Root Hairs As A Model System For Studying Plant Cell Growth

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Annals of Botany 88: 1±7, 2001 doi:10.1006/anbo.2001.1430, available online at http://www.idealibrary.com on B OTA N I CA L B R I E F I N G Root Hairs as a Model System for Studying Plant Cell Growth J U L I A FO R E M A N and L I A M D O L A N * Department of Cell and Developmental Biology, John Innes Centre, Norwich, NR4 7UH, United Kingdom Received: 10 January 2001 Returned for revision: 16 February 2001 Accepted: 20 March 2001 Root hairs are tip-growing projections that form on specialized epidermal cells. Physiological studies are identifying key transporters required for hair growth, and drug studies have been instructive in de®ning the role of the cytoskeleton in cell morphogenesis. Genetic analysis is identifying proteins involved in cell growth and the phenotypes of the mutants are instructive in de®ning the precise function of these proteins in cellular morphogenesis. Recent progress in our understandings of cell growth using the arabidopsis root hair as a model system is reviewed. # 2001 Annals of Botany Company Key words: Arabidopsis, root hair, trichoblast, actin, microtubules, cell wall, genetics, calcium, potassium, phosphorus. I N T RO D U C T I O N Root hairs are tip-growing tubular-shaped outgrowths that emerge from the basal end of specialized cells called trichoblasts (Dolan et al., 1994; Fig. 1). They are responsible for anchorage of the root and the uptake of water and nutrients. During the formation of Rhizobium symbioses they are the site of interaction between the bacterium and the legume host (Oldroyd, 2001). Arabidopsis is well established as a model system for plant biology, and the recent publication of its entire genome (Kaul et al., 2000) is accelerating molecular-based research into this organism. The structure of the arabidopsis root is simple and invariant, and provides a useful model for studying plant development. The epidermal layer is the outermost layer of the mature root and consists of two cell types. Cells that overlie the anticlinal walls between adjacent cortical cells di€erentiate into trichoblasts (root hair producing cells). Cells that overlie the periclinal walls di€erentiate into atrichoblasts (non-root hair producing cells) (Dolan et al., 1993). Laser abalation experiments have shown that it is positional information, not cell lineage, that de®nes cell fate (Berger et al., 1998). The root tip consists of three distinct organizational zones (Fig. 2). The meristematic zone contains the initial cells and dividing cells of the root. Trichoblasts are morphologically distinct from atrichoblasts in this early developmental stage. Proximal to this zone is the elongation zoneÐit is at this stage that root hairs initiate. The next zone is the di€erentiation zone in which elongated cells mature into fully di€erentiated cells and where root hairs grow and reach maturity (Dolan et al., 1993). Observations using time-lapse video microscopy, cryo-SEM and light microscopy have identi®ed three structural phases of hair development. The ®rst stage is the appearance of a bulge at * For correspondence. Fax ‡44 (0) 1603 450022, e-mail liam. [email protected] 0305-7364/01/070001+07 $35.00/00 F I G . 1. Cryo-SEM image of an arabidopsis root hair. Tip-growing tubular-shaped outgrowths are produced from the basal end of trichoblast cells. the basal end of the trichoblast. During the second stage, slow tip growth occurs from a selected site on the bulge. The third stage, which begins when the hair is between 20 and 40 mm in length, involves an increase in the rate of hair growth to between 1 and 2 mm per minute (Dolan et al., 1994). The intracellular zonation of arabidopsis root hairs is characteristic of tip-growing cells. The cytoplasm at the extreme tip of the hair contains a large number of secretory vesicles containing cell wall components. A variety of vesicle types, including coated vesicles and pleiomorphic vesicles, are present at the tip, showing that this region of the cell is active in membrane tracking and exocytosis. Below this is an organelle-rich region containing large # 2001 Annals of Botany Company 2 Foreman and DolanÐArabidopsis Root Hairs their membranes are incorporated into the plasma membrane (Galway et al., 1997). Treatment of root hairs with the cellulose synthase inhibitor, 2,6-dichlorobenzonitrile (DCB), results in cell rupture at the weakest point of the cell, the tip. It is probable that DCB treatment results in the inhibition of cell wall synthesis at the tip while the protoplast of the cell continues to grow. To identify mutations in genes encoding proteins required for cell wall synthesis, mutants with phenotypes resembling the DCB-treated roots were identi®ed. kojak (kjk) mutants initiate root hairs that rupture soon after initiation (Fig. 3G±I). KJK encodes a member of the D subfamily of cellulose synthase-like (CSL) proteins (AtCSLD3). The KJK/AtCSLD3 protein is similar in size to several other plant cellulose synthase catalytic subunit proteins (CESA). The four highly conserved sub-domains of the plant CESA and the arabidopsis CSLD proteins that characterize the b-glycosyl transferases are present in KJK/ AtCSLD3. Consistent with this view is the ®nding that the KJK protein is located on the endoplasmic reticulum. This suggests that it is required for the synthesis of non-cellulosic wall polysaccharides. The Kjk ÿ phenotype and the pattern of expression suggest that KJK acts early in the process of root hair growth. This is the ®rst member of the AtCSLD subfamily of proteins to be shown to have a function in cell growth (Favery et al., 2001). I O N I C R E G UL AT I O N O F RO OT H A I R G ROW T H F I G . 2. Cryo-SEM image of the elongation and di€erentiation zones of an arabidopsis root. Root hairs begin to emerge at the proximal end of the elongation zone of the root. Tip growth and cell di€erentiation occur in the di€erentiation zone. numbers of Golgi bodies and mitochondria, as well as some vesicular bodies. These cell structures are involved in the biosynthesis and transport of macromolecules. The area between the apical and sub-apical regions contains smooth and ribosome-coated endoplasmic reticulum. The vacuole occupies the basal region of the hair, and grows in concert with the expansion of the cell. Once growth has ceased, the vacuole extends to the hair tip (Galway et al., 1997). T H E C E L L WA L L An integral aspect of plant cell growth is the expansion of the cell wall in concert with the protoplast. The cell wall is a complex network of cellulose, hemicelluloses, pectins and structural proteins. During cell growth, some cell wall components, such as cellulose, are synthesized at the plasma membrane whilst others, such as pectin and xyloglucan, are synthesized in the endomembrane system and transported to the cell surface in vesicles (Favery et al., 2001). At the very tip of the hair, exocytosis of vesicles containing cell wall material causes the release of their contents into the matrix of the developing cell wall and It is well established that an internal hydrostatic pressure (turgor) is required for cellular expansion (Cosgrove, 1986). The plasma membrane H ‡ -ATPase has been shown to play a key role in the generation of turgor in root hairs by generating an electrochemical H ‡ gradient that drives the uptake of ions (Lew, 1991). There is also evidence that the H ‡ -ATPase causes acidi®cation of the cell wall, leading to cell wall loosening and thus allowing growth to occur (Taiz, 1984). In arabidopsis root hairs, it has been shown that a local acidi®cation of the cell wall at the site of the initial bulge is required for root hair initiation. The acidi®cation of the cell wall coincides with a localized cytoplasmic alkalization. This suggests that the acidi®cation of the wall is due to the movement of protons out of the cell and into the wall at the hair tip. However, this alkalization per se is not required for growth since dissipation of the alkaline region by treatment with weak acids such as propionic acid or butyric acid does not inhibit root hair initiation (Bibikova et al., 1998). This suggests that alkalization is merely the result of local depletion of protons due to the activity of the H ‡ -ATPase. Potassium Potassium is the major osmotically active cation in most plant cells. A net inward potassium current has been shown to exist in growing root hairs that is sucient to drive cellular expansion of the root hair (Lew, 1991). However, it has long been established that two K ‡ uptake mechanisms with distinct kinetic parameters reside in parallel on the Foreman and DolanÐArabidopsis Root Hairs 3 F I G . 3. Morphology of root hair mutants. A±I. Stereomicroscope images of arabidopsis root hair mutants. Bars ˆ 100 mm. H±I, Cryo-SEM images of kjk mutants. A, rhd1 mutantÐhairs have a wide bulbous swelling at the base. B, rhd2 mutantÐroot hair initiation occurs but tip growth is arrested, causing a stubby hair phenotype. C, rhd6 mutants have fewer root hairs than the wild-type. D, Wild-type. E, trh1 mutantÐ root hairs do not elongate causing a short hair phenotype. F, tip1 mutantÐhairs are initiated from a large bulge on the epidermal cell, from which one to four branches emerge, some of which undergo further branching. G, kjk mutantÐroot hairs initiate but rupture causing the trichoblast cells to die. H, Scanning electron micrograph of kjk root hairs soon after rupture. I, SEM of a kjk root hair after rupture. plasma membrane of roots (Maathuis and Sanders, 1993). One of these is a low anity pathway that has the characteristics of a channel-mediated transporter. The other is a high anity pathway that moves K ‡ into the cytosol against its electrochemical gradient. Studies into the mechanism of the high anity transport system by patchclamping root protoplasts suggest that it is independent of ATP and imply an H ‡ -coupled K ‡ transport with a ratio of 1 H ‡ : 1 K ‡ (Maathuis and Sanders, 1994). Furthermore, there is molecular evidence for at least six potassium transport systems in the arabidopsis root, AtKT, AKT1, SKOR1, AtKUP1, AtKT2 and AtHKT1 (Debrosses et al., 2001). Mutants with defects in potassium transport are providing insight into the role of potassium during root hair elongation. tiny root hair (trh1) mutants form short root hairs and frequently initiate more than one root hair per trichoblast (Fig. 3E). trh1 encodes a member of the AtKT (Arabidopsis thailana K ‡ transport) family of potassium carriers, previously designated AtKUP4 or AtKT2. The trh1 phenotype cannot be suppressed by high external KCl concentrations indicating an absolute requirement for TRH1/AtKT2 in root hair growth. Another potassium channel present in the arabidopsis root is the inward rectifying potassium channel AKT1. akt1 mutants develop longer root hairs than the wild-type when grown in the absence of external potassium, but develop shorter root hairs than the wild-type when grown in potassium concentrations above 10 mM (Debrosses et al., 2001). This suggests that TRH1/AtKT2 and AKT1 have di€erent functions in the growing hair cell. Calcium Calcium in¯ux through plasma membrane calcium channels is required for normal root-hair tip growth (Schiefelbein et al., 1992). Patch-clamping studies of root hair apices have recently identi®ed hyperpolarizationactivated calcium channels that are most active at the apex of root hairs. It is suggested that these channels are involved in the apical in¯ux of growing root hairs (VeÂry and Davies, 2000). A localized increase in the cytoplasmic calcium concentration, [Ca2‡ ]c , is formed as soon as hair growth initiates. root hair defective2 (rhd2) mutants have short root hairs (Fig. 3B) and lack the localized elevation in [Ca2‡ ]c at the hair tip (Wymer et al., 1997). This suggests that RHD2 activity is required either directly or indirectly to transport Ca2‡ from outside the cell into the cytoplasm at the hair tip. Evidence suggests Ca2‡ is not the primary factor in determining the direction of elongation, since arti®cially created [Ca2‡ ]c gradients and mechanical 4 Foreman and DolanÐArabidopsis Root Hairs obstacles only cause transient re-orientation of growth (Bibikova et al., 1997). T H E C Y TO S K E L E TO N The plant cytoskeleton consists of two components, actin micro®laments and microtubules. Together they form a dynamic cytoplasmic network that is involved with many aspects of cell growth and polarity. The cytoskeleton stabilizes structural features and is involved with cellular morphogenesis through controlling the directionality of growth. Axial bundles of ®lamentous actin are present in the root hairs of most plant species. The bundles are present throughout the hair and penetrate the apical dome (Lloyd et al., 1987). Actin ®laments (AFs) are composed of identical actin monomers (G-actin) arranged in a polar helical polymer. They play a key role as tracks for intracellular transport of organelles and vesicles and are involved in cytoplasmic streaming. The direction of cytoplasmic streaming is determined in part by the polarity of the AFs. In root hairs of the aquatic monocotyledon Hydrocharis, AF bundles are longitudinally orientated in the transvacuolar strand and the sub-cortical region. The AFs in each bundle are of uniform polarity and actin bundles in the transvacuolar strand and the sub-cortical region have opposite polarities. Since the polarity of the AFs determines the direction of cytoplasmic streaming, cytoplasmic streaming moves basipetally in the transvacuolar strand and is acropetal in the sub-cortical region. This suggests organelles and vesicles move towards the tip in the transvacuolar strand and away from the tip in the subcortical region (Tominaga et al., 2000). Consistent with this view is the observation that actin ®laments have their barbed ( fast growing) ends oriented towards the tip in the transvacuolar strands and are arranged with the pointed ends to the tip in the sub-cortical regions (Tominaga et al., 2000). The ®mbrin/villin family of proteins is involved in the bundling of actin ®laments with uniform polarity in animal cells. A 135 kDa actin bundling protein (135-ABP) from Hydrocharis, homologous to the arabidopsis actin bundling protein villin, has been shown to co-localize with AF bundles of Hydrocharis. 135-ABP is involved in the bundling of actin ®laments in vivo since microinjection of antiserum against 135-ABP into living root hair cells causes the transvacuolar strand to disappear. The thick ®lamentous actin bundles in the transvacuolar strand disperse into thin bundles that are displaced to the sub-cortical region. Displacement of the thin AF bundles suggests that a minimum thickness of actin bundles is required to maintain the structure of the transvacuolar strand (Tominaga et al., 2000). A role for actin micro®laments in the growth of root hairs has been shown using drugs that depolymerize actin micro®laments. The application of the actin antagonist latrunculin B causes inhibition of growth rates in arabidopsis and maize root hairs (Bibikova et al., 1999; Baluska et al., 2000). A complex microtubular network has been shown to be present in the root hairs of many species. Microtubules are composed of tubulin and are arranged in ®lamentous structures consisting of 13 proto®laments in a cylindrical array. Two networks of microtubules exist in root hairs: plasma membrane associated microtubules run from tip to base; internal bundles of microtubules are present in the cytoplasmic strands between the nucleus and the hair tip. These two systems are continuous, the endoplasmic bundles of microtubules branch within the apical dome and assume a cortical location (Lloyd et al., 1987). Using microtubule disrupting drugs, the microtubule cytoskeleton has been shown to play a role in controlling the directionality of root hair growth and morphology. The application of low levels (51 mM) of taxol, a microtubule-stabilizing drug, or oryzalin, a microtubule-depolymerizing drug, causes a waving of the root hair as it elongates. Increased levels (10 mM) of taxol or oryzalin cause root hairs to form several elongating branches on a single root hair. Growth rates are not a€ected by these drugs, suggesting an intact microtubule cytoskeleton is not required for tip growth but is required to maintain growth polarity and to stabilize a single growth point (Bibikova et al., 1999). Microtubulestabilizing and -depolymerizing drugs have similar e€ects, suggesting that it is microtubule dynamics that are important for controlling the directionality of growth. There is evidence that microtubules are involved in restricting the movement of growth machinery to the apex of an elongating root hair by restricting the movement of the tip-focused [Ca2‡ ]c gradient. In root hairs, an arti®cial [Ca2‡ ]c causes transient reorientation of growth only if applied across the apical, cytoplasm-rich region of the tip where growth is already occurring. In root hairs in which microtubules have been stabilized by treatment with taxol, a locally induced elevated [Ca2‡ ]c is sucient to form a new growth point at the site of the new gradient, even when this gradient is generated in the vacuolar region of the hair more than 10 mm away from the established tip-growing point (Bibikova et al., 1999). This suggests that microtubule dynamics are required to limit growth to a single point and to limit the active region of growth. Furthermore, root hairs show only a transient reorientation of growth and the [Ca2‡ ]c gradient in response to touch. Taxol-treated root hairs show redirected growth for an extended time in response to touch and a new growth point forms further away from the site of touch stimulation (Bibikova et al., 1999). This suggests that microtubules are involved in controlling the direction of growth. NUTRIENT DEFICIENCIES Phosphorus de®ciency substantially increases root hair length by increasing the growth rate and duration of hair growth. This response is local and occurs in response to phosphorus availability in the immediate environment of the root and is not related to the overall phosphorus status of the plant (Bates and Lynch, 1996). The greater length of root hairs is likely to increase phosphorus uptake since hair length is positively correlated with phosphorus uptake in a low phosphorus environment. For example, wild-type plants can acquire more phosphorus under limiting conditions than a short root hair mutant (rhd2; Fig. 3B) Foreman and DolanÐArabidopsis Root Hairs or a mutant with few root hairs (rhd6; Fig. 3C) (Bates and Lynch, 2000). There is also evidence that low phosphorus stress and high auxin concentrations elicit similar responses (Bates and Lynch, 1996). G E N E T I C A N A LY S I S O F RO OT H A I R G ROW T H Genetic analysis of root hair growth has led to the identi®cation of key proteins involved in di€erent aspects of cell elongation. Mutants with no root hairs de®ne genes involved in the earliest stages of hair outgrowth. rhl1, rhl2 and rhl3 have similar phenotypes: mutants have few hairs on their primary root. The outer epidermal walls of the mutants are perfectly smooth, with no sign of root hair initiation. All three mutants have pleiotropic phenotypes and plants are extreme dwarfs (Schneider et al., 1997). RHL1 encodes a small hydrophilic protein containing a nuclear localization signal, which targets the protein to the nucleus (Schneider et al., 1998). Its precise function in cell growth is unclear at present. RHD6 is involved in the assembly of the cellular components at the site of root hair initiation. rhd6 mutants have fewer hairs than wild-type roots, exhibit a basal shift of root hair emergence, and often have multiple root hairs originating from a single cell (Fig. 3C). However, the rhd6 mutation does not a€ect the growth of the root hairs: the few root hairs that form on rhd6 mutants are normal in all respects. The phenotype suggests that RHD6 is active at an early stage of root hair development and is not required later, during the growth of the root hair (Masucci and Schiefelbein, 1994). Double mutant analysis is consistent with this view, suggesting RHD6 acts before the genes required for root hair growth (Parker et al., 2000). The ROOT HAIR DEVELOPMENT1 (RHD1) protein is required for correct expansion of the hair cell. rhd1 mutant root hairs are similar in length to those of the wildtype, but the hairs have a wide bulbous region at their base (Fig. 3A). In some cases the entire epidermal wall is forced outward to form the basal portion of the hair. It is thought that the RHD1 gene product is involved in regulating the degree of epidermal cell wall loosening during root hair initiation (Schiefelbein and Somerville, 1990). A group of mutants has been identi®ed that have very short root hair phenotypes, similar to kjk, although hair tip rupture has not been reported for any of these mutants. rhd2, shaven1 (shv1), shv2 and shv3 mutant root hairs begin to emerge from the epidermal cells, but do not elongate, causing a `stubby' hair phenotype (Schiefelbein and Somerville, 1990; Parker et al., 2000). The phenotype suggests that the genes de®ned by these mutations are needed for the establishment of tip growth (Parker et al., 2000). rhd3 mutants have short root hairs that have a wavy appearance and are occasionally branched. Rather than elongating in a single direction, perpendicular to the root axis, the hairs elongate in a corkscrew fashion (Schiefelbein and Somerville, 1990). Experiments monitoring root hair growth by microbead labelling revealed that the wavy appearance of the hairs is due to irregular changes in the direction of root hair growth associated with di€erential 5 expansion about the root hair tip. The average volume of rhd3 root hairs is less than one-third that of wild-type hairs, suggesting that the RHD3 mutation is required for cell expansion. The vacuole is also smaller in rhd3 mutants than in wild-type plants. The RHD3 gene product is a novel 89 kD polypeptide containing a putative GTP-binding motif near its amino terminus (Wang et al., 1997). The biochemical function of the protein is unknown. rhd4 mutants produce short root hairs. The hairs vary in diameter along their length, forming bulges and constrictions during the elongation process. Occasionally root hairs are also branched. It has been suggested that the RHD4 protein is involved in cell wall deposition at the root hair tip (Schiefelbein and Somerville, 1990). Treatment of root hairs with taxol and oryzalin results in the formation of root hairs with branches and occasional swellings. A similar phenotype is found in can of worms1 (cow1) and tip growth defective 1 (tip1) mutants, suggesting that the respective wild-type genes may encode proteins involved in microtubule-mediated activities. cow1 mutants have shorter, wider root hairs than the wild-type, and some root hairs emerge from the same initiation site (Grierson et al., 1997). tip1 mutants also have short root hairs, approx. one-tenth the length of wild-type hairs (Fig. 3F) (Schiefelbein et al., 1993). Hairs are initiated from a large bulge on the epidermal cells, from which one to four branches emerge. Some of these branches undergo further branching. Tip growth of pollen tubes is also defective in tip1 mutants indicating that its activity is also required during tip growth in this cell type (Schiefelbein et al., 1993; Ryan et al., 1998). Several other mutants that have short root hairs and a range of di€erent shapes have been identi®ed: bristled1 (bst1), centipede1 (cen1), cen2, cen3 and supercentipede1 (scn1). These genes are required to control the shape of the root hair during tip growth. All of these mutants, with the exception of cen1, also produce multiple hairs, suggesting that these genes have a role in restricting the number of root hairs produced by tip growth from each hair-forming site (Parker et al., 2000). A large double mutant analysis has suggested that there is a complicated genetic network controlling root hair formation, with many of the genes having several functions (Parker et al., 2000). Few double mutant combinations displayed clear epistasis, suggesting that many of the genes act in parallel or independently of each other. Six epistatic interactions have been described for root hair morphogenesis genes. Double mutant analysis suggests that RHD6 acts before RHD2 and SHV1; RHD2 acts before COW1; COW1 acts before RHD4; and SHV1 acts before SHV2 and SHV3. In this genetic-based study, four main groups of genes were assigned to subsequent stages of root hair growth. The ®rst group of genes is those required for the beginning of root hair formation, this includes SHV3, CEN2, RHD3, SCN1 and TIP1 (Fig. 4). The main evidence for these genes acting at this stage is the synergistic e€ects between pairs of these genes that prevent root hair formation. The second group of genes is those required for swelling formation. There are only two genes in this class, TIP1 and RHD1 (Fig. 4). There is evidence that RHD1 and TIP1 act in parallel, since the rhd1 : tip1 double 6 Foreman and DolanÐArabidopsis Root Hairs TIP GROWTH TIP1, SCN1, COW1, RHD3, CEN1, CEN2, CEN3, BST1 TRANSITION TO TIP GROWTH RHD2, SHV1, SHV2, SHV3, TIP1, BST1, RHD3, CEN1, CEN2, CEN3, SCN1 SWELLING FORMATION TIP1, RHD1 HAIR INITIATION SHV3, CEN2, RHD3, SCN1, TIP1 F I G . 4. SEM image of the di€erentiation zone of an arabidopsis root showing the four groups of root hair morphogenesis genes, as classi®ed by Parker et al. (2000). Double mutant analysis has suggested that there is a complicated genetic network controlling root hair formation, with many genes having several functions. mutant does not have the characteristic increased swelling conferred by the tip1 mutation. Double mutant combinations with RHD1 and other root hair morphogenesis genes have shown additive interactions, suggesting RHD1 acts in parallel with these genes. The third class of genes is those required for the transition from swelling formation to tip growth. There are 11 genes in this class: RHD2, SHV1, SHV2, SHV3, TIP1, BST1, RHD3, CEN1, CEN2, CEN3 and SCN1 (Fig. 4). Two epistatic interactions have been identi®ed in this class of genes, suggesting that SHV2 acts after SHV1, and that SHV3 acts after RHD2. Other interactions are additive, suggesting that the genes work independently. The fourth class of genes is those required for root hair elongation by tip growth. This group of genes includes TIP1, SCN1, COW1, RHD3, CEN1, CEN2, CEN3 and BST1 (Fig. 4). The only epistatic interaction is the previously reported interaction between COW1 and RHD4, implying COW1 acts before RHD4. This suggests that most genes involved with tip growth act at least partly independently (Parker et al., 2000). This study has led to a complex model of the genetic contributions to root hair development, which will act as a working model for subsequent research. CO N C L U S I O N S The large range of arabidopsis root mutants are providing insights into the physiology of root hair growth and revealing the genetic basis of root hair morphogenesis. Although many are not yet cloned, they will provide an overwhelming wealth of information in the near future. Current research is bringing together physiological, cellular and genetically based studies of root hair growth to provide an extensive knowledge of how plant cells grow. AC K N OW L E D G E M E N T S J.F. is funded by the BBSRC on a Quota studentship. L.D. acknowledges receipt of funding from the BBSRC, European Union and the Gatsby Foundation. We are grateful to Kim Findlay and Paul Linstead for images in Figs 1, 2 and 4. We thank Georg Seifert for detailed discussions and suggestions that have immensely improved our manuscript. L I T E R AT U R E C I T E D Baluska F, Salaj J, Mathur J, Braun M, Jasper F, Samaj J, Chua NH, Barlow PW, Volkmann D. 2000. 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