Morphogenesis: Forcing the Tissue
2
Dispatches Morphogenesis: Forcing the Tissue How are the transcriptional events that control form actually transduced into the shape of an organism? Analysis of plant tissue mechanical properties shows that control of the extracellular matrix is key. Andrew Fleming Organisms are recognizable by their form. Developmental biology has been extraordinarily successful in identifying the transcriptional networks and signalling cascades controlling many aspects of morphogenesis, yet the question remains as to how the molecular events within the cells of an organism are actually transduced into a change of form at the supracellular level. In this issue of Current Biology, Peaucelle et al. [1] provide evidence that regulation of the mechanical properties of the extracellular matrix in plants is the means by which form is controlled. The growth of plant tissue (as in animals) is a physical process. However, plant cells, in contrast to those in animals, do not move relative to one another and, instead, are glued to each other via a highly complex and dynamic extracellular matrix — the plant cell wall. Individual cells within this matrix generate a hydrostatic turgor pressure which provides the force for growth. Turgor is balanced by forces within the enveloping cell wall so that when the forces are in equilibrium, no growth occurs. Growth can occur by either increasing the internal pressure or by controlled loosening of the cell wall, with the consensus opinion being that in most situations it is alteration in the mechanical properties of the cell wall that underpin morphogenic events in plants, such as organ initiation [2]. Organ formation occurs repeatedly at the shoot apical meristem, a group of self-renewing cells functionally equivalent to the stem cells in animal systems, which are located at the tip of the plant. At regular intervals, some of the daughter cells in the flank of the meristem become set aside (determined) to become leaves. The determination of organ primordia is marked both by changes in the expression of particular transcription factors and by an altered pattern of growth factors [3]. How, though, do these molecular changes lead to the initial outgrowth — the earliest morphogenic event of organ formation? A number of lines of evidence indicate the importance of cell wall properties in organ initiation [4–6], but a key missing piece in the puzzle has been the demonstration that changes in cell wall mechanical properties do actually occur during this process. Now, Peaucelle et al. [1], as well as the recent work of Milani et al. [7], provide evidence to support this hypothesis. The challenge in this area of research has been that the meristem is rather small (a dome of diameter <100 mm), and is generally hidden away from view by the leaves that have previously been generated. In addition, the curved surface of the meristem raises analytical problems not present in the study of flat materials (which is where most of the techniques for the analysis of mechanics at this scale have been developed). Finally, the cell wall is a highly complex and dynamic compound material whose mechanical properties may depend upon the fact that, normally, it is under stress by the hydrostatic forces of the cells that surround it. Clearly, this is not a trivial problem to address! The application of techniques such as atomic force microscopy to this problem, used by both recent investigations [1,7], does require a number of assumptions during data interpretation (the number of unknowns is rather high), but nevertheless, even allowing for these caveats, the recent results provide a novel insight into how the extracellular matrix can mechanically restrict or permit morphogenesis. Peaucelle et al. [1] measured the relative stiffness of the meristem surface, both in areas where organ formation was occurring and where organ formation is known never to occur. Moreover, by adjusting the geometry of the tip used to probe the tissue, they could gain information from both the surface and the underlying tissue. Their results indicate that the regions on the flanks of the meristem where organ formation occurs are less stiff than the central region (where organ formation never occurs). Using a transgenic approach, they altered the methylation status of a particular component of the cell wall (the pectins), which they had previously shown modulated the ability of the meristem to form organs [5]. When pectin methylation was decreased, the meristem became more pliant in a wider area, leading to organs being formed in a broader spatial region. Conversely, when pectin methylation was maintained at a relatively high level, the meristem became stiffer in a wider area and organ formation was inhibited. Thus, a picture emerges from these recent studies in which the meristem is mechanically defined by a central region that is relatively stiff in which it is more difficult for the tissue to bulge outwards, surrounded by a periphery in which the tissue is more pliant and susceptible to bulging (Figure 1). These regions are also marked at the molecular level by various transcriptional networks, suggesting that the one controls the other [8]. Identifying the precise molecular pathways and which modulators of the cell wall are the downstream targets of the transcriptional regulators is a clear target for future research. Within the pliant region of the meristem periphery, particular regions are selected to be the site of actual bulging (organ initiation). There is compelling evidence that auxin transport is key to site selection [9], with cell wall proteins such as pectin modifying enzymes and expansin involved in the downstream events that release the potential of the peripheral region to bulge [1,4,5]. Again, defining the precise molecular pathway and targets involved will be important for future research. One surprising observation made in the paper is that the mechanical properties of the underlying Current Biology Vol 21 No 20 R840
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Transcript of Morphogenesis: Forcing the Tissue
Current Biology Vol 21 No 20R840
Dispatches
Morphogenesis: Forcing the Tissue
How are the transcriptional events that control form actually transduced intothe shape of an organism? Analysis of plant tissue mechanical propertiesshows that control of the extracellular matrix is key.
Andrew Fleming
Organisms are recognizable by theirform. Developmental biology has beenextraordinarily successful in identifyingthe transcriptional networks andsignalling cascades controlling manyaspects of morphogenesis, yet thequestion remains as to how themolecular events within the cells of anorganism are actually transduced intoa change of form at the supracellularlevel. In this issue of Current Biology,Peaucelle et al. [1] provide evidencethat regulation of the mechanicalproperties of the extracellular matrix inplants is the means by which form iscontrolled.
The growth of plant tissue (as inanimals) is a physical process.However, plant cells, in contrast tothose in animals, do not move relativeto one another and, instead, are gluedto each other via a highly complex anddynamic extracellular matrix — theplant cell wall. Individual cells withinthis matrix generate a hydrostaticturgor pressure which provides theforce for growth. Turgor is balanced byforces within the enveloping cell wall sothat when the forces are in equilibrium,no growth occurs. Growth can occur byeither increasing the internal pressureor by controlled loosening of the cellwall, with the consensus opinion beingthat in most situations it is alteration inthe mechanical properties of the cellwall that underpin morphogenic eventsin plants, such as organ initiation [2].
Organ formation occurs repeatedlyat the shoot apical meristem, a group ofself-renewing cells functionallyequivalent to the stem cells in animalsystems, which are located at the tip ofthe plant. At regular intervals, some ofthe daughter cells in the flank of themeristem become set aside(determined) to become leaves. Thedetermination of organ primordia ismarked both by changes in theexpression of particular transcriptionfactors and by an altered pattern of
growth factors [3]. How, though, dothese molecular changes lead to theinitial outgrowth — the earliestmorphogenic event of organformation? A number of lines ofevidence indicate the importance ofcell wall properties in organ initiation[4–6], but a key missing piece in thepuzzle has been the demonstration thatchanges in cell wall mechanicalproperties do actually occur during thisprocess. Now, Peaucelle et al. [1], aswell as the recent work of Milani et al.[7], provide evidence to support thishypothesis.
The challenge in this area of researchhas been that the meristem is rathersmall (a dome of diameter <100 mm),and is generally hidden away from viewby the leaves that have previously beengenerated. In addition, the curvedsurface of the meristem raisesanalytical problems not present in thestudy of flat materials (which is wheremost of the techniques for the analysisof mechanics at this scale have beendeveloped). Finally, the cell wall isa highly complex and dynamiccompound material whose mechanicalproperties may depend upon the factthat, normally, it is under stress bythe hydrostatic forces of the cellsthat surround it. Clearly, this is nota trivial problem to address! Theapplication of techniques such asatomic force microscopy to thisproblem, used by both recentinvestigations [1,7], does requirea number of assumptions during datainterpretation (the number ofunknowns is rather high), butnevertheless, even allowing for thesecaveats, the recent results providea novel insight into how theextracellular matrix can mechanicallyrestrict or permit morphogenesis.
Peaucelle et al. [1] measured therelative stiffness of the meristemsurface, both in areas where organformation was occurring and whereorgan formation is known never tooccur. Moreover, by adjusting the
geometry of the tip used to probe thetissue, they could gain information fromboth the surface and the underlyingtissue. Their results indicate that theregions on the flanks of the meristemwhere organ formation occurs are lessstiff than the central region (whereorgan formation never occurs). Usinga transgenic approach, they altered themethylation status of a particularcomponent of the cell wall (the pectins),which they had previously shownmodulated the ability of themeristem toform organs [5]. When pectinmethylation was decreased, themeristem became more pliant ina wider area, leading to organs beingformed in a broader spatial region.Conversely, when pectin methylationwas maintained at a relatively highlevel, the meristem became stiffer ina wider area and organ formation wasinhibited.Thus, a picture emerges from these
recent studies in which the meristem ismechanically defined by a centralregion that is relatively stiff in which it ismore difficult for the tissue to bulgeoutwards, surrounded by a periphery inwhich the tissue is more pliant andsusceptible to bulging (Figure 1). Theseregions are also marked at themolecular level by varioustranscriptional networks, suggestingthat the one controls the other [8].Identifying the precise molecularpathways and which modulators of thecell wall are the downstream targets ofthe transcriptional regulators is a cleartarget for future research. Within thepliant region of themeristem periphery,particular regions are selected to be thesite of actual bulging (organ initiation).There is compelling evidence that auxintransport is key to site selection [9],with cell wall proteins such as pectinmodifying enzymes and expansininvolved in the downstream events thatrelease the potential of the peripheralregion to bulge [1,4,5]. Again, definingthe precise molecular pathway andtargets involved will be important forfuture research.One surprising observation made in
the paper is that the mechanicalproperties of the underlying
A B
D
F
H
C
E
G
Current Biology
Figure 1. Schematic diagram of the role oftissue mechanics in organ initiation
(A) The meristem is partitioned into a centralzone characterised by a relatively stiff extra-cellular matrix (red), surrounded by a periph-eral zone of relatively pliant material (beige).Regions within the peripheral zone aredemarcated for organ initiation (green) byan auxin-based patterning system. As a con-sequence of cell wall loosening in this region,morphogenesis occurs (B). If the centralregion of tissue stiffness extends into theflanks of the meristem (C), then, despite thepresence of the endogenous signals for leafinitiation, morphogenesis does not occursince the downstream cell wall effectorscannot overcome the preset local tissuemechanics (D). Similarly, if auxin signaling isectopically induced in the central zone (E),the local tissue stiffness blocks morphogen-esis (F). In contrast, ectopic signaling in theperipheral zone (G) leads to cell wall loos-ening and ectopic leaf initiation (H).
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extracellular matrix, rather than theepidermis, are important for the systemto function, whereas other work in thisarea has suggested the opposite[10,11]. The mechanical interactions ofcell layers are liable to be complex andtrying to define linear cause and effectmay be too simplistic an approach,with the meristem being set up asa truly integrated system. The furtherapplication of tools such as atomicforce microscopy will hopefully providemore data to provide a deeper insightinto this issue. A second surprise is thatthe stiffness response of the tissue toaltered pectin methylation status wasthe opposite of that expected fromextant models; decreased pectinmethylation is expected to make the
extracellular matrix stiffer, not morepliant. Although we have extensivedata on the composition of the plantcell wall, our understanding of howthese components fit together and howthey influence the mechanicalproperties of thematrix is largely basedon models that still need to bestringently tested [12].
Finally, much of developmentalbiology has viewed the process ofmorphogenesis as a one-way process(gene transcription leading to form), butthere are a number of lines of evidenceindicating that feedback loops mustoccur so that the transcriptionalapparatus is itself sensitive to andmodulated by the physical stressesand strains that underpinmorphogenesis [13,14]. These ideasare most advanced in animaldevelopment and differentiation[15,16], but the field is now opening upfor plant biologists working at theinterface of developmental mechanicsto explore this area and to close theloop of genetic regulation andmorphogenesis.
References1. Peaucelle, A., Braybrook, S.A., and Hofte, H.
(2011). Pectin-induced changes in cell wallmechanics underlie organ initiation inArabidopsis. Curr. Biol. 21, 1720–1726.
2. Cosgrove, D.J. (2005). Growth of the plant cellwall. Nat. Rev. Mol. Cell Biol. 6, 850–861.
3. Braybrook, S.A., and Kuhlemeier, C. (2010).How a plant builds leaves. Plant Cell 22,1006–1018.
4. Pien, S., Wyrzykowska, J., McQueen-Mason, S., Smart, C., and Fleming, A. (2001).Local expression of expansin induces the entireprocess of leaf development and modifies leafshape. Proc. Natl. Acad. Sci. USA 98,11812–11817.
5. Peaucelle, A., Louvet, R., Johansen, J.N.,Hofte, H., Laufs, P., Pelloux, J., and Mouille, G.(2008). Arabidopsis phyllotaxis is controlled bythe methyl-esterification status of cell-wallpectins. Curr. Biol. 18, 1943–1948.
6. Hamant, O., Heisler, M.G., Jonsson, H.,Krupinski, P., Uyttewaal, M., Bokov, P.,Corson, F., Sahlin, P., Boudaoud, A.,Meyerowitz, E.M., et al. (2008). Developmentalpatterning by mechanical signals inArabidopsis. Science 322, 1650–1655.
7. Milani, P., Gholamirad, M., Traas, J.,Arneodo, A., Boudaoud, A., Argoul, F., andHamant, O. (2011). In vivo analysis of local wallstiffness at the shoot apical meristem inArabidopsis using atomic force microscopy.Plant J. 67, 1116–1163.
8. Busch, W., Miotk, A., Ariel, F.D., Zhao, Z.,Forner, J., Daum, G., Suzaki, T., Schuster, C.,Schultheiss, S.J., Leibfried, A., et al. (2010).Transcriptional control of a plant stem cellniche. Dev. Cell 18, 849–861.
9. Reinhardt, D., Pesce, E.-R., Stieger, P.,Mandel, T., Baltensperger, K., Bennett, M.,Traas, J., Friml, J., and Kuhlemeier, C. (2003).Regulation of phyllotaxis by polar auxintransport. Nature 426, 255–260.
10. Savaldi-Goldstein, S., Peto, C., and Chory, J.(2007). The epidermis both drives andrestricts plant shoot growth. Nature 446,199–202.
11. Reinhardt, B., Hanggi, E., Muller, S., Bauch, M.,Wyrzykowska, J., Kerstetter, R., Poethig, S.,and Fleming, A.J. (2007). Restoration of DWF4expression to the leaf margin of a dwf4 mutantis sufficient to restore leaf shape but not size:the role of the margin in leaf development. PlantJ. 52, 1094–1104.
12. Geitmann, A. (2010). Mechanical modeling andstructural analysis of the primary plant cell wall.Curr. Opin. Plant Biol. 13, 693–699.
13. Green, P.B. (1994). Connecting gene andhormone action to form, pattern andorganogenesis: biophysical transductions. J.Exp. Bot. 45, 1775–1788.
14. Eyckmans, J., Boudou, T., Yu, X., andChen, C.S. (2011). A hitchhiker’s guide tomechanobiology. Dev. Cell 21, 35–47.
15. Gilbert, P.M., Havenstrite, K.L.,Magnusson, K.E., Sacco, A., Leonardi, N.A.,Kraft, P., Nguyen, N.K., Thrun, S., Lutolf, M.P.,and Blau, H.M. (2010). Substrate elasticityregulates skeletal muscle stem cell self-renewalin culture. Science 329, 1078–1081.
16. Engler, A.J., Sen, S., Sweeney, H.L., andDischer, D.E. (2006). Matrix elasticity directsstem cell lineage specification. Cell 126,677–689.
Department of Animal and Plant Sciences,University of Sheffield, Sheffield S10 2TN,UK.E-mail: [email protected]
DOI: 10.1016/j.cub.2011.08.052
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