JXB Advance Access originally published online on July 2, 2004
Journal of Experimental Botany 2004 55(403):1635-1641; doi:10.1093/jxb/erh193
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RESEARCH PAPER |
Lateral ABA transport in maize roots (Zea mays): visualization by immunolocalization
1Julius-von-Sachs-Institut für Biowissenschaften, Universität Würzburg, Julius-von-Sachs-Platz 2, 97082 Würzburg, Germany
2Institut für Botanik, Technische Universität, Schnittspahnstr. 3, 64287 Darmstadt, Germany
* To whom correspondence should be addressed. Fax: +49 931 888 6158. E-mail: hartung{at}botanik.uni-wuerzburg.de
Received 14 January 2004; Accepted 5 May 2004
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Abstract |
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The intensity of an ABA (abscisic acid) signal as a root-to-shoot signal, as well as its action on root hydraulic conductivity, strongly depends on the distribution of ABA during its radial transport across roots. Therefore ABA was visualized by immunolocalization with monoclonal ABA antibodies under conditions of lateral water flow induced by the application of a pressure gradient to the cut surface of the mesocotyl of maize seedlings. From the labelling of rhizodermis, hypodermis, cortical cells, and endodermis of roots of hydroponically (no exodermis) and aeroponically (with exodermis) grown seedlings it is concluded that the exodermis acts as a barrier to apoplastic transport that controls ABA uptake and efflux, but that the endodermis can easily be overcome via an apoplastic bypass. In longitudinal sections the strongest ABA signals originated from the root cap and the meristematic root tip, which is in agreement with the non-vacuolated cells of these tissues being an effective anion trap for ABA.
Key words: ABA immunolocalization, exodermis, lateral ABA transport, maize root sections
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Introduction |
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The role of abscisic acid as a long-distance stress signal has now been well established. When the soil is getting dryer ABA is synthesized within the roots, released to the xylem, and transported to the leaf blades where transpiration is restricted and to the leaf meristems, where leaf development is adapted to the environmental stress conditions (Sauter et al., 2001
). In addition to its synthesis, ABA may be of external origin, as released from other plant roots and from ABA-producing micro-organisms such as soil fungi. Under natural conditions, ABA and its conjugates were shown to be present in the soil solution of moist soils at concentrations up to 30 nM (Hartung et al., 1996; Sauter and Hartung, 2000).
The intensity of the ABA stress signal to the stem must also be regulated during lateral transport through the roots. Assuming that ABA is transported within the symplast, changes of water flow, mainly due to varying transpiration, will dilute or concentrate ABAxyl. An apoplastic bypass flow of ABA as shown by Freundl et al. (1998) could compensate for the dilutions caused by increased transpiration. The Casparian bands of the endodermis could form a first barrier to ABA transport. Once a second apoplastic barrier, the Casparian bands in the hypodermis have been formed, the uptake and lateral transport of external ABA are also affected (Hartung et al., 2002).
The aim of this study was to show lateral transport of ABA and its possible regulation in maize roots. Immunolocalization of ABA in cross-sections of maize roots, grown hydroponically (without exodermis), and aeroponically (with exodermis) was used to visualize the distribution of ABA under transpiring and non-transpiring conditions. For cytokinins and auxin in plant tumours, immunolocalization with monoclonal antibodies has been established by Veselov et al. (2003). Their technique was adapted and optimized for ABA detection in root tissues. Until now, few attempts have been undertaken to study ABA distribution in plant tissues by immunolocalization. Sossountzov et al. (1985) and Pastor et al. (1995) used polyclonal ABA antibodies for immunogold and immunofluorescence experiments to study the ABA compartmentation in green plant cells. Wächter et al. (2003) recently showed ABA accumulation in xylem parenchyma cells in plant tumours and stems. The present study describes for the first time ABA distribution in roots during its radial transport as induced by the application of a pressure gradient to the cut surface of the mesocotyl, thus imitating the suction force of the xylem. By applying this technique it was possible to demonstrate that a significant portion of external ABA from the soil is transported from the cortex directly across the endodermis into the metaxylem vessels and that the exodermis can prevent ABA loss into the rhizosphere.
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Materials and methods |
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Plant material
Seeds of maize (Zea mays L. cv. Helix, Kleinwanzlebener Saatzucht AG, Einbeck, Germany) were germinated on filter paper soaked with 0.5 mM CaSO4 at 26 °C in the dark for 4 d. Seedlings were transferred to aerated hydroponic or to aeroponic culture and kept in a greenhouse with an additional light source (mercury vapour lamp, 200 µmol m–2 s–1; day/night 16/8 h, 25/17 °C) for 7 d. The standard nutrient solution used was modified from that of Pirson and Seidel (1950)
and contained the following nutrients: 1.5 mM KH2PO4, 2.0 mM KNO3, 1.0 mM CaCl2, 1.0 mM MgSO4, 18 µM FeNaEDTA, 8.1 µM H3BO3, and 1.5 µM MnCl2 at pH 5.5. The conditions of hydroponic and aeroponic culture were previously described in detail (Freundl et al., 1998).
Immunolocalization of ABA
For immunolocalization studies 11-d-old Zea mays seedlings, grown either hydroponically or aeroponically, were used. The average length of the hydroponically grown roots was 32 cm, that of the aeroponically grown roots 44 cm. The total root surface area of the root system of the seedlings, measured with a computer-assisted system (scanwise 1.2.0.5
[EC]
, Agfa; WinRHIZO Pro V.2002b, Regent Instruments Inc.) was 87 cm2 and 120 cm2, respectively. A radial water flow was established either by transpiration in intact seedlings kept in the light or by applying a suction pressure of –0.06 MPa to the cut surface of excised root systems at the level of the mesocotyl emerging in an aerated nutrient solution, as described in detail by Freundl et al. (1998). In some of the experiments 100 nM (+)-cis-trans-ABA (Sigma Chemicals, Deisenhofen, Germany) was added to the medium surrounding the roots. This ABA concentration was used for the following reasons: (i) it can be expected in the apoplast of root tissues of stressed plants (Slovik et al., 1995); (ii) in the plants of this study it causes an increase of ABAxyl of up to 30 nM, which is in the range of moderately stressed maize plants in natural conditions (Sauter and Hartung, 2000); and (iii) this ABA concentration increases the water conductivity of maize roots substantially (Hose et al., 2000).
Tissue preparation
In order to immobilize ABA immediately by covalent binding to proteins after a suction experiment, root segments of different zones were fixed for at least 24 h with 3% (w/v) paraformaldehyde in 4% (w/v) 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC) containing 0.1% (v/v) Triton X-100 at 4 °C. After washing the fixed samples with stabilizing buffer (SB; 50 mM HEPES, 5 mM MgSO4, 5 mM EGTA, pH 6.9) and PBS (phosphate buffered saline: 137 mM NaCl, 2.7 mM KCl, 7.9 mM Na2HPO4, 1.5 mM KH2PO4, pH 7.3) to remove the fixative, the samples were dehydrated with a graded series of ethanol/PBS. Tissues were then infiltrated with increasing concentrations of Steedman's wax (a polyester with low melting point; polyethyleneglycol-distearate in 1-hexadecanol 9:1 (w/w); Vitha et al., 1997). Sections of 11 µm were prepared with a sliding microtome (HN 40, Jung, Heidelberg, Germany) and collected on poly-L-lysine-coated slides.
Immunocytochemistry
Sections were dewaxed in a series of ethanol from 30% to 100% in 20% intervals (no technical grade ethanol because of increased autofluorescence) and rinsed with SB, methanol (–20 °C) and PBS/1% Tween 20 (v/v). Before incubating overnight with the primary monoclonal ABA antibody (ABA-15-I-c-5; Agdia/Linaris, Wertheim-Bettingen, Germany), the sections were pretreated with rabbit serum for 1 h to reduce unspecific binding. The mouse monoclonal hybridoma antibody, raised against ABA–BSA conjugates, is highly specific to (+)-cis-trans-ABA (Weiler, 1982). After washing with PBS/Tween 20 the sections were incubated with the red Alexa conjugate 568 (568 goat anti-mouse IgG, H+L, Molecular Probes, Göttingen, Germany; excitation 568 nm, emission 598–608 nm) as a secondary antibody for 2 h. After washing with PBS and staining with toluidine blue (to quench the autofluorescence of the lignified cell walls; Peterson, 1988) and with aniline blue, sections were embedded in 1,4-diazabicyclo-(2,2,2)octane (DABCO; 25 mg ml–1 in PBS/glycerol 1+9). The covered slides were sealed with nail varnish. Sections were viewed with a confocal laser scanning microscope (CLSM; Zeiss LSM 5 Pascal 5, Axioskop 2 mot plus), excitation 543, HFT 543, NFT 635, emission (BP) 560–615). Images were converted to a false colour glow mode to improve the contrasts. In Figs 1i and 2h (see Results) the vertical bar shows a spectrum of colours from black (no signal) to white (strongest signal) indicating the intensity of fluorescence originating from the secondary antibody, Alexa 568-conjugate, and thus the increasing ABA concentrations. From the intensity of the signals absolute ABA concentrations cannot be concluded. However, ABA-transport experiments performed with the same system under comparable conditions (Freundl et al., 1998, 2000), as well as computer simulations about compartmental ABA concentrations in root cell compartments (Slovik et al., 1995) indicate the fluctuation range of ABA concentration. Several controls were included in order to show the specificity of the method. Some sections were treated with neither ABA antibody nor Alexa, others were treated with ABA antibody or with Alexa separately, others with ABA antibody presaturated with an excess of ABA (500 nM) and with Alexa. The experiments were performed 5–8 times with 60–80 tissue sections each.
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Results |
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The validity of ABA-immunolocalization
Although the monoclonal ABA antibody used in this study is highly specific to (+)-cis-trans-ABA (Mertens et al., 1985
) careful controls were essential to avoid artefacts and misinterpretations. Autofluorescence of the cell walls, especially of the rhizodermis, exodermis, and xylem elements may interfere (Fig. 1k). Therefore, toluidine blue was used to suppress autofluorescence (Fig. 1i, l, m, n), according to Peterson (1988). The control shown in Fig 1i, which was not incubated with the primary and secondary antibodies, was treated with toluidine blue. It did not show ABA signals. In addition, controls without the primary antibody (Fig. 1l) or the secondary antibody (Fig. 1m) did not emit fluorescence signals. When sections were treated with a monoclonal ABA antibody that had been preincubated with an excess of ABA (500 nM) no ABA signals were visible either (Fig. 1n). It is therefore concluded that fluorescence of the sections in Fig. 1a–i, l–n exclusively originate from ABA.
ABA in hydroponically grown maize roots
When ABA was absorbed from a water flow containing 100 nM ABA, created by a –0.06 MPa pressure gradient, the rhizodermis, hypodermis, endodermis, and the early metaxylem vessels with their surrounding xylem parenchyma cells showed strong ABA signals (Fig. 1b), while cortical cells and the late metaxylem vessels were extremely weakly labelled. In the rhizodermis and hypodermis, the radial cell walls exhibited the strongest label, together with the early metaxylem and protoxylem vessels in the stele.
When seedlings were grown without their caryopses, stronger ABA signals appeared in the xylem parenchyma cells and also in the cortical cells (Fig. 1c). When a radial water flow was created by leaf transpiration in intact, non-decapitated, seedlings (Fig. 1e), similar ABA signals were obtained to those under the artificial pressure gradient (Fig. 1b). By comparison, weak signals originated from endogenous ABA (Fig. 1g): only the rhizodermis and hypodermis were labelled.
In cross-sections between the root tip and the root hair zone (17–18 mm behind the root tip, see Fig. 2a, arrow), ABA signals were more homogeneously distributed over the sections and were more intensive than in the root hair zone (Fig. 2b, d).
ABA in aeroponically grown roots
When maize seedlings were grown aeroponically (Fig. 1d), i.e. under more natural conditions, Casparian bands were formed in the hypodermis. This barrier was shown to reduce uptake of external ABA and to prevent the release of internal ABA (Hose et al., 2000, 2001). Different from the endodermis, these barrier properties became evident by the strong labelling of the outer tangential cell walls of the exodermis and the rhizodermis, whereas the inner tangential walls of the exodermis only emitted weak signals (Fig. 2h). This was also obvious in the fluorescence intensity profiles of Fig. 3. In addition, the radial walls of the endodermis exhibited a particularly strong fluorescence (Fig. 2f, g). Together with the weak label of the inner and outer tangential walls of the endodermis, this is consistent with an apoplastic flow of the lipophilic ABA directly across the Casparian bands of the endodermis. Signals from endogenous ABA resembled those of hydroponically grown roots (Fig. 1h).
ABA in longitudinal sections
In longitudinal sections of maize seedlings, the strongest signals were emitted from the root cap and the meristematic root tip (Fig. 2b). Here the whole cells were labelled due to scarce vacuolization of the meristematic cells. The more basal regions fluoresce much less. From 5.9 mm onwards, stelar cells exhibited significant signals (Fig. 2c). The longitudinal sections indicate that the root tip apically from the root hair zone seems to be less important for radial and longitudinal basipetal ABA transport, although weak fluorescence in the intermediate zone could indicate that ABA is transported but not accumulated.
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Discussion |
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In the work presented here the distribution of ABA in the presence of a radial water flow was visualized for the first time. Besides endogenously synthesized ABA, roots also absorb external ABA produced by soil micro-organisms and released from other plant roots (Hartung et al., 1996
). External ABA passes the rhizodermis and the cortical cells predominantly along the lateral cell walls. In all experiments, the strongest ABA-specific signals originated from the cell walls together with the cytoplasmic layer. Vacuoles were not visibly labelled because the vacuolar ABA concentration is lower than the cytosolic by at least a factor of 10 (Kaiser et al., 1985) and because in the vacuoles the protein content, to which ABA could be bound with EDAC, is too low. When cells were not or only weakly vacuolized almost the whole cells were strongly labelled, as obvious from longitudinal (Fig. 2b) and cross-sections (Fig. 2d) obtained from 1.7–1.8 mm behind the tip. In this zone of the maize roots increased ABA concentrations maintain the growth of the root tip under conditions of water deficiency (Saab et al., 1990; Sharp et al., 1990). Whether ABA is exclusively transported in the cell walls or in the adjacent cytoplasmic layer as well cannot be recognized from the images presented here. In the endodermis again the radial cell walls were strongly labelled, including the Casparian bands. The inner and outer tangential walls showed much weaker signals. This is consistent with a direct ABA transport across the radial endodermal cell walls with an apoplastic bypass flow of water. After passing the endodermis, the early metaxylem vessels are rapidly reached. The early metaxylem vessels of maize roots in and slightly above the root hair zone were found to be in a developing stage and to conduct water at very low rates (Aubin et al., 1986). Their cytoplasmic layer will accumulate ABA arriving with the water flow before it is released into the functioning vessels and thus into the transpiration stream.
During its radial transport through the root, ABA also increases the hydraulic conductivity of the root. In the experiments shown here 100 nM ABA stimulated the radial water flow 2-fold, from 2.3x10–9 to 4.6x10–9 m3 m–2 s–1 (D Schraut, unpublished results). From the present results (Figs 1, 2) it can be concluded that the site of ABA action may be in the xylem parenchyma cells (XP) that surround the early and the developing metaxylem vessels. Water flux into roots and its flow into shoots is known to be dependent on both osmotic forces and transpiration. ABA apparently controls ion fluxes as well as water movement. Although the exact site of ABA action is still obscure, XP cells in the stele are very likely targets. Such XP cells in roots have been shown to be highly active in ion release into the xylem vessels (Läuchli et al., 1971). More recently voltage-dependent XP-located channels were found to respond to water stress, and ABA was found to inhibit K+ release from XP into the vessels through the outward rectifying K+ channels SKOR (Roberts, 1998; Gaymard et al., 1998; Roberts and Snowman, 2000). The assumption that ABA also controls water channels (Wan et al., 2004) would be in agreement with the findings of Otto and Kaldenhoff (2000), about a strong gene expression of water channels in the stele of tobacco. By contrast, Hose (2001) detected a higher gene expression of different water channels in the cortex of maize than in the stele. Assuming that those aquaporins are under ABA control, it is suggested that here the hypodermis and exodermis, including the adjacent cortical cells, are the best candidates for additional sites of ABA action. This applies particularly to roots of aeroponically grown seedlings.
The radial ABA transport is diminished in the aeroponically grown roots when the hypodermis exhibits Casparian bands. Freundl et al. (2000) and Hose et al. (2000) showed that the exodermis is a barrier to radial ABA transport, for both efflux and influx. Barrier properties must exist in tissue zones with strong fluorescence, i.e. the rhizodermis and the outer tangential walls of the exodermis (Figs 1d, 2g). In these zones ABA could also accumulate at such barriers. The signals from the inner tangential exodermal walls were much weaker (Fig. 3b). The labelling of the endodermis showed undisturbed flux of ABA directly into the metaxylem vessels. Different from the hydroponically grown roots, cortical cells and the living stelar cells of aeroponically grown roots exhibited stronger fluorescence. This may be a result of the weak leakage of internal and external ABA to the medium, caused by the Casparian bands of the exodermis.
In conclusion, cross-sections and longitudinal sections of maize seedling roots, together with carefully performed control experiments, can be used to visualize the radial transport pathways of ABA. These data are consistent with earlier findings with ABA-flux analyses on the barrier properties of the exo- and endodermis.
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Acknowledgements |
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We are grateful to Dr Markus Langhans (TU Darmstadt) for his valuable advice with immunostaining, to Professor EW Weiler (University Bochum) for the generous supply of ABA antibodies, to Robin Wacker (University Würzburg) for help with the microtome, to Bianca Röger for skilful technical help, and to the Deutsche Forschungsgemeinschaft (Ha 963-11/1) for financial support.
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