Plant Cell Growth and Elongation

Abstract

Irreversible cell expansion is an essential process underlying plant growth and development. Growth begins with cell wall loosening which induces wall stress relaxation which in turn generates the reduced water potential that is needed for water uptake and volumetric expansion of the cell. Most growing cell walls consist of a cellulose scaffold embedded in a matrix of polysaccharides classified as pectins and hemicelluloses. Models of the growing cell wall are tentative hypotheses about how cell wall components are linked together to make a strong yet extensible wall and have implications about the mechanism of wall expansion. Recent evidence indicates that wall loosening by expansins and specific endoglucanases may be limited to specific regions (‘biomechanical hotspots’) where cellulose microfibrils come into close contact. The growing cell wall is dynamically modified by enzymes that change the structure of pectins and hemicelluloses, thereby altering their interactions with each other and with cellulose. Growth cessation is correlated with reduced expression of genes that promote wall loosening and changes in matrix polysaccharides that lead to a less extensible cell wall.

Key Concepts:

  • Irreversible cell expansion is an essential aspect of plant growth and morphogenesis.

  • Surface expansion of the cell wall may be highly localised, as in tip‐growing cells, or more evenly dispersed over the cell wall surface (‘diffuse growth’), occurs pattern common in most cells of the plant body.

  • Most cells undergo a relatively brief period of rapid cell expansion after they leave the meristem and before they differentiate into mature cells.

  • Expansive growth of plant cells requires simultaneous uptake of water into the cell and irreversible expansion of the cell wall.

  • Cell growth begins with cell wall loosening which leads to ‘stress relaxation’ of the cell wall which in turn creates the water potential difference needed for water uptake by the cell, resulting in the physical enlargement of the cell.

  • The growing cell wall is composed of a network of cellulose microfibrils embedded in pectins and hemicelluloses that make up the wall matrix; these materials combine to form a load‐bearing structure that controls cell mechanics and physically limits cell growth.

  • Deposition of new polymers to the wall is usually coordinated with surface expansion, but these are separable processes.

  • Plant cell walls enlarge more rapidly at low pH (‘acid growth’), a process that is mediated by nonenzymatic proteins named α‐expansins.

  • Several classes of enzymes modify the structure of pectins and hemicelluloses in the cell wall, but it is not clear whether such activity modulates growth in most cases.

  • Cessation of cell enlargement likely involves multiple processes, including tightening of the matrix‐cellulose network and reduced expression of wall‐loosening proteins.

Keywords: cell enlargement; cell wall structure; pectins; hemicelluloses; cellulose; expansin; acid growth; wall stress relaxation; wall loosening; cell water uptake

Figure 1.

Patterns of surface expansion in various cell types and the net orientation of cellulose microfibrils in different walls. (a) Walls with randomly oriented cellulose generally expand isodiametrically by diffuse growth. Note that cellulose microfibrils do not increase in length but spread out as the wall expands. (b) The cellulose microfibrils in the side walls in this cell are oriented in a net circumferential direction. Expansion of these walls results in a highly biased pattern of diffuse growth, with axial extension greatly exceeding increase in girth. Note that the actual density of celluose microfibrils in cell walls is much higher than what is illustrated in these diagrams. (c) Tip growth differs from diffuse growth in that wall expansion is highly focused to the hemispherical dome at the tip of the cell (e.g. in pollen tube or root hair). The local pattern of surface expansion may be observed by measuring the distance between marks placed on the cell (small blue squares) at time intervals. Marks on most of the tube do not separate from one another, indicating the lack of surface expansion over most of the tube. Only marks at the tip become separated as a result of local surface expansion.

Figure 2.

Growth biophysics in a nutshell. (a) Sketch representing the sequence of events that couple cell wall expansion and water uptake: (1) Osmosis in plant cells results in turgor pressure that stretches the cell wall and creates a counter balancing tensile stress in the wall. (2) Wall loosening results in a subtle slippage or cutting of load‐bearing parts of the cell wall, causing a simultaneous reduction in wall stress and turgor pressure. (3) The reduction in turgor pressure displaces the cell from water potential equilibrium, initiating an influx of water, physical enlargement of the cell, and a restoration of cell turgor. In cells growing at a steady rate, wall relaxation and water influx occurs simultaneously with steady, small displacement from water potential equilibrium whose value depends on hydraulic conductance of the water pathway. (b) A mechanical representation of the processes outlined in (a): (1) The wall is represented by two mechanical elements, an elastic spring (red) and a sliding component (blue). The tensile force in the wall, graphically indicated by the extension of the springs, arises from cell tugor pressure. (2) Stress relaxation results from a sliding of the blue components, resulting in a contraction of the springs and a reduction in wall stress, without a change in the total length of the wall (black dotted lines). (3) As water flows into the cell to restore turgor towards its equilibrium value, the wall expands by stretching of the elastic elements, with a consequent restoration of wall stress.

Figure 3.

The plant primary cell wall is made of cellulose microfibrils embedded in pectins and hemicellulose. (a) The fibrillar organisation of the cell wall is readily seen in images of the newly deposited wall surface visualised by atomic force microscopy. The thinnest fibrillar structures are cellulose microfibrils, which are closely aligned and bundled in short segments. Note the layered texture of the wall, with the orientation of cellulose microfibrils differing for each lamella. The scale bar=500 nm. (b) An individual microfibril is made of a long ribbon of approximately 18–24 glucans tightly packed into a highly ordered array. (c) Molecular structure of a small part of a cellulose microfibril, showing a short stretch of two β1,4‐linked D‐glucans and the hydrogen bonds (dotted lines) within each chain and between adjacent chains. The resulting microfibril has great tensile strength and the surface can bond noncovalently to matrix polymers, particularly hemicelluloses such as xyloglucan and arabinoxylan. (d) Two pathways of cell wall synthesis. Cellulose microfibrils are manufactured at the cell surface by synthase complexes embedded in the plasma membrane. Pectins and hemicelluloses are produced in the Golgi apparatus and delivered to the cell surface by vesicles that fuse with the plasma membrane and extrude their contents. Reproduced with permission from Zhang et al. (). © Springer.

Figure 4.

(a) In a new model that accounts for the extensibility of primary cell walls, the load‐bearing connections between cellulose microfibrils are proposed to be restricted to short regions, dubbed ‘biomechanical hotspots’, where microfibrils come into close contact with one another. Reproduced with permission from Park YB and Cosgrove DJ (). © American Society of Plant Biologists. (b) A depiction of two cellulose microfibrils in cross section (green, blue) bonded together via a xyloglucan chain (red). Reproduced with permission from Zhao et al. (). © Springer.

Figure 5.

Structural analysis of expansin–cell wall interactions indicate that expansins bind to selective cellulose sites in the cell wall. (a) The interactions between a bacterial expansin (red) and a short cellulose fragment (green) have been worked out in detail by X‐ray crystallography of expansin–ligand complexes (Georgelis et al., ). The binding to cellulose is mediated by a set of three aromatic residues arranged in a line on one half of the protein. (b) The interaction of bacterial expansin with a cellulose microfibril is modelled by molecular dynamics simulation with support from solid‐state NMR (Wang et al., ). Expansins target specific domains of cellulose with a different crystalline structure than bulk cellulose. The exact molecular basis for the subsequent stress relaxation is still uncertain.

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Cosgrove, Daniel J(Nov 2014) Plant Cell Growth and Elongation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001688.pub2]