Apoplastic Transport of Abscisic Acid Through Roots of Maize: EEct of the Exodermis
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Planta (2000) 210: 222±231 Apoplastic transport of abscisic acid through roots of maize: eect of the exodermis Elenor Freundl1, Ernst Steudle2, Wolfram Hartung1 1Julius-von-Sachs-Institut fuÈ r Biowissenschaften der UniversitaÈ tWuÈ rzburg, Lehrstuhl Botanik I, Julius-von-Sachs Platz 2, 97082 WuÈ rzburg, Germany 2Lehrstuhl fuÈ r P¯anzenoÈ kologie der UniversitaÈ t Bayreuth, UniversitaÈ tsstraûe 30, 95440 Bayreuth, Germany Received: 26 January 1999 / Accepted: 26 May 1999 Abstract. The exodermal layers that are formed in Introduction maize roots during aeroponic culture were investigated with respect to the radial transport of cis-abscisic acid In a recent publication, we analysed the radial trans- (ABA). The decrease in root hydraulic conductivity port of water and cis-abscisic acid (ABA) through (Lpr) of aeroponically grown roots was stimulated 1.5- excised roots of young maize and sun¯ower plants, and fold by ABA (500 nM), reaching Lpr values of roots the factors and mechanisms that determine the intensity lacking an exodermis. Similar to water, the radial ¯ow of of the ABA signal in the root xylem (Freundl et al. ABA through roots (JABA) and ABA uptake into root 1998). The results prove that, to some extent, ABA can tissue were reduced by a factor of about three as a result be transported apoplastically across the root cylinder of the existence of an exodermis. Thus, due to the by solvent drag with the transpiration stream into cooperation between water and solute transport the xylem vessels. This conclusion is in line with that of development of the ABA signal in the xylem was not Steudle and Peterson (1998) who have recently sum- aected. This resulted in unchanged re¯ection marised the evidence for an apoplastic transport of coecients for roots grown hydroponically and solutes, such as nutrients. In the endodermis, the aeroponically. Despite the well-accepted barrier proper- apoplastic passage of solutes may be restricted to ties of exodermal layers, it is concluded that the arrays where Casparian bands are not yet mature, such endodermis was the more eective ®lter for ABA. Owing as in the root tip, or where root initials break through to concentration polarisation eects, ABA may accu- the endodermis. In addition, due to low amounts of mulate in front of the endodermal layer, a process suberin the maize endodermis may not be as imperme- which, for both roots possessing and lacking an able as expected (Zeier and Schreiber 1998; Schreiber exodermis, would tend to increase solvent drag and et al. 1999). In the earlier paper by Freundl et al. hence ABA movement into the xylem sap at increased (1998), we argued that the apoplastic transport of ABA water ¯ow (JVr). This may account for the higher ABA could compensate for the dilution resulting from water concentrations found in the xylem at greater pressure uptake, at least to some extent. Hence, apoplastic ABA dierence. transport in the root could aect the root-to-shoot signal of ABA. Key words: Abscisic acid ± Exodermis ± Hydraulic The experiments by Freundl et al. (1998) were per- conductivity ± Root (ABA transport) ± Water transport ± formed with plants that were cultivated hydroponically Zea and were lacking a Casparian band in the hypodermis (Peterson 1988; Zimmermann and Steudle 1998). When grown in soil, most plants, however, develop a Caspar- ian band which may play an important role during the uptake of water and nutrient ions and their retainment within the root (Perumalla et al. 1990; Damus et al. 1997; Enstone and Peterson 1998). The same should be true for solutes produced in the root such as the stress Abbreviations and symbols: ABA = cis-abscisic acid; CABA = o hormone ABA. ABA concentration in the medium; CABA = ABA concentration in x With respect to ABA one may speculate that, the xylem; Lp = hydraulic conductivity of root; r = re¯ec- r ABA depending on the rate of transpiration, this compound tion coecient for ABA; JABA = ABA ¯ow per unit root surface area; JVr = volume ¯ow per unit root surface area could be taken up from the soil solution which contains Correspondence to: W. Hartung; Fax: 49 (931) 888 6158; ABA at concentrations ranging between 1 and 10 nM E-mail: [email protected] (Hartung et al. 1996). On the other hand, release of E. Freundl et al.: Apoplastic transport of abscisic acid through roots of maize: eect of the exodermis 223 endogenous ABA to the soil solution may be retarded by pictures (Intas Colour LC 100C low light camera; GoÈ ttingen, the exodermis, resulting in increased ABA concentra- Germany). Lengths of primary roots were measured with a ruler. tions in the root apoplast. In the present paper, we deal Root surface areas were determined as described in detail by with the role of the exodermis and how this transport Freundl et al. (1998). barrier may aect the uptake of ABA from the root Measurement of xylem sap ¯ow induced by subatmo- medium. We have compared the radial transport of spheric pressures (vacuum). Seedlings were cut at a distance of ABA across maize roots that had developed a complete 20 mm above the root base. Excised roots with the mesocotyl still exodermis with those lacking it. External ABA concen- attached were ®xed to a capillary using a pressure-tight silicone seal trations of 5±500 nM have been applied to the root ®xed by a screw (Freundl et al. 1998). The root medium was medium. This includes the range of naturally occurring aerated. Suction applied to the root system caused xylem sap ¯ow ABA concentrations in the soil. Although measured into a calibrated capillary. The subatmospheric pressure was raised in steps of 0.02 MPa from zero to )0.08 MPa. At each pressure responses are quite variable and sometimes contradic- step, a steady water ¯ow across the root system was waited for. tory, it is known from the literature that ABA aects the Usually, the steady state was attained after 15±30 min. Water ¯ow )1 hydraulic conductivity of roots and hence root water (JVr in m s ) was measured for each step of change in pressure to )1 )1 ¯ow (Markhart et al. 1979; Glinka 1980; Fiscus 1981). calculate hydraulic conductivity Lpr in m s MPa for roots from This, in turn, would also modulate the ABA signal in the aeroponically and hydroponically grown maize plants (Zimmer- xylem. Therefore, changes in root hydraulics have been mann and Steudle 1998). The overall driving force contained an osmotic and a hydraulic measured as well. Rather than applying pressure to the (hydrostatic) component. The former was estimated from the root medium, cut surfaces of maize mesocotyls have dierence in concentration between xylem sap (at steady state at a been subjected to vacuum to induce water ¯ows which given constant hydrostatic pressure gradient) and medium using a simulate conditions in an intact transpiring plant re¯ection coecient of the root of rsr 0:6 for the nutrients in the (Freundl et al. 1998). From the results we conclude that xylem sap and medium since (Steudle and Frensch 1989; the exodermis remarkably alters the transport properties Zimmermann and Steudle 1998): of maize roots. JVr LprPr rsr px po Eq. 1 Here, px means the osmotic pressure of the xylem sap and po the osmotic concentration of the root medium. It held that px po. Materials and methods The correction for the osmotic component would be substantial at low ¯ow rates with a non-linear relationship between water ¯ow Plant material. Seeds of maize (Zea mays L. cv. Garant FAO and driving force (Fiscus 1975). However, this could be omitted, 240; Asgrow, Bruchsal, Germany) were germinated on ®lter paper since the relations were linear and water ¯ows created by osmotic gradients should be small in maize (see Discussion). soaked in 0.5 mM CaSO4 for 4±6 d at 21 °C in the dark. Maize seedlings developed roots of a length of up to 110 mm and primary The root medium was aerated. Root pressure was observed in leaves of a length of up to 30 mm. Some of the seedlings were roots in non-aerated and aerated media using dierent intensities transferred to aerated hydroponic culture vessels as described by of air bubbling. When aeration was switched on, there were no Freundl et al. (1998). Others were grown in mist culture reductions in root pressure, indicating that air bubbling did not (aeroponics) using the same nutrient solution. The latter seedlings cause leakages resulting from cracks and other damage (data not were ®xed by pieces of foam rubber in holes on top of a cubic PVC shown). The Lpr was determined for each individual root system, ®rst in the absence and then, subsequently, in the presence of ABA box of 1 m3 (Zimmermann and Steudle 1998). Roots protruded from the holes and grew into the box. At the bottom of the box, a in the medium. Concentrations used for Lpr measurements were centrally placed timer-regulated air conditioner produced mist for either zero or 500 nM of ABA. Xylem sap was collected with a 10 h a day. (Defensor, Axair; NuÈ rnberg, Germany). Apart from syringe from the capillary attached to the root system. Just that, growing conditions were similar to those used during before raising the vacuum by another step, osmotic concentrations hydroponic culture. Plants employed in experiments were grown of media and xylem sap were measured by freezing-point either in hydroponic or aeroponic culture for 7 d. Roots from depression (Osmomat 030, Gonotec, Berlin, Germany). Through- plants raised hydroponically became 80±400 mm long. Overall out the paper, the atmosphere has been used as a reference (zero shoot length was 100±250 mm.