4.3.2  Cell wall expansion

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Figure 4.18 High resolution scanning electron micrograph of the primary cell wall of onion (Allium cepa L.). The root has been saponin treated then freeze—fractured to reveal the inner face of a cell wall. Cellulose microfibrils (cmf) and gel matrix components (gm) that cross-link these microfibrils are arrowed, revealing most microfibrils lying at a similar angle within the wall. The upper left corner of the image shows a remaining fragment of plasma membrane, with microtubules running diagonally across the membrane (Courtesy of P. Vesk)

Inflow of water results in cell expansion (growth) as the prim-ary cell wall stretches to accommodate water uptake. Cell walls are, however, not infinitely extensible; directionally controlled cell expansion under pressure sets plant cells apart from animal cells. So, an increase in plant cell volume is achieved through coordination of many events: cell walls yield to P, solute and water fluxes are initiated, membranes surrounding the vacuoles and cytoplasm expand and new wall and membrane components are synthesised. Cell wall yielding is of special significance in plant growth because all plant cells are encased in a matrix of wall polymers (Table 4.2) which resists expansion sufficiently to generate pressures within the cell contents but yields sufficiently to allow cell expansion in growth zones (Figure 4.18; see also Case study 4.2). Expansion of plant cells is intriguing because wall yielding, an extracellular process, is so exquisitely coordinated by events within the cell. So subtle is this coordination that different walls of a single cell generally have different extension rates, even though each wall is subject to the same P. The cytoskeleton is a central player in co-ordination of wall expansion; intracellular microtubular arrays influence orientation of cellulose microfibrils in the wall. Microfibrils do not stretch longitudinally so growth can only proceed normal to the microfibril axes (Figure 4.19). In this way, cytological events help shape cells.

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Figure 4.19 Model of cell wall expansion. (a) Cellulose microfibrils oriented normal to the axis of a cylindrical cell undergoing longitudinal elongation. A typical epidermal cell would grow with this strong polarity. (b) Microfibrils (hatched) are shown in parallel arrays, joined by a loose and a tight hemicellulose polymer which are each hydrogen bonded to the microfibril. Arrows denote P-driven separation of microfibrils. (c) After a period of growth, increasing numbers of hemicellulosic polymers develop tension as the cell extends, giving rise to wall pressure. Note the lengthening of the distance spanned by the polymers through time. New wall polymers are normally secreted from cells to maintain wall mass (From Passioura and Fry 1992; reproduced with permission of Journal of Experimental Botany)

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Figure 4.20 Stress relaxation in a slowly growing pea epicotyl measured with the pressure-block technique. The atmospheric pressure which must be applied to a pea seedling to block growth increases as tissues ‘relax’.The initial rate of relaxation (dashed line) is used to calculate applied pressure required to overcome ΔΨ which is set up by steady-state tissue expansion (prior to pressure application). Subsequent relaxation (after 2-4 min) reveals the applied pressure required to counter effective pressure generated within cells to drive growth (P - Pth) (From Cosgrove 1993; reproduced with permission of The Plant Cell))

Cell wall yielding and water uptake occur simultaneously in a growing cell but they can be uncoupled from one another, giving an appreciation of the hierarchy in which they occur. An ingenious set of experiments outlined below indicates that the yielding of cell walls to P, often termed wall relaxation, actually predisposes a cell to water influx and volume expansion. Normally, the restoration of P follows as active solute accumu-lation (e.g. through ion pumps) raises intracellular osmotic pressure (P). That is, changes to cell walls appear to precede water uptake. Two lines of work by Cosgrove and colleagues at Pennsylvania State University have led to this conclusion. In one case pea epicotyls (young shoots) were kept in a humid atmosphere but cut off from a continuous water supply in order to prevent water uptake. Cells of the growing zone relaxed (P decreased) whereas those of the non-growing zone did not relax. This could be detected because no water was available to restore P. This suggests that cell wall relaxation occurs even in the absence of water uptake: a consequent loss of P would normally be overcome by uptake of xylem sap or soil moisture if it was available. A second series of experiments involved the ‘pressure block’ technique in which gas pressure was used to block growth by annulling cell P. Experimentally, this can be done by sealing stems of intact plants into a gas-tight chamber and increasing the pressure of the atmosphere around the tissues. A plot of pressure applied versus growth (Figure 4.20) shows that the applied pressure required to block growth increased over time, suggesting that cell walls relaxed steadily through time even though growth had ceased. Again, this is evidence for a relaxation of cell walls leading to growth rather than an uptake of water driving wall expansion and growth. To translate this into a cellular context, cell wall yielding to pressure exerted by the plasma membrane is more likely to be the primary event in growth than osmotically driven influx of water. In practice, these events seem to occur simultaneously and might be considered as partners which sustain cell growth.

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