Métabolisme du γ-Aminobutyrate chez Agaricus bisporus

γ-Aminobutyrate synthesis, the first step in this pathway, is catalysed by L-glutamate-1-carboxy-lyase (E.G. 4.1.1.15). The purification procedure and some of in vitro properties of this enzyme isolated from fruit-bodies of Agaricus bisporus 19 Lge were investigated. Glutamate decarboxylase has been partially purified from a homogenate by a combination of ammonium sulfate fractionation and hydroxylapatite column chromatography. All kinetic studies were carried out manometrically in a nitrogen atmosphere at 35°C by conventional Warburg technique. The decarboxylase is a pyridoxal-phosphate requiring enzyme. The pH optimum was found to be between 5.5 and 5.6 and the Km value for glutamate was calculated to be 4 × 10^-2M from a Lineweaver-Burk plot. Of the amino acids tested the enzyme is specific for glutamate. γ-Aminobutyrate is not carboxylated to form glutamate. Inhibition by malate, malonate, α-ketoglutarate and NAD were found to be non-competitive with respect to glutamate, and those by succinate to be uncompetitive. Fractionation of the subcellular components shows that the enzyme is localized in the hyaloplasm. The results are discussed in relation to the γ-aminobutyrate bypath, a probable shunt to the Krebs cycle.     

Structural changes in the innercortex cells of soybean root nodules are induced by short-term exposure to high salt or oxygen concentrations

After a 2 h exposure of intact soybean nodules to high concentrations of NaCl (100mol m_?3) or oxygen (8OkPa O₂), morphometric computations carried out using an image analysis technique on semi-thin sections showed that both treatments induced a decrease in the area of the inner-cortex cells, which were then characterized by a tangential elongation. In contrast, no significant change in area occurred in the middle-cortex cells although their elongation decreased. Electron microscopic observations showed that in the inner-cortex cells changes included the presence of wall infoldings, an enlarged periplasmic space and a lobate nucleus whose chromatin distribution differed from that of the control. Structural changes also occurred in the endoplasmic reticulum, microbodies, mitochondria and plastids. From several of these changes, which are similar to those noted in osmocontractil cells in response to external stimuli, it can be hypothesized that the inner cortex may provide a potential mechanism for the control of oxygen diffusion through the nodules.     

Calcium Involvement In the IAA-induced Leaflet Opening of Cassia fasciculata

Opening of Cassia fasciculata leaflets was induced in darknessafter application of indole-3-acetic acid (IAA). This movementwas obtained with concentrations from 10^–6 M to 10^–4M, after a corresponding time-lag ranging from 120 to 30 min.IAA (5x10^–5 M) allowed leaflet opening at all the pH valuestested (from 3·5 to 7·5), the largest aperturebeing obtained at pH 60 in MES 2·5 mM. Our data suggesta functional involvement of calcium in the regulation of theturgor variations occurring in the pulvinar motor cells duringIAA-induced leaflet opening which occurs in darkness: indeed,this movement was inhibited by the Ca²⁺ chelator EGTA (thisinhibition was reversed by CaCl²) or by antagonists (LaCl³,TMB-8); on the contrary, the IAA-opening was enhanced by ionophoreA 23187. Calcium mobilization through specific channels was tested usingantagonists such as verapamil and nifedipine: at physiologicaldoses, these compounds did not significantly affect leafletresponse. The possibility that calcium could originate frominternal stores was checked using lithium chloride which isknown to block the phosphatidylinositol cycle in animal cells.This compound hindered auxin-induced opening for concentrationshigher than 5x10^–4 M. The calcium-binding protein calmodulinwas shown to be implicated in the IAA-induced response sinceopening was inhibited in a concentration-dependent manner aftertreatment with compound 48/80 and with W-7. Key words: Cassia fasciculata, auxin, calcium, second messenger, turgor regulation     

Comparative Histocytology of the Petiole and the Main Pulvinus in Mimosa pudica L.

In Mimosa pudica, the main pulvinus, which brings about leafmovements, presents unusual structural characteristics in comparisonwith the petiole. Peculiar cellular features which exist inthe cortex, epidermis, parenchyma and endodermal regions includethe shape of the cells, their disposition and the location ofthe organelles. The central cylinder of the petiole is surrounded only by afew parenchyma layers whereas the central cylinder of the pulvinusforms a narrow central core enclosed in numerous cortical parenchymalayers. The phloem of the pulvinus contains collenchymatouscells towards the outside and possesses companion cells withwall ingrowths; these phloem members do not exist in the petiole.Xylem and protoxylem parenchyma cells of the petiole possesswall ingrowths which do not occur in homologous cells of thepulvinus. Moreover the pith of the pulvinus is composed of smallfibriform elements similar to the xylem fibriform elements ofthe organ. The structures observed may facilitate exchanges between cellsin the petiole and in the pulvinus. The predominant functionsof the organs relative to lateral and longitudinal transferof nutrients and conduction of stimuli are discussed. Mimosa pudica L., sensitive plant, pulvinus, ultrastructure, conduction of stimuli, leaf movement     

Metabolisme du γ-aminobutyrate chez Agaricus bisporus III. La succinate-semialdehyde:NAD(P)+ oxydoreductase

Metabolism of γ-Aminobutyrate in Agaricus bisporus. III. The Succinate-Semialdehyde: NAD (P)⁺ Oxidoreductase. The succinate-semialdehyde:NAD(P)⁺ oxidoreductase (E.C. 1.2.1.16) is responsible for the second step in the catabolism of γ-aminobutyrate: the irreversible enzymatic conversion of succinic semialdehyde (SSA) to succinate. Succinate semialdehyde dehydrogenase was extracted from mitochondrial fraction of fruit-bodies of Agaricus bisporus Lge. The mitochondrial pellet was sonicated and centrifuged at 110,000 g; the supernatant obtained was designated the “crude extract”. The enzyme was extremely unstable on storage, unless 1 mM EDTA and 20% glycerol were added. Kinetic studies were carried out at 30°C, and the formation of NADH or NADPH was followed by measuring increase of absorbance at 340 nm with a spectrophotometer. The dehydrogenase was completely inactive when the reaction was run in the absence of thiol and was more active with NAD⁺ than with NADP⁺. In the “crude extract” the activity with NADP⁺ had a pH optimum between 8.6 and 9.1 and the K_m values for SSA and NADP⁺ were 2.0 × 10^?4M and 1.4 × 10^?4M respectively. The pH optimum with NAD⁺ was found between 8.6 and 8.8 and the K_m value for SSA is 4.8 × 10^?4M and for NAD⁺ 2.0 × 10^?3M. With NAD⁺, the kinetic values (pH, K_m) of the “crude extract” chromatographed on hydroxylapatite were unchanged. Inhibition by thiamine pyrophosphate (TPP) was uncompetitive with respect to NAD⁺, those by malate, ATP, ADP and NADPH non-competitive and that by NADH competitive. These results and the fact that activity with NAD⁺ was lost more slowly than with NADP⁺ indicate the possibility of at least two mitochondrial succinate-semialdehyde dehydrogenases, even though the activities of this enzyme assayed with NAD⁺ and NADP⁺ respectively were not able to be separated from each other by hydroxylapatite column chromatography. Some speculations on the metabolic regulation of this dehydrogenase and considerations on the significance of these results in the physiology of respiration in Agaricus bisporus Lge are given.     

Effects of Colchicine, Vinblastine, Cytochalasin B and Phalloidin on the Seismonastic Movement of Mimosa pudica Leaf and on Motor Cell Ultrastructure

Fleurat-Lessard, P., Roblin, G., Bonmort, J. and Besse, C. 1988.Effects of colchicine, vinblastine, cytochalasin B and phalloidinon the seismonastic movement of Mimosa pudica leaf and on motorcell ultrastructure.—J. exp. Bot. 39: 209–221. Colchicine at 1 x 10^–3 mol dm^–3 does not affectthe seismonastic movement of Mimosa pudica leaves but disruptsmicrotubules in motor cells. Vinblastine at 5 x 10^–3 moldm^–3 does not affect this movement and partly disruptsmicrotubules. Vinblastine at 1 x 10^–4 mol dm^–3 alwaysdisrupts microtubules, even after a 12 h reversibility whenthe movement is restored. These drugs, applied at the same respectiveconcentrations, do not alter cytoplasmic and vacuolar fibrils.Cytochalasin B and phalloidin alter the seismonastic movementof Mimosa leaves when applied at concentrations of 1.25 x 10^–3and 2.4 x 10^–4 mol dm^–3 respectively. These drugs,used at the same respective concentrations, also affect themotor cell structure and, in particular, modify the arrangementand the structure of the fibrils but they do not destroy themicrotubules. These data suggest that microtubules are not directly involvedin the seismonastic reaction whereas fibrils, formed by thin(3.0 nm wide) filaments, may be implicated in this reaction. Key words: Colchicine, cytochalasin B, phalloidin, Mimosa pudica, motor cells, vinblastine     

STRUCTURAL AND ULTRASTRUCTURAL FEATURES OF CORTICAL CELLS IN MOTOR ORGANS OF SENSITIVE PLANTS

(1) The movements are only expressed in motor cells, regardless of the nature of the stimulation or its point of application. Therefore, these cells have structures capable of traducing the different stimulation-induced messages which are received in parts incapable of movement. (2) K⁺, Cl^- and Ca²⁺ are the major ions. Their fluxes have been followed during nyctinastic movements as well as during stimuli-induced movements. At the moment, the location and the role of these ions are being studied. (3) The movement results from the integrated activity of all (n) motor cells in the pulvini (i.e. n > 350 × 10³ in primary pulvini 3 mm long and 1·9 mm thick). (4) The motor cell is a full-grown cell whose osmotic activity induces turgor variations allowing foliar movements. (5) The motor cell is a highly differentiated cell, which, up to now, has never been able to dedifferentiate in order to produce callus. (6) The motor cell has original features in its apoplastic compartment (large meatuses, wall foldings, large periplasm with membranes) and in its symplastic compartment (double vacuolar apparatus, morphological polarity given by the tannin vacuole location near the nucleus, abundant mitochondria). (7) Its cytoskeleton includes microtubules, cytoplasmic and vacuolar fibrils (in particular in the tannin vacuole), and a wall with special properties. (8) The motor cell is supposed to contain contractile proteins, whose nature and location are being investigated. (9) The shape change of the motor cell is obvious after pulvinar bending. This change is probably associated with a volume change in several intracellular compartments (vacuole, mitochondria, vesicles). (10) At the cellular and subcellular level the same general features are observed in motor cells of non-seismonastic and of seismonastic species. Probably, functional differences depend upon differences occurring at the molecular level. (11) The motor cell is an interesting model for the study of the osmoregulation mechanism in plant cells, to test the effect of toxic products, in particular to find their optimal efficiency in the circadian cycle.     

Ultrastructural Features of Cortical Parenchyma Cells ('Motor Cells') in Stamen Filaments of Berberis canadensis Mill. and Tertiary Pulvini of Mimosa pudica L.

The comparative study of Berberis stamen filaments and Mimosatertiary pulvini shows many common features in their corticalparenchyma ultrastructure. In both mature organs, cells haveunusual sinuous walls with numerous plasmodesmata, tannins inthe vicinity of the nucleus, microfilaments and microfibrilsin tannin vacuoles. Nevertheless, in Mimosa pulvini, the abaxialcells have thicker walls than the adaxial cells while in stamenfilaments the reverse is true; therefore the dissymmetric organizationof the organ walls is the opposite. We will discuss: (1) the problems of chemical fixation of excitableplant cells; (2) relations between the direction of bendingand the thickness of cell walls; (3) the location of tanninsand (4) the existence of microfilaments and microfibrils. Key words: Berberis canadensis Mill., berberry, Mimosa pudica L., sensitive plant, Stamen, Pulvinus, Ultrastructure, Seismonastic movement     

Absorption of Some Amino Acids by Sporophytes Isolated from Polytrichum formosum and Ultrastructural Characteristics of the Haustorium Transfer Cells

The absorption of ¹⁴C-labelled amino acids (glycine, threonineand -aminoisobutyric acid) by the isolated sporophyte of Polytrichumformosum takes place mainly in the haustorium. The isolationof the sporophyte does not alter the absorption capacity ofits haustorium nor its ultrastructure, in particular that ofits peripheral transfer cells. amino acids, transfer cells, sporophyte, Polytrichum formosum, haustorium