Fine-tuning by strigolactones of root response to low phosphate

Strigolactones are plant hormones that regulate the development of different plant parts. In the shoot,they regulate axillary bud outgrowth and in the root,root architecture and root-hair length and density. Strigolactones are also involved with communication in the rhizosphere,including enhancement of hyphal branching of arbuscular mycorrhizal fungi. Here we present the role and activity of strigolactones under conditions of phosphate deprivation.Under these conditions,their levels of biosynthesis and exudation increase,leading to changes in shoot and root development. At least for the latter,these changes are likely to be associated with alterations in auxin transport and sensitivity. On the other hand,strigolactones may positively affect plant–mycorrhiza interactions and thereby promote phosphate acquisition by the plant. Strigolactones may be a way for plants to fine-tune their growth pattern under phosphate deprivation.     

Nutritional deprivation increases intracellular phosphate and polyphosphate in poultry litter microflora

AIMS: To determine if mixed microflora from poultry litter accumulates phosphate when deprived of carbon and energy or nitrogen sources. METHODS AND RESULTS: Microbial enrichments from poultry litter capable of metabolizing ammonia, amino acids, and glucose were subjected to nutritional deprivation and the effects on intracellular phosphate levels were determined. Results indicate deprivation of glucose yields a 38 and 50% increase in intracellular phosphate and polyphosphate levels, respectively. Deprivation of nitrogen sources did not result in significant intracellular phosphate accumulation. CONCLUSIONS: Micro-organisms normally present in poultry litter respond to carbohydrate deprivation by accumulating intracellular phosphate. SIGNIFICANCE AND IMPACT OF THE STUDY: Poultry litter typically contains significant levels of phosphate which contribute to environmental pollution when applied to land. Phosphate is highly mobile in soils and often drains into local watersheds following rain events. This study raises the possibility that poultry litter micro-organisms may have the capacity to sequester phosphate, which could delay or diminish phosphate run-off.     

Adaptation to phosphate deprivation in osteoblast-like cells

The rat osteosarcoma cell line UMR-106–01 has an osteoblast-like phenotype. When grown in monolyer culture these cells transport inroganic phosphate and L-alanine via Na⁺-dependent transport systems. Exposure of these cells to a low phosphate medium for 4 h produced a 60–70 per cent increase in Na⁺-dependent phosphate uptake compared to control cells maintained in medium with a normal phosphate concentration. In contrast, Na⁺-dependent alanine uptake and Na⁺-independent phosphate uptake were not changed during phosphate deprivation. The increased phosphate uptake was due, in part, to an increased V_max and was blocked completely by pretreatment with cycloheximide (70 μM). In these cells recovery of intracellular pH after acidification with NH₄Cl is due primarily to the Na⁺/H⁺ exchange system. The rate of this recovery process, monitored with a pH sensitive indicator (BCECF), was decreased by more than 50 per cent in phosphate-deprived cells compared to controls indicating that Na⁺/H⁺ exchange was inhibited during phosphate deprivation.     

The metabolic flux phenotype of heterotrophic Arabidopsis cells reveals a flexible balance between the cytosolic and plastidic contributions to carbohydrate oxidation in response to phosphate limitation

Understanding the mechanisms that allow plants to respond to variable and reduced availability of inorganic phosphate is of increasing agricultural importance because of the continuing depletion of the rock phosphate reserves that are used to combat inadequate phosphate levels in the soil. Changes in gene expression, protein levels, enzyme activities and metabolite levels all point to a reconfiguration of the central metabolic network in response to reduced availability of inorganic phosphate, but the metabolic significance of these changes can only be assessed in terms of the fluxes supported by the network. Steady‐state metabolic flux analysis was used to define the metabolic phenotype of a heterotrophic Arabidopsis thaliana cell culture grown on a Murashige and Skoog medium containing 0, 1.25 or 5 mm inorganic phosphate. Fluxes through the central metabolic network were deduced from the redistribution of ¹³C into metabolic intermediates and end products when cells were labelled with [1‐¹³C], [2‐¹³C], or [¹³C₆]glucose, in combination with ¹⁴C measurements of the rates of biomass accumulation. Analysis of the flux maps showed that reduced levels of phosphate in the growth medium stimulated flux through phosphoenolpyruvate carboxylase and malic enzyme, altered the balance between cytosolic and plastidic carbohydrate oxidation in favour of the plastid, and increased cell maintenance costs. We argue that plant cells respond to phosphate deprivation by reconfiguring the flux distribution through the pathways of carbohydrate oxidation to take advantage of better phosphate homeostasis in the plastid.     

Modeling linear and variable growth in phosphate limited suspension cultures of Opium poppy

In examining the growth kinetics of cell suspensions of opium poppy (Papaver somniferum), the increase in biomass with time was observed to be linear over the entire batch growth period of up to 20 days. Although batch growth profiles were reproducible utilizing the same inoculum, growth rates varied tremendously when experiments were inoculated with cells from different flasks. Both of these phenomena are difficult to explain with conventional batch growth models. In a series of a experiments, phosphate was determined to be the growth-rate-limiting substrate. By expressing growth rate in terms of the intracellular reserves of phosphorus, a growth model which expresses kinetics in terms of the intracellular phosphorus contents of the cells is shown to predict both linear growth character and inoculum dependent variability in growth. The stationary phase phosphate content of seven plant suspension cultures of different plant species was found to be comparable to phosphorus levels of phosphate-starved poppy cells, which suggests that phosphate limitation may be common for plant tissue culture. The applicability of this model to other biological systems which display similar batch growth patterns when subjected to inorganic nutrient deprivation is discussed.     

Growth Inhibition in Suspension-Cultured Rice Cells under Phosphate Deprivation Is Mediated through Putrescine Accumulation

The effects of phosphate deprivation on the growth and polyamine levels of suspension-cultured rice (Oryza sativa) cells were investigated. When rice suspension cells were deprived of phosphate, cell growth was markedly inhibited. Phosphate deprivation resulted in a higher putrescine level and lower spermidine and spermine levels in rice suspension cells. The growth of rice cells cultured in the absence of phosphate did not recover as a result of spermidine and spermine addition. D-Arginine and [alpha]-methylornithine, inhibitors of putrescine biosynthesis, caused a reduced level of putrescine in rice suspension cells cultured under phosphate deprivation. The growth of rice cells cultured in the absence of phosphate was completely recovered after the addition of D-arginine but not [alpha]-methylornithine. Our results indicate that putrescine accumulation is a factor causing growth inhibition of suspension-cultured rice cells under phosphate deprivation.     

Influence of cytokinins on the expression of phosphate starvation responsive genes in Arabidopsis

The increase in the ratio of root growth to shoot growth that occurs in response to phosphate (Pi) deprivation is paralleled by a decrease in cytokinin levels under the same conditions. However, the role of cytokinin in the rescue system for Pi starvation remains largely unknown. We have isolated a gene from Arabidopsis thaliana (AtIPS1) that is induced by Pi starvation, and studied the effect of cytokinin on its expression in response to Pi deprivation. AtIPS1 belongs to the TPSI1/Mt4 family, the members of which are specifically induced by Pi starvation, and the RNAs of which contain only short, non-conserved open reading frames. Pi deprivation induces AtIPS1 expression in all cells of wild-type plants, whereas in the pho1 mutant grown on Pi-rich soils, AtIPS1 expression in the root was delimited by the endodermis. This supports the view that pho1 is impaired in xylem loading of Pi, and that long-distance signals controlling the Pi starvation responses act via negative control. Exogenous cytokinins repress the expression of AtIPS1 and other Pi starvation-responsive genes in response to Pi deprivation. However, cytokinins did not repress the increase in root-hair number and length induced by Pi starvation, a response dependent on local Pi concentration rather than on whole-plant Pi status. Our results raise the possibility that cytokinins may be involved in the negative modulation of long-distance, systemically controlled Pi starvation responses, which are dependent on whole-plant Pi status.     

Functional interactions between potassium and phosphate homeostasis in Saccharomyces cerevisiae

Maintenance of ion homeostatic mechanisms is essential for living cells, including the budding yeast Saccharomyces cerevisiae. Whereas the impact of changes in phosphate metabolism on metal ion homeostasis has been recently examined, the inverse effect is still largely unexplored. We show here that depletion of potassium from the medium or alteration of diverse regulatory pathways controlling potassium uptake, such as the Trk potassium transporters or the Pma1 H⁺‐ATPase, triggers a response that mimics that of phosphate (Pi) deprivation, exemplified by accumulation of the high‐affinity Pi transporter Pho84. This response is mediated by and requires the integrity of the PHO signaling pathway. Removal of potassium from the medium does not alter the amount of total or free intracellular Pi, but is accompanied by decreased ATP and ADP levels and rapid depletion of cellular polyphosphates. Therefore, our data do not support the notion of Pi being the major signaling molecule triggering phosphate‐starvation responses. We also observe that cells with compromised potassium uptake cannot grow under limiting Pi conditions. The link between potassium and phosphate homeostasis reported here could explain the invasive phenotype, characteristic of nutrient deprivation, observed in potassium‐deficient yeast cells.     

New aspects on phosphate sensing and signalling in Saccharomyces cerevisiae

The mechanism involved in the cellular phosphate response of Saccharomyces cerevisiae forms part of the PHO pathway, which upon expression allows a co-ordinated cellular response and adaptation to changes in availability of external phosphate. Although genetic studies and analyses of the S. cerevisiae genome have produced much information on the components of the PHO pathway, little is known about how cells sense the environmental phosphate level and the mechanistic regulation of phosphate acquisition. Recent studies emphasize different levels in phosphate sensing and signalling in response to external phosphate fluctuations. This review integrates all these findings into a model involving rapid and long-term effects of phosphate sensing and signalling in S. cerevisiae. The model describes in particular how yeast cells are able to adjust phosphate acquisition by integrating the status of the intracellular phosphate pools together with the extracellular phosphate concentration.     

Ammonium accumulation in relation to growth of rice cell suspension under phosphate deprivation

Suspension-cultured rice cells growth was markedly inhibited and ammonium content increased when rice cells were deprived of phosphate. When rice cells were cultured at increasing concentrations of ammonium chloride, ammonium content increased, however, no significant inhibition of cell growth was observed. Addition of D-arginine, an inhibitor of putrescine biosynthesis, resulted in a complete recovery of growth in rice cells under phosphate deprivation, but did not decrease the content of ammonium. Our results indicate that the growth inhibition induced by phosphate deprivation is not associated with ammonium accumulation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phosphate deprivation induces transfer of DGDG galactolipid from chloroplast to mitochondria

In many soils plants have to grow in a shortage of phosphate, leading to development of phosphate-saving mechanisms. At the cellular level, these mechanisms include conversion of phospholipids into glycolipids, mainly digalactosyldiacylglycerol (DGDG). The lipid changes are not restricted to plastid membranes where DGDG is synthesized and resides under normal conditions. In plant cells deprived of phosphate, mitochondria contain a high concentration of DGDG, whereas mitochondria have no glycolipids in control cells. Mitochondria do not synthesize this pool of DGDG, which structure is shown to be characteristic of a DGD type enzyme present in plastid envelope. The transfer of DGDG between plastid and mitochondria is investigated and detected between mitochondria-closely associated envelope vesicles and mitochondria. This transfer does not apparently involve the endomembrane system and would rather be dependent upon contacts between plastids and mitochondria. Contacts sites are favored at early stages of phosphate deprivation when DGDG cell content is just starting to respond to phosphate deprivation.     

Alteration of cylindrospermopsin production in sulfate- or phosphate-starved cyanobacterium Aphanizomenon ovalisporum

The effect of sulfate and phosphate deprivation on cell growth and cylindrospermopsin level was studied in Aphanizomenon ovalisporum ILC-164. Sulfate starvation induced a characteristic reduction of cylindrospermopsin pool size on the basis of cell number and unit of dry mass of culture. Phosphorous starvation of A. ovalisporum cultures induced a lesser reduction of cylindrospermopsin pool size. This divergence in the pool size of cylindrospermopsin may be the consequence of different growth rate. To show the metabolic changes concomitant with reduction of cylindrospermopsin pool size were obtained by measurement of ATP sulfurylase and alkaline phosphatase activity. The present study is the first concerning the cylindrospermopsin content under sulfate starvation and discusses it in relation to phosphorous starvation.     

Expression analysis suggests novel roles for members of the Pht1 family of phosphate transporters in Arabidopsis

The completion of the Arabidopsis thaliana genome has revealed that there are nine members of the Pht1 family of phosphate transporters in this species. As a step towards identifying the role of this gene family in phosphorus nutrition, we have isolated the promoter regions from each of these genes, and fused them to the reporter genes beta-glucuronidase and/or green fluorescent protein. These chimeric genes have been introduced into A. thaliana, and reporter gene expression has been assayed in plants grown in soil containing high and low concentrations of inorganic phosphate (Pi). Four of these promoters were found to direct reporter gene expression in the root epidermis, and were induced under conditions of phosphate deprivation in a manner similar to previously characterised Pht1 genes. Other members of this family, however, showed expression in a range of shoot tissues and in pollen grains, which was confirmed by RT-PCR. We also provide evidence that the root epidermally expressed genes are expressed most strongly in trichoblasts, the primary sites for uptake of Pi. These results suggest that this gene family plays a wider role in phosphate uptake and remobilisation throughout the plant than was previously believed.     

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.     

The introduction of a phytase gene from Bacillus subtilis improved the growth performance of transgenic tobacco

Phytate, the main form of phosphorus storage in plant seeds, is well known to be an anti-nutrient and a major source of phosphorus pollution in animal manure. To improve phosphorus bio-availability, we introduced a recently characterized phytase from Bacillus subtilis into the cytoplasm of tobacco cells. Although the introduction of acid fungal phytase from Aspergillus niger in previous studies did not result in any phenotypic changes in tobacco, here we show that a tobacco line transformed with a neutral phytase exhibited phenotypic changes in flowering, seed development, and response to phosphate deficiency. The transgenic line showed an increase in flower and fruit numbers, small seed syndrome, lower seed IP6/IP5 ratio, and enhanced growth under phosphate-starvation conditions compared with the wildtype. The results suggest that the over-expression of Bacillus phytase in the cytoplasm of tobacco cells shifts the equilibrium of the inositol phosphate biosynthesis pathway, thereby making more phosphate available for primary metabolism. The approach presented here can be applied as a strategy for boosting productivity in agriculture and horticulture.     

Phosphate availability affects the tonoplast localization of PLDzeta2, an Arabidopsis thaliana phospholipase D

Under phosphate deprivation, higher plants change their lipid composition and recycle phosphate from phospholipids. A phospholipase D, PLDzeta2, is involved in this recycling and in other cellular functions related to plant development. We investigated the localization of Arabidopsis PLDzeta2 by cell fractionation and in vivo GFP confocal imaging. AtPLDzeta2 localizes to the tonoplast and the Nter regulatory domain is sufficient for its sorting. Under phosphate deprivation, AtPLDzeta2 remains located in the tonoplast but its distribution is uneven. We observed PLDzeta2-enriched tonoplast domains preferentially positioned close to mitochondria and beside chloroplasts. In absence of PLDzeta2, membrane developments were visualized inside vacuoles.     

Initiation of Yeast Sporulation by Partial Carbon, Nitrogen, or Phosphate Deprivation

In this paper we show that partial deprivation of a carbon source, a nitrogen source, or phosphate in the presence of all other nutrients needed for growth initiates meiosis and sporulation of Saccharomyces cerevisiae homothallic strain Y55. For carbon deprivation experiments, cells were grown in synthetic medium (pH 5.5) containing an excess of one carbon source and then transferred to the same medium containing different concentrations of the same carbon source. In the case of transfer to different acetate concentrations, the log optical density at 600 nm increased at the previous rate until the cells had used up all of the acetate, whereupon the cells entered a stationary phase and did not sporulate. The same was observed with ethanol. In contrast, at different concentrations of dihydroxy-acetone or pyruvate, cells grew at different rates and sporulated optimally at intermediate concentrations (50 to 75 mM). The response to galactose was similar but reflected the presence of a low-affinity galactose transport system and the induction of a high-affinity galactose transport system. Cells could also sporulate when a glucose medium ran out of glucose, apparently because they initiated sporulation during the subsequent lag period and then used the produced ethanol as a carbon source. For phosphate deprivation experiments, cells growing with excess ethanol or pyruvate and phosphate were transferred to the same medium containing limiting amounts of phosphate. First, they used up the intracellular phosphate reserves for rapid growth, and then they sporulated optimally when an intermediate concentration (30 μM) of phosphate had been added to the medium. For nitrogen deprivation experiments, cells grown with excess acetate, ethanol, or pyruvate and NH₄⁺ were transferred to the same medium from which all nitrogen had been removed. These cells sporulated well in acetate medium but poorly in ethanol and pyruvate media. However, the sporulation frequency in the latter media could be increased greatly by adding intermediate concentrations (1 mM) of the slowly metabolizable amino acids glycine, histidine, or phenylalanine. If one assumes that the sporulation response to partial deprivation of carbon-, nitrogen-, or phosphorus-containing compounds reflects control by a single metabolite, the intracellular concentration of this metabolite may decide at the START position (G1 phase) of the cell cycle whether a/α cells enter mitosis or meiosis.     

磷缺失引起小麦叶片中脂含量的变化与脂的合成和磷脂酰甘油的降解有关

对生长在不同磷营养水平条件下小麦(Triticum aestivum var.Zhongyou 9507)叶片中光合膜脂含量变化的原因进行了研究.通过对生长在不同磷营养水平条件下9 d龄和16 d龄小麦叶片中光合膜脂含量的分析,发现在磷缺失培养条件下,小麦光合膜脂的相对含量发生了很大变化,这种变化与小麦叶龄密切相关.在16d龄小麦植株中,第一片叶为老叶,第二片叶为较老叶,而第三片叶为新叶,PG和MGDG在叶片中的相对含量从新叶到老叶逐渐下降,而DGDG和SQDG含量逐渐上升;在磷缺失条件下,16 d龄小麦第一叶片中PG的含量(2.5%)远远低于其在9 d龄第一叶片中的含量(5.5%).以上结果说明,磷缺失引起小麦叶片中脂含量的变化不仅与脂合成有关,而且与PG的降解有关;新生叶片中PG含量减少的主要原因是由于磷供应不足,从而影响了PG的合成;而PG的降解则是老叶中PG含量下降的主要原因.