Relationship between Energy-dependent Phosphate Uptake and the Electrical Membrane Potential in Lemna gibba G1

High rates of phosphate uptake into phosphate-starved Lemna gibba L. G1 were correlated with a high membrane potential (pd = −220 millivolts). In plants maintaining a low pd (−110 millivolts), the uptake rate was only 20% of that of high-pd plants. At the onset of phosphate transport, the membrane of high-pd plants was transiently depolarized. This effect was much smaller in low-pd plants. Light stimulated phosphate uptake and the repolarization upon phosphate-induced depolarization, especially in plants grown without sucrose. The phosphate uptake rate was optimal at pH 6 and decreased with increasing pH, corresponding to the phosphate-induced pd changes. Phosphate starvation stimulated the uptake and increased the phosphate-induced depolarization, thus indicating that phosphate uptake depends on the intracellular phosphate level. It is suggested that uptake of monovalent phosphate in Lemna gibba proceeds by an H⁺ cotransport dependent on the proton electrochemical potential difference and, hence, on the activity of an H⁺ -extrusion pump.     

Nitrate uptake by bean (Phaseolus vulgaris L.) roots under phosphate deficiency

Bean (Phaseolus vulgaris L.) plants were cultured for 19 d on complete or on phosphate deficient culture media. Low inorganic phosphate concentration in the roots decreased ATP level and nitrate uptake rate. The mechanisms which may control nitrate uptake rate during phosphate deficiency were examined. Plasma membrane enriched fractions from phosphate sufficient and phosphate deficient plants were isolated and compared. The decrease in total phospholipid content was observed in plasma membranes from phosphate deficient roots, but phospholipid composition was similar. No changes in ATPase and proton pumping activities measured in isolated plasma membrane of phosphate sufficient and phosphate deficient bean roots were noted. The electron microscope observations carried out on cortical meristematic cells of the roots showed that active ATPases were found in plasma membrane of both phosphate sufficient and phosphate deficient plants. The decrease in inorganic phosphate concentration in roots led to increased nitrate accumulation in roots, accompanied by a corresponding alterations in NO₃ distribution between shoots and roots. Nitrate reductase activity in roots of phosphate deficient plants estimated in vivo and in vitro was reduced to 50–60% of the control. The increased NO₃ concentration in root tissue may be explained by decreased NR activity and lower transport of nitrate from roots to shoots. Therefore, the reduction of nitrate uptake during phosphate starvation is mainly a consequence of nitrate accumulation in the roots.     

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.     

Effects of phosphate stress on the rate of phosphate uptake during resupply to deficient tomato plants

Plants of Lycopersicon esculentum Mill. P. I. 120262 show an increased phosphate uptake rate per unit dry weight of root after as little as one day of growth in solutions lacking phosphate. The reversibility of this response in plants experiencing various degrees of phosphate stress was investigated by measuring the depletion of phosphate from solutions over 3-h intervals. Measurements were made at three times in the first 30 h after phosphate was resupplied. Reversibility decreased as the level of phosphate stress increased. The phosphate uptake rate was returned to the level of controls after 30 h of phosphate resupply in plants grown without phosphate for one or three days. Plants grown without phosphate for five or seven days had uptake rates 26 and 40% higher than controls, respectively, after the same period of phosphate resupply. Internal phosphate concentrations after 30 h of phosphate resupply were equal to or greater than the controls in all plants. These results are consistent with a simple reversible feedback of phosphate status on phosphate transport in slightly stressed plants, but such a mechanism seems inadequate to explain the responses observed in more severely stressed plants.     

Effects of sulphate and pH on the plant-availability of phosphate adsorbed on goethite

The adsorption of phosphate on metal (hydr)oxides may be influenced by the pH and by the adsorption of other ions. In this study, the influence of sulphate and pH on phosphate adsorption on goethite and the availability to plants of adsorbed phosphate was examined. Maize plants were grown on suspensions of goethite with adsorbed phosphate, containing the same total amount of phosphate and either 0.11 mM or 2.01 mM sulphate at pH 3.7, 4.6 or 5.5. The uptake of phosphorus by the plants increased with the larger sulphate concentration and decreasing pH. Mean P uptake in the treatment with 2.01 mM sulphate and pH 3.7 was 55 µmol plant^-1, whereas in the treatment with 0.11 mM sulphate and pH 5.5 it was 2 µmol plant^-1. Batch adsorption experiments using³² P and speciation modelling of ion adsorption showed that in the presence of sulphate, the phosphate concentration in solution strongly increased with decreasing pH, due to competitive adsorption between sulphate and phosphate on goethite. Modelled phosphate concentrations in solution in the uptake experiment were all below 0.6 µM and correlated well with the observed P uptake. This correlation indicates that the strong influence of the sulphate concentration and pH on the plant-availability of adsorbed phosphate results from the competition between sulphate and phosphate for adsorption on goethite.     

植物磷稳态的调控机制

磷是植物必需的重要营养元素之一,是生物大分子的重要组成部分,在植物生命过程中发挥着不可或缺的作用.维持体内磷稳态对于植物的生长发育和环境应答至关重要.多种信号分子参与调控植物对磷的吸收和转运.植物维持磷稳态主要包括土壤磷的活化、磷的吸收、转运、存储和再利用等过程,涉及磷胁迫响应、转录因子调节、miRNA调节、菌根共生、...     

Adventitious rooting of conifers: influence of physical and chemical factors

In conifers, vegetative propagation of superior genotypes is the most direct means for making large genetic gains, because it allows a large proportion of genetic diversity to be captured in a single cycle of selection. There are two aims of vegetative propagation, namely large-scale multiplication of select genotypes and production of large numbers of plants from scarce and costly seed that originates from controlled seed orchard pollinations. This can be achieved, in some species, either through rooted cuttings or rooted microshoots, the latter regenerated through tissue culture in vitro. Thus far, both strategies have been used but often achieved limited success mainly because of difficult and inefficient rooting process. In this overview of technology, we focus on the progress in defining the physical and chemical factors that help the conifer cuttings and microshoots to develop adventitious roots. These factors include plant growth regulators, carbohydrates, light quality, temperature and rooting substrates/media as major variables for development of reliable adventitious rooting protocols for different conifer species.     

Factors of plant nutrient availability relevant to soil testing

Summary In most arable soils the nitrate availability depends mainly on the quantity of nitrate present in the rooting zone at the beginning of the growing season. Easily mineralizable organic N and the release of non-exchangeable NH₄ from clay minerals may in addition control the nitrogen availability during a season. In flooded soils, ammonium is the major form of nitrogen absorbed by plants. Ammonium dynamics in these soils is similar to that of potassium. The availability of both is controlled mainly by the intensity and buffering power for ammonium or potassium, respectively. Basically, intensity of the supply and buffering power for phosphate are the main factors determining the phosphate availability. The determination of the phosphate buffer power, especially in the root zone, however, remains to be difficult. Soil test methods should take into consideration the major factors and processes relevant to the availability of a particular plant nutrient.     

Physiological changes in, and phosphate uptake by potato plants during development of, and recovery from phosphate deficiency

The development of phosphate deficiency (P-stress) was observed in rooted sprouts of Solanum tuberosum L. cv. Desiree growing in solutions without phosphate. Shoot growth was inhibited by P-stress within 3 to 5 days of terminating the phosphate supply, while significant effects on root growth were not recorded until 7 to 9 days. Thus, the shoot:root dry weight ratio decreased from 4.3 to 2.6 over a 10-day period. Growth in the absence of an exogenous phosphate supply progressively diluted the phosphorus in the plant. The proportional decrease in concentration was similar in roots and shoots over a 7-day period, even though the former were growing more quickly. The potential for phosphate uptake per unit weight of root increased rapidly during the first 3 days of P-stress. When the plants were provided subsequently with a labelled, 1 mol m^?3 phosphate solution, the absorption rate was 3 to 4-fold greater than that of control plants which had received a continuous phosphate supply. The increased rate of uptake by P-stressed plants was accounted for by an increase (3-fold) in the V_max of system 1 for phosphate transport and by a marked increase in the affinity of the system for phosphate (decrease in K_m). In the early stages of P-stress, before marked changes in growth were measured, the proportion of labelled phosphate translocated to the shoots increased slightly relative to the controls when a phosphate supply was restored. In the later stages of stress a greater proportion was retained in the root system of P-stressed plants than in that of controls. In plants with roots divided between solutions containing or lacking a phosphate supply, the increased absorption rate was determined by the general demand for phosphate in the plant and not by the P-status of the particular root where uptake was measured. By contrast, the poportion translocated was strongly dependent on the P-status of the root. The restoration of a phosphate supply to P-stressed plants was marked by a rapid increase in the P concentration in snoots and roots which returned to levels similar to unstressed controls within 24 h. The enhanced uptake rate persisted for at least 5 days, resulting in supra-normal concentrations of P in both shoots and roots, and in the formation of extensive necrotic areas between the veins of mature leaves. Autoradiographs showed accumulations of ³²P in these lesions and at the points where guttation droplets formed on leaves.     

Quantitative evaluation of the role of organic acid exudation in the mobilization of rock phosphate by rape

Phosphorus-deficient rape plants appear to acidify part of their rhizosphere by exuding malic and citric acid. A simulation model was used to evaluate the effect of measured exudation rates on phosphate uptake from Mali rock phosphate. The model used was one on nutrient uptake, extended to include both the effect of ion uptake and exudation on rhizosphere pH and the effect of rhizosphere pH on the solubilization of rock phosphate. Only the youngest zones of the root system were assumed to exude organic acids. The transport of protons released by organic acids was described by mass flow and diffusion. An experimentally determined relation was used describing pH and phosphate concentration in the soil solution as a function of total soil acid concentration. Model parameters were determined in experiments on organic acid exudation and on the uptake of phosphate by rape from a mixture of quartz sand and rock phosphate. Results based on simulation calculations indicated that the exudation rates measured in rape plants deficient in phosphorus can provide the roots with more phosphate than is actually taken up. Presence of root hairs enhanced the effect of organic acid exudation on calculated phosphate uptake. However, increase of root hair length without exudation as an alternative strategy to increase phosphate uptake from rock phosphate appeared to be less effective than exudation of organic acids. It was concluded that organic acid exudation is a highly effective strategy to increase phosphate uptake from rock phosphate, and that it unlikely that other rhizosphere processes play an important role in rock phosphate mobilization by rape.     

ars1, an Arabidopsis mutant exhibiting increased tolerance to arsenate and increased phosphate uptake

Arsenic is one of the most toxic pollutants at contaminated sites, yet little is known about the mechanisms by which certain plants survive exposure to high arsenic levels. To gain insight into the mechanisms of arsenic tolerance in plants, we developed a genetic screen to isolate Arabidopsis thaliana mutants with altered tolerance to arsenic. We report here on the isolation of a mutant arsenic resisant 1 (ars1) with increased tolerance to arsenate. ars1 germinates and develops under conditions that completely inhibit growth of wild-type plants and shows a semi-dominant arsenic resistance phenotype. ars1 accumulates levels of arsenic similar to that accumulated by wild-type plants, suggesting that ars1 plants have an increased ability to detoxify arsenate. However, ars1 plants produce phytochelatin levels similar to levels produced by the wild type, and the enhanced resistance of ars1 is not abolished by the gamma-glutamylcysteine synthetase inhibitor l-buthionine sulfoxime (BSO). Furthermore, ars1 plants do not show resistance to arsenite or other toxic metals such as cadmium and chromium. However, ars1 plants do show a higher rate of phosphate uptake than that shown by wild-type plants, and wild-type plants grown with an excess of phosphate show increased tolerance to arsenate. Traditional models of arsenate tolerance in plants are based on the suppression of phosphate uptake pathways and consequently on the reduced uptake of arsenate. Our data suggest that arsenate tolerance in ars1 could be due to a new mechanism mediated by increased phosphate uptake in ars1. Models discussing how increased phosphate uptake could contribute to arsenate tolerance are discussed.     

Control of Net Uptake of Nutrients by Regulation of Influx in Barley Plants Recovering from Nutrient Deficiency

Rates of influx and net uptake of nitrate, phosphate and sulphatewere measured in intact barley plants, and concurrent effluxwas obtained by difference. Net uptake of these anions variedwidely depending on the nutrient status of the plants, and thedifferences in net uptake could be accounted for almost entirelyby changes in influx. Efflux played only a minor role in regulatingnet uptake of nitrate, phosphate or sulphate during recoveryfrom N-, P-, or S-deficiency. Nitrate influx and short-term ammonium absorption by N-deficientbarley plants were closely correlated, and varied in parallelwith rates of net uptake of nitrate or ammonium by similar plants.Again, it would seem that net uptake of ammonium is controlledpredominantly by changes in the rate of influx.Copyright 1993,1999 Academic Press Hordeum vulgare, barley, nutrient absorption, influx, nitrate, phosphate, sulphate, ammonium     

Adenine nucleotide translocase-dependent anion transport in pea chloroplasts

Pea chloroplasts were found to take up actively ATP and ADP and exchange the external nucleotides for internal ones. Using carrier-free [14C]ATP, the rate of nucleotide transport in chloroplasts prepared from 12-14-day-old plants was calculated to be 330 mumol ATP/g chlorophyll/min, and the transport was not affected by light or temperature between 4 and 22 degrees C. Adenine nucleotide uptake was inhibited only slightly by carboxyatractylate, whereas bongkrekic acid was nearly as effective an inhibitor of the translocator in pea chloroplasts as it was in mammalian mitochondria. There was no counter-transport of adenine nucleotides with substrates carried on the phosphate translocator including inorganic phosphate, 3-phosphoglycerate and dihydroxyacetone phosphate. However, internal or external phosphoenolpyruvate, normally considered to be transported on the phosphate carrier in chloroplasts, was able to exchange readily with adenine nucleotides. Furthermore, inorganic pyrophosphate which is not transported by the phosphate carrier initiated efflux of phosphoenolpyruvate as well as ATP from the chloroplast. These findings illustrate some interesting similarities as well as differences between the various plant phosphate and nucleotide transport systems which may relate to their role in photosynthesis.     

Understanding plant rooting patterns in semi‐arid systems: an integrated model analysis of climate,soil type and plant biomass

Aim A consistent set of root characteristics for herbaceous plants growing in water‐limited environments has been developed based on compilations of global root databases, but an overall analysis of why these characteristics occur is still missing. The central question in this study is whether an ecohydrological model which assumes that rooting strategies reflect maximization of transpiration can predict the variations in rooting strategies of plants in dry environments. Location Arid ecosystems across the globe. Methods A model was used to explore interactions between plant biomass, root–shoot allocation, root distribution, rainfall, soil type and water use by plants. Results Model analyses showed that the predicted shifts in rooting depth and root–shoot allocation due to changes in rainfall, soil type and plant biomass were quite similar to observed shifts. The model predicted that soil type, annual rainfall and plant biomass each had strong effects on the rooting strategies that optimize transpiration, but also that these factors have strong interactive effects. The process by which plants compete for water availability (soil evaporation or drainage) especially affected the depth distribution of roots in the soil, whereas the availability of rainfall mainly affected the optimal root–shoot allocation strategy. Main conclusions The empirically observed key patterns in rooting characteristics of herbaceous plant species in arid environments could be explained in this theoretical study by using the concept of hydrological optimality, represented here by the maximization of transpiration.     

Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.     

Short-term phosphate uptake kinetics in Zostera noltii Hornem: a comparison between excised leaves and sediment-rooted plants

Short-term phosphate uptake by excised leaves of Zostera noltii Hornem. as well as by leaves of sediment-rooted plants were characterized and compared in a kinetic framework. Time courses of phosphate disappearance were measured over a wide range of initial substrate concentrations. Phosphate uptake determined by this perturbation method did not follow Michaelis-Menten kinetics. Both excised leaves and sediment-rooted plants exhibited a biphasic uptake pattern as a function of phosphate concentration. However, rooted plants showed higher uptake rates and accumulated higher amounts of phosphate than excised leaves. The results point out the importance of the structural and functional coupling between shoots and underground parts during the nutrient foliar uptake processes. Our study also indicates that Zostera noltii leaves function as a phosphate sink in the water column.A second objective of this work is to compare the perturbation and the multiple flask methods in determining the uptake kinetic parameters. The obtained results support that both methods provide valuable and complementary information in determining the uptake rates.     

Influence of phosphate-stress on phosphate absorption and translocation by various parts of the root system of Hordeum vulgare L. (barley)

Plants of Hordeum vulgare (barley) were grown initially in a solution containing 150 M phosphate and then transferred on day 6 to solutions with (+P) and without (-P) phosphate supplied. After various times plants from these treatments were supplied with labelled phosphate. Analysis of plant growth and rates of labelled phosphate uptake showed that a general enhancement of uptake and translocation was found, in plants which had been in the-P solution, several days before the rate of dry matter accumulation was affected. Subsequently a detailed analysis of phosphate uptake by segments of intact root axes showed that the enhancement of phosphate uptake by P-stress occurred first in the old and mature parts of the seminal root axis and last in the young zones 1 cm from the root apex. During this transition period there were profound changes in the pattern of P absorption along the length of the root. Most of the additional P absorbed in response to P-stress was translocated to the shoot, particularly in older zones of the axis. Enhancement of phosphate uptake in young zones of nodal axes occurred at an earlier stage than in seminal axes. The results are related to the P-status of shoots and root zones and discussed in relation to the general control by the shoot of phosphate transport in the root.     

Ion Uptake Efficiency of Sunflower Roots

The term ion uptake efficiency is used for the rate of uptake of a particular ion from nutrient solutions holding a standard concentration of that ion (0.5 mM sulphate, 1.5 mM phosphate or 2.0 mM rubidium). The uptake efficiency for rubidium and phosphate in roots of intact sunflower plants depended on the salt status of the plants and on the concentration of the ion under investigation in pretreatment solutions. The effect of pretreatment was a rapid process causing differences of more than 300% in ion uptake efficiency within 1 h, depending on the composition of the pretreatment solution. At concentrations above 0.1 mM the rate of uptake of rubidium in the root was higher than the net potassium uptake necessary for adequate growth. The rate of sulphate uptake was related to potassium uptake but not to phosphate uptake. It is suggested that ion uptake of the roots is regulated by internal factors as well as by direct interactions between the medium and the absorbing surfaces.     

Analysis of phosphate acquisition efficiency in different Arabidopsis accessions

The morphological and physiological characteristics of Arabidopsis accessions differing in their phosphate acquisition efficiencies (PAEs) when grown on a sparingly soluble phosphate source (hydroxylapatite) were analyzed. A set of 36 accessions was subjected to an initial PAE evaluation following cultivation on synthetic, agarose-solidified media containing potassium phosphate (soluble) or hydroxylapatite (sparingly soluble). From the five most divergent accessions identified in this way, C24, Co, and Cal exhibited high PAEs, whereas Col-0 and Te exhibited low PAEs. These five accessions were analyzed in detail. Significant differences were found in root morphology, phosphate uptake kinetics, organic acid release, rhizosphere acidification, and the ability of roots to penetrate substrates. Long root hairs at high densities, high uptake per unit root length, and high substrate penetration ability in the efficient accessions C24 and Co mediate their high PAEs. The third accession with high PAE, Cal, exhibits a high shoot-to-root ratio, long roots with long root hairs, and rhizosphere acidification. These results are consistent with previous observations and highlight the suitability of using Arabidopsis accessions to identify and isolate genes determining the PAE in plants.