Contents
I. Introduction  424
II. The phosphorus conundrum 424
III. Adaptations to low P 424
IV. Uptake of P  424
V. P deficiency alters root development and function 426
VI. P deficiency modifies carbon metabolism 431
VII. Acid phosphatase 436
VIII. Genetic regulation of P responsive genes 437
IX. Improving P acquisition 439
X. Synopsis  440


Summary
Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants.

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Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource

Oxygen deficiency in the rooting zone occurs with poor drainage after rain or irrigation, causing depressed growth and yield of dryland species, in contrast with native wetland vegetation that tolerates such conditions. This review examines how roots are injured by O2 deficiency and how metabolism changes during acclimation to low concentrations of O2. In the root apical meristem, cell survival is important for the future development; metabolic changes under anoxia help maintain cell survival by generating ATP anaerobically and minimizing the cytoplasmic acidosis associated with cell death. Behind the apex, where cells are fully expanded, ethylene-dependent death and lysis occurs under hypoxia to form continuous, gas-filled channels (aerenchyma) conveying O2 from the leaves. This selective sacrifice of cells may resemble programmed cell death and is distinct from cell death caused by anoxia. Evidence concerning alternative possible mechanisms of anoxia tolerance and avoidance is presented.

OXYGEN DEFICIENCY AND ROOT METABOLISM: Injury and Acclimation Under Hypoxia and Anoxia

AGPase, ADP glucose pyrophosphorylase
GS, glutamine synthetase
GOGAT, glutamate : oxoglutarate amino transferase
NADP-ICDH, NADP-dependent isocitrate dehydrogenase
NR, nitrate reductase
OPPP, oxidative pentose phosphate pathway
3PGA, glycerate-3-phosphate
PEPCase, phosphoenolpyruvate carboxylase
Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
SPS, sucrose phosphate-synthase

This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO2]. The initial stimulation of photosynthesis in elevated [CO2] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation, organic acid synthesis and starch accumulation, to modulate the sugar-mediated repression of the expression of genes involved in photosynthesis, and to modulate whole plant events including shoot–root allocation, root architecture and flowering. Third, increased rates of growth in elevated [CO2] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids. Fourth, interpretation of experiments in elevated [CO2] requires that the nitrogen status of the plants is monitored. The suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevated [CO2] and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated [CO2], and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar-mediated repression of gene expression, end-product feedback of photosynthesis, nitrogen-induced senescence, or ontogenetic drift? Is the accumulation of starch a passive response to increased carbohydrate formation, or is it triggered by changes in the nutrient status? How do changes in sugar production and inorganic nitrogen assimilation interact in different conditions and at different stages of the life history to determine the response of whole plant growth and allocation to elevated [CO2]?

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.1999.00386.x

The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background

This review discusses the organization and regulation of the glycolytic pathway in plants and compares and contrasts plant and nonplant glycolysis. Plant glycolysis exists both in the cytosol and plastid, and the parallel reactions are catalyzed by distinct nuclear-encoded isozymes. Cytosolic glycolysis is a complex network containing alternative enzymatic reactions. Two alternate cytosolic reactions enhance the pathway's ATP yield through the use of pyrophosphate in place of ATP. The cytosolic glycolytic network may provide an essential metabolic flexibility that facilitates plant development and acclimation to environmental stress. The regulation of plant glycolytic flux is assessed, with a focus on the fine control of enzymes involved in the metabolism of fructose-6-phosphate and phosphoenolpyruvate. Plant and nonplant glycolysis are regulated from the "bottom up" and "top down," respectively. Research on tissue- and developmental-specific isozymes of plant glycolytic enzymes is summarized. Potential pitfalls associated with studies of glycolytic enzymes are considered. Some glycolytic enzymes may be multifunctional proteins involved in processes other than carbohydrate metabolism.

The organization and regulation of plant glycolysis

A vestigial, nonphotosynthetic plastid has been identified recently in protozoan parasites of the phylum Apicomplexa. The apicomplexan plastid, or “apicoplast,” is indispensable, but the complete sequence of both the Plasmodium falciparum and Toxoplasma gondii apicoplast genomes has offered no clue as to what essential metabolic function(s) this organelle might perform in parasites. To investigate possible functions of the apicoplast, we sought to identify nuclear-encoded genes whose products are targeted to the apicoplast in Plasmodium and Toxoplasma. We describe here nuclear genes encoding ribosomal proteins S9 and L28 and the fatty acid biosynthetic enzymes acyl carrier protein (ACP), β-ketoacyl-ACP synthase III (FabH), and β-hydroxyacyl-ACP dehydratase (FabZ). These genes show high similarity to plastid homologues, and immunolocalization of S9 and ACP verifies that the proteins accumulate in the plastid. All the putatively apicoplast-targeted proteins bear N-terminal presequences consistent with plastid targeting, and the ACP presequence is shown to be sufficient to target a recombinant green fluorescent protein reporter to the apicoplast in transgenic T. gondii. Localization of ACP, and very probably FabH and FabZ, in the apicoplast implicates fatty acid biosynthesis as a likely function of the apicoplast. Moreover, inhibition of P. falciparum growth by thiolactomycin, an inhibitor of FabH, indicates a vital role for apicoplast fatty acid biosynthesis. Because the fatty acid biosynthesis genes identified here are of a plastid/bacterial type, and distinct from those of the equivalent pathway in animals, fatty acid biosynthesis is potentially an excellent target for therapeutics directed against malaria, toxoplasmosis, and other apicomplexan-mediated diseases.

http://www3.botany.ubc.ca/keeling/PDF/98api.pdf

Nuclear-encoded proteins target to the plastid in Toxoplasma gondii and Plasmodium falciparum

We have investigated the regulation of sucrose storage in cell-suspension cultures of sugarcane. When grown in batch culture, sucrose accumulation commences after about 5 d, when the nitrogen supply is exhausted. Sucrose storage is also induced by decreasing the nitrogen supply to cells growing in a chemostat. The measured activity of sucrose-phosphate synthase is high enough to account for the rate of sucrose accumulation, provided precautions are taken to avoid the hydrolysis of UDP during the assay. The cells contained high sucrose-synthase activity but pulsing experiments with [14C]glucose and unlabelled fructose indicated that this enzyme did not contribute substantially to the synthesis of sucrose, because the glucosyl and fructosyl moieties of sucrose were equally labelled. Several lines of evidence demonstrate the presence of a cycle in which sucrose is synthesized and degraded simultaneously; sucrosephosphate-synthase activity doubles during the phase when the cells are actively storing sucrose but activity is also high after storage has ceased, or when the sucrose is being remobilised; pulse experiments with [14C]fructose also showed that sucrose synthesis occurs not only during the storage phase, but also after storage has stopped and during the rapid mobilisation of sucrose; the cells contain high activities of sucrose synthase and alkaline invertase and these are both at a maximum when sucrose storage is occurring; even during the storage phase. [14C]fructose pulses lead to labelling of free glucose which is evidence for rapid synthesis and degradation of sucrose. It is proposed that the rate and extent of sucrose storage is regulated by this cycle of synthesis and degradation. Measurements of enzyme activities and metabolite levels are presented, and it is discussed which factors could contribute to the regulation of these two opposing fluxes and, hence, the rate of net sucrose storage and mobilisation.

Sucrose storage in cell suspension cultures of Saccharum sp. (sugarcane) is regulated by a cycle of synthesis and degradation.

We have investigated whether sucrose accumulation in heterotrophic cell-suspension cultures of Chenopodium rubrum L. is regulated by a cycle in which sucrose is simultaneously synthesised and degraded. Net sucrose accumulation was measured by monitoring the sucrose content, unidirectional synthesis was monitored by supplying pulses of [14C] glucose, and unidirectional degradation was estimated from the difference between unidirectional synthesis and net accumulation. When 50 mM glucose was supplied to carbohydrate-depleted cells there was a rapid net accumulation of sucrose, which stopped after 24 h. The incorporation of 14C into sucrose was similar to the initial rate of net sucrose accumulation, but rapid 14C incorporation continued after the cells had stopped accumulating sucrose. A method was developed to rapidly separate sucrose-phosphate synthase (SPS) from uridine-diphosphate-hydrolysing activities which interfered with the assay. The cells contained enough SPS activity to catalyse the observed rate of sucrose synthesis. SPS activity increased in cells which had stopped accumulating sucrose, and the enzyme became less sensitive to inhibition by inorganic phosphate. Sucrose synthase and alkaline invertase activity were four- and twofold higher than SPS activity, and both degradative enzymes increased in cells which had stopped accumulating sucrose. Sucrose synthase is strongly modulated by the concentration of sucrose and by competitive feedback regulation by fructose in these cells. It is concluded that sucrose accumulation ceases in these cells because the rate of degradation of sucrose increases until it matches the rate of synthesis. It is discussed how this cycle is regulated, and how it may interact with the substrate cycle between triose-phosphates and hexose-phosphates (Hatzfeld and Stitt, 1990, Planta 180, 198–204). These cycles allow sucrose turnover to respond in a highly sensitive manner to small changes in the balance between the supply of sucrose and the demand for carbon for respiration and biosynthesis in the cell.

Cytosolic cycles regulate the turnover of sucrose in heterotrophic cell-suspension cultures of Chenopodium rubrum L.

Independent Changes of Inorganic Pyrophosphate and the Atp Adp or Utp Udp Ratios in Plant-Cell Suspension-Cultures

Experiments were carried out to determine whether pyrophosphate: fructose-6-phosphate phosphotransferase (PFP) catalyses the rapid recycling of triose phosphates that is found in the cytosol of heterotrophic cell cultures of Chenopodium rubrum L. (W.-D. Hatzfeld, M. Stitt, 1990, Planta, 180, 198–204). Oxygen uptake, carbohydrate turnover, fructose 2,6-bisphosphate (Fru2,6bisP), glycolytic intermediates, adenine and uridine nucleotides, pyrophosphate and the activity of PFP and glycolytic enzymes were monitored for 48 h after subculturing carbohydrate-depleted cells onto glucose. Immediately after transfer there was an increase in the amount of Fru2,6bisP, and of the hexose phosphate. The triose phosphates, fructose-1,6-bisphosphate and inorganic pyrophosphate increased gradually over the next 24 h. This was accompanied by a tripling in the extractable activity of PFP, but not of phosphofructokinase. The activity of fructose-1,6-bisphosphatase was 20–50fold lower than that of PFP. It is calculated that the activity of PFP is high enough to catalyse the observed rate of cycling between the triose phosphates and the hexose phosphates, based on the measured Vmax capacity of the enzyme, the known kinetic properties, and the measured levels of its reactants and Fru2,6bisP. The changes in the levels of Fru2,6bisP were not correlated with the rate of respiration. Instead, the rate of O2 uptake was inversely related to the phosphoenolpyruvate level, showing that pyruvate kinase or phosphoenolpyruvate carboxylase are regulating the use of glucose for respiration. There was also no relation between Fru2,6bisP, and partitioning to sucrose or starch. It is proposed that the main function of the cycle in these cells is to maintain high levels of inorganic pyrophosphate and triose phosphates, which are necessary for the remobilisation of sucrose and for biosynthesis in the plastid, and that ‘coarse’ and ‘fine’ control of PFP play an important role in regulating this cycle.

Fructose-2,6-bisphosphate, metabolites and 'coarse' control of pyrophosphate: fructose-6-phosphate phosphotransferase during triose-phosphate cycling in heterotrophic cell-suspension cultures of Chenopodium rubrum.

The subcellular compartmentation of nucleoside diphosphate kinase (EC 2.7.4.6) and the uridine nucleotides has been studied in leaves. Membrane filtration of barley (Hordeum vulgare L.) leaf mesophyll protoplasts and differential centrifugation of spinach (Spinacia oleracea L.) leaf extracts showed that about half the nucleoside diphosphate kinase is present in the cytosol. The activity is adequate to account for the turnover of UTP and UDP during photosynthetic sucrose synthesis. Nonaqueous density gradient centrifugation of freeze-stopped, lyophilized spinach leaf material showed that the uridine nucleotides are predominantly located in the cytosol and that the cytosolic UDP-glucose pool is considerably larger than the UTP or UDP pools.

Jane Dancer

Papers

Sucrose storage in cell suspension cultures of Saccharum sp. (sugarcane) is regulated by a cycle of synthesis and degradation.

Cytosolic cycles regulate the turnover of sucrose in heterotrophic cell-suspension cultures of Chenopodium rubrum L.

Independent Changes of Inorganic Pyrophosphate and the Atp Adp or Utp Udp Ratios in Plant-Cell Suspension-Cultures

Fructose-2,6-bisphosphate, metabolites and 'coarse' control of pyrophosphate: fructose-6-phosphate phosphotransferase during triose-phosphate cycling in heterotrophic cell-suspension cultures of Chenopodium rubrum.

Subcellular compartmentation of uridine nucleotides and nucleoside-5' -diphosphate kinase in leaves.