Biomedical subjects

B J Reger

Publications and source records attributed to B J Reger.

8 recordsLinked to original sources

PubMed1981

Photosynthetic Characteristics of Portulaca grandiflora, a Succulent C(4) Dicot : CELLULAR COMPARTMENTATION OF ENZYMES AND ACID METABOLISM.

The succulent, cylindrical leaves of the C(4) dicot Portulaca grandiflora possess three distinct green cell types: bundle sheath cells (BSC) in radial arrangement around the vascular bundles; mesophyll cells (MC) in an outer layer adjacent to the BSC; and water storage cells (WSC) in the leaf center. Unlike typical Kranz leaf anatomy, the MC do not surround the bundle sheath tissue but occur only in the area between the bundle sheath and the epidermis. Intercellular localization of photosynthetic enzymes was characterized using protoplasts isolated enzymatically from all three green cell types.Like other C(4) plants, P. grandiflora has ribulose 1,5-bisphosphate carboxylase and the decarboxylating enzyme, NADP(+)-malic enzyme, in the BSC. Unlike other C(4) plants, however, phosphoenolpyruvate carboxylase, pyruvate, Pi dikinase, and NADP(+)-malate dehydrogenase of the C(4) pathway were present in all three green cell types, indicating that all are capable of fixing CO(2) via phosphoenolpyruvate carboxylase and regenerating phosphoenolpyruvate. Other enzymes were about equally distributed between MC and BSC similar to other C(4) plants. The enzyme profile of the WSC was similar to that of the MC but with reduced activity in most enzymes, except mitochondrion-associated enzymes.Intracellular localization of enzymes was studied in organelles partitioned by differential centrifugation using mechanically ruptured mesophyll and bundle sheath protoplasts. Phosphoenolpyruvate carboxylase was a cytosolic enzyme in both cells; whereas, ribulose 1,5-bisphosphate carboxylase and NADP(+)-malic enzyme were exclusively compartmentalized in the bundle sheath chloroplasts. NADP(+)-malate dehydrogenase, pyruvate, Pi dikinase, aspartate aminotransferase, 3-phosphoglycerate kinase, and NADP(+)-triose-P dehydrogenase were predominantly localized in the chloroplasts while alanine aminotransferase and NAD(+)-malate dehydrogenase were mainly present in the cytosol of both cell types. Based on enzyme localization, a scheme of C(4) photosynthesis in P. grandiflora is proposed.Well-watered plants of P. grandiflora exhibit a diurnal fluctuation of total titratable acidity, with an amplitude of 61 and 54 microequivalent per gram fresh weight for the leaves and stems, respectively. These changes were in parallel with changes in malic acid concentration in these tissues. Under severe drought conditions, diurnal changes in both titratable acidity and malic acid concentration in both leaves and stems were much reduced. However, another C(4) dicot Amaranthus graecizans (nonsucculent) did not show any diurnal acid fluctuation under the same conditions. These results confirm the suggestion made by Koch and Kennedy (Plant Physiol. 65: 193-197, 1980) that succulent C(4) dicots can exhibit an acid metabolism similar to Crassulacean acid metabolism plants in certain environments.

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PubMed1979

Distribution of Photosynthetic Enzymes between Mesophyll, Specialized Parenchyma and Bundle Sheath Cells of Arundinella hirta.

Arundinella hirta L. is a C(4) plant having an unusual C(4) leaf anatomy. Besides mesophyll and bundle sheath cells, A. hirta leaves have specialized parenchyma cells which look morphologically like bundle sheath cells but which lack vascular connections and are located between veins, running parallel to them. Activities of phosphoenolpyruvate and ribulose-1,5-bisphosphate carboxylases and phosphoenolpyruvate carboxykinase, NADP-and NAD-malic enzymes were determined for whole leaf extracts and isolated mesophyll protoplasts, specialized parenchyma cells, and bundle sheath cells. The data indicate that A. hirta is a NADP-malic enzyme type C(4) species. In addition, specialized parenchyma cells and bundle sheath cells are enzymatically alike. Compartmentation of enzymes followed the C(4) pattern with phosphoenolpyruvate carboxylase being restricted to mesophyll cells while ribulose-1,5-bisphosphate carboxylase and decarboxylating enzymes were restricted to bundle sheath and specialized parenchyma cells.

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PubMed1977

Model for variable light sensitivity in imbibed dark-dormant seeds.

The level of light-induced germination of the seed of common purslane (Portulaca oleracea L.) and curly dock (Rumex crispus L.) changes with dark incubation time prior to brief, low energy, red light treatment. The rate at which phytochrome-far red-absorbing form (Pfr) acts in the light-induced population of seeds was measured by quantitating per cent reversals of the red light effect with saturating far red light exposures at successive times after the red light exposure. A linear positive correlation was found between this rate and the final germination level. These results are compatible with a model involving changing levels, during dark incubation, of a component with which Pfr interacts. In this model, germination is initiated after attainment of a certain level of interaction between Pfr and this component. These findings also support the view that the Pfr to Pr decay rate constant and total phytochrome level are stable during dark incubation.

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PubMed1975

Polysome Formation in Light-sensitive Common Purslane Seeds.

Common purslane (Portulaca oleracea L.) seeds show light-controlled dormancy. Ribosome profiles from dark-incubated seeds consist of 22 to 26% polysomes. Light induces germination and stimulates polysome formation during the 12-hour lag period preceding radicle protrusion. Polysome levels increase to 29, 35, and 41% with exposure to 3, 6, and 9 hours of light, respectively. Although polysomes form on imbibition in the dark, 6 hours of light stimulates a significant increase in polysome formation which is probably related to early stages of radicle elongation.

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PubMed1972

Protein synthesis by isolated etioplasts and chloroplasts from pea and wheat and the effects of chloramphenicol and cycloheximide.

Etioplasts capable of incorporating (14)C-leucine into protein have been isolated from dark-grown pea and wheat plants. The requirements for leucine incorporation for etioplasts were similar to those for chloroplasts. An ATP-generating system, Mg(2+), and GTP were required. The amino-acid-incorporation activity of etioplasts from wheat was comparable to that of chloroplasts on an RNA basis, whereas the activity of pea etioplasts was about 50% of the activity of pea chloroplasts. The incorporation of leucine into protein by etioplasts and chloroplasts from pea and wheat was inhibited by chloramphenicol, and to a slight extent by cycloheximide.

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PubMed1972

Light-stimulated Production of a Chloroplast-localized System for Protein Synthesis in Euglena gracilis.

Chloroplasts and proplastids isolated respectively from autotrophic and dark-adapted cells of Euglena gracilis strain Z incorporated (14)C-l-leucine into protein. In each case the incorporation was inhibited by chloramphenicol (50% inhibition at about 5 mug/ml for chloroplasts and 30 mug/ml for proplastids), but not appreciably by cycloheximide at concentrations up to 200 mug/ml. Chloroplasts from autotrophic cells incorporated leucine into protein at rates of about 10 pg leucine per mg RNA in one minute, but isolated proplastids were only 5 to 10% as active. When dark-adapted cells were illuminated there was little increase in the activity of the chloroplast fraction during the first 12 hr. Between 12 and 24 hr, when there was a rapid increase in the rate of synthesis of chlorophyll, the capacity of the chloroplast fraction for protein synthesis increased markedly. Suppression of the formation of a chloroplast-localized system for protein synthesis by treating the cells with chloramphenicol and the lack of such an effect with cycloheximide suggests that certain of the proteins which form part of a functional chloroplast system for protein synthesis are themselves synthesized within the chloroplasts.

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PubMed1970

Identification and origin of some chloroplast aminoacyl-tRNA synthetases and tRNAs.

Light-grown, wild type Euglena gracilis contains two aminoacyl-tRNA synthetases for both phenylalanine and isoleucine. Only one of the two synthetases for each amino acid is found in isolated chloroplasts, as are the light-induced phenylalanine and isoleucine tRNAs. In each case the light-induced chloroplast tRNAs can only be acylated by the chloroplast synthetases. The chloroplast isoleucyl-tRNA synthetase is light-inducible and cannot be detected in dark-grown cells or in cells of the bleached mutant W(3)BUL. The presence of the chloroplast phenylalanyl-tRNA synthetase in W(3)BUL, which contains no chloroplast DNA or structure, indicates that this chloroplast enzyme is synthesized in the cytoplasm and is coded by nuclear genes.

Amino Acids↗

PubMed1970

The photosynthetic response to a shift in the chlorophyll a to chlorophyll B ratio of chlorella.

The chlorophyll a:b ratio was shifted in Chlorella vannielii by varying the illuminance under which the cells were cultured-the ratio increased from 2.9, 3.0, 4.0, and 4.8 to 6.2, respectively, at 100, 300, 900, 2,700 and 6,000 foot candles. The 6,000-foot candle cells retained an optimal growth rate at the chlorophyll a:b ratio of 6.2 which was the upper limit of normal growth. Comparisons were made between the 300-and 6,000-foot candle cultures to determine the significance to the photosynthetic mechanism of a shift in the chlorophyll a:b ratio.The high light cells (6,000 foot candles) contained only one-tenth the total amount of chlorophyll of the low light cells (300 foot candles) based on dry weight. The total chlorophyll per cell of the high light cells was one-fifth of that in the low light cells. Electron micrographs indicated differences in chloroplast structure. An average of five or six thylakoids composed a granum-like region of the low light chloroplasts, whereas only a pair of thylakoids at most was found in the high light chloroplasts. The high light chloroplasts had more starch. On a dry weight basis, the high light cells had a respiration rate 3 times that of the low light cells. Based on chlorophyll, the respiration rate of the high light cells was 26 times greater. Based on dry weight, the oxygen evolution for both cultures was essentially the same at 6,000 foot candles; however, at 300 foot candles the rate for the low light cells was about 5 times faster than that of the high light cells. With chlorophyll as the index, the rates of the high light cells were higher than those of the low light cells-7 times faster at 6,000 and 2 times faster at 300 foot candles. At 10,000 foot candles, the low light-grown cells underwent photooxidation, whereas the high light grown cells photosynthesized at a rate slightly higher than at 6,000 foot candles.Action spectra of system II (oxygen evolution) from a modulated polarograph indicated photochemical participation of chlorophyll b in the high light deficient cells, although the participation was much less than in the low light cells. Enhancement was 1.11 for the low light cells, and 1.05 for the high light cells.In order to account for the energy balance in the two cultures it was concluded that perhaps oxidative phosphorylation supplemented a reduced photophosphorylation in the high light cells. Experiments with peroxyacetyl nitrate support the view that cyclic photophosphorylation is less in the chlorophyll b-deficient cells. Chlorophyll b served also to broaden absorption for the photosynthetic unit-a detrimental role when cells are illuminated above saturation.

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