Hydrostatic Pressure in Cell Growth and Function

Opinion TRENDS in Plant Science Vol.12 No.3

Life under pressure: hydrostatic pressure in cell growth and function

Laura Zonia and Teun Munnik

Department of Plant Physiology, Swammerdam Institute for Life Sciences, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, Netherlands

H2O is one of the most essential molecules for cellular osmotic pressure, cell volume and hydraulic conductivity in life. Cell volume, osmolality and hydrostatic pressure are driving cell shape restructuring and growth [5–14]. In this tightly controlled by multiple signaling cascades and Opinion article, we will present the case for hydrodynamic they drive crucial cellular functions ranging from exocy- control of plant cell growth. tosis and growth to apoptosis. Ion fluxes and cell shape To build this case we will briefly discuss some important restructuring induce asymmetries in osmotic potential aspects of cell hydrodynamics that support this alternative across the plasma membrane and lead to localized model: the initial events in cell volume perception, mech- hydrodynamic flow. Cells have evolved fascinating strat- anisms that initiate cell volume recovery, and several egies to harness the potential of hydrodynamic flow to functions regulated or driven by cell volume that are perform crucial functions. Plants exploit hydrodynamics important for growth. We will discuss evidence from to drive processes including gas exchange, leaf position- animal systems where much work has been done, but will ing, nutrient acquisition and growth. This paradigm is focus on the similarities discovered to date in plant cells. extended by recent work that reveals an important role The last section will discuss recent work on the role of for hydrodynamics in pollen tube growth. volume and pressure in pollen tube tip growth. It is hoped that the concepts presented in this Opinion article will The pivotal role of water in the origin, function and stimulate new ways of thinking about plant cell growth and proliferation of cellular life function. Life is all about aqueous chemistry and reactions that occur at surfaces and interfaces. The unique physical Sensors and controllers in cell volume regulatory properties of water not only promoted the emergence of networks cellular life but also set limits on effective cell dimensions Mechanosensitive ion channels within which viability and reproduction can be maintained Water surrounds cells, penetrates into the plasma [1]. It was crucial for cell function that osmolality, mem- membrane and cytosol, and affects the local geometry of brane tension and hydrostatic pressure (see Glossary) the lipid bilayer and membrane proteins [15]. Ions or osmo- were tightly controlled. Thus, early in evolution, cells were lytes perturb the aqueous network and affect membrane confronted with the problem of how to control their volume tension and osmotic potential. In bacteria, milli-osmolar and regulate the flow of water across the membrane. It is changes in water concentration are sufficient to shift the believed that solutions to this problem arose with the osmotic pressure across the plasma membrane and generate earliest protocells, and these formed the basis of volume stretch and compression forces along the plane of the lipid regulation during the development of cellular complexity bilayer [16]. These forces gate mechanosensitive (MS) ion [1]. The list of processes that are controlled by cell volume and hydrodynamics reads like the book of life itself – it includes growth and proliferation, membrane transport, Glossary exocytosis, endocytosis, cell shape changes, hormone sig- Anisotropic: exhibiting unequal properties along different axes. naling, metabolism, excitability, neural communication, Electro-osmosis: the movement of a polar fluid through a selectively perme- cell migration, nutrient delivery, waste filtration, necrosis able membrane or porous material, or along a charged surface, under the influence of an electric field. and apoptosis [2,3]. Hydraulic conductivity: the movement of fluid through pores or confined For cells enclosed by a cell wall, growth is dependent on spaces under pressure, depending on the intrinsic permeability of the material deposition of new wall materials at the cell surface coupled and on the degree of saturation. Hydrodynamics: the dynamics of fluids in motion; the forces exerted by fluids with osmotic pressure to drive expansion of the cell. There in motion. has been some debate about whether plant cell growth is Hydrostatic pressure: the pressure exerted by a static fluid, depends on the primarily controlled by modulations of cell wall stiffness or fluid depth, density and gravity; independent of shape, total mass or surface area of the fluid. by changes in osmotic pressure (Box 1). Deposition of new Osmotic potential: the potential for water to move across a selectively wall materials is not in itself sufficient to promote growth permeable membrane. [4]. However, recent work is revealing a crucial role for Osmotic pressure: the force exerted against a selectively permeable membrane by a solution that has different solute concentrations on either side of the membrane. Viscoelastic: a material that exhibits both viscous and elastic properties, and Corresponding author: Zonia, L. ([email protected]). shows time-dependent strain when a stress is applied. Available online 12 February 2007. www.sciencedirect.com 1360-1385/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2007.01.006 Opinion TRENDS in Plant Science Vol.12 No.3 91

[22]. It has been proposed that aquaporins might function Box 1. What drives plant cell growth – cell wall loosening or as osmosensors and regulators of cell volume in animal, osmotic pressure? plant, fungal and bacterial cells [23]. Plants express more There is agreement that both cell wall properties and turgor aquaporin homologs than are expressed by animals, pressure have roles in growth, but there is disagreement about reflecting the importance of regulating water flow for plant what drives the initial event of cell enlargement. Current theory cells. Plant aquaporins have high levels of expression in considers that cell wall loosening occurs first, which then allows expansion of the cell wall because of the decreased rigidity. As cells undergoing large volume changes or high water flux expansion starts there is a simultaneous decrease in intracellular rates, including pollen coat and stomatal guard cells pressure, which then promotes passive entry of H2O down its [24,25]. osmotic potential gradient, causing further expansion of the cell [32,77,78]. There is considerable evidence to support this theory. Expansin proteins promote cell wall stress relaxation, and their Activation of ion fluxes during cell volume perturbation expression and activity correlate with growth [79]. Experimental and recovery techniques that increase cell wall rigidity in pollen tubes also There is a dynamic balance between the osmotic potential decrease growth rates [32,59,80]. However, a recent cytomechanical across the plasma membrane and cell volume, which is study revealed that there is no significant change in the viscoelastic dependent on intracellular metabolic flux and extracellu- properties of the pollen tube cell wall in the apical growth zone during repeated deformations [56]. This suggests that cycles of cell lar conditions. In neuronal cells undergoing intense acti- wall loosening cannot be the driving force for oscillatory growth. vation, high ionic flux rates cause localized changes in the Furthermore, recent analyses succeeded in resolving previously osmotic balance across the plasma membrane that induces undetected data and show that increased pressure induces expan- water flow out of the cells and invagination of the plasma sion of the cell wall and drives growth in Chara cells [5,6].The membrane [26]. During homeostatic regulation in many mechanics of cell wall expansion in the apical dome is similar for root hairs, Chara and Nitella rhizoids, and fungal sporangia in spite cell types, any perturbations to the osmotic balance rapidly of differences in cell wall composition, indicating that it is an activate mechanisms involved in cell volume recovery. The emergent feature of tip growth [81]. Thus, there is evidence that first responses involve changes in ion flux across the pressure induces growth in plant cells. A new model called loss of plasma membrane, which can be detected in animal, plant stability (LOS) has been proposed [7,8]. The LOS model considers and fungal cells within 1–10 min during volume recovery that turgor pressure increases gradually to a critical point that is determined by the cell wall geometry and material properties, at (Figure 1) [2,27–29]. Changes in ionic flux rates rapidly which point the pressure causes a loss of stability of the cell wall result in H2O flow across the plasma membrane and induce that manifests as wall extension and growth. A recent examination changes in cell volume, and vice versa. Hyperosmotic shifts of pollen tube growth extends this work and reveals a role for or cell shrinkage induce K+,ClÀ and Na+ uptake, and H+ hydrodynamic flow. This work shows that a driving force for growth in pollen tubes is mediated by cyclical volume increases in the efflux, which cause H2O to flow into the cells and leads to apical region [9]. Cell volume increases are known to promote regulatory volume increase. Conversely, hypo-osmotic exocytosis (see text), and turgor pressure can drive polysaccharide shifts or cell swelling induce K+ and ClÀ efflux and H+ insertion into cell walls and promote cell wall assembly [10,11].In influx, which cause H2O to flow out of the cells and leads to summary, there is increasing evidence that osmotic pressure and regulatory volume decrease. Cell swelling and increased hydrodynamics have crucial roles in driving plant cell growth. hydrostatic pressure induce influx of extracellular Ca2+ in animal cells [2,30], and there is evidence that swelling channels (MscS, MscL, TRP) that function as osmotic safety induces Ca2+ influx in pollen tubes [31].Ca2+ influx at the valves and initiate processes leading to cell volume recovery apex is an important regulator of pollen tube growth [32]. [16,17]. MS channels arose early in evolution and are pre- sent across all three domains of Archaea, Bacteria and Activation of signaling cascades during cell volume Eukarya. Indeed, pressure-induced activation of MS chan- perturbation and recovery nels is postulated to be one of the first signal transduction Secondary responses to cell volume changes include cascades that arose with the onset of cellular life [16,17].In activation of downstream signaling pathways that target plants, MS ion channels have been identified that are a plethora of cell functions (Figure 1) [2,33–36]. Numerous involved in cell volume perception and regulation. Arabi- studies have shown that the levels of signaling lipids are dopsis mesophyll cells have a MS anion channel that is increased by osmotic stress. Hypo-osmosis and cell swelling specifically gated by convex curvature of the plasma mem- induce increases in phosphatidic acid in pollen tubes, uni- brane, such as occurs during cell swelling [18,19]. Tobacco cellular green algae and erythrocytes. Hyperosmosis and suspension cells contain a MS anion channel that mediates cell shrinkage induce increases in phosphatidylinositol-bis- fluxes great enough to induce cell volume changes [19]. phosphates in animal and plant cells. Osmotic stress also Furthermore, an MscS-like channel was shown to control activates MAPK cascades and induces gene expression. Cell plastid size and shape in Arabidopsis [20]. Interestingly, the volume changes impact cytoskeletal organization, and Rho bacterial MscS channel is pressure-gated, voltage-depen- GTPases are a major point of convergence to integrate dent, and selective for anions [17]. A MS Ca2+ channel has membrane signals and cytoskeletal organization. In animal been identified in lily pollen tubes that is thought to mediate cells, there is increasing evidence that Rho GTPases are Ca2+ influx at the apex [21]. regulated by cell volume and intracellular ionic strength [2,37,38]. Rac is activated by both cell swelling and cell Aquaporins shrinking, and specificity appears to be linked to subcellular Plasma membrane permeability to water is greatly localization. Rho is activated by cell shrinking, and its enhanced by aquaporin channels that allow the flow of presence is required for signaling cascades that are acti- H2O (and some small neutral solutes) across membranes vated by cell swelling. The mechanisms involved in Rho www.sciencedirect.com 92 Opinion TRENDS in Plant Science Vol.12 No.3

Figure 1. Cell volume regulatory networks in (a) animal and (b) plant cells. First responses to hypertonic shrinking and hypotonic swelling are mediated by changes in ion fluxes across the membrane, which drive the influx or efflux of H2O and lead to cell volume recovery. Hypertonic shrinking in plant cells causes the protoplast plasma membrane to retract from the cell wall (plasmolysis), remaining attached by thin cytoplasmic projections called Hechtian strands. Secondary responses to cell volume changes are mediated by phospholipid signaling cascades, protein kinase cascades and Rho GTPases. Question marks denote known functions in animal cells that might be possible but have yet to be tested in plant cells. Abbreviations: AA, arachadonic acid; cAMP, cyclic AMP; DAG, diacylglycerol; DGPP, diacylglycerol pyrophosphate; ERK1/2, extracellular signal-regulated kinases; mTOR, mammalian target of rapamycin; N, nucleus; PI3K, phosphoinositide 3-kinase; PtdIns(3,4)P2, phosphatidylinositol (3,4)- bisphosphate; PtdIns(3,5)P2, phosphatidylinositol (3,5)-bisphosphate; PtdIns(4,5)P2, phosphatidylinositol (4,5)-bisphosphate; PtdOH, phosphatidic acid; SIMK, stress- induced MAPK; V, vacuole.

GTPase activation in response to cell volume changes are is crucial for the control of transpiration and gas exchange. currently under investigation. In plant cells, recent work This reversible hydrodynamic loading–unloading is largely indicates the importance of Rho/Rac GTPases for growth driven by the accumulation or loss of K+ and ClÀ [43]. and development [39]. Future research should investigate a During reversible cell swelling, the intracellular osmolar- potential link between Rho/Rac GTPases and volume regu- ity can change by as much as 50% [44]. Surface area varies lation in plant cells. linearly with cell volume changes and can increase by 40– 50% [45]. Furthermore, the swelling–shrinking cycles of Hydrodynamic flow drives crucial cellular functions guard cells can oscillate rapidly, enabling tight control of Cell volume crucially affects membrane tension and stomatal aperture size [46]. In summary, these results curvature, molecular crowding in the cytosol, and intra- indicate that ion flux changes rapidly modulate cell cellular ionic strength [40–42]. Because of this, processes volume. ranging from growth and proliferation to necrosis and There is a strong correlation between membrane apoptosis are controlled by cell volume in Bacteria and tension and vesicle insertion or retrieval in many different Eukarya [2,3]. Furthermore, many cell types have evolved cell types. Increases in cell volume or membrane tension fascinating strategies that enable them to harness the stimulate exocytosis, whereas decreases in cell volume or potential of osmotic gradients and hydrodynamic flow to membrane tension stimulate endocytosis [47]. The situ- drive crucial cellular functions. Some functions occur more ation is similar in guard cells. Cell volume increases drive readily under certain cell volume conditions, such as exo- exocytosis and surface area expansion, whereas cell cytosis, endocytosis, neural activation and enzymatic reac- volume decreases drive endocytosis and surface area con- tions. Some functions in plants and fungi can only be traction [44,45]. There is also evidence for constitutive driven by exploiting hydrodynamic flow, such as stomatal membrane turnover in guard cells [44]. Continuous mem- guard cell opening and closing, leaf pulvini motor organ, brane turnover might enable plant cells to respond rapidly mechanical traps of carnivorous plants, and fungal appres- to small shifts in the osmotic potential across the mem- sorial penetration. A survey of cellular functions that are brane and, thus, control membrane tension. driven by or dependent on cell volume is presented in Table 1. In this Opinion article, we will briefly discuss a Vectorial hydrodynamic flow and hydrostatic pressure well-characterized plant cell system that is driven by surges can trigger localized cell expansion and growth hydrodynamics – stomatal aperture and membrane There is increasing evidence that cell shape restructuring dynamics in guard cells. and polarized growth are initiated by transiently non- Guard cells undergo osmotically induced shrinking and equilibrated hydrostatic pressure surges pushing against swelling and thereby control stomatal aperture size, which the cell boundary [9,12–14]. Recent work in animal and www.sciencedirect.com Opinion TRENDS in Plant Science Vol.12 No.3 93

Table 1. A survey of cellular functions driven by or dependent on cell volume and hydrodynamics Organism Function Refs Vertebrate Human Exocytosis and endocytosis [47] Human PLD-mediated exocytosis and endocytosis [62] Human Neural activation in visual cortex [63] Human Kidney filtration of plasma [64] Human Liver metabolism [65] Human, rat, mouse, Xenopus Proliferation and apoptosis [66,67] Human, pig, rat Rho/Rac GTPase activation and reorganization of actin cytoskeleton [37,38] Mouse Melanoma blebbing [12] Mouse Neurotransmitter release at neuromuscular junction [68] Chick embryo Neural axon growth [50] Plant Arabidopsis, Vicia, Maize Exocytosis and endocytosis in guard cells [44,45] Nicotiana, Chara Cell growth [5,6,9] Chara Polysaccharide insertion into cell walls; cell wall deposition and assembly [10,11] Vicia, Arabidopsis, Nicotiana Stomatal aperture, stomatal oscillations [43,46] Samanea saman Leaf positioning [69] Nicotiana tabacum Leaf unfolding [70] Nicotiana tabacum Anther dehiscence [25] Venus flytrap Trap closure [71] Slime mold Dictyostelium Blebbing [13] Fungi Neurospora crassa Hyphal mass flow [51] Magnaporthe grisea Appressorial penetration of host tissues [72,73] Ascobolus immersus Pressurized spore dissemination [74] Bacteria E. coli Growth [75] E. coli Chemotaxis [76] bacterial cells indicates that the cytoplasm is highly mass cytoplasmic flow and hyphal extension in Neurospora non-uniform and structured as a porous contractile elastic [51]. As noted above, hydrostatic pressure surges might network composed of cytoskeletal filaments and orga- drive bulge formation in root hairs and pollen tubes [9,14]. nelles, infiltrated with an electrolytic interstitial fluid, Below we consider pollen tube growth. similar to a fluid-filled sponge [12,42]. It is proposed that the anisotropic nature of these structural networks, Properties of cytoplasm and cell wall electrolyte pathways and pools ultimately results in vec- The pollen tube apical region differs from distal regions in torial electrochemical gradients within the cell [42]. The several key parameters. Actin arrays are present as a fine result of this is that hydrostatic pressure cannot instan- cortical fringe in the apical dome and as longitudinal taneously propagate throughout the cytoplasm and is filaments in the distal tube [52]. The distribution of highly dependent on the hydraulic and electrochemical vesicles and organelles differ in that the apical dome is conductivity of the network [12,42]. Thus, hydrostatic enriched with vesicles whereas organelles are largely pressure can be transiently non-equilibrated within the excluded [53]. Oscillations in ion fluxes that are correlated cell. In regions with weaker cortical cytoskeletal arrays, a with growth rate oscillations are prevalent at the apex and hydrostatic pressure surge can push the plasma membrane along the tube flanks in the apical region [32,54,55]. This outward to create a bulge [12,13]. In pollen tubes and root might confer transient and localized electro-osmotic hairs, there is evidence that the apex has weaker cortical activity during the growth cycle. The apical region behaves arrays than distal regions because plasmolysis induces viscoelastically whereas the distal region behaves elasti- retraction of the protoplast from the apex (Figure 2a) cally and with greater stiffness [56]. The tips of fungal [48,49]. Bulge formation has been modeled as the initial hyphae also have greater viscoelasticity and hydrophilicity event for polarized growth of root hairs, trichomes and than distal regions [57]. Finally, it is the apical 50 mm pavement cells [14]. region that undergoes volume changes in response to Several cell types grow by an anisotropic process called osmotic or ionic perturbations (Figure 2a) [33,58]. Taken tip growth, including neurons, pollen tubes, root hairs and together, these results indicate that the cytoplasm of pol- fungal hyphae. There have been relatively few studies of len tubes is highly non-uniform and that apical and distal the role of osmotic pressure and cell volume in tip-growing domains are likely to have different hydraulic conduc- cells, but the most recent data indicate a role for hydro- tivities. dynamics. Growth rates of neuronal axons were increased Manipulation of the osmotic potential difference across by as much as sixfold by hypo-osmotic shifts in the extra- the plasma membrane rapidly modulates apical volume, cellular medium, whereas hyperosmotic shifts reduced cell wall expansion, and pollen tube growth rates (see growth rates [50]. Osmotic gradients have a role in driving also Figure 2a) [9,33,58]. Increasing the culture medium www.sciencedirect.com 94 Opinion TRENDS in Plant Science Vol.12 No.3

Figure 2. Hydrodynamics in pollen tube growth. (a) The pollen tube growth rate oscillator depends on apical volume and pressure. In isotonic conditions, the chart shows that the average growth cycle period is 50 s. Upon shifts to hypertonic conditions, apical shrinking occurs and cells can undergo plasmolysis at the apex; the chart shows that average growth cycle period is 100 s. Upon shifts to hypotonic conditions, apical swelling occurs; the chart shows that average growth cycle period is 25 s. (b) D2O affects pollen tube morphology and growth. Pollen tubes were germinated for 2 h in normal medium and then grown for 12 h in normal medium (Control) or in D2O-based culture medium (D2O). D2O flows through cells with different kinetics than those of H2O, reducing growth rates and inducing cell wall thickening, branching, and irregular pollen tube diameters. (c) Proposed model for apical volume oscillations in pollen tubes. The chart shows an example of two cycles of pollen tube growth. The growth rate decreases during phase I, whereas phase II has maximally increasing growth rate. During phase I, electro-osmosis and hydraulic conductivity drive H2O into the pollen tube and lead to gradual volume increase in the apical region. Apical swelling and increased pressure at the apical plasma membrane induce stress relaxation of the cell wall and trigger the start of the growth cycle. During phase II, vectorial mass flow and regulatory volume decrease mechanisms result in H2O efflux at the apex, which drives cell elongation and growth. Scale bars = 10 mm. Portions of (a) and (b) reproduced, with permission, from Ref. [9]. osmolality and stiffness induce decreases in pollen tube cell wall mechanical properties in different osmotic growth and decrease the abundance of cell wall pectins environments and during different phases of cellular acti- [59]. These results suggest that the mechanical properties vation. Expansins and other wall-modifying enzymes of the cell wall might be adjusted to balance rather than would have particularly important roles in rapidly growing control the dynamic interaction between extracellular and cells to maintain optimum cell wall extensibility with intracellular pressures. This would enable fine-tuning of respect to this dynamic balance. www.sciencedirect.com Opinion TRENDS in Plant Science Vol.12 No.3 95

Cell volume, hydrodynamics, and hydrostatic pressure function as a self-organizing, cyclic and dissipative surges mechanism that sets the frequency of growth rate oscil- Pollen tube cell volume has a crucial role in fertilization, is lations. When cycles of regulatory volume decrease– exquisitely sensitive to the osmotic potential difference increase are uncoupled, growth rate would be stochastic. across the plasma membrane and is correlated with growth When the cycles are coupled, hydrostatic pressure surges [9,33,58,60]. When a pollen tube reaches the ovule it stops would provide a motive force that enables the pollen tube to growing and undergoes a massive increase in volume and penetrate the stigma and grow through the style like a pressure that causes the cell to explode at the apex, hydraulic drill. propelling the sperm cells into the ovule sac [60]. Apical swelling is a rapid and generalized result of growth inhi- Conclusions and perspectives bition, suggesting that during normal growth there is a Hydrodynamics and hydrostatic pressure are among the mechanism that dissipates or expends this pressure most fundamental physical properties that determine cell buildup. form and function. Cells rapidly respond to changes in A recent investigation of apical volume in growing volume and osmotic potential differences across the Nicotiana tabacum pollen tubes revealed an important plasma membrane. Hydrostatic pressure changes can gen- role for hydrodynamics (Figure 2) [9]. The apical region erate stretch or compression forces along the plasma mem- undergoes volume oscillations that have the same fre- brane and activate mechanosensitive ion channels. This quency as growth rate oscillations but are phase-shifted simple switch is postulated to be one of the oldest sensory by 1808, so that the start of the growth cycle occurs as transduction processes that evolved with the onset of volume reaches maximum, and growth rate reaches maxi- cellular life. There is increasing evidence that hydrodyn- mum as volume reaches minimum (Figure 2a, Isotonic). amics and/or hydrostatic pressure have important roles in Experimental manipulation of apical volume is sufficient cell shape and structure, exocytosis and growth, in animal to reset the growth rate oscillator [9]. Hypertonic treat- and plant cells, algae, slime molds, oomycetes and fungal ment decreases pressure and causes the average growth hyphae [5,6,9–14,44,45,51]. period to double, whereas hypotonic treatment increases Evidence is emerging that the cytosol consists of a pressure and causes the average growth period to halve highly anisotropic and elastic network composed of cyto- (Figure 2a). Osmotic manipulation of volume is sufficient to skeletal arrays and organelles, infiltrated with interstitial reset the growth rate oscillation frequency (period) – not electrolytic pathways and electrochemical gradients just the amplitude – which indicates that hydrodynamics [12,42]. Because of this, localized hydrostatic pressure is has a crucial function in driving the pollen tube growth rate strongly dependent on the local hydraulic conductivity of oscillator. When extracellular H2O in the culture medium the network. All the available evidence indicates that 2 is replaced with H2O (deuterium oxide, D2O), pollen tube apical and distal regions of pollen tubes differ in their growth and cell morphology are adversely affected hydraulic conductivity. Osmotic potential gradients and/ (Figure 2b). Transfer back into H2O medium reverses or electro-osmosis are likely factors that induce apical the effects on growth and morphology [9]. Vectorial mass volume oscillations and hydrostatic pressure surges in flow of cytoplasm toward the apex has been observed in pollen tubes. It has been proposed that oscillations in fungal hyphae and pollen tubes [51,58]. There is evidence membrane ion transport could occur in any plant cell or from Raman microscopy that polarized growth is main- tissue under suitable conditions [61]. It will be interesting tained at least in part by the flow of H2O out of the pollen to discover if hydrostatic pressure surges are a general tube apex [9]. mechanism of plant cell growth.

Hydrodynamics and pollen tube growth Acknowledgements These results lead to a new model for pollen tube growth We apologize to scientists whose work could not be cited due to space limitations. We thank Michel Haring for critical reading of the (Figure 2c). This model proposes that growth rate oscil- manuscript. Research in T.M.’s laboratory is currently supported by the lations are driven by cycles of vectorial hydrodynamic flow Netherlands Organization for Scientific Research (NWO 813.06.003, that cause volume increases and decreases in the apical 863.04.004 and 864.05.001). region. Hydrodynamic flow is likely to result from electro- osmotic activity and hydraulic conductivity in the apical References region [9,32,33,52–55,58]. Volume oscillations would spon- 1 Morris, C.E. (2002) How did cells get their size? Anat. Rec. 268, 239– 251 taneously emerge from activation of cell volume recovery 2 Wehner, F. et al. (2003) Cell volume regulation: osmolytes, osmolyte mechanisms (Figure 1) that cycle around an attractor for transport, and signal transduction. Rev. Physiol. Biochem. Pharmacol. optimal intracellular osmolality. Volume increases result 148, 1–80 in gradually increasing hydrostatic pressure surges that 3 Hoffmann, E.K. et al., eds (2006) Cell volume control in health and disease, 12th International Symposium, Acta Physiol. 187, 1–352 promote exocytosis, activate mechanosensitive channels 4 Roy, S. et al. (1998) Effects of Yariv phenylglycoside on cell wall and trigger cell wall stress relaxation and the start of assembly in the lily pollen tube. Planta 204, 450–458 the growth cycle. Apical swelling simultaneously activates 5 Proseus, T.E. et al. (2000) Turgor, temperature and the growth of plant mechanisms involved in regulatory volume decrease cells: using Chara corallina as a model system. J. Exp. Bot. 51, 1481– (Figure 1b, hypotonic swelling). Vectorial mass flow and 1494 6 Proseus, T.E. and Boyer, J.S. (2006) Identifying cytoplasmic input to hydrodynamic efflux at the apex drive polarized cell the cell wall of growing Chara corallina. J. Exp. Bot. 57, 3231–3242 elongation while also promoting compensatory hydrodyn- 7 Wei, C. and Lintilhac, P.M. (2003) Loss of stability – a new model for amic influx along distal regions. These dynamics would stress relaxation in plant cell walls. J. Theor. Biol. 224, 305–312 www.sciencedirect.com 96 Opinion TRENDS in Plant Science Vol.12 No.3

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Plant Science Conferences in 2007

CSHL Meeting: Plant Genomes 15–18 March 2007 Cold Spring Harbor Laboratory, New York, USA http://meetings.cshl.edu/meetings/plants07.shtml

Model Legumes Congress (MLC2007) 24–28 March 2007 Tunis (Tunisia) http://www.ecopark.rnrt.tn/mlc2007/

SEB Main Meeting 2007 31 March – 4 April 2007 Glasgow, UK http://www.sebiology.org/Meetings/pageview.asp?S=2&mid=91

Royal Society Discussion Meeting - Revealing how Nature uses Sunlight to Split Water 23–24 April 2007 London, UK http://www.royalsoc.ac.uk/

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