The growing cell wall in plants is a thin, strong and pliant extracellular layer, composed of cellulose microfibrils embedded in a hydrated matrix that is made of complex polysaccharides and a small amount of structural proteins.
A cellulose microfibril consists of ∼36 (1,4)-linked β-D-glucans that crystallize into a strong, thin ribbon ∼4 nm in diameter. Microfibrils are synthesized by large membrane complexes that contain three isoforms of cellulose synthase, which are glucosyltransferases that are encoded by CESA genes.
A membrane-bound endoglucanase, which is called KORRIGAN, is required for proper formation of the cellulose microfibril, perhaps serving a trimming function.
Matrix polysaccharides, which include cellulose-binding glycans (hemicelluloses) and acid polysaccharides (pectins), are synthesized in the Golgi apparatus and, after secretion into the wall, become integrated with the existing wall by enzymatic and spontaneous crosslinking mechanisms. Recent studies have identified the genes encoding glycosyltransferases that synthesize some of the glycosidic linkages in xyloglucans, mannans and pectins.
Cell-wall enlargement begins with wall stress relaxation, which reduces cell turgor pressure, which in turn draws water into the cell to increase cell volume and restore both turgor and wall stress.
Expansins are a group of nonenzymatic wall proteins that induce wall stress relaxation and cell wall extension (creep) in cell walls. They mediate 'acid growth' of cell walls and might function by disrupting the noncovalent linkages that hold microfibrils in the cell wall.
Xyloglucan is a key polymer that links microfibrils together. Xyloglucan hydrolases have the potential for loosening cell walls and causing cell-wall enlargement, but it is still unclear whether plant enzymes function in this way in vivo.
Xyloglucan endotransglucosylase cuts and joins xyloglucans. The consequences of this biochemical activity for wall properties depend on the state of xyloglucan in the cell wall.
Plant cellulases might digest the noncrystalline regions of cellulose microfibrils and release trapped xyloglucans, resulting in increased wall extensibility and cell growth.
It has been proposed that the hydroxyl radical functions as a novel wall-loosening agent that nonenzymatically cuts wall polysaccharides. Whether it is produced in sufficient amounts to cause wall loosening and whether it has sufficient specificity for action remain to be answered.
Plant cells encase themselves within a complex polysaccharide wall, which constitutes the raw material that is used to manufacture textiles, paper, lumber, films, thickeners and other products. The plant cell wall is also the primary source of cellulose, the most abundant and useful biopolymer on the Earth. The cell wall not only strengthens the plant body, but also has key roles in plant growth, cell differentiation, intercellular communication, water movement and defence. Recent discoveries have uncovered how plant cells synthesize wall polysaccharides, assemble them into a strong fibrous network and regulate wall expansion during cell growth.
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I gratefully acknowledge the research contributions of numerous students and colleagues, particularly D. M. Durachko, M. Perich, L.-C. Li, S. McQueen-Mason, T. Shcherban, S. Yuang, and Y. Wu. Our research is supported by grants from the National Science Foundation, the National Institutes of Health and the Department of Energy.
The author declares no competing financial interests.
The contents of living cells, including cytoplasm and nucleus.
Slow, time-dependent, irreversible extension, in which the microfibrils and associated matrix polysaccharides slowly slide within the wall, therefore increasing its surface area.
A tissue that comprises a group of specialized cells that are involved in the transport of water and solutes in vascular plants. Mature xylem vessels essentially contain only the cell wall.
Region of rapid cell division on a plant; it is where cell initials (or 'stem' cells) are maintained and organogenesis starts. Root meristems, shoot meristems and flower meristems fit this description.
A membrane-bound cellular compartment, usually filled with a dilute watery solution. Mature plant cells often have very large central vacuoles.
- WALL LOOSENING
Modification of the cell wall that enables it to extend in response to the wall stress that is generated by cell turgor.
- PRIMARY CELL WALL
The flexible extracellular matrix that is deposited while the cell is expanding.
- SECONDARY CELL WALL
The flexible extracellular matrix that is deposited while the cell is still expanding is known as the primary cell wall. When expansion ceases, a secondary wall is sometimes laid down inside the primary wall, making it stronger.
- CELLULOSE MICROFIBRIL
A tough, inelastic fibre wrapped in layers (lamellae) within the plant cell wall. Composed of (1,4)-linked β-D-glucosyl residues.
Group of complex polysaccharides that are extracted from the cell wall by hot water, dilute acid or calcium chelators. They include homogalacturonan, rhamnogalacturonans I and II, galactans, arabinans and other polysaccharides.
Group of complex polysaccharides, including xyloglucans, xylans and mannans, that are extracted from plant cell walls by use of strong alkali; characteristically they bind tightly to the surface of cellulose and have a backbone made up of (1,4)-β-D-glycans that resembles cellulose.
Property that indicates how readily gases, liquids and other materials can penetrate an object.
- MIDDLE LAMELLA
The thin layer that connects two plant cells and is rich in pectin.
- MATRIX POLYSACCHARIDES
Complex polysaccharides found in the space between cellulose microfibrils. They are traditionally divided into pectins and hemicelluloses.
- TURGOR PRESSURE
Force generated by water pushing outward on the plasma membrane and plant cell wall, that results in plant rigidity. The loss of turgor pressure causes wilting.
- STEROL GLUCOSIDE
A molecule consisting of a sterol that is linked to glucose through a glycosidic bond.
A polymer built up of sugar residues connected by glycosidic bonds; β-D- identifies the particular stereochemical configuration of the sugar.
Enzymes that cut a glycan and ligate one of the fragments to the free end of another polymer, usually of the same type.
- DOUBLE-LABELLING EXPERIMENTS
Experiments in which two different tags (such as the radioisotopes 3H and 14C) are incorporated in and attached to a molecule. Useful for tracing the origin and fate of a molecule that undergoes complicated processing.
- WALL STRESS RELAXATION
Reduction in mechanical stress in the cell-wall network, because of slippage or scission of load-bearing polymers in the cell wall; wall loosening stimulates wall stress relaxation.
- HYDROXYL RADICAL
The most active form of reactive oxygen species, consisting of a free hydroxyl group in which oxygen is missing an electron in its outermost shell. It is a strong oxidant that can steal an electron from — and thereby damage — polysaccharides, proteins, lipids, nucleic acids and other classes of organic molecules.
- ACID GROWTH
Faster cell elongation under acidic conditions.
Wall loosening proteins that induce wall stress relaxation and irreversible wall extension in a pH-dependent manner, but they do not hydrolyze wall polymers.
- COORDINATE BOND
Chemical bond involving the sharing of a pair of electrons, each supplied by one atom.
The process by which old parts of a plant break off naturally (for example, leaves).
- BIAXIAL STRAIN ASSAYS
Procedures in which a material is stretched not in one direction, but in two directions, as happens with the membrane of an expanding balloon.
The stem region of a seedling below the cotyledons (seed leaves).
- TRACHEARY ELEMENTS
Specialized cells in the xylem of vascular plants that are responsible for the conductance of water as well as providing mechanical support.
Genes or gene families that originated from a common ancestral sequence by a duplication event, not involving speciation.
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