Fructosyl Transfer between 1-Kestose and Sucrose in Wheat Leaves

The labeling pattern of the sugar moieties of 1-kestose after in vivo pulse labeling with ¹⁴CO₂ was not the same as that after in vitro labeling with ¹⁴C-sucrose. The two fructosyl residues of 1-kestose had similar specific radioactivities after in vitro synthesis, but after in vivo radiolabeling the specific radioactivity of the terminal fructosyl moiety was significantly less than the internal fructosyl moiety. Evidence is presented that the uneven specific radioactivity of in vivo radiolabeling results from enzymatic transfer of terminal fructosyl residue from 1-kestose to sucrose.     

Incorporation of proline and aromatic amino acids into cell walls of maize coleoptiles

Sections excised from maize coleoptiles incorporated radioactivity from proline, tyrosine, and phenylalanine into structural components of the cell wall. Only about 2% of radioactivity from proline taken up by sections was incorporated into cell wall; about 24% of that incorporated was in hydroxyproline and the rest remained in proline. In contrast, as much as 40% of the radioactivity from phenylalanine and 30% from tyrosine was incorporated into cell wall material. Most of this radioactivity was in saponifiable ferulic acid. Small amounts of p-coumaric and diferulic acid were found, but only a small fraction of the hemicellulose can possibly be immobilized directly through cross-linking of diferulic esters. Substantial amounts of radioactivity from aromatic amino acids remained insoluble after strong alkali extractions of wall material, and a large fraction of polysaccharide was solubilized by dilute alkali following oxidation of phenolics by acidic NaClO₂. Hence, hemicellulosic material in the cell walls of maize coleoptiles may be organized and cross-linked primarily through alkali-resistant etherified aromatics.     

The plant cytoskeleton-cell-wall continuum

Two of the most challenging mysteries of morphogenesis are how cells receive positional information from neighbouring cells and how receipt of this information triggers events that initiate cell differentiation. The concept that the cytoskeleton and éxocellular matrix' (ECM) form an interactive scaffold for perception and transduction of positional information is relatively new. Research is beginning to indicate that a continuous cytoskeleton-ECM scaffold may be a feature of all eukaryotic cells and that many of the molecules participating in this structure may be shared by plants, fungi and animals.     

Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth

Advances in determination of polymer structure and in preservation of structure for electron microscopy provide the best view to date of how polysaccharides and structural proteins are organized into plant cell walls. The walls that form and partition dividing cells are modified chemically and structurally from the walls expanding to provide a cell with its functional form. In grasses, the chemical structure of the wall differs from that of all other flowering plant species that have been examined. Nevertheless, both types of wall must conform to the same physical laws. Cell expansion occurs via strictly regulated reorientation of each of the wall's components that first permits the wall to stretch in specific directions and then lock into final shape. This review integrates information on the chemical structure of individual polymers with data obtained from new techniques used to probe the arrangement of the polymers within the walls of individual cells. We provide structural models of two distinct types of walls in flowering plants consistent with the physical properties of the wall and its components.     

Dynamic changes in cell surface molecules are very early events in the differentiation of mesophyll cells from Zinnia elegans into tracheary elements

The Zinnia mesophyll cell system consists of isolated leaf mesophyll cells in culture that can be induced, by auxin and cytokinin, to transdifferentiate semi-synchronously into tracheary elements (TEs). This system has been used to establish the precise time point at which the TE cell fate becomes determined, and then changes have been looked for in cell-wall composition and architecture that are associated with the establishment of competence, determination, and differentiation with the transition from primary to secondary cell wall formation. At very early stages in this time course, changes in the repertoire of proteins and polysaccharides both in the cell wall and secreted into the culture medium were found. Changes in the secretion of pectic polysaccharides, xyloglucans and arabinogalactan proteins (AGPs) have been detected using the monoclonal antibodies JIM 7, CCRC-M1 and JIM 13, that recognize these three classes of cell-wall molecule, respectively. Twenty-four hours before secondary thickenings are visible, an AGP is present in the primary walls of a subpopulation of cells, and is secreted into the culture medium. This molecule is present in the secondary thickenings of mature TEs but not in their surrounding primary walls. Methyl-esterified pectic polysaccharides are present in all cell walls and are secreted into the culture medium throughout the time course of differentiation, though at an increased rate in inductive medium. However, sugar and linkage analysis of culture media shows that a relatively unbranched rhamnogalacturonan is enriched in inductive medium around the time of determination and increases rapidly in concentration. The amount of fucosylated xyloglucan in cell walls increases during the time course, but appears in inductive medium 24 h earlier than in control medium and may have a subtly different structure. The fucose-containing epitope on the xyloglucan disappears abruptly and entirely from inductive medium 6 h before any secondary thickenings are visible in the cells. The disappearance of the epitope is correlated with secretion of several hydrolytic enzyme activities. In Zinnia leaves, the mesophyll cell walls contain neither the fucosylated xyloglucan nor the AGP, although methylesterified pectin is present. All three epitopes are expressed in the vascular bundles, and the AGP is specifically localized in the xylem cells. Fucosylated xyloglucan is also present in the epidermal tissue, and the AGP is present in guard cells. The dynamic behaviour of these specific cell-wall molecules is tightly correlated with differentiation events in vitro, and can be clearly distinguished from the production of new wall material found in expanding and elongating cells. The precise timing of the appearance and disappearance of these proteins and polysaccharides compared with the point of cell-fate determination provides us with a series of cell-surface markers for cell states at very early times in the transdifferentiation pathway.     

The growth physics and water relations of red-light-induced germination in lettuce seeds

Using lettuce (Lactuca sativa L., cv. Grand Rapids) embryos in osmotica, we have demonstrated that when the growth rates of the embryonic axes of seeds treated with red (R) or far-red (FR) light are equalized, the axes of R-treated seeds develop a 3.4-bar decrease in water potential (paper No. III).As axial growth begins, reserve protein and phytin decrease rapidly, concomitant with increases in reducing sugars, -amino nitrogen, and inorganic and esterified soluble phosphates. However, no differences between the axes of R-and FR-treated seeds are found with respect to the changes in these compounds, indicating that these changes arise as a result of growth and are not under immediate phytochrome control. Little change in the total lipid content is found in either treatment. The axes of FR-treated seeds hydrolyze endogenous sucrose at a greater rate thant those of R-treated seeds. Axes of R-treated seeds accumulate K⁺ and Na⁺ to a greater extent than those of FR-treated seeds. When potassium salts are added to the incubation medium, R induces increased K⁺ uptake by the axis and greater medium acidification by the axis. Malate and other organic acids and acidic amino acids increase at equal rates in both treatments, indicating that inorganic anions may also be taken up to balance the ionic charges. The results are compatible with the assumption that changes in the osmotic and pressure potentials of the embryonic axes of R-treated seeds are the result of a phytochrome-stimulated proton pump which, in whole dormant seeds, would initiate water-potential changes allowing the embryos to overcome the mechanical restraint of the surrounding seed layers, resulting in germination.Abbreviations FR far-red light - PEG polyethylene glyeol 4000 - Pfr far-red-absorbing form of phytochrome - R red light III=Carpita et al. 1979     

The influence of plant growth regulators on the growth of the embryonic axes of red- and far-red-treated lettuce seeds

The influence of several plant growth regulators on the growth of the embryonic axes from red- and far-red-(R- and FR-)treated lettuce (Lactuca sativa L., cv. Grand Rapids) seeds was examined; as shown previously, the water potential of the axes from R-treated seeds has been lowered by 3.5–5.6 bars compared to that in axes from FR-treated ones. Kinetin and abscisic acid (ABA), when included in the incubation medium, reduced the elongation of the axes whereas fusicoccin stimulated it; however, these effects were the same in axes of both R- and FR-treated seeds. In contrast, elongation of axes from FR-treated seeds was stimulated by gibberellic acid (GA₃, but elongation of axes from R-treated ones was not affected by this hormone. This latter result indicates that gibberellins may be involved in the phytochrome-mediated growth responses in lettuce axes.When the root caps of the embryos were removed prior to light treatment, R was still able to induce a water-potential decrease in the embryonic axes, indicating that at least a portion of the active Pfr resides in the axis and not the root cap.Abbreviations ABA abscisic acid - FR far red light - GA₃ gibberellic acid - PEG polyethylene glycol - Pfr far-red-absorbing form of phytochrome - R red light     

Changes in Esterification of the Uronic Acid Groups of Cell Wall Polysaccharides during Elongation of Maize Coleoptiles

Cell walls of grasses have two major polysaccharides that contain uronic acids, the hemicellulosic glucuronoarabinoxylans and the galactosyluronic acid-rich pectins. A technique whereby esterified uronic acid carboxyl groups are reduced selectively to yield their respective 6,6-dideuterio neutral sugars was used to determine the extent of esterification and changes in esterification of these two uronic acids during elongation of maize (Zea mays L.) coleoptiles. The glucosyluronic acids of glucuronoarabinoxylans did not appear to be esterified at any time during coleoptile elongation. The galactosyluronic acids of embryonal coleoptiles were about 65% esterified, but this proportion increased to nearly 80% during the rapid elongation phase before returning to about 60% at the end of elongation. Methyl esters accounted for about two-thirds of the total esterified galacturonic acid in cell walls of unexpanded coleoptiles. The proportion of methyl esters decreased throughout elongation and did not account for the increase in the proportion of esterified galactosyluronic acid units during growth. The results indicate that the galactosyluronic acid units of grass pectic polysaccharides may be converted to other kinds of esters or form ester-like chemical interactions during expansion of the cell wall. Accumulation of novel esters or ester-like interactions is coincident with covalent attachment of polymers containing galactosyluronic acid units to the cell wall.     

Effects of 35 C heat treatments on photosensitive grand rapids lettuce seed germination

Grand Rapids lettuce (Lactuca sativa L.) seeds were given 35 C heat treatments to increase photodormancy in a subsequent 20 C dark period. Short heat treatments (1-5 hours) induced a significant germination percentage increase of from 16% to over 50% depending on seed lot. With longer heat treatments dark germination percentage was gradually reduced to zero. If given at the end of 35 C, far red or red followed by far red further increased the amount of dark germination.     

Intercellular communication in the rat anterior pituitary gland: an in vivo and in vitro study

The concept of "stimulus-secretion coupling" suggested by Douglas and co-workers to explain the events related to monamine discharge by the adrenal medulla (5, 7) may be applied to other endocrine tissues, such as adrenal cortex (36), pancreatic islets (4), and magnocellular hypothalamic neurons (6), which exhibit a similar ion-dependent process of hormone elaboration. In addition, they share another feature, that of joining neighbor cells via membrane junctions (12, 26, and Fletcher, unpublished observation). Given this, and the reports that hormone secretion by the pars distalis also involves a secretagogue-induced decrease in membrane bioelectric potential accompanied by a rise in cellular [Ca++] (27, 34, 41), it was appropriate to test the possibility that cells of the anterior pituitary gland are united by junctions.