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-

Plasma membrane-rich fractions were prepared from maize coleoptiles by low-shear homogenization and differential and sucrose-gradient centrifugation. Plasma membrane fragments were identified using a specific cytochemical stain based on phosphotungstic acid prepared in chromic acid. In a comparison of 10 different cell fractions of varying plasma membrane content, the N-1-napthylphthalamic-acid (NPA)-binding activity of the fractions was directly proportional to the content of plasma membrane. The NPA binding appears to be strong K M between 10(-8) and 10(-7) M) but non-covalent. NPA is known to inhibit auxin transport efficiently and quickly. Thus, the results are consistent with the localization of auxin transport sites at the plasma membrane of plant cells.

No abstract

Spirogyra Link (1820) is an anabranched filamentous green alga that forms free‐floating mats in shallow waters. It occurs widely in static waters such as ponds and ditches, sheltered littoral areas of lakes, and stow‐flowing streams. Field observations of its seasonal distribution suggest that the 70‐μm‐wide filament form of Spirogyra should have a cool temperature and high irradiance optimum for net photosynthesis. Measurements of net photosynthesis and respiration were marie at 58 combinations of tight and temperature in a controlled environment facility. Optimum conditions were 25°C and 1500 μmol photons m−2 s−1, at which net photosynthesis averaged 75.7 mg O2 gdm−1 h−1. Net photosynthesis was positive at temperatures from 5° to 35°C at most irradiances except at combinations of extremely low irradiances and high temperatures (7 and 23 μmol photons m−2 s−1 at 30°C and 7, 23, and 35 μmol photons m−2 s−1 at 35°C). Respiration rates increased with both temperature and prior irradiance. Light‐enhanced respiration rates were significantly greater than dark respiration rates following irradiances of 750 μmol photons m−2 s−1 or greater. Polynomials were fitted to the data to generate response surfaces; such response surfaces can be used to represent net photosynthesis and respiration in ecological models. The data indicate that the alga can tolerate the cool water and high irradiances characteristic of early spring but cannot maintain positive net photosynthesis under conditions of high temperature and low light (e.g. when exposed to self‐shading).

Using recently developed techniques for solubilization of RNA polymerase from soybean chromatin and isolation of plasma membrane fractions from plants we can show the presence of a transcriptional factor specifically released from the membranes by auxin, 2,4-dichlorophenoxyacetic acid. The nonauxin, 3,5-dichlorophenoxyacetic acid, does not release the factor, but subsequent exposure of the membranes to auxin results in its release. Factor activity could not be demonstrated in fractions devoid of plasma membranes. The presence of a regulatory factor for RNA polymerase associated with plant plasma membrane and specifically released by auxin provides a mechanism whereby both rapid growth responses and delayed nuclear changes could be derived from a common auxin receptor site associated with plasma membrane.Stimulation of cell enlargement by auxin in soybean hypocotyls is accompanied by increases in chromatin-bound RNA polymerase activity (1). We previously isolated a factor that stimulates chromatin-bound RNA polymerase from control tissue but not from auxin-treated tissue (2). Such a factor might interact with auxin, with subsequent transfer of either an auxin-factor complex or a modified factor across the nuclear membrane. Transcription factors have also been described by Matthysse and Phillips (3) and Venis (4).The plasma membrane is a primary site of hormone activity (5-9). We reasoned that these lines of evidence were compatible with interaction of auxin with specific receptor sites on the plasma membrane to release or modify a factor that controls transcription. In this study, plasma membrane fractions isolated from soybean hypocotyls were treated with phys- RNA Polymerase Solubilization and Assay of Activity. RNA polymerase was solubilized from isolated chromatin preparations and assayed as described (11) by the Whatman DE81 disc technique of Blatti et al. (12). a-Amanitin was added immediately before addition of the enzyme.Membrane Isolations. Plasma membrane and other cell fractions were isolated by the procedure of Lembi et al. (8) modified as follows. About 25 g of hypocotyl tissue were homogenized in 40 ml of a freshly prepared medium consisting of 0.1 M K2HPO4, 1.0 mM dithiothreitol, and 20 mM EDTA in centrifuged (100,000 X g, 90 min) coconut milk (pH 6.5) (coconut milk medium) and containing 0.5 M sucrose. Homogenates were prepared with a Polytron 20ST (Kinematica, Lucerne, Switzerland) homogenizer (13) and centrifuged in 50-ml tubes for 12 min at 20,000 X g (Sorvall RC2-B, HB-4 rotor) to remove cell walls, nuclei, plastids, mitochondria, microbodies and large membrane fragments. The supernatant containing microsomes and most of the cytoplasmic membranes (dictyosomes, plasma membrane fragments, etc.) was then layered onto a step gradient containing equal volumes of 0.65, 0.8, 1.0, 1.2, and 1.3 M sucrose in coconut milk medium having sucrose equivalent densities of 0.8, 1.0, 1.2, 1.4, and 1.5 M, respectively. The gradient was centrifuged for 90 min at 100,000 X g. Membrane fractions were recove...