Nature of the Inorganic Carbon Species Actively Taken Up by the Cyanobacterium Anabaena variabilis

The nature of the inorganic carbon (C_i) species actively taken up by cyanobacteria CO₂ or HCO₃⁻ has been investigated. The kinetics of CO₂ uptake, as well as that of HCO₃⁻ uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO₂ was higher than that for HCO₃⁻. Though the calculated V_max was the same in both cases, the maximum rate of uptake actually observed was higher when HCO₃⁻ was supplied. C_i uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO₂ was the species supplied. Observations of photosynthetic rate as a function of intracellular C_i level (following supply of CO₂ or HCO₃⁻ for 5 seconds) led to the inference that HCO₃⁻ is the species which arrives at the inner membrane surface, regardless of the species supplied. When the two species were supplied simultaneously, mutual inhibition of uptake was observed.

On the basis of these and other results, a model is proposed postulating that a carboic anhydrase-like subunit of the C_i transport apparatus binds CO₂ and releases HCO₃⁻ at or near a membrane porter. The latter transports HCO₃⁻ ions to the cell interior.

     

Metabolism of alpha-Ketoglutarate by Roots of Woody Plants

The uptake and metabolism of α-ketoglutarate-5-¹⁴C by peach, apple, and privet root tissues were studied over various time intervals. As much as 80% of the absorbed ¹⁴C appeared as ¹⁴CO₂ in 320 minutes in peach roots. Apple and privet roots were less effective in this conversion with the bulk of the ¹⁴C found in the organic acid fraction. This indicates differences in organic acid metabolism among species of woody plants.

The ¹⁴C accumulated in malate earlier and in larger quantities than in citrate. Both glutamate and aspartate were labeled in 10 minutes and glutamate was labeled as early as 3 minutes. The labeling pattern does not clearly distinguish between the synthesis of glutamate by glutamic dehydrogenase or by transamination with oxaloacetate.

The rapid metabolism of α-ketoglutarate to glutamate by the 3 species studied indicates the presence of enzyme systems important in amino acid synthesis in the roots of woody plants.

     

Inorganic Nitrogen Metabolism in Barley Roots Under Poorly Aerated Conditions

Nitrate reduction, nitrite reduction, and ammonium assimilationwere measured over c. 24 h in excised sterile barley roots,in air or under low oxygen tensions. Partial anoxia had relativelylittle effect, but the pathway of nitrogen assimilation wasseverely inhibited during complete anoxia, when the uptake ofnitrate ceased. Much of the nitrate which was present in theroots at the time of excision was apparently unavailable forassimilation. None of the reactions of the pathway served inplace of oxygen as an electron acceptor under anaerobic conditions.The concentration of nitrate in the external solution duringgrowth and during the experimental treatments had no directeffect on anaerobic ethanol formation, although an indirecteffect was noted which was due to variations in the carbohydratecontent of the tissue.     

Lactate Is Released and Taken Up by Isolated Rabbit Vagus Nerve During Aerobic Metabolism

Abstract: To determine if lactate is produced during aerobic metabolism in peripheral nerve, we incubated pieces of rabbit vagus nerve in oxygenated solution containing d -[U-¹⁴C]glucose while stimulating electrically. After 30 min, nearly all the radioactivity in metabolites in the nerve was in lactate, glucose 6-phosphate, glutamate, and aspartate. Much lactate was released to the bath: 8.2 pmol (µg dry wt)⁻¹ from the exogenous glucose and 14.2 pmol (µg dry wt)⁻¹ from endogenous substrates. Lactate release was not increased when bath P o ₂ was decreased, indicating that it did not come from anoxic tissue. When the bath contained [U-¹⁴C]lactate at a total concentration of 2.13 m M and 1 m M glucose, ¹⁴C was incorporated in CO₂ and glutamate. The initial rate of formation of CO₂ from bath lactate was more rapid than its formation from bath glucose. The results are most readily explained by the hypothesis that has been proposed for brain tissue in which glial cells supply lactate to neurons.     

Assimilation of NO(3) Taken Up by Plants in the Light and in the Dark

An experiment was conducted to determine the extent that NO₃⁻ taken up in the dark was assimilated and utilized differently by plants than NO₃⁻ taken up in the light. Vegetative, nonnodulated soybean plants (Glycine max L. Merrill, `Ransom') were exposed to ¹⁵NO₃⁻ throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing ¹⁴NO₃⁻, with plants sampled subsequently at each light/dark transition over 3 days. The rates of ¹⁵NO₃⁻ absorption were nearly equal in the light and dark (8.42 and 7.93 micromoles per hour, respectively); however, the whole-plant rate of ¹⁵NO₃⁻ reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). The lower rate of reduction in the dark was associated with both substantial retention of absorbed ¹⁵NO₃⁻ in roots and decreased efficiency of reduction of ¹⁵NO₃⁻ in the shoot. The rate of incorporation of ¹⁵N into the insoluble reduced-N fraction of roots in darkness (1.10 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant ¹⁵NO₃⁻ reduction in darkness.

A large portion of the ¹⁵NO₃⁻ retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of ¹⁵NO₃⁻ acquired during the light period. Taking into account reduction of NO₃⁻ from all endogenous pools, it was apparent that plant reduction in a given light period (~13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO₃⁻ (8.42 micromoles per hour) during that period. The primary source of substrate for NO₃⁻ reduction in the dark was exogenous NO₃⁻ being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO₃⁻ in the light occurred in the root system.

     

Regulation of Photosynthetic Metabolism by Low-Temperature Treatment of Roots of Single-Rooted Soybean Plants

The single-rooted leaf of soybean (Glycine max L. Merr.) wasused to study source-sink relationships in photosynthesis. Theroots grown in a culture solution at 20 ? 1?C were treated withlow temperatures (minimum 6?C) for 4 days to change the source/sinkbalance. This treatment suppressed the increase of root dryweight, but by 2 days after the end of the treatment, the leafdry weight showed a 1.7-fold increase mainly due to the accumulationof starch and sucrose. In the roots, both contents also increasedsignificantly. The sucrose and starch contents in leaves andthe sucrose content in roots began decreasing immediately afterthe treatment was stopped, while the starch content in rootsremained at the high level. The leaf area did not change throughoutthe experimental period. The low-temperature treatment of theroots caused an abrupt decrease in the photosynthetic activity(41% of the initial value by day 4) with slow recovery afterthe stop treatment was stopped. A significant negative correlationwas found between the photosynthetic rate and the sucrose contentin the leaves given the low-temperature treatment, but no significantcorrelation was found between the rate and the starch content.This result indicates that the photosynthetic metabolism ismore directly influenced by the amount of sucrose in the cytoplasmthan by the starch content in the chloroplast when the source/sinkbalance is changed by low-temperature treatment to suppressroot growth. (Received May 6, 1986; Accepted December 6, 1986)     

Selenocysteine Is Selectively Taken Up by Red Blood Cells

Both organic and inorganic forms of selenium are utilized in the biosynthesis of selenoproteins. Selenite is taken up by red blood cells and then returned to the plasma after reduction, but little is known about the metabolic fate of selenocysteine. We found that selenocysteine was taken up into red blood cells without decomposition into selenide.     

三种湿地植物的生长及根系溶解性有机碳分泌物研究

研究了美人蕉(Canna indica Linn.)、风车草(Cyperus flabelliformis Rottb.)和水鬼蕉(Hymenocallis littoralis (Jack) Salisb.)3种湿地植物在人工气候室水培条件下的根系溶解性有机碳分泌物分泌量及其与生长的关系.结果表明,风车草和美人蕉的植...     

Interactions between Glucose and Inorganic Carbon Metabolism in Chlorella vulgaris Strain UAM 101

Chlorella vulgaris strain UAM 101 has been isolated from the effluent of a sugar refinery. This alga requires glucose to achieve maximal growth rate even under light saturating conditions. The growth rate of cultures grown on light + CO₂ + glucose (3.16 per day) reaches the sum of those grown on light + CO₂ (1.95 per day) and on dark + glucose (1.20 per day). Unlike other Chlorella strains, uptake of glucose (about 2 micromoles per milligram dry weight per hour) was induced to the same extent in the light and dark and was not photosensitive. The rate of dark respiration was not affected by light and was strongly stimulated by the presence of glucose (up to about 40% in 4 hours). The rate of photosynthetic O₂ evolution was measured as a function of the CO₂ concentration. These experiments were conducted with cells which experienced different concentrations of CO₂ or glucose during growth. The maximal photosynthetic rate was inhibited severely by growing the cells in the presence of glucose. A rather small difference in the apparent photosynthetic affinity for extracellular inorganic carbon (from 10-30 micromolar) was found between cells grown under low and high CO₂. Growth with glucose induced a reduction in the apparent affinity (45 micromolar) even though cells had not been provided with CO₂. Experiments performed at different pH values indicate CO₂ as the major carbon species taken from the medium by Chlorella vulgaris UAM 101.     

Carbon Dioxide Fixation by Barley Roots

The non-volatile, 80 per cent.ethanol-soluble products of fixationhave been investigated in excised roots, using C^14O₂ and radiochromatography. The main radioactive compounds separated were malic, citric(or iso-citric), aspartic, and glutamic acids, asparagine andglutamine. Less activity was present in serine, tyrosine, -ketoglutaricacid, and alanine, and in a number of unidentified compounds. The uptake of C^14O₂ was inhibited by virtually anaerobic conditions. From the above observations it is considered likely that C¹⁴is transformed through the reactions of the tricarboxylic acidcycle. C¹⁴ in the soluble fraction was markedly increased by maintainingthe root material in water rather than in a nutrient solutionprior to exposure to C^14O₂ This increase was chiefly in malicacid.     

Metabolism of Inorganic N Compounds by Ammonia-Oxidizing Bacteria

ABSTRACT

Ammonia oxidizing bacteria extract energy for growth from the oxidation of ammonia to nitrite. Ammonia monooxygenase, which initiates ammonia oxidation, remains enigmatic given the lack of purified preparations. Genetic and biochemical studies support a model for the enzyme consisting of three subunits and metal centers of copper and iron. Knowledge of hydroxylamine oxidoreductase, which oxidizes hydroxylamine formed by ammonia monooxygenase to nitrite, is informed by a crystal structure and detailed spectroscopic and catalytic studies. Other inorganic nitrogen compounds, including NO, N₂O, NO₂, and N₂ can be consumed and/or produced by ammonia-oxidizing bacteria. NO and N₂O can be produced as byproducts of hydroxylamine oxidation or through nitrite reduction. NO₂ can serve as an alternative oxidant in place of O₂ in some ammonia-oxidizing strains. Our knowledge of the diversity of inorganic N metabolism by ammonia-oxidizing bacteria continues to grow. Nonetheless, many questions remain regarding the enzymes and genes involved in these processes and the role of these pathways in ammonia oxidizers.     

Micronutrient Cation Sorption by Roots and Uptake by Plants

The sorption of ferric iron, copper, zinc and manganese by wheatseedling roots and by discs of cellulose filter paper was measured.The magnitude of sorption at pH 5-0 was Fe(III) > Cu(II)> Zn(II) > Mn(II). Sorption of Cu(II), Zn(II) and Mn(II)increased with increasing pH whilst sorption of Fe(III) decreased.The patterns of sorption are discussed in the light of our knowledgeof the hydrolysis of the metal ions. It is suggested that metalsadsorbed on root surfaces may be remobilized by organic ligandswhich leak from the root cells. Where an external liquid diffusionpath away from the root does not exist, soluble metal ligandcomplexes might accumulate in the water free space and superficialwater film of the root, thus facilitating their uptake intoroot cells and translocation within the plant. Under such conditionsthe amounts of metal translocated to the shoots of wheat seedlingsare shown to be related to the amounts of metal adsorbed bytheir roots. Key words: Adsorption, Micronutrients, Roots     

Inorganic Carbon Uptake by Chlamydomonas reinhardtii

The rates of CO₂-dependent O₂ evolution by Chlamydomonas reinhardtii, grown with either air levels of CO₂ or air with 5% CO₂, were measured at varying external pH. Over a pH range of 4.5 to 8.5, the external concentration of CO₂ required for half-maximal rates of photosynthesis was constant, averaging 25 micromolar for cells grown with 5% CO₂. This is consistent with the hypothesis that these cells take up CO₂ but not HCO₃⁻ from the medium and that their CO₂ requirement for photosynthesis reflects the K_m(CO₂) of ribulose bisphosphate carboxylase. Over a pH range of 4.5 to 9.5, cells grown with air required an external CO₂ concentration of only 0.4 to 3 micromolar for half-maximal rates of photosynthesis, consistent with a mechanism to accumulate external inorganic carbon in these cells. Air-grown cells can utilize external inorganic carbon efficiently even at pH 4.5 where the HCO₃⁻ concentration is very low (40 nanomolar). However, at high external pH, where HCO₃⁻ predominates, these cells cannot accumulate inorganic carbon as efficiently and require higher concentrations of NaHCO₃ to maintain their photosynthetic activity. These results imply that, at the plasma membrane, CO₂ is the permeant inorganic carbon species in air-grown cells as well as in cells grown on 5% CO₂. If active HCO₃⁻ accumulation is a step in CO₂ concentration by air-grown Chlamydomonas, it probably takes place in internal compartments of the cell and not at the plasmalemma.     

Utilization of Inorganic Carbon by Ulva lactuca

Thalli discs of the marine macroalga Ulva lactuca were given inorganic carbon in the form of HCO₃⁻, and the progression of photosynthetic O₂ evolution was followed and compared with predicted O₂ evolution as based on calculated external formation of CO₂ (extracellular carbonic anhydrase was not present in this species) and its carboxylation (according to the K_m(CO₂) of ribulose-1,5-bisphosphate carboxylase/oxygenase), at two different pHs, assuming a photosynthetic quotient of 1. The K_m(inorganic carbon) was some 2.5 times lower at pH 5.6 than at the natural seawater pH of 8.2, whereas V_max was similar under the two conditions, indicating that the unnaturally low pH per se had no adverse effect on U. lactuca's photosynthetic performance. These results, therefore, could be evaluated with regard to differential CO₂ and HCO₃⁻ utilization. The photosynthetic performance observed at the lower pH largely followed that predicted, with a slight discrepancy probably reflecting a minor diffusion barrier to CO₂ uptake. At pH 8.2, however, dehydration rates were too slow to supply CO₂ for the measured photosynthetic response. Given the absence of external carbonic anhydrase activity, this finding supports the view that HCO₃⁻ transport provides higher than external concentrations of CO₂ at the ribulose-1,5-bisphosphate carboxylase/oxygenase site. Uptake of HCO₃⁻ by U. lactuca was further indicated by the effects of potential inhibitors at pH 8.2. The alleged band 3 membrane anion exchange protein inhibitor 4,4′-diisothiocyanostilbene-2,2′disulphonate reduced photosynthetic rates only when HCO₃⁻ (but not CO₂) could be the extracellular inorganic carbon form taken up. A similar, but less drastic, HCO₃⁻-competitive inhibition of photosynthesis was obtained with Kl and KNO₃. It is suggested that, under ambient conditions, HCO₃⁻ is transported into cells at defined sites either via facilitated diffusion or active uptake, and that such transport is the basis for elevated internal [CO₂] at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation.     

Metabolism of Arginine and Ornithine in Suspension-Cultured Cells and Aseptic Roots of Intact Plants of Atropa belladonna

Suspension-cultured cells and aseptically cultured roots ofintact plants of Atropa belladonna L. removed tropane alkaloidprecursors arginine (Arg) and ornithine (Orn) at nearly an equalrate from the feeding medium. A great part of Arg- and Orn-derived¹⁴C-label was found in ethanol-insoluble compounds, mostly inproteins already after 2 h feeding. Ethanol-soluble label inthe roots was found mainly in amino acids (e.g. glutamine, Gln)after 2 h feeding, and after 20 h also in some intermediatesof the urea cycle (e.g. argininosuccinate). In suspension cultures, subculturing of the initiation callusdecreased both the uptake of the basic amino acids tested andtheir binding on to the apoplastic space. After 20 h feedingwith Arg more label was found in organic acids in stationaryphase suspension cultures with repressed alkaloid synthesisthan in roots producing alkaloids. The growth phase and passagenumber also affected into which amino acids the label was incorporated.When the initiation callus was young (the 3rd passage), theintermediates of the urea cycle were actively labelled, butwhen the initiation callus was older (the 8th passage) and thesuspension formed roots, especially Gln was labelled. Only tracesof -N-methylornithine were detected in feeding experiments withOrn and Arg. Considerable arginase activity with a high pH optimumwas observed in cell suspensions and roots of A. belladonna. Key words: Atropa, arginine, ornithine, roots, suspension culture     

Molybdenum Metabolism in Plants

Abstract: Among the micronutrients essential for plant growth and for microsymbionts, Mo is required in minute amounts. However, since Mo is often sequestered by Fe- or Al-oxihydrox-ides, especially in acidic soils, the concentration of the water-soluble molybdate anion available for uptake by plants may be limiting for the plant, even when the total Mo content of the soil is sufficient. In contrast to bacteria, no specific molybdenum uptake system is known for plants, but since molybdate and sulfate behave similarly and have similar structure, uptake of molybdate could be mediated unspecifically by one of the sulfate transporters. Transport into the different plant organs proceeds via xylem and phloem. A pterin-bound molybdenum is the cofactor of important plant enzymes involved in redox processes: nitrate reductase, xanthine dehydrogenase, aIdehyde oxidase, and probably sulfite oxidase. Biosynthesis of the molybdenum cofactor (Moco) starts with a guanosine-X-phos-phate. Subsequently, a sulfur-free pterin is synthesized, sulfur is added, and finally molybdenum is incorporated. In addition to the molybdopterin enzymes, small molybdopterin binding proteins without catalytic function are known and are probably involved in the storage of Moco. In symbiotic systems the nitrogen supply of the host plant is strongly influenced by the availability of Mo in soil, since both bacterial nitrogenase and NADPH-dependent nitrate reductase of mycorrhizal fungi are Mo enzymes.     

The Roots of Carnivorous Plants

Carnivorous plants may benefit from animal-derived nutrients to supplement minerals from the soil. Therefore, the role and importance of their roots is a matter of debate. Aquatic carnivorous species lack roots completely, and many hygrophytic and epiphytic carnivorous species only have a weakly devel-oped root system. In xerophytes, however, large, extended and/or deep-reaching roots and sub-soil shoots develop. Roots develop also in carnivorous plants in other habitats that are hostile, due to flood-ing, salinity or heavy metal occurance. Information about the structure and functioning of roots of car- nivorous plants is limited, but this knowledge is essential for a sound understanding of the plants’ physiology and ecology. Here we compile and summarise available information on: (1) The morphology of the roots. (2) The root functions that are taken over by stems and leaves in species without roots or with poorly developed root systems; anchoring and storage occur by specialized chlorophyll-less stems; water and nutrients are taken up by the trap leaves. (3) The contribution of the roots to the nutrient supply of the plants; this varies considerably amongst the few investigated species. We compare nutrient uptake by the roots with the acquisition of nutri-ents via the traps. (4) The ability of the roots of some carnivorous species to tolerate stressful conditions in their habitats; e.g., lack of oxygen, saline conditions, heavy metals in the soil, heat during bushfires, drought, and flooding     

Carbon Metabolism Alterations in Sunflower Plants Infected with the Sunflower Chlorotic Mottle Virus

Sunflower chlorotic mottle virus (SuCMoV) causes chlorotic mottling symptoms and important growth reductions and yield losses in sunflower (Helianthus annuus L., cv. Contiflor 7). This paper describes the effects of SuCMoV on some aspects of carbon metabolism of sunflower plants. After symptoms became evident, CO₂ fixation rates decreased, nevertheless, soluble sugars and starch increased in infected leaves. High H₂O₂ accumulation, lipid peroxidation and chlorophyll degradation were, like the other changes, observed only after symptom expression. Increased soluble carbohydrate accumulation was not related to changes in α‐amylase (EC 3.2.1.1) activity, nor in the activities of enzymes associated with sugar import and hydrolysis such as invertase (EC 3.2.1.26) and sucrose synthase (EC 2.4.1.13), suggesting it did not derive from starch hydrolysis nor increased sugar import. Rather, it may derive from recycling of cell components associated with the development of oxidative damage. The physiological alterations caused by this virus share many common features with the development of senescence.