Occurence and regulation of Calvin cycle enzymes in non-autotrophic Beggiatoa strains

A field isolate of Bradyrhizobium japonicum which failed to attach polarly or firmly to soybean roots was compared with the laboratory isolate I-110 for its relative rate of growth, piliation, attachment and nodulation. Both isolates grew at a comparable rate in yeast extract-gluconate medium as well as in soybean root exudate, produced comparable amounts of soybean lectin binding polysaccharide, infected through curled root hairs and developed effective nodules. Approximately 5% of cells in cultures of 110 possessed pili but none were detected in cultures of 1007 by electron microscopy. Light microscopic observations of root hairs from roots exposed to 1007 and 110 inoculum showed no polarly attached cells of 1007 and approximately 100 cells of 110 polarly attached per mm root hair length. Plate counting of firmly bound cells released by sonication indicated that the number of 1007 cells firmly adhering was at least 1000-fold lower than the number of 110 cells attached. The significance of polar, firm and weak attachment in the initiation of symbiotic interactions is discussed.Dedicated to the menory of Harry E. Calvert     

Photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica was examined by measuring whole-canopy CO₂ gas exchange and chlorophyll (Chl) a fluorescence of plants growing near Palmer Station along the Antarctic Peninsula. Both species had negligible midday net photosynthetic rates (P_n) on warm, usually sunny, days (canopy air temperature [T_c]> 20°C), but had relatively high P_n on cool days (T_c<10°C). Laboratory measurements of light and temperature responses of P_n showed that high temperature, not visible irradiance, was responsible for depressions in P_n on warm sunny days. The optimal leaf temperatures (T_l) for P_n in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive P_n at 0°C T_l, which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal P_n, and we estimate that their low-temperature compensation points occurred at ?2°C T_l (C. quitensis) and ?3°C (D. antarctica). Because of the strong warming trend along the peninsula over recent decades and predictions that this will continue, we were particularly interested in the mechanisms responsible for their negligible rates of P_n on warm days and their unusually low high-temperature compensation points (i.e., 26°C in C. quitensis and 22°C in D. antarctica). Low P_n at supraoptimal temperature (25°C) appeared to be largely due to high rates of temperature-enhanced respiration. However, there was also evidence for direct impairment of the photosynthetic apparatus at supraoptimal temperature, based on Chl fluorescence and P_n/intercellular CO₂ concentration (c_i) response curve analyses. The breakpoint or critical temperature (T_cr) of minimal fluorescence (F_o) was ≈42°C in both species, which was well above the temperatures where reductions in P_n were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for P_n. The optimal T_l for photochemical quenching (q_p) and the quantum yield of photosystem II (PSII) electron transfer (φ_PSII) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower q_p and greater non-photochemical quenching (q_NP), but had little effect on F_o, maximal fluorescence (F_m) or the ratio of variable to maximal fluorescence (F_v/F_m) in both species. In addition, carboxylation efficiencies or initial slopes of their P_n/c_i response were lower at supraoptimal temperatures, suggesting reduced activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Although continued warming along the peninsula will increase the frequency of supraoptimal temperatures, T_c at our field site averaged 4.3°C and was below the temperature optima for P_n in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of P_n.     

Dependence of the Calvin cycle activity on kinetic parameters for the interaction of non-equilibrium cycle enzymes with their substrates

Kinetic model studies and control analyses of the Calvin photosynthesis cycle have been performed to characterize the dependence of the cycle activity on maximum velocities and Km values for the interaction of the non-equilibrium cycle enzymes and ATP synthetase with their substrates under conditions of light and carbon dioxide saturation. The results show that Km values have no major influence on the cycle activity at optimal concentrations of external orthophosphate. The maximum cycle activity is controlled mainly by the catalytic capacities of ATP synthetase and sedoheptulose-bisphosphatase, and is close to the maximum cycle flux that can be supported by these two enzymes.     

Photosynthetic and respiratory acclimation and growth response of Antarctic vascular plants to contrasting temperature regimes

Air temperatures have risen over the past 50 yr along the Antarctic Peninsula, and it is unclear what impact this is having on Antarctic plants. We examined the growth response of the Antarctic vascular plants Colobanthus quitensis (Caryophyllaceae) and Deschampsia antarctica (Poaceae) to temperature and also assessed their ability for thermal acclimation, in terms of whole-canopy net photosynthesis (P(n)) and dark respiration (R(d)), by growing plants for 90 d under three contrasting temperature regimes: 7°C day/7°C night, 12°C day/7°C night, and 20°C day/7°C night (18 h/6 h). These daytime temperatures represent suboptimal (7°C), near-optimal (12°C), and supraoptimal (20°C) temperatures for P(n) based on field measurements at the collection site near Palmer Station along the west coast of the Antarctic Peninsula. Plants of both species grown at a daytime temperature of 20°C had greater RGR (relative growth rate) and produced 2.2-3.3 times as much total biomass as plants grown at daytime temperatures of 12° or 7°C. Plants grown at 20°C also produced 2.0-4.1 times as many leaves, 3.4-5.5 times as much total leaf area, and had 1.5-1.6 times the LAR (leaf area ratio; leaf area:total biomass) and 1.1-1.4 times the LMR (leaf mass ratio; leaf mass:total biomass) of plants grown at 12° or 7°C. Greater RGR and biomass production at 20°C appeared primarily due to greater biomass allocation to leaf production in these plants. Rates of P(n) (leaf-area basis), when measured at their respective daytime growth temperatures, were highest in plants grown at 12°C, and rates of plants grown at 20°C were only 58 (C. quitensis) or 64% (D. antarctica) of the rates in plants grown at 12°C. Thus, lower P(n) per leaf area in plants grown at 20°C was more than offset by much greater leaf-area production. Rates of whole-canopy P(n) (per plant), when measured at their respective daytime growth temperatures, were highest in plants grown at 20°C, and appeared well correlated with differences in RGR and total biomass among treatments. Colobanthus quitensis exhibited only a slight ability for relative acclimation of P(n) (leaf-area basis) as the optimal temperature for P(n) increased from 8.4° to 10.3° to 11.5°C as daytime growth temperatures increased from 7° to 12° to 20°C. There was no evidence for relative acclimation of P(n) in D. antarctica, as plants grown at all three temperature regimes had a similar optimal temperature (10°C) for P(n). There was no evidence for absolute acclimation of P(n) in either species, as rates of P(n) in plants grown at a daytime temperature of 12°C were higher than those of plants grown at daytime temperatures of 7° or 20°C, when measured at their respective growth temperatures. The poor ability for photosynthetic acclimation in these species may be associated with the relatively stable maritime temperature regime during the growing season along the Peninsula. In contrast to P(n), both species exhibited full acclimation of R(d), and rates of R(d) on a leaf-area basis were similar among treatments when measured at their respective daytime growth temperature. Our results suggest that in the absence of interspecific competition, continued warming along the Peninsula will lead to improved vegetative growth of these species due to (1) greater biomass allocation to leaf-area production (as opposed to improved rates of P(n) per leaf area) and (2) their ability to acclimate R(d), such that respiratory losses per leaf area do not increase under higher temperature regimes.     

The effect of acute high light and low temperature stresses on the ascorbate–glutathione cycle and superoxide dismutase activity in two Dunaliella salina strains

Dunaliella species accumulate carotenoids and their role in protection against photooxidative stress has been investigated extensively. By contrast, the role of other antioxidants in this alga, has received less attention. Therefore, the components of the ascorbate–glutathione cycle, along with superoxide dismutase (E.C. 1.15.1.1) and peroxidase (E.C. 1.11.1.11) activity were compared in two strains of Dunaliella salina. Strain IR‐1 had two‐fold higher chlorophyll and β‐carotene concentration than Gh‐U. IR‐1 had around four‐fold higher superoxide dismutase, ascorbate peroxidase and pyrogallol peroxidase activities than Gh‐U on a protein basis. Ascorbate and glutathione concentrations and redox state did not differ between strains and there was little difference in the activity of ascorbate–glutathione cycle enzymes (monodehydroascorbate reductase [E.C. 1.6.5.4], dehydroascorbate reductase [E.C. 1.8.5.1] and glutathione reductase [E.C. 1.8.1.7]). The response of these antioxidants to high light and low temperature was assessed by transferring cells from normal growth conditions (28°C, photon flux density of 100 μmol m^?2 s^?1)to 28°C/1200 μmol m^?2 s^?1; 13°C/100 μmol m^?2 s^?1; 13°C/1200 μmol m^?2 s^?1 and 28°C/100 μmol m^?2 s^?1 for 24 h. Low temperature and combined high light‐low temperature decreased chlorophyll and β‐carotene in both strains indicating that these treatments cause photooxidative stress. High light, low temperature and combined high light‐low temperature treatments increased the total ascorbate pool by 10–50% and the total glutathione pool by 20–100% with no consistent effect on their redox state. Activities of ascorbate–glutathione cycle enzymes were not greatly affected but all the treatments increased superoxide dismutase activity. It is concluded that D. salina can partially adjust to photooxidative conditions by increasing superoxide dismutase activity, ascorbate and glutathione.     

Characterization of antifreeze activity in Antarctic plants

Deschampsia antarctica and Colobanthus quitensis are the only vascular plants to have colonized the Maritime Antarctic, which is characterized by its permanently low temperature and frequent summer frosts. To understand how the plants survive freezing temperatures year-round, antifreeze activity was assayed in apoplastic extracts obtained from both non-acclimated and cold-acclimated Antarctic plants. By observing the shape of ice crystals grown in dilution series of the extracts, it was found that D. antarctica had antifreeze activity, but C. quitensis did not. D. antarctica exhibited antifreeze activity in the non-acclimated state and this activity increased after cold acclimation. The antifreeze activity in D. antarctica was labile to proteolysis and high temperature, active over a wide pH range, and associated with molecules greater than 10 kDa in molecular weight. These results show that D. antarctica produces antifreeze proteins that are secreted into the apoplast. When examined by SDS-PAGE, the apoplastic extracts from cold-acclimated D. antarctica exhibited 13 polypeptides. It is concluded that D. antarctica accumulates AFPs as part of its mechanism of freezing tolerance. Moreover, this is the first plant in which antifreeze activity has been observed to be constitutive.     

Involvement of thylakoid membranes in supramolecular organisation of Calvin cycle enzymes in Anacystis nidulans

The cells of unicellular photosynthetic cyanobacterium Anacystis nidulans were permeated with lysozyme, toluene, toluene-triton, toluene-triton-lysozyme. Transmission electron microscopy of semi-thin sections (500 nm) using TEM at 160 kV showed that cells permeated with only lysozyme or toluene showed the typical concentric arrangement of thylakoid membranes. However, when toluene-treated cells were further treated with triton and lysozyme the thylakoid membranes were disrupted. Sequential reactions of Calvin cycle were studied in the differentially permeated cells in vivo, using various intermediates such as 3-PGA, GA-3-P, FDP, SDP, R-5-P, RuBP and cofactors like ATP, NADPH depending on the requirement. RuBP and R-5-P + ATP dependent activities could be observed in all types of permeated cells. Sequential reactions of the entire Calvin cycle using 3-PGA could be detected in the cells that had retained the internal organisation of the thylakoid membranes after permeation and were lost on disruption of this organisation. Light dependent CO2 fixation could be detected only in the cells permeated with lysozyme. This activity was abolished in the cells after treatment with toluene. The results suggested that the integrity of thylakoid membranes may be essential for the organisation of sequential enzymes of the Calvin cycle in vivo and facilitate their functioning.     

Effects of long-term action of high temperature and high light on the activity and energy interaction of both photosystems in tomato plants

The acclimation to high light, elevated temperature, and combination of both factors was evaluated in tomato (Solanum lycopersicum cv. M82) by determination of photochemical activities of PSI and PSII and by analyzing 77 K fluorescence of isolated thylakoid membranes. Developed plants were exposed for six days to different combinations of temperature and light intensity followed by five days of a recovery period. Photochemical activities of both photosystems showed different sensitivity towards the heat treatment in dependence on light intensity. Elevated temperature exhibited more negative impact on PSII activity, while PSI was slightly stimulated. Analysis of 77 K fluorescence emission and excitation spectra showed alterations in the energy distribution between both photosystems indicating alterations in light-harvesting complexes. Light intensity affected the antenna complexes of both photosystems stronger than temperature. Our results demonstrated that simultaneous action of high-light intensity and high temperature promoted the acclimation of tomato plants regarding the activity of both photosystems in thylakoid membranes.     

Measuring in vivo elasticities of Calvin cycle enzymes: network structure and patterns of modulations

To measure the kinetics of enzymes, the proteins are usually assayed in vitro after isolation from their parent organisms. We make an attempt to show how one might determine enzyme elasticities in an intact system by a multiple modulation approach. Certain target enzymes are modulated in their activities and the changes in metabolite concentrations and flux rates upon the modulations are used to calculate the enzyme elasticities. Central to this approach is that the modulations must be independent of each other, and an algorithm is developed for finding all independent modulations that allow determining the elasticities of a given enzyme. This approach is applied to a mass-action model of the Calvin cycle. The goal is to determine the elasticities of as many enzymes as possible by modulating the activities of as few of them as possible. It is shown that the elasticities of 20 (out of 22) Calvin cycle enzymes can be determined by modulating just five reactions. Moreover, visualization of independence of modulations may be used to decompose the Calvin cycle into several sections that are independent of each other regarding flow of matter and information.     

Xanthophyll cycle induction by anaerobic conditions under low light in Chlamydomonas reinhardtii

The effect of anaerobiosis on the induction of the xanthophyll cycle was investigated in Chlamydomonas reinhardtii. The results showed that, anaerobiosis obtained by either sulfur starvation or by bubbling nitrogen in the culture grown in complete medium induced the xanthophyll cycle even when cultures were exposed to low light conditions. The zeaxanthin content reached 35 mmol mol^?1 Chl a, after 110 h in anaerobic sulfur-starved cultures, and 30 mmol mol^?1 Chl a within 24 h in sulfur replete cultures bubbled with nitrogen. Both starved and non-starved cultures grown under aerobic conditions, did not exhibit any sizeable increase in the zeaxanthin content. Chlorophyll fluorescence measurements revealed a decrease in the maximum photochemical quantum yield of PSII (F_v/F_m) by more than 50 %. The chlorophyll fluorescence kinetics (OJIP) analysis showed a strong rise at the J-step indicating a strong reduction of Q_A. Our findings demonstrated that anaerobiosis in low light exposed cultures induced the xanthophyll cycle through a strong increase of the level of plastoquinone pool reduction, which was associated to the formation of a trans-thylakoid membranes proton gradient, while in dark anaerobic cultures, no appreciable induction of xanthophyll cycle could be observed, despite the sizeable increase in non–photochemical quenching.     

Physiological and molecular mechanisms of brassinosteroid-induced tolerance to high and low temperature in plants

Brassinosteroids (BRs) are plant hormones that were isolated for the first time in the 1970s. This group currently includes more than 70 compounds that differ in their structure and physiological activity. BRs are present in plants in a free form or in the form of conjugates. BRs are known as plant growth regulators, but they also play a role in the plant response to environmental stresses. In the case of plants that are exposed to low/high temperature, exogenous BRs can counteract growth inhibition and reduce biomass losses as well as increase plant survival. BRs show a multidirectional activity in regulating the metabolism of plants exposed to extreme temperatures. The following BRs actions can be distinguished: changes in membrane physicochemical properties, regulation of the expression of selected genes (including stress-responsive genes), as well as indirect effects on metabolism through other hormones or signalling molecules (such as hydrogen peroxide). This review summarizes the current knowledge about the effects of BRs on the physiological and biochemical processes that occur in plants during exposure to low or high temperatures.     

On the regulatory significance of inhibitors acting on non-equilibrium enzymes in the Calvin photosynthesis cycle

Control analyses and kinetic model studies have been performed in order to obtain quantitative information on the regulatory significance of 12 experimentally well-documented inhibitory interactions of Calvin cycle intermediates with the four non-equilibrium cycle enzymes. Evidence is presented to show that none of these interactions contributes significantly to the cycle flux control over the range of external orthophosphate concentrations where the reaction cycle shows close to optimal activity. Contrary to what has been generally supposed, the examined inhibitions appear to be of little interest for our understanding of the biological regulation of the Calvin photosynthesis cycle under conditions of light and carbon dioxide saturation.     

Onset of morning activity in bumblebee foragers under natural low light conditions

Foraging on flowers in low light at dusk and dawn comes at an additional cost for insect pollinators with diurnal vision. Nevertheless, some species are known to be frequently active at these times. To explore how early and under which light levels colonies of bumblebees, Bombus terrestris, initiate their foraging activity, we tracked foragers of different body sizes using RFID over 5 consecutive days during warm periods of the flowering season. Bees that left the colony at lower light levels and earlier in the day were larger in size. This result extends the evidence for alloethism in bumblebees and shows that foragers differ in their task specialization depending on body size. By leaving the colony earlier to find and exploit flowers in low light, larger‐sized foragers are aided by their more sensitive eyes and can effectively increase their contributions to the colony''s food influx. The decision to leave the colony early seems to be further facilitated by knowledge about profitable food resources in specific locations. We observed that experience accrued over many foraging flights determined whether a bee started foraging under lower light levels and earlier in the morning. Larger‐sized bees were not more experienced than smaller‐sized bees, confirming earlier observations of wide size ranges among active foragers. Overall, we found that most foragers left at higher light levels when they could see well and fly faster. Nevertheless, a small proportion of foragers left the colony shortly after the onset of dawn when light levels were below 10 lux. Our observations suggest that bumblebee colonies have the potential to balance the benefits of deploying large‐sized or experienced foragers during dawn against the risks and costs of foraging under low light by regulating the onset of their activity at different stages of the colony''s life cycle and in changing environmental conditions.     

Litter dynamics of low and high tannin sericea lespedeza plants under field conditions

Sericea lespedeza [Lespedeza cuneata (Dum. Cours) G. Don.] is a perennial legume with high polyphenol (tannin) concentration. The decomposition and nutrient release of sericea lespedeza residues with two tannin levels (high = 22% or low = 16%), which were either placed on the soil surface or buried at a depth of 5 cm were studied. An experiment was carried out at two sites for 2 yr. Litter mass loss and nutrient (N, P, K, Ca, Mg, Cu, Fe, Mn, Zn, B, Mo, Co) release were recorded. Litter mass loss was higher for the low-tannin residues and for those buried. From the beginning of the experiments, the C : N ratio was lower than 25 : 1 and N was mineralized for most of the studied cases; this means that N increased in availability to plants. The release of the rest of the studied nutrients was higher at the depth of 5 cm than on the soil surface and higher for low-tannin residues than for high-tannin. The observed accumulation of some nutrients in the first months presumably depended on microbial activity. The rate of release was higher for the macronutrients than for the micronutrients, with the exception of B, which had the same behaviour as the macronutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

高温强光胁迫对苹果果皮PPO活性的影响

苹果果实日烧是一种普遍发生的生理病害,最常见的特征之一就是在果实表面出现褐变。通常认为PPO与植物的酶促褐变密切相关。研究了自然和控制条件下,高温强光胁迫对果实PPO活性的影响,以便揭示高温强光胁迫下苹果果实褐变与PPO活性之间的联系。结果表明：高温和强光胁迫与果皮PPO活性变化密切相关。就树冠不同方位而言,西南方位是高温和强光胁迫最严重的区域,其外围裸露果实的PPO活性也最强。在一定范围内,随着处理温度和光照强度的升高,果皮PPO活性也逐渐增强。短时间剧烈升温能够引起PPO活性骤然上升。在同样程度的高温胁迫下,提高环境湿度有利于抑制果皮PPO活性,从而减轻褐变症状的发生。室内外试验一致证实：果实日烧褐变现象与高温强光胁迫下果皮组织PPO活性大幅度提高有直接关系。     

Integration of light and temperature signaling pathways in plants

As two of the most important environmental factors, light and temperature regulate almost all aspects of plant growth and development. Under natural conditions, light is accompanied by warm temperatures and darkness by cooler temperatures, suggesting that light and temperature are tightly associated signals for plants. Indeed, accumulating evidence shows that plants have evolved a wide range of mechanisms to simultaneously perceive and respond to dynamic changes in light and temperature. Notably, the photoreceptor phytochrome B (phyB) was recently shown to function as a thermosensor, thus reinforcing the notion that light and temperature signaling pathways are tightly associated in plants. In this review, we summarize and discuss the current understanding of the molecular mechanisms integrating light and temperature signaling pathways in plants, with the emphasis on recent progress in temperature sensing, light control of plant freezing tolerance, and thermomorphogenesis. We also discuss the questions that are crucial for a further understanding of the interactions between light and temperature signaling pathways in plants.