Cell-Wall-bound Enzymes

Cell-wall-bound enzymes in monocots have been less extensively studied than in dicots. Moreover, the majority of studies with this class of plants has been conducted with oat coleoptiles. 

Cell-wall-bound glycosidases present in these coleoptiles include / -d-galactosidase, / -D-glucosidase, / -D-xylosidase, / -L-fucosidase, and Ct-D-mannosidase the enzymes were reported to be activated by auxin and by hydrogen ions, and, therefore, they may be involved in cell growth.243 A contrary claim has been made by Evans244 that neither cell-wall-bound 

In maize-root tips, high specific activities of/ -D-galactosidase, a- and jff-D-glucosidase, N-acetyl-/ -D-glucosaminidase, acid phosphatase, and phosphodiesterase (EC 3.1.4.1) are found in the cell-wall fraction.246 

There is thus some evidence for the tight binding of enzymes, especially glycosidases, to cell walls in both dicots and monocots. The nature and localization of these enzymes suggest that they may, perhaps, play a role in wall breakdown and such other processes as elongation growth. The membrane systems of plant cells are known to be involved in the transport, and introduction, of polysaccharides into the cell wall247-249 enzymes localized in the wall may also play a part in the metabolism of these polymers when they are transferred from the membrane system to the wall. 

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The poor activities of pectic enzymes in the cultivation medium led us to prove the cell cytosole and the cell walls for these activities. The cytosole contains only traces of polygalacturonase activity, but the suspension of cell walls established the activity which seems to be widely sufficient for yeast growth and development. The characterization of this cell wall bound enzymes will be the object of our next studies. 

Possible functions for peptidophosphogalactomannan and the new lipopeptidophosphogalactomannan were suggested. It was proposed that phosphogalactomannan may protect, at the time of synthesis as well as later, those enzymes which are destined to become cell wall bound or extracellular. The relationship between structure, function and biosynthesis are discussed. 

Association of the cell wall-bound jS-D-fructofuranosidase of sugar beet seedlings with structural polysaccharides of the wall was investigated. After sequential extraction with boiling water, 0.5% H4edta and 24% KOH from the wall of sugar beet, the polysaccharides were prepared as pectic substances I and II, and hemicellulose. Solubilization of the enzyme in the presence of the polysaccharides and the profiles on gel filtration of the enzyme with the poly-... 

Plant synthates, primarily intended for the developing seed, are the main nutrition source to the fungus (Mower and Hancock, 1975) which obviously is exploited at about 5 dpi depending on the ergot species. To use this natural sink, several enzymes are secreted such as the cell-wall bound inducible fructosyltransferase (invertase) (Bassett et ah, 1972 Taber, 1985 Tudzynski et ai, 1995) and the fungal foot is developed structurally for attaching and absorbing (Luttrell, 1980). While intense exudation of honeydew is reported to occur without penetration... 

The vacuole comprising most mature plant cells contains secondary metabolites and enzymes. The cell envelope is composed of cellulosic cell walls bound by pectinaceous middle lamella. A phospholipid-bilayer membrane is located on the inside of the cell wall (plasmalemma), as well as around the vacuole (tonoplast). Disruption of the cell or vacuole membrane leads to the release of hydrolytic enzymes which cause the degradation of pectins or cellulose. This in turn weakens the cell wall and tissue structure and leads to a loss of firmness and crispiness due to a dissipation of the intracellular osmotic pressure (turgor) (Taiz and Zeiger, 2007). 

In our investigations, we also detected the sorption of isoPO from potato, Arabidopsis and wheat, by calcium pectate. Moreover, we observed the binding with calcium pectate of potato PO from the fraction of proteins ionically bound with cell walls. It is likely that the ability of some PO isoforms to bind with pectin ensures the spatial proximity of these enzymes to the sites of the initiation of lignin synthesis and that these "pectin-specific" isoforms take part in this process. 

When cell-wall fragments are incubated in molar NaCl, ionically bound proteins are released into the incubation medium. All investigated crude cell extracts deesterified Citrus pectin (Table 2) but the deesterification rates were clearly higher when the enzymes were still bound to the cell walls, indicating a major loss of activity during the solubilization process. 

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