Genotypical Differences in Aluminum Resistance of Maize Are ...

Genotypical Differences in Aluminum Resistance of MaizeAre Expressed in the Distal Part of the Transition Zone. Is

Reduced Basipetal Auxin Flow Involved in Inhibition of RootElongation by Aluminum?1

Malte Kollmeier, Hubert H. Felle, and Walter J. Horst*

Institute of Plant Nutrition, University of Hannover, Herrenhauser Strasse 2, D–30419 Hannover, Germany(M.K., W.J.H.); and Institute of General Botany and Plant Physiology, University of Giessen,

Senckenbergstrasse 17–21, D–35390 Giessen, Germany (H.H.F.)

Short-term Al treatment (90 mM Al at pH 4.5 for 1 h) of the distaltransition zone (DTZ; 1–2 mm from the root tip), which does notcontribute significantly to root elongation, inhibited root elongationin the main elongation zone (EZ; 2.5–5 mm from the root tip) to thesame extent as treatment of the entire maize (Zea mays) root apex.Application of Al to the EZ had no effect on root elongation. Highergenotypical resistance to Al applied to the entire root apex, andspecifically to the DTZ, was expressed by less inhibition of rootelongation, Al accumulation, and Al-induced callose formation,primarily in the DTZ. A characteristic pH profile along the surfaceof the root apex with a maximum of pH 5.3 in the DTZ wasdemonstrated. Al application induced a substantial flattening of thepH profile moreso in the Al-sensitive than in the Al-resistant culti-var. Application of indole-3-acetic acid to the EZ but not to themeristematic zone significantly alleviated the inhibition of rootelongation induced by the application of Al to the DTZ. Basipetaltransport of exogenously applied [3H]indole-3-acetic acid to themeristematic zone was significantly inhibited by Al application tothe DTZ in the Al-sensitive maize cv Lixis. Our results provideevidence that the primary mechanisms of genotypical differences inAl resistance are located within the DTZ, and suggest a signalingpathway in the root apex mediating the Al signal between the DTZand the EZ through basipetal auxin transport.

The initial effect of Al toxicity is the inhibition of rootelongation, a dramatic effect occurring within minutes af-ter application. It is generally accepted that the root apexplays the major role in Al perception and response (forrecent reviews, see Delhaize and Ryan, 1995; Horst, 1995;Kochian, 1995; Taylor, 1995; Rengel, 1996). This is welldemonstrated by the fact that: (a) Al accumulation as anindicator of Al sensitivity occurs in the distal parts of theroot apex (Delhaize et al., 1993a; Llugany et al., 1994;Sivaguru and Horst, 1998); (b) 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 (c) calloseformation, as a sensitive marker of Al sensitivity, is in-duced primarily in apical cells of the outer cortex (Wisse-meier et al., 1987; Zhang et al., 1994; Wissemeier and Horst,1995; Sivaguru and Horst, 1998). However, the question ofthe primary target of the Al effect remained open untilrecently.

Ryan et al. (1993) showed that the root tip was most Alsensitive. Sivaguru and Horst (1998) presented evidencethat the distal transition zone (DTZ) is the most Al-susceptible zone of the primary root of the Al-sensitivemaize (Zea mays) cv Lixis. Subsequently, Sivaguru et al.(1999a) and Horst et al. (1999) showed that Al leads toalterations in the organization of microtubules and actinmicrofilaments, which were most severe in the DTZ. Al-though it is still a matter of debate whether Al affects theroot symplastically or apoplastically, evidence increasesthat the apoplast plays the major role in Al perception(Horst, 1995). Al binds strongly to the cell wall of rootepidermal and cortical cells (Delhaize et al., 1993a), andHorst et al. (1999) demonstrated the role of pectin contentin different apical root zones of maize for the Al response.Apart from negative charge properties of the cell wall andits cation exchange capacity, the exudation of organic com-pounds complexing Al (Delhaize et al., 1993b; Basu et al.,1994; Pellet et al., 1995) and the maintenance of higherapoplastic pH (Degenhardt et al., 1998) may play an addi-tional or even more important role in the response ofdifferent species and genotypes to Al.

In this study we extend our former findings about thespatial sensitivity of the primary root of maize on genotyp-ical differences between the Al-sensitive maize cv Lixis andthe Al-resistant cv ATP-Y. We paid particular attention tothe relationship between root surface pH and root-growthdynamics as affected by Al. Furthermore, we consideredthe role of indole-3-acetic acid (IAA) in Al toxicity, testingthe hypothesis that Al might influence basipetal IAA trans-port in the root cortex, which is considered to contributeto the regulation of root growth and development (Hasen-stein and Evans, 1988; Kerk and Feldman, 1994; Rueggeret al., 1997).

1 This work was supported by the Deutsche Forschungsgemein-schaft within the Special Research Program 717 (“The Apoplast ofHigher Plants”).

* Corresponding author; e-mail [email protected]; fax 49–511–762–3611.

Plant Physiology, March 2000, Vol. 122, pp. 945–956, www.plantphysiol.org © 2000 American Society of Plant Physiologists

945 www.plantphysiol.orgon April 16, 2018 - Published by Downloaded from

Copyright © 2000 American Society of Plant Biologists. All rights reserved.

http://www.plantphysiol.org

MATERIALS AND METHODS

Plant Material, Growth Conditions, andExperimental Treatments

Seeds of the maize (Zea mays L.) cv ATP-Y, which isclassified as Al resistant, and cv Lixis, which is classified asAl sensitive by Llugany et al. [1994]; Horst et al. 1997), weresoaked in tap water for 8 h. Selected seeds of equal size andshape were germinated in filter paper rolls moistened withnutrient solution for 3 d in a growth chamber under con-trolled environment conditions with 70% relative air hu-midity, 30°C/28°C day/night temperature, a day/nightcycle of 16 h/8 h, and a 300 mm m22 s21 photon fluxdensity.

All experiments were conducted with intact plants. Uni-form seedlings with primary root lengths of about 7 to 10cm were selected for the experiments. Seedlings wereadapted to low pH by stepwise lowering of pH over 24 hbefore the beginning of the experiments.

Local Al treatment was provided by agarose blocks con-taining nutrient solution and 0.6% (w/v) low-gelling-temperature agarose (Fluka, Deisenhofen, Germany) (1.2%for the electrophysiological experiments and measure-ments of the partial root elongation). The nutrient solutionhad the following composition: CaSO4, 250 mm; KNO3, 400mm; MgSO4, 100 mm; FeEDDHA, 20 mm; MnSO4, 1 mm;ZnSO4, 0.2 mm; CuSO4, 0.2 mm; KH2PO4, 10 mm; H3BO3, 8mm; (NH4)6Mo7O24, 0.1 mm; and NH4NO3, 200 mm (pH 4.3).

For the study on the short-term effects of Al, the treat-ment duration was 1 h in all experiments at 30°C temper-ature in a growth chamber, except for the electrophysio-logical experiments, which were conducted under a 21°C to23°C ambient temperature. The Al supply was a 90 mmmonomeric Al obtained from an Al atomic spectroscopystandard solution (AlCl3z6H2O, 1,000 mg L21, Fluka). Toachieve a final monomeric Al concentration of 90 mm in theagarose gel, 300 mm Al was added to the cooled solution(measured by means of the aluminon method according toKerven et al. [1989]; see also Sivaguru and Horst, 1998).

Determination of Callose in 1-mm Root Segments

Al was applied to specific apical root zones using thepolyvinylchloride (PVC) block system previously de-scribed by Sivaguru and Horst (1998). After the Al treat-ment, 2-cm root tips were fixed in 96% (v/v) ethanol toavoid the formation of wound callose. The segments werethen dissected, blotted dry, and transferred to Eppendorfcups containing 1 mL of 1 m NaOH. Each sample contain-ing two 1-mm root segments was ultrasonicated (BandelinSonopuls, Bandelin Electronics, Berlin) for 40 s. Subse-quently, the cups containing the samples were heated in an80°C water bath for 20 min to solubilize the callose from thedisintegrated cell walls, and were then centrifuged for 12min at 12,000 min21 at room temperature. Callose concen-trations in the supernatant were quantified fluorometri-cally (Hitachi f2000, Hitachi, Tokyo; excitation at 393 nmand emission at 484 nm) according to the method describedby Kauss (1989) using aniline blue. The Al-induced callose

content was calculated from the callose content of Al-treated segments minus the callose content in segmentsfrom control roots not treated with Al.

Determination of Al in 1-mm Root Segments

After three brief rinses of the excised root tips in ultra-pure water (18.3 MV, E-pure, D4642, Barnstead, Dubuque,IA), individual 1-mm root segments were dissected underultrapure water from fresh root tips within 30 min after theAl treatment period. The root segments were placed intoEppendorf cups (two segments each) containing 500 mL ofultrapure water, frozen, and kept at 220°C until analysis.For the Al analysis the samples were transferred into10-mL Teflon cups, and the ultrapure water was evapo-rated in a heating block at 120°C. The samples were dis-solved in 1 mL of ultrapure concentrated HNO3 and wetoxidized at 190°C until the acid had completely evapo-rated. The ash was dissolved in 500 mL of ultrapure HNO3

(1:30 in ultrapure water). The samples were analyzed for Alusing a graphite furnace atomic absorption spectrometer(UNICAM 939 QZ, Analytical Technology, Cambridge,UK) with Zeeman background compensation. Instrumentaladjustments were optimized for the highest sensitivity. TheAl contents of Al-treated root segments were corrected formean Al contents from blanks and root segments nottreated with Al (control).

Electrophysiology

A modified electrophysiological setup was used (Petersand Felle, 1999). It was turned by an angle of 90°, allowingundisturbed positive gravitropism of the root. The seedlingitself remained intact. The electrical setup for the fabrica-tion and application of ion-sensitive microelectrodes hasbeen described by Felle and Bertl (1986) and Felle (1994,1998). pH-selective micropipettes were pulled on a Getrainstrument (vertical) from borosilicate tubing with solidfilament (Hilgenberg, Malsfeld, Germany). Tip diameterswere 3 to 4 mm. The tips were blunt and heat-polished.Before filling, the micropipette (apical 4 cm) was bent in a30° angle, allowing measurement in the modified setup. Togive the ion-sensitive sensor in the tip sufficient firmness tostay in place for repeated use, the cocktail (Fluka) wasdissolved in a mixture of 40 mg of PVC per mL of tetra-hydrofuran (THF) at a ratio of 30:70 (v/v). After evapora-tion of the THF, the remaining firm gel was topped withthe undiluted sensor cocktail, followed by the referencesolution (0.5 m KCl and 1 mm 2-([N-morpholino])-ethanesulfonic acid [MES], pH 6.0).

The micropipettes were connected through a Ag/AgClhalf-cell to a high-impedance amplifier (FD 223, WorldPrecision Instruments, Sarasota, FL). Signals were recordedon a chart recorder (L 2200, Linseis, Selb, Germany). Ex-periments were carried out under constant perfusion (1 mLmin21) in a specially designed plexiglass chamber at roomtemperature (22°C223°C). The measurement solution con-sisted of distilled water containing 200 mm each of CaCl2and KCl, and 1 mm MES. The pH was adjusted to 4.5. ThepH profiles were measured at a distance of 20 mm from the

946 Kollmeier et al. Plant Physiol. Vol. 122, 2000

www.plantphysiol.orgon April 16, 2018 - Published by Downloaded from Copyright © 2000 American Society of Plant Biologists. All rights reserved.


root surface and in 200-mm intervals for the first 5 apicalmm. Control measurements were carried out 15 min afterthe roots had been attached to the measurement cuvette.Only roots showing the specific basic pH profile wereselected for further experiments. The Al treatment lasted 60min at a concentration of 90 mm. Either the whole root orspecific 1-mm root zones were treated with Al using aspecially designed adjustable agarose block in plexiglassthat was movable along the root in the plexiglass cuvette.

Partial Elongation of 1-mm Root Zones

The experimental setup for the electrophysiological ex-periments allowed simultaneous measurement of pH pro-files and elongation growth of specific 1-mm root zones.Before the seedlings were attached to the measurementcuvette the root was marked with ink (Bruynzeel Perma-nent fine, Bruynzeel, The Netherlands) dots in 1-mm inter-vals. The distances between the dots and the width of thedots were measured at the beginning and during the courseof the experiment, and then the elongation rate of thespecific root zone was calculated. The ink used did notdetach during the course of the experiment and did notaffect root elongation or disturb the pH measurements, asconfirmed in preliminary experiments. The precision of themeasurements was 20 mm at a 64-fold magnificationagainst a scale. Due to the vertical setup of the measure-ment cuvette, root bending did not occur.

Exogenous Application of IAA and Determination of RootElongation Rate

The setup for the determination of the effect of exoge-nous IAA application on Al-induced inhibition of rootgrowth was a modification of the PVC block system de-scribed by Sivaguru and Horst (1998) for the specific pur-poses of the experiments conducted. This included asmoothly moveable tray on which the PVC plates to whichthe plants were attached could be moved along the binoc-ular (20-fold magnification; Stemi SV8, Zeiss, Oberkochen,Germany). Gravity-induced curvature did not complicatethe measurements due to the vertical attachment of theseedlings to the PVC plates. Ninety micromolar monomericAl or 10 mm of the IAA transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) was applied via agarose blocksto the 1- to 2-mm apical root zone, while agarose blocks(1.2% [w/v] agarose) containing nutrient solution and 1027

m IAA were positioned either around the meristematiczone (MZ) (0–1 mm) or the main elongation zone (EZ)(2.5–3.5 mm). This IAA concentration has been selected asmost enhancing root elongation from a range of prelimi-nary experiments in which IAA concentrations have beenvaried from 1029 to 1023 m (data not shown).

Exogenous Application of [3H]IAA and Localization inSpecific Root Segments

Al (90 mm monomeric) or 10 mm of the IAA transportinhibitors TIBA or N-1-naphthylphthalamic acid (NPA)was applied to the DTZ of primary roots of intact plants in

the PVC system as described before. [3H]IAA (777 GBq/mmol, Amersham Pharmacia Biotech, Freiburg, Germany)was applied to the MZ in 64-mL agarose blocks (1.2% [w/v]with nutrient solution) for 30 min. Then, 2 3 1028 mtritiated IAA was mixed with untritiated IAA (Sigma,Deisenhofen, Germany) to reach a final IAA concentrationof 1027 m. After application, the roots were carefully re-moved from the system and rinsed in distilled water for 3 sfrom the base to the tip. Afterward, the segments were cutand frozen in 4 mL of distilled water in separate scintilla-tion vials overnight. After reaching room temperatureagain, 8 mL of scintillation cocktail (Lumasafe Plus, LumacLSC B.V., Groningen, The Netherlands) was added. Radio-activity was determined in a liquid scintillation counter(RackBeta 1217, LKB Wallac OY, Turku, Finland) allowing10 min of integration time per specimen.

RESULTS

Effect of Al on Root Growth

When Al was applied to the entire root apex, the root-elongation rate was significantly inhibited in both cultivars

Figure 1. Effect of Al application (90 mM Almono in 0.6% [w/v]agarose gel) for 1 h to the entire root apex or to specific 1-mm rootsegments on the root elongation rate of primary roots of the maize cvATP-Y (Al-resistant) and cv Lixis (Al-sensitive). Values are means offive independent replicates 6 SD. The results shown are representa-tive of three independent experiments. Different letters indicate sig-nificant differences at P , 0.05 (Tukey’s test). White bars, 0 mM Al;black bars, 90 mM Al.

Root Apical Spatial Aluminum Sensitivity and Auxin Flow in Maize 947



Figure 2. Al contents of specific 1-mm segmentsof the primary root apex of the Al-sensitivemaize cv Lixis (E) and the Al-resistant cv ATP-Y(F). Ninety micromolar Almono was applied for1 h in agarose gel to specific 1-mm root zones,as indicated by arrows. Values are means ofthree independent replicates 6 SD. The resultsshown are representative of two independentexperiments. Stars indicate significant genotyp-ical differences at P , 0.05 (Tukey’s test).

Figure 3. Al-induced callose formation in spe-cific 1-mm segments of the primary root apex ofthe Al-sensitive maize cv Lixis (E) and the Al-resistant cv ATP-Y (F). Ninety micromolar Al-mono was applied for 1 h in agarose gel to spe-cific 1-mm root zones, as indicated by arrows.Values are means of five independent repli-cates 6 SD. The results shown are representativeof three independent experiments. Stars indicatesignificant genotypical differences at P , 0.05(Tukey’s test).




(Fig. 1). However, growth inhibition was more severe in cvLixis (44.4%) than in cv ATP-Y (18.2%), reflecting thegreater Al resistance of cv ATP-Y. Al treatment of the 1- to2-mm apical root zone (the DTZ) led to a slightly strongerinhibition of root elongation than the treatment of thewhole root apex (cv Lixis: 55.5%; cv ATP-Y: 27.3%). Fur-thermore, the genotypical differences in Al resistance weremaintained. Al treatment of the 0- to 1-mm root zonesignificantly reduced root elongation only in the Al-sensitive cv Lixis; treatment of the 2.5- to 3.5-mm root zonedid not affect root elongation in either cultivar.

Effect of Local Al Application on Al Content andCallose Formation

The particular Al sensitivity of the 1- to 2-mm apical rootzone and the higher Al sensitivity of cv Lixis compared

with cv ATP-Y was related to significantly higher Al accu-mulation (Fig. 2) and Al-induced callose formation (Fig. 3)when Al was applied to this apical root zone. Al applica-tion to the 0- to 1-mm zone also led to enhanced Al accu-mulation and callose formation; however, the cultivars didnot differ significantly. Al application to the more basalroot zones only led to slight or no increases in Al andcallose contents, respectively.

Effect of Al on Partial Elongation of Specific 1-mmRoot Zones

For the measurement of the effect of Al application to theentire root apex or to individual 1-mm root zones on thepartial elongation rate of individual apical root zones, aspecially developed cuvette was used, which also allowedthe simultaneous measurement of pH profiles along theroot apex. In both maize cultivars the main EZ was 2.5 to 5mm behind the root apex (Fig. 4). When Al was applied tothe entire root apex, root elongation was inhibited over the

Figure 4. Effect of Al supply (90 mM Almono, 1 h) to the entire rootapex or specific 1-mm apical root zones on partial elongation rates ofapical 1-mm root segments of the primary roots of the maize cvATP-Y (Al-resistant) and cv Lixis (Al-sensitive). Values are means offive independent measurements 6 SD. The results shown are repre-sentative of two independent experiments. Different letters indicatesignificant differences at P , 0.05 (Tukey’s test). F, Control (Al); E 0to 1 mm zone; �, 1 to 2 mm zone; ƒ, 2.5 to 3.5 mm zone; f, entireapex.

Figure 5. Effect of Al application to the entire root apex on the rootsurface (distance 20 mm) pH profiles along the primary root of themaize cv ATP-Y (Al-resistant) and cv Lixis (Al-sensitive). Ninetymicromolar Almono was supplied in a solution containing KCl andCaCl2 at 200 mM each for 15 (�), 30 (f), and 60 (l) min. F, Control(2Al) The bulk solution pH was adjusted to 4.5. The values aremeans of three independent replicates 6 SD. The results shown arerepresentative of three independent experiments.




entire EZ. The higher Al resistance of cv ATP-Y was ex-pressed as a less-severe inhibition over the entire zone. Theapplication of Al only to the 1- to 2-mm apical root zone, azone that only slightly contributes to root elongation, pro-duced the same inhibition of root elongation. The genotyp-ical differences in Al sensitivity were similarly expressed.Application of Al to the 0- to 1-mm and the 2.5- to 3.5-mmroot zones induced significantly less or no inhibition ofroot elongation, respectively.

Effect of Al on the pH Profile along the Root Surface

Roots of both maize cultivars growing in the absence ofAl at pH 4.5 in the bulk solution developed a typical andstable pH profile at the root surface along the 5-mm rootapex (Fig. 5). The pH increased steeply, from 4.8 at the roottip to about 5.3 at a 1-mm distance from the tip, dropped tothe bulk solution pH at 3 mm from the tip, and then rose

Figure 6. Effect of Al application (90 mM Almono in agarose gel) for 1 hto specific root zones on the root surface (distance 20 mm) pHprofiles along the primary roots of the maize cv ATP-Y (F; Al-resistant) and cv Lixis (E; Al-sensitive). The pH of the bulk solutioncontaining KCl and CaCl2 at 200 mM was adjusted to 4.5. Values aremeans of three independent replicates 6 SD. The results shown arerepresentative of three independent experiments. The shaded areasindicate the zone of Al application.

Figure 7. Effect of exogenous IAA supply (0.1 mM in 1.2% [w/v]agarose blocks containing nutrient solution) for 1 h to different apicalroot zones (A, MZ 0–1 mm; B, EZ 2.5–3.5 mm) on Al-inducedinhibition of elongation of primary roots of the maize cv ATP-Y(Al-resistant) and cv Lixis (Al-sensitive). Ninety micromolar mono-meric Al (gray bars) or 10 mM TIBA (black bars) in 0.6% (w/v) agarosegel was applied to the 1-to 2-mm root zone (DTZ). White bars,Control (2Al). Values are means of five independent replicates 6 SD.The results shown are representative of three independent experi-ments. Different letters indicate significant differences at P , 0.05(Tukey’s test).




again to pH 4.7 in the 4- to 5-mm zone. Al application foronly 15 min induced a substantial flattening of the pHprofile in both cultivars. Longer Al treatment increased thiseffect only in the Al-sensitive cv Lixis. The application of Alto the 1- to 2-mm apical root zone induced a similar flat-tening of the pH profile along the root apex (Fig. 6). Theflattening was more pronounced in cv Lixis, especially in

the 1- to 2-mm root zone. Application of Al to the 0- to1-mm or the 2.5- to 3.5-mm zones did not affect the pHprofiles in either cultivar.

Effect of Exogenous IAA Application on Al-InducedInhibition of Root Growth

Application of IAA to the 0- to 1-mm MZ as well as to the2.5- to 3.5-mm EZ significantly enhanced root elongation inboth maize cultivars (Fig. 7A). Al applied to the 1- to 2-mmzone (the DTZ) reduced the root elongation rate in cv Lixissignificantly more than in cv ATP-Y, as shown above.When IAA was applied to the MZ of Al-treated roots,absolute root elongation was significantly enhanced only inthe Al-resistant cv ATP-Y. Root elongation relative to theIAA-treated control without Al was not significantly af-fected in either cultivar (Table I). However, when IAA wasapplied to the EZ, Al-induced inhibition of root elongationwas significantly reduced (Fig. 7B, Table I). TIBA treatmentof the DTZ led to an inhibition of root elongation compa-rable to that caused by Al in both cultivars (Fig. 7, Table I).Genotypical differences did not occur. The TIBA effectcould be compensated significantly in both cultivars byIAA application to the EZ.

Effect of Al on Uptake and Distribution of [3H]IAA

Within 30 min of treatment, basipetal [3H]IAA transportdid not go beyond the first 10 mm of the root apex in eithercultivar. The application of either TIBA or NPA to the DTZled to a significant decrease in basipetal transport of[3H]IAA. Total (Fig. 8) and relative contents (Fig. 9) in theEZ (2–5 mm) were significantly lower than in the non-treated control roots. In contrast to the EZ, in the MZ andDTZ (0–2 mm), [3H]IAA contents consistently accumu-lated. Roots treated with Al showed effects similar to thosetreated with the IAA transport inhibitors: accumulation inmore apical root zones and lower contents of [3H]IAA inthe EZ especially in the Al-sensitive cultivar. The effects ofAl and IAA transport inhibitors applied to the DTZ onbasipetal [3H]IAA transport from the MZ to the EZ areillustrated in Figure 9.

Figure 8. Effect of Al on uptake and basipetal distribution of [3H]IAAin the apex of primary roots of the maize cv ATP-Y (Al-resistant) andcv Lixis (Al-sensitive). Application of 90 mM monomeric Al, 10 mM

NPA, or 10 mM TIBA in nutrient solution, pH 4.3, in 0.6% (w/v)agarose gel to the DTZ for 30 min. Control roots were treated onlywith nutrient solution in agarose blocks, pH 4.3. [3H]IAA (0.1 mM in1.2% [w/v] agarose blocks containing nutrient solution) was appliedto the MZ for 30 min. Values are means of five independent repli-cates 6 SD. The results shown are representative of three independentexperiments. Different letters indicate significant differences at P ,0.05 (Tukey’s test).

Table I. Effect of exogenous application of 1027 M IAA in agarose blocks to the apical root zonesMZ (0–1 mm) or EZ (2.5–3.5 mm) on inhibition of root elongation induced by application of 90 mM

monomeric AI or 10 mM TIBA to the DTZ (1–2 mm) relative to control roots treated only with nutri-ent solution in agarose blocks

Values are the means of five independent replicates 6 SD. Different letters indicate significantdifferences at P , 0.05 (Tukey test).

IAA ApplicationZone

Treatment to DTZ

Inhibition of Root Elongation

ATP-Y (AI-resistant) Lixis (AI-sensitive)

2IAA 1IAA 2IAA 1IAA

%

MZ 90 mM AI 41.8 6 6.5b 34.2 6 3.7b 55.1 6 5.3a 60.8 6 4.9a10 mM TIBA 45.9 6 11.5a 46.1 6 6.8a 47.3 6 9.6a 55.4 6 5.8a

EZ 90 mM AI 42.3 6 3.7b 31.6 6 2.3c 52.8 6 5.3a 38.4 6 3.8b10 mM TIBA 48.3 6 11.1a 25.9 6 4.6b 45.0 6 10.0a 21.1 6 7.4b




DISCUSSION

The setup for the determination of partial elongation ofspecific 1-mm root zones revealed that the main EZ islocated within 2.5 to 5 mm from the root tip, with a max-imum at 4 mm (Fig. 4), thus confirming the findings of Piletet al. (1983), Collings et al. (1992), Evans and Ishikawa(1997), and Blancaflor et al. (1998). The 1- to 2-mm zonecontributed only very little to root elongation, so it appearsjustified to classify this zone as DTZ. In this root zone, cellsare changing their mitotic mode and undergo a prepara-tory phase for rapid elongation (Baluska et al., 1996). Thiszone is most responsive to a variety of environmentalstimuli such as gravitropism and auxin (Meuwly and Pilet,1991; Ishikawa and Evans, 1993), as well as thigmomor-phism (Ishikawa and Evans, 1990).

In the present study we confirmed our previous results(Sivaguru and Horst, 1998; Sivaguru et al., 1999a) andverify and substantiate that the DTZ is the most Al-sensitive apical root zone in maize. Application of Al to theDTZ inhibited root elongation similarly to Al application tothe whole root apex. The application of Al to the 0- to 1-mmroot zone (the MZ) was significantly less inhibitory, andapplication to the adjacent EZ (2.5–3.5 mm from the roottip) was not inhibitory at all (Figs. 1 and 4). The lower Alsensitivity of the MZ may be due to protection of the MZthrough root-cap mucilage, which strongly binds Al (Ar-chambault et al., 1996), thus protecting the MZ from Alinjury (Horst et al., 1982). The low Al sensitivity of the EZis not yet understood, but appears to be in agreement withthe much lower Al sensitivity of stationary phase com-pared with log phase cells (Sivaguru et al., 1999b).

The cultivar differences in Al resistance were only clearlyexpressed when Al was applied to either the whole rootapex or to the DTZ (Figs. 1 and 4), clearly indicating thatthis root zone plays an outstanding role in the expressionof Al toxicity and Al resistance mechanisms in maize roots.

The high Al sensitivity of the DTZ and genotypical dif-ferences in Al sensitivity were characterized by higher Alaccumulation and Al-induced callose formation (Figs. 2and 3). This is in agreement with results showing a positiverelationship between Al sensitivity and Al accumulation inroot tips (Rincon and Gonzales, 1992; Delhaize et al., 1993a;Llugany et al., 1994) and Al-induced callose formation(Wissemeier et al., 1987; Zhang et al., 1994; Horst et al.,1997). Our results are consistent with the view that Alresistance requires exclusion of Al from the apoplast andAl sensitivity is due to enhanced binding of Al to Al-sensitive binding sites of the apoplast (Blamey et al., 1992;Grauer and Horst, 1992; Horst et al., 1997).

Externally applied Al rapidly binds to the cell walls ofroot cells (Zhang and Taylor, 1989; Blamey et al., 1990;Delhaize et al., 1993a), where the main binding sites are thenegative charges on carboxylic groups of the pectic matrix.These charges yield an electrical potential gradient deter-mining binding and distribution of ions in the apoplast(Kinraide, 1993). Horst et al. (1999) showed that the spatialAl sensitivity of the root apex is positively correlated withthe pectin contents of the root zones, with the exception ofthe MZ, but those results might have been due to contam-

ination of the MZ with mucilage. The role of pectin contentand its degree of methylation (and, thus, negative charge)on Al toxicity and resistance were further confirmed by N.Schmohl and W.J. Horst (unpublished results). Binding ofAl may lead to impairment of the physical properties of thecell wall, leading to changes in extensibility and permeabil-ity (Blamey et al., 1993; Pritchard, 1994). It may also lead tomechanical stress, which could transduce a signal to thecytoskeleton via transmembrane proteins assumed to con-nect the cell wall and cytoskeleton (Nick, 1999), inducingsevere disintegration of the microtubule and actin network(Blancaflor et al., 1998; Horst et al., 1999; Sivaguru et al.,1999a).

Our results do not support the recently expressed viewthat Al accumulation in root tip vacuoles is a major mech-anism of Al resistance (Vazquez et al., 1999). However, wedo not exclude the possibility that longer-term adaptationto Al supply will also require such a tolerance mechanism.However, in a normally growing root (2 mm h21), the cellswithin the DTZ will not be exposed to Al for more than 1 h.

Cell wall pH is difficult to measure directly (Degenhardtet al., 1998; Peters et al., 1998), so it has been frequentlycalculated indirectly from pH changes in the incubationmedium. However, these measurements cannot be ex-pected to yield results that can be directly referred to as cellwall or apoplastic pH (Peters et al., 1998, and refs. therein).Using ion-sensitive microelectrodes, as proposed by Felleand Bertl (1986) and Felle (1994, 1998), we measured rootsurface pH directly in the 20-mm distance from the surfaceand related it to root growth in specific zones of the pri-mary root. The pH profile we found is in agreement withthe findings of Pilet et al. (1983), Collings et al. (1992), andFelle (1998) in the maize root apoplast and is consistent forboth cultivars. It is particularly remarkable, however, thatthe roots seem to be capable of maintaining this patternwithin a vast range of bulk solution pH from 7.9 (Felle,1998) over 6.8 (Pilet et al., 1983) to 4.5 (our results). Thefinding that the roots maintained this pH profile even after24 h of adaptation to low pH shows that it is not a transienteffect due to seedling manipulation in the experimentalsetup. Effects of gravistimulation were also excluded bythe vertical setup. It is of particular interest that the maizeroot is capable of either acidifying or alkalizing specificroot zones depending on the pH of the surrounding me-dium, because the root response involved must thereforebe understood as a powerful adaptation mechanism totransient changes in medium pH. This view is in agreementwith findings by Yan et al. (1998) showing that maize rootsare capable of coping with extremely low external pH ifcarefully adapted.

Miyasaka et al. (1989), Ryan et al. (1992), and Collings etal. (1992) explained the pH pattern along the rhizoplane asresulting from endogenous ionic currents traversing theroot tip. Collings et al. (1992) discussed the role of thecurrents observed in transducing the gravitropic stimulusfrom the root cap to the EZ. By removing K1, Na1, Ca21,or Cl2 ions from the bathing solution or by applying theCa21 channel blocker lanthanum, they could demonstratethat the currents measured were primarily composed ofprotons, a finding that is in accordance with Ryan et al.




(1992). Ca21 influx and Cl2 efflux contribute to these cur-rents to a lesser extent (Iwabuchi et al., 1989; Ryan et al.,1992).

At pH 4.5, an increase in rhizosphere pH of 0.1 to 0.2units leads to considerable decrease of phytotoxic Al31

activity (Martell and Motekaitis, 1989; Kinraide, 1991), andthe capability of roots to increase rhizosphere pH has been

suggested as a possible mechanism of Al resistance (Mi-yasaka et al., 1989; Degenhardt et al., 1998). This is difficultto reconcile with the particularly high Al uptake and Alsensitivity of the DTZ and the highest pH at the rootsurface observed in this study (Figs. 2 and 5). Rather, itappears that the pH increase primarily led to less compe-tition of Al31 with H1 for apoplastic binding sites, thus

Figure 9. Effect of Al on [3H]IAA distributionrelative to entire IAA uptake into the apex ofprimary roots of the maize cv ATP-Y (Al-resistant) and cv Lixis (Al-sensitive). Applicationof 90 mM monomeric Al, 10 mM NPA, or 10 mM

TIBA in nutrient solution, pH 4.3, in 0.6% (w/v)agarose gel to the DTZ for 30 min. Control rootswere treated only with nutrient solution in aga-rose blocks, pH 4.3. [3H]IAA (0.1 mM in 1.2%[w/v] agarose blocks containing nutrient solu-tion) was applied to the MZ for 30 min. Valuesare means of five independent replicates 6 SD.The results shown are representative of threeindependent experiments. Different letters indi-cate significant differences at P , 0.05 (Tukeytest).




enhancing Al toxicity, as suggested by Grauer and Horst(1992) and Kinraide (1993). In this context it is important toconsider that Al application to the entire root apex orspecifically to the DTZ led to a flattening of the alkalizationpeak (1–2 mm) in both cultivars after 15 min of applicationand remained relatively constant for the succeeding 45min. This may explain why in the presence of Al, the pHincrease in the DTZ was not sufficient to inactivate Al31

especially in the Al-sensitive cultivar, where Al applicationled to a greater flattening of the pH peak in the DTZ thanin the Al-resistant cultivar. However, we believe that thegenotypical differences are the consequence rather than thecause for the differences in Al resistance, a conclusionshared by Miyasaka et al. (1989) after measuring ion fluxeswith lower spatial resolution in the rhizosphere of twowheat cultivars differing in Al resistance.

The alkalization peak in the DTZ may be due to: (a)enhanced anion uptake, e.g. an H1/anion symport, whichis in agreement with the findings of Collings et al. (1992)and Ryan et al. (1992) comparing two wheat cultivars dif-fering in Al resistance; (b) enhanced respiration and thusbicarbonate production; and (c) enhanced release of or-ganic acid anions, which at the low ambient pH in theapoplast will bind free protons. However, the release oforganic anions appears unlikely to account for alkalizationin the Al-sensitive cultivar, because it is well establishedthat this will lead to Al resistance through complexation, asshown for malate in wheat (Delhaize et al., 1993b; Basu etal., 1994; Ryan et al., 1995a, 1995b), citrate in maize (Pelletet al., 1995), and oxalate in buckwheat (Zheng et al., 1998).

The capacity of the Al-resistant cultivar to better main-tain the alkalization peak in the DTZ may be explained bytwo factors: (a) the physiological properties for mainte-nance of ionic currents are less affected by Al in the Al-resistant than in the Al-sensitive cultivar (Miyasaka et al.,1989) or (b) an Al exclusion mechanism is switched on inthe Al-resistant cultivar promoting release of organic acidanions. They might bind free protons in the rhizosphereand hence lead to an increase in surface pH. At the pre-vailing cytoplasmic pH (.7.0) both malate and citrate aremainly present in dissociated form as anions. Since theexternal concentration of these anions is much lower andbecause of the negative membrane potential, transport ofthese anions into the external solution is a thermodynam-ically passive process along a steep electrochemical gradi-ent. Hence, gating of anion channels permeable for theseanions would allow considerable efflux within a short time(Schmidt and Schroeder, 1994; Papernik and Kochian, 1997;Ryan et al., 1997). Even though there is agreement aboutthe role of adequate cell wall pH in root elongation (Rayleand Cleland, 1992; Zieschang et al., 1993; Kutschera, 1994;Felle, 1998), a causal relation between the changes in pHprofile observed and Al-induced inhibition of root growthremains a matter of debate (Ryan et al., 1992).

The finding that Al applied to the DTZ, which does notcontribute significantly to root elongation, severely inhibitscell elongation in the EZ not in contact with Al (Fig. 4)strongly suggests a signaling pathway mediating the Alsignal between DTZ and EZ. It is clearly established thatcoordinated auxin transport is involved in the regulation of

root growth, morphology, and the gravitropic growth re-sponse (Hasenstein and Evans, 1988; Kaufmann et al., 1995;Evans and Ishikawa, 1997; Ruegger et al., 1997; Muller etal., 1998). Auxin is transported from auxin-synthesizingshoot tissues via the phloem toward the root apical meris-tems, where it is proposed to be unloaded from the centralstele into cortical and epidermal cells and then translocatedbasipetally to the EZ (Hasenstein and Evans, 1988; Estelle,1998). Inhibition of basipetal auxin flow has been impli-cated in severe effects on root growth and morphology.These include swelling of root tips through uncontrolledpericlinal divisions (Blancaflor and Hasenstein, 1995; Rueg-ger et al., 1997) as a consequence of auxin accumulation inthe MZ and DTZ and suppression of the gravitropic re-sponse (Muller et al., 1998; Hasenstein et al., 1999) due toinsufficient auxin accumulation and thus growth inhibitionon the lower side of the bending root.

Similar effects on growth and morphological changeshave been reported in Al-treated roots (Blancaflor et al.,1998; Sivaguru et al., 1999a). Although Hasenstein andEvans (1988) clearly demonstrated inhibition of basipetaltransport of [3H]IAA in maize roots by Al, the implicationsof these results in the understanding of Al rhizotoxicityhave not been followed up on to date. The results pre-sented in this study confirm those findings by showing thatlocal application of Al, as well as TIBA or NPA, to the DTZled to reduced [3H]IAA contents in the EZ, while contentsrelative to control roots were elevated in the MZ/DTZ(Figs. 8 and 9). The involvement of Al/auxin transportinteraction in the expression of Al-induced inhibition ofroot elongation is further substantiated by the result thatexogenous application of IAA in root-growth-stimulatingdoses to the EZ alleviated inhibition of root elongationcaused by Al or TIBA applied to the DTZ (Fig. 7; Table I).The stronger inhibition of [3H]IAA transport by Al in theAl-sensitive compared with the Al-resistant cultivar sup-ports this assumption.

Although the results presented here suggest an involve-ment of auxin-transport inhibition in the expression of Altoxicity, the mechanism by which Al affects root apicalbasipetal IAA transport needs to be further elucidated.

In conclusion, the results shown convey substantial evi-dence that the DTZ of the maize primary root plays anoutstanding role in the Al response, and that the primarymechanisms of genotypical differences in Al resistance arelocated within the DTZ. Furthermore, they provide circum-stantial evidence for the existence of a signaling pathway inthe root apex mediating the Al signal between DTZ and EZthrough alterations in basipetal auxin transport. The nec-essary elucidation of the physiological processes responsi-ble for Al sensitivity and for genotypical differences in Alresistance of the DTZ and of the Al signal is the subject ofour ongoing research.

ACKNOWLEDGMENTS

The maize cv Lixis was kindly donated by Force Limagrain(Montpellier, France), and cv ATP-Y by Dr. Charles The (Institutde la Recherche Agronomique pour le Development, Cameroon).We also thank Rudiger Sachse (Zentrum fur Strahlenschutz and




Radiookologie, Hannover, Germany) for his assistance on the Wal-lac RackBeta.

Received July 27, 1999; accepted November 9, 1999.

LITERATURE CITED

Archambault DJ, Zhang G, Taylor GJ (1996) Accumulation of Alin root mucilage of an Al-resistant and an Al-sensitive cultivar ofwheat. Plant Physiol 112: 1471–1478

Baluska F, Volkmann D, Barlow PW (1996) Specialized zones ofdevelopment in roots. View from the cellular level. Plant Physiol112: 3–4

Basu U, Godbold D, Taylor GJ (1994) Aluminum resistance inTriticum aestivum associated with enhanced exudation of malate.J Plant Physiol 144: 747–753

Blamey FPC, Asher CJ, Kerven GL, Edwards DG (1993) Factorsaffecting aluminum sorption by calcium pectate. Plant Soil 149:87–94

Blamey FPC, Edmeades DC, Wheeler DM (1990) Role of cation-exchange capacity in differential aluminum tolerance of Lotusspecies. J Plant Nutr 13: 729–744

Blamey FPC, Edmeades DC, Wheeler DM (1992) Empirical mod-els to approximate calcium and magnesium amelioration effectsand genetic differences in aluminium tolerance in wheat. PlantSoil 144: 281–287

Blancaflor EB, Hasenstein KH (1995) Time course and auxinsensitivity of cortical microtubule reorientation in maize roots.Protoplasma 185: 72–82

Blancaflor EB, Jones DL, Gilroy S (1998) Alterations in the cy-toskeleton accompany aluminum-induced growth inhibitionand morphological changes in primary roots of maize. PlantPhysiol 118: 159–172

Collings DA, White RG, Overall RL (1992) Ionic current changesassociated with the gravity-induced bending response in rootsof Zea mays L. Plant Physiol 100: 1417–1426

Degenhardt J, Larsen PB, Howell SH, Kochian LV (1998) Alumi-num resistance in the Arabidopsis mutant alr-104 is caused by analuminum-induced increase in rhizosphere pH. Plant Physiol117: 19–27

Delhaize E, Craig S, Beaton CD, Bennet RJ, Jagadish VC, RandallPJ (1993a) Aluminum tolerance in wheat (Triticum aestivum L.).I. Uptake and distribution of aluminum in root apices. PlantPhysiol 103: 685–693

Delhaize E, Ryan PR (1995) Aluminum toxicity and tolerance inplants. Plant Physiol 107: 315–321

Delhaize E, Ryan PR, Randall PJ (1993b) Aluminum tolerance inwheat (Triticum aestivum L.). II. Aluminum-stimulated excretionof malic acid from root apices. Plant Physiol 103: 695–702

Estelle M (1998) Polar auxin transport: new support for an oldmodel. Plant Cell 10: 1775–1778

Evans ML, Ishikawa H (1997) Cellular specifity of the gravitropicmotor response in roots. Planta 203: 115–122

Felle HH (1994) The H1/Cl2 symporter in root hair cells ofSinapsis alba. Plant Physiol 106: 1131–1136

Felle HH (1998) The apoplastic pH of Zea mays root cortex asmeasured with pH-sensitive microelectrodes: aspects of regula-tion. J Exp Bot 49: 987–995

Felle HH, Bertl A (1986) The fabrication of H1-selective liquid-membrane microelectrodes for use in plant cells. J Exp Bot 37:1416–1428

Grauer UE, Horst WJ (1992) Modeling cation amelioration ofaluminium toxicity. Soil Sci Soc Am J 56: 166–172

Hasenstein KH, Blancaflor EB, Lee JS (1999) The microtublecytoskeleton does not integrate auxin transport and gravitro-pism in maize roots. Physiol Plant 105: 729–738

Hasenstein KH, Evans ML (1988) Effects of cations on hormonetransport in primary roots of Zea mays. Plant Physiol 86: 890–894

Horst WJ (1995) The role of the apoplast in aluminium toxicity andresistance of higher plants: a review. Z Pflanzenernahr Bodenk158: 419–428

Horst WJ, Puschel A-K, Schmohl N (1997) Induction of calloseformation is a sensitive marker for genotypic aluminium sensi-tivity in maize. Plant Soil 192: 23–30

Horst WJ, Schmohl N, Kollmeier M, Baluska F, Sivaguru M(1999) Does aluminium inhibit root growth of maize throughinteraction with the cell wall—plasma membrane—cytoskeletoncontinuum? Plant Soil 215: 163–174

Horst WJ, Wagner A, Marschner H (1982) Mucilage protects rootmeristems from aluminium injury. Z Pflanzenphysiol 109:95–103

Ishikawa H, Evans ML (1990) Electrotropism of maize roots: roleof the root cap and relationship by gravitropism. Plant Physiol94: 913–918

Ishikawa H, Evans ML (1993) The role of the distal elongationzone in the response of maize roots to auxin and gravity. PlantPhysiol 102: 1203–1210

Iwabuchi A, Yano M, Shimuzu H (1989) Development of extra-cellular electric pattern around Lepidium roots: its possible rolein root growth and gravitropism. Planta 183: 381–390

Kaufmann PB, Wu LL, Brock TG, Kim D (1995) Hormones andthe orientation of growth. In PJ Davies, ed, Plant Hormones:Physiology, Biochemistry and Molecular Biology. Kluwer Aca-demic Publishers, Dordrecht, The Netherlands, pp 547–571

Kauss H (1989) Fluorimetric measurement of callose and other1,3-b-glucans. In HF Linskens, JF Jackson, eds, Modern Methodsof Plant Analysis: Volume 10 Plant Fibers. Springer-Verlag, Ber-lin, pp 127–137

Kerk N, Feldman L (1994) The quiescent center in roots of maize:initiation, maintenance and role in organization of the rootapical meristem. Protoplasma 183: 100–106

Kerven GL, Edwards DG, Asher CJ, Hallman PS, Kokot S (1989)Aluminum determination in soil solution. II. Short term calori-metric procedures for the measurement of inorganic monomericaluminium in the presence of organic acid ligands. Aust J SoilRes 27: 91–102

Kinraide TB (1991) Identity of the rhizotoxic aluminium species.Plant Soil 134: 167–178

Kinraide TB (1993) Aluminum enhancement of plant growth inacid rooting media: a case of reciprocal alleviation of toxicity bytwo toxic cations. Physiol Plant 88: 619–625

Kochian LV (1995) Cellular mechanisms of aluminum toxicity andresistance in plants. Annu Rev Plant Physiol Plant Mol Biol 46:237–260

Kutschera U (1994) The current status of the acid-growth hypoth-esis. New Phytol 126: 549–569

Llugany M, Massot N, Wissemeier AH, Poschenrieder C, HorstWJ, Barcelo J (1994) Aluminum tolerance of maize cultivars asassessed by callose production and root elongation. Z Pflanzen-ernahr Bodenk 157: 447–451

Martell AE, Motekaitis RJ (1989) Coordination chemistry andspecification of Al(III) in aqueous solution. In TE Lewis, ed,Environmental Chemistry and Toxicology of Aluminum. LewisPublishers, Chelsea, MI, pp 3–17

Meuwly P, Pilet EP (1991) Local treatment with indole-3-aceticacid influences differential growth responses in Zea mays L.roots. Planta 185: 58–64

Miyasaka SC, Kochian LV, Shaff JE, Foy CD (1989) Mechanismsof aluminum tolerance in wheat. An investigation of genotypicdifferences in rhizosphere pH, K1, and H1 transport, and root-cell membrane potentials. Plant Physiol 91: 1188–1196

Muller A, Guan C, Galweiler L, Tanzler P, Huijser P, MarchantA, Parry G, Bennett A, Wisman E, Palme K (1998) AtPIN2defines a locus of Arabidopsis for root gravitropism control.EMBO J 17: 6903–6911

Nick P (1999) Signals, motors, morphogenesis: the cytoskeleton inplant development. Plant Biol 1: 169–179

Papernik LA, Kochian LV (1997) Possible involvement of Al-induced electrical signals in Al tolerance in wheat. Plant Physiol115: 657–667

Pellet DM, Grunes DL, Kochian LV (1995) Organic acid exuda-tion as an aluminum-tolerance mechanism in maize (Zea maysL.). Planta 196: 788–795




Peters WS, Felle HH (1999) The correlation of profiles of surfacepH and relative elemental growth rates in maize roots. PlantPhysiol 121: 905–912

Peters WS, Luthen H, Bottger M, Felle H (1998) The temporalcorrelation of changes in apoplast pH and growth rate in maizecoleoptile segments. Aust J Plant Physiol 25: 21–25

Pilet PE, Versel JM, Mayor G (1983) Growth distribution andsurface pH patterns along maize roots. Planta 158: 398–402

Pritchard J (1994) The control of cell expansion in roots. NewPhytol 127: 3–26

Rayle DL, Cleland R (1992) The acid growth theory of auxin-induced cell elongation is alive and well. Plant Physiol 99:1271–1274

Rengel Z (1996) Uptake of aluminium in plant cells. New Phytol134: 389–406

Rincon M, Gonzales RA (1992) Aluminum partitioning in intactroots of aluminum-tolerant and aluminum-sensitive wheat(Triticum aestivum L.) cultivars. Plant Physiol 99: 1021–1028

Ruegger M, Dewey E, Hobbie L, Brown D, Bernasconi P, TurnerJ, Muday G, Estelle M (1997) Reduced naphthylphthalamic acidbinding in the tir3 mutant of Arabidopsis is associated with aregulation in polar auxin transport and diverse morphologicaldefects. Plant Cell 9: 745–757

Ryan PR, Delhaize E, Randall PJ (1995a) Characterization ofAl-stimulated efflux of malate from the apices of Al-tolerantwheat roots. Planta 196: 103–110

Ryan PR, Delhaize E, Randall PJ (1995b) Malate efflux from rootapices and tolerance to aluminum are highly correlated inwheat. Aust J Plant Physiol 22: 531–536

Ryan PR, DiTomaso JM, Kochian LV (1993) Aluminum toxicity inroots: an investigation of spatial sensitivity and the role of theroot cap. J Exp Bot 44: 437–446

Ryan PR, Shaff JE, Kochian LV (1992) Aluminum toxicity in roots:correlations among ionic currents, ion fluxes, and root elonga-tion in aluminum-sensitive and aluminum-tolerant wheat culti-vars. Plant Physiol 99: 1193–1200

Ryan PR, Skerrett M, Findlay GP, Delhaize E, Tyerman SD(1997) Aluminum activates an anion channel in the apical cells ofwheat roots. Proc Natl Acad Sci USA 94: 6547–6552

Schmidt C, Schroeder JI (1994) Anion selectivity of slow anionchannels in the plasma membrane of guard cells. Plant Physiol106: 383–391

Sivaguru M, Baluska F, Volkmann D, Felle HH, Horst WJ (1999a)Impacts of aluminum on the maize cytoskeleton: short-termeffects on the distal part of the transition zone. Plant Physiol 119:1–10

Sivaguru M, Horst WJ (1998) The distal part of the transition zoneis the most aluminium-sensitive apical root zone of Zea mays L.Plant Physiol 116: 155–163

Sivaguru M, Yamamoto Y, Matsumoto H (1999b) Differentialimpacts of aluminium on microtubules depends on the growthphase in suspension cultured tobacco cells. Physiol Plant 107:110–119

Taylor GJ (1995) Overcoming barriers to understanding the cellu-lar basis of aluminum resistance. Plant Soil 171: 89–103

Vazquez MD, Poschenrieder C, Corrales I, Barcelo J (1999)Change in apoplastic aluminum during the initial growth re-sponse to aluminum by roots of a tolerant maize variety. PlantPhysiol 119: 435–444

Wissemeier AH, Horst WJ (1995) Effect of calcium supply onaluminium-induced callose formation, its distribution and per-sistence in roots of soybean (Glycine max (L.) Merr.). J PlantPhysiol 145: 470–476

Wissemeier AH, Klotz F, Horst WJ (1987) Aluminium-inducedcallose synthesis in roots of soybean (Glycine max L.). J PlantPhysiol 129: 487–492

Yan F, Feuerle R, Schaffer S, Fortmeier H, Schubert S (1998)Adaptation of active proton pumping and plasmalemma AT-Pase activity of corn roots to low root medium pH. Plant Physiol117: 311–319

Zhang G, Hoddinott J, Taylor GJ (1994) Characterisation of 1–3-b-glucan (callose) synthesis in roots of Triticum aestivum in re-sponse to aluminium toxicity. J. Plant Physiol 144: 229–234

Zhang G, Taylor GJ (1989) Kinetics of aluminum uptake by ex-cised roots of aluminum-tolerant and aluminum-sensitive culti-vars of Triticum aestivum L. Plant Physiol 91: 1094–1099

Zheng SJ, Ma JF, Matsumoto H (1998) High aluminum resistancein buckwheat. 1. Al-induced specific secretion of oxalic acidfrom root tips. Plant Physiol 117: 745–751

Zieschang HE, Kohler K, Sievers A (1993) Changing proton con-centrations at the surfaces of gravistimulated Phleum roots.Planta 190: 546–554




Genotypical Differences in Aluminum Resistance of Maize Are ...

Documents

Transcript of Genotypical Differences in Aluminum Resistance of Maize Are ...