The effect of plant hormones on phosphate uptake and translocation in maize roots

In short-term (1 h) uptake experiments GA₃(10^-5M) stimulated P_i uptake into maize root cortex cells by 28.7 %, Ethrel (10^-3M) inhibited it by 18.5 % and BA, IAA, and ABA were inactive. In long-term (5 h) experiments ABA remained inactive, GA₃ lost its stimulatory effect, and BA (5. 10^-6M), IAA (10^-4 -10^-5M), and Ethrel (10^-3 -5. 10^-4M) decreased P_i uptake. When the hormones were present only during 3 h preincubation (“augmentation”) period ABA was inactive, GA₃ slightly raised and BA, IAA, and Ethrel slowed down subsequent P_i uptake. BA(10^-7 –10^-5M) decreased xylem sap volume flow and P_i translocation. ABA in all tested concentrations (10^-8 –10^-5M) reduced exudation rate and P_i translocation, its effect declining with time. IAA effect strongly depended on concentration used and on application time and varied from strong inhibition to moderate stimulation of both volume flow and P_i translocation. GA₃ (10^-7M) slightly stimulated xylem volume flow but inhibited phosphate translocation. Ethrel (10^-4 and 10^-5M) increased both parameters, but P_i transloeation much more than volume flow. IAA, BA, and ABA influenced volume flow and P transloeation to the same extent leaving P_i concentration in the xylem sap unchanged. GA₃ and Ethrel influence P_i concentration in the xylem sap and it is thus probable that these hormones regulate release of phosphate ions into the xylem sap.     

The kinetics of calcium uptake by roots

Summary Barley leaves extracted 35 min after a 15-min pulse of 10^-6 M ¹⁴C-labelled cis,trans-(±)-abscisic acid (ABA) showed very little breakdown of the ABA. Leaves extracted 2 h after a 30-min pulse of 10^-5 M labelled ABA showed at least 34% degradation of the hormone. The pattern of degradation was very similar for leaves kept in either light or darkness following the labelling pulse, and the products formed resembled those described in tomato fruit and bean axes. No detectable isomerization to the physiologically inactive trans,trans-(±)-ABA isomer occurred in these leaves during the 2-h period. ABA-induced stomatal closure is partially reduced 30 min after the pulse. The reversal cannot be attributed to catabolism of the hormone and must be explained by removal of the hormone into storage sites where it cannot act on the stomata.     

The influence of salinity on phosphate uptake and distribution in lupin roots

Treeby, M. T. and van Steveninck, R. F. M. 1988. The influence of salinity on phosphate uptake and distribution in lupin roots. - Physiol. Plant. 72: 617–622.
The uptake and distribution of phosphate in lupin ( Lupinus luteus L. cv. Weiko III) roots under moderate salt (NaCl) stress was studied. Vacuolar inorganic phosphate (PJ concentrations in high phosphate plants were decreased by salt, although whole root P| was unaffected. In low phosphate plants, vacuolar P_i was unaffected by salt while whole root P_i was increased. Phosphate uptake was not altered by salt in high phosphate plants, but was depressed in low phosphate plants. These observations lead to the conclusion that in high phosphate plants P_i accumulates in cytoplasm and/or stele, ultimately giving rise to phosphate toxicity in shoots. Increasing phosphate supply had no effect on Na⁺ accumulation in root cell vacuoles in the epidermis or cortex, but the concentration of Cl⁻ in endodermal vacuoles was lowered.     

Multiphasic uptake of phosphate by corn roots

Abstract The concentration dependence of phosphate uptake was studied using root sections of corn (Zea mays L. cv. Ganga 5). Detailed and wide-range (57 concentrations in the range 1 μmol m^?3-75 mol m^?3), precise (average SEM < 2.5%, n= 6) and reproducible (similar patterns in three independent experiments and for 5, 10, 15, 20, 25 and 30°C) data revealed six (or seven) concentration-dependent phases separated by ‘jumps’ or sharp breaks. These transitions were independent of temperature and occurred over relatively narrow concentration ranges (0.0001–0.0004, 0.08–0.31, 1.0–3.5, (7.5–10), 18–20 and 57–59 mol m^?3). The intermediate phases obeyed Michaelis-Menten kinetics, whereas sigmoidal kinetics were observed at lower concentrations. Uptake within each of the two highest phases increased more rapidly with increasing external phosphate concentration than predicted from Michaelis-Menten kinetics but also saturated more rapidly. The latter finding is not consistent with free diffusion across the plasmalemma at high external phosphate concentrations. Kinetic models yielding continuous isotherms, e.g. the sum of one or two Michaelis-Menten terms and a diffusion term, cannot account for the data.     

Oligomycin inhibition of phosphate uptake and ATP labeling in excised maize roots

ATP labeling by newly absorbed ³²P in excised maize roots was reduced 34% by the presence of oligomycin during a 4-min uptake period with no reduction in rate of phosphorus absorption. Longer exposure to oligomycin, during pretreatment periods or longer uptake periods, reduced phosphorus absorption and further reduced ATP synthesis. In these tissues it appears that oligomycin inhibits ATP production at the mitochondria, that ATP is the energy source for phosphorus uptake at the plasmalemma, and that a depletion in the ATP supply causes a reduced rate of uptake.     

Mechanism of sulfate uptake by excised maize roots

The mechanism of uptake of sulfate by excised maize roots (Zea mays L. cv. Ganga-5) was investigated. It was found that in the concentration range l × 10^?9 M ? 5 × 10^?2 M, the sulfate uptake could be represented by a single multiphasic isotherm having four phases. Each phase covered a limited concentration range and obeyed Michaelis-Menten kinetics, which indicates mediation in sulfate uptake by a structure which changes characteristics (kinetic constants) at certain discrete salts concentrations (transition points).     

The influence of mechanical impedance on the growth of maize roots

Summary Maize roots were grown between 1 mm glass beads on which a pressure of 40 kPa was applied. The roots were supplied with a constant flow of aerated nutrient solution. Compared with roots grown in a nutrient solution, the impeded crown roots showed a reduction in length of about 75%, whereas the diameter was about 50% increased.These changes in root morphology have been attributed to changes in cell wall structure of the cortex cells, which also occur as a result of the influence of ethylene.It is suggested that ethylene acts as an intermediate factor in the effect of mechanical impedance on root growth.     

Temperature dependence of the concentration kinetics of absorption of phosphate and potassium in corn roots

The effect of temperature on respiration and kinetics of H₂PO₄⁻ and K⁺ uptake in corn roots was determined in the range of 2 to 42 C. The response of uptake to temperature, determined from Q₁₀ and activation energy (Ea) data, for the anion and the cation differ significantly, especially in the range of uptake mechanism (Mech.) I. At 2.5 micromolar the Ea for K⁺ uptake below the 13 C transition is 29.3 kilocalories per mole. As the K⁺ concentration is increased, Ea declines and at 0.25 millimolar is 21.6 kilocalories per mole. Accompanying this change in Ea is a shifting of the apparent transition temperature from 13 to 17 C. Above the temperature transition the Ea's for K⁺ uptake in the Mech. I range are quite low (3.0) and this value is unchanged by increases of K⁺ concentration to 0.25 millimolar. In the range of Mech. II above 1 millimolar K⁺ the temperature transitions are not seen and plots become linear. The Ea's show an increasing trend from 4.7 at 1 millimolar to 6.1 at 50 millimolar. The uptake of H₂PO₄⁻ is much more temperature sensitive having a constant Ea at concentrations in the Mech. I range below the 13 C temperature transition. The Arrhenius plots reveal a second transition at 22 C and the Ea for this segment is 21.0. Above the second transition the Ea remains high (10.0) and is constant in the range of Mech. I. In the range of Mech. II there is a concentration dependent decline in Ea for H₂PO₄⁻ uptake (22.7 at 1.0 millimolar to 1.0 at 50 millimolar). There is no definable low temperature transition at these concentrations. Ion uptake is found to be much more sensitive to low temperature than respiration in this chill-sensitive species. The data suggest that the low temperature reduction of ion transport is more closely related to restriction of function of active transport systems than to either respiration or membrane permeability.     

Cadmium-induced sulfate uptake in maize roots

The effect of cadmium (Cd) on high-affinity sulfate transport of maize (Zea mays) roots was studied and related to the changes in the levels of sulfate and nonprotein thiols during Cd-induced phytochelatin (PC) biosynthesis. Ten micromolar CdCl(2) in the nutrient solution induced a 100% increase in sulfate uptake by roots. This was not observed either for potassium or phosphate uptake, suggesting a specific effect of Cd(2+) on sulfate transport. The higher sulfate uptake was not dependent on a change in the proton motive force that energizes it. In fact, in Cd-treated plants, the transmembrane electric potential difference of root cortical cells was only slightly more negative than in the controls, the external pH did not change, and the activity of the plasma membrane H(+)-ATPase did not increase. Kinetics analysis showed that in the range of the high-affinity sulfate transport systems, 10 to 250 microM, Cd exposure did not influence the K(m) value (about 20 microM), whereas it doubled the V(max) value with respect to the control. Northern-blot analysis showed that Cd-induced sulfate uptake was related to a higher level of mRNA encoding for a putative high-affinity sulfate transporter in roots. Cd-induced sulfate uptake was associated to both a decrease in the contents of sulfate and glutathione and synthesis of a large amount of PCs. These results suggest that Cd-induced sulfate uptake depends on a pretranslational regulation of the high-affinity sulfate transporter gene and that this response is necessary for sustaining the higher sulfur demand during PC biosynthesis.     

Nature of the calcium effect in phosphate uptake by barley roots

Summary The rates of phosphate uptake of young intact plants were measured in solution at concentrations between 3.2×10^–7 M and 3.2×10^–5 M KH₂PO₄. The addition of 0.005 to 0.01M CaCl₂ increased the uptake rates to values of up to 4 times those of controls. The increases in uptake relative to the controls were most marked at the lowest phosphate concentrations. The addition of KCl also increased the uptake rates, but to a much smaller extent. The results are analyzed by the enzyme kinetic theory, and it is concluded that the uptake of phosphate at low ionic strengths is impeded by negative potentials at the root surface.The experimental results in this letter have appeared in a thesis.     

Inhibition of phosphate uptake in corn roots by aluminum-fluoride complexes

F forms stable complexes with Al at conditions found in the soil. Fluoroaluminate complexes (AlF(x)) have been widely described as effective analogs of inorganic phosphate (Pi) in Pi-binding sites of several proteins. In this work, we explored the possibility that the phytotoxicity of AlF(x) reflects their activity as Pi analogs. For this purpose, (32)P-labeled phosphate uptake by excised roots and plasma membrane H(+)-ATPase activity were investigated in an Al-tolerant variety of maize (Zea mays L. var. dwarf hybrid), either treated or not with AlF(x). In vitro, AlF(x) competitively inhibited the rate of root phosphate uptake as well as the H(+)-ATPase activity. Conversely, pretreatment of seedlings with AlF(x) in vivo promoted no effect on the H(+)-ATPase activity, whereas a biphasic effect on Pi uptake by roots was observed. Although the initial rate of phosphate uptake by roots was inhibited by AlF(x) pretreatment, this situation changed over the following minutes as the rate of uptake increased and a pronounced stimulation in subsequent (32)Pi uptake was observed. This kinetic behavior suggests a reversible and competitive inhibition of the phosphate transporter by fluoroaluminates. The stimulation of root (32)Pi uptake induced by AlF(x) pretreatment was tentatively interpreted as a phosphate starvation response. This report places AlF(3) and AlF(4)(-) among Al-phytotoxic species and suggests a mechanism of action where the accumulation of Pi-mimicking fluoroaluminates in the soil may affect the phosphate absorption by plants. The biochemical, physiological, and environmental significance of these findings is discussed.     

Saturable uptake of indol-3yl-acetic Acid by maize roots

The uptake of 5-[³H]indol-3yl-acetic acid (IAA^*) by segments of Zea mays L. roots was measured in the presence of nonradioactive indol-3yl-acetic acid (IAA°) at different concentrations. IAA uptake was found to have a nonsaturable component and a saturable part with (at pH 5.0) an apparent K_m of 0.285 micromolar and apparent V_max 55.0 picomoles per gram fresh mass per minute. These results are consistent with those which might be expected for a saturable carrier capable of regulating IAA levels. High performance liquid chromatography analyses showed that very little metabolism of IAA^* took place during 4 minute uptake experiments. Whereas nonsaturable uptake was similar for all 2 millimeter long segments prepared within the 2 to 10 millimeter region, saturable uptake was greatest for the 2 to 4 millimeter region. High levels of uptake by stelar (as compared with cortical) segments are partly attributable to the saturable carrier, and also to a high level of uptake by nonsaturable processes. The carrier may play an essential role in controlling IAA levels in maize roots, especially the accumulation of IAA in the apical region. The increase in saturable uptake toward the root tip may also contribute to the acropetal polarity of auxin transport.     

Influence of phosphate status on phosphate uptake kinetics of maize (Zea mays) and soybean (Glycine max)

To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03–100 µmol P L^-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, I_max, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, I_max was negatively related to % P in shoots. Michaelis constants, K_m, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, C_min, where net influx equals zero, was found between 0.06 and 0.3 µmol L^-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of I_max, when internal P concentration decreases. Changes of K_m and C_min were of minor importance.     

Inhibition of phosphate uptake in barley roots by hydroxy-benzoic acids

The influence of 15 hydroxy-benzoic acids upon active inorganic phosphate absorption by barley roots was examined. For each compound an inhibition constant (k_i) was determined, i.e. the concentration of compound required to bring about a 50% inhibition of absorption. The k_i values of the benzoic acids were strongly correlated with their octanol—water partition coefficients and their pK_a values. This suggests that the inhibition of normal membrane functions, brought about by benzoic acids, results from a generalized increase in cell membrane permeability. Salicylate derivatives were generally more inhibitory than would be predicted from their partition coefficients; their pronounced toxicity probably arises from structural impediments to their detoxication.     

Amyloplast sedimentation kinetics in gravistimulated maize roots

Amyloplast sedimentation in gravistimulated maize (Zea mays L.) roots was measured using the change in angle from the center of the cell to each amyloplast as an index of sedimentation. Using tissue fixed after gravistimulation, the relationship between mean amyloplast angle and the duration of gravistimulation was found to be linear when plotted on a logarithmic time scale. Extrapolated values for the onset of angular change are 5.9 s after the start of gravistimulation for the entire population of amyloplasts and 11.8 s for lead amyloplasts. By multiplying the instantaneous angular velocity (in radians) by the cell center to amyloplast radius, it is possible to calculate the initial sedimentation velocity to be 19.1 m min^-1 at 5.9 s. During sedimentation, the mean amyloplast angles surpass the calculated cell corner angle of 123° at 2.2 min for all amyloplasts and at 19 s for lead amyloplasts near the new lower wall. Thus, substantial sedimentation occurs within the presentation time, calculated to be 4.1 min. These kinetics are consistent with several hypotheses of graviperception.Symbol t_p presentation time     

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 ophiobolin a on ion leakage and hexose uptake by maize roots

Ophiobolin A, a sesterterpene metabolite of Helminthosporium maydis, Nisikado and Miyake, stimulates net leakage of electrolytes and glucose from maize (Zea mays L.) seedling roots. Treatment of the roots with ophiobolin A at a concentration of 10 mug/ml (25 mum) inhibits uptake of 10 mm 2-deoxyglucose by 50% and of 0.5 mm 2-deoxyglucose by 85%. Compartmental analysis of the efflux of 3-O-methylglucose failed to show a similar effect of ophiobolin A on the rate of efflux of hexose. The inhibition of uptake is not reversible by washing. There is no difference in the effects on roots from cytoplasmic male sterile or normal cytoplasm plants, and exposure of carrot (Daucus carota L.) root discs to ophiobolin A also causes inhibition of 2-deoxyglucose uptake by this tissue.     

Regulation of phosphate uptake kinetics inOscillatoria agardhii

In order to study phosphate uptake kinetics the cyanobacteriumOscillatoria agardhii was grown in continuous culture under a phosphorus limitation. The affinity of the uptake system reflected in the initial slope of the uptake rate versus external substrate concentration curve (dV/ds) was found to be unaffected by the growth wate.The maximum phosphate uptake rate (V _m ) decreased as the growth rate was increased. Attempts were made to relate the decrease ofV _m to the increase in phosphorus content of the cells that occurred a higher growth rates. Accumulation of phosphate during pulse experiments indeed resulted in a decrease ofV _m . However feedback regulation ofV _m by accumulated phosphorus was found to occur only to a small extent in steady state growing cells. The main part of the regulation of the activity of the phosphate uptake system seemingly is determined by a long term process that is, at least longer than 2 h. The presence of short term feedback inhibition by accumulated phosphorus on the activity of the uptake system provides an explanation of the phenomenon thatOscillatoria agardhii is not able to grow at near _max growth rates under a phosphorus limitation.     

Nitrate uptake ability by maize roots during and after drought stress

The effects of different intensities and durations of soil drought and re-watering on the nitrate uptake ability of maize roots were studied. Plants were grown in split-root containers with one part of the root system subjected to different intensities and durations of soil drought and re-watering while the other part of the root system was continuously watered to 23% (w/w) soil water content (70% water capacity). Experiments were performed in split-root containers to maintain a high growth rate, thus ensuring high nutrient demand of the shoot irrespective of the soil water regime. To avoid limitation of nitrate uptake by transport processes in the dry soil, and to ensure a uniform ¹⁴N/¹⁵N ratio at the root surface, ¹⁵N was applied to the roots by placing them into an aerated nutrient solution with 0.5 mM Ca(¹⁵NO₃)₂. Shoot elongation and biomass were only slightly affected by drought in one root compartment when the soil in the other root compartment was kept wet. Therefore, the growth-related nutrient demand of the shoot remained at a high level. At moderate levels of soil drought (10% w/w water content) the ability of the roots for N-uptake was not affected even after 10 d of drought. N-uptake ability was reduced to about 20% of the well-watered control only when the soil water content was decreased to 5%. Total soluble sugar content of the roots increased with increasing soil drought, indicating that low N-uptake ability of roots subjected to severe soil drought was not caused by low assimilate supply from the shoot. Nitrate uptake ability of roots maintained in very dry soil (5% soil water content w/w) even for a prolonged period of 8 d, recovered within 3 d following re-watering. Root growth increased one day after re-watering. A short-term experiment with excised roots formerly subjected to severe soil drought showed that nitrate uptake ability recovered in old and young root segments after 2 d of re-watering. Obviously, the increase in N-uptake ability after re-watering was caused not only by new root growth but also by recovery of the uptake ability of formerly stressed roots.