Structure and chemical composition of endodermal and rhizodermal/hypodermal walls of several species

CWM, isolated cell wall material
ECW, isolated endodermal cell walls
G, guaiacyl monomer
H, p-hydroxyphenyl monomer
HCW, isolated hypodermal cell walls
RHCW, isolated rhizodermal and hypodermal cell walls
S, syringyl monomer
XV, isolated xylem vessels

Endodermal cell walls of the three dicotyledoneous species Pisum sativum L., Cicer arietinum L. and Ricinus communis L. were isolated enzymatically and analysed for the occurrence of the biopolymers lignin and suberin. From P. sativum, endodermal cell walls in their primary state of development (Casparian strips) were isolated. Related to the dry weight, these isolates contained equal amounts of suberin (2·5%) and lignin (2·7%). In contrast, the endodermal cell walls of C. arietinum and R. communis, which were nearly exclusively in their secondary state of development, contained significantly higher proportions of suberin (10–20%) and only traces of lignin (1–2%). The results of the chemical analyses were supported by a microscopic investigation of Sudan III-stained root cross-sections, showing a Casparian strip restricted to the radial walls of the endodermis of P. sativum and well-pronounced red suberin lamellae in C. arietinum and R. communis roots. Compared with recently investigated monocotyledoneous species, higher amounts of suberin by one order of magnitude were detected with the secondary state of development of dicotyledoneous species. Furthermore, the carbohydrate and protein contents of primary (Clivia miniata Reg. and Monstera deliciosa Liebm.), secondary (C. arietinum and R. communis) and tertiary endodermal cell walls (Allium cepa L. and Iris germanica L.) were determined. The relative carbohydrate content of secondary endodermal cell walls was low (14–20%) compared with the content of primary (42–50%) and tertiary endodermal cell walls (60%), whereas the protein content of isolated endodermal cell walls was high in primary (13%) and secondary (8%) and low in tertiary endodermal cell walls (0·9–2%). The results presented here indicate that the quantitative chemical composition of primary, secondary, and tertiary endodermal cell walls varies significantly. Finally, cell wall proteins are described as an additional important constituent of endodermal cell walls, with the highest concentrations occurring in primary (Casparian strips) and secondary endodermal cell walls.     

Fourier transform infrared-spectroscopic characterisation of isolated endodermal cell walls from plant roots: chemical nature in relation to anatomical development

The chemical nature of enzymatically isolated endodermal cell walls from Cicer arietinum L., Clivia miniata Reg. and Iris germanica L. was studied by FTIR (Fourier transform infrared) spectroscopy. Observed frequencies were assigned to functional groups present in the cell wall and relative amounts of the biopolymers suberin and lignin, cell wall carbohydrates and proteins were determined. Infrared absorption spectra indicated structural characteristics for the three different developmental states of the isolated endodermal cell wall: primary endodermis with Casparian strips (state I), secondary endodermis with suberin lamellae (state II), and tertiary endodermis with U-shaped cell wall depositions (state III). The data obtained from this study are compared with previous results obtained by chemical degradation of isolated endodermal cell walls and subsequent determination of monomeric degradation products by gas chromatography and mass spectrometry. It is concluded that FTIR spectroscopy represents a direct and nondestructive method suitable for the rapid investigation of isolated plant cell walls. Furthermore, the observation that the suberin-assigned absorption bands disappeared after transesterification of the samples with BF₃-methanol confirmed that suberin is completely degraded by this treatment. Received: 20 February 1999 / Accepted: 25 May 1999     

Chemical analysis and immunolocalisation of lignin and suberin in endodermal and hypodermal/rhizodermal cell walls of developing maize (Zea mays L.) primary roots

The composition of suberin and lignin in endodermal cell walls (ECWs) and in rhizodermal/hypodermal cell walls (RHCWs) of developing primary maize (Zea mays L.) roots was analysed after depolymerisation of enzymatically isolated cell wall material. Absolute suberin amounts related to root length significantly increased from primary ECWs (Casparian strips) to secondary ECWs (suberin lamella). During further maturation of the endodermis, reaching the final tertiary developmental state characterised by the deposition of lignified secondary cell walls (u-shaped cell wall deposits), suberin amounts remained constant. Absolute amounts of lignin related to root length constantly increased throughout the change from primary to tertiary ECWs. The suberin of Casparian strips contained high amounts of carboxylic and 2-hydroxy acids, and differed substantially from the suberin of secondary and tertiary ECWs, which was dominated by high contents of ω-hydroxycarboxylic and 1,ω-dicarboxylic acids. Furthermore, the chain-length distribution of suberin monomers in primary ECWs ranged from C₁₆ to C₂₄, whereas in secondary and tertiary ECWs a shift towards higher chain lengths (C₁₆ to C₂₈) was observed. The lignin composition of Casparian strips (primary ECWs) showed a high syringyl content and was similar to lignin in secondary cell walls of the tertiary ECWs, whereas lignin in secondary ECWs contained higher amounts of p-hydroxyphenyl units. The suberin and lignin compositions of RHCWs rarely changed with increasing root age. However, compared to the suberin in ECWs, where C₁₆ and C₁₈ were the most prominent chain lengths, the suberin of RHCWs was dominated by the higher chain lengths (C₂₄ and C₂₆). The composition of RHCW lignin was similar to that of secondary-ECW lignin. Using lignin-specific antibodies, lignin epitopes were indeed found to be located in the Casparian strip. Surprisingly, the mature suberin layers of tertiary ECWs contained comparable amounts of lignin-like epitopes. Received: 19 August 1998 / Accepted: 3 February 1999     

The Casparian Strip of Clivia miniata Reg. Roots: Isolation,Fine Structure and Chemical Nature*

The root endodermis of Clivia miniata Reg. was successfully isolated using the cell wall degrading enzymes cellulase and pectinase. The enzymes did not depolymerize those regions of the primary cell walls of anticlinal endodermal root cells where the Casparian strips were located. Since the endodermis of C. miniata roots remained in its primary developmental state over the whole root length, endodermal isolates essentially represented Casparian strips. Thus, sufficient amounts of isolated Casparian strips could be obtained to allow further detailed investigations of the isolates by microscopic, histochemical and analytical methods. Scanning electron microscopy revealed the reticular structure of the Casparian strips completely surrounding the central cylinder of the roots. Whereas in younger parts of the root only the anticlinal cell walls of the endodermis remained intact in the isolates, in older parts of the root the periclinal walls also restricted enzymatic degradation due to the deposition of lignin. Extracts of the isolates with organic solvents did not reveal any wax-like substances which might have been deposited within the cell wall forming a transport barrier, as is the case with cutin and suberin. However, several histochemical and analytical methods (elemental analysis and FTIR spectroscopy) showed that the chemical nature of the Casparian strips of C. miniata roots can definitely be a lignified cell wall. These findings are in complete agreement with studies carried out at the beginning of this century on the chemical nature of the Casparian strips of several other plant species. The implications of these results concerning apoplasmatic transport of solutes and water across Casparian strips are discussed.     

Chemical composition and ultrastructure of broad bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) nodule endodermis in comparison to the root endodermis

Ultrastructure and development of apoplastic barriers within indeterminate root nodules formed by Vicia faba L. were examined by light and electron microscopy. The nodule outer cortex is separated from the inner cortex by a heavily suberized nodule endodermis, which matures in submeristematic regions and possesses suberin lamellae. Unsuberized passage cells are present near vascular strands, which are surrounded by a vascular endodermis attached on the inner side of the nodule endodermal cell walls. The vascular endodermis appears immediately below the meristematic apex in developmental state I (Casparian bands), gradually develops suberin lamellae, and attains developmental state II at the base of the nodule. For chemical analysis apoplastic barrier tissues were dissected after enzymatic digestion of non-impregnated tissues. Root epidermal and endodermal cell walls as well as nodule outer cortex could be isolated as pure fractions; nodule endodermal cell walls could not be separated from vascular endodermal cell walls and enclosed xylem vessels. Gas chromatography-flame ionization detection and gas chromatography-mass spectrometry were applied for quantitative and qualitative analysis of suberin and lignin in isolated cell walls of these tissues. The suberin content of isolated endodermal cell walls of nodules was approximately twice that of the root endodermal cell walls. The suberin content of the nodule outer cortex and root epidermal cell walls was less than one-tenth of that of the nodule endodermal cell wall. Substantial amounts of lignin could only be found in the nodule endodermal cell wall fraction. Organic solvent extracts of the isolated tissues revealed long-chain aliphatic acids, steroids, and triterpenoid structures of the lupeol type. Surprisingly, extract from the outer cortex consisted of 89% triterpenoids whereas extracts from all other cell wall isolates contained not more than 16% total triterpenoids. The results of ultrastructural and chemical composition are in good correspondence and underline the important role of the examined tissues as apoplastic barriers.     

Chemical composition of Casparian strips isolated from Clivia miniata Reg. roots: evidence for lignin

Endodermal cell walls and xylem vessels were isolated enzymatically from Clivia miniata Reg. roots. Transmission-electron-microscopic investigation of cross-sections of intact C. miniata roots and scanning-electron-microscopic investigation of isolated endodermal cell walls indicated that the root endodermis of C. miniata is essentially in its primary state of development. Isolated Casparian strips and xylem vessels were subjected to two different degradation methods usually applied to prove the existence of lignin, namely, cupric oxide oxidation and thioacidolysis. The reaction products obtained were typical aromatic derivatives of the natural lignin precursors coniferyl and sinapyl alcohols, and, in traces, of p-coumaryl alcohol, indicating the occurrence of lignin in the polymers from both Casparian strips and xylem vessels. The qualitative chemical compositions of the polymers from the two sources were similar, whereas the quantitative compositions were different, indicating that the molecular structure of the lignin polymer in the Casparian strips was different from that in the xylem vessels. Thus, for the first time, direct chemical evidence has been obtained that Casparian strips of C. miniata roots contain lignin as a major cell wall polymer.The author is indebted to Prof. Dr. G. Krohne (Zentrale Abteilung für Elektronenmikroskopie, Universität Würzburg, Germany) and to Prof. Dr. R. Guggenheim (Labor für Rasterelektronenmikroskopie, Universität Basel, Schweiz) for offering the opportunity for transmission-electron-microscopic and low-temperature scanning-electron-microscopic investigations, respectively. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.     

Endodermal cell-cell contact is required for the spatial control of Casparian band development in Arabidopsis thaliana

Background and Aims

Apoplasmic barriers in plants fulfil important roles such as the control of apoplasmic movement of substances and the protection against invasion of pathogens. The aim of this study was to describe the development of apoplasmic barriers (Casparian bands and suberin lamellae) in endodermal cells of Arabidopsis thaliana primary root and during lateral root initiation.

Methods

Modifications of the endodermal cell walls in roots of wild-type Landsberg erecta (Ler) and mutants with defective endodermal development – scarecrow-3 (scr-3) and shortroot (shr) – of A. thaliana plants were characterized by light, fluorescent, confocal laser scanning, transmission and cryo-scanning electron microscopy.

Key Results

In wild-type plant roots Casparian bands initiate at approx. 1600 µm from the root cap junction and suberin lamellae first appear on the inner primary cell walls at approx. 7000–8000 µm from the root apex in the region of developing lateral root primordia. When a single cell replaces a pair of endodermal and cortical cells in the scr-3 mutant, Casparian band-like material is deposited ectopically at the junction between this ‘cortical’ cell and adjacent pericycle cells. Shr mutant roots with an undeveloped endodermis deposit Casparian band-like material in patches in the middle lamellae of cells of the vascular cylinder. Endodermal cells in the vicinity of developing lateral root primordia develop suberin lamellae earlier, and these are thicker, compared wih the neighbouring endodermal cells. Protruding primordia are protected by an endodermal pocket covered by suberin lamellae.

Conclusions

The data suggest that endodermal cell–cell contact is required for the spatial control of Casparian band development. Additionally, the endodermal cells form a collet (collar) of short cells covered by a thick suberin layer at the base of lateral root, which may serve as a barrier constituting a ‘safety zone’ protecting the vascular cylinder against uncontrolled movement of water, solutes or various pathogens.     

Growth and Differentiation of Root Endodermis in Primula acaulis Jacq.

Adventitious roots of Primula acaulis Jacq. are characterized by broad cortex and narrow stele during the primary development. Secondary thickening of roots occurs through limited cambial growth together with secondary dilatation growth of the persisting cortex. Close to the root tip, at a distance of ca. 4 mm from the apex, Casparian bands (state I of endodermal development) within endodermal cells develop synchronously. During late, asynchronous deposition of suberin lamellae (state II of endodermal development), a positional effect is clearly expressed - suberization starts in the cells opposite to the phloem sectors of the vascular cylinder at a distance of 30 – 40 mm from the root tip. The formation of secondary walls in endodermis (state III of endodermal development) correlates with the beginning of secondary growth of the root at a distance of ca. 60 mm. Endodermis is the only cortical layer of primrose, where not only cell enlargement but also renewed cell division participate in the secondary dilatation growth. The original endodermal cells additionally divide anticlinally only once. Newly-formed radial walls acquire a typical endodermal character by forming Casparian bands and deposition of suberin lamellae. A network of endodermal Casparian bands of equal density develops during the root thickening by the tangential expansion of cells and by the formation of new radial walls with characteristic wall modifications. These data are important since little attention has been paid up till now to the density of endodermal network as a generally significant structural and functional trait of the root. This revised version was published online in July 2006 with corrections to the Cover Date.     

The development of the endoder mis andPhi layer of apple roots

Summary Suberin lamellae and a tertiary cellulose wall in endodermal cells are deposited much closer to the tip of apple roots than of annual roots. Casparian strips and lignified thickenings differentiate in the anticlinal walls of all endodermal andphi layer cells respectively, 4–5 mm from the root tip. 16 mm from the root tip and only in the endodermis opposite the phloem poles, suberin lamellae are laid down on the inner surface of the cell walls, followed 35 mm from the root tip by an additional cellulosic layer. Coincidentally with this last development, the suberin and cellulose layers detach from the outer tangential walls and the cytoplasm fragments. 85 mm from the root tip the xylem pole endodermis (50% of the endodermis) develops similarly, but does not collapse. 100–150 mm from the root tip, the surface colour of the root changes from white to brown, a phellogen develops from the pericycle and sloughing of the cortex begins. A few secondary xylem elements are visible at this stage.Plasmodesmata traverse the suberin and cellulose layers of the endodermis, but their greater frequency in the outer tangential and radial walls of thephi layer when compared with the endodermis suggests that this layer may regulate the inflow of water and nutrients to the stele.     

Review article. Apoplastic barriers in roots: chemical composition of endodermal and hypodermal cell walls

Based on the characterization of the chemical composition of endodermal and hypodermal cell walls isolated from seven monocotyledonous and three dicotyledonous plant species, a model of the composition of apoplastic barriers in roots is proposed. Depending on the species, endodermal and hypodermal cell walls of roots contained varying amounts of the biopolymers suberin, lignin, cell wall proteins, and carbohydrates. Although analysis of the chemical composition of these apoplastic barriers of roots is now possible, it is pointed out that conclusions from these data concerning the functional properties of these cell walls can not easily be drawn. However, in analogy to suberized periderms it is argued that the suberin should play a role in establishing an apoplastic transport barrier in roots, albeit not a perfect barrier. Furthermore, due to the combined occurrence of suberin, lignin and cell wall proteins it is argued that endodermal and hypodermal cell walls also have an important function as barriers towards pathogens. Finally, it is pointed out that additional experimental approaches combining the investigation of transport properties and of the chemical composition of apoplastic transport barriers in roots are necessary before the function of endodermal and hypodermal cell walls in roots can be fully understood.     

Chemical composition of apoplastic transport barriers in relation to radial hydraulic conductivity of corn roots (Zea mays L.)

The hydraulic conductivity of roots (Lp_r) of 6- to 8-d-old maize seedlings has been related to the chemical composition of apoplastic transport barriers in the endodermis and hypodermis (exodermis), and to the hydraulic conductivity of root cortical cells. Roots were cultivated in two different ways. When grown in aeroponic culture, they developed an exodermis (Casparian band in the hypodermal layer), which was missing in roots from hydroponics. The development of Casparian bands and suberin lamellae was observed by staining with berberin-aniline-blue and Sudan-III. The compositions of suberin and lignin were analyzed quantitatively and qualitatively after depolymerization (BF₃/methanol-transesterification, thioacidolysis) using gas chromatography/mass spectrometry. Root Lp_r was measured using the root pressure probe, and the hydraulic conductivity of cortical cells (Lp) using the cell pressure probe. Roots from the two cultivation methods differed significantly in (i) the Lp_r evaluated from hydrostatic relaxations (factor of 1.5), and (ii) the amounts of lignin and aliphatic suberin in the hypodermal layer of the apical root zone. Aliphatic suberin is thought to be the major reason for the hydrophobic properties of apoplastic barriers and for their relatively low permeability to water. No differences were found in the amounts of suberin in the hypodermal layers of basal root zones and in the endodermal layer. In order to verify that changes in root Lp_r were not caused by changes in hydraulic conductivity at the membrane level, cell Lp was measured as well. No differences were found in the Lp values of cells from roots cultivated by the two different methods. It was concluded that changes in the hydraulic conductivity of the apoplastic rather than of the cell-to-cell path were causing the observed changes in root Lp_r. Received: 17 March 1999 / Accepted: 22 June 1999     

Suberin lamella development in maize seedling roots grown in aerated and stagnant conditions

Hypoxia can stimulate the development of a suberized exodermis in aquatic plants; however, its influence on this aspect of terrestrial root development is sparsely documented. To determine the effects of hypoxia on maize (Zea mays cv. Seneca Horizon) roots, seedlings were grown in vermiculite (VERM), aerated hydroponics (AER), stagnant hydroponics with agar (STAG), or aerated hydroponics with agar (AERAG). The endo- and exodermis were examined for wall modifications. Lateral root emergence and aerenchyma formation were documented qualitatively. The endodermal Casparian band formation was unaffected by treatment. Endodermal and exodermal suberin lamella formation was earliest and most extensive in VERM. Suberization, especially in the exodermis of aerated treatments, was depressed in all hydroponic media. In comparison with AER, STAG exodermal lamellae were increased, but endodermal lamellae were decreased. Since the suberized exodermis forms a barrier to radial oxygen loss from roots to the medium, its stimulation in STAG roots (which also developed extensive aerenchyma) would help retain oxygen in the root. The reduction of endodermal lamellae should facilitate oxygen diffusion into the stele. Clearly, the response to environmental conditions is variable within individual cortical cell layers. Additionally, the observed patterns of lamellae, aerenchyma and lateral root development indicate a tight radial co-ordination of root development.     

The structure of barley roots in relation to the transport of ions into the stele

Summary Three successive states are recognized in the development of endodermal cells in seminal and nodal axes and primary lateral roots of barley: 1. Casparian bands in the anticlinal walls; 2. suberin lamella around the whole inner face of the wall; 3. unevenly deposited cellulosic wall thickening. These states develop asynchronously, the cells adjacent to the protoxylem pole cells always being last to mature. All cells have progressed to at least the secondary state by 32 cm from the tip in seminal axes, 48 cm from the tip in nodal axes, but only 6 cm from the tip in primary laterals. The asynchronous development gives the appearance of passage cells adjacent to the protoxylem pole cells, although all cells eventually attain the same state and degree of wall thickening. Long distance transport of calcium shows a close correlation with the incidence of suberin lamellae in the three types of root examined; it is suggested that formation of a suberin lamella effectively blocks calcium movement into the stele and, therefore, long distance transport. Plasmodesmata are present in comparable frequencies through both tangential and radial endodermal walls; they appear to maintain intercellular continuity until a late stage in development.     

植物凯氏带形成分子机制及功能特点的研究进展

凯氏带位于被子植物初生根内皮层细胞,环绕细胞1周,是与质膜紧密结合的非极性带状增厚结构。凯氏带作为植物根中离子径向运输障碍,调节离子的质外体吸收途径,迫使土壤中的离子通过内皮层细胞膜,选择性地进入中柱。凯氏带发现于1865年,但直至拟南芥凯氏带蛋白的发现和凯氏带阻滞作用物质基础被揭示,凯氏带的形成机理和功能才逐渐为人们所认知。凯氏带的物质基础为木质素,其形成需要由凯氏带蛋白和受体激酶组成的合成平台。细胞内部的木质素单体经ABCG载体运输到凯氏带的形成区,经ESB1dirigent蛋白、RBOHF氧化酶和PER64过氧化物酶等催化,合成木质素。该文对近年来国内外有关凯氏带形成的分子机制和功能特点方面的研究进展进行综述,为进一步理解和解析凯氏带的形成机理和功能提供参考。     

Casparian strips in needles are more solute permeable than endodermal transport barriers in roots of Pinus bungeana

The structure and development of endodermal Casparian strips in Pinus bungeana needles and roots were studied by scanning electron microscopy and fluorescence microscopy. Primary pit fields (PFs) frequently occurred in radial walls of Casparian strips isolated from needles, whereas PFs were never detected in Casparian strips from roots. Formation of Casparian strips in needles as well as roots started at the outer parts of the radial walls and they finally occupied the entire radial walls of the endodermis. Fourier transform infrared (FTIR) spectroscopy of Casparian strips isolated from roots revealed significant absorption bands characteristic for suberin. However, in Casparian strips of needles, evidence for suberin was rarely detected by FTIR spectroscopy. The apoplastic permeability of Casparian strips in needles and roots was probed by the apoplastic tracers calcofluor and berberine. Casparian strips in roots efficiently blocked the apoplastic transport (AT) of calcofluor and berberine. Casparian strips in needles blocked the AT of calcofluor, but diffusion of berberine was not inhibited and berberine thiocyanate crystals were detectable in the vascular tissue of the needles. From the data presented, it must be concluded that Casparian strips in needles, which are characterized by the absence of suberin, are more solute permeable compared with Casparian strips in roots.     

Fine structure of isolated and non-isolated potato tuber periderm

Cell walls of the periderm of native potato tuber (Solanum tuberosum L. cv. Primura) consist of a primary wall, a suberized secondary wall and a tertiary wall. With a mixture of pectinase and cellulase intact periderm membranes can be isolated. Isolation does not affect fine structure. It is suggested that the lignin in the middle lamellae and primary walls prevents the enzymes from digesting pectinaceous materials and cellulose. In specimens fixed with OsO₄, the suberized walls appear as alternating electrondense and electron-lucent lamellae. This lamellar architecture is not altered by extraction with chloroform. Therefore, the current view that the electronlucent lamellae consist of soluble lipids (waxes) can no longer be maintained. It is argued that the lamellation is a property of the suberin itself, and the suberized wall consists of alternating layers of suberins differing in polarity. A hypothesis of suberin assembly from sub-units is advanced and the subunits are shown for the first time.     

Casparian strips in needles of Pinus bungeana: isolation and chemical characterization

By using cell wall degrading enzymes, Casparian strips were for the first time isolated from Pinus bungeana needle endodermis. They appeared as a fine network, similar to those isolated from roots. Fourier transform infrared spectroscopic analysis provided evidence that the Casparian strips were impregnated with lignin, suberin, cellulose and cell wall proteins.     

Development of the Hypodermal Casparian Band in Corn and Onion Roots

A hypodermal Casparian band develops 40–50 mm from theroot tip in corn and 30–40 mm from the root tip in onion.In both plants, the endodermal Casparian band matures about20 mm closer to the root tip than the hypodermal Casparian band.Using the apoplastic fluorescent dye, Calcofluor white M2R (CFW),a permeability barrier could be distinguished in the radialwalls of the hypodermis 40–50 mm from the root tip incorn and onion. In progressively younger regions of the roots,CFW was first excluded from the outer tangential hypodermalwalls and the inner tangential epidermal walls, then the radialepidermal walls so that in very young regions only the outertangential epidermal walls were permeated. In contrast to CFW,the symplastic fluorescent dye, uranin, was translocated fromthe epidermis into the stele at all distances tested (5.0–50mm from the root tips). CFW and uranin at a concentration of0.01% proved nontoxic to corn and onion roots on the basis ofroot growth tests. Key words: Zea mays, Casparian band, Hypodermis, Allium cepa     

Root Endodermis and Exodermis: Structure, Function, and Responses to the Environment

Roots of virtually all vascular plants have an endodermis with a Casparian band, and the majority of angiosperm roots tested also have an exodermis with a Casparian band. Both the endodermis and exodermis may develop suberin lamellae and thick, tertiary walls. Each of these wall modifications has its own function(s). The endodermal Casparian band prevents the unimpeded movement of apoplastic substances into the stele and also prevents the backflow of ions that have moved into the stele symplastically and then were released into its apoplast. In roots with a mature exodermis, the barrier to apoplastic inflow of ions occurs near the root surface, but prevention of backflow of ions from the stele remains a function of the endodermis. The suberin lamellae protect against pathogen invasion and possibly root drying during times of stress. Tertiary walls of the endodermis and exodermis are believed to function in mechanical support of the root, but this idea remains to be tested. During stress, root growth rates decline, and the endodermis and exodermis develop closer to the root tip. In two cases, stress is known to induce the formation of an exodermis, and in several other cases to accelerate the development of both the exodermis and endodermis. The responses of the endodermis and exodermis to drought, exposure to moist air, flooding, salinity, ion deficiency, acidity, and mechanical impedance are discussed.