Journal of Experimental Botany

This Article Abstract FREE Full Text (PDF) E-letters: Submit a response Alert me when this article is cited Alert me when E-letters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (58) Request Permissions Disclaimer Google Scholar Articles by White, P. J. Search for Related Content PubMed PubMed Citation Articles by White, P. J. Agricola Articles by White, P. J. Social Bookmarking          What's this?

Journal of Experimental Botany, Vol. 52, No. 358, pp. 891-899, May 1, 2001
© 2001 Oxford University Press

Opinion Papers

The pathways of calcium movement to the xylem

Philip J. White¹

Department of Plant Genetics and Biotechnology, Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK

Received 31 July 2000; Accepted 1 December 2000

Abstract

Calcium is an essential plant nutrient. It is acquired from the soil solution by the root system and translocated to the shoot via the xylem. The root must balance the delivery of calcium to the xylem with the need for individual root cells to use [Ca²⁺]_cyt for intracellular signalling. Here the evidence for the current hypothesis, that Ca²⁺ travels apoplastically across the root to the Casparian band which it then circumvents via the cytoplasm of the endodermal cell, is critically reviewed. It is noted that, although Ca²⁺ channels and Ca²⁺-ATPases are present and could catalyse Ca²⁺ influx and efflux across the plasma membrane of endodermal cells, their transport capacity is unlikely to be sufficient for xylem loading. Furthermore, there seems to be no competition, or interactions, between Ca²⁺, Ba²⁺ and Sr²⁺ for transport to the shoot. This seems incompatible with a symplastic pathway involving at least two protein-catalysed transport steps. Thus, a quantity of purely apoplastic Ca²⁺ transport to the xylem is indicated. The relative contributions of these two pathways to the delivery of Ca²⁺ to the xylem are unknown. However, the functional separation of symplastic Ca²⁺fluxes (for root nutrition and cell signalling) and apoplastic Ca²⁺ fluxes (for transfer to the shoot) would enable the root to fulfil the demand of the shoot for calcium without compromising intracellular [Ca²⁺]_cyt signals. This is also compatible with the observed correlation between transpiration rate and calcium delivery to the shoot.

Key words: Arabidopsis thaliana, barium, calcium, Casparian band, endodermis, strontium.

Introduction

Calcium (Ca) is an essential plant nutrient. It fulfils a variety of structural roles, and functions as counter-cation for inorganic and organic anions in the vacuole (Marschner, 1995). In addition, the cation (Ca²⁺) is essential for cell division and expansion, and explicit perturbations in cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) link specific environmental or developmental stimuli to their appropriate physiological responses (White, 1998, 2000). Calcium is required in large amounts, and the Ca content of healthy plant tissues generally exceeds 0.1–1% dry matter. Dicotyledeonous plants require more Ca in their tissues than do monocotyledonous plants (Islam et al., 1987), which has been attributed to a larger cation exchange capacity of their cell walls (Kirkby and Pilbeam, 1984).

Plants acquire Ca primarily from the soil solution through the root system, and the shoot is supplied via the xylem. In an actively growing plant the Ca flux to the xylem is high. For example, a cereal seedling with a shoot/root FW ratio of 2 and a relative growth rate of 0.2 d^-1 must translocate Ca at rates approximating 40 nmol h^-1 g^-1 FW root to maintain its shoot Ca content at 0.1% dry weight (White, 1998). The root must balance this delivery of Ca to the xylem with the necessity for individual root cells to use [Ca²⁺]_cyt for intracellular signalling (White, 1998; Kiegle et al., 2000b). A low (submicromolar) [Ca²⁺]_cyt must be maintained in all root cells and [Ca²⁺]_cyt must increase rapidly when challenged by appropriate stimuli. These properties may be compromised by high nutritional Ca²⁺ fluxes through root cells unless they can be controlled exquisitely. In this article the possible mechanisms by which the root might effect xylem Ca loading, without compromising cell signalling, are discussed. A particular emphasis is placed on the pathways of Ca²⁺ movement through the root to the xylem.

Root anatomy and the structure and development of the endodermis

The root is composed of many different cell types. Each cell type appears specialized to specific tasks (Clarkson, 1991; Maathuis et al., 1998). It is commonly suggested that the outer cell layers of the root, the epidermis and cortex, are involved primarily in the acquisition of water and mineral nutrients. These are separated from the cells of the stele, which are devoted to the loading, unloading and long-distance transport of solutes in the xylem and phloem, by the endodermal cell layer (Fig. 1). The part of the root outside the plasma membrane is termed the apoplast. This includes cell walls, intercellular spaces and the lumena of tracheary elements. The interconnected cytoplasm of root cells is termed the symplast. In principle, solutes may reach the xylem either symplastically, by entering root cells and moving from cell to cell through plasmodesmata, or apoplastically, without traversing a single plasma membrane. However, in the mature root, an hydraulic separation of cortical and stelar apoplasts is imposed by the presence of an extracellular structure termed the Casparian band. This structure, which is present on the anticlinal (transverse and radial longitudinal) walls of endodermal cells, is present in all roots of vascular plants and is thought to act as a barrier to the apoplastic movement of solutes. (There appears to be a single exception to this statement. An endodermal Casparian band was not found in the roots of Lycopodium: Damus et al., 1997.) A Casparian band of some description is present throughout the root, including the regions that are considered most permeable to water and solutes, except for a few millimetres at the extreme root tip.

View larger version (128K): [in this window] [in a new window]   Fig. 1. Schematic views of the development of the endodermis and its hypothetical impact on the pathway of calcium movement to the xylem (Clarkson, 1993). Initially, the Casparian band develops as a modification of the anticlinal (transverse and radial longitudinal) walls between endodermal cells (State I). This is assumed to block apoplastic (cell wall) movement of calcium to the xylem, and to force the calcium destined for the xylem to enter the root symplast, through Ca2+ channels in the accessible plasma membrane of the endodermal cell. It is then assumed that this symplastic Ca2+ is pumped into the stelar apoplast by Ca2+ ATPases in the plasma membranes of cells within the stele. Subsequently, layers of aliphatic suberin polymers are deposited over the entire inner surface of the walls surrounding endodermal cells (State II), and the endodermal cell becomes buried by the deposition of tertiary cell walls (State III). The deposition of suberin lamellae isolates the endodermal cell protoplast from the apoplast, preventing Ca2+ influx to endodermal cells and, thereby, calcium transport to the xylem.

 
The endodermis develops in three stages (Fig. 1; Esau, 1965; Clarkson and Robards, 1975; Peterson and Cholewa, 1998; Schreiber et al., 1999). Initially, a Casparian band is deposited within the transverse and radial longitudinal walls of the endodermal cells (State I), which becomes firmly attached to the plasma membrane of these cells. The Casparian band is composed primarily of lignin, but also contains some suberin (Schreiber et al., 1999). All cells in the endodermis develop a Casparian band approximately synchronously, following cell elongation and coincident with (or slightly prior to) the maturation of the protoxylem (Esau, 1965; Peterson and Lefcourt, 1990). At a variable distance from the apex, layers of aliphatic suberin polymers are deposited over the entire inner surface of the walls surrounding endodermal cells (State II). This event frequently coincides with the emergence of lateral roots. The layers of suberin create a lamellar structure that isolates the endodermal cell protoplast from the apoplast and subsequently becomes buried by the deposition of lignified, carbohydrate cell walls (State III). These latter stages in the development of the endodermis proceed more rapidly in cells near the phloem compared to those near the xylem and ‘passage cells’, which remain in State I whilst those around them proceed to State III, can frequently be observed. It is speculated that passage cells are important in the transfer of ions to the stele in mature roots (Peterson and Enstone, 1996), because they become the only cells to expose a plasma membrane to the external solution (and therefore capable of ion uptake) when the epidermis and cortex die. The presence of suberin lamellae is thought to prevent endodermal cells from taking up solutes from the apoplast, but it is not thought to influence solute movement within the apoplast (Peterson and Cholewa, 1998).

A symplastic pathway for calcium movement to the xylem

The immediate and substantial drop in root pressure upon injury of the endodermis indicates that the Casparian band is the main barrier to the radial movement of ions across the root (Peterson et al., 1993; Steudle et al., 1993). Further, it has been proposed that Ca²⁺ cannot enter the stele via the apoplast when the Casparian band is present (Clarkson and Robards, 1975; Clarkson, 1984, 1991, 1993; Marschner, 1995). This view is based on (i) the hydrophobic chemistry of the Casparian band, (ii) the rapid penetration of divalent cations (such as Ca²⁺ and Sr²⁺) and of cell impermeant tracers (UO²⁺₂, La³⁺, hydrophilic dyes) into the cortex but not the stele of plant roots and (iii) the observation that Ca²⁺ delivery to the xylem is increased in regions of the root where the Casparian band is discontinuous or ruptured.

Calcium delivery to the xylem is maximal in the apical zone of the root (Fig. 2), which comprises both immature and recently-matured root cells. In this region, endodermal cells possess a Casparian band (State I). Although there is no direct evidence that either lignin or suberin are effective barriers to the movement of water and dissolved solutes (Schrieber et al., 1999), it is frequently assumed that these Casparian bands are impermeable to Ca²⁺ and, therefore, that Ca²⁺ must bypass the Casparian band symplastically prior to its delivery into the xylem (Fig. 1). It is further assumed that, since Ca²⁺ mobility in the symplast is restricted by low [Ca²⁺]_cyt, most radial movement of Ca across the root will occur through the apoplast up to the Casparian band. However, it should be noted that Ca²⁺ is only a small fraction of the total cytoplasmic Ca pool and Ca chelates might move symplastically. Since the shortest symplastic route that circumvents the Casparian band is through the cytoplasm of the endodermal cell, it is argued that Ca²⁺ will utilize this pathway to minimize any compromise of [Ca²⁺]_cyt signalling mechanisms (Clarkson, 1984, 1993).

View larger version (19K): [in this window] [in a new window]   Fig. 2. The profile of calcium translocation to the shoot along the seminal root of marrow (Cucurbita pepo L.), and its relationship with the development of the endodermis. Calcium translocation is maximal in the root apical zone, where the endodermal cells possess a Casparian band (State I endodermis) and xylem vessels first become fully conductive, and declines as the endodermal cells become increasingly suberised (State II endodermis). Results were obtained by enclosing short (3.5 mm) sections of seminal roots in flow cells containing 0.15 mM Ca2+ labelled with carrier-free 85Sr for 6 h (Harrison-Murray and Clarkson, 1973).

 
Upon the deposition of suberin lamellae in the endodermis (State II and III), Ca delivery to the xylem is severely restricted, even though the endodermal cells are still symplastically connected to the cortex and stele by functional plasmodesmata. This appears to confirm the absence of a symplastic pathway for Ca movement between the cortex and stele. The observation that Ca movement to the xylem decreases as endodermal cells become inaccessible from the apoplast is held to support the view that Ca destined for the xylem enters the symplast exclusively through endodermal cells.

In basal regions of the root some Ca may reach the xylem through the apoplast as lateral roots emerge. During lateral root formation, the endodermal cells divide and the newly formed radial walls lack a Casparian band (Peterson and Lefcourt, 1990). However, Casparian bands are deposited at the base of lateral root primordia and rapidly become continuous with those of the endodermis of the parent root. Similarly, although the endodermal Casparian band may lose its function as an apoplastic barrier when roots undergo secondary (thickening) growth, this occurs only after it is replaced by the periderm (Esau, 1965; Weerdenburg and Peterson, 1984).

In the roots of many plant species, particularly when grown in soil, an exodermis (hypodermis) develops that has both Casparian bands and suberin lamellae with chemical compositions and properties similar to those of the endodermis (Perumalla et al., 1990; Peterson and Perumalla, 1990; Damus et al., 1997; Schreiber et al., 1999). However, the integrity of the exodermal Casparian band is disrupted by the emergence of lateral roots, and its development is patchy, absolutely dependent upon environmental conditions (it does not form readily in plants grown hydroponically) and frequently subsequent to the suberization of the endodermis (Enstone and Peterson, 1992, 1997, 1998). Thus, in most cases, a complete Casparian band in the exodermis will be limited to regions where Ca delivery to the xylem would be minimal even in its absence, and so this structure is unlikely to influence Ca movement to the xylem. Consistent with this view, Zimmermann and Steudle (1998) observed that the transport to the xylem of the apoplastic dye PTS (trisodium 3-hydroxy-5,8,10-pyrene-trisulphonate) was not affected by the presence of an exodermal Casparian band.

A scheme for Ca delivery to the xylem based on exclusive Ca²⁺ transport through endodermal cells is found in many textbooks (Clarkson, 1991; Marschner, 1995). In this scheme, Ca²⁺ is considered to travel apoplastically across the root to the Casparian band. To circumvent the Casparian band, it is proposed that Ca²⁺ enters the cytoplasm of the endodermal cell through Ca²⁺-permeable channels. It is then actively effluxed from the symplast by the plasma membrane Ca²⁺-ATPases or Ca²⁺/H⁺ antiporters of cells within the stele. In theory, the necessity for Ca²⁺ to traverse the plasma membrane of endodermal cells would allow the root to control the rate and selectivity of Ca transport to the shoot. The selectivity of the apoplastic route, which is determined by the ion exchange properties of the cell wall, is minimal. However, by diverting the cation flow through the cell, transport proteins in the cell membranes can selectively catalyse the movement of Ca²⁺ to the xylem (Clarkson, 1991). This could benefit the plant by restricting the movement of toxic cations to the shoot. However, several lines of evidence suggest that this simple scheme might be incorrect. In the next section the thermodynamic and kinetic feasibility of the symplastic transport pathway will be assessed. Then the apparent absence of selectivity or competition between divalent cations for transport to the shoot (both of which are characteristics of protein-mediated transport pathways) will be considered.

The thermodynamic and kinetic feasibility of delivering Ca²⁺ to the xylem exclusively through endodermal cells

The scheme for Ca delivery to the xylem based on exclusive transport through endodermal cells requires the presence of sufficient plasma membrane Ca²⁺ channels on the cortical side and sufficient Ca²⁺ efflux transporters on the stelar side of the Casparian band to catalyse the observed Ca²⁺ flux to the xylem.

There is a considerable electrochemical gradient driving Ca²⁺ influx through Ca²⁺ channels into all root cells (White, 1998; Kiegle et al., 2000b). Several types of Ca²⁺-permeable channels have been recorded in the plasma membrane of root cells. These include depolarization-activated Ca²⁺ channels (White, 1998, 2000), hyperpolarization-activated Ca²⁺ channels (Kiegle et al., 2000a; Véry and Davies, 2000), voltage-insensitive cation (VIC) channels (Davenport and Tester, 2000), and outward-rectifying cation (KORC or NORC) channels (De Boer, 1999). In addition, it is likely that both mechanosensitive (stretch-activated) and second messenger-activated Ca²⁺ channels are also present (White, 1998; Leng et al., 1999). All these channels have been attributed roles in intracellular signalling. Since plasma membrane depolarization is a non-specific response to many stimuli, depolarization-activated Ca²⁺channels are thought to generate a universal signal priming plant cells for an immediate response (White, 1998). Hyperpolarization-activated Ca²⁺ channels are necessary for Ca²⁺-influx to (hyperpolarized) elongating root cells, such as cells in the elongation zone and root hair cells, and may determine the rate and direction of root cell elongation (Kiegle et al., 2000a; Véry and Davies, 2000). Mechanosensitive channels are thought to play a role in the regulation of turgor, and in determining the allometry of cell expansion and morphogenesis (White, 1998). The autoregulatory properties ensuing from Ca²⁺ influx through KORC and NORC has suggested the control of non-specific ion efflux from root cells (De Boer, 1999). Second messenger-activated Ca²⁺ channels have been implicated in defence responses and voltage-insensitive Ca²⁺ channels are probably required to maintain a constant, basal [Ca²⁺]_cyt in the resting cell (White, 1998). The possibility that any of these channels contribute to nutritional Ca fluxes has not been considered. Indeed, little is known even about their distribution within the root. However, it has been estimated that the density of Ca²⁺ channels in the plasma membrane of endodermal cells required to catalyse the observed Ca²⁺ flux to the xylem would be in excess of 2 channels µm^-2 (White, 1998). This value is high, but not inconceivable.

The presence of Ca²⁺-ATPases in the plasma membrane of root cells has been demonstrated both biochemically (Evans and Williams, 1998; Giesler et al., 2000) and electrophysiologically (Felle et al., 1992). It is thermodynamically feasible for this transport mechanism to pump Ca²⁺ from the cytoplasm to the stelar apoplast. However, there is some doubt as to whether Ca²⁺-ATPases could catalyse sufficient Ca²⁺ flux to the xylem to support the nutritional demands of the shoot (White, 1998). If the Ca²⁺ flux to the xylem occurred exclusively through endodermal cells, the density of Ca²⁺-ATPase required would exceed the protein packing capacity of the plasma membrane. Indeed, even if many cell-types within the stele contributed to Ca²⁺ efflux from the symplast to xylem, the mechanism would still be kinetically challenged. Thus, it is unlikely that a symplastic pathway alone could deliver the necessary physiological Ca²⁺ flux to the xylem.

The observation that the Ca²⁺ concentration of xylem sap exuding from excised roots may exceed that of the medium bathing the roots when plants are grown in solutions containing submillimolar Ca²⁺ (Lazaroff and Pitman, 1966; White et al., 1992) is often cited as evidence for active Ca²⁺ transport to the stele. However, this phenomenon can be attributed simply to the stelar apoplast having an electrical potential more negative than the bathing medium and, thereby, concentrating cations in the xylem (Shone, 1968; De Boer, 1999).

An apoplastic bypass for calcium transport to the xylem

If Ca²⁺ reached the xylem by a symplastic pathway, its flux would be expected to show the characteristics of protein-catalysed transport. Thus, symplastic Ca²⁺ transport to the xylem should be selective, there should be competition for transport between permeant cations, and the transport process should saturate at high concentrations of permeant cations.

The Ca²⁺ channels in the plasma membrane of root cells show significant discrimination between permeant cations (White, 1998; Véry and Davies, 2000). In general, Ba²⁺ is more permeable than either Sr²⁺ or Ca²⁺. This is consistent with observations that the accumulation ratios for Sr/Ca and Ba/Ca in plant roots exceed those in the solution bathing the roots (Russell, 1963; Andersen, 1967; Moore et al., 1998), and that divalent cations compete for uptake into excised roots (Epstein and Leggett, 1954). Similarly, the Ca²⁺-ATPases of plants, fungi and animals exhibit a high specificity for the transported cation, preferring Ca²⁺ to Ba²⁺ or Sr²⁺ (Liang et al., 1997; Mandal et al., 2000). For transport to the shoot, by contrast, there appears to be no selectivity between Ca²⁺, Ba²⁺ and Sr²⁺ (Fig. 3). Indeed, it is frequently observed that the Ca:Ba:Sr ratio in the shoot is identical to that of the solution to which roots are exposed (Collander, 1941; Menzel and Heald, 1955; Bowen and Dymond, 1956; Young and Rasmusson, 1966; Moore et al., 1998), and that there is a close correlation between the accumulation of Ca, Sr and Ba by plant species grown in the same substrate (Russell, 1963; Andersen, 1967; Wyttenbach et al., 1995; Veresoglou et al., 1996). This is found for all plant species and is not influenced by plant development. It has also been observed that the presence of Ba or Sr in an agar substrate did not affect the shoot Ca content of Arabidopsis plants, and vice versa (Fig. 3). These observations imply that there is no competition between these divalent cations for transport to the shoot, despite the fact that they compete for uptake into root cells (Epstein and Leggett, 1954). Since the cation transporters on cell membranes are selective, these observations are not consistent with the symplastic pathway through the root being an exclusive route for Ca²⁺, Ba²⁺ and Sr²⁺ movement to the xylem.

View larger version (24K): [in this window] [in a new window]   Fig. 3. The divalent cation content of shoots of Arabidopsis thaliana (Columbia, gl1) plants grown for 21 d in 0.8% (w/v) agar containing a complete nutrient medium (based on Murashige and Skoog, 1962) with either 3 mM Ca (closed symbols) or 10 mM Ca (open symbols), and varying concentrations of Ba or Sr. Nutrient media lacked sulphate in all experiments with Ba. (A) The shoot Sr content (circles) increased linearly with increasing Sr content of the agar. Similarly, the shoot Ca content (squares) increased with increasing Ca content of the agar. The Ca content of the agar had no effect on shoot Sr content, and increasing the Sr content of the agar had little effect on the shoot Ca content. (B) The shoot Ba content (circles) increased linearly with increasing Ba content of the agar. The shoot Ca content (squares) similarly increased with increasing Ca content of the agar. The Ca content of the agar had no effect on shoot Ba content, and increasing the Ba content of the agar had little effect on the shoot Ca content. (C, D) The Ba : Ca and the Sr : Ca ratios in the shoot tissues were identical to those in the agar. Data are means from three or four experiments (HC Bowen and PJ White, unpublished data).

 
The accumulation of Ca, Ba and Sr by shoot tissue is often linearly related to the concentrations of these cations in the nutrient solution (Fig. 3; Rediske and Selders, 1953; Russell and Squire, 1958; English and Barker, 1987). This contrasts markedly with divalent cation uptake by root cells (Epstein and Leggett, 1954), influx through Ca²⁺ channels (White et al., 2000) and Ca²⁺-ATPase activities (Evans and Williams, 1998), which show hyperbolic relationships with increasing divalent cation concentrations. The absence of a saturable component to the transport process, which might be expected if a transport protein was involved, again suggests an apoplastic route to the xylem. Furthermore, the significant effect of transpiration on Ca²⁺ fluxes to the shoot (Lazeroff and Pitman, 1966; Marschner, 1995) may be taken as additional, circumstantial evidence of significant apoplastic fluxes to the xylem. Apoplastic Ca²⁺ would be translocated with the transpiration stream by solvent drag, and the rate at which Ca²⁺ is delivered to the xylem would depend on the rate of apoplastic water flow.

Further discussion and perspectives

The textbook scheme suggests that Ca²⁺ destined for the xylem must traverse the root symplast at some stage on its journey (Fig. 1). Moreover, it specifically suggests that Ca²⁺ travels apoplastically across the root to the Casparian band, which it then circumvents by entering the symplast of the endodermal cell through Ca²⁺ channels before being actively effluxed to the stele through Ca²⁺-ATPases. In theory, this would allow the root to control the rate and selectivity of Ca transport to the shoot. However, several lines of evidence suggest that this scheme is incorrect. Firstly, although the pathway is thermodynamically feasible, and endodermal cells are likely to be furnished with appropriate Ca²⁺ transporters, there is doubt as to whether the plasma membrane of endodermal cells could physically contain sufficient Ca²⁺-ATPase proteins to catalyse the observed Ca²⁺ flux to the xylem. Secondly, there is no selectivity, or interactions, between Ca²⁺, Ba²⁺ and Sr²⁺ for transport to the shoot, and the concentrations of these cations in shoot tissue are linearly related to their concentrations in the nutrient medium. This suggests that these divalent cations are unlikely to be transported across any plasma membrane (which would impart selectivity, competition and saturation phenomena) prior to their delivery to the xylem. A substantial apoplastic bypass is, therefore, indicated for the transport of Ca to the shoot.

If the Casparian band is permeable to divalent cations, it is also likely to be permeable to other ions and solutes. It is unsurprising, therefore, that an apoplastic bypass for transport to the xylem has also been suggested for Na⁺ (Yadav et al., 1996), Cl^– (Storey and Walker, 1999) and ABA (Freundl et al., 1998) based on the strong correlation between their transport and that of tracers for apoplastic solute movement. It is also indicated by root reflection coefficients () for solutes less than unity. Although the endodermis is a significant barrier to solute movement (Peterson et al., 1993; Steudle et al., 1993), reflection coefficients for solute movement across roots are generally lower than those for transport across cell membranes (Steudle et al., 1987, 1993; Rüdinger et al., 1994; Frensch et al., 1996; Freundl et al., 1998). These observations prompted Steudle and co-workers to suggest an explicit ‘composite transport’ model for solute movement to the xylem (Steudle et al., 1993; Steudle and Peterson, 1998). This model considers the root as a complex composite structure, in which parallel pathways (symplastic and apoplastic) with contrasting transport properties contribute to the total radial transport of solutes. It is consistent with low root reflection coefficients for solutes and with the greater passive permeability and lower reflection coefficients observed in root segments closest to the apex (Frensch et al., 1996). It is also consistent with the observation that increasing water flow decreases the root reflection coefficients for solutes. At rates of high transpiration, more water will move to the xylem through an apoplastic pathway (Steudle and Peterson, 1998), and this will promote apoplastic solute transport. At low transpiration rates, such as during the night or during stress conditions (drought, high salinity, nutrient deprivation) the apoplastic pathway will be less used, and transmembrane solute fluxes will dominate.

Several suggestions can be made for the location of this apoplastic bypass. First, there may be a high proportion of purely apoplastic solute flux in the root apex where the endodermis has not yet developed (Steudle and Peterson, 1998). Second, there may be a significant apoplastic flux in regions where lateral roots penetrate the endodermis (Clarkson, 1993). Third, it has been speculated that the Casparian band, even though it is the major barrier to radial solute fluxes through the apoplast (Peterson et al., 1993), is not a particularly effective barrier (Steudle et al., 1993; Frensch et al., 1996; Schreiber et al., 1999).

The hypothesis that Ca reaches the xylem solely by bypassing the Casparian band through the symplast of the endodermal cell rests critically on the assumption that the Casparian Band is impermeable to Ca²⁺. This assumption has not been verified by direct experimentation, and it has been suggested that wax-free Casparian bands are imperfect barriers to apoplastic fluxes of water and dissolved solutes (Sanderson, 1983; Steudle et al., 1993; Steudle and Peterson, 1998; Schreiber et al., 1999). However, if the Casparian band in the State I endodermis was measurably permeable to Ca²⁺, and its permeability was reduced during the development of suberin lamellae (State II endodermis), then the longitudinal profile of Ca movement to the xylem could be reinterpreted as arising (at least in part) from apoplastic transport to the xylem. Such a reduction in the permeability of the Casparian band might be effected by changes in the chemical composition of its lignin, suberin or wax components during the transition to State II endodermis (Schreiber et al., 1999; Zeier et al., 1999). If this occurred, then the properties of Ca transport to the shoot (i.e. lack of selectivity, and absence of competition between cations or flux saturation) could be reconciled with the development of the endodermis.

The operation of two parallel pathways for Ca movement to the xylem could imply a functional separation of symplastic and apoplastic Ca²⁺ fluxes within roots. Symplastic Ca²⁺ fluxes would be important for root nutrition and cell signalling, whilst apoplastic Ca²⁺ fluxes would contribute to Ca transfer to the xylem. This arrangement is consistent with the contrasting changes observed in the specific activity of Ca²⁺ fluxes from radiolabelled root cells to the external medium and the xylem (White et al., 1992) and compatible with reports that plants respond to environmental stresses by modifying the apoplastic barriers in roots. One of the consequences of a dominant apoplastic pathway for Ca²⁺ movement to the xylem would be difficulties in controlling the magnitude and selectivity of cation fluxes to the shoot. In this context, increased endodermal suberization in response to the presence of 100 µM Cd²⁺ or 100 mM Na⁺ in the solution bathing the roots can be interpreted as an acclimatory response restricting both symplastic and apoplastic movement of toxic cations to the xylem (Schrieber et al., 1999).

In conclusion, there are likely to be two parallel pathways across roots delivering Ca²⁺to the xylem. These are considered to be (1) through the symplast of endodermal cells or (2) entirely through the apoplast. The relative magnitudes of Ca²⁺ fluxes through these two pathways are unknown. Although the textbook suggests that the symplastic pathway dominates, there is circumstantial evidence that apoplastic transport of Ca²⁺to the xylem may also occur. Indeed, utilising an apoplastic pathway for Ca²⁺ movement to the xylem may be a preferred option by which the root can fulfil the demand of the shoot for Ca without compromising intracellular [Ca²⁺]_cyt signals. To understand the physiology of divalent cation relations in plants, it will be necessary to estimate the relative magnitudes of Ca²⁺fluxes through the symplastic and apoplastic pathways and how these are affected by developmental and environmental parameters.

Acknowledgments

I thank Helen Bowen, Martin Broadley, Mike Malone, Richard Napier, and Kathryn Woolaway (HRI), Ruth Rowlands and Anna Dudley (Rank Prize Funds Vacation Students), Malcolm Bennett and Sean May (Nottingham) for their contributions to the ideas presented here. I thank Marc Knight (Oxford) and Julia Davies (Cambridge) for providing unpublished manuscripts. My work is supported by the Biotechnology and Biological Sciences Research Council (UK).

Notes

¹ Fax: +44 1789 470552. E-mail: philip\|[hyphen]\|j.white{at}hri.ac.uk

References

Andersen AJ. 1967. Investigations on the plant uptake of fission products from contaminated soils. I. Influence of plant species and soil types on the uptake of radioactive strontium and caesium. (Danish Atomic Energy Commission Research Establishment, Risö.) Risö Report 170, 1–32.

Bowen HJM, Dymond JA. 1956. The uptake and distribution of calcium and strontium by plants from soils and nutrient solutions. Journal of Experimental Botany 7, 264–272.[Abstract/Free Full Text]

Clarkson DT. 1984. Calcium transport between tissues and its distribution in the plant. Plant, Cell and Environment 7, 449–456.

Clarkson DT. 1991. Root structure and sites of ion uptake. In: Waisel Y, Eschel A, Kalkafi U, eds. Plant roots: the hidden half. New York: Marcel Dekker, 417–453.

Clarkson DT. 1993. Roots and the delivery of solutes to the xylem. Philosophical Transactions of the Royal Society (London) B 341, 5–17.

Clarkson DT, Robards AW. 1975. The endodermis, its structural development and physiological role. In: Torrey JG, Clarkson DT, eds. The development and function of roots. London: Academic Press, 415–436.

Collander R. 1941. Selective absorption of cations by higher plants. Plant Physiology 16, 691–720.[Free Full Text]

Damus M, Peterson RL, Enstone DE, Peterson CA. 1997. Modifications of cortical cell walls in roots of seedless vascular plants. Botanica Acta 110, 190–195.

Davenport R, Tester M. 2000. A weakly voltage-dependent, non-selective cation channel mediates toxic sodium influx in wheat. Plant Physiology 122, 823–834.[Abstract/Free Full Text]

De Boer AH. 1999. Potassium translocation into the root xylem. Plant Biology 1, 36–45.

English JE, Barker AV. 1987. Ion interactions in calcium-efficient and calcium-inefficient tomato lines. Journal of Plant Nutrition 10, 857–869.

Enstone DE, Peterson CA. 1992. The apoplastic permeability of root apices. Canadian Journal of Botany 70, 1502–1512.

Enstone DE, Peterson CA. 1997. Suberin deposition and band plasmolysis in the corn (Zea mays L.) root exodermis. Canadian Journal of Botany 75, 1188–1199.

Enstone DE, Peterson CA. 1998. Effects of exposure to humid air on epidermal viability and suberin deposition in maize (Zea mays L.) roots. Plant, Cell and Environment 21, 837–844.

Epstein E, Leggett JE. 1954. The absorption of alkaline earth cations by barley roots: kinetics and mechanism. American Journal of Botany 41, 785–791.[Web of Science]

Esau K. 1965. Plant anatomy, 2nd edn. New York: John Wiley & Sons.

Evans DE, Williams LE. 1998. P-type calcium ATPases in higher plants—biochemical, molecular and functional properties. Biochimica et Biophysica Acta 1376, 1–25.[Medline]

Felle HH, Tretyn A, Wagner G. 1992. The role of the plasma membrane Ca²⁺-ATPase in Ca²⁺ homeostasis in Sinapis alba root hairs. Planta 188, 306–313.

Frensch J, Hsiao TC, Steudle E. 1996. Water and solute transport along developing maize roots. Planta 198, 348–355.

Freundl E, Steudle E, Hartung W. 1998. Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem. Planta 207, 8–19.[Web of Science]

Giesler M, Axelsen KB, Harper JF, Palmgren MG. 2000. Molecular aspects of higher plant P-type Ca²⁺-ATPases. Biochimica et Biophysica Acta 1465, 52–78.[Medline]

Harrison-Murray RS, Clarkson DT. 1973. Relationships between structural development and absorption of ions by the root system of Cucurbita pepo. Planta 114, 1–16.[Web of Science]

Islam AKMS, Asher CJ, Edwards DG. 1987. Response of plants to calcium concentration in flowing solution culture with chloride or sulphate as the counter-ion. Plant and Soil 98, 377–395.

Kiegle E, Gilliham M, Haseloff J, Tester M. 2000a. Hyperpolarization-activated calcium currents found only in cells from the elongation zone of Arabidopsis thaliana roots. The Plant Journal 21, 225–229.[Web of Science][Medline]

Kiegle E, Moore CA, Haseloff J, Tester MA, Knight MR. 2000b. Cell-type-specific calcium responses to drought, salt and cold in the Arabidopsis root. The Plant Journal 23, 267–278.[Web of Science][Medline]

Kirkby EA, Pilbeam DJ. 1984. Calcium as a plant nutrient. Plant, Cell and Environment 7, 397–405.

Lazeroff N, Pitman MG. 1966. Calcium and magnesium uptake by barley seedlings. Australian Journal of Biological Sciences 19, 991–1005.

Leng Q, Mercier RW, Yao W, Berkowitz GA. 1999. Cloning and first functional characterization of a plant cyclic nucleotide-gated cation channel. Plant Physiology 121, 753–761.[Abstract/Free Full Text]

Liang F, Cunningham KW, Harper JF, Sze H. 1997. ECA1 complements yeast mutants defective in Ca²⁺ pumps and encodes an endoplasmic reticulum-type Ca²⁺-ATPase in Arabidopsis thaliana. Proceedings of the National Academy of Sciences, USA 94, 8579–8584.[Abstract/Free Full Text]

Maathuis FJM, May ST, Graham NS, Bowen HC, Jelitto TC, Trimmer P, Bennett MJ, Sanders D, White PJ. 1998. Cell marking in Arabidopsis thaliana and its application to patch-clamp studies. The Plant Journal 15, 843–851.[Web of Science][Medline]

Mandal D, Woolf TB, Rao R. 2000. Manganese selectivity of Pmr1, the yeast secretory pathway ion pump, is defined by Residue Gln⁷⁸³ in transmembrane segment 6. Residue Asp⁷⁷⁸ is essential for cation transport. The Journal of Biological Chemistry 275, 23933–23938.[Abstract/Free Full Text]

Marschner H. 1995. Mineral nutrition of higher plants, 2nd edn. London: Academic Press.

Menzel RG, Heald WR. 1955. Distribution of potassium, rubidium, cesium, calcium and strontium within plants grown in nutrient solutions. Soil Science 80, 287–293.

Moore HC, Trimmer P, Bennett MJ, Thomas BJ, White PJ. 1998. Barium tolerant mutants of Arabidopsis thaliana. In: Abstracts of the 11^th international workshop on plant membrane biology. Experimental Biology Online, 207.

Murashige T, Skoog F. 1962. A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiologia Plantarum 15, 473–497.

Perumalla CJ, Peterson CA, Enstone DE. 1990. A survey of angiosperm species to detect hypodermal Casparian bands. I. Roots with a uniserate hypodermis and epidermis. Botanical Journal of the Linnean Society 103, 93–112.

Peterson CA, Cholewa E. 1998. Structural modifications of the apoplast and their potential impact on ion uptake. Zeitschrift fur Pflanzenernährung Bodenkunde 161, 521–531.

Peterson CA, Enstone DE. 1996. Functions of passage cells in the endodermis and exodermis of roots. Physiologia Plantarum 97, 592–598.

Peterson CA, Lefcourt BEM. 1990. Development of endodermal Casparian bands and xylem in lateral roots of broad bean. Canadian Journal of Botany 68, 2729–2735.

Peterson CA, Murrmann M, Steudle E. 1993. Location of the major barriers to water and ion movement in young roots of Zea mays L. Planta 190, 127–136.

Peterson CA, Perumalla CJ. 1990. A survey of angiosperm species to detect hypodermal Casparian bands. II. Roots with a multiserate hypodermis or epidermis. Botanical Journal of the Linnean Society 103, 113–125.

Rediske JH, Selders AA. 1953. The absorption and translocation of strontium by plants. Plant Physiology 28, 594–605.[Free Full Text]

Rüdinger M, Hallgren SW, Steudle E, Schulze E-D. 1994. Hydraulic and osmotic properties of spruce roots. Journal of Experimental Botany 45, 1413–1425.[Abstract/Free Full Text]

Russell RS. 1963. The extent and consequences of the uptake by plants of radioactive nuclides. Annual Review of Plant Physiology 14, 271–294.[Web of Science]

Russell RS, Squire HM. 1958. The absorption and distribution of strontium in plants. I. Preliminary studies in water culture. Journal of Experimental Botany 9, 262–276.[Abstract/Free Full Text]

Sanderson J. 1983. Water uptake by different regions of the barley root. Pathways of radial flow in relation to development of the endodermis. Journal of Experimental Botany 34, 240–253.[Abstract/Free Full Text]

Schreiber L, Hartmann K, Skrabs M, Zeier J. 1999. Apoplastic barriers in roots: chemical composition of endodermal and hypodermal cell walls. Journal of Experimental Botany 50, 1267–1280.[Abstract/Free Full Text]

Shone MGT. 1968. Electrochemical relations in the transfer of ions to the xylem sap of maize roots. Journal of Experimental Botany 19, 468–485.[Abstract/Free Full Text]

Steudle E, Murrmann M, Peterson CA. 1993. Transport of water and solutes across maize roots modified by puncturing the endodermis. Further evidence for the composite transport model of the root. Plant Physiology 103, 335–349.[Abstract]

Steudle E, Oren R, Schulze E-D. 1987. Water transport in maize roots. Measurement of hydraulic conductivity, solute permeability and of reflection coefficients of excised roots using the root pressure probe. Plant Physiology 84, 1220–1232.[Abstract/Free Full Text]

Steudle E, Peterson CA. 1998. How does water get through roots? Journal of Experimental Botany 49, 775–788.[Abstract/Free Full Text]

Storey R, Walker RR. 1999. Citrus and salinity. Scientia Horticulturae 78, 39–81.

Veresoglou DS, Barbayiannis N, Matsi T, Anagnostopoulos C, Zalidis GC. 1996. Shoot Sr concentrations in relation to shoot Ca concentrations and to soil properties. Plant and Soil 178, 95–100.

Véry A-A, Davies JM. 2000. Hyperpolarization-activated calcium channels at the tip of Arabidopsis root hairs. Proceedings of the National Academy of Sciences USA 97, 9801–9806.[Abstract/Free Full Text]

Weerdenburg CA, Peterson CA. 1984. Effect of secondary growth on the conformation and permeability of the endodermis of broad bean (Vicia faba), sunflower (Helianthus annuus) and garden balsam (Impatiens balsamina). Canadian Journal of Botany 62, 907–910.

White PJ. 1998. Calcium channels in the plasma membrane of root cells. Annals of Botany 81, 173–183.[Abstract/Free Full Text]

White PJ. 2000. Calcium channels in higher plants. Biochimica et Biophysica Acta 1465, 171–189.[Medline]

White PJ, Banfield J, Diaz M. 1992. Unidirectional Ca²⁺ fluxes in roots of rye (Secale cereale L.). A comparison of excised roots with roots of intact plants. Journal of Experimental Botany 43, 1061–1074.[Abstract/Free Full Text]

White PJ, Piñeros M, Tester M, Ridout MS. 2000. Cation permeability and selectivity of a root plasma membrane calcium channel. Journal of Membrane Biology 174, 71–83.[Web of Science][Medline]

Wyttenbach A, Bajo S, Bucher J, Furrer V, Schleppi P, Tobler L. 1995. The concentration of Ca, Sr, Ba and Mn in successive needle age classes of Norway spruce [Picea abies (L.) Karst.]. Trees 10, 31–39.

Yadav R, Flowers TJ, Yeo AR. 1996. The involvement of transpirational bypass flow in sodium uptake by high- and low-sodium-transporting lines of rice developed through intravarietal selection. Plant, Cell and Environment 19, 329–336.

Young WI, Rasmusson DC. 1966. Variety differences in strontium and calcium accumulation in seedlings of barley. Agronomy Journal 58, 481–483.[Abstract/Free Full Text]

Zeier J, Ruel K, Ryser U, Schreiber L. 1999. Chemical analysis and immunolocalization of lignin and suberin in endodermal and hypodermal/rhizodermal cell walls of developing maize (Zea mays L.) primary roots. Planta 209, 1–12.[Web of Science][Medline]

Zimmermann HM, Steudle E. 1998. Apoplastic transport across young maize roots: effect of the exodermis. Planta 206, 7–19.

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				P. Senadheera, R. K. Singh, and F. J. M. Maathuis Differentially expressed membrane transporters in rice roots may contribute to cultivar dependent salt tolerance J. Exp. Bot., July 1, 2009; 60(9): 2553 - 2563. [Abstract] [Full Text] [PDF]

				M. Kerton, H. J. Newbury, D. Hand, and J. Pritchard Accumulation of calcium in the centre of leaves of coriander (Coriandrum sativum L.) is due to an uncoupling of water and ion transport J. Exp. Bot., January 1, 2009; 60(1): 227 - 235. [Abstract] [Full Text] [PDF]

				J. S. Busse, S. Ozgen, and J. P. Palta Influence of Root Zone Calcium on Subapical Necrosis in Potato Shoot Cultures: Localization of Injury at the Tissue and Cellular Levels J. Amer. Soc. Hort. Sci., September 1, 2008; 133(5): 653 - 662. [Abstract] [Full Text] [PDF]

				P. J. White, H. C. Bowen, B. Marshall, and M. R. Broadley Extraordinarily High Leaf Selenium to Sulfur Ratios Define 'Se-accumulator' Plants Ann. Bot., July 1, 2007; 100(1): 111 - 118. [Abstract] [Full Text] [PDF]

				F. J. M. Maathuis The role of monovalent cation transporters in plant responses to salinity J. Exp. Bot., March 1, 2006; 57(5): 1137 - 1147. [Abstract] [Full Text] [PDF]

				T. Kamiya, T. Akahori, M. Ashikari, and M. Maeshima Expression of the Vacuolar Ca2+/H+ Exchanger, OsCAX1a, in Rice: Cell and Age Specificity of Expression, and Enhancement by Ca2+ Plant Cell Physiol., January 1, 2006; 47(1): 96 - 106. [Abstract] [Full Text] [PDF]

				S. Ding, T. Liang, C. Zhang, J. Yan, and Z. Zhang Accumulation and fractionation of rare earth elements (REEs) in wheat: controlled by phosphate precipitation, cell wall absorption and solution complexation J. Exp. Bot., October 1, 2005; 56(420): 2765 - 2775. [Abstract] [Full Text] [PDF]

				L. C. HO and P. J. WHITE A Cellular Hypothesis for the Induction of Blossom-End Rot in Tomato Fruit Ann. Bot., March 1, 2005; 95(4): 571 - 581. [Abstract] [Full Text] [PDF]

				M. L. Hayter and C. A. Peterson Can Ca2+ Fluxes to the Root Xylem Be Sustained by Ca2+-ATPases in Exodermal and Endodermal Plasma Membranes? Plant Physiology, December 1, 2004; 136(4): 4318 - 4325. [Abstract] [Full Text] [PDF]

				R. Storey and R. A. Leigh Processes Modulating Calcium Distribution in Citrus Leaves. An Investigation Using X-Ray Microanalysis with Strontium as a Tracer Plant Physiology, November 1, 2004; 136(3): 3838 - 3848. [Abstract] [Full Text] [PDF]

				E. Cholewa and C. A. Peterson Evidence for Symplastic Involvement in the Radial Movement of Calcium in Onion Roots Plant Physiology, April 1, 2004; 134(4): 1793 - 1802. [Abstract] [Full Text] [PDF]

				M. R. Broadley, H. C. Bowen, H. L. Cotterill, J. P. Hammond, M. C. Meacham, A. Mead, and P. J. White Phylogenetic variation in the shoot mineral concentration of angiosperms J. Exp. Bot., February 1, 2004; 55(396): 321 - 336. [Abstract] [Full Text] [PDF]

				P. J. WHITE and M. R. BROADLEY Calcium in Plants Ann. Bot., October 1, 2003; 92(4): 487 - 511. [Abstract] [Full Text] [PDF]

				M. R. Broadley, H. C. Bowen, H. L. Cotterill, J. P. Hammond, M. C. Meacham, A. Mead, and P. J. White Variation in the shoot calcium content of angiosperms J. Exp. Bot., May 1, 2003; 54(386): 1431 - 1446. [Abstract] [Full Text] [PDF]

				L. Kerkeb and U. Kramer The Role of Free Histidine in Xylem Loading of Nickel in Alyssum lesbiacum and Brassica juncea Plant Physiology, February 1, 2003; 131(2): 716 - 724. [Abstract] [Full Text] [PDF]

				Q. Leng, R. W. Mercier, B.-G. Hua, H. Fromm, and G. A. Berkowitz Electrophysiological Analysis of Cloned Cyclic Nucleotide-Gated Ion Channels Plant Physiology, February 1, 2002; 128(2): 400 - 410. [Abstract] [Full Text] [PDF]

				E. Hose, D.T. Clarkson, E. Steudle, L. Schreiber, and W. Hartung The exodermis: a variable apoplastic barrier J. Exp. Bot., December 1, 2001; 52(365): 2245 - 2264. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) E-letters: Submit a response Alert me when this article is cited Alert me when E-letters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (58) Request Permissions Disclaimer Google Scholar Articles by White, P. J. Search for Related Content PubMed PubMed Citation Articles by White, P. J. Agricola Articles by White, P. J. Social Bookmarking          What's this?

Online ISSN 1460-2431 - Print ISSN 0022-0957

Copyright © 2005 Society for Experimental Biology

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics