Essay 12.1 Metabolic Flexibility Helps Plants to Survive Stress

Essay 12.1 Metabolic Flexibility Helps Plants to Survive Stress

William C. Plaxton, Professor of Biology & Biochemistry, Queen’s University, Kingston, Ontario

September, 2010

Introduction

Owing to their sessile lifestyle, plants have evolved numerous adaptations to help them cope with unavoidable abiotic and biotic stresses that are imposed upon them in their natural environment. For example, plants frequently alter their growth and developmental patterns so as to alleviate some of the unfavorable environmental changes that they are exposed to. Thus, increased allocation of biomass to roots typically occurs in dry or mineral nutrient-deficient conditions. There is little doubt that this type of developmental plasticity is a key aspect to the survival of plants in extreme environments. However, it is equally apparent that certain biochemical and metabolic adaptations of plants also facilitate their growth and/or survival under sub-optimal environmental conditions. For instance, in contrast to animals, plants can often accomplish the same step in a metabolic pathway in several different ways. This so-called ‘metabolic flexibility’ is perhaps best exemplified by a wide variety of genetic engineering experiments that have partially or fully eliminated individual enzymes traditionally considered to be essential, and yet the resulting transgenic plants were able to grow and develop more or less normally (Plaxton and Podestá 2006).

Plant Respiratory Metabolism Represents a Central Feature of Plant Metabolic Flexibility

Plants utilize sucrose and starch as the principle substrates for respiration, and can fully oxidize these fuels to CO2 and H2O via the standard pathways of glycolysis, the citric acid cycle, and the mitochondrial electron transport chain (miETC). However, there are at least four remarkable attributes in the organization and associated bioenergetic features of plant respiratory metabolism that are not commonly seen in other organisms.

Remarkable Attribute #1. Plant glycolysis exists in the plastid and cytosol, with the parallel reactions catalyzed by distinct nuclear-encoded isozymes (see textbook Figure 12.1; Plaxton and Podestá 2006). The prime functions of glycolysis in darkened chloroplasts and non-photosynthetic plastids are to participate in the breakdown of starch as well as to generate carbon skeletons, reductants and ATP for anabolic pathways such as fatty acid synthesis and N-assimilation.

Remarkable Attribute #2. An extraordinary feature of plant carbohydrate metabolism is that the cytosolic glycolytic pathway in itself is a complex network containing parallel enzymatic reactions at the level of sucrose, fructose-6-phosphate, glyceraldehyde-3-phosphate and phosphoenolpyruvate (PEP) metabolism (see textbook Figure 12.3). Each bypass reaction of plant cytosolic glycolysis circumvents a classical glycolytic reaction that is dependent upon an adenine nucleotide or inorganic orthophosphate (Pi) as a cosubstrate. As discussed below, this flexibility allows the preferential utilization of inorganic pyrophosphate (PPi) as an alternate energy donor, particularly when cellular ATP pools become diminished during stresses such as anoxia and nutritional Pi starvation.

Remarkable Attribute #3. Respiratory O2 consumption by the plant miETC can be mediated by the ATP-producing cytochrome pathway or by a non-energy conserving alternative pathway that involves the rotenone-insensitive NAD(P)H dehydrogenase bypasses to Complex I and the cyanide-resistant alternative oxidase (AOX) bypass to Complex III and IV (see textbook Figure 12.8).

Remarkable Attribute #4. A number of bypass reactions to the plant citric acid cycle exist and include: (i) an NADP+-specific isocitrate dehydrogenase within the mitochondrial matrix that can circumvent the NAD+-specific isocitrate dehydrogenase of the conventional citric acid cycle, (ii) the glyoxylate cycle of germinating oil seeds which necessitates two key enzymes (isocitrate lyase and malate synthase) that collectively bypass both decarboxylating reactions of the citric acid cycle, thereby allowing gluconeogenesis from stored fats to occur (see textbook Figure 12.19), and (iii) the gamma-amino butyric acid (GABA) shunt which may function as an alternative, NAD+-independent, mechanism for glutamate entry into the citric acid cycle. GABA, a four carbon non-protein amino acid, may represent a significant proportion of the free amino acid pool in plant cells subjected to abiotic or biotic stresses (Plaxton and Podestá 2006). Various stresses initiate a signal transduction pathway in which increased cytosolic Ca2+ activates a calmodulin-dependent glutamate decarboxylase leading to a marked elevation in intracellular GABA levels. After the stress is relieved, GABA can be transported into the mitochondrion and enter the TCA cycle via its conversion to succinate through the sequential action of GABA transaminase and succinic semialdehyde dehydrogenase.

The alternative reactions of glycolysis, the citric acid cycle, and miETC are believed to endow plants with crucial metabolic flexibility that facilitates their development and acclimation to unavoidable abiotic stresses. An ongoing and challenging problem has been to elucidate the respective function(s), control, and relative importance of the many alternative reactions of plant respiration.

Pyrophosphate Permits Microbes and Plants to Conserve ATP

Inorganic pyrophosphate (PPi) is a byproduct of a host of biosynthetic reactions, including the polymerization reactions involved in the final steps of macromolecule synthesis. One dogma of cellular bioenergetics (stemming from animal studies) is that the anhydride bond of PPi is never utilized to perform cellular work since it is immediately hydrolyzed by a highly active soluble inorganic pyrophosphatase (Figure 1). However, the large amounts of PPi produced during biosynthesis are not always wasted, but may be employed by various anaerobic microorganisms as well as the plant cytosol to enhance the energetic efficiency of several cellular processes.

Figure 1   Pyrophosphate hydrolysis is catalyzed by inorganic pyrophosphatase. In all animals and many microbes the large amount of PPi that is generated as a byproduct of anabolism is immediately hydrolyzed by an abundant inorganic pyrophosphatase in a highly exergonic reaction. However, in some microbes and the plant cytosol, PPi produced during biosynthesis is not wasted since: (1) there is little or no inorganic pyrophosphatase present (this allows PPi to accumulate), and (2) PPi-dependent enzymes exist that can use the PPi instead of ATP to do useful cellular work.

Awareness of the importance of PPi in the bioenergetics of some organisms originated from research on energy-poor anaerobic microorganisms such as the bacteria Priopionibacterium shermanii and the parasitic protist Entamoeba histolytica (the latter causing amoebic dysentery in humans). These species have no ATP-dependent phosphofructokinase (ATP-PFK), but instead convert fructose-6-phosphate to fructose-1,6-bisphosphate via a PPi-dependent phosphofructokinase (PPi-PFK) (Figure 2A). Similarly, they lack pyruvate kinase, but instead employ PPi to convert PEP and AMP into pyruvate, ATP and Pi via pyruvate, Pi dikinase (PPDK), thereby converting the bond energy of PPi into a high-energy phosphate of ATP. As outlined in Figure 2B, the PPDK reaction results in the net creation of two ‘ATP equivalents’ (i.e., two ‘high energy’ phosphoanhydride bonds). This is clearly evident when the reaction catalyzed by PPDK is summed with that catalyzed by the ubiquitous and highly abundant adenylate kinase (Figure 2B). Owing to their use of PPi-PFK and PPDK, these organisms are able to yield 5 ATP per glucose oxidized to two molecules of pyruvate, with a net expenditure of 3 PPi recycled from biosynthetic reactions (Figure 2C). This clearly represents a considerable bioenergetic advantage for obligate anaerobes such as P. shermanii and E. histolytica, since the net ATP yield for classical glycolysis as occurs in animals is only two ATP per glucose converted to two molecules of pyruvate.

Figure 2   Nucleoside phosphate and pyrophosphate metabolism in the glycolytic pathway of a eukaryotic anaerobic amoeba Entamoeba histolytica. A. The glycolytic pathway of E. histolytica. B. PPDK uses PEP and PPi to catalyze the conversion of AMP to ATP (i.e., the production of two high energy phosphoanhydride bonds = two ‘ATP equivalents’). C. The ATP and PPi balance sheet for E. histolytica glycolysis. Abbreviations are as described in the text or as follows: 1,3-DPGA, 1,3-diphosphoglycerate; Fru-1,6-P2, fructose-1,6-bisphosphate; Fru-6-P, fructose-6-phosphate; Glu-6-P; glucose-6-phosphate; 3-PGA, 3-phosphoglycerate.

Pyrophosphate: An Autonomous Energy Donor of the Plant Cytosol

In contrast to animal cells, the plant cytosol lacks soluble inorganic pyrophosphatase and consequently contains PPi concentrations of up to about 0.5 mM. Moreover, the PPi level of the plant cytosol is remarkably insensitive to abiotic stresses such as anoxia or Pi starvation, or following the addition of respiratory poisons, which elicit significant reductions in cellular ATP pools (Plaxton and Podestá 2006). How stressed versus non-stressed plant cells maintain a relatively constant level of cytosolic PPi remains enigmatic. Anabolism, and hence the rate of PPi production, would generally be more prevalent under non-stressed conditions. However, even during stresses such as anoxia or Pi starvation, PPi would continue to be generated (albeit at a lower rate) during the biosynthesis of essential macromolecules (proteins, nucleic acids, membranes, polysaccharides) needed to support even diminished growth, and/or to replace the macromolecules that may have become damaged or worn out. This is indicated by the fact that cytosolic PPi levels remain fairly stable in plants that have been exposed to abiotic stresses that bring about significant reductions in cellular adenylate pools.

Plant PPi-PFK Catalyzes A Net Glycolytic Flux in the Plant Cytosol

The discovery in 1979 of the strictly cytosolic PPi-PFK in plants, and its potent activation by low concentrations of the regulatory metabolite fructose-2,6-bisphosphate led to a surge of research on the role of PPi in plant metabolism. It is now evident that PPi-PFK is an adaptive plant enzyme whose activity and subunit composition are dependent upon a variety of environmental, developmental, species and tissue-specific cues. Of note are reports describing the production of transgenic plants exhibiting significantly lower expression of PPi-PFK, or overexpression of mammalian 6-phosphofructo-2-kinase (the enzyme responsible for the synthesis of PPi-PFK’s allosteric activator, fructose-2,6-bisphosphate) (reviewed by Plaxton and Podestá 2006). Metabolite studies of these transgenic plants indicated that PPi-PFK catalyzes a net glycolytic flux in vivo (although PPi-PFK theoretically catalyzes a readily reversible reaction, in contrast to the irreversible ATP-PFK reaction).

Sucrose Synthase and the H+-Pumping Pyrophosphatase Also Employ PPi to Circumvent ATP-dependent Processes of the Plant Cytosol

Apart from PPi-PFK, PPi could be employed as an alternative energy donor for two other processes of the plant cytosol that are normally dependent upon ATP (Figure 3):

(1) Sucrose conversion to hexose-phosphates can proceed via the ATP-dependent invertase pathway or via the PPi-dependent sucrose synthase pathway.

(2) Active transport of protons into the vacuole from the cytosol can be carried out by separate ATP- or PPi-dependent H+-pumps located in the tonoplast membrane.

Figure 3   A model suggesting several adaptive metabolic processes (indicated by bold arrows) that may promote the survival of Pi-starved vascular plants. Alternative pathways of cytosolic glycolysis, mitochondrial electron transport, and tonoplast H+-pumping may facilitate respiration and vacuolar pH maintenance by Pi-deficient plant cells because they negate the dependence on adenylates and Pi, the levels of which become markedly depressed during severe Pi starvation. Roots secrete organic acids produced by PEP carboxylase to increase the availability of mineral-bound Pi (by solubilizing Ca-, Fe-, and Al-phosphates). A key component of this model is the critical secondary role played by "metabolic Pi recycling systems" during Pi deprivation. Enzymes that catalyze the numbered reactions are as follows: 1, hexokinase; 2, fructokinase; 3, nucleoside diphosphate kinase; 4, UDP-glucose pyrophosphorylase; 5, phosphoglucose isomerase; 6, phosphoglucose mutase; 7, NAD-dependent G3P dehydrogenase (phosphorylating); and 8, 3-PGA kinase. Abbreviations are as described in the legend for Figure 2, or as follows: ATP-PFK, ATP-dependent phosphofructokinase; DHAP, dihydroxyacetone phosphate; G3P, glyceraldehyde-3-phosphate; Glu-1-P; glucose-1-phosphate; MDH, malate dehydrogenase; OAA, oxaloacetate; PK, pyruvate kinase; UQ, ubiquinone. (Click image to enlarge.)

Plants Upregulate Alternative PPi-dependent Enzymes in Response to Stress

Tolerance to stresses such as anoxia and nutritional Pi starvation appears to depend upon a combination of morphological and metabolic adaptations that are both species and tissue specific. For example, flooding-intolerant species such as pea readily succumb to anoxia, whereas other plants, notably rice, as well as many wetland plants can germinate and grow new shoots even after several weeks of anoxia. That PPi-powered processes may be a crucial facet of the metabolic adaptations of plants to environmental extremes that lead to depressed ATP (but not PPi) pools is indicated by the significant upregulation of sucrose synthase, UDP-glucose pyrophosphorylase, PPi-PFK, PPDK, and the tonoplast H+-pyrophosphatase by anoxia or hypoxia, extended Pi-starvation, and/or cold stress (Plaxton and Podestá 2006, Li et al. 2008). The use of PPi-dependent cytosolic bypasses is believed to help plants survive these stresses by circumventing ATP-limited reactions while conserving diminished cellular ATP pools.

Sucrose metabolism of anoxic rice seedlings was shown to proceed mainly through sucrose synthase and UDP-glucose pyrophosphorylase with nucleoside diphosphate kinase facilitating the cycling of uridilates needed for operation of this pathway (reaction 3 of Figure 3) (Plaxton and Podestá 2006). Assuming that PPi is a byproduct of anabolism, no ATP is needed for the conversion of sucrose to hexose-phosphates via the sucrose synthase/UDP-glucose pyrophosphorylase pathway, whereas 2 ATPs are needed for the invertase/hexokinase pathway. Mertens (1991) argued that PPi-PFK functions in glycolysis in anoxic rice seedlings, since both PPi-PFK activity and the level of its activator fructose-2,6-bisphosphate are increased, while the activity of ATP-PFK declines. Similarly, PPDK represents a third enzyme that can employ PPi to catalyze a glycolytic bypass reaction in the cytosol of plant cells (Figure 3). This enzyme is highly expressed in mesophyll chloroplasts of C4 leaves where it functions in the PPi producing direction to regenerate PEP from pyruvate as part of the C4 photosynthetic cycle. However, PPDKs having a non-photosynthetic function are expressed as both cytosolic and plastidic isoforms in C3 plant cells (Plaxton and Podestá 2006). Although the metabolic roles of PPDK in C3 plants remain unclear, the cytosolic PPDK has been hypothesized to function as a glycolytic bypass to pyruvate kinase in non-green hypoxic rice tissues (i.e., developing seed endosperm and flooded roots), as well as Pi-starved maize roots so as to enhance the ATP yield of glycolysis (Plaxton and Podestá 2006, Li et al. 2008). PPDK could confer a considerable bioenergetic advantage for ATP-depleted plant cells, since as discussed above and outlined in Figure 2 this enzyme catalyzes the net creation of two ATP equivalents in the glycolytic direction, as opposed to the single ATP generated by the pyruvate kinase reaction. Hence, the overall net yield of ATP obtained during glycolytic fermentation of sucrose is increased from 4 to 12 if sucrose is metabolized via the sucrose synthase/UDP-glucose pyrophosphorylase, PPi-PFK, and PPDK bypasses, relative to the invertase/hexokinase, ATP-PFK, and pyruvate kinase pathway of classical glycolysis (Figure 3).

Alternative PPi-dependent cytosolic reactions of plant glycolysis and tonoplast H+-pumping confer a considerable bioenergetic advantage that can extend the survival time of plant cells that have become ATP-depleted owing to unavoidable environmental stresses such as cold temperature, anoxia/hypoxia, salt stress, or Pi starvation. This has been corroborated by studies of mutant or transgenic plants exhibiting altered levels of PPi-dependent enzymes and pathways. For example: (i) a 60% reduction in root PPi levels of transgenic potato plants overexpressing a bacterial soluble inorganic pyrophosphatase resulted in decreased plant growth and overall vitality during hypoxia stress, lower ATP levels, and an impaired ability to resume aerobic growth following four days of hypoxia (Mustroph et al. 2005); (ii) a crucial role for sucrose synthase in maintaining glycolysis in hypoxic maize roots was established by examination of mutants deficient in sucrose synthase activity (Ricard et al. 1998); and (iii) overexpression of the tonoplast H+-pumping pyrophosphatase resulted in transgenic plants that outperformed controls when subjected to nutritional Pi deprivation or salt stress (Gao et al., 2006, Yang et al. 2007).

Metabolic Flexibility of Cytosolic Glycolysis and Tonoplast H+-pumping during Phosphate Starvation

Orthophosphate (Pi) is the preferentially assimilated form of P for organisms such as plants that acquire their minerals directly from the environment. Pi plays a central role in virtually all major metabolic processes in plants, including photosynthesis and respiration. Despite its importance, Pi is one of the least available nutrients in many terrestrial and aquatic environments. In soil, Pi is often complexed to Al3+, Ca3+, or Fe3+, and thus occurs in an insoluble mineral form that renders it unavailable to plants. The massive use of Pi fertilizers in agriculture demonstrates how the free Pi levels of most soils are suboptimal for plant growth. Worldwide reserves of rock phosphate—our major source of Pi fertilizers in agriculture—have been projected to become exhausted within the next 50 to 75 years. Thus, research on plant metabolic adaptations to Pi deficiency is of significant practical importance. This research could lead to the development of rational strategies for the application of biotechnology to reduce or eliminate our current overreliance on expensive, polluting, and nonrenewable Pi fertilizers.

Pi-starved plants have evolved multiple and complex strategies to obtain limiting Pi. This includes increasing the efficiency of root Pi uptake via the upregulation of high-affinity Pi transporters of the plasma membrane, as well as inducing a wide variety of Pi scavenging and recycling enzymes. Examples of the latter include: (i) root secretion of Pi-starvation inducible nucleases and purple acid phosphatases to hydrolyze Pi from soil-localized nucleic acids and Pi-monoesters (Tran and Plaxton 2008), and (ii) intracellular Pi scavenging and conservation by replacing membrane phospholipids with non-phosphorus containing galacto- or sulfonyl lipids (Tjellström et al. 2008). Nevertheless, extended Pi deprivation inevitably leads to a marked decrease (up to 50-fold) in cytoplasmic Pi levels, and consequent large (up to 80%) reductions in intracellular levels of ATP and related nucleoside phosphates. This would be expected to reduce carbon flux through the enzymes of classical glycolysis that are dependent upon adenylates or Pi as co-substrates (Figure 3). Despite depleted intracellular Pi and adenylate pools, Pi-deprived plants must continue to generate energy and carbon skeletons for key metabolic processes. As indicated in Figure 3, at least seven Pi- and adenylate-independent glycolytic bypass enzymes (sucrose synthase, UDP-glucose pyrophosphorylase, PPi-PFK, non-phosphorylating NADP-glyceraldehyde-3-phosphate dehydrogenase, PEP carboxylase, PPDK, and PEP phosphatase) in addition to the PPi-dependent H+-pump of the tonoplast membrane have been reported to be significantly upregulated following Pi starvation of plant cells (Plaxton and Podestá 2006, Li et al. 2008). It has been hypothesized that these enzymes represent Pi starvation-inducible bypasses to adenylate or Pi-dependent enzymes (i.e., invertase/hexokinase, ATP-PFK, phosphorylating NAD-glyceraldehyde-3-phosphate dehydrogenase, pyruvate kinase, and tonoplast H+-ATPase) thereby facilitating glycolytic flux and vacuolar pH maintenance during severe Pi stress when the intracellular levels of Pi and adenylates may be very low. Furthermore, as Pi exerts reciprocal allosteric effects on the activity of ATP-PFK (potent activator) and PPi-PFK (potent inhibitor) (Figure 3), the large decrease in cytoplasmic Pi levels that occurs during long term Pi starvation has also been proposed to promote the in vivo PPi-PFK activity while curtailing that of ATP-PFK (Plaxton and Podestá 2006). Pi is also a byproduct of the reactions catalyzed by the bypass enzymes PPi-PFK, PEP carboxylase, PPDK, PEP phosphatase, and tonoplast H+-pyrophosphatase (Figure 3). Thus, the upregulation of these five enzymes by Pi starvation could play a dual role during Pi stress. They may facilitate glycolysis or tonoplast H+-pumping by bypassing adenylate-dependent reactions, while simultaneously generating free Pi for its reassimilation into the metabolism of the Pi-starved cells via photo- or oxidative phosphorylation.

PEP Carboxylase Plays an Important Role during Pi Starvation

PEP carboxylase is a ubiquitous and tightly regulated plant cytosolic enzyme encoded by a small gene family that has diverse functions including:

(i) the fixation of atmospheric CO2 in CAM and C4 photosynthesis, and

(ii) the anaplerotic replenishment of citric acid cycle intermediates that have been consumed in biosynthesis and N-assimilation.

However, together with cytosolic malate dehydrogenase and the mitochondrial NAD-dependent malic enzyme, PEP carboxylase can also function as a glycolytic enzyme by indirectly bypassing the reaction catalyzed by cytosolic pyruvate kinase (Figure 3). Enhanced levels of PEP carboxylase mRNA, protein, and/or activity during Pi-starvation have been reported for diverse species including oilseed rape, Arabidopsis thaliana, tomato, and white lupin (Plaxton and Podestá 2006, Gregory et al. 2009). The PEP carboxylase-malate dehydrogenase-malic enzyme bypass of cytosolic pyruvate kinase has been hypothesized to be of particular importance during nutritional Pi deprivation when pyruvate kinase activity may become ADP limited (Plaxton and Podestá 2006). PEPC induction has also been correlated with the synthesis and consequent excretion of large amounts of malic and citric acids by roots during Pi stress. This increases Pi availability to the roots by acidifying the soil to solubilize otherwise inaccessible sources of mineralized Pi. Despite considerable biochemical and transcriptomic evidence for PEPC’s participation in plant acclimation to Pi starvation, little research has been performed to document the specific PEPC isozymes(s) upregulated during Pi stress nor the relationship between cellular Pi nutrition and the enzyme’s in vivo phosphorylation status. However, a recent study indicated that the parallel induction and in vivo phosphorylation-activation of the PEP carboxylase isozyme AtPPC1 plays an important role in the metabolic adaptations of Pi-starved Arabidopsis (Gregory et al. 2009).

Growth and dark respiration rates of leaves of transgenic tobacco lacking leaf cytosolic pyruvate kinase were largely unaffected (relative to wild type controls), which proves that plants (unlike animals or yeast) have metabolic bypasses to cytosolic pyruvate kinase (Grodzinski et al. 1999). These results reflect the amazing flexibility of plant PEP metabolism, which is probably an evolutionary adaptation to the stresses that plants are frequently exposed to in their natural environment.

Alternative Pathways of Plant Mitochondrial Electron Transport Also Contribute to the Survival of Pi-Starved Plants

Respiratory O2 consumption by plant mitochondria can be mediated by the energy conserving cytochrome pathway (as in animals) or by the non-energy conserving alternative pathway (Figure 3) (also see textbook Figure 12.8, and Web Topic 12.3). The significant reductions in cellular Pi and ADP levels that follow severe Pi deprivation will impede respiratory electron flow through the cytochrome pathway at the sites of coupled ATP synthesis. However, the presence of non-proton motive pathways of electron transport could provide a mechanism whereby respiratory flux is maintained under conditions when the availability of ADP and/or Pi are restrictive (i.e., during severe Pi deprivation). Plants acclimate to Pi stress by increased engagement of the non-energy conserving (i.e., rotenone- and cyanide-insensitive) alternative pathways of the miETC (Figure 3) (Plaxton and Podestá 2006). Moreover, increased levels of the AOX protein have been reported to occur following Pi stress of some plant species. This allows continued functioning of the citric acid cycle and respiratory electron transport chain with limited ATP production and may thereby contribute to the survival of Pi-deficient plants. By preventing severe respiratory restriction, the AOX prevents both undesirable redirections in carbon metabolism as well as the excessive generation of harmful reactive O2 species in the mitochondrion of Pi starved plants (see Web Essay 12.7). This has been corroborated by the impaired growth and metabolism of Pi-starved transgenic tobacco suspension cells that cannot synthesize a functional AOX protein (Plaxton and Podestá 2006). Northern blot and proteomic analyses indicated that the absence of AOX led to the increased expression of proteins normally associated with oxidative stress in the Pi-deprived transgenic tobacco cells. By draining excess electrons, AOX also helps to minimize the production of damaging ROS that otherwise arises when ubiquinone becomes over-reduced.

High-throughput Transcript and Proteome Profiling Indicates that Pi starvation Inducibility of Glycolytic and Mitochondrial Electron Transport Bypass Proteins Is Widespread in Plants

The recent application of high throughput transcript and proteome profiling technologies has allowed researchers to simultaneously catalogue the effects of Pi deficiency on the expression of many genes and proteins in plants such as Arabidopsis, rice, maize, and white lupin. Interestingly, the enhanced expression of alternative glycolytic enzymes (such as sucrose synthase, UDP-glucose pyrophosphorylase, PPi-PFK, and PPDK) and miETC proteins (such as AOX) that do not require Pi or adenylates as co-substrates has been observed (Plaxton and Podestá 2006, Li et al. 2008).

Conclusions

This web essay considered how the unique flexibility of plant carbohydrate and energy metabolism may help plants to cope with unavoidable environmental stresses that may be imposed upon them. Much research has focused on the molecular biology of plant stress responses, and this has revealed the ability of plants to respond to stress by altered gene expression leading to the synthesis of various stress-inducible proteins. Somewhat less attention has been devoted to the organization and control of intermediary metabolism as pertains to the acclimation of plants to stress. However, studies of plant metabolic responses to extreme environments have revealed some remarkably adaptive mechanisms that may serve to limit the deleterious consequences of stress. Although these adaptations are by no means identical in all plants, certain aspects are conserved in a wide variety of species from very different environments. A better understanding of the extent to which changes in flux through alternative enzymes and pathways influences plant stress tolerance is of significant practical interest. This knowledge is relevant to the applied efforts of agricultural biotechnologists to engineer transgenic crops exhibiting an improved resistance to abiotic stress, including nutritional Pi deprivation.

References

Gao, F., Gao, Q., Duan, X., Yue, G., Yang, A., and Zhang, R. (2006) Cloning of an H+-PPase gene from Thellungiella halophila and its heterologous expression to improve tobacco salt tolerance. J. Exp. Bot. 57: 3259–3270

Gregory, A. L., Hurley, B. A., Tran, H. T., Valentine, A. J., She, Y-M., Knowles, V. L, and Plaxton, W. C. (2009) In vivo regulatory phosphorylation of the phosphoenolpyruvate carboxylase AtPPC1 in phosphate-starved Arabidopsis thaliana. Biochem. J. 420: 57–65

Grodzinski, B., Jiao, J., Knowles, V. L., and Plaxton, W. C. (1999) Photosynthesis and carbon partitioning in transgenic tobacco plants deficient in leaf pyruvate kinase. Plant Physiol. 120: 887–895

Li, K., Xu, C., Li, Z., Zhang, K., Yang, A., and Zhang, J. (2008) Comparative proteome analyses of phosphorus responses in maize (Zea mays L.) roots of wild-type and a low-P-tolerant mutant reveal root characteristics associated with phosphorus efficiency. Plant J. 55: 927–939

Mertens, E. (1991) Pyrophosphate-dependent phosphofructokinase: an anaerobic glycolytic enzyme? FEBS Lett. 285: 1–5

Mustroph, A., Albrecht, G., Hajirezaei, M., Brimm, B., and Biemelt, S. (2005) Low levels of pyrophosphate in transgenic potato plants expressing E. coli pyrophosphatase lead to decreased vitality under oxygen deficiency. Annals of Botany 96: 717–726

Plaxton, W. C., and Podestá, F. E. (2006) The functional organization and control of plant respiration. Crit. Rev. Plant Sci. 25: 159–198

Ricard, B., Van Toai, T., Chourey, P., and Saglio, P. (1998) Evidence for the critical role of sucrose synthase for anoxic maize roots using a double mutant. Plant Physiol. 116: 1323–1331

Tjellström, H., Andersson, M. X., Larsson, K. E., and Sandelius, A. S. (2008) Membrane phospholipids as a phosphate reserve: the dynamic nature of phospholipid-to-digalactosyl diacylglycerol exchange in higher plants. Plant Cell Environ. 31: 1388–1398

Tran, H. T., and Plaxton, W. C. (2008) Proteomic analysis of alterations in the secretome of Arabidopsis thaliana suspension cells subjected to nutritional phosphate deficiency. Proteomics 20: 4317–4326

Yang, H., Knapp, J., Koirala, P., Rajagopal, D., Peer, W. A., Silbart, L. K., Murphy, A., and Gaxiola, R. A. (2007) Enhanced phosphorus nutrition in monocots and dicots over-expressing a phosphorus-responsive type I H+-pyrophosphatase. Plant Biotech. J. 5: 735–745.