Apoplastic transport of abscisic acid through roots of maize: effect of the exodermis

The exodermal layers that are formed in maize roots during aeroponic culture were investigated with respect to the radial transport of cis-abscisic acid (ABA). The decrease in root hydraulic conductivity (Lp_r) of aeroponically grown roots was stimulated 1.5-fold by ABA (500 nM), reaching Lp_r values of roots lacking an exodermis. Similar to water, the radial flow of ABA through roots (J_ABA) and ABA uptake into root tissue were reduced by a factor of about three as a result of the existence of an exodermis. Thus, due to the cooperation between water and solute transport the development of the ABA signal in the xylem was not affected. This resulted in unchanged reflection coeffcients for roots grown hydroponically and aeroponically. Despite the well-accepted barrier properties of exodermal layers, it is concluded that the endodermis was the more effective filter for ABA. Owing to concentration polarisation effects, ABA may accumulate in front of the endodermal layer, a process which, for both roots possessing and lacking an exodermis, would tend to increase solvent drag and hence ABA movement into the xylem sap at increased water flow (J_Vr). This may account for the higher ABA concentrations found in the xylem at greater pressure difference. Received: 26 January 1999 / Accepted: 26 May 1999     

Stress induction of abscisic acid in maize roots

Moderate water stresses in the range 0 to −0.6 MPa applied with PEG 6000 to excised roots of Zea mays L. var. LG 11 induced increases of up to four-fold in the amount of abscisic acid (ABA) determined in the tissue after a 12 h period of xylem exudation. The ABA concentration of xylem exudate collected after a 2 h water stress also increased by up to four-fold. Salt stresses, induced with NaCl solutions, resulted in similar increases in the ABA concentrations. ABA concentrations in both root tissue and xylem exudate were highest 4 h after removal of the stress and then declined over a subsequent 8 h period. These results are interpreted in support of the concept that root-produced ABA may have a role in the fine control of the plant's water balance.     

Inhibition of ion accumulation in maize roots by abscisic acid

Summary An inhibition of root growth, a decrease in the amount of potassium (as ⁸⁶Rb) and phosphate (³²P) accumulation by the root, and a partial depolarization of transmembrane electropotential were observed to develop with a similar time course and to a similar extent when intact maize (Zea mays L.) roots were treated with 10^-5 M abscisic acid (ABA). Potassium uptake was inhibited by ABA when excised, low-salt roots were bathed in KCl, KH₂PO₄, or K₂SO₄. ABA did not affect the ATP content of the tissues, the activity of isolated mitochondria, nor the activity of microsomal K⁺-stimulated ATPases.     

Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem

The radial movement of cis-abscisic acid (ABA) has been investigated in young excised roots of Zea mays L. and Helianthus annuus L. which were grown hydroponically. In addition to the symplastic path, ABA was largely translocated across the root apoplast by solvent drag with the water in the transpiration stream. On the apoplastic path ABA may even cross the endodermis. Depending on the ABA concentration of the medium (range: 5–500 nM) and in the root apoplast, the solvent-drag component of the flow of ABA counteracted the dilution of ABA in the xylem caused by transpirational water flow. Acidification of the rhizosphere and of the root apoplast increased the apoplastic transport component. In sunflower, the apoplastic flow of ABA was significantly weaker than in maize roots. This was also indicated by the larger apparent reflection coefficient (σ_ABA) of sunflower roots for ABA (sunflower: σ_ABA = 0.97 ± 0.02, n = 6 roots; maize: σ_ABA = 0.68 ± 0.06, n = 6 roots; ±SD). For both species, σ_ABA was smaller than unity. Root reflection coefficients were affected by factors such as pH, ABA concentration of the medium, and by the suction force applied to excised root systems. Due to the complex composite structure of the permeation barrier in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected the absolute value of the reflection coefficient, σ_ABA has been termed `apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) modify the intensity of the root-to-shoot signal which is influenced by an apoplastic bypass flow of ABA. The latter may be substantially affected by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper. Received: 25 February 1998 / Accepted: 16 July 1998     

Effects of water stress on growth, osmotic potential and abscisic acid content of maize roots

Under water stress conditions, induced by mannitol solutions (0 to 0.66 M ) applied to the apical 12 mm of intact roots of Zea mays L. (cv. LG 11), a growth inhibition, a decrease in the osmotic potential of the cell sap and a significant accumulation of abscisic acid (ABA) were observed. When the roots were placed in a humid atmosphere after the stress, the growth rate increased again, even if elongation had been totally inhibited. Under a stress corresponding to an osmotic potential of -1.09 MPa in the solution, growth was totally inhibited, which means that the root cell turgor pressure was reduced to the yield threshold. These conditions led to the largest accumulation of ABA. The effect of water stress on the level of ABA was studied for three parts of the root. The greatest increase in ABA (about 10 fold) was obtained in the growth zone and this increase was apparently independent of the hydrolysis of the conjugated form. With a mannitol treatment of 1 h equivalent to a stress level of -1.39 MPa, a 4-fold increase in ABA efflux into the medium was obtained. These results suggest that there are interactions between water stress, root growth, osmotic potential and the ABA level. The growth under conditions of stress and the role of endogenous ABA in the control of plant metabolism, specially in the growth zone, are discussed.     

A possible stress physiological role of abscisic acid conjugates in root-to-shoot signalling

Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC-transporters. Because of its extremely hydrophilic properties, ABA-GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA-GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress.     

Radial transport of water and abscisic acid (ABA) in roots of Zea mays under conditions of nutrient deficiency

Radial water (J(V)) and abscisic acid (ABA) flows (J(ABA)) through maize root seedlings have been investigated under different conditions of nutrient deficiency. Whereas J(V) was reduced under nitrogen deficiency, potassium deficiency stimulated J(V). A substantial increase of J(ABA) was observed in roots kept under potassium deficiency. The observed changes of J(V) might have resulted from changed barrier properties of the endodermis. Nitrogen and potassium deficiency also caused an accumulation of endogenous ABA in root tissues. Under all conditions studied, except under K(+)-deficiency, external ABA (100 nM) caused an increase of J(V). The data of this study were used to analyse the relations between internal and endogenous root ABA, J(V), and J(ABA). The internal ABA of root tissues was positively correlated with J(V) and was highly significant (P <0.001 for internal and P=0.03 for endogenous root ABA) within the range 2-300 pmol g(-1) FW. It was also highly positively correlated to the radial ABA flows. There was also a highly positive correlation between J(V) and J(ABA). The data of this study indicate, for the first time, the relations between internal ABA, water, and ABA flows. Independent of treatment with external ABA, an ABA transport by solvent drag across the endodermis is confirmed.     

Absorption and long distance transport by isolated stele of maize roots in relation to the dual mechanisms of ion absorption

Summary Ion absorption and transport by intact roots, isolated cortex and isolated stele were compared shortly after tissue isolation and after aging. Absorption isotherms in the low and in the high concentration range show that in stripped-stele, which absorbs at a very low rate immediately after isolation, the capacity of system 1 but not system 2 is built up with aging. In agreement with this result analysis of individual fluxes across plasmamembrane and tonoplast reveals that only the influx from the medium into the cytoplasm increases considerably with aging of stele. Changes observed in aging excised roots and in isolated cortex are much less significant. In spite of the increase of absorption with aging by isolated stele, long distance transport, which is essentially passive through freshly stripped stele, decreases with aging. The reported results reflect the marked permeability of the plasmamembrane of fresh isolated stele, and demonstrate the importance of the cortex as a tissue collecting ions for long distance transport. New evidence for the theory of symplasmatic transport of ions into the xylem vessels is thus provided.

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Zusammenfassung Die Ionenaufnahme durch intakte Wurzeln, isolierte Rinde und isolierte Zentralzylinder und der Ferntransport durch intakte und entrind ete Wurzeln wurden verglichen, und zwar kurz nach der Isolierung und nach Altern der Gewebe. Frisch isolierte Zentralzylinder akkumulieren Io nen nur in ganz geringem Maße oder überhaupt nicht. Von den beiden Systemen der metabolischen Ionenaufnahme, die in einem niedrigen (System 1: bis 0,5 meq/l) und in einem hohen Konzentrationsbereich (System 2: 1-50 meq/l) die Geschwindigkeit der Ionenaufnahme durch intakte Wurzeln und isolierte Wurzelrinde bestimmen, entwickelt sich während des Alterungsprozesses in isolierten Zentralzylindern System 1, nicht aber System 2.In Übereinstimmung mit diesem Befund zeigt die Analyse der Einzelfluxe am Plasmalemma und am Tonoplasten, daß nur der Influx aus der Außenlösung in das Cytoplasma beim Altern der Zentralzylinder beträchtlich ansteigt. Veränderungen beim Altern von abgeschnittenen, intakten Wurzeln und isolierter Rinde sind viel weniger ausgeprägt.Obwohl die Ionenaufnahme beim Altern isolierter Zentralzylinder steigt, verringert sich der Ferntransport, der bei frisch isolierten Zentralzylindern rein passiv ist. Die mitgeteilten Ergebnisse zeigen die ausgeprägte Permeabilität frisch isolierter Zentralzylinder und demonstrieren die Bedeutung der Wurzelrinde als ein Gewebe, das Ionen für den Ferntransport sammelt. Auf diese Weise werden neue Anhaltspunkte für die Theorie des symplasmatischen Transportes der Ionen in die Gefäße gewonnen.

     

Radial transport of ions in roots

Measurements were made of the relative amounts of ⁸⁶Rb, ³⁶Cl, and ³²P accumulated in the cortex and stele of intact roots of corn (Zea mays), either detached or attached to their shoots. Both 4- and 7-day-old roots accumulated as much or more ⁸⁶Rb in the stele as in the cortex. In experiments with ³⁶Cl, cortex and stele accumulated the same amount, except for 4-day-old and 7-day-old attached roots, in which the cortex contained more ³⁶Cl than the stele after 23 hr. An additional study of ³²P uptake showed greater accumulation in the cortex than the stele for a short period of time, but as much in the stele as in the cortex after 8 to 24 hr. Transport of ⁸⁶Rb, ³⁶Cl, and ³²P into the xylem exudate increased with increasing accumulation of these ions in stele and cortex of the root. These experiments show no consistent difference between cortex and stele of intact corn roots with respect to their ability to accumulate several kinds of ions.     

Jasmonic acid transient accumulation is needed for abscisic acid increase in citrus roots under drought stress conditions

Phytohormones are central players in sensing and signaling numerous environmental conditions like drought stress. In this work, an experimental system based on severe drought was established and hormone profiling together with gene expression of key enzymes involved in abscisic acid (ABA) and jasmonic acid (JA) biosynthesis was studied in roots of citrumelo CPB 4475 (a commercial citrus rootstock) plants. JA concentration transiently increased after a few hours of stress, returning to control levels 30 h after the onset of the condition. A more progressive ABA accumulation was observed, with the onset of this increase at the same time or right after the JA transient accumulation. Molecular data suggested that, at least, part of the hormonal regulation takes place at the biosynthetic level. These observations also pointed to a possible involvement of JA on ABA biosynthesis under stress. To test this hypothesis, JA and ABA biosynthesis were chemically inhibited and subsequently phenotypes rescued by the addition of exogenous hormones. Results showed that the early JA accumulation was necessary for the subsequent ABA increase in roots under stress whereas the opposite could not be stated. The model includes a burst of JA in roots of citrus under severe drought stress conditions that leads to a more progressive ABA accumulation that will induce later plant responses. The present work adds a new level of interaction between JA and ABA at the biosynthetic level that together with the previously described interaction between signal transduction cascades of the two hormones would allow plants to fine‐tune specific responses to different stimuli.     

Patterns of auxin and abscisic acid movement in the tips of gravistimulated primary roots of maize

Because both abscisic acid (ABA) and auxin (IAA) have been suggested as possible chemical mediators of differential growth during root gravitropism, we compared with redistribution of label from applied ³H-IAA and ³H-ABA during maize root gravitropism and examined the relative basipetal movement of ³H-IAA and ³H-ABA applied to the caps of vertical roots. Lateral movement of ³H-ABA across the tips of vertical roots was non-polar and about 2-fold greater than lateral movement of ³H-IAA (also non-polar). The greater movement of ABA was not due to enhanced uptake since the uptake of ³H-IAA was greater than that of ³H-ABA. Basipetal movement of label from ³H-IAA or ³H-ABA applied to the root cap was determined by measuring radioactivity in successive 1 mm sections behind the tip 90 minutes after application. ABA remained largely in the first mm (point of application) whereas IAA was concentrated in the region 2–4 mm from the tip with substantial levels found 7–8 mm from the tip. Pretreatment with inhibitors of polar auxin transport decreased both gravicurvature and the basipetal movement of IAA. When roots were placed horizontally, the movement of ³H-IAA from top to bottom across the cap was enhanced relative to movement from bottom to top whereas the pattern of movement of label from ³H-ABA was unaffected. These results are consistent with the hypothesis that IAA plays a role in root gravitropism but contrary to the idea that gravi-induced asymmetric distribution of ABA contributes to the response.     

Effect of zeatin on the growth and indolyl-3-acetic acid and abscisic acid levels in maize roots

Elongation, indolyl-3-acetic acid (IAA) and abscisic acid (ABA) levels, – gas chromatography-mass spectrometry quantification –, in the elongating zone were analysed for maize ( Zea mays L., Cv. LG11) roots immersed in buffer solution with or without zeatin (Z). The effect of Z depends on the initial extension rate of roots. The slower growing roots are more strongly inhibited by Z (10⁻⁷−10₋₅ M ) and they show a greater increase in IAA and ABA content. When compared to the rapidly growing roots, the larger reactivity of the 'slow'ones cannot be attributed to a higher Z uptake as shown when using [¹⁴C]-Z. It is suggested that Z could regulate root elongation by acting on the IAA and/or ABA level. The comparative action of these two hormones is discussed.     

Long-distance stress and developmental signals associated with abscisic acid signaling in environmental responses

Flowering plants consist of highly differentiated organs, including roots, leaves, shoots and flowers, which have specific roles: root system for water and nutrient uptake, leaves for photosynthesis and gas exchange and reproductive organs for seed production. The communication between organs through the vascular system, by which water, nutrient and signaling molecules are transported, is essential for coordinated growth and development of the whole plant, particularly under adverse conditions. Here, we highlight recent progress in understanding how signaling pathways of plant hormones are associated with long-distance stress and developmental signals, with particular focus on environmental stress responses. In addition to the root-to-shoot peptide signal that induces abscisic acid accumulation in leaves under drought stress conditions, we summarize the diverse stress-responsive peptide signals reported to date to play a role in environmental responses.     

Differential molecular responses to abscisic acid and osmotic stress in viviparous maize embryos

Substantial quantities of mRNA encoding the abundant Em polypeptide accumulate, in planta, in developing embryos of maize (Zea mays L.). By contrast, accumulation of Em mRNA is only barely detectable in embryos with the vp-5/vp-5 genotype [an abscisic acid (ABA)-deficient viviparous phenotype]. Em mRNA is not detectable within viviparous embryos of the vp-1/vp-1 genotype that are non-responsive to ABA. Culture of immature wild-type and vp-5/vp-5 embryos in the presence of exogenous ABA or of an osmotically active agent prevents precocious germination and results in expression of the Em genes. When vp-1/vp-1 embryos are cultured under similar conditions, only the application of osmotic stress prevents precocious germination. However, Em mRNA does not accumulate either in ABA-treated or stressed, arrested embryos, indicating a requirement for ABA perception through a VP-1-mediated mechanism for Em gene expression. Nevertheless, vp-1/vp-1 embryos do show both ABA and stress responses at the molecular level. Treatment with ABA causes the accumulation of mRNA encoding a polypeptide of approx. 30 kDa, whilst osmotic stress induces the accumulation both of a 30-kDa polypeptide and a set of approx. 20-kDa polypeptides. This indicates the existence of discrete, parallel ABA and stress response pathways in developing maize embryos.Abbreviations ABA abscisic acid - cDNA copy-DNA - DAP days after pollination - kDa kilodaltons - MS Murashige and Skoog medium - LEA late embryogenesis abundant - NEpHGE non-equilibrium pH gradient gel electrophoresis - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis     

Gas chromatography-mass spectrometric determinations of abscisic acid levels in the cap and the apex of maize roots

Quantitative analyses of abscisic acid (ABA) in different parts of maize root tips (Zea mays L. cv. Kelvedon 33) were performed by mass fragmentography using the hexadeuterated analog of ABA as internal standard. It was found that the cap and the apex contained 36.1 g and 66.5 g ABA kg^–1 fresh weight, respectively. The possibility that the growth regulator formed in the cap and inhibiting the elongation of the extending zone of the root is ABA is discussed.Abbreviations ABA abscisic acid - ABA-D₆ hexadeuterated ABA - ABA-Me and ABA-D₆-Me methyl esters of ABA and ABA-D₆, respectively - GC-MS gas chromatograph(y)-mass spectrometry/spectrometer - IAA indol-3-yl-acetic acid - MF mass fragmentography - TMS trimethylsilyl     

Water stress and indol-3yl-acetic acid content of maize roots

Water-stress conditions were applied to the apical 12 mm of intact or excised roots ofZea mays L. (cv. LG 11) using mannitol solutions (0 to 0.66 M) and changes in weight, water content, growth and IAA level of these roots were investigated. With increasing stress a decrease in growth, correlated with an increased IAA level, was observed. The largest increase in IAA (about 2.7-fold) was found in the apical 5 mm of the root and was obtained under a stress corresponding to an osmotic potential of −1.39 MPa in the solution. This stress led to an isotonic state in the cells after 1 h. When the duration of water stress (−1.09 MPa) was increased to 2 or 3 h, no further increase in the IAA content was observed in the root segments. This indicated that there was no correlation between a hypothetical passive penetration of mannitol in the cells and IAA content. Indol-3yl-acetic acid rose to the same level in excised as in intact roots. In both cases, IAA accumulation was apparently independent of the hydrolysis of the conjugated form. The caryopsis and shoot seem not to be necessary to induce the increase of the IAA level in the roots during water stress (−1.09 MPa). Therefore, there seems to be a high rate of IAA biosynthesis in excised maize roots under water-stress conditions. Exodiffusion of IAA was observed during an immersion in either buffer or stress (−1.09 MPa) solution. In both cases, this IAA efflux into the medium represented about 50% of the endogenous level. Considering the present results, IAA appears to play an important part in the regulation of maize root metabolism and growth under water deficiency.     

Osmotic stress and abscisic acid reduce cytosolic calcium activities in roots of Arabidopsis thaliana*

Cytosolic Ca²⁺· ([Ca²⁺]_i, and elongation growth were measured in the roots of Arabidopsis thaliana. Exposure of plant tissues to high NaCl and abscisic acid (ABA) concentrations results in a reduction in the rate of growth, but the mechanism by which growth is inhibited is not understood. Both NaCl and ABA treatments are known to influence [Ca²⁺]_i, and in this study we measured the effects of salinity and ABA on [Ca²⁺]_i in cells from the meristematic region of Arabidopsis roots. The Ca²⁺-sensitive dye Fura-2 and ratiometric techniques were used to measure [Ca²⁺]_i in cells of the root meristem region. Resting [Ca²⁺]_i was found to be between 100 and 200 μmol m^?3 in roots of untreated plants. Resting [Ca²⁺]_i changed in response to changes in the [Ca²⁺] surrounding growing roots. An increase of external [Ca²⁺] increased [Ca²⁺]_i; conversely, a decrease of external [Ca²⁺] decreased [Ca²⁺]_i. Exposure of roots to NaCl caused a rapid reduction of [Ca²⁺]_i, a response that was proportional to the external NaCl concentration. Thus, as the NaCl concentration was increased, [Ca²⁺]_i in root meristematic cells decreased. Root elongation was also inhibited in proportion to the external NaCl concentration, with maximal inhibition occurring at 120 mol m^?3 NaCl. The [Ca²⁺]_i of root meristem cells also changed in response to ABA, and the magnitude of the effect of ABA was dependent upon ABA concentration. Treatment with 0.2 mmol m^?3 ABA caused a momentary increase in [Ca²⁺]_i followed by a decrease after 15 min, but 10 mmol m^?3 ABA caused an immediate decline in [Ca²⁺]_i. There was a strong positive correlation between [Ca²⁺]_i and root elongation rates. Experiments with the ABA-deficient Arabidopsis mutant aba-3 indicated that the reduction in [Ca²⁺]_i brought about by NaCl was unlikely to be mediated via changes in endogenous ABA. Experiments with solutes such as sorbitol, KCl and NaNO₃ indicated that the effects of NaCl could be mimicked by other solutes and was not specific for NaCl.