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Plant Physiol. (1998) 116: 155-163
The Distal Part of the Transition Zone Is the Most
Aluminum-Sensitive Apical Root Zone of Maize1
Mayandi Sivaguru2 and
Walter J. Horst*
Department of Plant Sciences, School of Biological Sciences,
Madurai Kamaraj University, Madurai 625 021, India (M.S.); and Institute of Plant Nutrition, University of Hannover,
Herrenhäuser Strasse 2, D-30419 Hannover, Germany (W.J.H.)
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ABSTRACT |
For a
better understanding of Al inhibition of root elongation, knowledge of
the morphological and functional organization of the root apex is a
prerequisite. We developed a polyvinyl chloride-block technique to
supply Al (90 µm monomeric Al) in a medium containing agarose to individual 1-mm root zones of intact seedlings of maize (Zea mays L. cv Lixis). Root elongation was measured
during a period of 5 h. After Al treatment, callose (5 h) and Al
(1 h) contents of individual 1-mm apical root segments were determined. For comparison, callose and Al levels were also measured in root segments after uniform Al supply in agarose blocks to the 10-mm root
apex. Only applying Al to the three apical 1-mm root zones inhibited
root elongation after 1 h. The order of sensitivity was 1 to
2 > 0 to 1 > 2 to 3 mm. In the 1- to 2-mm root zone high levels of Al-induced callose formation and accumulation of Al was
found, independently of whether Al was applied to individual apical
root zones or uniformly to the whole-root apex. We conclude from these
results that the distal part of the transition zone of the root apex,
where the cells are undergoing a preparatory phase for rapid elongation
(F. Balu ka, D. Volkmann, P.W. Barlow [1996] Plant Physiol 112:
3-4), is the primary target of Al in this Al-sensitive maize cultivar.
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INTRODUCTION |
It is now generally accepted that the root apex plays a major role
in the Al-perception and -response mechanisms (for recent reviews, see
Delhaize and Ryan, 1995 ; Horst, 1995; Kochian, 1995; Taylor, 1995;
Rengel, 1996). This is especially well demonstrated by the fact that Al
sensitivity is characterized by enhanced accumulation of Al in the root
apex (Delhaize et al., 1993a; Llugany et al., 1994; Samuels et al.,
1997); Al-resistance mechanisms, such as the release of Al-complexing
organic compounds, are confined mainly to the root apex (Horst et al.,
1982; Delhaize et al., 1993b; Pellet et al., 1995); and callose
formation, a sensitive marker of Al toxicity, is primarily induced in
apical root cortical cells (Wissemeier et al., 1987; Zhang et al.,
1994; Wissemeier and Horst, 1995).
In the past the root apex has been divided into three different zones:
the root cap, the MZ, and the EZ. Bennet and Breen (1991a) attributed a
major role to the root cap in the perception of Al toxicity in maize
(Zea mays L.). However, Ryan et al. (1993) concluded from
their divided-chamber experiments that the MZ (0-3 mm behind the root
cap), and not the root cap, was the most Al-sensitive site in maize.
Based on short-term studies of Al effects on cell division and cell
elongation in root apices of soybean, Horst and Klotz (1990) attributed
the inhibition of root elongation by Al to the inhibition of cell
elongation rather than cell division. However, whether Al primarily
interferes with processes related to cell division or cell elongation
has not yet been unequivocally clarified.
In recent years our knowledge of the morphological and functional
organization of the root apex has grown substantially. On the basis of
morphological, cytological, and physiological characteristics, Ishikawa
and Evans (1993, 1995, and refs. therein) and Baluka et al.
(1996, and refs. therein) classified different zones of the maize root
apex on a millimeter scale (the subdivision of EZ into apical EZ and
central EZ is arbitrarily defined by Ishikawa and Evans [1993]) for
maize plants grown in humid air under similar conditions, as in this
study: MZ, 0 to 1.7; distal EZ or TZ, 1.7 to 3.4; apical EZ, 3.4 to
3.9; central EZ, 3.9 to 5.6; and EZ, >5.6. It appeared to us that for
a better understanding of the inhibition of root elongation by Al and
Al resistance, consideration of this spatial organization of the root
apex is a prerequisite. In this report we have tested the responses of
the first 5 mm of the maize root apex individually to study the
differential responses of these zones to Al.
The presented experiments are based on the study of Ryan et al. (1993).
However, we tried to avoid some of the shortcomings of that study by:
(a) using agarose instead of agar as the culture medium (Calba et al.,
1996); (b) increasing the spatial resolution by applying Al to 1-mm
intact root segments; and (c) keeping the seedlings in a vertical
position during the treatment to avoid interference with the
physiological responses to gravity and hormones (Hasenstein et al.,
1988; Ishikawa and Evans, 1993). In addition to growth, induction of
callose formation, the most sensitive indicator of Al toxicity
(Wissemeier et al., 1987), and accumulation of Al in 1-mm sections
along the root apex were measured.
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MATERIALS AND METHODS |
Growth Conditions
Seeds of maize (Zea mays L. cv Lixis) that has been
classified as Al sensitive (Llugany et al., 1994) were soaked in warm tap water for 8 h. Then the water was decanted and the seeds were held at 4 to 6°C for 20 h in darkness to induce synchronized
germination (Shen-Miller et al., 1978). The seeds were germinated
between filter paper-Styrofoam sandwiches for 6 d in a growth
chamber under controlled environmental conditions with 70% RH, 30°C
day and night temperature, and 300 µmol m 2
s1 photon flux density during the 16-h d.
Uniform seedlings were then transferred to the PVC blocks especially
designed to fit the experimental requirements (Fig
1A). Five seedlings were mounted on the
PVC block with different apical root positions placed into the
horizontal 1-mm slit, which was vertically sealed with petroleum jelly
(Fig. 1B). Low-gelling agarose (0.6%) dissolved in NS by heating was
poured into the horizontal slit using a fine-tipped Pasteur pipette
just before solidification to completely embed the 1-mm root segments.
For the Al treatments, 300 µm Al from an Al atomic
spectroscopy standard stock solution (1000 mg
L1, Fluka) was added to the cooled agarose
solution to achieve a final monomeric Al concentration (aluminon
method, according to Kerven et al. [1989]) of 90 µm,
measured after filtration through a 0.4-µm cellulose acetate filter
(Sartorius, Hayward, CA). This agarose solution was eluted from the
solidified agarose gel (see above), which was incubated for 2 d
under room temperature, frozen at  20°C for 120 min, and then
centrifuged for 3 min at 12,000g.
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| Figure 1.
The experimental setup of the PVC-block technique
used in the present study (A). One PVC block consisted of five vertical slits, allowing us to mount five seedlings simultaneously at five different ATS positions (B). The horizontal slits were filled with
agarose medium after positioning the roots and sealing the vertical
slits with petroleum jelly. The setup was immersed in an upright
position into NS up to the level indicated by the arrow. Once the
seedlings were in place, the roots were covered with filter paper and
kept moistened by frequent spraying with NS. ATS in relation to
described morphological and physiological organization of the root apex
(see introduction) are shown in B. AEZ, Apical EZ; CEZ, central EZ.
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Great care was taken to keep the whole-root system moist during all
manipulations and the subsequent treatment period. This was achieved by
soaking the plant-supporting Styrofoam and filter paper in NS, covering
the roots with NS-moistened filter paper, and spraying profusely with
NS using a hand sprayer. The NS used for the preparation of the agarose
medium had the following composition (µm):
CaSO4, 250; KNO3, 400;
MgSO4, 100; MnSO4, 1;
ZnSO4, 0.1; CuSO4, 0.2;
KH2PO4, 10;
H3BO3, 8;
(NH4)6Mo7O24,
0.1; and NH4NO3, 200. The
pH of the NS was adjusted to 4.3 with 0.1 n HCl. The mounting of the plants on one horizontally positioned PVC block never
took more than 5 to 10 min. Afterward, the PVC block was placed in an
upright position into plastic troughs containing NS to the level marked
by an arrow in Figure 1A and placed in a growth chamber under the
conditions described above. At the end of the treatment period, 1 h for Al uptake and 5 h for the determination of root-elongation
rate and callose formation, 5-cm-long root tips were dissected under
double-distilled water and then fixed in 96% ethanol. In addition to
the PVC-block technique, which allowed the Al treatment of 1-mm root
segments, parallel experiments were performed in which the whole apical
10-mm root tips of intact seedlings were inserted in agarose blocks (50 mm × 50 mm) in small plastic trays with or without Al under
conditions similar to those described above.
Determination of Root-Elongation Rate
Root-elongation rate was calculated from measurements of root
lengths at 1-h intervals under 40× magnification against a scale, and
by having the horizontal slit of the PVC block as the reference line.
The precision of every measurement was 125 µm. Because the roots were
positioned vertically, gravity-induced curvature did not complicate the
measurements.
Determination of Callose in Root Segments
Roots fixed in 96% ethanol to avoid formation of wound callose
were briefly washed and kept in a Petri dish containing
double-distilled water. Under a stereomicroscope (Stemi SV8, Zeiss)
with 20× magnification, roots were cut into 10 consecutive 1-mm
segments starting at the apex, including the cap, using a razor blade.
Two (Al treatment of individual 1-mm root segments) or 10 (Al treatment
of the entire root apex) root segments from the identical DFT positions
were pooled together to increase the precision of callose
determination. After dissection, the segments were carefully blotted
dry and transferred immediately to Eppendorf tubes containing 1 m NaOH. Callose levels were estimated following the method
of Köhle et al. (1985), which was modified slightly to increase
its sensitivity. Briefly, each sample containing the similarly treated
root segments in NaOH was ultrasonicated directly (Bandalin Sonopuls,
Bandalin Electronics, Berlin, Germany) for 1 min. Subsequently, the
samples were placed in a water bath (80°C) for 30 min to solubilize
the callose and then centrifuged for 15 min at 12,000g at
room temperature. Callose concentration in the supernatant was
quantified fluorimetrically (f-2000, excitation at 393 nm and emission
at 484 nm; Hitachi, Tokyo, Japan) with decolorized aniline blue using
Pachyman (250-690 polymerization grade, Calbiochem) as a reference.
The callose contents of 1-mm Al treatments were expressed as Al-induced
callose, which means that the callose levels of respective control
segments (Al) were subtracted from the Al-treated root segments.
Callose contents are expressed as Pachyman equivalents on a millimeter basis.
Determination of Al in Root Segments
After a brief rinse of the excised roots in upw (18.2 M ,
E-pure, D4642, Barnstead, Dubuque, IA), individual 1-mm root segments were dissected under upw from fresh root tips within 30 min after the
treatment period. The root segments were individually placed into
Eppendorf cups containing 250 µL of upw, frozen, and kept at 20°C
until analysis. For the Al analysis the root segments including the upw
were transferred to 5-mL Teflon cups and heated on a hot plate to
120°C to evaporate the upw. Then 500 µL of concentrated, double-distilled, ultra-pure HNO3 was added, and
the temperature increased to 240°C for the digestion of the root
segments until the acid was completely evaporated. Because of the low
Al concentrations in the samples (from 0.002 to 0.35 nmol), great care
had to be taken to avoid Al contamination while preparing the samples.
This was achieved by using upw and 1:30 (v/v)
HNO3:upw-washed Teflon labware. The ash was
dissolved in 2 mL of 1:30 (v/v) HNO3:upw. Samples
were analyzed for Al using a UNICAM 939 QZ graphite furnace atomic
absorption spectrophotometer (Analytical Technology Inc., Cambridge,
UK) with Zeeman background compensation. Instrumental adjustments were
optimized for sensitivity. Al contents of Al-treated root segments were
corrected for mean Al contents from blanks and root segments not
treated with Al (control).
Statistical Analysis
Results are presented from a representative of six (root
elongation), two (Al content), and two (callose content) experiments. Root-elongation experiments consisted of five independent replicates for each ATS and control position. All other experiments had three to
six independent replicates. The statistical package SAS (version 6.11, SAS Institute, Cary, NC) was used for the calculation of the
se and comparisons of the means (Tukey's test).
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RESULTS |
Root Elongation
In the first approach Al was applied to the entire root apex (10 mm) using the agarose-block technique. In the presence of 90 µm Al, root elongation was inhibited as early as 1 h
after the commencement of the Al treatment. Root-elongation rate was reduced to about 50% of the control over the entire 5-h Al-treatment duration (Fig. 2, A and B). Application
of Al to individual 1-mm segments of the root apex led to inhibition of
root elongation especially when the first 3-mm apical root segments
were exposed to Al (Fig. 3). It appeared
that the immediate (0-2 h) comparative sensitivity to Al between the
root segments was 1 to 2 > 0 to 1 > 2 to 3 mm. With longer
duration of the Al treatment, the roots seem to resume elongation
growth. The immediate response and recovery from the initial Al
response becomes distinct when root-elongation rates after 1 h are
compared (Fig. 4). A differential
response pattern between the root segments is apparent. Application of Al to the 0- to 1-mm segment becomes increasingly inhibitory to root
elongation, whereas the inhibition is initially (after 1 h)
maximum when Al is applied to the 1- to 2-mm segment (significant at
P < 0.05 for the comparison of comparable root
segments between ATS positions). With longer Al treatment duration,
root-elongation rates nearly completely recovered: after 5 h of Al
supply, root-elongation rate was no longer significantly different from
the untreated control. Although there was a tendency toward decreased
root-elongation rates when Al was applied to the 3- to 4-mm and 4- to
5-mm root segments, this difference was generally insignificant
compared with the untreated controls.
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| Figure 2.
Effect of Al supply (5 h) in agarose medium to the
entire (10 mm) root apex on the root elongation (A) and elongation rate (B) of seedlings of cv Lixis. Means ± se are of five
independent replicates. Means with different letters are significantly
different (P < 0.05, Tukey test) with
regard to the Al effect.
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| Figure 3.
Effect of Al supply (5 h) in agarose medium
(PVC-block technique) to individual 1-mm apical root segments on the
root elongation of seedlings of cv Lixis. A, ATS 0 to 1 mm; B, 1 to 2 mm; C, 2 to 3 mm; D, 3 to 4 mm; and E, 4 to 5 mm. Means ± se are of five independent replicates.
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| Figure 4.
Effect of Al supply (5 h) in agarose medium
(PVC-block technique) to individual 1-mm apical root segments on the
root-elongation rate of seedlings of cv Lixis. Means ± se are of five independent replicates. Means with different
letters are significantly different (P < 0.05, Tukey test) with regard to the Al effect.
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Because the roots continued to grow during the 5-h Al treatment period,
the Al-treatment zone moved along the root. Therefore, in Figure
5 root-elongation rates are presented
in relation to the root segment in contact with Al after every 1 h
of Al treatment. This presentation clearly shows that the root becomes
increasingly sensitive to Al when the treatment zone moved into the 1- to 2.5-mm section and increasingly recovered when it moved out of this
zone. Only one value (0- to 1-mm Al-treated segment after 4 h)
does not fit this pattern. It is further confirmed that application of
Al, not only to the 3- to 5-mm root segments but also up to 20 mm, did
not lead to significant inhibition of root elongation.
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| Figure 5.
Effect of Al supply (5 h) in agarose medium
(PVC-block technique) to individual 1-mm apical root segments on the
root-elongation rate of seedlings of cv Lixis. Root length was measured
at 1-h intervals and root-elongation rates were calculated. Initially (time 0) Al was applied to the 1- to 5-mm apical root segments. The
position of the ATS after every 1 to 5 h of treatment is
visualized by the hatched areas. The numbers represent mean elongation
rates expressed as percent over control for the previous 1-h period. Numbers with asterisks (*) are significantly different (P < 0.05, Tukey test).
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Callose Formation
Callose formation is a sensitive indicator of Al injury to roots.
In a preliminary approach Al was applied by incubating the whole 10-mm
root apex using agarose blocks (Fig.
6). Callose contents were higher in
Al-treated roots by a factor of about 10. They showed a sharp maximum
(factor 3) in the root segment for 1 to 2 mm and then declined steadily
toward more basal parts of the root. Also, in the controls without Al a
slight peak of callose content could be seen.
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| Figure 6.
Effect of Al supply (5 h) in agarose medium
(agarose blocks) to the 10-mm root apex on callose contents (Pachyman
equivalents [PE]) of individual 1-mm root segments of seedlings of cv
Lixis. Means ± se are of three independent
replicates, each comprising 10 1-mm root segments of identical DFTs.
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When Al was applied for 5 h to individual 1-mm root segments, the
pattern of Al-induced callose formation showed a similar pattern along
the root apex only when Al was applied to the first two apical root
segments (Fig. 7). Generally, the callose
content was lower by a factor of 2, probably because of the fact that the individual root segments of the growing root were in contact with
Al for a shorter period of time, compared with the preliminary experiment. Application of Al to root segment 1 (Fig. 7A) and segment 2 (Fig. 7B) led to maximum callose formation in root segments 2 and 3, respectively. Al-induced callose formation gradually decreased toward
more basal root segments but remained significantly different from
zero. Application of Al to more basal apical root segments (Fig. 7,
C-E) did not induce callose formation in the treated or adjacent root
segments. There was a tendency for enhanced callose formation only in
the basal root segments.
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| Figure 7.
Effect of Al supply (5 h) in agarose medium
(PVC-block technique) to 1-mm apical root segments on Al-induced
callose contents (expressed as Pachyman equivalents [PE]) of
individual 1-mm root segments of seedlings of cv Lixis. A, ATS 0 to 1 mm; B, 1 to 2 mm; C, 2 to 3 mm; D, 3 to 4 mm; and E, 4 to 5 mm.
Means ± se are of six independent replicates, each
comprising four 1-mm root segments of identical DFTs. Means with
different letters are significantly different (P < 0.05, Tukey test) with regard to the Al effect.
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Al Content
In contrast to the experiments described above, to focus on the
initial Al effects on root elongation Al was applied only for 1 h
(compare Figs. 3 and 4). In a first approach, Al was uniformly applied
to the whole 10-mm apex using agarose blocks (Fig.
8). There was a maximum Al content in the
root segment at 1 to 2 mm, sharply declining to control (without Al)
levels toward the apex and more-basal root segments. The same pattern
of distribution of Al between the root segments was found when Al was
individually applied only to the 1- to 2-mm segment (Fig.
9B). Treatment of the other root segments
did not lead to significantly higher Al contents in the treated
compared with the untreated root segments, although a slight
increase in the treated 3- to 5-mm segments (Fig. 9, C-E) and a
broader peak in the three apical segments could be noticed when Al
was applied to the 0- to 1-mm segment (Fig. 9A).
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| Figure 8.
Effect of Al supply (1 h) in agarose medium
(agarose blocks) to the 10-mm root apex on Al contents of individual
1-mm root segments of seedlings of cv Lixis. Means ± se are of six independent replicates, each comprising one
1-mm root segment.
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| Figure 9.
Effect of Al supply (1 h) in agarose medium
(PVC-block technique) to 1-mm apical root segments on Al contents of
individual 1-mm root segments of seedlings of cv Lixis. A, ATS 0 to 1 mm; B, 1 to 2 mm; C, 2 to 3 mm; D, 3 to 4 mm; and E, 4 to 5 mm.
Means ± se are of six independent replicates each
comprising one 1-mm root segment. Means with different letters are
significantly different (P < 0.05, Tukey
test) with regard to the Al effect.
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DISCUSSION |
With the present experimental setup, we have tried to avoid some
of the shortcomings of the previous study by Ryan et al. (1993) on the
spatial sensitivity of maize roots to Al. First, the seedlings were
kept in a vertical position to avoid any disturbance of the root
response to gravity, such as modification of the pH gradients
(Monshausen et al., 1996), Ca, and auxin flow (Hasenstein and Evans,
1988; Ishikawa and Evans, 1992, 1993), which might interfere with the
response of the roots to Al. Second, agarose was used instead of agar,
because the phytotoxic level of Al in the agarose, but not in the agar
solution, is comparable to NS (Calba et al., 1996). Third, the total Al
supply to individual root segments was increased considerably by the
greater agarose volume in contact with the root zone. Nevertheless, a
much higher Al concentration had to be applied for rapid inhibition of
maize root elongation than in NS (Hasenstein and Evans, 1988; Ryan et al., 1993), because in NS, a much larger pool of Al is in direct contact with the root. Fourth, Al could be applied to 1-mm root segments, which allowed us to increase the resolution of the Al sensitivity of the root apex, which was necessary to relate the Al
response corresponding to the spatial organization of the root apex
(Fig. 1B) (Ishikawa and Evans, 1993, 1995; Baluka et al., 1994,
1996).
We are aware of the fact that the length of the different developmental
apical root zones may vary according to the experimental conditions, as
was shown by Ishikawa and Evans (1995). Therefore, we placed our
emphasis on the creation of experimental conditions similar to those
described by Ishikawa and Evans (1993). It is out of the scope of the
present study to verify the accuracy of the spatial organization of the
maize root apex for our experimental conditions, because the
conclusions drawn by us do not require accuracy at the 0.1 mm level.
A shortcoming of the technique applied here might be the fact that the
site of Al treatment moved during the course of the experiment because
of root growth. However, as shown in Figure 5, the dynamics of the
Al/root-zone interaction provided further support to our suggestion
that the distal part of the EZ is the most Al-sensitive site of the
root apex: (a) application of Al to the 1- to 2-mm, and not the 0- to
1-mm, root segment led to the most severe inhibition of root
elongation; (b) movement of the Al-treated segment from the root tip
into the 1- to 2.5-mm position enhanced inhibition of root elongation,
and movement out of this zone led to alleviation of this inhibition;
and (c) recovery from Al injury was faster after Al exposure to the 2- to 3-mm root segment compared with the 1- to 2-mm root segment.
This suggestion is strongly supported by the following results of the
present study:
(a) The maximum induction of callose formation by Al occurs in this
root zone (Figs. 6 and 7). Induction of callose formation has been
shown to be a sensitive response to Al in roots and indicative of
genotypic differences in Al sensitivity in soybean (Wissemeier and
Horst, 1995), wheat (Zhang et al., 1994), and maize (Llugany et al.,
1994; Horst et al., 1997).
(b) The high Al sensitivity of the DTZ is related to preferential Al
accumulation in this root zone (1-2 mm) when Al is supplied uniformly
to the whole-root apex (Fig. 8) or specifically to this root zone (Fig.
9) as well. Accumulation of Al in the apical 0- to 2-mm root zone has
recently been shown to be indicative of genotypic Al sensitivity in
wheat (Samuels et al., 1997). This is in agreement with the
classification of the maize cultivar used in this study as Al sensitive
(Llugany et al., 1994; Horst et al., 1997). Al resistance seems to be
related to the plant's capacity to protect this sensitive root zone
from Al, e.g. by the release of organic acids such as malate in wheat
(Delhaize et al., 1993b; Ryan et al., 1995) and citrate in maize
(Pellet et al., 1995).
These authors have demonstrated that the root apex is the most
important site of organic acid excretion. However, based on the results
presented in this study it is desirable to differentiate between the
different apical root zones of the root apex. The reason for
preferential Al accumulation in the DTZ compared with the main MZ (0-1
mm) and EZ is not known. Reduced Al uptake by the main MZ might be
attributable to excretion of mucilage, which strongly binds Al
(Archambault et al., 1996) and protects the root apex from Al injury
(Horst et al., 1982). In the EZ, in contrast to the DTZ and TZ, an
enhanced release of protons, leading to acidification of the apoplast
(Monshausen et al., 1996) and thereby facilitating root growth
(Weisenseel et al., 1992; Zieschang et al., 1993), could reduce the
binding of Al to negative charges (Grauer and Horst, 1992; Kinraide,
1993).
The results presented do not support the hypothesis put forward by
Bennet and Breen (1991b) that the root cap is the most sensitive
perceptor of the Al signal. In contrast, they are in agreement with the
results of Ryan et al. (1993), who showed that in horizontally growing
maize roots the removal of the root cap did not modify the response of
the roots to Al, and concluded that the root zone 0 to 3 mm behind the
root cap is most sensitive to Al. In our study we could localize the
most Al-sensitive root zone even more precisely: following the proposed
spatial organization of the root apex (Fig. 1B) the most Al-sensitive
zone is the DTZ (Baluka et al., 1996) or distal part of the
distal EZ (Ishikawa and Evans, 1993, 1995). Particularly, this zone
(DTZ) is characterized by a switch from cell division to cell
elongation by the presence of cells that are changing their mitotic
mode and undergoing a preparatory phase for rapid elongation. It is
noteworthy that the TZ is the zone most responsive to a variety of
environmental stimuli other than Al, such as gravitropism and auxin
(Meuwly and Pilet, 1991; Ishikawa and Evans, 1993), thigmotropism
(Ishi-kawa and Evans, 1992), mechanical impedance (Ishikawa and
Evans, 1990), and drought stress (Sharp et al., 1988). Although our
results indicate a special role of the distal part of the TZ in the
expression of Al toxicity, we do not exclude that the proximal part of
the MZ may also be involved.
Al accumulation in the DTZ led to a rapid inhibition of root elongation
within less than 1 h (Figs. 4B and 9). This is difficult to
reconcile with a direct effect of Al on rapid cell elongation in the
EZ. Therefore, it is necessary to envisage interference of Al in the
DTZ with the signaling system involved in the regulation of cell
elongation. Earlier work by Hasenstein et al. (1988) also points to the
same conclusion, although it must be interpreted with care because of
the extremely high Al concentration and the pH buffer used. They showed
that application of Al to one side of the cap of a vertically growing
maize root induced curvature away from the Al source. In a subsequent
study Hasenstein and Evans (1988) presented evidence that this Al
effect was related to localized inhibitory effects of Al on basipetal
auxin transport in the root cortex.
In conclusion, on the basis of the presented results we suggest that
the DTZ of the root apex is the primary target of Al in an Al-sensitive
maize cultivar. Further elucidation of Al toxicity and resistance
requires focus to be placed on this root zone and on the signaling
pathways between it and the EZ.
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FOOTNOTES |
1
This research was supported by a grant from the
German Research Foundation to W.J.H., and from an Indo (Ministry of
Human Resource Development, Department of Education, Government of
India)-German postdoctoral fellowship awarded by the German Academic
Exchange Service, Bonn, to M.S.
2
Present address: Institute of Plant Nutrition,
University of Hannover, Herrenhäuser Strasse 2, D-30419 Hannover,
Germany.
*
Corresponding author; e-mail horst{at}mbox.pflern.uni-hannover.de; fax
49-511-7623611.
Received June 2, 1997;
accepted September 8, 1997.
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ABBREVIATIONS |
Abbreviations:
ATS, Al-treated segment.
DFT, distance from tip
(including cap).
DTZ, distal part of the transition zone.
EZ, elongation zone.
MZ, meristematic zone.
NS, nutrient solution.
PVC, polyvinyl chloride.
TZ, transition zone.
upw, ultra-pure water.
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ACKNOWLEDGMENTS |
We sincerely thank Hock Werner (Institute for Horticultural
Engineering, University of Hannover) for his patience in manufacturing the PVC blocks to our specifications, Lutz Collet for his support during the refinement of the Al analysis, and the anonymous reviewers for their critical comments on the manuscript.
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LITERATURE CITED |
Archambault DJ,
Zhang G,
Taylor GJ
(1996)
Accumulation of Al in root mucilage of an Al-resistant and an Al-sensitive cultivar of wheat.
Plant Physiol
112:
1471-1478
[Abstract]
Baluka F,
Barlow PW,
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Abstract
Introduction