Loosening of plant cell walls by expansins
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Expansin
Turgor pressure
Plant cell
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Expansin
Turgor pressure
Plant cell
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Growing plant cell walls characteristically exhibit a property known as 'acid growth', by which we mean they are more extensible at low pH (< 5) (1). The plant hormone auxin rapidly stimulates cell elongation in young stems and similar tissues at least in part by an acid-growth mechanism (2, 3). Auxin activates a H(+) pump in the plasma membrane, causing acidification of the cell wall solution. Wall acidification activates expansins, which are endogenous cell wall-loosening proteins (4), causing the cell wall to yield to the wall tensions created by cell turgor pressure. As a result, the cell begins to enlarge rapidly. This 'acid growth' phenomenon is readily measured in isolated (nonliving) cell wall specimens. The ability of cell walls to undergo acid-induced extension is not simply the result of the structural arrangement of the cell wall polysaccharides (e.g. pectins), but depends on the activity of expansins (5). Expansins do not have any known enzymatic activity and the only way to assay for expansin activity is to measure their induction of cell wall extension. This video report details the sources and preparation techniques for obtaining suitable wall materials for expansin assays and goes on to show acid-induced extension and expansin-induced extension of wall samples prepared from growing cucumber hypocotyls. To obtain suitable cell wall samples, cucumber seedlings are grown in the dark, the hypocotyls are cut and frozen at -80 degrees C. Frozen hypocotyls are abraded, flattened, and then clamped at constant tension in a special cuvette for extensometer measurements. To measure acid-induced extension, the walls are initially buffered at neutral pH, resulting in low activity of expansins that are components of the native cell walls. Upon buffer exchange to acidic pH, expansins are activated and the cell walls extend rapidly. We also demonstrate expansin activity in a reconstitution assay. For this part, we use a brief heat treatment to denature the native expansins in the cell wall samples. These inactivated cell walls do not extend even in acidic buffer, but addition of expansins to the cell walls rapidly restores their ability to extend.
Expansin
Turgor pressure
Elongation
Epicotyl
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SUMMARY Pollen tubes live a life on a razor’s edge. They must maintain cell wall integrity whilst growing towards the ovule at extraordinary speed but explosively burst at just the right moment to release the sperm cells—with fatal consequences for reproduction if things go wrong. The precisely controlled growth of the pollen tube depends on the fine-tuned balance between the expansive force of turgor pressure and the restraining effect of the cell wall. Currently, it is not well understood how the composition of the cell wall affects its mechanical properties. Using Arabidopsis mutants, we have investigated these interactions by combining experimental and simulation techniques to determine instantaneous and time-dependent mechanical parameters. This allowed, for the first time, the quantification of the effects of cell wall biochemistry on turgor pressure and cell wall elasticity and to predict their effects on growth rate. Our systems biology approach is widely applicable to study the implications of mechanical stress on growth.
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Expansive
Plant cell
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Flowers of <em>Oenothera hookeri</em> Torr. et Gray, <em>Oe. brevistylis</em> and <em>Oe. lamarkiana</em> de Vries were pollinated after anthesis by insects. <em>Oe. biennis</em> L., <em>Oe. suaveolens</em> Desf and sulfurea were selfpollinated in the buds. Pollen morphology was slightly different: <em>Oe. hookeri</em> have regular, triporated pollen, often germinating through two pores; in <em>Oe. suaveolens</em> many pollen grains had callose patches on the intine; in <em>Oe. brevistylis</em> tetraporated pollen were more often than in other species; in <em>Oe. lamarckiana</em> many pollen grains were empty; in <em>Oe. biennis</em> and <em>Oe. suaveolens</em> pollen grain size and viability varied. The pollen tube growth and fertilization were similar in 5 species and can be considered as typical for <em>Oenothera</em>. In the ovary pollen tubes branched and changed their growth direction. Near micropyle they formed short branches to the inner integument. In the nucellus the pollen tube became swollen.
Oenothera
Anthesis
Callose
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Plant cells have a rigid cell wall that constrains internal turgor pressure yet extends in a regulated and organized manner to allow the cell to acquire shape. The primary load-bearing macromolecule of a plant cell wall is cellulose, which forms crystalline microfibrils that are organized with respect to a cell's function and shape requirements. A primary cell wall is deposited during expansion whereas secondary cell wall is synthesized post expansion during differentiation. A complex form of asymmetrical cellular differentiation occurs in Arabidopsis seed coat epidermal cells, where we have recently shown that two secondary cell wall processes occur that utilize different cellulose synthase (CESA) proteins. One process is to produce pectinaceous mucilage that expands upon hydration and the other is a radial wall thickening that reinforced the epidermal cell structure. Our data illustrate polarized specialization of CESA5 in facilitating mucilage attachment to the parent seed and CESA2, CESA5 and CESA9 in radial cell wall thickening and formation of the columella. Herein, we present a model for the complexity of cellulose biosynthesis in this highly differentiated cell type with further evidence supporting each cellulosic secondary cell wall process.
Secondary cell wall
Mucilage
Turgor pressure
Microfibril
Expansin
Plant cell
Osmotic pressure
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Turgor pressure
Plant cell
Tip growth
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Turgor pressure
Pectin
Tip growth
Lilium
Osmotic concentration
Osmotic pressure
Osmoregulation
Plant cell
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Callose
Turgor pressure
Tip growth
Plant cell
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Summary Plant cell expansion is controlled by a fine‐tuned balance between intracellular turgor pressure, cell wall loosening and cell wall biosynthesis. To understand these processes, it is important to gain in‐depth knowledge of cell wall mechanics. Pollen tubes are tip‐growing cells that provide an ideal system to study mechanical properties at the single cell level. With the available approaches it was not easy to measure important mechanical parameters of pollen tubes, such as the elasticity of the cell wall. We used a cellular force microscope ( CFM ) to measure the apparent stiffness of lily pollen tubes. In combination with a mechanical model based on the finite element method ( FEM ), this allowed us to calculate turgor pressure and cell wall elasticity, which we found to be around 0.3 MP a and 20–90 MP a, respectively. Furthermore, and in contrast to previous reports, we showed that the difference in stiffness between the pollen tube tip and the shank can be explained solely by the geometry of the pollen tube. CFM , in combination with an FEM ‐based model, provides a powerful method to evaluate important mechanical parameters of single, growing cells. Our findings indicate that the cell wall of growing pollen tubes has mechanical properties similar to rubber. This suggests that a fully turgid pollen tube is a relatively stiff, yet flexible cell that can react very quickly to obstacles or attractants by adjusting the direction of growth on its way through the female transmitting tissue.
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Elasticity
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The cell wall is important for pollen tube growth, but little is known about the molecular mechanism that controls cell wall deposition in pollen tubes. Here, the functional characterization of the pollen-expressed Arabidopsis cellulose synthase-like D genes CSLD1 and CSLD4 that are required for pollen tube growth is reported. Both CSLD1 and CSLD4 are highly expressed in mature pollen grains and pollen tubes. The CSLD1 and CSLD4 proteins are located in the Golgi apparatus and transported to the plasma membrane of the tip region of growing pollen tubes, where cellulose is actively synthesized. Mutations in CSLD1 and CSLD4 caused a significant reduction in cellulose deposition in the pollen tube wall and a remarkable disorganization of the pollen tube wall layers, which disrupted the genetic transmission of the male gametophyte. In csld1 and csld4 single mutants and in the csld1 csld4 double mutant, all the mutant pollen tubes exhibited similar phenotypes: the pollen tubes grew extremely abnormally both in vitro and in vivo, which indicates that CSLD1 and CSLD4 are not functionally redundant. Taken together, these results suggest that CSLD1 and CSLD4 play important roles in pollen tube growth, probably through participation in cellulose synthesis of the pollen tube wall.
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Callose
Double fertilization
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