conditions. The cultivar Nobre is sensitive to high temperatures, whereas Diplomata shows better crop yield under high temperatures. Plants were grown in a greenhouse prior to transferring to a controlled environment cabinet for the temperature treatments. In a first experiment, 30 days-old plants were subjected to a short exposure (1 day) at temperatures that varied from 9 °C to 39 °C. Diplomata had lower net CO₂ assimilation rate (A) at 15 °C and 21 °C, but higher from 27 °C to 39 °C. Photosynthetic parameters calculated from modeling the response of A to the intercellular CO₂ concentration suggested that the different temperature responses of the two genotypes are caused by different rates of diffusion of CO₂ to the assimilation site, not by differences in biochemical limitations of photosynthesis. While stomatal conductance (g_s) did not differ between the genotypes, mesophyll conductance (g_m) was slightly greater for Nobre at 15 °C, but much higher in Diplomata from 21 °C to 39 °C. In a second experiment, no difference was observed in biomass accumulation between the two genotypes after growth for 24 days under a 35/20 °C (day/night) regime. Hence, the differences in photosynthesis did not cause variation in plant growth at the vegetative stage. The differential genotypic response of g_m to temperature suggests that g_m might be an important limitation to photosynthesis in Nobre, the common bean genotype sensitive to elevated temperature. However, more studies are needed employing other methods for g_m evaluation to validate these results.     

Rapid responses of C14 clone of Eucalyptus globulus to root drought stress: Time-course of hormonal and physiological signaling

The responses of juvenile plants of forest crops to drought stress are a key stage in the survival of forest populations. In this work, a suitable experimental system to study the early drought resistance mechanisms and signaling in a drought-tolerant clone (C14) of Eucalyptus globulus Labill is proposed. This system, using hydroponic culture and an osmotic agent, polyethylene glycol 8000, was demonstrated to induce severe stress in the root area, affecting the responses of the plantlets at the aerial level. These responses were very fast, beginning only 3 h after the induction of stress, and the results highlight the roles of xylematic abscisic acid (ABA) and pH changes over other signals, such as cytokinins, as early chemical signals in rapid water stress. The relationship between these chemical factors, ABA and pH, and the physiological and water parameters observed were significant, supporting their proposed principal role. This work aids our understanding of underlying responses to hydrological limitations of forest crops, and provides valuable information for further physiological and molecular studies of water stress in this and other tree species.     

The single-process biochemical reaction of Rubisco: a unified theory and model with the effects of irradiance, CO₂ and rate-limiting step on the kinetics of C₃ and C₄ photosynthesis from gas exchange

Photosynthesis is the origin of oxygenic life on the planet, and its models are the core of all models of plant biology, agriculture, environmental quality and global climate change. A theory is presented here, based on single process biochemical reactions of Rubisco, recognizing that: In the light, Rubisco activase helps separate Rubisco from the stored ribulose-1,5-bisphosphate (RuBP), activates Rubisco with carbamylation and addition of Mg²⁺, and then produces two products, in two steps: (Step 1) Reaction of Rubisco with RuBP produces a Rubisco-enediol complex, which is the carboxylase-oxygenase enzyme (Enco) and (Step 2) Enco captures CO₂ and/or O₂ and produces intermediate products leading to production and release of 3-phosphoglycerate (PGA) and Rubisco. PGA interactively controls (1) the carboxylation-oxygenation, (2) electron transport, and (3) triosephosphate pathway of the Calvin-Benson cycle that leads to the release of glucose and regeneration of RuBP. Initially, the total enzyme participates in the two steps of the reaction transitionally and its rate follows Michaelis-Menten kinetics. But, for a continuous steady state, Rubisco must be divided into two concurrently active segments for the two steps. This causes a deviation of the steady state from the transitional rate. Kinetic models are developed that integrate the transitional and the steady state reactions. They are tested and successfully validated with verifiable experimental data. The single-process theory is compared to the widely used two-process theory of Farquhar et al. (1980. Planta 149, 78-90), which assumes that the carboxylation rate is either Rubisco-limited at low CO₂ levels such as CO₂ compensation point, or RuBP regeneration-limited at high CO₂. Since the photosynthesis rate cannot increase beyond the two-process theory's Rubisco limit at the CO₂ compensation point, net photosynthesis cannot increase above zero in daylight, and since there is always respiration at night, it leads to progressively negative daily CO₂ fixation with no possibility of oxygenic life on the planet. The Rubisco-limited theory at low CO₂ also contradicts all experimental evidence for low substrate reactions, and for all known enzymes, Rubisco included.