Journal of Experimental Botany, Vol. 52, No. 358, pp. 1051-1061,
May 1, 2001
© 2001 Oxford University Press
Original Papers |
Frequencies of plasmodesmata in Allium cepa L. roots: implications for solute transport pathways
Department of Biology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Received 21 August 2000; Accepted 27 December 2000
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Abstract |
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Plasmodesmatal frequencies (PFs) were analysed in Allium cepa L. roots with a mature exodermis (100 mm from the tip). For all interfaces within the root, the numbers of plasmodesmata (PD) µm-2 wall surface (Fw) were calculated from measurements of 60 walls on ultrathin sections. For tissues ranging from the epidermis up to the stelar parenchyma, the frequencies were also expressed as total PD numbers mm-1 root length (Fn), which is most instructive for considering the radial transport of ions and photosynthates (because the tissues were arranged in concentric cylinders). The Fn values were constantly high at the interfaces of exodermis–central cortex, central cortex–endodermis and endodermis–pericycle (4.05x105, 5.13x105, and 5.64x105, respectively). If the plasmodesmata are functional, a considerable symplastic transport pathway exists between the exodermis and pericycle. Two interfaces had especially low PFs: epidermis–exodermis (Fn=8.96x104) and pericycle–stelar parenchyma (Fn=6.44x104). This suggests that there is significant membrane transport across the interface of epidermis–exodermis (through short cells) and direct transfer of ions from pericycle to protoxylem vessels. In the phloem, the highest PF was detected at the metaphloem sieve element–companion cell interface (Fw=0.42), and all other interfaces had much lower PFs (around 0.10). In the pericycle, the radial walls had a high PF (Fw=0.75), a feature that could permit lateral circulation of solutes, thus facilitating ion (inward) and photosynthate (outward) delivery.
Key words: Allium cepa L., phloem unloading, plasmodesmata, plasmodesmatal frequency, root, symplastic transport, transmission electron microscopy.
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Introduction |
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Intercellular transport between living plant cells is mediated by plasmodesmata (PD). These cytoplasmic channels provide a low-resistance pathway that is available for a wide range of ions and molecules (typical size exclusion limit <1 kDa). PD are dynamic structures that can be regulated by a number of internal and external factors (Overall and Blackman, 1996
; Ding et al., 1999). When dealing with symplastic transport across a tissue, what is observed is the collective, rather than the individual, functioning of PD on that particular interface. Therefore, the frequency of functional PD is the most important parameter that will determine the direction, extent and rate of symplastic transport under given conditions. Data regarding plasmodesmatal frequencies (PFs) must be obtained with transmission electron microscopy (TEM); the time required for this approach has largely constrained efforts to perform large-scale surveys. Available frequency information has been principally produced for leaves from decades of painstaking endeavour to elucidate photosynthate transport processes (Gamalei, 1991; Van Bel, 1993; Van Bel and Oparka, 1995; Turgeon, 1996). Also, exciting advances have been made, yet again, in leaves, in the structural and functional regulation of PD during macromolecule transport (Ding et al., 1999; Lucas, 1999; Oparka and Santa Cruz, 2000). In comparison, PD in root systems have received limited attention. In a recent study of the Arabidopsis thaliana L. root apical meristem, a tissue-specific pattern of PD distribution was observed, and dye-coupling experiments established a correlation between this pattern and the potential for symplastic diffusion of small molecules (Zhu et al., 1998). For root tissues proximal to the root tip, only a very few species have been examined and in these, PFs were measured only for selected interfaces (Robards et al., 1973; Robards and Jackson, 1976; Warmbrodt, 1985a, b, 1986a; Kurkova, 1989; Wang et al., 1995). To date, a complete picture of symplastic connections in any root has been lacking.
Special consideration is needed for some plants that have an exodermis in their roots. The exodermis is an outermost cortical layer that develops Casparian bands (Peterson and Perumalla, 1990) and suberin lamellae (Kroemer, 1903; Von Guttenberg, 1968). The Casparian bands, since they are in the anticlinal walls, do not affect the roots’ symplastic transport in the radial direction. The suberin lamellae, lying all around the cells’ protoplasts, may or may not affect the symplastic connections within the root, according to the type of the exodermis. In the uniform exodermis (all cells elongate), as seen in Zea mays L. (the only species of this category examined by TEM), suberin lamellae do not interfere with the symplastic continuity of the layer (Wang et al., 1995). However, in the dimorphic exodermis (with long and short cells alternating along the axis of the root), as in Citrus sp. (Walker et al., 1984) and Allium cepa L. (Ma and Peterson, 2000), suberin lamella deposition in long cells severs all their PD. Accordingly, the short cells (without suberin lamellae) must play a paramount role in the symplastic fluxes (of ions and photosynthate-derived nutrients). In an earlier paper, an overall view of PD relationships (but not details of frequencies) of A. cepa roots was provided (Ma and Peterson, 2000). A closer examination of PD distribution in the exodermis (short cells), as well as in all other tissues, will provide further insights into the symplastic connections in mature roots.
There are some other major issues that remain unclarified, one of which concerns xylem loading. Are the xylem vessels loaded by stelar parenchyma (cells intervening between the xylem and phloem strands) or by pericycle or by both? (See Fig. 1 for locations of these and other cell types.) Most authors have assumed that ions are transported from the cortex to the stelar parenchyma from whence they are finally transferred into the vessels (Sanderson, 1975; Stelzer et al., 1975; Robards and Clarkson, 1976; Clarkson, 1993). This idea gained support from experiments in which stelar parenchyma cells proved to be capable of actively accumulating ions from the cortex in Z. mays (Läuchli et al., 1971a, b, 1974a, b). However, studies on Hordeum vulgare L. roots suggested that the pericycle might play a major role in xylem loading (Vakhmistrov et al., 1972; Kurkova et al., 1974; Vakhmistrov, 1981). In the latter species, the pericycle–stelar parenchyma interface had a much lower PF than the pericycle–endodermis interface, and it was envisaged that the majority of ions would proceed from the pericycle directly to the xylem vessels, rather than through the stelar parenchyma cells (Vakhmistrov et al., 1972; Kurkova et al., 1974; Vakhmistrov, 1981). In the present study, the relative significance of these two tissues (stelar parenchyma and pericycle) at this critical point of radial ion transport will be examined in Allium cepa L. roots.
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Phloem unloading in roots is another field that is poorly understood. In the root tip, symplastic transport (phloem unloading and post-phloem transport) is intensive to sustain cell division and growth (Dick and ap Rees, 1975
; Oparka et al., 1994; Zhu et al., 1998). More mature zones (proximal to the tip) are apparently weaker sinks for photosynthates but here all the living cells still need a continuous supply of photosynthates for their normal respiratory activities. Phloem unloading and post-phloem transport of photosynthates in mature zones could be accomplished by a symplastic pathway in several species examined (Giaquinta et al., 1983; Warmbrodt, 1985a, b, 1986a, b). However, in A. thaliana, no symplastic phloem unloading was observed under normal conditions of root growth (Wright and Oparka, 1997). In the present study, it was noted whether or not the PD associated with the phloem were structurally normal, and what the related PFs may imply for solute transfer.
There are several ways of expressing PFs, three of which were used in the present study. (1) The number of PD µm-2 wall surface (Fw). This is the basic and most commonly used value (as in most of the studies cited above). Fw is most useful for a comparison of cell interfaces to predict their relative capacity for symplastic transfer and, thus, is applicable to the phloem region of the root where the paths concerned are very short. (2) The number of PD on a tissue interface over a unit root length (Fn). This treatment is more instructive than the previous one for predicting symplastic transport capacity for tissues ranging from epidermis to pericycle. This is simply because all the tissues (except for the central cortex) are organized into concentric cylinders; the interface areas of which are determined by their radii (Fig. 1
). The interfaces will be traversed by both ions (in the inward direction moving from the soil solution to the stele) and photosynthates (in the outward direction moving from the stele to the cortex and epidermis). (3) The number of PD µm-2 tissue interface cylinder (Ft). The values of Ft, which may or may not be equal to Fw (see Materials and methods), was employed in the present study for the purpose of comparing the present results to related literature data that were expressed in this way.
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Materials and methods |
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Plant materials
Bulbs of Allium cepa L. cv. Ebeneezer were planted in moist vermiculite as described previously (Ma and Peterson, 2000
). At 7–14 d, root systems were collected. Any adhering vermiculite was gently rinsed away from the roots prior to further sampling.
Transmission electron microscopy
Root segments were excised 100 mm from the tip for TEM. (For a full account of the technique, see Ma and Peterson, 2000.) At this distance, the roots had a mature exodermis, i.e. Casparian bands had developed in both short and long cells, and the latter also had suberin lamellae. The section thickness was set at 8x10-2 µm.
Calculation of tissue interface areas
Free-hand cross-sections were made 100 mm from the root tip and photographed (using colour slide films). Diameters (and thus areas) of individual tissue interfaces were measured from the transparencies under a dissecting microscope. The surface area of exodermal short cells was obtained from tangential sections of the exodermis. These data were used to calculate Fn (below).
Calculation of plasmodesmatal frequencies
PFs were calculated in three different ways. The rationale for each was presented in the Introduction.
PD µm-2 wall (Fw)
Calculations were performed following the formula of Gunning (in Robards, 1976):
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), the diameters of PD were measured from TEM negatives (mean of 5 PD at their largest dimensions in the longitudinal view). To obtain N and L, 60 walls were randomly sampled from 5–30 non-serial ultrathin sections (which were cut from 5–15 roots). On each wall, N was counted directly in the microscope. The same wall was digitized at x2600 by the program Analysis 2.0 (Soft-Imaging Service GmbH) and its length was subsequently measured with another program, Northern Exposure (Empix Imaging, Inc.).
PD mm-1 root length (Fn)
The interfaces measured were: (1) epidermis–exodermis, (2) exodermis–central cortex, (3) central cortex–endodermis, (4) endodermis–pericycle, and (5) pericycle–central stele (Fig. 1). It should be noted that this approach is not suitable for the central cortex, because intercellular spaces had developed and the cells were not arranged in concentric cylinders. The formula is
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D is area (µm2) of the cylinder (D, diameter, in µm) into which each tissue interface fits. Fw, corresponding values obtained from formula 1.
Clearly, formula 2 was not applicable to interfaces 1 and 2 (above) since intact exodermal PD did not occur in the tangential walls of exodermal long cells but only of short cells (Ma and Peterson, 2000). Therefore,
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). Fw, corresponding value obtained from formula 1.
Interface 5 is highly heterogeneous, consisting of three sub-interfaces, i.e. pericycle–stelar parenchyma, pericycle–companion cells, and pericycle–protophloem sieve element(s). For the first sub-interface, the Fn value was calculated from
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Number of PD µm-2 tissue interface (Ft)
The formula for this calculation is
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) because, for each, only one cell type occurred on either side of the interface. Values of Ft were not estimated for the central cortex for the reasons stated above.
Statistics
Paired comparisons t-tests were performed at 
=0.05 (Sokal and Rohlf, 1981) for the values of Fw of the phloem region and some other interfaces (see Results). Analyses were not done for Ft and Fn, since these values are secondary in nature (i.e. derived from the means of Fw).
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Results |
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Plasmodesmatal frequencies in tissues from epidermis to stelar parenchyma
The PD at the interfaces examined were similar in diameter (ranging from 59 to 65 nm) but different in frequency. Considering that the radial symplastic path spans between the epidermis and the stelar parenchyma, the interfaces of concern were ranked, on the basis of their Fw values, in the following order: central cortex>short cells–central cortex>endodermis–pericycle>central cortex–endodermis>epidermis–short cell>pericycle–stelar parenchyma. Related data are displayed in Table 1
. To provide a clearer visual indication of PD distribution, a plasmodesmogram (Van Bel and Oparka, 1995) was constructed, based on the Fw values (Fig. 2).
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The Fn values were rather constant in the region from the exodermis up to the pericycle, in spite of the uncertainty about the central cortex (Table 1
). For this latter tissue, although it is theoretically possible to obtain an Fn value, the estimate would be misleading if applied to any explanation of radial transport, due to the presence of intercellular spaces and the cells' irregular arrangement. Nevertheless, the abundance of PD in the cell walls (Fw) of the central cortex apparently renders it efficient for symplastic transport. At the outermost interface of the root, epidermis–exodermis, a very small number of PD was observed. To provide an even sharper picture of PD distribution along the radial path (across the concentric cylinders of tissue interfaces), this set of data was ranked by ratios (Fig. 2). In this diagram, two different treatments were made for interface 5 (as indicated in Fig. 1). First, it was assumed that all PD between the pericycle and its adjacent inner living cells are functional; the ratios are shown in the first file of numbers. Second, based on the assumption that the pericycle–phloem interface is non-functional in transferring ions, the radial symplastic pathway is then reduced to the pericycle–stelar parenchyma interface. In this case, the second file of numbers applies.
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Plasmodesmatal frequencies associated with the stele and the endodermis
The root usually contained six phloem strands (Fig. 1). In each, the components and their spatial relationships were as depicted in Fig. 3A. Each strand contained 1 or 2 protophloem sieve elements, 3–5 metaphloem sieve elements and their associated companion cells. At its outer tangential side, the phloem was in contact with the pericycle by 1 or 2 protophloem sieve elements and 3–4 companion cells. On its flanks, the phloem was connected to the stelar parenchyma by companion cells. Metaphloem sieve elements were associated neither with the pericycle nor with the stelar parenchyma. In the phloem, companion cells had the densest cytoplasm of any cell type, characterized by numerous mitochondria and ER profiles (Fig. 3B, C, D, E). Pericycle cells had denser cytoplasm than stelar parenchyma cells (Fig. 3B). Those stelar parenchyma cells deep in the stele had a thin cytoplasm and few organelles, comparable to immature metaxylem vessel members.
All PD in the entire phloem region were structurally normal (as in other areas of the root, see Ma and Peterson, 2000) and did not appear to be damaged or blocked (e.g. Fig. 3E). In general, the PFs (Fw) in this region were much lower than in the tissues external to the stele (Table 2). Within the phloem, the values were very uniform except for the one at the metaphloem sieve element–companion cell interface (Table 2; Fig. 4), but this was not significantly different (at =0.05). At the phloem–pericycle interface, the Fn value was estimated in the following way. As seen in a cross-section of the root (Fig. 1), the phloem was connected to about half of the inner tangential surface area of the pericycle; this interface area was shared between companion cells and protophloem sieve elements at a ratio of about 4:1 (Fig. 3A). The Fn values for these two sub-interfaces were estimated at 4.61x104 and 5.76x103, respectively, totalling 5.19x104. This number was added to that of the pericycle–stelar parenchyma interface to express the total Fn on the entire pericycle inner face (Table 1).
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The pericycle and endodermis each exhibited unique patterns of PD distribution. Pericycle cells had very high PFs (Fw) in both their radial and outer tangential walls, but significantly lower PF (at
=0.05) in their inner tangential walls (Fig. 2; Table 2). In the endodermis, the PFs on the outer and inner tangential walls were high and comparable to each other, but the radial walls had an extremely low PF which was significantly lower (at =0.05) than that of corresponding walls of pericycle (Fig. 2, Table 2).
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Discussion |
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The present study provided a complete assessment of PFs that were associated with the symplastic transport, both inward (for ions) and outward (for photosynthates), in Allium cepa L. roots. This is the first such comprehensive treatment for any root system.
Plasmodesmatal frequencies: implications for ion transport
Symplastic transport is made possible in A. cepa roots by the presence of PD in all tissue interfaces. Previous understanding of symplastic transport in the root has been largely based on studies of the cortex (Anderson and Reilly, 1968; Ginsburg and Ginzburg, 1970a, b; Jarvis and House, 1970; Baker, 1971; Clarkson and Sanderson, 1974; Van Iren and Boers-Van de Sluijs, 1980). The present results on A. cepa roots strongly support a symplastic pathway across this tissue, since its PF (Fw) was rather high (Table 1). In addition, this major, if not the only, pathway is most likely to occur both near the root surface (from short cells to central cortex) and deep in the root (from central cortex to endodermis to pericycle); high and constant PD numbers (Fn) were found on these interfaces (Table 1; Fig. 2).
Short cells apparently take up ions from the apoplast, as indicated by the following observations. As shown in Table 1, the exodermis (through short cells) was connected with the epidermis with a much lower PF than with the cortex. When Ft values were compared (2.77x10-2 versus 1.68x10-1; Table 1), there was a 6-fold difference. When Fn values (which are more instructive for examining radial transport) were compared, the difference was also obvious (5-fold). Assuming that a constant flow of material is sustained from the soil solution to the cortex and the PD function close to their capacity, there should be a supplementary mechanism(s) that makes the flow possible across the epidermis–short cells interface. It is hypothesized that ions diffuse through the epidermal walls and enter the short cells at their outer tangential plasma membranes. Short cells normally display features typical of metabolically active cells (Peterson and Enstone, 1996; Ma and Peterson, 2000). In extreme situations, short cells exhibit elevated potential for ion uptake by developing wall ingrowths (Atriplex hastata L., Kramer et al., 1978; A. cepa, Wilson and Robards, 1980). In both Citrus sp. (Walker et al., 1984) and A. cepa (Barrowclough and Peterson, 1994, Kamula et al., 1994), the epidermis tends to die; the movement of ions from the apoplast to the symplast of short cells then becomes the only pathway for ion uptake in this zone of the root. In the Z. mays exodermis (uniform, all cells symplastically connected to adjacent tissues), the PFs on its outer and inner tangential walls are not as different as in A. cepa (Table 2). Therefore, even if the hypothesized supplementary mechanism exists in Z. mays, it is unlikely that it plays a role as significant as in A. cepa.
The PFs of the endodermis (on its outer and inner tangential walls) have important implications for the function of the endodermis and also of the root as a whole, especially in ion transport. It is generally believed that the endodermal Casparian bands divert apoplastic flows of solutes from the central cortex across the endodermal outer tangential plasma membrane into the symplast (of the endodermis). Supposing that all solutes in the endodermis are then constantly moved to the pericycle symplastically, this interface would be expected to have a higher PF (Fw) than the outer side. In H. vulgare, indeed, the value (Fw) is doubled (0.75 versus 0.37) (as recorded in Robards et al., 1973; see Table 2). But in another work on the same species, the difference was trivial (Warmbrodt, 1985a; Table 2). Due to this discrepancy in the two studies, a firm conclusion will require a thorough re-examination of the system. In both A. cepa, and Z. mays, the inner and outer tangential endodermal walls had comparable PFs (Table 2). It is interesting to note that in these latter species, the exodermis (with Casparian bands) is an effective apoplastic barrier (Peterson, 1998). It is tempting to postulate that symplastic transport across the cortex is the principal mechanism in both species. In A. cepa, the high PF (Fw, Table 1) would facilitate such movement. In this connection, since H. vulgare is a non-exodermal species (Perumalla et al., 1990), a relatively significant apoplastic pathway across the cortex would be expected. Ideally, a comparison of Fn values of exodermal and non-exodermal species would be more informative; unfortunately, these values are lacking in the literature.
The PFs at the periphery of the stele are important in considering the relative significance of pericycle and stelar parenchyma in loading the xylem. In H. vulgare roots it was found that the pericycle was poorly connected by PD to the internal stele tissues (Vakhmistrov et al., 1972). They suggested that ions would move directly from the pericycle to the xylem vessels, rather than through the internal stele tissues (see also the last section of this Discussion). It is inferred from a later contribution (Vakhmistrov, 1981) that the ‘internal stele tissues’ are equivalent to ‘stelar parenchyma’. Although Warmbrodt (Warmbrodt, 1985a) was unable to confirm the results of Vakhmistrov et al. (Vakhmistrov et al., 1972), the latter authors’ results were in general agreement with those of Robards and Jackson (Robards and Jackson, 1976; see Table 2). In further support of Vakhmistrov's idea, a recent study of H. vulgare showed that the pericycle had more intense expression of the plasma membrane H+-ATPase than the stelar parenchyma (Samuels et al., 1992). Assuming that the PD at the pericycle–phloem interface are non-functional in transferring ions, then the frequency of proposed functional PD (at the pericycle–stelar parenchyma interface) is even lower in A. cepa than in H. vulgare (Fig. 4; Table 2). If, on the other hand, the pericycle–phloem PD were functional, then ions could diffuse into the phloem. However, ions in the phloem would need to cross the companion cells–stelar parenchyma interface before entering the vessels (Figs 1, 3A); this interface is unlikely to support a significant amount of symplastic transport in view of the extremely low PF (Table 2). Now the major concern is how the xylem is loaded. For A. cepa roots, there is evidence that an active step exists from the symplast to the xylem (for Cl-; Hodges and Vaadia, 1964), which is probably achieved by a proton pump (Clarkson and Hanson, 1986). The exact location of the active step, however, has not been clearly determined; it could be in the stelar parenchyma or the pericycle or both. Traditionally, an active role has been designated to the xylem parenchyma (Läuchli et al., 1971a, b, 1974a, b; De Boer, 1999). However, this term has frequently been used to denote a collective of stelar parenchyma and pericycle. From the physiological point of view, there is a need to separate these two tissues. They are different from each other both in their ontogeny and in their spatial relationships to adjacent tissues, which in turn could well reflect their different functional roles.
On phloem unloading and post-phloem transport of photosynthates in mature roots
Both outside and inside the phloem, transport of photosynthates can be achieved by PD. Across the root cortex, there is physiological evidence for this kind of transport in several species (Dick and ap Rees, 1975; Giaquinta et al., 1983; Fisher and Oparka, 1996). Inside the phloem, it seems to be a common feature that companion cells are the principal receivers of the translocated photosynthates from the shoot through sieve tubes (Warmbrodt, 1985a, b). These ideas gained support from the present analysis of PFs (Fig. 4; Table 2). Yet, as to the initial steps of the post-phloem transport (or, more specifically, post-companion cell transport), variations might occur among species. For instance, in H. vulgare (Warmbrodt, 1985a), Z. mays (Warmbrodt, 1985b) and some other species (Patrick and Offler, 1996), the preferred symplastic paths would be from companion cells to stelar parenchyma cells and to the pericycle, along a decreasing PF gradient. A parallel solute concentration gradient was detected in H. vulgare (Warmbrodt, 1986b) and Z. mays (Warmbrodt, 1987). Although A. cepa shares certain similarities with H. vulgare (Warmbrodt, 1985a) and Z. mays (Warmbrodt, 1985b) with respect to phloem construction, no such preference is expected on an anatomical basis in the former since its PD were distributed approximately evenly around the phloem (Fig. 4; Table 2).
By what pathway are photosynthates unloaded from the phloem into the pericycle in the mature zone of the root? Unlike the richness of literature on phloem unloading in expanding leaves and storage organs, there is a paucity of knowledge regarding the root (Patrick, 1990; Fisher and Oparka, 1996). In A. cepa, the interfaces of phloem–pericycle and stelar parenchyma–pericycle had Fn=5.19x104 and 1.25x104 PD mm-1 root, respectively. The PD appeared structurally normal (Fig. 3E). Assuming the PD on both interfaces are functional in transferring photosynthates to the pericycle, the combined value (6.44x104, close to that at the exodermis–epidermis interface, Table 1) would support a significant amount of symplastic transport. In the mature root zone of A. thaliana, the PD at the interface of phloem and surrounding cells are held closed under normal conditions; they open only during lateral primordium formation (Oparka et al., 1995) or upon application of metabolic inhibitors, as detected by fluorescent tracer dyes (Wright and Oparka, 1997). Tracer experiments also demonstrated functional phloem isolation in stems of several species (Hayes et al., 1985; Aloni and Peterson, 1990; Van Bel and Kempers, 1990; Van Bel and Van Rijen, 1994). In support of these results, the expression of the A. thaliana AtSUC2 sucrose-H+ symporter has been localized exclusively in the companion cells of root and stem (and some other organs, Truernit and Sauer, 1995; Stadle and Sauer, 1996). A similar expression pattern of sucrose-H+ symporter was found in Plantago major L. (Stadle et al., 1995). The new results are indicative of apoplastic unloading through companion cells. If this is the case in roots, photosynthates in the apoplast will have to enter the symplast internal to the endodermal Casparian band (an apoplastic barrier) prior to their outward transport. Several questions concerning phloem unloading in roots remain to be investigated: (1) is a sucrose H+-symporter present in all plant roots and, more particularly, in their mature zones? (2) is the carrier functional and, if so, how is it regulated? and (3) how are the PD regulated along with the carriers?
A further note on the pericycle
In addition to the involvement of PD on the tangential walls in the radial transport as discussed above, the pericycle may also conduct ions and photosynthates across its radial walls. Based on results obtained from H. vulgare, Vakhmistrov suggested that this cell layer could act as an ‘annular collector/disperser’ (Vakhmistrov et al., 1972; Vakhmistrov, 1981). In brief, substances received from the endodermis tend to move symplastically across the radial walls (with a high PF), toward those pericycle cells that lie near the xylem vessels (i.e. the ‘annular collector’ role; Fig. 5A) where the concentration of ions is kept low by the transpiration stream. In this scenario, it was assumed that, in the stele, little symplastic transport of ions would occur toward the vessels (see the previous section). At the same time, the pericycle functions as an ‘annular disperser’ (Fig. 5B) in the outward transport of photosynthates (Kurkova et al., 1974; Vakhmistrov, 1981). This is based on the following consideration. Physically, the pericycle cells that are connected with the phloem (and, to a lesser extent, probably also with the stelar parenchyma, see above) are able to receive substances from the phloem (if the PD are functional). But, these substances are not directly accessible to those pericycle cells that are in contact with the xylem (see also Figs 1 and 4). Thus, some of the photosynthates would move directly to the adjacent endodermal cells, and the rest would tend to equilibrate across the pericycle via PD, reaching the xylem-associated pericycle cells, and then the adjacent endodermal cells. The present results on A. cepa roots are in favour of this model. In Z. mays, a large number of PD was also observed in pericycle radial walls (Warmbrodt, 1985b; see also Table 2). Vakhmistrov's brilliant insights deserve to be better known and research should be extended to the examination of other species to see if this model is generally applicable, both structurally and physiologically.
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Conclusions |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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The findings reported here show that PD connections are present across all root tissues of A. cepa. At the epidermis–short cells interface, both symplastic transport and transmembrane transport are anticipated. The pericycle could play a significant role in symplastic transport of ions and photosynthates, not only directly (along the radial path) but also indirectly (by circulation in the tangential direction across radial walls). Symplastic photosynthate distribution could occur in the phloem region and beyond. These results will be useful for a functional examination of ion and photosynthate movement across the roots of A. cepa and other species.
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Acknowledgments |
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We thank Mr Dale Weber (University of Waterloo, Waterloo) for his expert support with the TEM, Dr William Diehl-Jones (University of Waterloo) for instructions on cell wall measurement, Dr Tony Robards (University of York, UK) for advice on plasmodesmatal frequency calculation, Dr Pat Newcombe (University of Waterloo) for statistical advice, and Ms Daryl Enstone (University of Waterloo) for her assistance throughout the research. The Natural Sciences and Engineering Research Council of Canada provided funding to CAP, and the University of Waterloo awarded Graduate Student Scholarships to FM.
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Notes |
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1 Present address: Department of Plant Biology and Plant Biotechnology Centre, The Ohio State University, Columbus, Ohio 43210, USA.
2 To whom correspondence should be addressed. Fax: +1 519 746 0614. E-mail: cpeterso{at}uwaterloo.ca
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Abbreviations |
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Fn, number of plasmodesmata mm-1 root length (at individual tissue interfaces); Ft, number of plasmodesmata µm-2 tissue interface; Fw, number of plasmodesmata µm-2 wall surface; PD, plasmodesmata; PF, plasmodesmatal frequency; TEM, transmission electron microscopy..
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References |
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Aloni R, Peterson CA. 1990. The functional significance of phloem anastomoses in stems of Dahlia pinnata Cav. Planta 182, 583–590.
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