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Journal of Plant Physiology 164 (2007) 1395—1409

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REVIEW

The epidermal-growth-control theory of stemelongation: An old and a new perspective

U. Kutscheraa,�, K.J. Niklasb

aInstitute of Biology, University of Kassel, Heinrich-Plett-Str. 40, D-34109 Kassel, GermanybDepartment of Plant Biology, Cornell University, Ithaca, NY 14853, USA

Received 1 July 2007; accepted 15 August 2007

KEYWORDSAuxin action;Cell elongation;Cuticle;Epidermis andgrowth

ee front matter & 2007jplph.2007.08.002

s: CM, cuticular membring author. Tel.: +49 561ess: [email protected]

SummaryThe botanist G. Kraus postulated in 1867 that the peripheral cell layers determinethe rate of organ elongation based on the observation that the separated outer andinner tissues of growing stems spontaneously change their lengths upon isolationfrom each other. Here, we summarize the modern version of this classical concept,the ‘‘epidermal-growth-control’’ or ‘‘tensile skin’’ theory of stem elongation. First,we present newly acquired data from sunflower hypocotyls, which demonstrate thatthe expansion of the isolated inner tissues is not an experimental artefact, asrecently claimed, but rather the result of metabolism-independent cell elongationcaused by the removal of the growth-controlling peripheral walls. Second, wepresent data showing that auxin-induced elongation of excised stem segments isattributable to the loosening of the thick epidermal walls, which provides additionalevidence for the ‘‘epidermal-growth-control concept’’. Third, we show that thecuticle of aerial organs can be thin and mechanically weak in seedlings raised at highhumidity, but thick and mechanically important for organs growing under relativelydry air conditions. Finally, we present a modified model of the ‘‘tensile skin-theory’’that draws attention to the mechanical and physiological roles of (a) the thickened,helicoidal outer cell walls, (b) the mechanical constraint of a cuticle, and(c) the interactions among outer and inner cell layers as growth is coordinated byhormonal signals.& 2007 Elsevier GmbH. All rights reserved.

Elsevier GmbH. All rights reserved.

ane, IT, inner tissues, OEW, outer epidermal wall, OT, outer tissues804 4467; fax: +49 561 804 4009.(U. Kutschera).

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U. Kutschera, K.J. Niklas1396

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1396Verification of classical experiments with modern techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1397The sunflower hypocotyl as a model system: is the expansion of the isolated inner tissues anexperimental artefact? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398The organismic perspective and architecture of the outer cell walls . . . . . . . . . . . . . . . . . . . . . . . . 1401The plant cuticle: structure and function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1402Auxin action and gravitropic upward bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1404Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1406Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1407References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1407

Introduction

There are two highly polarized ways of concep-tualizing the biology of multicellular organisms.The first subscribes to the classical cell theory ofthe 19th century, which views the multicellularorganism as a composite of independently operat-ing cells such that understanding the functions ofdifferent cell types is sufficient to adduce thephysiology of the entire organism (Morton, 1981;Baluska et al., 2004). The second perspective viewsbiological phenomena such as growth and develop-ment as the emergent properties of the wholeliving, multicellular system. To our knowledge, theeminent botanist W. Hofmeister (1867) was the firstto introduce this alternative view to the classicalcell theory in his famous textbook The Plant Cell,wherein he concluded that meristems (rather thanindividual cells) regulate and determine growth anddevelopment. This basic idea, which is now calledthe ‘‘organismal concept of multicellularity’’, statesthat a comprehensive understanding of growth,development, and physiological processes cannotbe deduced from the behaviour of individual cells,but rather requires knowledge of the functional (andcoordinated) properties of tissues, tissue systems,and organs (Kaplan, 1992; Lucas et al., 1993).

Although the organismal concept was first for-malized by Hofmeister (1859) in his The Plant Cell,it was predicated in large part on his earlierexperimental analyses of the tensile stresses thatdevelop in the outer cell layers of growing organs asa result of the expansion/elongation of internaltissues. J. Sachs (1865) described Hofmeister’s‘‘tissue tension concept’’ (Gewebespannung) twoyears earlier in his Experimental Physiology ofPlants. This concept was confirmed by Kraus (1867,1869), who argued that the inner tissues (IT) of thestem (especially the pith) provide the driving forcefor elongation, whereas the outer cell layersdetermine the rate of organ growth by imposing amechanical constraint.

Although the organismic and tissue tensionconcepts quickly became part of the standardintellectual repertoire of 19th and early 20thcentury plant biology (e.g., Sachs, 1882; Darwinand Acton, 1894; Pfeffer, 1904; Jost, 1908), bothperspectives subsequently lost their popularity inpart because of the discovery of the growthhormone auxin and the introduction of the oatcoleoptile as a standard model system. Indeed, it isfair to say that the basic tenets of the cell theoryonce again became the central ‘‘dogma’’ of a newresearch agenda. For example, the title of aninfluential review article on organ growth by Heyn(1940) was The Physiology of Cell Elongation(underline added), whereas, three decades later,Cleland (1971) published an important, compre-hensive summary of the same topic that wasentirely focused on the behaviour and propertiesof individual cells. This shift in perspective con-tinued unchallenged until a series of articlespresented direct measurements of the tensileforces occurring in epidermal peels as well asturgor determinations for coleoptiles, internodes,and hypocotyls (Kutschera, 1987, 1989, 1991, 1992,1994, 1995; Kutschera et al., 1987) that wereintegrated into a new biophysical model (laterdubbed by Peters and Tomos (1996a, b) the‘‘Kutschera epidermal-growth-control hypoth-esis’’). Although this model was subsequentlycorroborated by a number of workers (e.g., Niklasand Paolillo, 1997, 1998), the role of the epidermisin regulating organ elongation has remained con-troversial and, with some notable exceptions (e.g.,Schopfer, 2006; Savaldi-Goldstein et al., 2007), hasbeen largely ignored in the current literature (e.g.,Cosgrove, 1999, 2005; Zonia and Munnik, 2007).

The aim of this article is to review in some detailthe history of Hofmeister’s tissue tension hypoth-esis and to summarize the experimental evidenceand theoretical underpinnings of the Kutschera‘‘epidermal-growth-control concept’’. Specifically,we re-evaluate Hofmeister’s seminal experiments

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The epidermal-growth-control theory of stem elongation 1397

and present recently acquired, unpublished obser-vations on sunflower stems that corroborate andexpand upon his conclusions. Additionally, weexplore new biomechanical data for the mechan-ical role of the cuticle, which lead us to suggest abiophysically more comprehensive model for plantorgan elongation under natural conditions.

Verification of classical experiments withmodern techniques

Although the phenomenon of ‘‘tissue tension’’was described qualitatively by 18th and 19thcentury botanists (for historical reviews, seeKutschera, 1989; Peters and Tomos, 1996b; Niklas,

Figure 1. Photograph of sunflower (Helianthus annuus) planout with stem segments excised from the upper region of theinto water (B). The pith was separated by a corkborer frowater (C). (B and C adapted from Pfeffer, 1904 and from Jos

1992), as noted, it was Hofmeister’s (1859, 1860)now classical experiments that first shed quantita-tive light on the physiological and mechanicalphenomena operating within the outer cell layerof growing organs (stems and petioles) of higherplants such as sunflower (Figure 1A).

Two of the most important of these experimentsare illustrated in Figure 1B and C: (1) theimmediate outward re-curvature of median slicesthrough internodes when dissected, which is evenmore evident when internodes are placed in water(Figure 1B) and (2) the rapid extension of the pith(exposed by removing peripheral tissues with acorkborer) when internodes are submerged inwater (Figure 1C). By virtue of the first experiment,Hofmeister concluded that the peripheral celllayers contract as internal tissues expand when

ts in the field (A) and two classical experiments carriedaxis. A slice of the stem was split lengthwise and placedm the outer tissues; it is extending in the presence oft, 1908).

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hydrated. Although he used the internode of Coleusblumei as his model system, Sachs (1865, 1882)pointed out that the same results could be achievedusing different species, e.g., sunflower stems. Inthe second experiment, Hofmeister quantitativelyestimated the physical stresses exerted by theturgid pith against the epidermis (and subepider-mal cell layers). After adding water, he noted thatthe pith rapidly elongated by up to 20% ofinternodal length, thus concluding that the hy-drated pith is held in compression by the epider-mal–subepidermal cell layers, which in turn areheld in tension.

Muller (1880) subsequently constructed an appara-tus that could quantitatively estimate (albeit cru-dely) the longitudinal stresses exerted by thehydrated pith of the Helianthus stem against theouter cell layers (Figure 1C). He showed that acolumn of excised pith slices, split longitudinally andconfined within a glass cylinder, exerted a pressure of13.5atm (1.35MPa) against a weight applied to thetop of the tissue mass. Pfeffer (1904) popularized thisobservation in his influential textbook. Heinich(1909), one of Pfeffer’s students, later demonstratedthat the ‘‘release of tissue tension’’ in Helianthus(Figure 1C) and other plant species differs from thegrowth process of the intact stem with respect to itssensitivity to low temperature and the supply ofatmospheric oxygen. Specifically, he showed that thepith maintained its capacity to elongate even whenplaced in ice water (0–3 1C) and anaerobic conditionsthat caused stem growth to cease entirely. Likewise,Peters and Tomos (2000) showed that tissue elonga-tion is insensitive to cyanide (KCN), an inhibitor ofoxidative phosphorylation and hence cellular ATPproduction, for at least 2h after its application.

Thus, Heinich’s observation that the pith elongateswhen exposed to water that is entirely independentof the metabolism of the cells was confirmed.Curiously, however, Peters and Tomos (2000) did notattribute the extension of the internal tissues to arelease of compressive forces exerted by theperipheral cell layers. Rather, they argued that theextension of internal tissues was ‘‘an effect of wateruptake’’ and that the concept of tissue tension isincompatible with the theory of growth regulators asa source of control of organ extension (Peters andTomos, 2000). These assertions will be discussed ingreater detail later (see next section). However, herewe note that (1) their inferences ignore Hofmeister’soriginal observation that the pith extends in lengthimmediately after a turgid internode is excised,albeit at a slower rate when compared to whenexposed to water and (2) the tissue tension theorywas never purported to explain the direct origin ofthe regulation of extension growth.

In a related manner, based on detailed histolo-gical and developmental observations on the inter-nodes of sweetgum (Liquidamber styraciflua),Brown et al. (1995a, b) concluded that cell divisionsand expansion/extension in the developing pithprovide the ‘‘driving force’’ for internodal elonga-tion and that it is an oversimplification that ‘‘thepassive extension of peripheral tissues, especiallythe epidermis, controls the rate of growth in axialorgans’’ (Brown et al., 1995a, p. 776). Theseauthors also concluded that the rapid productionand differentiation of vascular tissues in elongatinginternodes ‘‘contribute significantly to the devel-opment of tensile forces ascribed only to outerperipheral tissues in herbaceous plants’’ (Brownet al., 1995b, p. 781). The first of these conclusionswas based in large part on two observations: (1) asinternodes attained their maximum rates of elon-gation, the highest rates of cell division occur in thepith and cortex (and continue until the cessation ofinternode growth) and (2) concomitant reducedrates of cell division in peripheral internode tissuesare associated with a significant increase in cellelongation rates in the epidermis and subepidermis(cortical collenchyma) (Brown et al., 1995a). Bothof these observations are consistent with thetheoretical expectation that tissues located nearthe longitudinal axis of internodes experiencecompressive stresses as peripheral tissues areplaced in longitudinal (and circumferential) ten-sion, and neither observation provides a basis forinferring a priori which of these two featuresconstrains or ‘‘drives’’ internode growth in length.Nevertheless, Brown et al. (1995a, b) are entirelyjustified in arguing for a more synoptic mechanicalview of axial organ growth, one that involves acomplex stress–strain dynamic among differentadjoining tissues in which all tissues and cell typesparticipate.

More recently, Passioura and Boyer (2003) used theconcept of tissue tension from the epidermis todevelop a time- and position-dependent model ofstem growth incorporating both water uptake andturgor-driven cell expansion. The model correctlypredicted the measured organ dynamics, based on thephysics of tissue tension as described in the Nitellainternode through corresponding experiments.

The sunflower hypocotyl as a modelsystem: is the expansion of the isolatedinner tissues an experimental artefact?

In the classical plant physiology literature (Sachs,1865, 1882; Darwin and Acton, 1894; Pfeffer, 1904;

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Figure 3. Peeling experiments with excised hypocotylsegments of 4-day-old etiolated sunflower seedlings (seeFigure 2A) that demonstrate the existence of tissuestresses. Intact segment, 10mm in length; isolated stripof outer tissues (OT); peeled stem segment (inner tissues,IT), cut to a length of 10mm and placed for 1 h in water.The isolated IT rapidly elongates (+15% within 2 h),whereas the separated peripheral cell layers (OT, cellskilled during the process of peeling) spontaneouslycontract (�9%) due to the relaxation of the wall stress(arrows). Scale bar ¼ 1mm.


Jost, 1908), seedlings of sunflower (Helianthusannuus) were frequently used as a model organism.However, as a consequence of the discovery of thegrowth hormone auxin (indole-3-acetic acid, IAA),which was based on experiments with grass (Avena)coleoptiles, the Helianthus hypocotyl (Figure 2)quickly became one of the ‘‘dicot model systems’’of choice for the analysis of the mechanism oforgan elongation (see Heyn, 1940). Here we discusshow this model system shows that the expansion ofisolated internal tissues is not an experimentalartefact (as claimed by Peters and Tomos, 2000)and that the inner tissues are held in compressionby a peripheral ‘‘tensile skin’’ (Figure 3).

The Helianthus hypocotyl is composed of fourbasic tissues: the epidermis, which consists of asingle layer of densely packed cells covered by asuperficial layer of cutin, the waxy cuticle (seeFigure 8); the cortex, which consists of �10–15layers of structurally unspecialized cells (parench-yma); six vascular bundles, which consist of phloemand xylem tissues (each containing fibres); and thepith, which is composed of relatively large par-enchymous cells. Peeled epidermal strips thereforeconsist of 2–3 cell layers (the epidermis and 1–2layers of the cortex; Figure 4), denoted here as theouter tissues (OT). The IT consist of the remainingcortex, vascular tissues, and pith (Kutschera,1990). In addition, the diameter of the OT cells isonly 1/2 to 1/4 that of the cells that constitute thecortex and pith (Figures 4 and 7; Kutschera, 1990).

Figure 4. Light micrograph of a transverse sectionthrough the middle of a hypocotyl segment (Figure 3)from which part of the outer tissues were peeled off(upper quarter of the section). The micrograph wasprepared as described by Kutschera (1990). Co ¼ cortex,Ep ¼ epidermis, IT ¼ inner tissues, OT ¼ outer tissues,Va ¼ vascular tissue. Scale bar ¼ 50 mm.

Figure 2. Photograph of 4-day-old etiolated sunflowerseedlings and hypocotyl segments that were split long-itudinally and placed for 1 h into water. The seedlingswere grown at 100% relative humidity in continuousdarkness and either maintained vertically (A) or placedfor 3 h in horizontal position to induce gravitropic upwardbending (B). Note that the split halves curved outward,with the exception of the lower half of the curved stem.g ¼ vector of gravity. Scale bar ¼ 5mm.

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Figure 2 shows etiolated sunflower seedlings thatwere grown for 4 days in darkness at 100% relativehumidity (25 1C). Under these conditions the stemreaches a growth rate of 1.6mm/h. Using split orpeeled segments, the phenomenon of ‘‘tissuetension’’ can be illustrated (Figures 2 and 3) andquantified by means of a miniaturized pressureprobe. This technique shows that the average cellturgor in the OT is not significantly different fromthat of the IT (Kutschera and Kohler, 1992).However, the osmotic pressure of the tissue sap inthe epidermal layer is higher than that in the IT.Thus, the OT water potential (C) is more negativethan that of the IT (Table 1), indicating that theHelianthus hypocotyl can be regarded as a hydro-statically uniform cylinder with a radial waterpotential difference (DC) between the OT andIT that drives the movement of water from theinternal tissues into the epidermis.

It should be noted that Nonami and Boyer (1993)observed uniform turgor across the soybean hypo-cotyl, just as Kutschera and Kohler (1992) did insunflower stems. A radial water potential gradientdeveloped not from turgor but rather from theosmotic pressure of the cells (Boyer, 1988; Boyerand Silk, 2004).

The mechanical effects of this radial waterpotential gradient are consistent with the classicalexperiments of Sachs (1865), Kraus (1867), Muller(1880), Heinich (1909) and others who showed thatpeeled IT cylinders of stems rapidly elongate,particularly when placed in water. This apparent‘‘release of tissue tension’’ led these workers toconclude that the outer cell layers constrain theelongation of the inner cells. Also, because thepeeled OT, which consist of non-turgid (killed)cells, elastically shrink upon isolation by up to 10%(Figure 3), an ancillary insight was that the IT aremaintained in a state of compression. Using avariety of axial organs (petioles, peduncles, grass

Table 1. Water relations of the outer tissues (OT) andinner tissues (IT) of 4-day-old dark-grown sunflowerseedlings, measured in the sub-apical 1 cm region ofwhole plants

OT IT

Turgor pressure (MPa) 0.4970.01 0.4870.01Osmotic pressure (MPa) 0.6370.02 0.5670.01Water potential (MPa) �0.14 �0.08

Turgor and osmotic pressure were determined in the epidermis/sub-epidermis or in the cortex (see Fig. 4) as described byKutschera and Kohler (1994). The water potential was calculatedby difference (turgor minus osmotic pressure). Data representmeans (7s.e.m.) of nine independent measurements each(unpublished results).

coleoptiles, etc.), more recent workers cameto the same conclusion (Firn and Digby, 1977;Kutschera, 1987, 1989, 1991, 1992, 1994, 1995,2000; Kutschera et al., 1987; Kutschera and Briggs,1987, 1988a, b; Niklas, 1992; Niklas and Paolillo,1997, 1998; Schopfer, 2006).

Figure 5. Spontaneous tissue elongation in sunflowerstem segments upon removal of the expansion-limitingepidermal cell layer. Subapical hypocotyl segments(initial lengths: 15mm) from 4-day-old dark-grown (A)or irradiated seedlings (B) were bisected longitudinally.Non-peeled (intact) or peeled semicylinder sections wereplaced between the clamps of an extensiometer (dis-tance: 10mm) as described by Kutschera (2000). Afterthe addition of water (arrows) the time course of tissueelongation was recorded. The average velocities (V) ofelongation of intact (J) and peeled (K) semicylindersover the first 8min are shown in the insets (means7s.e.m. of 9 independent measurements each).

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As noted, these conclusions were rejected byPeters and Tomos (2000) who examined the expan-sion of IT-slices cut from sunflower hypocotyls andconcluded that ‘‘isolation-induced inner tissueexpansion is not an elastic response to a releasefrom compressive stress, but rather is an effect ofwater uptake’’. In an accompanying theoreticalpaper, Peters et al. (2000) reinforced this view andimplicitly attributed the ‘‘release of tissue ten-sion’’ to an experimental artefact. However, recentnew experiments (outlined in Figures 5 and 6) usingsubapical longitudinally bisected hypocotyl sec-tions (initial length: 15mm) from dark-grown (orirradiated) sunflower seedlings (see Figures 2A and7B) show that these assertions are invalid.

In these experiments, the elongation of intact orpeeled semi-cylinders in water was determinedwith an extensiometer (as described by Kutschera,2000). Although intact semi-cylinders can take upwater via their cut surfaces, they display onlyminor, transient elongation. In contrast, peeledsemi-cylinders rapidly elongate in water (Figures5A, B) and manifest kinetics similar to those ofpeeled hypocotyl segments (Kutschera, 1991, 1992,

Figure 6. Relative rate of water loss of intact and peeledsemicylinder sections excised from the hypocotyls of 4-day-old dark-grown (A) or irradiated sunflower seedlings(B). The segments were fixed between the clamps of anextensiometer (distance: 10mm) and a solution ofpolyethylene glycol (PEG) with an osmotic pressure of1.0MPa was added (arrows). The time courses of tissueshrinking were recorded for non-peeled hypocotyl seg-ments (control, dashed lines), intact (non-peeled) andpeeled semi-cylinder segments. Average rates of tissueshrinking over the first 4min (V) are given (means7s.e.m. of 9 measurements each).

1995, 2000; Kutschera and Kohler, 1992, 1994;Hodick and Kutschera, 1992).

Clearly, the critical question raised by theseobservations is ‘‘To what extent is the rapidelongation response of the peeled semi-cylindersdue to an enhanced rate of water uptake of thetissue?’’ To answer this question, experiments wereconducted to estimate the relative rate of waterloss (which is a measure for the hydraulic con-ductivity) of intact vs. peeled semi-cylinders(Figure 6A, B). These data conclusively demon-strate that the kinetics of tissues shrinkage (i.e.,water efflux) of non-peeled (intact) and peeledsemi-cylinders after the addition of the osmoticumpolyethylene glycol (PEG, mol. wt. 8000, osmoticpressure ¼ 1.0MPa, see Kutschera, 1991) were notidentical over the first 4min. The rates of tissueshrinking (in mm/min) were enhanced by about 75%in response to peeling. Yet, over the same timeinterval, the rates of tissue elongation of non-peeled (intact) vs. peeled semi-cylinders wereenhanced by 400–600% (Figure 5A, B; insets).

The organismic perspective andarchitecture of the outer cell walls

Our results (Figures 5 and 6) collectively showthat rapid tissue elongation of peeled hypocotylsegments placed in water is neither attributable tothe leakage of solutes (Kutschera and Kohler, 1992)nor exclusively to an enhancement of waterconductivity. It is reasonable therefore to concludethat (contra Peters and Tomos, 2000) the ‘‘releaseof tissue tension’’ as depicted in Figures 1–3 is notan experimental artefact but rather the result ofphysical stresses (tension/compression) that devel-op between outer and inner cell layers in the intactorgan. In addition, these experimental data, intandem with anatomical inspection, reveal that thedogmatic ‘‘cell-centred’’ view is inappropriate forrepresentative axial organs like the hypocotyl andthat an ‘‘organismic’’ approach is required, one inwhich different tissues operate as a mechanicallyand physiologically integrated system (see Brownet al., 1995a, b, who reached similar conclusions).Indeed, this organismic perspective must be ex-tended into the sub-cellular domain of organstructure and anatomy, particularly with regard tothe cell wall architecture (Niklas, 1989, 1992).

For example, in a series of studies, it was shownthat the outer epidermal periclinal wall (OEW) ofstems and coleoptiles can be much thicker than thewalls of the internal tissues. The electron micro-graph depicted in Figure 7A shows that the OEW of

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Figure 7. Thickness of the cell walls in the sunflowerhypocotyl. Transmission electron micrograph of a trans-verse section through the peripheral six cell layers (A) inthe sub-apical region (arrow) of the hypocotyl of a 4-day-old sunflower seedling (B) that was grown for 3 days indarkness and thereafter irradiated for 1 day withcontinuous white light. The methods are described byKutschera (1990). Average wall thicknesses are indicated(arrows). Co ¼ cortex, Ep ¼ epidermis, OEW ¼ outerepidermal wall, SEp ¼ sub-epidermis. Scale bars ¼ 10 mm(A) and 5mm (B).

Figure 8. Section through a hypocotyl of a 4-day-oldsunflower seedling (see Figure 7B) showing the outerepidermal wall that is covered by a thin cuticle. Thetransmission electron micrograph was prepared as de-scribed by Kutschera (1990). Cu ¼ cuticle, Cy ¼ cyto-plasm, Mi ¼ mitochondrion, OEW ¼ outer epidermalwall, Va ¼ vacuole. Scale bar ¼ 2 mm.


light-grown sunflower hypocotyls is approximately2 mm thick, whereas a typical cell wall in the cortexis only ca. 0.2 mm thick. The thinnest walls arethose in the pith (�0.1 mm; see Kutschera, 1992).Moreover, Hodick and Kutschera (1992) have shownthat the OEW of the sunflower hypocotyl is amultilayered, helicoidal structure composed ofalternating layers of cellulose microfibrils that arelongitudinally and transversely oriented with re-spect to the axis of the stem (Figure 8; for similaranalyses, see Niklas and Paolillo, 1997; Kerstenset al., 2001; Suslov and Verbelen, 2006). In contrast,

the thin walls of the IT are unilayered with cellulosemicrofibrils that are oriented transversely withrespect to the direction of elongation growth(Kutschera, 2000). It should be noted that cellulose,which is composed of neutral D-glucan chains, formsmicrofibrils that represent the mechanical ‘‘skele-ton’’ within the primary cell wall (Taiz, 1984; Fry,2000, 2004; Somerville, 2006). These skeletalelements are embedded into a ‘‘matrix’’ that iscomposed of Ca2+-rich pectins, hemicelluloses, andglycoproteins (Fry, 2000, 2004). It is obvious thatthe cellulose architecture of the peripheral walls,with microfibrils oriented in transverse and long-itudinal direction with respect to the direction ofgrowth, determine the mechanical propertiesof the hydrostatically inflated axial organ (seeFigure 10A). This topic has been discussed in detailby Taiz (1984), Kutschera (2000), Kerstens et al.(2001), Schopfer (2006), Suslov and Verbelen (2006)and others.

Finally, it should be noted that the epidermallayer of different plant organs is characterized byspecialized structures such as stomata etc. thatgrow in coordination with the epidermal groundcells. We refer to Martin and Glover (2007)for further information on this aspect of plantdevelopment.

The plant cuticle: structure and function

The mechanical role of the cuticle, whichcovers the OEW (and, depending on the species,

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Figure 9. Scanning electron micrograph showing epicu-ticular waxes (ECW) on the surface of epidermal cells in asunflower seedling (A). Scale bar ¼ 10 mm. Schematicrepresentation of the cuticularized epidermis (B). Thecuticular membrane (CM), consisting of the cuticularproper (CP) and the cuticular layer (CL), develops on theouter epidermal (periclinal) wall (OEW) and can extendinto the epidermal anticlinal walls.


subepidermal cell walls) is also important (Figure 8).Pfeffer (1904, p. 73) was the first to draw atten-tion to the mechanical attributes of thecuticle. Under the heading ‘‘Strains in the cuticleand cell-wall’’, he affirmed ‘‘that the cuticle isunder strain’’ and referred to experiments carriedout with the ‘‘isolated outer wall of the epider-mis’’. Unfortunately, quantitative data on themechanical properties of the cuticle were unavail-able until a century later. However, recentlyacquired data indicate that the cuticle of plantsthat grow under natural conditions (relatively dryair) can be remarkably strong in tension. Hence, itcan mechanically contribute significantly to theability of the OEW to resist tensile deformations asorgans ‘‘inflate’’ hydrostatically, either as a resultof transient changes in organ turgor or as a result ofgrowth in mass (see Wiedemann and Neinhuis,1998; Matas et al., 2004a, b). Equally important,recent mechanical experiments indicate that thecuticle of fruits evinces pronounced viscoelasticbehaviour, i.e., its rate of tensile deformationdepends on both the duration and magnitude ofapplied stresses (Matas et al., 2004a, b).

The cuticle typically consists of an external layerof epicuticular waxes (ECW) (Figure 9A) overlying acomparatively thin layer of saponifiable lipids (thecuticle proper, CP), which covers an inner layer ofwaxes and fibrous polysaccharides embedded in acutin matrix (the cuticular layer, CL). The CP andCL comprise the cuticular membrane (CM) thatdevelops within and is part of the peripheralprimary cell wall (Esau, 1977) (Figure 9B). Thestructure of the CM and the extent to whichit extends beneath the epidermis vary amongspecies (Jeffree, 1996). For example, although itis frequently confined to the outer periclinal andanticlinal walls of the epidermis, the CM maydevelop in subepidermal cell walls. Significantstructural and chemical differences in the CM alsoexist across and within species (Kolattukudy, 1996;Wiedemann and Neinhuis, 1998; Matas et al.,2004a, b). These differences can significantly affectthe physiological and mechanical performance ofthe CM in terms of reducing uncontrolled waterloss, the entry of pathogenic organisms and variousorganic compounds, and the deleterious effects ofexcessive sunlight (see Riederer, 1990; Wiedemannand Neinhuis, 1998; Riederer and Schreiber, 2001).In seedlings that were raised in growth chambers at100% relative humidity (Figure 2), the cuticleremains thin (ca. 0.2 mm, see Figure 8) andmechanically weak. However, in plants raised atlower humidities (i.e., more negative water poten-tials of the air) this ‘‘waxy skin’’ can become asthick as the OEW itself or even considerably thicker.

Wiedemann and Neinhuis (1998) have shown that inleaves and fruits that have reached their final sizethe cuticle has a thickness of 3–13 mm and cantherefore mechanically stabilize the mature plantorgan.

Engineering theory and experimental evidenceshow that the outermost periclinal wall layers ofthe epidermis experience the maximum tensilestresses in any transverse section through a primaryorgan that is fully turgid (Pfeffer, 1904; Kutschera,1989; Niklas, 1992; Niklas and Paolillo, 1997, 1998).The CM therefore can function as a tensileperipheral ‘‘lipid skin’’, depending on its thicknessand material properties (see Figures 9 and 10B, C).Evidence for this proposition comes from themechanical analyses of surgically isolated epider-mal peels and cellulase/pectinase-isolated CM fromthe fruits of greenhouse-grown tomato plants(Lycopersicon esculentum) cultivars differing in

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their susceptibility to cracking when fully ripe(Petracek and Bukovac, 1995; Matas et al.,2004a, b). Samples were tested under uniaxialtension to determine their rate of creep, plasticand instantaneous elastic strains, breaking stress(strength), and work of fracture. The fruit peels ofdifferent cultivars exhibited pronounced viscoelas-tic and strain-hardening or strain-softening beha-viour, and differed significantly in rheologicalresponses and magnitudes of material properties.Peels from crack-resistant cultivars crept lessrapidly and accumulated more plastic strains (butless rapidly), were stiffer and stronger, and had alarger work of fracture than peels isolated fromcrack-prone cultivars. The CM also differed, e.g.,crack-prone cultivars strain-softened at forces thatcaused crack-resistant cultivars to strain-harden.The mechanical behaviour of peels and their CMcorrelated with anatomical differences, e.g., theCM of crack-resistant cultivars extends into sub-epidermal cell layers, whereas the CM of crack-prone cultivars is poorly developed below theepidermis.

Based on their analyses, Matas et al. (2004a, b)proposed a simple rheological model for the tomatofruit wall that attributes strain-hardening to the

Figure 10. Updated version the ‘‘epidermal-growth-controsegment through an axial organ (stem, coleoptile, petiole) (Aouter tissues (including epidermis ¼ Ep) with thickness th ¼the outer surface of the turgid segment differ in magnitude w(c–d) ¼ 2 s (a–b), indicating that the segment is prone tomaterial (helicoidal walls). l ¼ longitudinally, t ¼ transveepidermal wall (OEW). The growth (elongation) of epidermcuticle (Cu) is shown schematically (B, C). Since the cells ofwalled epidermal cells, there must be hormonal signals that

passive re-alignment of macromolecular fibrilseither in the CM or its associated cell walls (seeFigure 10B, C). The question whether or not thiswall model also applies to stems and coleoptilesthat grow under natural conditions is currentlyunder investigation. However, it is clear that asorgans grow, new cell wall materials and new cellsare added such that the tensile strains in the outertissue layers are relieved.

Auxin action and gravitropic upwardbending

Seventy years ago, Went and Thimann (1937)published their monograph entitled Phytohormoneswherein they summarized the field of auxin (IAA)research with special reference to a ‘‘monocot’’model system, the Avena sativa coleoptile. Inaddition to the straight-growth Avena-assay, theauthors introduced the ‘‘pea (Pisum sativum)test’’, which is based on a general property ofelongating organs, particularly stems, that, whensplit longitudinally in the growing zone, curveoutwards in water and inwards in auxin solution.

l theory’’ of stem elongation. Biophysical model of a). Organ segment with external radius R and a region ofR�r (IT ¼ inner tissues). Tensile strains (s) developing onhen measured in the directions a–b and c–d such that slongitudinal rupture unless composed of an anisotropicrsely oriented cellulose microfibrils within the outeral cells of an aerial organ that is covered by a thickthe extensible IT elongate at the same rate as the thick-coordinate the growth process within the entire organ.

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The outward curvature is due to tissue tension. Theinward curvature ‘‘is a differential growth phenom-enon of complex nature’’ (Went and Thimann 1937,p. 54). Nevertheless, the role of the epidermal celllayer as the primary target of IAA action has beenignored by subsequent generations of auxin phy-siologists (see Heyn, 1940 and Lockhart, 1960 forclassical reviews and Cleland, 1971 and Taiz, 1984for more recent accounts).

Firn and Digby (1977) were the first to demon-strate that, in hypocotyl sections of etiolatedsunflower seedlings (Figures 2 and 3), the outer2–3 cell layers (OT) represent the auxin-responsivetissue of the organ with respect to the initiation ofgrowth. They observed that peeled sections (i.e.,isolated IT-cylinders) rapidly elongate in water dueto the ‘‘release of tissue tension’’, but thesesamples were no longer responsive to applied IAA.These authors concluded that the rapid elongationin water was not an experimental artefact.Schopfer (1993) and others have shown thatsegments from which the cuticle is removed byuse of a ‘‘polishing cloth’’ elongate at the samerate as untreated control segments, which indi-cates that, at least in this ‘‘segment modelsystem’’, the thin cuticle does not restrict the rateof organ elongation.

More recently, Edelmann and Kutschera (1993)confirmed Firn and Digby’s (1977) finding that theOT represent the primary target tissue of auxinaction in Helianthus stems. In addition, theyshowed that IAA acts by loosening of the growth-limiting outer walls (enhancement in wall plasti-city) and that the pressure exerted by the ITagainstthe peripheral cell layers is identical with theaverage turgor of the cortical cells. In theseexperiments, tissue pressure was estimated by anosmotic equilibrium method (described in detail byKutschera, 1991, 1992, 1995); osmotic pressure ofthe tissue sap and cell turgor were determined with

Table 2. Effect of auxin (indole-3-acetic acid, IAA) at optimpressure of the tissues sap, cell turgor in the cortex, pressurlayers and plastic extensibility of the epidermal walls in 5-da

Treatment Length (mm) Osmotic pressure(MPa)

Tur(MP

Initial 15.270.5 0.5170.01 0.46 h�IAA 16.770.8

(+10%)0.4670.01(�10%)

0.4(�9

6 h+IAA 19.971.5(+31%)

0.3670.01(�29%)

0.3(�2

Sections, 15mm in length, were incubated for 1 h on distilled water(darkness, 25 1C). The changes within 6 h were calculated (%). Data(adapted from Edelmann and Kutschera, 1993; supplemented by unp

a miniaturized pressure probe (Kutschera andKohler, 1994). Cell wall mechanical properties(elastic and plastic deformation of non-peeledsemicylinders) were quantified by use of a con-stant-load extensiometer (Lockhart, 1960; Edel-mann, 1995; Kutschera, 2000).

Unpublished data that supplement the resultsof Edelmann and Kutschera (1993) are sum-marized in Table 2. These data show that, in theabsence of external osmotica (i.e., pure water),the changes in segment length (DL in %) overa growth period of 6 h are approximately +10% and+30% in the absence (�IAA) and presence (+IAA)of auxin, respectively. These DL-values arelargely identical with the corresponding declinesin the osmotic pressure of the tissue sap, cell turgorin the cortex, and the pressure exerted by theIT-cylinder against the peripheral walls (tissuepressure).

The following three general conclusions emergefrom the data summarized in Table 2. First, duringshort-term growth in auxin solution, hypocotyl cellsmaintain a negative water potential (C) of about�0.04 to �0.05MPa. This corresponds to the DC-value between the OT and IT of intact 4-day-oldsunflower hypocotyls (see Table 1 for calculationof C). Second, because cell turgor (Pv) and theaverage tissue pressure of the stem segments(7IAA) are numerically equivalent, the turgorpressure of the IT-cells is displaced to (and henceborne by) the thick peripheral wall(s) of theepidermis and sub-epidermis. By the same token,the protoplasts of the epidermal cells (OT) in intacthypocotyls are characterized by turgor pressuresequivalent to those of the IT (Table 1). It shouldbe noted that in these ‘‘peeling experiments’’(Figure 3) the epidermal cells were killed(Pv ¼ 0). Yet, OT-stripes contract by about 10%upon isolation owing to a relaxation of the stress inthe peripheral walls. And, third, the data shown in

al concentration on the growth of intact sections, osmotice exerted by the inner tissues against the peripheral celly-old dark-grown sunflower seedlings

gor pressurea)

Tissue pressure(MPa)

Plasticextensibility(mm10 g�1min�1)

570.01 0.4570.02 12373170.02%)

0.4070.02(�11%)

12875(+4%)

270.019%)

0.3170.01(�31%)

23079(+87%)

(initial) and thereafter transferred to H2O7IAA (10 mM) for 6 hrepresent means (7s.e.m.) of 9 independent experiments eachublished results).

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Table 2 demonstrate that the Helianthus hypocotylsegments (Figure 3) incubated in H2O7IAA behavelike osmometers or ‘‘giant cells’’ analogous toNitella internodal cells (Proseus and Boyer, 2006),in which water uptake results in a correspondingdilution of the internal (‘‘cell’’) sap, accompaniedby a proportional decline in the hydrostatic (turgorand tissue) pressure.

All of these observations are in accordance withcorresponding results obtained with excised seg-ments from coleoptiles (oat, maize) and peaepicotyls that were incubated in IAA (Kutschera,1987, 1994, 2001a, b; Kutschera et al., 1987;Kutschera and Briggs, 1987, 1988a, b; Dietz et al.,1990; Masuda, 1990; Schopfer, 1993, 2006). Collec-tively, therefore, these data show that auxinregulates the rate of growth in stems and coleop-tiles by an enhancement in plastic extensibility ofthe peripheral walls via biochemical mechanismsthat are still under investigation (Kutschera,2001a, b, 2003, 2006; Schopfer, 1993, 2006; Schop-fer and Liszkay, 2006). It should be noted that this‘‘epidermis-growth-control theory’’ proposed byKutschera (1987) implies that there exist complexinteractions between the growth-limiting outercells and the extensible cells of the IT that elongateat the same rate in the intact organ (Figure 10A–C).In a recent paper, Savaldi-Goldstein et al. (2007)proposed a molecular model illustrating the ‘‘cross-talk’’ between epidermal and inner tissues ingrowing shoots of Arabidopsis thaliana. However,more work is required to further elucidate theseputative signals from the expanding epidermis tothe IT.

As pointed out by Went and Thimann (1937), theanalysis of auxin action was associated with theexperimental study of tropisms of stems andcoleoptiles. During gravitropic upward bending(‘‘geotropic curvature’’) of intact sunflower hypo-cotyls, the growth of the upper side (epidermis) ofthe shoot ceases following gravistimulation. Thisresponse to gravity is accompanied by a markedacceleration in the rate of cell elongation on thelower side of the hypocotyl (Edelmann, 2001;Kutschera, 2001b). Figure 2B shows that theapparent tension in the OT of the upper side (nogrowth) is higher than in the vertically growingstem and that tissue tension in the elongated lowerhalf or ‘‘semi-cylinder’’ is zero (no outwardcurvature). Therefore, large (differential) changesin the mechanical properties of the OT occur in theupper vs. lower half during gravitropic upwardbending (Hejnowicz, 1997). These changes, whichhave not been characterized in detail in etio-lated stems of H. annuus, are currently underinvestigation.

Concluding remarks

We have argued that a biophysical analogy can bedrawn among sunflower hypocotyls, the Avenacoleoptile, Chara or Nitella internodal cells, orany similar biological structure because they (1) areall parts of organisms that grow irreversibly in sizeby the uptake of water facilitated by hormonallyorchestrated wall-loosening processes, (2) achieveand maintain their mature size and shape by virtueof the stiffening of walls, (3) obey basic mechanicallaws that show unequivocally that the outermostelements in the walls of any pressure containmentvessel experience the largest tensile stresses, and(4) thus resist and contribute to the internalpressure generating these stresses; (5) all havewalls containing long-chain polymers that can berealigned by tensile forces, and, by virtue ofgrowth and development, (6) undergo structuraland physiological changes that must simultaneouslyaccommodate and prefigure subsequent develop-mental states experienced by cell walls. This basicanalogy is consistent with the most recent andsophisticated theories for the hormonal control ofcell wall behaviour (see Cosgrove, 1999, 2005).Importantly, it also lies at the heart of the‘‘epidermal-growth-control model’’ as originallyproposed 20 years ago (Kutschera, 1987, 1989,1991, 1992, 1994, 1995, 2000, 2001a, b, 2003;Kutschera et al., 1987).

We also argue that, by virtue of their logic andscope, this analogy and the Kutschera model are notoversimplifications (contra Brown et al., 1995a, b;Peters and Tomos, 1996a, b, 2000) because theyprovide a biophysical appraisal of the instantaneousfunctional status of a biological structure in theoperative state of regulated growth. That is, bothoffer a phenomenological description that providespredictions about behaviour that do not requiredetailed knowledge about the physiological status ofthe system. By so doing, the analogy and the modelapply equally to growing organs experiencing wallloosening and non-growing organs that are stiffeningas they achieve their final size.

For example, the experiments on auxin action(see Table 2) are in accordance with the modelproposed two decades ago (Kutschera, 1987) andshow that, over a period of 6 h, hormone-mediatedorgan elongation is brought about by a mechanicalloosening (plasticity increase) of the growth-limit-ing peripheral walls, resulting in water uptake,tissue expansion, and hence growth of the multi-cellular system (Masuda, 1990; Kutschera, 2003,2006; Schopfer, 2006). Likewise, surgical destruc-tion of the external cell layers of mature axialorgans results in an immediate reduction in organ

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bending stiffness in excess of that predicted by thenumber of damaged epidermal cells, therebydemonstrating that the peripheral cell layers actas a ‘‘tensile skin’’ that provides mechanicalsupport (Niklas and Paolillo, 1997, 1998).

These and other experiments reviewed here andelsewhere show that the physiological and mechan-ical phenomena attending the loosening of epider-mal–subepidermal cell walls (as well as the walls ofinner tissues) come into play as organs grow in sizeirreversibly, whereas these same phenomena becomeprogressively inoperative for organs that have ceasedgrowing but nevertheless experience transient andreversible changes in turgor. Hence, the predictionsof the ‘‘epidermal-growth-control model’’ are tem-porally dependent on subcellular as well as cellular,tissue, and organ properties, and hormonal status,which can only be understood by considering theorganism as a whole (see Kaplan, 1992; Lucas et al.,1993; Niklas, 1997, 2000; Kutschera and Niklas, 2004,2005 for an evolutionary perspective of this topic).

In summary, the epidermal-growth-control modelis consistent with (1) biomechanical theory, (2)recent concepts concerning cell wall architectureand physiological behaviour, (3) novel data on themechanical properties of the plant cuticle, and (4)all classical (and recent) observations concerningthe mechanical and physiological behaviour ofwhole or excised portions removed from activelygrowing or fully mature plant parts (Figure 10A–C).Attempts to discredit the model on the basis of itbeing an ‘‘oversimplification’’ fail to recognize thedifference between a phenomenological versus amechanistic explanation of biological systems.More important, they also endorse a linear cause-and-effect logic for analysing plant growth anddevelopment, which are better understood as theemergent properties of dynamic networks ofphysical and physiological/biochemical processes.

Acknowledgements

We thank Dr. D.J. Paolillo (Cornell University) forhis comments on an early draft of this paper. Thecooperation of the authors is supported by theAlexander von Humboldt-Stiftung (AvH), Bonn(Germany).

References

Baluska F, Volkmann D, Barlow PW. Eukaryotic cells andtheir cell bodies: cell theory revised. Ann Bot2004;94:9–32.

Boyer JS. Cell enlargement and growth-induced waterpotentials. Physiol Plant 1988;73:311–6.

Boyer JS, Silk W. Hydraulics of plant growth. Funct PlantBiol 2004;31:761–73.

Brown CL, Sommer HE, Pienaar LV. The predominant roleof the pith in the growth and development ofinternodes in Liquidamber styraciflua (Hamamelida-ceae). I. Histological basis of compressive and tensilestresses in developing primary tissues. Am J Bot1995a;82:769–76.

Brown CL, Sommer HE, Pienaar LV. The predominant roleof the pith in the growth and development ofinternodes in Liquidamber styraciflua (Hamamelida-ceae). II. Pattern of tissues stress and response ofdifferent tissues to specific surgical procedures. Am JBot 1995b;82:777–81.

Cleland RE. Cell wall extension. Annu Rev Plant Physiol1971;22:197–222.

Cosgrove DJ. Enzymes and other agents that enhance cellwall extensibility. Annu Rev Plant Physiol Plant MolBiol 1999;50:391–416.

Cosgrove DJ. Growth of the plant cell wall. Nat Rev MolCell Biol 2005;6:850–61.

Darwin F, Acton EH. Practical physiology of plants.Cambridge: Cambridge University Press; 1894.

Dietz A, Kutschera U, Ray PM. Auxin enhancement ofmRNAs in epidermis and internal tissues of the peastem and its significance for control of elongation.Plant Physiol 1990;93:432–8.

Edelmann HG. Wall extensibility during hypocotylgrowth: a hypothesis to explain elastic-induced wallloosening. Physiol Plant 1995;95:296–303.

Edelmann HG. Lateral redistribution of auxin is not themeans for gravitropic differential growth of coleop-tiles: a new model. Physiol Plant 2001;112:119–26.

Edelmann HG, Kutschera U. Tissue pressure and cell-wallmetabolism in auxin-mediated growth of sunflowerhypocotyls. J Plant Physiol 1993;142:467–73.

Esau K. Anatomy of seed plants, 2nd ed. New York: Wiley;1977.

Firn R, Digby J. The role of the peripheral cell layers inthe geotropic curvature of sunflower hypocotyls: anew model of shoot geotropism. Aust J Plant Physiol1977;4:337–47.

Fry SC. The growing plant cell wall: chemical andmetabolic analysis, 2nd ed. New York: Longman;2000.

Fry SC. Primary cell wall metabolism: tracking thecareers of wall polymers in living plant cells. NewPhytol 2004;161:641–75.

Heinich K. Uber die Entspannung des Markes im Gewe-beverband und sein Wachstum im isolierten Zustand.Jb Wiss Bot 1909;46:207–69.

Hejnowicz Z. Graviresponse in herbs and trees: a majorrole for the redistribution of tissue and growthstresses. Planta 1997;203:S136–46.

Heyn ANJ. The physiology of cell elongation. Bot Rev1940;6:515–74.

Hodick D, Kutschera U. Light-induced inhibition of elong-ation growth in sunflower hypocotyls. Biophysical and

ARTICLE IN PRESS


ultrastructural investigations. Protoplasma 1992;168:7–13.

Hofmeister W. Uber die Beugungen saftreicher Pflan-zentheile nach Erschutterung. Ber Verh Ges WissLeipzig 1859;11:175–204.

Hofmeister W. Uber die durch die Schwerkraft bestimm-ten Richtungen von Pflanzentheilen. Ber Verh Ges WissLeipzig 1860;12:175–213.

Hofmeister W. Die Lehre von der Pflanzenzelle (Handbuchder physiologischen Botanik, Bd. I). Leipzig: WEngelmann; 1867.

Jeffree CE. Structure and ontogeny of plant cuticles. In:Kerstiens G, editor. Plant cuticles: an integratedfunctional approach. Oxford: BIOS Scientific Publish-ers; 1996. p. 33–82.

Jost L. Vorlesungen uber Pflanzenphysiologie. 2. Auflage.Jena: Gustav Fischer; 1908.

Kaplan D. The relationship of cells to organisms in plants:problem and implications of an organismal perspec-tive. Int J Plant Sci 1992;153:S28–37.

Kerstens S, Decraemer WF, Verbelen J-P. Cell walls at theplant surface behave mechanically like fiber-rein-forced composite materials. Plant Physiol 2001;127:381–5.

Kolattukudy PE. Biosynthetic pathways of cutin andwaxes, and their sensitivity to environmental stresses.In: Kerstiens G, editor. Plant cuticles: an integratedfunctional approach. Oxford: BIOS Scientific Publish-ers; 1996. p. 83–108.

Kraus G. Ueber die Ursachen der Formanderungenetiolierender Pflanzen. Jb Wiss Bot 1869;7:206–60.

Kraus G. Die Gewebespannung des Stammes und ihreFolgen. Bot Zeitung 1867;25:105–42.

Kutschera U. Cooperation between outer and innertissues in auxin-mediated plant organ growth. In:Cosgrove DJ, Knievel DP, editors. Physiology of cellexpansion during plant growth. Rockville: AmericanSociety of Plant Physiologists; 1987. p. 215–26.

Kutschera U. Tissue stresses in growing plant organs.Physiol Plant 1989;77:157–63.

Kutschera U. Cell-wall synthesis and elongation growth inhypocotyls of Helianthus annuus L. Planta 1990;181:316–23.

Kutschera U. Determination of the longitudinal tissuestresses in the growing and non-growing regions ofsunflower hypocotyls. J Plant Physiol 1991;138:460–5.

Kutschera U. The role of the epidermis in the control ofelongation growth in stems and coleoptiles. Bot Acta1992;105:246–52.

Kutschera U. The current status of the acid-growthhypothesis. New Phytol 1994;126:549–69.

Kutschera U. Tissue pressure and cell turgor in stems andcoleoptiles: implications for the organismal theory ofmulticellularity. J Plant Physiol 1995;146:126–32.

Kutschera U. Cell expansion in plant development. RevBrasil Fisiol Veg 2000;12:65–95.

Kutschera U. Stem elongation and cell wall proteins inflowering plants. Plant Biol 2001a;3:466–80.

Kutschera U. Gravitropism of axial organs in multicellularplants. Adv Space Res 2001b;27:851–60.

Kutschera U. Auxin-induced cell elongation in grasscoleoptiles: a phytohormone in action. Curr Top PlantBiol 2003;4:27–46.

Kutschera U. Acid growth and plant development.Science 2006;311:952–3.

Kutschera U, Briggs WR. Differential effect of auxin on invivo extensibility of cortical cylinder and epidermis inpea internodes. Plant Physiol 1987;84:1361–6.

Kutschera U, Briggs WR. Growth, in vivo extensibility, andtissue tension in developing pea internodes. PlantPhysiol 1988a;86:306–11.

Kutschera U, Briggs WR. Interaction between corticalcylinder and epidermis during auxin-mediated growthin pea internodes. Plant Sci 1988b;54:23–8.

Kutschera U, Kohler K. Turgor and longitudinal tissuepressure in hypocotyls of Helianthus annuus L. J ExpBot 1992;43:1577–81.

Kutschera U, Kohler K. Cell elongation, turgor andosmotic pressure in developing sunflower hypocotyls.J Exp Bot 1994;45:591–5.

Kutschera U, Niklas KJ. The modern theory of biologicalevolution: an expanded synthesis. Naturwissenschaf-ten 2004;91:255–76.

Kutschera U, Niklas KJ. Endosymbiosis, cell evolution,and speciation. Theory Biosci 2005;124:1–24.

Kutschera U, Bergfeld R, Schopfer P. Cooperation ofepidermis and inner tissues in auxin-mediated growthof maize coleoptiles. Planta 1987;170:168–80.

Lockhart J. Cell extension. In: Bonner J, Varner JE,editors. Plant biochemistry. New York: AcademicPress; 1960. p. 826–49.

Lucas WJ, Ding B, Van der Schoot C. Plasmodesmata andthe supracellular nature of plants. New Phytol 1993;125:435–76.

Matas AJ, Cobb ED, Bartsch JA, Paolillo JR, Niklas KJ.Biomechanics and anatomy of Lycopersicon esculen-tum fruit peels and enzyme-treated samples. Am J Bot2004a;91:352–60.

Matas AJ, Cobb ED, Bartsch JA, Paolillo JR, Niklas KJ.Crack resistance in cherry tomato fruit correlates withcuticular membrane thickness. Hortic Sci 2004b;39:1354–8.

Martin C, Glover BJ. Functional aspects of cell patterningin aerial epidermis. Curr Opin Plant Biol 2007;10:70–82.

Masuda Y. Auxin-induced cell elongation and cell wallchanges. Bot Mag Tokyo 1990;103:345–70.

Morton AG. History of botanical science. London:Academic Press; 1981.

Muller NJC. Handbuch der Allgemeinen Botanik. ErsterTeil: Anatomie und Physiologie der Gewachse. Heidel-berg: Carl Winters Universitatsbuchhandlung; 1880.

Niklas KJ. Mechanical behaviour of plant tissues asinferred from the theory of pressurized cellular solids.Am J Bot 1989;76:929–37.

Niklas KJ. Plant biomechanics. An engineering approachto plant form and function. Chicago and London: TheUniversity of Chicago Press; 1992.

Niklas KJ. The evolutionary biology of plants. Chicago andLondon: The University of Chicago Press; 1997.

ARTICLE IN PRESS


Niklas KJ. The evolution of plant body plans – abiomechanical perspective. Ann Bot 2000;85:411–38.

Niklas KJ, Paolillo DJ. The role of the epidermis as astiffening agent in Tulipa (Liliaceae) stems. Am J Bot1997;84:735–44.

Niklas KJ, Paolillo DJ. Preferential states of longi-tudinal tension in the outer tissues of Taraxacumofficinale (Asteraceae) peduncles. Am J Bot 1998;85:1068–81.

Nonami H, Boyer JT. Direct demonstration of a growth-induced water potential gradient. Plant Physiol 1993;102:13–9.

Passioura JB, Boyer JS. Tissue stresses and resistance towater flow conspire to uncouple the water potential ofthe epidermis from that of the xylem in elongatingplant stems. Funct Plant Biol 2003;30:325–34.

Peters WS, Tomos AD. The epidermis still in control? BotActa 1996a;109:264–7.

Peters WS, Tomos AD. The history of tissue tension. AnnBot 1996b;77:657–65.

Peters WS, Tomos AD. The mechanical state of ‘‘innertissues’’ in the growing zone of sunflower hypocotylsand the regulation of its growth rate followingexcision. Plant Physiol 2000;123:605–12.

Peters WS, Hagemann W, Tomos AD. What makes plantsdifferent? Principles of extracellular matrix functionin ‘soft’ plant tissues. Comp Biochem Physiol A 2000;125:151–67.

Petracek PD, Bukovac MJ. Rheological properties ofenzymatically isolated tomato fruit cuticle. PlantPhysiol 1995;109:675–9.

Pfeffer W. Pflanzenphysiologie. Ein Handbuch der Lehrevom Stoffwechsel in der Pflanze. Zweiter Band. 2.Auflage. Leipzig: W. Engelmann; 1904.

Proseus TE, Boyer JS. Identifying cytoplasmic input to thecell wall of growing Chara corallina. J Exp Bot2006;57:3231–42.

Riederer M. Estimating partitioning and transport oforganic chemicals in the foliage/atmosphere system:discussion of a fugacity-based model. Environ SciTechnol 1990;24:829–37.

Riederer M, Schreiber L. Protecting against water loss:analysis of the barrier properties of plant cuticles.J Exp Bot 2001;52:2023–32.

Sachs J. Handbuch der Experimental-Physiologie derPflanzen (Handbuch der physiologischen Botanik, Bd.IV). Leipzig: W. Engelmann; 1865.

Sachs J. Vorlesungen uber Pflanzen-Physiologie. Leipzig:W. Engelmann; 1882.

Savaldi-Goldstein S, Peto C, Chory J. The epidermis bothdrives and restricts plant shoot growth. Nature2007;446:199–202.

Schopfer P. Determination of auxin-dependent pHchanges in coleoptile cell walls by a null-pointmethod. Plant Physiol 1993;103:351–7.

Schopfer P. Biomechanics of plant growth. Am J Bot2006;93:1415–25.

Schopfer P, Liszkay A. Plasmamembrane-generated re-active oxygen intermediates and their role in cellgrowth of plants. BioFactors 2006;28:73–81.

Somerville C. Cellulose synthesis in higher plants. AnnuRev Cell Dev Biol 2006;22:57–78.

Suslov D, Verbelen J-P. Cellulose orientation determinesmechanical anisotropy in onion epidermis cell walls.J Exp Bot 2006;57:2187–92.

Taiz L. Plant cell extension: regulation of cell wallmechanical properties. Annu Rev Plant Physiol1984;35:585–657.

Went F, Thimann KV. Phytohormones. New York: TheMacMillan Company; 1937.

Wiedemann P, Neinhuis C. Biomechanics of isolated plantcuticles. Bot Acta 1998;111:28–34.

Zonia L, Munnik T. Life under pressure: hydrostaticpressure in cell growth and function. Trends PlantSci 2007;12:90–7.

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