Accumulation of phosphate, sulfate and sucrose by excised Phloem tissues

Sucrose-induced stomatal closure is conserved across evolution

As plants evolved to function on land, they developed stomata for effective gas exchange, for photosynthesis and for controlling water loss. We have recently shown that sugars, as the end product of photosynthesis, close the stomata of various angiosperm species, to coordinate sugar production with water loss. In the current study, we examined the sugar responses of the stomata of phylogenetically different plant species and species that employ different photosynthetic mechanisms (i.e., C3, C4 and CAM). To examine the effect of sucrose on stomata, we treated leaves with sucrose and then measured their stomatal apertures. Sucrose reduced stomatal aperture, as compared to an osmotic control, suggesting that regulation of stomata by sugars is a trait that evolved early in evolutionary history and has been conserved across different groups of plants.

Functions and regulation of phosphate starvation-induced secreted acid phosphatases in higher plants

Phosphorus is essential for plant growth and development, but levels of inorganic phosphate (Pi), the major form of phosphorus that plants assimilate, are quite limiting in most soils. To cope with Pi deficiency, plants trigger a suite of adaptive responses, including the induction and secretion of acid phosphatases (APases). In this article, we describe how Pi starvation-induced (PSI) APases are analyzed, and we provide a brief historical review of their identification. We then discuss the current understanding of the functions of PSI-secreted APases and how these APases are regulated at the molecular level. Finally, we provide a perspective on the future direction of research in this field.

Does Don Fisher's high-pressure manifold model account for phloem transport and resource partitioning?

The pressure flow model of phloem transport envisaged by Münch (1930) has gained wide acceptance. Recently, however, the model has been questioned on structural and physiological grounds. For instance, sub-structures of sieve elements may reduce their hydraulic conductances to levels that impede flow rates of phloem sap and observed magnitudes of pressure gradients to drive flow along sieve tubes could be inadequate in tall trees. A variant of the Münch pressure flow model, the high-pressure manifold model of phloem transport introduced by Donald Fisher may serve to reconcile at least some of these questions. To this end, key predicted features of the high-pressure manifold model of phloem transport are evaluated against current knowledge of the physiology of phloem transport. These features include: (1) An absence of significant gradients in axial hydrostatic pressure in sieve elements from collection to release phloem accompanied by transport properties of sieve elements that underpin this outcome; (2) Symplasmic pathways of phloem unloading into sink organs impose a major constraint over bulk flow rates of resources translocated through the source-path-sink system; (3) Hydraulic conductances of plasmodesmata, linking sieve elements with surrounding phloem parenchyma cells, are sufficient to support and also regulate bulk flow rates exiting from sieve elements of release phloem. The review identifies strong circumstantial evidence that resource transport through the source-path-sink system is consistent with the high-pressure manifold model of phloem transport. The analysis then moves to exploring mechanisms that may link demand for resources, by cells of meristematic and expansion/storage sinks, with plasmodesmal conductances of release phloem. The review concludes with a brief discussion of how these mechanisms may offer novel opportunities to enhance crop biomass yields.

The Arabidopsis purple acid phosphatase AtPAP10 is predominantly associated with the root surface and plays an important role in plant tolerance to phosphate limitation

Induction of secreted acid phosphatase (APase) is a universal response of higher plants to phosphate (Pi) limitation. These enzymes are thought to scavenge Pi from organophosphate compounds in the rhizosphere and thus to increase Pi availability to plants when Pi is deficient. The tight association of secreted APase with the root surface may make plants more efficient in the utilization of soil Pi around root tissues, which is present in organophosphate forms. To date, however, no systematic molecular, biochemical, and functional studies have been reported for any of the Pi starvation-induced APases that are associated with the root surface after secretion. In this work, using genetic and molecular approaches, we identified Arabidopsis (Arabidopsis thaliana) Purple Acid Phosphatase10 (AtPAP10) as a Pi starvation-induced APase that is predominantly associated with the root surface. The AtPAP10 protein has phosphatase activity against a variety of substrates. Expression of AtPAP10 is specifically induced by Pi limitation at both transcriptional and posttranscriptional levels. Functional analyses of multiple atpap10 mutant alleles and overexpressing lines indicated that AtPAP10 plays an important role in plant tolerance to Pi limitation. Genetic manipulation of AtPAP10 expression may provide an effective means for engineering new crops with increased tolerance to Pi deprivation.

Onset of Phloem Export from Senescent Petals of Daylily

During senescence, petals of attached daylily (Hemerocallis hybrid cv Cradle Song) flowers lost 95% sugar and 65% dry weight over the first 24 h, with 30% of dry weight loss coming from nonsugar components. Detaching flowers did not delay senescence, but halted loss of carbohydrate and amino acid, suggesting that loss in the intact state was due to phloem export. Petal autolysis occurred mainly in the interveinal parenchyma, causing vascular strands to begin separating from the petal mass. Such vascular strands still stained with tetrazolium and accumulated sucrose, indicating a retained viability. Their sucrose accumulation rates were high in comparison with those of other plant tissues, and the accumulated product was mainly sucrose. Sucrose synthesis took place in the senescent petal, and sucrose was the principal sugar in phloem exudate, whereas hydroxyproline and glutamine were the main transport amino acids. [14C]Sucrose applied to attached senescent flowers was rapidly translocated to other parts of the plant, particularly developing flower buds. Thus, onset of phloem export allowed most of the soluble carbohydrate and amino acid in the senescing flower to be retrieved by the plant. Additional salvaged material came from proteins and possibly from structural carbohydrate. Over a 12-h period, the flower switched from acting as a strong carbohydrate sink during expansion to become a strong source during senescence. This rapid reversal offers potential for phloem transport studies.

Factors which affect the amount of inorganic phosphate, phosphorylcholine, and phosphorylethanolamine in xylem exudate of tomato plants

Phosphate in the xylem exudate of tomato (Lycopersicon esculentum) plants was 70 to 98% inorganic phosphate (Pi), 2 to 30% P-choline, and less than 1% P-ethanolamine. Upon adding (32)Pi to the nutrient, Pi in xylem exudate had the same specific activity within 4 hours. P-choline and P-ethanolamine reached the same specific activity only after 96 hours. The amount of Pi in xylem exudate was dependent on Pi concentration in the nutrient and decreased from 1700 to 170 micromolar when Pi in the nutrient decreased from 50 to 2 micromolar. The flux of 0.4 nmoles organic phosphate per minute per gram fresh weight root into the xylem exudate was not affected by the Pi concentration in the nutrient solution unless it was below 1 micromolar. During 7 days of Pi starvation, Pi in the xylem exudate decreased from 1400 to 130 micromolar while concentrations of the two phosphate esters remained unchanged.The concentration of phosphate esters in the xylem exudate was increased by addition of choline or ethanolamine to the nutrient solution, but Pi remained unchanged. Upon adding [(14)C]choline to the nutrient, 10 times more [(14)C]P-choline than [(14)C]choline was in the xylem exudate and 85 to 90% of the ester phosphate was P-choline. When [(14)C]ethanolamine was added, [(14)C]P-ethanolamine and [(14)C]ethanolamine in the xylem sap were equal in amount. P-choline and P-ethanolamine accumulated in leaves of whole plants at the same time and the same proportion as observed for their flux into the xylem exudate. No relationship between the transport of P-choline and Pi in the xylem was established. Rather, the amount of choline in xylem exudate and its incorporation into phosphatidylcholine in the leaf suggest that the root is a site of synthesis of P-choline and P-ethanolamine for phospholipid synthesis in tomato leaves.

Characteristics of sulfate transport across plasmalemma and tonoplast of carrot root cells

Compartmental analysis of (35)SO(4) (2-) exchange kinetics is used to obtain SO(4) (2-) fluxes and compartment contents in carrot (Daucus carota L.) storage root cells, where 2 to 5% of the SO(4) (2-) taken up is reduced to organic form. The necessary curve fitting is verified by (a) consistency between ;content versus time' and ;rate versus time' plots of washout data; (b) agreement between loading and washout kinetics; and (c) correct identification of the fastest exchange phase as being from extracellular spaces.Sulfate is actively transported up an electrochemical potential gradient at both plasmalemma and tonoplast. The plasmalemma influx is from 2 to 10 times higher than the tonoplast influx, is much greater than the SO(4) (2-) reduction rate, and would not limit the rate of either. This is consistent with the finding that the plasmalemma influx is not regulated by internal SO(4) (2-) or cysteine (Cram 1982 Plant Sci Lett, in press).Both SO(4) (2-) influxes rise with only limited saturation as the external SO(4) (2-) concentration increases up to 50 millimolarity. Both effluxes appear to be passive, with extensive recycling in the plasmalemma influx pump. SO(4) (2-) permeability is about 10(-11) meter per second at both membranes.The high, nonlimiting fluxes of SO(4) (2-) at the plasmalemma relative to the tonoplast (found also in Lemna; Thoiron, Thoiron, Demarty, Thellier 1981 Biochim Biophys Acta 644: 24-35) contrasts with SO(4) (2-) fluxes in bacteria and with Cl(-) fluxes in plant cells. Their implications for work on characteristics and regulation of SO(4) (2-) uptake in roots and tissue cultures are discussed.

Non-uniformities in the metabolism of excised leaves and leaf discs

In intact tobacco and Chinese-cabbage (Brassica pekinensis) leaves an "ageing" process begins as soon as the leaves are excised. The terminal few millimetres of the petiole increasingly preempt materials such as phosphate and uracil taken up by the leaf. Actinomycin D treatment blocks this process and leads to increased uptake of such materials into the lamina.Immediately after excision there is a radial "geographical" gradient, in the ability of excised leaf discs to accumulate phosphate and uracil from solution. Tissue near the cut edge accumulates much more of these materials than that near the centre, and total nucleic acids isolated from the outer zone have a higher specific activity.Over the first day after excision there is a marked increase in this ability of the outer tissue of discs to accumulate labelled precursors but the changes taking place with time are complex and variable. Specific activity of total nucleic acids increases somewhat less than the increased uptake of labelled precursor. Actinomycin D becomes very unevenly distributed in leaf discs floated on solutions of the drug. These various effects are of sufficient magnitude to affect the interpretation of studies using excised leaf tissues.

Translocation and Metabolic Conversion of C-Labeled Assimilates in Detached and Attached Leaves of Phaseolus vulgaris L. in Different Phases of Leaf Expansion

Leaf excision greatly affected the actual levels of (14)C-assimilates in laminas and petioles of primary bean leaves (Phaseolus vulgaris L.) following a transport period. However, it did not affect the percentage of starch in the insoluble residue; starch decreased from 20% of the insoluble residue after exposure to (14)CO(2) to 3% after 5 hr in both attached and detached leaves. The transition from import to export of attached and detached leaves was at the same stage, i.e., when the cotyledons were 63 to 85% depleted. The composition of the (14)C-assimilates in importing leaves was different from that in exporting leaves. In the former, only 5% of the soluble label was free sugar, while 74% was free sugar in the latter. The failure of importing leaves to export was not due to the labeled substances being nontransferable. Extracts from importing leaves applied to exporting leaves were exported; these extracts were high in amino acids and organic acids but low in free sugar. However, exporting leaves exposed to (14)CO(2) appeared to export sugars more readily than amino acids. Cotyledon excision did not delay transition of leaves from import to export. Actually, excision seemed to enhance slightly the transition of the primary leaves from import to export.

Influence of calcium on sodium and potassium absorption by fresh and aged bean stem slices

The influence of Ca on the aging processes of bean stem (Phaseolus vulgaris) slices and on the absorption of K and Na by fresh and aged slices was investigated. In the presence of Ca, fresh tissue showed a preferential Na uptake. The preference for Na over K resulted from a differential depressive effect of Ca on absorption of these two ions. In aged tissue Na uptake was also depressed, but K absorption was accelerated, with a net result of a much greater absorption of K than Na.The presence of Ca in the aging medium promoted the development of K-absorbing capacity as well as an increase in the rate of respiration but did not influence the loss of capacity to absorb Na as tissue aged. This, along with the demonstration that protein synthesis is involved in the development of K-absorbing capacity by aging tissue, suggests that Ca may have an effect on basic physiological processes concerned with development of ion absorption by aging tissue. The influences that Ca may have on the physical and physiological aspects of ion transport are discussed.

Kinetics of C-14 Translocation in Soybean: III. Theoretical Considerations

Based largely on data from soybean, some mathematical models are derived to describe the transport kinetics of (14)C-photosynthate. The effects of leaf size, leaf shape, and translocation velocity on the rate of tracer efflux from the leaf are considered, and it is shown that the duration of these effects will approximate the time required for tracer to reach the petiole from the farthest point of the leaf. This duration is designated as the "kinetic size" of the leaf. Although its effect will be slight in the case of soybean (about 2 to 3 minutes), a considerable effect of the kinetic size will be found in the case of large leaves, or when the translocation velocity is low.Source pool kinetics in soybean are described by a two-compartment model, one compartment representing a photosynthetic compartment and the second (the source pool) a nonphotosynthetic compartment next to the veins. The kinetics in the petiole are approximated by a two-compartment model representing the translocation stream and tissues outside the translocation stream. A combination of the models predicts fairly accurately the translocation kinetics observed in soybean.The models are compared with others in the literature. Although the assumptions are in substantial agreement with those made by Evans, Ebert, and Moorby, they are inconsistent with the model based on the movement of transcellular strands presented by Canny and Phillips.

Phosphorus compounds in translocating Phloem

Phosphate-(32)P was introduced into a turnip leaf, and 3 hr later, the vascular bundles were stripped from the petiole and their phosphate ester pattern was studied. The pattern did not alter along their length and was like that of other tissues. Pumpkin leaves were painted with phosphate-(32)P; and later, the petioles were cut, the sieve tube exudates were collected and their phosphate ester patterns were studied. Exudates collected after 10 min had a high proportion of their (32)P present in P(i) and nucleoside triphosphates, while exudates collected after long translocation times (4-22 hr) had a lower proportion in these, and a higher proportion in hexose monophosphates and UDP glucose. In general, the ester patterns were like those of other tissues. The results indicate that sieve tubes are metabolically active, and that P(i) is the primary form in which phosphorus moves in the phloem.

Leaf structure and translocation in sugar beet

Anatomical and ultrastructural details of a translocating 10-cm leaf of sugar beet (Beta vulgaris L. var. Klein Wanzleben) were correlated with translocation rate data. The minor veins were found to be 13 times as extensive as the major veins and measure 70 cm/cm(2) leaf lamina. Measurements disclosed that a 33-mu length of minor vein services 29 mesophyll cells with the result that translocate moves an average of 73 mu or 2.2 cell diameters during transport from mesophyll cells to a minor vein. High-resolution, freeze-dry autoradiography revealed that assimilates accumulate in organelle-rich cells of the minor vein phloem. Correlation of phloem volume and loading rate for minor veins yielded an uptake rate of 735 mumoles of sucrose per g fresh weight of phloem. The arrangement and structural features of minor veins appeared to be consistent with the concept that vein loading precedes translocation.

[The kinetics of ion uptake by young and old branches of mnium cuspidatum]

Isotherms of K(Rb)-, Cl- and SO4-uptake by young and old branches of the moss Mnium cuspidatum were investigated. Old moss gametophytes from the 1966 vegetation period were collected in the forests surrounding Darmstadt from February to mid-April 1967 and from the 1967 season in late September 1967. Young plants were sampled from mid-April to the end of May 1967 and they were also grown by water culture of old plants.Both young and old branches have hyperbolic isotherms of ion uptake in the low concentration range (0-0.5 mM) (Fig. 1-3), which slightly differ in K mand V max (Table). Isotherms in the high range (1-10 mM), however, are drastically different, changing from linear or exponential with young moss branches to hyperbolic with old gametophytes (Figs. 1-3).The linear or exponential high-range isotherm obtained with young moss plants is compared with other examples reported in the literature (Fig. 4). As the leaflets of the moss plants, which constitute 2/3 of the fresh weight of the material used in the experiments, have well developed vacuoles, the correlation between hyperbolic isotherms and vacuolation does not apply here (Fig. 4a, TORII and LATIES, 1966).The change in shape of the high-range moss isotherm with age resembles the change from exponential to hyperbolic kinetics in isolated potato discs during washing (Fig. 4b, LATIES, MACDONALD and DAINTY, 1964). The events triggered by isolation of potato discs from the interior of the tuber may be similar to the changes in the moss material under the control of the terminal bud, which is only active in the young branches.The suggested influence of the active terminal bud of young moss plants on the ion absorption process of cells in the tissue may be related to effects of growth substances on translocation reported in the literature and may point to a direct effect of these regulatory systems on membrane function.In this respect the comparison of corn root stele and cortex is of interest. Isolated steles, both freshly isolated and after washing, have exponential isotherms in the high range (Fig. 4c), whereas cortex displays a hyperbolic isotherm which changes little with ageing (LüTTGE and LATIES, 1967). In contrast to the case in potato and moss materials, this phenomenon is not simply due to ageing but involves morphogenetic differences.Temperature is another factor which influences the shape of the high range isotherm. All examples discussed so far refer to experiments at room temperature. At low temperatures high-range isotherms for proximal root tissue or aged potato discs have an exponential shape (TORII and LATIES, 1966; LATIES, MACDONALD and DAINTY, 1964). It thus appears that the exponential isotherm of young moss branches indicates that as in freshly isolated potato discs or in corn root stele the metabolic high-range uptake system is not developed.

Levels of phosphate esters in spirodela

The duckweed Spirodela oligorrhiza was grown in sterile nutrient solutions that contained 1 mm phosphate-(32)P at various specific activities. In solutions with activities higher than 2 muc per mumole per ml, plant growth was inhibited after a time, and the physical appearance of the plants was affected. The critical level of radiation, at which growth was first affected, corresponded to 5 kilorads.Plants were grown for 9 days (5 generations) in a culture solution containing phosphate at 0.5 muc per mumole per ml (radiation load approx 0.5 kilorads) so that all phosphorus-containing materials in the tissue became uniformly labeled. The various radioactive compounds were extracted, chromatographed, identified, and their radioactivity was measured. From this radioactivity plus the specific activity of the supplied phosphate, the amount of each compound was calculated. The data constitute a complete balance-sheet for phosphorus in a plant tissue. The identity of 98% of the phosphorus in the tissue was determined. Inorganic phosphate (32,700 mmumoles/g fr wt) was the predominant phosphorus-containing compound; RNA (5100 mmumoles P/g fr wt) was the main organic phosphate; phosphatidyl choline (1600 mmumoles/g fr wt) was the main phospholipid, and glucose-6-phosphate (500 mmumoles/g fr wt) the main acid-soluble phosphate ester. Amounts of other phosphorus compounds are given.

[Investigations on the distribution and transport of ions in plant tissues with the electron probe X-ray micronalyser : II. Long-distance transport of ions to pea fruits]

Translocation and distribution of K, Ca, Sr and P in the fruit stalk and pods of Pisum sativum were studied by means of the electron probe X-ray micronalyser.Long-distance transport through the fruit stalk of K and P as well as of Ca and Sr takes mainly place in sieve tubes. Therefore the theory of MüNCH (1930) concerning the supply of substances via the phloem to seeds of weakly transpiring fruits is confirmed for several important ions. A fairly small Ca supply to the sieve tubes seems to be the reason that the transport of Ca in sieve tubes and its content in seeds are relatively low. Ca and Sr are also translocated in xylem vessels, mainly to the dorsal suture of the pods; there they accumulate as sulfate in the xylem tissues of the central vein.In addition to the longitudinal translocation there is also a lateral transport outwards from the conducting tissues. The heaviest depositions of minerals are located in the cell walls of sclerenchyma outside of the vascular bundles. These depositions consist mainly of Ca-Sr-phosphate in the fruit stalk and the dorsal suture and of Ca-Sr-sulfate as well as K-phosphate in the ventral suture. The cortical cells of the fruit stalk contain some crystals of Ca-Sr-oxalate.

[Investigations on the distribution and transport of ions in plant tissues with the X-Ray microanalyzer : I. Experiments on Vegetative Organs of Zea Mays]

1. The distribution of K, Ca, Sr and P in the vegetative organs of Zea Mays was examined. 2. The ions were detected localized in cryostat sections with the X-ray microanalyzer. 3. In meristems (root tip, intercalary meristem in internodes) P is distributed rather uniformly, and K is mainly accumulated in tissues which are about to undergo or are undergoing differentiation. The content of Ca and Sr in meristematic cells is low. 4. Root hairs accumulate ions intensively. 5. The vascular bundle of the root and the big bundles in the sheath and midrib of the oldest leaves show a high ion content. 6. In the vascular bundles, the ions are mainly localized in sclerenchyma and xylem. The phloem always contains much P and in part it also contains a considerable amount of K. 7. The ions are mainly localized to the cell walls in differentiated cells. Cations are partly adsorbed to cell walls or may be bound chemically. Part of the Ca and Sr deposited in cell walls is found as phosphate. 8. A barrier to lateral transport of ions across the root is located in the plasmalemma of the outermost cortex layer. After the transfer across the barrier, ions move in the symplams to the vascular tissue. 9. Translocation of K and P in the root is basipetal in xylem and acropetal in phloem; translocation of Ca and Sr is mainly basipetal. 10. The ions are accumulated in the oldest leaves. P and K may be retranslocated from these leaves in the phloem and later on redistributed to younger leaves. 11. The deposition of ions in the epidermis and bundlesheaths of the shoot is interpreted as salt exeretion.

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

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.

The mechanism of stomatal action

Recent reviews have denied the applicability of the classical theory of stomatal movement. The newer explanations are shown to be incorrect, and the major objections to the classical theory invalid. Nevertheless, the classical theory needs to be modified. If the decisive factor is assumed to be carboxylic acid (RCOOH) rather than CO2 concentration, all the known facts can be explained. Two predictions of this modified classical theory were vindicated. The proposed relationship of stomatal opening to RCOOH concentration is illustrated schematically.

Sites of accumulation in excised Phloem and vascular tissues

Excised pieces of vascular bundle and phloem tissue were allowed to accumulate radioactive phosphate and sulfate, and were then sectioned and autoradiographed so as to detect the sites of accumulation. Special methods were needed to prevent any diffusion of the radioisotope. Some autoradiographs obtained are presented. In excised celery vascular bundles, the most radioactive area and hence the most actively accumulating tissue was the young secondary phloem at the sides of the bundle. In intact plants, the same tissue was the most active in translocating. In excised apple phloem there was some variation in behavior, but again the young secondary phloem was generally the most actively accumulating tissue. Accumulation activities of individual cells in the phloem and vascular tissue were compared. It appeared that all cell types, ray, phloem and xylem parenchyma, cambial cells and sieve tubes, accumulated at least 5 times more actively than did the cortical parenchyma cells. The sieve tubes were among the most actively accumulating cells present, accumulating 20 times more actively than the cortical parenchyma cells. It is concluded that accumulation processes have a primary role to play in the mechanism of phloem transport.