Isolation of Mesophyll Cells from Sedum telephium Leaves

Kinetics for glutamine-synthetase inhibition by phosphinothricin and measurement of other enzyme activities in situ in isolated asparagus cells using a freeze-thaw technique

Kinetics for the inhibition of glutamine synthetase (EC 6.3.1.2) in situ by the herbicidal glutamate analogue, phosphinothricin, have been generated, and produce an inhibitor dissociation constant (Ki) of 6.5 μM. This has been achieved through the development of a rapid technique for the isolation of mesophyll cells from the cladophylls of young asparagus (Asparagus sprengeri) plants to provide starting material for the direct measurement of enzyme activities in situ. A modification of the technique developed by Rhodes and Stewart (Planta 118, 133-144 (1974) for the direct determination of enzyme activities in higher-plant tissues has been applied to these asparagus cells. Treatment of the cells by a single freezing in liquid nitrogen for a very short period (10 s), followed by thawing, alters the permeability of cell and organelle membranes allowing enzymes to become accessible to many small molecules, and yet remain concentrated and active within the cell. The activities of enzymes known to be located specifically in the organelles as well as the cytoplasm can be measured in asparagus cells treated in this way. Comparisons have been made between the activity and inhibition of glutamine synthetase in situ, and the enzyme isolated and partially purified from asparagus cells by fast protein liquid chromatography. Similarities in Km and Ki values obtained between these two emphasize the efficacy of the freeze-thaw technique. There is only a single glutamine-synthetase isoenzyme in asparagus mesophyll cells, which copurifies with the one normally associated with the chloroplast (GS2).

Mechanical separation of palisade and spongy-parenchyma cells from the leaves of mesomorphic dicotyledons for photosynthetic studies

Palisade and spongy-parenchyma cells were isolated from the leaves of a number of mesomorphic dicotyledons by simply brushing the upper and lower sides, respectively, of the leaves with nylon brushes. Cross-contamination with the opposite cell type was minimal and both cell types were photosynthetically as active as leaf discs. The rates and early products of CO2 incorporation in the two cell types isolated from Zinnia elegans Jacq. plants grown in full natural light were the same, indicating that the photosynthetic physiology of the two cell types is quite similar.

Photosynthesis in enzymatically isolated leaf cells from the CAM plant Sedum telephium L

A technique has been developed for the enzymatic isolation of leaf cells from the Crassulacean acid-metabolism plant Sedum telephium. The cells exhibited high activity in both (14)CO2 incorporation (30-70 μmol CO2 mg(-1) chlorophyll h(-1)) and O2 evolution in the presence of bicarbonate (60-110 μmol O2 mg(-1) chlorophyll h(-1)). Half-maximum saturation of (14)CO2 incorporation occurred at a bicarbonate concentration of ca. 2 mM (20 μM CO2) at pH 8.4 and 30°C. Two types of light-dependent O2 evolution are reported: O2 evolution in the absence of exogenously supplied bicarbonate (endogenous O2 evolution), and bicarbonate-stimulated O2 evolution. Oxygen evolution in the presence of approximately ambient concentrations of CO2 appeared to be a combination of the endogenous O2 evolution and O2 evolution from fixation of the exogenously supplied CO2.

Effects of temperature on the hill reaction and photophosphorylation in isolated cactus chloroplasts

Chloroplasts isolated from Opuntia polyacantha Haw. (Cactaceae) are capable of noncyclic electron transport and ATP synthesis. Hill reaction rates, measured by O(2) evolution or by ferricyanide reduction, increase with increasing temperature to approximately 40 C. The temperature optimum of NADP reduction is 42 C while the optimum for noncyclic photophosphorylation is 35 C. NADP-linked phosphorylation exhibits a higher coupling ratio (P/e(2)) than ferricyanide-linked photophosphorylation. The temperature optima for photochemical energy production correlate with photosynthetic properties of Crassulacean acid metabolism (CAM) plants and are discussed in relation to the operation of CAM at high tissue temperature.

Equilibration of Label in Malate during Dark Fixation of CO(2) in Kalanchoë fedtschenkoi

In vitro studies of dark (14)CO(2) fixation with isolated cell aggregates of Kalanchoë fedtschenkoi showed that malate synthesized after 20 sec is predominantly (85 to 92%) labeled at carbon 4, while after 20 min only 65 to 69% of the radioactivity was located in this position. The intramolecular labeling pattern of malate could not be changed by supplementing the cells with carboxylation reaction substrates such as ribulose diphosphate or phosphoenolpyruvate. The kinetic decline of label at carbon 4 of malate occurs independently of CO(2) fixation, since 4-(14)C-labeled aspartate fed to the cells gave rise to malate labeled 62% at carbon 4 after 20 min. Furthermore, the cells were capable of converting fed malate to fumarate. It is concluded that synthesis of malate during dark CO(2) fixation is accomplished by a single carboxylation step via phosphoenolpyruvate carboxylase and labeling patterns observed in malate are a consequence of the action of fumarase.

Respiration and Gas Exchange in Stem Tissue of Opuntia basilaris

Respiration and gas exchange in the light were studied manometrically with tissue slices from stem material of Opuntia basilaris Engelm. and Bigel. Dark respiration rates were greater in young stems than in mature stems. The timing of the experiment in the day/night cycle influences the magnitude and pattern of respiration and gas exchange in the light. Net dark respiration has a temperature optimum between 35 and 40 C, and is maintained at 60% of the control rate in tissue equilibrated with experimental osmotic potentials of -25 bars. Net gas exchange in the light is regulated by the titratable acidity of the tissue and by the tissue temperature. Increased rates of net CO(2) evolution and net O(2) consumption occur in the light with high levels of titratable acidity and high temperatures. An efflux of CO(2) and influx of O(2) occur following light/dark transitions. These patterns are reversed following dark/light transitions. Similar results were demonstrated at 15, 25, and 35 C, and are interpreted as a mechanism of adaptation to desert environments.

Physiological studies with isolated leaf cells: early products of photosynthesis and their metabolic interconversions

A number of plants have been surveyed with respect to isolation by mild grinding in large quantities of leaf cells. The extent of recovery of mesophyll cells per unit leaf area was found to vary with plant species and the method of grinding. Greater than 70% recovery was obtained from the leaves of Canna indica L., Crotalaria Laburnifolia L., and Thunbergia grandiflora Roxb.By pulse-chase time course experiments, the photosynthetically fixed primary carbon compounds of bean leaf cells were not converted into the ethanol-insoluble fraction. About 25% of total (14)C-photoassimilates were found to leak out into the incubation medium. In contrast, Euglena and Chlorella cells incorporated their primary photosynthetic products into cellular macromolecules and the amount of "leak" was very little. (14)C-Leucine supplied to the bean cells was absorbed readily and incorporated into the trichloroacetic acid insoluble fraction.

Carbon dioxide fixation by epidermal and mesophyll tissues of Tulipa and Commelina

Rates of (14)CO2 fixation by epidermal tissue of Tulipa gesneriana (tulip) and Commelina diffusa are only slightly higher in the light than in the dark while in mesophyl tissues rates are much greater in the light. The first products of (14)CO2 fixation by epidermal tissue of Tulipa gesneriana and C. diffusa in the light and dark are malate and aspartate. In addition to these dominating dicarboxylic acids, 3-phosphoglyceric acid and sugar phosphates appear in the light, while in the dark only the amino acids, glutamate and glutamine become labelled. Mesophyll tissue of tulip and C. diffusa, however, gives typical CO2 fixation patterns of the labelled products of C3 plants. Furthermore, a period of dark (14)CO2 fixation followed by a light (12)CO2 chase carried out with epidermal tissue suggested that malate can act has the precursor of phosphorylated compounds of the Calvin cycle and consequently of starch. The data are consistent with the view that guard cells are able to exhibit Crassulacean acid metabolism.

C/C ratio changes in crassulacean Acid metabolism plants

(13)C/(12)C ratios have been found in totally combusted leaves of Crassulacean acid metabolism plants to range from -14 to -33 delta (13)C per thousand compared with a limestone standard. Crassulacean acid metabolism plants apparently utilize both ribulose-1, 5-diphosphate carboxylase and phosphoenolpyruvate carboxylase to assimilate atmospheric CO(2) and, depending on environmental conditions, have (13)C/(12)C ratios indicative of either carboxylase or to any intermediate value. The degree of discrimination against (13)C and the resultant (13)C/(12)C ratio from the photosynthetically fixed CO(2) is influenced by environmental conditions and is not a specific and fixed characteristic of a Crassulacean acid metabolism plant. Certain Crassulacean acid metabolism plants may shift their ratios as much as 17 delta (13)C per thousand in specific environments.