Biosynthesis of glucans occurred in cell-free fractions isolated from onion stem (Allium cepa L.) enriched in either dictyosomes or plasma membranes. l8-1,3-and p-1,4-Glucans were synthesized in differing proportions and at different rates as the concentration of uridine diphosphoglucose or the proportion of dictyosomes or plasma membrane varied. At low (1.5 uM) UDP-glucose concentrations synthesis of alkaliinsoluble glucan was correlated with abundance of dicytosomes; most of the substrate utilized by plasma membrane was for glycolipid synthesis. At high (1 mM) UDP-glucose concentration, the synthesis of alkali-insoluble glucans correlated with the abundance of plasma membrane. Substrate enhancement of g-1,4-glucan synthesis in dictyosome fractions was less than proportional to increases in substrate concentration. In contrast, ,-1,4-glucan synthesis by plasma membrane was more than proportionately increased. At high substrate concentrations the synthesis of 6-1 ,3-glucans predominated in both dictyosome and plasma membrane fractions. The results show that the capacity to synthesize glucans resides in both Golgi apparatus and plasma membranes of onion stem, but that the plasma membrane has the greatest capacity for synthesis of alkali-insoluble glucans at high UDP-glucose concentrations.In vivo studies of cell wall biogenesis indicate that cellulose synthesis occurs at the cell surface, possibly at the surface of the plasma membrane (28,34,39,44,52), while pectic and hemicellulosic polysaccharides are synthesized by the Golgi apparatus (1,5,10,18,27,40). In contrast, Ray et al. (36) found UDP-glucose: /3-1,4-glucan glucosyltransferase (3-1,4-glucan synthetase) activity to be localized in a membrane frac-
It is known that auxin induces the uptake of water by plant tissues. Three principal suggestions have been made concerning the mechanism of such auxininduced net water uptake. The first proposes that auxin in some manner plasticizes the cell wall. The second suggests that auxin brings about active synthesis of cell wall material. These two mechanisms suppose osmotic entry of water into the cell in response to loweredl wall pressure. The thirdI is that auxin brings about a non-osmotic transport of water into the tissue. Thimann (22) has grouped these into two hypotheses in the form of models. One model visualizes a pump (active transport) and the other a piston arrangement (cell wall pressure reduction). Such active transport has been rigorously defined by Rosenberg (21) Three general basal media were used: 1) potassium maleate buffer (0.0025 M, pH 4.8) with or without potassium indoleacetate (5 mg/l); 2) potassium maleate buffer, sucrose (0.09 M), MnSO4 (100 mg/l), arginine (100 mg/i) and with or without potassium indoleacetate (5 mg/l); 3) redistilled water with or without indoleacetic acid (IAA) (5 mg/l, pH 5.0 to 5.5), potassium-free. All solutions were made up with redistilled water.In the following report, the terms hypotonic and hypertonic are used in referring to solutions external to the cell. These terms are defined as follows: hvpertonic = solution whose OP is greater than OP1; hypotonic =solution whose OP is less than OPi. It The respiratory rate of Avena coleoptile sections is depressed in the presence of increasing concentration of an external solute, as is shown in figure 2. www.plantphysiol.org on April 7, 2019 -Published by Downloaded from
Heat treatments of two minutes at 46 to 47°C to root systems of Nicotiana rustica and Phaseolus vulgaris affected roots and shoots. Xylem exudate of Phaseolus was collected, and it was found that heat treatment reduced cytokinin levels and increased abscisic acid levels in the exudate. Shoot and root growth of both species was reduced. Root membrane integrity of Nicotiana was measured and was found to be impaired. It is suggested that the changes in hormone activity due to heat treatment regulate the reduction in shoot growth. Cell wall metabolism and glucosyl transferases making β‐glucans were investigated in Phaseolus leaves. Incorporation of 14C from 14CO2 into wall constituents was slightly inhibited but neither photosynthesis nor extracted β‐glucan synthetases were affected during the first 12 hours after treatment.
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