Factors influencing the isotopically exchangeable phosphate in soils

Summary The labile phosphate in a non-calcareous and a slightly calcareous soil was determined by isotopic exchange in the presence and absence of 0.001 molar solutions of a chelating and a non-chelating organic anion. The rate of isotopic exchange curves were analyzed graphically to sub-divide the labile phosphate into 3 or 4 fractions. The half-lives of exchange for the rapid, medium and slow fractions were between 0.3 to 1.6 hours, 1.8 to 8.6 hours and 25.8 to 46.1 hours respectively. In addition, an instantaneously-exchanging component was sometimes observed.In the presence of the citrate ion, the total labile phosphate was increased in the non-calcareous soil and decreased in the calcareous soil, whereas the diethyl barbiturate ion (DEB) anions decreased the total labile phosphate in both soils. In general, the citrate ion increased the rapid and the medium fractions whereas the DEB anion either did not affect them or decreased them. Again, whereas citrate always increased the phosphate in solution, the effect of DEB anions depended on the soil. The major effect of the organic anions was to greatly decrease the slowly-exchanging fraction in both soils.The half-lives of exchange for the rapid, medium and slow fractions were in the order no organic anion > citrate > barbiturate and the rate constants for a first-order mechanism were in the order no organic anion < barbiturate < citrate.Small but significant differences were observed between the two soils.     

The influence of method of watering on phosphate uptake by ryegrass grown in pots: Effect on the L-value

Summary The influence of method of watering (overhead or sub-irrigation) on quantity, location and specific activity of phosphate taken up by ryegrass was examined in a pot experiment on two contrasting soils.It is concluded that watering method has no influence on L value provided that P-32 is uniformly distributed. Where, however watering regime leads to uneven root activity the L-value may be modified by uneven distribution of P-32.Sub-irrigation with occasional overhead watering is likely to provide the most satisfactory moisture regime.     

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.     

The phosphate uptake mechanism

The slow rate of diffusion of phosphate in soil results in a zone of depletion of phosphate ions in solution around the roots of plants in low phosphate soils. Transfer of phosphate to the site of uptake into the root symplasm limits phosphate uptake in such soils. This transfer involves movement across the depletion zone and through the root apoplasm. The apoplasm is made up of the cell walls of epidermal and cortical cells, together with the associated intercellular spaces. Although the pores in the open latticework of these cell walls permit movement of nutrients around cells, they increase the path length across which phosphate ions have to diffuse. The structural components and net negative charges of the cell walls also influence the effective concentrations of phosphate in the apoplasm. This concentration may be further modified by excreted organic compounds around cell walls and the presence of micro-organisms that use such compounds as carbon sources. A membrane on the inner surface of the cell wall, the plasmalemma, separates the apoplasm from the symplasm. Uptake of nutrients into the root symplasm occurs through transporter proteins embedded in this membrane. Understanding of the mechanisms by which phosphate is transported across the plasmalemma into the plant symplasm has advanced considerably over the past 4 years due to the application of molecular techniques. Genes encoding the transporters involved in this process have been isolated from a number of plant species. These transporters belong to a family of membrane proteins characterized by having 12 membrane-spanning domains arranged in a '6+6' configuration. H₂PO₄ ^– ions, together with protons, are transported through this protein. This transport process is driven by the potential across the membrane maintained by the action of a H⁺-ATPase, the `proton pump', that extrudes protons to the outer surface of the membrane. The expression of genes encoding high-affinity root phosphate transporters is regulated by the phosphorus (P) status of the plant. The transduction pathway involved in this regulation is not known at present. It is a systemic response rather than a localized response, however, the overall phosphate status of the plant being the controlling factor. Under phosphate stress, the expression of genes encoding these phosphate transporters is up-regulated. This results in a greater number of transporter proteins in the plasmalemma and enhanced phosphate uptake rates, if phosphate is available at the membrane surface. Uptake occurs around the root tip, into epidermal cells with their associated root hairs and into cells in the outer layers of the root cortex. Further back along the root axis, phosphate can also be taken up by transfer from mycorrhizal fungi to root cortical cells.Strategies for increasing nutrient uptake by overexpressing genes encoding high-affinity phosphate transporters are likely to be mainly applicable to situations where a reasonable phosphate concentration can be maintained at the outer surface of the plasmalemma. Maintaining such a concentration is a major problem in the phosphate deficient soils of the semi-arid tropics (SAT), so emphasis in these soils is on strategies to improve the movement of phosphate to the surface of the plasmalemma. There may be scope, however, for manipulating the expression of genes involved in the internal mobilisation of phosphate within the plant, thereby improving phosphate utilisation.     

Studies of ‘Available’ and isotopically exchangeable sulphur in some North Queensland soils

Summary Using ³⁵S-sulphate, the specific activity of various sulphur fractions in some diverse North Queensland soils has been followed for up to 185 days in a glasshouse experiment. The sulphur extracted with 0.01 M calcium phosphate was from the same pool as that used by the test plants, and since near full recovery of added ³⁵SO₄ was obtained initially, this fraction is comparable to the L-value. On the other hand, 0.5 M NaHCO₃ removed some soil sulphur that was not available to the plants.Liming caused an initial increase in the phosphate extractable fraction, the sulphur seemingly being released from the NaHCO₃ extractable fraction, but decreased sulphate sorption also contributed to the increase in S uptake by the plants upon liming. re]19750507     

Membrane potential dependence of Fe(III) uptake by mouse duodenum

Intestinal iron uptake by mouse duodenal fragments is inhibited in the absence of oxygen and glucose from the incubation medium and by a variety of metabolic inhibitors. The mechanism of energy coupling to iron uptake is, however, unclear. In vitro experiments using duodenal fragments showed Fe3+ uptake to be markedly inhibited, in a reversible fashion, by the replacement of incubation medium Na+ by K+. Addition of phloridzin to the medium failed to affect iron uptake, suggesting that the above effect was not a consequence of reduced glucose uptake. Substitution of Na+ by Rb+ also potently reduced duodenal iron uptake. Replacement of medium NaCl by either mannitol or choline chloride had no significant effect on Fe3+ uptake, thus excluding the possibility of the Fe3+ uptake process being Na+-dependent. Similar observations were made with duodenal fragments from animals with enhanced Fe3+ absorption, due to chronic hypoxia. Valinomycin (1-5 microM) increased the uptake of both glucose and Fe3+. Higher concentrations (22.5 microM) of the ionophore were inhibitory. In vivo studies (tied-off segments) using Rb+-containing medium confirmed the inhibitory effects of univalent cations on Fe3+ absorption. Enhanced absorption of Fe3+ was also demonstrable in vivo, with low concentrations of valinomycin and nigericin added to the luminal medium. These observations suggest that the Fe3+ uptake process may be dependent on the brush-border membrane potential.     

The metabolic fate of isotopically labeled trimethylamine-N-oxide (TMAO) in humans

Trimethylamine-N-oxide (TMAO) is associated with chronic disease risk. However, little is known about the metabolic fate of dietary TMAO. This study sought to quantitatively elucidate the metabolic fate of orally consumed TMAO in humans. As part of a crossover feeding study, healthy young men (n=40) consumed 50-mg deuterium-labeled methyl d9-TMAO (d9-TMAO), and enrichments of TMAO and its derivatives were measured in blood for 6 h, urine and stool, as well as skeletal muscle in a subset of men (n=6). Plasma d9-TMAO was detected as early as 15 min, increased until 1 h and remained elevated through the 6-h period. TMAO exhibited an estimated turnover time of 5.3 h, and ~96% of the dose was eliminated in urine by 24 h, mainly as d9-TMAO. No d9-TMAO was detected in feces. Notably, d9-TMAO and d9-trimethylamine were detected in skeletal muscle (n=6) at 6 h, and the enrichment ratio of d9-TMAO to d9-trimethylamine was influenced by a genetic variant in flavin-containing monooxygenase isoform 3 (FMO3 G472A). These results suggest that the absorption of orally consumed TMAO is near complete and does not require processing by gut microbes. TMAO exhibits fast turnover in the circulation with the majority being eliminated in urine within 24 h. A small portion of the dose, however, is taken up by extrahepatic tissue in a manner that appears to be under the influence of FMO3 G472A polymorphism. This trial was registered at clinicaltrials.gov as NCT02558673.     

Effect of dimethyl sulfoxide (DMSO) on zinc availability (L-value), growth and metabolic activities of rice plants

Summary Effects of soil application (0.01, 0.1 and 1%) and foliar sprays (0.001, 0.01 and 0.1%) of dimethyl sulfoxide (DMSO) and soil application of Zn (10 and 20 ppm) on growth, yield, photosynthetic pigments and some enzymatic activities of rice (Oryza sativa L. variety Jaya) were investigated in a Zn-deficient soil under pot culture trials. Control plants showed typical Zn deficiency symptoms, very low dry matter and chlorophyll contents and significantly lower activities of carbonic anhydrase and tryptophan synthetase. Application of 10 or 20 ppm Zn to the soil markedly improved plant growth, chlorophyll content, enzymatic activities, Zn availability (L-value), and grain yield. Application of 1% DMSO to the soil proved to be severly phytotoxic for plant growth and dry matter accumulation. Application of lower doses of DMSO to the soil (0.01 and 0.1%) or as foliar sprays (0.001 and 0.01%), however, slightly increased dry weights of all plant parts at 45 days after transplanting. Grain yield was significantly increased by all DMSO treatments, except 1% soil application which completely suppressed the grain formation.Enzymatic activities and chlorophyll and carotenoid accumulation showed concentration-dependent stimulation or inhibition by DMSO treatments. Carbonic anhydrase activity was significantly increased by most of the DMSO treatments whereas tryptophan synthetase activity was stimulated only by the lowest dose of soil (0.01%) and foliar (0.001%) applications. Chlorophyll content was stimulated by lower doses of foliar application of DMSO (0.001 and 0.01%) but the other DMSO treatments brought about a concentration-dependent decrease in chlorophyll accumulation. Carotenoid accumulation was inversely related to that of chlorophylls. Chlorophyll: carotenoid ratio was increased by all DMSO and Zn treatments except 1% soil application of DMSO. Zn availability (L-value) of the soil was increased by all DMSO treatments. Zn content of leaf blades was positively correlated with chlorophyll content (r=0.579), tryptophan synthetase (r=0.700) and carbonic anhydrase (r=0.537) activities but negatively correlated with that of carotenoid content (r=–0.896). The possible mechanisms of DMSO on plants are discussed in relation to its possible agricultural utility.Publication No. 811 under Journal Series of the Experiment Station, G.B. Pant of Agriculture & Technology, Pantnagar (Nainital), India.     

Carrier-mediated uptake of glyphosate in broad bean (Vicia faba) via a phosphate transporter

The possibility that the herbicide glyphosate (N-phosphonomethylglycine) may be taken up in plant cells via a phosphate transporter of the plasma membrane was investigated using protoplasts of broad bean leaves ( Vicia faba L.). Phosphonoformic acid, a powerful inhibitor of phosphate transport in animal cells, was first demonstrated to be a competitive inhibitor of phosphate uptake inbroad bean protoplasts. Glyphosate was able to inhibit phosphate uptake into the protoplasts, and to protect partially the phosphate transporter from inhibition by phosphonoformic acid. Concentration dependence studies showed that glyphosate uptake exhibited a saturable phase at low glyphosate concentrations (0. 5 to 3 μ M ), superimposed by a linear uptake at higher concentrations (up to 100 μ M ). Inhibition of glyphosate uptake by para -chloromercuribenzene sulphonic acid, sodium azide and carbonyl-cyanide- m -chlorophenylhydrazone was much stronger at 1 than at 100 μ M glyphosate. Kinetics indicated that the saturable component of glyphosate transport was competitively inhibited by either phosphate or phosphonoformic acid. It is concluded that glyphosate can be absorbed via a phosphate transporter of the plasma membrane     

Characterization of the Rhizobium (Sinorhizobium) meliloti high- and low-affinity phosphate uptake systems.

Genetic studies have suggested that Rhizobium (Sinorhizobium) meliloti contains two distinct phosphate (Pi) transport systems, encoded by the phoCDET genes and the orfA-pit genes, respectively. Here we present data which show that the ABC-type PhoCDET system has a high affinity for Pi (Km, 0.2 microM) and that Pi uptake by this system is severely inhibited by phosphonates. This high-affinity uptake system was induced under Pi-limiting conditions and was repressed in the presence of excess Pi. Uptake via the OrfA-Pit system was examined in (i) a phoC mutant which showed increased expression of the orfA-pit genes as a result of a promoter-up mutation and (ii) a phoB mutant (PhoB is required for phoCDET expression). Pi uptake in both strains exhibited saturation kinetics (Km, 1 to 2 microM) and was not inhibited by phosphonates. This uptake system was active in wild-type cells grown with excess Pi and appeared to be repressed when the cells were starved for Pi. Thus, our biochemical data show that the OrfA-Pit and PhoCDET uptake systems are differentially expressed depending on the state of the cell with respect to phosphate availability.     

The effect of aluminium on phosphate uptake by three isolated ectomycorrhizal fungi

Three wheat cultivars with different tolerances against free aluminium were grown monoxenically in association with Azospirillum brasilense. In situ nitrogen fixation, measured with the acetylene reduction assay, was higher by the aluminium-tolerant cultivars than by the sensitive cultivar. The transfer of fixed nitrogen to the host plant, determined by the ¹⁵N dilution technique, was also significantly higher in the aluminium-resistant wheat plants. The total accumulation of fixed nitrogen in the host plants due to an A. brasilense inoculation varied from approximately 13% to 17% of the total nitrogen in the root and 2.9% to 3.9% of the nitrogen in the shoot.The quantity and quality of exudates released in liquid nutrient solution were analysed separately for two of the wheat cultivars, one aluminium-tolerant and one aluminium-sensitive. After 29 days of growth the aluminium-tolerant plants exudated significantly higher total amounts of carbon than aluminium-sensitive plants. No differences between the two cultivars existed in the carbon exudation rate per gram dry root.Much higher concentrations of low molecular dicarboxylic acids i.e. succinic, malic and oxalic acid, were found in the exudates of aluminium-tolerant plants. Dicarboxylic acids are potential chelating compounds for positively charged metals such as aluminium and they may play an important role in protecting the plant against aluminium incorporation. They are also very suitable substrates for Azospirillum spp. It is therefore suggested that these factors may be causing the higher associative nitrogen fixation rates which was found in the aluminium-tolerant wheat cultivars.     

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.     

Cell-cycle dependence of dolichyl phosphate biosynthesis

The cell-cycle dependence of dolichyl phosphate biosynthesis has been investigated in mouse L-1210 cells fractionated by centrifugal elutriation. Dolichyl phosphate levels increased linearly through the cell cycle, reaching a value in late S phase twice that of early G1. The cell-cycle dependences of four dolichyl phosphate metabolizing enzymes have been measured: cis-prenyltransferase, CTP-dependent dolichol kinase, dolichyl phosphatase, and dolichyl pyrophosphatase. The kinase, the cis-prenyltransferase, and the pyrophosphatase showed cell-cycle variations, increasing through G1 to a maximum in S phase while the monophosphatase activity was cell-cycle independent. The rate of accumulation of dolichyl phosphate was not affected by growing the cells in mevalonolactone-supplemented media. The evidence presented is consistent with models in which either the cis-prenyltransferase or the kinase/phosphatase couple (or both) regulates the levels of dolichyl phosphate in the cell.     

Oxalate dependence of calcium uptake kinetics of rabbit skeletal muscle microsomes (fragmented sarcoplasmic reticulum)

The initial rate of oxalate-facilitated Ca²⁺ uptake by skeletal microsomes depends on both Ca²⁺ and oxalate concentrations in the medium. The apparent K_m for Ca²⁺ increases with increasing oxalate concentration, indicating that Ca²⁺ uptake can involve a carrier-mediated transport system.