The Role of the Epidermis in the Control of Elongation Growth in Stems and Coleoptiles

The peripheral cell wall(s) of stems and coleoptiles are 6 to 20 times thicker than the walls of the inner tissues. In coleoptiles, the outer wall of the outer epidermis shows a multilayered, helicoidal cellulose architecture, whereas the walls of the parenchyma and the outer wall of the inner epidermis are unilayered. In hypocotyls and epicotyls both the epidermal and some subepidermal walls are multilayered, helicoidal structures. The walls of the internal tissues (inner cortex, pith) are unilayered, with cellulose microfibrils oriented primarily transversely. Peeled inner tissues rapidly extend in water, whereas the outer cell layer(s) contract on isolation. This indicates that the peripheral walls limit elongation of the intact organ. Experiments with the pressure microprobe indicate that the entire organ can be viewed as a giant, turgid cell: the extensible inner tissues exert a pressure (turgor) on the peripheral wall(s), which bear the longitudinal wall stress of the epidermal and internal cells. Numerous studies have shown that auxin induces elongation of isolated, intact sections by loosening of the growth-limiting peripheral cell wall(s). Likewise, the effect of light on reduction of stem elongation and cell wall extensibility in etiolated seedlings is restricted to the peripheral cell layers of the organ. The extensible inner tissues provide the driving force (turgor pressure), whereas the rigid peripheral wall(s) limit, and hence control, the rate of organ elongation.     

Stem Elongation and Cell Wall Proteins in Flowering Plants

Abstract: The growth of stems (hypocotyls, epicotyls) and stem-like organs (coleoptiles) in developing seedlings is largely due to the elongation of cells in the sub-apical region of the corresponding organ. According to the organismal concept of plant development, the thick outer epidermal wall, which can be traced back to the peripheral cell wall of the zygote, creates a sturdy organ sheath that determines the rate of stem elongation. The cells of the inner tissues are the products of secondary partitioning of one large protoplast; these turgid, thin-walled cells provide the driving force for organ growth. The structural differences between these types of cell walls are described (outer walls: thick, sturdy, helicoidal cellulose architecture; inner walls: thin, extensible, transversely-oriented cellulose microfibrils). On the basis of these facts, current models of cell wall loosening (and wall stiffening) are discussed with special reference to the expansin, enzymatic polymer remodelling and osmiophilic particle hypothesis. It is concluded that the exact biochemical mechanism(s) responsible for the coordinated yielding of the growth-controlling peripheral organ wall(s) have not yet been identified.     

Buckling of inner cell wall layers after manipulations to reduce tensile stress: observations and interpretations for stress transmission

The inner layer of the cell wall in tissues that are under tensile stress in situ, e.g. epidermis and collenchyma of etiolated sunflower hypocotyls, shows a pattern of transverse folds when the tissues are detached and plasmolysed. This can be observed by Nomarski imaging of inner surfaces of the outer cell walls and electron microscopy of longitudinal sections after peeling the epidermis and bathing it in plasmolysing solutions. The folds are apparently caused by buckling of the inner layer due to the longitudinal compressive force exerted on this layer by the outer wall layer, when it shrinks after the removal of the longitudinal tensile stresses. In these stresses, two components can be distinguished: the tissue stress, disappearing on peeling, and that caused directly by turgor pressure, disappearing in hyperosmotic solution. Investigation of the buckling indicates that the outer layer of the cell wall transmits in situ most of the longitudinal tensile stress in the wall. The common concept that the inner layer of the wall is the region bearing most stress and therefore regulating growth can still be valid with respect to the transverse stress component.     

Tensile Tissue Stress Affects the Orientation of Cortical Microtubules in the Epidermis of Sunflower Hypocotyl

Abstract In turgid multicellular organs, it is convenient to differentiate between the two kinds of tensile forces acting in cell walls as a result of turgor pressure. The primary forces occur both in situ and in cells isolated from the organ, whereas the secondary forces occur only in situ. The latter are an unavoidable physical consequence of the variation in mechanical parameters of tissues forming layers or strands. The most rigid tissue is under maximal tensile force, whereas the least rigid is under maximal compressive force. These forces cause tissue stresses (that is, certain tissues are under tensile stress, whereas others are under compressive stress in the organ). The primary and secondary forces result in primary and secondary stress in cell walls, respectively. The anisotropy of the primary stress is a function of cell shape. For instance, in cylindric cells the anisotropy expressed as the ratio of longitudinal to transverse stresses is 0.5. The anisotropy of the secondary stress is a function of the compound structure of the organ. For example, in the epidermis of sunflower hypocotyl, the longitudinal secondary stress is much higher than the transverse stress. The primary and secondary stresses are superimposed, and, as a consequence, the stress anisotropy in the outer thick walls of epidermal cells is greater than 1. These outer epidermal walls transmit most of the tissue stress. When the epidermis is peeled but remains turgid, only primary stress remains, but loading of the peel can reestablish the original stress anisotropy. We studied the effect of stress anisotropy changes on the orientation of cortical microtubules (CMTs) in the sunflower hypocotyl epidermis. We showed that changes in stress anisotropy cause the CMT orientation to change in the direction of maximal wall stress. In situ, the relatively high tensile tissue stress in the epidermis causes maximal stress in the longitudinal direction and relatively steep CMT orientation. When the tissue stress is removed from the epidermis by peeling, the CMTs tend to reorient toward the transverse direction, which is the direction of maximal stress in the primary component. On application of external longitudinal stress, to substitute for tissue stress, CMTs tend to reorient in the longitudinal direction. However, a relatively high rate of plastic strain is caused by the stress applied to the peel in an acid medium. This produces a less steep orientation of CMTs. It appears that the change in stress anisotropy orients the CMT in the direction in which the stress is maximal after the change, but there is also some effect of the growth rate on the orientation. Received 4 January 2000; accepted 10 February, 2000     

Cooperation of epidermis and inner tissues in auxin-mediated growth of maize coleoptiles

The function of the epidermis in auxinmediated elongation growth of maize (Zea mays L.) coleoptile segments was investigated. The following results were obtained: i) In the intact organ, there is a strong tissue tension produced by the expanding force of the inner tissues which is balanced by the contracting force of the outer epidermal wall. The compression imposed by the stretched outer epidermal wall upon the inner tissues gives rise to a wall-pressure difference which can be transformed into a water-potential difference between inner tissues and external medium (water) by removal of the outer epidermal wall. ii) Peeled segments fail to respond to auxin with normal growth. The plastic extensibility of the inner-tissue cell walls (measured with a constant-load extensiometer using living segments) is not influenced by auxin (or abscisic acid) in peeled or nonpeeled segments. It is concluded that auxin induces (and abscisic acid inhibits) elongation of the intact segment by increasing (decreasing) the extensibility specifically in the outer epidermal wall. In addition, tissue tension (and therewith the pressure acting on the outer epidermal wall) is maintained at a constant level over several hours of auxin-mediated growth, indicating that the inner cells also contribute actively to organ elongation. However, this contribution does not involve an increase of cell-wall extensibility, but a continuous shifting of the potential extension threshold (i.e., the length to which the inner tissues would extend by water uptake after peeling) ahead of the actual segment length. Thus, steady growth involves the coordinated action of wall loosening in the epidermis and regeneration of tissue tension by the inner tissues. iii) Electron micrographs show the accumulation of striking osmiophilic material (particles of approx. 0.3 m diameter) specifically at the plasma membrane/cell-wall interface of the outer epidermal wall of auxin-treated segments. iv) Peeled segments fail to respond to auxin with proton excretion. This is in contrast to fusicoccin-induced proton excretion and growth which can also be readily demonstrated in the absence of the epidermis. However, peeled and nonpeeled segments show the same sensitivity to protons with regard to the induction of acid-mediated in-vivo elongation and cell-wall extensibility. The observed threshold at pH 4.5–5.0 is too low to be compatible with a second messenger function of protons also in the growth response of the inner tissues. Organ growth is described in terms of a physical model which takes into account tissue tension and extensibility of the outer epidermal wall as the decisive growth parameters. This model states that the wall pressure increment, produced by tissue tension in the outer epidermal wall, rather than the pressure acting on the inner-tissue walls, is the driving force of growth.Abbreviations and symbols E _el, E _pl elastic and plastic in-vitro cell-wall extensibility, respectively - E _tot E _el+E _pl - FC fusicoccin - IAA indole-3-acetic acid - IT inner tissue - ITW inner-tissue walls - OEW outer epidermal wall - osmotic pressure - P wall pressure - water potential     

Modification of chemical properties of cell wall polysaccharides in the inner tissues by white light in relation to the decrease in tissue tension in Pisum sativum epicotyls

When white light irradiation inhibits shoot growth in derooted pea ( Pisum sativum L. cv. Alaska) cuttings, it decreases tissue tension, a driving force for shoot growth, by making the cell wall of the inner tissues mechanically rigid. To elucidate the mechanism by which light affects the mechanical properties of the cell wall in the inner tissues, its effect on the chemical properties of the cell walls was studied by analyzing qualitatively and quantitatively cell wall polysaccharides in the epdidermis and inner tissue of pea epicotyls grown in both dark and light. The amount of polysaccharides per subhook in the cell walls of both tissues increased during a 4-h dark incubation. Light suppressed the increase in hemicellulosic (HC)-II and cellulosic polysaccharides in the inner tissues, while it did not affect the increase in other wall fractions in either the epidermal or subepidermal tissues. No light effect was observed on the neutral sugar compositions of pectin, HC-I or HC-II fractions in either of the tissues. Light increased the mass-average molecular mass of xyloglucan and rhamnoarabinogalactan in HC-II fractions in the inner tissues, while such an effect was not observed in the epidermis. These facts suggest that the light-induced decrease in the tissue tension in pea epicotyls is caused by an increase in the molecular size of xyloglucan, rhamnoarabinogalactan in the HC-II fraction and/or the suppression of the synthesis of HC-II and cellulosic polysaccharides in the inner tissues.     

Axis elongation can occur with net longitudinal orientation of wall microfibrils

During the initial phases of elongation of pea internodes, oat and rice coleoptiles, oat mesocotyls, soybean hypocotyls and dandelion peduncles, net transverse orientation of cellulose wall microfibrils (Mfs) was found in the outer epidermal wall. This paper demonstrates that in all these axes, with the exception of rice coleoptile, net longitudinal orientation of microfibrils occurs in the outer epidermal wall in portions of the axes that were still elongating at the time of sampling. The timing of the transition to net longitudinal orientation and whether the transition proceeded acropetally or basipetally varied with the type of axis under study. The variability of the relationship between extension and the transition from net transverse to net longitudinal orientation suggests that factors other than extension are important in determining the transition. Layers of longitudinal wall microfibrils may be added to the extending epidermal wall to bolster its tensile strength commensurate with its function during and after extension. Attention is drawn to the parallels between the concept of tissue tension in growing axes and the concept that the epidermis functions as a stressed skin in the support of mature plant parts in primary growth.     

Cortical microtubules optimize cell-wall crystallinity to drive unidirectional growth in Arabidopsis

The shape of plants depends on cellulose, a biopolymer that self-assembles into crystalline, inextensible microfibrils (CMFs) upon synthesis at the plasma membrane by multi-enzyme cellulose synthase complexes (CSCs). CSCs are displaced in directions predicted by underlying parallel arrays of cortical microtubules, but CMFs remain transverse in cells that have lost the ability to expand unidirectionally as a result of disrupted microtubules. These conflicting findings suggest that microtubules are important for some physico-chemical property of cellulose that maintains wall integrity. Using X-ray diffraction, we demonstrate that abundant microtubules enable a decrease in the degree of wall crystallinity during rapid growth at high temperatures. Reduced microtubule polymer mass in the mor1-1 mutant at high temperatures is associated with failure of crystallinity to decrease and a loss of unidirectional expansion. Promotion of microtubule bundling by over-expressing the RIC1 microtubule-associated protein reduced the degree of crystallinity. Using live-cell imaging, we detected an increase in the proportion of CSCs that track in microtubule-free domains in mor1-1, and an increase in the CSC velocity. These results suggest that microtubule domains affect glucan chain crystallization during unidirectional cell expansion. Microtubule disruption had no obvious effect on the orientation of CMFs in dark-grown hypocotyl cells. CMFs at the outer face of the hypocotyl epidermal cells had highly variable orientation, in contrast to the transverse CMFs on the radial and inner periclinal walls. This suggests that the outer epidermal mechanical properties are relatively isotropic, and that axial expansion is largely dependent on the inner tissue layers.     

The growing outer epidermal wall: design and physiological role of a composite structure

Background: The cells of growing plant organs secrete an extracellular fibrouscomposite (the primary wall) that allows the turgid protoplaststo expand irreversibly via wall-yielding events, which are regulatedby processes within the cytoplasm. The role of the epidermisin the control of stem elongation is described with specialreference to the outer epidermal wall (OEW), which forms a ‘tensileskin’. Novel Facts: The OEW is much thicker and less extensible than the walls ofthe inner tissues. Moreover, in the OEW the amount of celluloseper unit wall mass is considerably greater than in the innertissues. Ultrastructural studies have shown that the expandingOEW is composed of a highly ordered internal and a diffuse outerhalf, with helicoidally organized cellulose microfibrils inthe inner (load-bearing) region of this tension-stressed organwall. The structural and mechanical backbone of the wall consistsof helicoids, i.e. layers of parallel, inextensible cellulosemicrofibrils. These ‘plywood laminates’ containcrystalline ‘cables’ orientated in all directionswith respect to the axis of elongation (isotropic material).Cessation of cell elongation is accompanied by a loss of order,i.e. the OEW is a dynamic structure. Helicoidally arranged extracellularpolymers have also been found in certain bacteria, algae, fungiand animals. In the insect cuticle crystalline cutin nanofibrilsform characteristic ‘OEW-like’ herringbone patterns. Conclusions: Theoretical considerations, in vitro studies and computer simulationssuggest that extracellular biological helicoids form by directedself-assembly of the crystalline biopolymers. This spontaneousgeneration of complex design ‘without an intelligent designer’evolved independently in the protective ‘skin’ ofplants, animals and many other organisms.     

Relationship between microtubule orientation and patterns of cell expansion in developing cortical cells of Lemna minor roots

In actively growing cortical cells in the elongation zone of Lemna minor L. roots, both longitudinal (radial and tangential) and transverse walls expand in both length and width. The longitudinal walls of the three types of cortical cells in the root (i.e. outer, middle and inner) showed the largest expansion in the longitudinal axis. In contrast, the inner cortical cells exhibited the least expansion in width, whereas the middle cortical cells displayed the largest expansion in width. Thus, the profiles of the expansion of longitudinal walls were characteristic for the three types of cortical cells. In this study, both the orientation of cortical microtubule (MT) arrays and their dynamic reorientation, and the density of cortical MTs, were documented and correlated to the patterns of cell wall expansion. Significantly, transverse arrays of cortical MTs were most prominent in the radial walls of the inner cortical cells, and least so in those of the middle cortical cells. Toward the base of roots, beyond the elongation zone, the orientation of cortical MTs shifted continuously from transverse to oblique and then to longitudinal. In this case, the rate of shift in the orientation of cortical MTs along the root axis was appreciably faster in the middle cortical cells than in the other two types of cortical cells. Interestingly, the continuous change in cortical MT orientation was not confirmed in the transverse walls which showed much smaller two-dimensional expansion than the radial walls. Additionally, the presence of fragmented or shortened cortical MTs rapidly increased concomitantly with the decrease of transversely oriented cortical MTs. This relationship was especially prominent in the transverse walls of the inner cortical cells, which displayed the least expansion among the three types of cortical cells investigated. In the root elongation zone, the density of cortical MTs in the inner cortical cells was about three times higher than that in the other two cortical cell types. These results indicate that in the early stage of cell expansion, the orientation of cortical MTs determines a preferential direction of cell expansion and both the shifting orientation and density of cortical MTs affect the magnitude of expansion in width of the cell wall.     

Dynamic microtubules under the radial and outer tangential walls of microinjected pea epidermal cells observed by computer reconstruction

By microinjecting rhodamine-conjugated pig brain tubulin into living pea stem epidermal cells it has been possible to follow cortical microtubules beneath the outer tangential wall (OTW) as they re-orientate from a transverse to a longitudinal alignment. Earlier immunofluorescence studies on fixed material have shown that parallel cortical microtubules circumnavigate the cell forming apparently continuous arrays which are transverse, oblique or longitudinal to the cell's long axis. If the array re-orientates as a whole then microtubules along the radial walls would be expected to share the alignment of those on the tangential walls. There are, however, reports that microtubules beneath the outer tangential wall have a different orientation from microtubules at the radial cell walls, raising important questions about the construction and behaviour of the array. Using computer-rotated stacks of optical sections collected by confocal scanning laser microscopy it has been possible to display the microtubules along radial as well as tangential walls of the same microinjected cells. These observations demonstrate for living epidermal cells that when microtubules are aligned longitudinally at the outer epidermal wall they remain oblique or transverse at the radial walls. The array may not therefore re-orientate as a whole but seems to undergo re-organization on only one cell face. However, despite the differing angles between the OTW and radial walls microtubules still form patterns which at the level of the confocal microscope are continuous from one cell face to another, around the cell.
It is concluded that some organizing principle attempts to establish overall organization at the cellular level but that this can be perturbed by local re-organization of dynamic microtubules in subcellular domains. This study emphasizes the importance of the outer epidermal wall and its associated cytoskeleton in initiating changes in the direction of cell expansion.     

Cell-wall tension of the inner tissues of the maize coleoptile and its potential contribution to auxin-mediated organ growth

Plant organs such as maize (Zea mays L.) coleoptiles are characterized by longitudinal tissue tension, i.e. bulk turgor pressure produces unequal amounts of cell-wall tension in the epidermis (essentially the outer epidermal wall) and in the inner tissues. The fractional amount of turgor borne by the epidermal wall of turgid maize coleoptile segments was indirectly estimated by determining the water potential ^* of an external medium which is needed to replace quantitatively the compressive force of the epidermal wall on the inner tissues. The fractional amount of turgor borne by the walls of the inner tissues was estimated from the difference between -^* and the osmotic pressure of the cell sap (_i) which was assumed to represent the turgor of the fully turgid tissue. In segments incubated in water for 1 h, -^* was 6.1–6.5 bar at a _i of 6.7 bar. Both -^* and _i decreased during auxin-induced growth because of water uptake, but did not deviate significantly from each other. It is concluded that the turgor fraction utilized for the elastic extension of the inner tissue walls is less than 1 bar, i.e. less than 15% of bulk turgor, and that more than 85% of bulk turgor is utilized for counteracting the high compressive force of the outer epidermal wall which, in this way, is enabled to mechanically control elongation growth of the organ. This situation is maintained during auxin-induced growth.Abbreviations and Symbols _i osmotic pressure of the tissue - ₀ external water potential - ^* water potential at which segment length does not change - IAA indole-3-acetic acid - ITW longitudinal inner tissue walls - OEW outer epidermal wall - P turgor Supported by Deutsche Forschungsgemeinschaft (SFB 206).     

Orientation of Wall Microfibrils in Avena Coleoptiles and Mesocotyls and in Pisum Epicotyls

Microfibrils (MFs) on the inner surface of the walls of Avenacoleoptile and mesocotyl cells and of Pisum epicotyl cells wereexamined by a replica method. In the elongating epidermis ofthese three organs, cells having MFs that were transverse, obliqueor longitudinal to the elongation axis were intermingled. Inthe elongating parenchymal tissues, all cells deposited MFstransversely. In non-elongating cells of Avena coleoptiles andPisum epicotyls, the orientation of MFs on the inner wall surfaceof both epidermal and parenchymal cells was more longitudinalthan in elongating cells. These observations on the orientationsof MFs are compatible with those our previously reported observationson the orientations of microtubules (MT) (Iwata and Hogetsu1988). Disruption of MTs of Avena coleoptiles by treatment withamiprophosmethyl caused changes in the orientation of depositionof MFs. These results support the idea that MFs are usuallyco-aligned with MTs in organ cells and that the orientationof MFs is controlled by MTs. The averaged direction of MFs, visualized under polarized light,showed a clear difference between the epidermal and inner-tissuecell walls in the elongating regions of the three organs. Inalmost all elongating and non-elongating epidermal cells, theaveraged direction of MFs was longitudinal, while it was transversein all inner-tissue cells. (Received December 16, 1988; Accepted April 28, 1989)     

Light-induced inhibition of elongation growth in sunflower hypocotyls

Summary The long-term effects of white light (WL) on epidermal cell elongation and the mechanical properties and ultrastructure of cell walls were investigated in the subapical regions of hypocotyls of sunflower seedlings (Helianthus annuus L.) that were grown in darkness. Upon transition to WL a drastic inhibition of epidermal cell elongation was observed. However, the mechanical properties of the inner tissues (cortex, vascular bundles, and pith) were unaffected by WL. Thus, the light-induced decrease in cell wall plasticity measured on entire stems occurs exclusively in the peripheral tissues (epidermis and 2 to 3 subepidermal cell layers).An electronmicroscopic investigation of the epidermal cell walls showed that they are of the helicoidal type with the direction of microfibrils monotonously changing during deposition. This cell wall type was identified by the appearance of arced patterns of microfibrils in cell walls sectioned oblique to the plane of their synthesis. WL irradiation did not change the periodicity of this pattern nor the thickness of the lamellae. Thus, the inhibition of cell elongation was not caused or accompanied by a shift in the direction of microfibril deposition in the growth-limiting outer tissues. However, cell wall thickness, the number of lamellae and hence the amount of cellulose oriented parallel and transverse to the longitudinal cell axis increased in WL. This may account for the effect of WL on the reduction of cell wall plasticity and growth.Abbreviations D darkness - PATAg periodic acid-thiocarbohydracide-silver protein - WL white light     

Fine structural observations on the epidermis

Summary The organization of the wall of epidermal cells in the petiole of species of Apium, Eryngium, Rumex, and Abutilon as well as that of the epidermis of Avena coleoptile has been investigated. The outer and inner tangential walls consist of layers in which the cellulose microfibrils are oriented alternately parallel or transverse to the longitudinal cell axis. This organization resembles that previously described for collenchyma cell walls (Wardrop, 1969; Chafe, 1970). On the radial (anticlinal) walls the orientation of the microfibrils is transverse and these appear continuous with the layers of transverse orientation of the outer and inner tangential walls. Variation in thickness of the outer tangential, and radial, and inner tangential walls appears to result from the variation in thickness of those layers in which the microfibrils have a longitudinal orientation. The extent to which these observations can interpreted in terms of some type of modified multi-net growth is discussed.     

Seed coat development, anatomy and scanning electron microscopy of Harpagophytum procumbens (Devil's Claw), Pedaliaceae

Seed coat development of Harpagophytum procumbens (Devil's Claw) and the possible role of the mature seed coat in seed dormancy were studied by light microscopy (LM), transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Very young ovules of H. procumbens have a single thick integument consisting of densely packed thin-walled parenchyma cells that are uniform in shape and size. During later developmental stages the parenchyma cells differentiate into 4 different zones. Zone 1 is the multi-layered inner epidermis of the single integument that eventually develops into a tough impenetrable covering that tightly encloses the embryo. The inner epidermis is delineated on the inside by a few layers of collapsed remnant endosperm cell wall layers and on the outside by remnant cell wall layers of zone 2, also called the middle layer. Together with the inner epidermis these remnant cell wall layers from collapsed cells may contribute towards seed coat impermeability. Zone 2 underneath the inner epidermis consists of large thin-walled parenchyma cells. Zone 3 is the sub-epidermal layers underneath the outer epidermis referred to as a hypodermis and zone 4 is the single outer seed coat epidermal layer. Both zones 3 and 4 develop unusual secondary wall thickenings. The primary cell walls of the outer epidermis and hypodermis disintegrated during the final stages of seed maturation, leaving only a scaffold of these secondary cell wall thickenings. In the mature seed coat the outer fibrillar seed coat consists of the outer epidermis and hypodermis and separates easily to reveal the dense, smooth inner epidermis of the seed coat. Outer epidermal and hypodermal wall thickenings develop over primary pit fields and arise from the deposition of secondary cell wall material in the form of alternative electron dense and electron lucent layers. ESEM studies showed that the outer epidermal and hypodermal seed coat layers are exceptionally hygroscopic. At 100% relative humidity within the ESEM chamber, drops of water readily condense on the seed surface and react in various ways with the seed coat components, resulting in the swelling and expansion of the wall thickenings. The flexible fibrous outer seed coat epidermis and hypodermis may enhance soil seed contact and retention of water, while the inner seed coat epidermis maintains structural and perhaps chemical seed dormancy due to the possible presence of inhibitors.     

Total epidermal cell walls of pea stems respond differently to auxin than does the outer epidermal wall alone

The effect of auxin on cell wall mass in the epidermis of third internodes of Pisum sativum L. cv. Alaska grown in dim red light was investigated using epidermal peels, to determine whether epidermal peels reflect the behavior of the outer epidermal cell wall. In contrast to the outer epidermal wall itself, where auxin caused thinning in proportion to growth (M.S. Bret-Harte et al, 1991, Planta 185, 462–471), auxin promoted an increase in wall mass in epidermal peels from treated internode segments in the absence of exogenously supplied sugar. The percentage gain in mass was smaller than the percentage elongation, however, so mass per unit length decreased in peels from auxin-treated segments. Epidermal peels from auxin-treated segments gained more wall mass than control peels even when adhering internal tissue at the basal end of the peel was removed. Epidermal peels also had a gross composition different from that of the outer wall alone (M.S. Bret-Harte and L.D. Talbott, 1993, Planta 190, 369–378). These discrepancies can be explained by the observation that the outer wall makes up only 30% of the mass of the epidermal peel. It appears that the inner walls of the epidermis, and walls of the outer layer of cortical cells that remain attached to the epidermis during peeling, nearly maintain their thickness by biosynthesis while the outer wall loses mass as previously described (Bret-Harte et al. 1991). These results indicate that epidermal peels may not be a good system for examining the biochemical and physiological properties of the outer epidermal cell wall.I would like to thank Dr. Peter M. Ray, of Stanford University, for the use of experimental facilities, helpful discussions, and technical and editorial assistance, Dr. Winslow R. Briggs, of the Carnegie Institute of Washington, for helpful discussions and for the use of experimental facilities, Dr. Paul B. Green, of Stanford University, for financial support, and Dr. Wendy K. Silk, of the Department of Land, Air, and Water Resources, University of California, Davis, for financial support. This work was supported by a National Science Foundation Graduate Fellowship, National Science Foundation grant DCB8801493 to Paul B. Green, and the generosity of Wendy K. Silk in the final writing.     

Transverse viscoelastic extension in nitella: I. Relationship to growth rate

Transverse viscoelastic extensibility was measured directly in isolated walls of Nitella internode cells. Cell walls extended transversely exhibit a yield point which is approximately twice the yield point in the longitudinal direction. Walls from young, growing cells are four to seven times more extensible longitudinally than transversely, while walls from mature, nongrowing cells are only two times more extensible longitudinally. Although longitudinal extensibility decreases drastically with the decrease in the growth rate, lateral extensibility is constant through development. There is a discrepancy between the lateral growth rate and transverse creep, since the lateral growth rate is not constant. However, the degree of wall anisotropy observed is consistent with the view that the transversely oriented cellulose microfibrils act as a “reinforcing filler” in Nitella cell walls.     

Cell wall traits that influence plant development,immunity, and bioconversion

The architecture of the plant cell wall is highly dynamic, being substantially re‐modeled during growth and development. Cell walls determine the size and shape of cells and contribute to the functional specialization of tissues and organs. Beyond the physiological dynamics, the wall structure undergoes changes upon biotic or abiotic stresses. In this review several cell wall traits, mainly related to pectin, one of the major matrix components, will be discussed in relation to plant development, immunity and industrial bioconversion of biomass, especially for energy production. Plant cell walls are a source of oligosaccharide fragments with a signaling function for both development and immunity. Sensing cell wall damage, sometimes through the perception of released damage‐associated molecular patterns (DAMPs), is crucial for some developmental and immunity responses. Methodological advances that are expected to deepen our knowledge of cell wall (CW) biology will also be presented.