Metabolic consequences of overproduction of phosphoenolpyruvate carboxylase in C3 plants

Prospects and perspectives: inferring physiological and regulatory targets for CAM from molecular and modelling approaches

BACKGROUND AND SCOPE

This review summarizes recent advances in our understanding of Crassulacean Acid Metabolism (CAM) by integrating evolutionary, ecological, physiological, metabolic and molecular perspectives. A number of key control loops which moderate the expression of CAM phases, and their metabolic and molecular control, are explored. These include nocturnal stomatal opening, activation of phosphoenolpyruvate carboxylase by a specific protein kinase, interactions with circadian clock control, as well as daytime decarboxylation and activation of Rubisco. The vacuolar storage and release of malic acid and the interplay between the supply and demand for carbohydrate reserves are also key metabolic control points.

FUTURE OPPORTUNITIES

We identify open questions and opportunities, with experimentation informed by top-down molecular modelling approaches allied with bottom-up mechanistic modelling systems. For example, mining transcriptomic datasets using high-speed systems approaches will help to identify targets for future genetic manipulation experiments to define the regulation of CAM (whether circadian or metabolic control). We emphasize that inferences arising from computational approaches or advanced nuclear sequencing techniques can identify potential genes and transcription factors as regulatory targets. However, these outputs then require systematic evaluation, using genetic manipulation in key model organisms over a developmental progression, combining gene silencing and metabolic flux analysis and modelling to define functionality across the CAM day-night cycle. From an evolutionary perspective, the origins and function of CAM succulents and responses to water deficits are set against the mesophyll and hydraulic limitations imposed by cell and tissue succulence in contrasting morphological lineages. We highlight the interplay between traits across shoots (3D vein density, mesophyll conductance and cell shrinkage) and roots (xylem embolism and segmentation). Thus, molecular, biophysical and biochemical processes help to curtail water losses and exploit rapid rehydration during restorative rain events. In the face of a changing climate, we hope such approaches will stimulate opportunities for future research.

Nitrogen assimilation and photorespiration become more efficient under chloride nutrition as a beneficial macronutrient

Chloride (Cl^-) and nitrate ( NO 3 - ) are closely related anions involved in plant growth. Their similar physical and chemical properties make them to interact in cellular processes like electrical balance and osmoregulation. Since both anions share transport mechanisms, Cl^- has been considered to antagonize NO 3 - uptake and accumulation in plants. However, we have recently demonstrated that Cl^- provided at beneficial macronutrient levels improves nitrogen (N) use efficiency (NUE). Biochemical mechanisms by which beneficial Cl^- nutrition improves NUE in plants are poorly understood. First, we determined that Cl^- nutrition at beneficial macronutrient levels did not impair the NO 3 - uptake efficiency, maintaining similar NO 3 - content in the root and in the xylem sap. Second, leaf NO 3 - content was significantly reduced by the treatment of 6 mM Cl^- in parallel with an increase in NO 3 - utilization and NUE. To verify whether Cl^- nutrition reduces leaf NO 3 - accumulation by inducing its assimilation, we analysed the content of N forms and the activity of different enzymes and genes involved in N metabolism. Chloride supply increased transcript accumulation and activity of most enzymes involved in NO 3 - assimilation into amino acids, along with a greater accumulation of organic N (mostly proteins). A reduced glycine/serine ratio and a greater ammonium accumulation pointed to a higher activity of the photorespiration pathway in leaves of Cl^--treated plants. Chloride, in turn, promoted higher transcript levels of genes encoding enzymes of the photorespiration pathway. Accordingly, microscopy observations suggested strong interactions between different cellular organelles involved in photorespiration. Therefore, in this work we demonstrate for the first time that the greater NO 3 - utilization and NUE induced by beneficial Cl^- nutrition is mainly due to the stimulation of NO 3 - assimilation and photorespiration, possibly favouring the production of ammonia, reductants and intermediates that optimize C-N re-utilization and plant growth. This work demonstrates new Cl^- functions and remarks on its relevance as a potential tool to manipulate NUE in plants.

Overexpression of Setaria italica phosphoenolpyruvate carboxylase gene in rice positively impacts photosynthesis and agronomic traits

C₄ plants have the inherent capacity to concentrate atmospheric CO₂ in the vicinity of RuBisCo, thereby increasing carboxylation, and inhibiting photorespiration. Carbonic anhydrase (CA), the first enzyme of C₄ photosynthesis, converts atmospheric CO₂ to HCO₃^-, which is utilized by PEPC to produce C₄ acids. Bioengineering of C₄ traits into C₃ crops is an attractive strategy to increase photosynthesis and water use efficiency. In the present study, we isolated the PEPC gene from the C₄ plant Setaria italica and transferred it to C₃ rice. Overexpression of SiPEPC resulted in a 2-6-fold increment in PEPC enzyme activity in transgenic lines with respect to non-transformed control. Photosynthetic efficiency was enhanced in transformed plants, which was associated with increased ФPSII, ETR, lower NPQ, and higher chlorophyll accumulation. Water use efficiency was increased by 16-22% in PEPC transgenic rice lines. Increased PEPC activity enhanced quantum yield and carboxylation efficiency of PEPC transgenic lines. Transgenic plants exhibited higher light saturation photosynthesis rate and lower CO₂ compensation point, as compared to non-transformed control. An increase in net photosynthesis increased the yield by (23-28.9%) and biomass by (24.1-29%) in transgenic PEPC lines. Altogether, our findings indicate that overexpression of C_4-specific SiPEPC enzyme is able to enhance photosynthesis and related parameters in transgenic rice.

Transcriptome changes associated with apple (Malus domestica) root defense response after Fusarium proliferatum f. sp. malus domestica infection

BACKGROUND

Apple replant disease is a soilborne disease caused by Fusarium proliferatum f. sp. malus domestica strain MR5 (abbreviated hereafter as Fpmd MR5) in China. This pathogen causes root tissue rot and wilting leaves in apple seedlings, leading to plant death. A comparative transcriptome analysis was conducted using the Illumina Novaseq platform to identify the molecular defense mechanisms of the susceptible M.26 and the resistant M9T337 apple rootstocks to Fpmd MR5 infection.

RESULTS

Approximately 518.1 million high-quality reads were generated using RNA sequencing (RNA-seq). Comparative analysis between the mock-inoculated and Fpmd MR5 infected apple rootstocks revealed 28,196 significantly differentially expressed genes (DEGs), including 14,572 up-regulated and 13,624 down-regulated genes. Among them, the transcriptomes in the roots of the susceptible genotype M.26 were reflected by overrepresented DEGs. MapMan analysis indicated that a large number of DEGs were involved in the response of apple plants to Fpmd MR5 stress. The important functional groups identified via gene ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment were responsible for fundamental biological regulation, secondary metabolism, plant-pathogen recognition, and plant hormone signal transduction (ethylene and jasmonate). Furthermore, the expression of 33 up-regulated candidate genes (12 related to WRKY DNA-binding proteins, one encoding endochitinase, two encoding beta-glucosidases, ten related to pathogenesis-related proteins, and eight encoding ethylene-responsive transcription factors) were validated by quantitative real-time PCR.

CONCLUSION

RNA-seq profiling was performed for the first time to analyze response of apple root to Fpmd MR5 infection. We found that the production of antimicrobial compounds and antioxidants enhanced plant resistance to pathogens, and pathogenesis-related protein (PR10 homologs, chitinase, and beta-glucosidase) may play unique roles in the defense response. These results provide new insights into the mechanisms of the apple root response to Fpmd MR5 infection.

Prokaryotic Expression of Phosphoenolpyruvate Carboxylase Fragments from Peanut and Analysis of Osmotic Stress Tolerance of Recombinant Strains

Phosphoenolpyruvate carboxylase (PEPC) is a ubiquitous cytosolic enzyme that catalyzes the irreversible β-carboxylation of phosphoenolpyruvate (PEP) in presence of HCO₃⁻ to produce oxaloacetate (OAA) during carbon fixation and photosynthesis. It is well accepted that PEPC genes are expressed in plants upon stress. PEPC also supports the biosynthesis of biocompatible osmolytes in many plant species under osmotic stress. There are five isoforms of PEPC found in peanut (Arachis hypogaea L.), namely, AhPEPC1, AhPEPC2, AhPEPC3, AhPEPC4, and AhPEPC5. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the gene expression patterns of these AhPEPC genes were different in mature seeds, stems, roots, flowers, and leaves. The expression of all the plant type PEPC (PTPCs) (AhPEPC1, AhPEPC2, AhPEPC3, and AhPEPC4) was relatively high in roots, while the bacterial type PEPC (BTPC) (AhPEPC5) showed a remarkable expression level in flowers. Principal component analysis (PCA) result showed that AhPEPC3 and AhPEPC4 are correlated with each other, indicating comparatively associations with roots, and AhPEPC5 have a very close relationship with flowers. In order to investigate the function of these AhPEPCs, the fragments of these five AhPEPC cDNA were cloned and expressed in Escherichia coli (E. coli). The recombinant proteins contained a conserved domain with a histidine site, which is important for enzyme catalysis. Results showed that protein fragments of AhPEPC1, AhPEPC2, and AhPEPC5 had remarkable expression levels in E. coli. These three recombinant strains were more sensitive at pH 9.0, and recombinant strains carrying AhPEPC2 and AhPEPC5 fragments exhibited more growth than the control strain with the presence of PEG6000. Our findings showed that the expression of the AhPEPC fragments may enhance the resistance of transformed E. coli to osmotic stress.

Cloning and functional identification of a Chilo suppressalis-inducible promoter of rice gene, OsHPL2

BACKGROUND

Promoters play a key role in driving insect-resistant genes during breeding of transgenic plants. In current transgenic procedures for breeding rice resistance to striped stem borer (Chilo suppressalis Walker, SSB), the constitutive promoter is used to drive the insect-resistant gene. To reduce the burden of constitutive promoters on plant growth, isolation and identification of insect-inducible promoters are particularly important. However, few promoters are induced specifically by insect feeding.

RESULTS

We found rice hydroperoxide lyase gene (OsHPL2) (LOC_Os02g12680) was upregulated after feeding by SSB. We subsequently cloned the promoter of OsHPL2 and analysed its expression pattern using the β-glucuronidase (GUS) reporter gene. Histochemical assays and quantitative analyses of GUS activity confirmed that P _HPL2 :GUS was activated by SSB, but did not respond to brown planthopper (Nilaparvata lugens Stål, BPH) infestation, mechanical wounding or phytohormone treatments. A series of 5' truncated assays were conducted and three positive regulatory regions (-1452 to -1213, -903 to -624, and -376 to -176) induced by SSB infestation were identified. P2R123-min 35S and P2TR2-min 35S promoters linked with cry1C of transgenic plants showed the highest levels of Cry1C protein expression and SSB larval mortality.

CONCLUSION

We identified an SSB-inducible promoter and three positive internal regions. Transgenic rice plants with the OsHPL2 promoter and its positive regions driving cry1C exhibited the expected larvicidal effect on SSB. Our study is the first report of an SSB-inducible promoter that could be used as a potential resource for breeding insect-resistant transgenic crops. © 2020 Society of Chemical Industry.

Transgenic maize phosphoenolpyruvate carboxylase alters leaf-atmosphere CO2 and 13CO2 exchanges in Oryza sativa

The engineering process of C4 photosynthesis into C3 plants requires an increased activity of phosphoenolpyruvate carboxylase (PEPC) in the cytosol of leaf mesophyll cells. The literature varies on the physiological effect of transgenic maize (Zea mays) PEPC (ZmPEPC) leaf expression in Oryza sativa (rice). Therefore, to address this issue, leaf-atmosphere CO2 and 13CO2 exchanges were measured, both in the light (at atmospheric O2 partial pressure of 1.84 kPa and at different CO2 levels) and in the dark, in transgenic rice expressing ZmPEPC and wild-type (WT) plants. The in vitro PEPC activity was 25 times higher in the PEPC overexpressing (PEPC-OE) plants (~20% of maize) compared to the negligible activity in WT. In the PEPC-OE plants, the estimated fraction of carboxylation by PEPC (β) was ~6% and leaf net biochemical discrimination against 13CO2[Formula: see text] was ~ 2‰ lower than in WT. However, there were no differences in leaf net CO2 assimilation rates (A) between genotypes, while the leaf dark respiration rates (Rd) over three hours after light-dark transition were enhanced (~ 30%) and with a higher 13C composition [Formula: see text] in the PEPC-OE plants compared to WT. These data indicate that ZmPEPC in the PEPC-OE rice plants contributes to leaf carbon metabolism in both the light and in the dark. However, there are some factors, potentially posttranslational regulation and PEP availability, which reduce ZmPEPC activity in vivo.

In vivo phosphoenolpyruvate carboxylase activity is controlled by CO2 and O2 mole fractions and represents a major flux at high photorespiration rates

Phosphenolpyruvate carboxylase (PEPC)-catalysed fixation of bicarbonate to C₄ acids is commonly believed to represent a rather small flux in illuminated leaves. In addition, its potential variation with O₂ and CO₂ is not documented and thus is usually neglected in gas-exchange studies. Here, we used quantitative NMR analysis of sunflower leaves labelled with ¹³ CO₂ (99% ¹³ C) under controlled conditions and measured the amount of ¹³ C found in the four C-atom positions in malate, the major product of PEPC activity. We found that amongst malate ¹³ C-isotopomers present after labelling, most molecules were labelled at both C-1 and C-4, showing the incorporation of ¹³ C at C-4 by PEPC fixation and subsequent redistribution to C-1 by fumarase (malate-fumarate equilibrium). In addition, absolute quantification of ¹³ C content showed that PEPC fixation increased at low CO₂ or high O₂ , and represented up to 1.8 μmol m^-2  s^-1 , that is, 40% of net assimilation measured by gas exchange under high O₂ /CO₂ conditions. Our results show that PEPC fixation represents a quantitatively important CO₂ -fixing activity that varies with O₂ and/or CO₂ mole fraction and this challenges the common interpretation of net assimilation in C₃ plants, where PEPC activity is often disregarded or considered to be constant at a very low rate.

Primary carbon metabolism-related changes in cucumber exposed to single and sequential treatments with salt stress and bacterial infection

This study examines how salt stress interacts with bacterial infection at the metabolic level. We measured chlorophyll a fluorescence as well as profiles of phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), NADP-isocitrate dehydrogenase (NADP-ICDH) and fumarase activities, malic and citric acids contents and the expression of NADP-ICDH and NADP-ME in the organ-dependent (root vs leaves) response of cucumber plants exposed to individual or sequential action of salt stress (50 mM or 100 mM NaCl) and Pseudomonas syringae pv lachrymans (Psl). NaCl treatment, Psl infection and the combination of these stresses caused disturbances in the activity of photosystem II which were suggested to specifically transmit the stress signals. PEPC and NADP-ME were induced in cucumber plants under stress, confirming that in C3 plants they function in defence responses. The profiles of malate and citrate contents, PEPC as well as NADP-ICDH and NADP-ME activities and gene expression in response to a combination of salt and pathogen stresses differed from those provoked by individual stress with respect to the direction, intensity and timing. The results indicated that the most pronounced defence response related to the readjustment of the carbon metabolism was observed in the leaves of plants exposed to combined stress. Intense activity changes of NADPH-generating enzymes, NADP-ICDH and NADP-ME, characterized the tailored response to combined stress and could be important for the integration of defence mechanisms between organs.

Physiological investigation of C4-phosphoenolpyruvate-carboxylase-introduced rice line shows that sucrose metabolism is involved in the improved drought tolerance

We compared the drought tolerance of wild-type (WT) and transgenic rice plants (PC) over-expressing the maize C₄PEPC gene, which encodes phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) gene, and evaluated the roles of saccharide and sugar-related enzymes in the drought response. Pot-grown seedlings were subjected to real drought conditions outdoors, and the yield components were compared between PC and untransformed wild-type (WT) plants. The stable yield from PC plants was associated with higher net photosynthetic rate under the real drought treatment. The physiological characters of WT and PC seedlings under a simulated drought treatment (25% (w/v) polyethylene glycol-6000 for 3 h; PEG 6000 treatment) were analyzed in detail for the early response of drought. The relative water content was higher in PC than in WT, and PEPC activity and the C₄-PEPC transcript level in PC were elevated under the simulated drought conditions. The endogenous saccharide responses also differed between PC and WT under simulated drought stress. The higher sugar decomposition rate in PC than in WT under drought analog stress was related to the increased activities of sucrose phosphate synthase, sucrose synthase, acid invertase, and neutral invertase, increased transcript levels of VIN1, CIN1, NIN1, SUT2, SUT4, and SUT5, and increased activities of superoxide dismutase and peroxidase in the leaves. The greater antioxidant defense capacity of PC and its relationship with saccharide metabolism was one of the reasons for the improved drought tolerance. In conclusion, PEPC effectively alleviated oxidative damage and enhanced the drought tolerance in rice plants, which were more related to the increase of the endogenous saccharide decomposition. These findings show that components of C₄ photosynthesis can be used to increase the yield of rice under drought conditions.

Towards efficient photosynthesis: overexpression of Zea mays phosphoenolpyruvate carboxylase in Arabidopsis thaliana

Plants with C4 photosynthesis are efficient in carbon assimilation and have an advantage over C3 photosynthesis. In C4 photosynthesis, the primary CO₂ fixation is catalyzed by phosphoenolpyruvate carboxylase (PEPC). Here, we show that overexpression of Zea mays PEPC cDNA, under the control of ³⁵S promoter, in Arabidopsis thaliana resulted in ~7-10 fold higher protein abundance and ~7-10 fold increase in PEPC activity in the transgenic lines than that in the vector control. We suggest that overexpression of PEPC played an anaplerotic role to increase the supply of 4-carbon carboxylic acids, which provided carbon skeletons for increased amino acid and protein synthesis. Higher protein content must have been responsible for increased metabolic processes including chlorophyll biosynthesis, photosynthesis, and respiration. Consequently, the PEPC-overexpressed transgenic plants had higher chlorophyll content, enhanced electron transport rate (ETR), lower non-photochemical quenching (NPQ) of chlorophyll a fluorescence, and a higher performance index (PI) than the vector control. Consistent with these observations, the rate of CO₂ assimilation, the starch content, and the dry weight of PEPC-overexpressed plants increased by 14-18 %, 10-18 %, and 6.5-16 %, respectively. Significantly, transgenics were tolerant to salt stress as they had increased ability to synthesize amino acids, including the osmolyte proline. NaCl (150 mM)-treated transgenic plants had higher variable to maximum Chl a fluorescence (F _v/F _m) ratio, higher PI, higher ETR, and lower NPQ than the salt-treated vector controls. These results suggest that expression of C4 photosynthesis enzyme(s) in a C3 plant can improve its photosynthetic capacity with enhanced tolerance to salinity stress.

Differential response of NADP-dehydrogenases and carbon metabolism in leaves and roots of two durum wheat (Triticum durum Desf.) cultivars (Karim and Azizi) with different sensitivities to salt stress

Wheat (Triticum durum Desf.) is a common Mediterranean species of considerable agronomic importance. Salinity is one of the major threats to sustainable agricultural production mainly because it limits plant productivity. After exposing the Karim and Azizi durum wheat cultivars, which are of agronomic significance in Tunisia, to 100mM NaCl salinity, growth parameters (dry weight and length), proline content and chlorophylls were evaluated in their leaves and roots. In addition, we analyzed glutathione content and key enzymatic activities, including phosphoenolpyruvate carboxylase (PEPC), NADP-isocitrate dehydrogenase (NADP-ICDH), NADP-malic enzyme (NADP-ME), glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH), involved in the carbon metabolism and NADPH-generating system. The sensitivity index indicates that cv Karim was more tolerant to salinity than cv Azizi. This higher tolerance was corroborated at the biochemical level, as cv Karim showed a greater capacity to accumulate proline, especially in leaves, while the enzyme activities studied were differentially regulated in both organs, with NADP-ICDH being the only activity to be unaffected in all organs. In summary, the data indicate that higher levels of proline accumulation and the differential responses of some key enzymes involved in the carbon metabolism and NADPH regeneration contribute to the salinity tolerance mechanism and lead to increased biomass accumulation in cv Karim.

Tissue-specific expression and post-translational modifications of plant- and bacterial-type phosphoenolpyruvate carboxylase isozymes of the castor oil plant, Ricinus communis L

This study employs transcript profiling together with immunoblotting and co-immunopurification to assess the tissue-specific expression, protein:protein interactions, and post-translational modifications (PTMs) of plant- and bacterial-type phosphoenolpyruvate carboxylase (PEPC) isozymes (PTPC and BTPC, respectively) in the castor plant, Ricinus communis. Previous studies established that the Class-1 PEPC (PTPC homotetramer) of castor oil seeds (COS) is activated by phosphorylation at Ser-11 and inhibited by monoubiquitination at Lys-628 during endosperm development and germination, respectively. Elimination of photosynthate supply to developing COS by depodding caused the PTPC of the endosperm and cotyledon to be dephosphorylated, and then subsequently monoubiquitinated in vivo. PTPC monoubiquitination rather than phosphorylation is widespread throughout the castor plant and appears to be the predominant PTM of Class-1 PEPC that occurs in planta. The distinctive developmental patterns of PTPC phosphorylation versus monoubiquitination indicates that these two PTMs are mutually exclusive. By contrast, the BTPC: (i) is abundant in the inner integument, cotyledon, and endosperm of developing COS, but occurs at low levels in roots and cotyledons of germinated COS, (ii) shows a unique developmental pattern in leaves such that it is present in leaf buds and young expanding leaves, but undetectable in fully expanded leaves, and (iii) tightly interacts with co-expressed PTPC to form the novel and allosterically-desensitized Class-2 PEPC heteromeric complex. BTPC and thus Class-2 PEPC up-regulation appears to be a distinctive feature of rapidly growing and/or biosynthetically active tissues that require a large anaplerotic flux from phosphoenolpyruvate to replenish tricarboxylic acid cycle C-skeletons being withdrawn for anabolism.

The role of proteins in C(3) plants prior to their recruitment into the C(4) pathway

Our most productive crops and native vegetation use a modified version of photosynthesis known as the C(4) pathway. Leaves of C(4) crops have increased nitrogen and water use efficiencies compared with C(3) species. Although the modifications to leaves of C(4) plants are complex, their faster growth led to the proposal that C(4) photosynthesis should be installed in C(3) crops in order to increase yield potential. Typically, a limited set of proteins become restricted to mesophyll or bundle sheath cells, and this allows CO(2) to be concentrated around the primary carboxylase RuBisCO. The role that these proteins play in C(3) species prior to their recruitment into the C(4) pathway is addressed here. Understanding the role of these proteins in C(3) plants is likely to be of use in predicting how the metabolism of a C(3) leaf will alter as components of the C(4) pathway are introduced as part of efforts to install characteristics of C(4) photosynthesis in leaves of C(3) crops.

The remarkable diversity of plant PEPC (phosphoenolpyruvate carboxylase): recent insights into the physiological functions and post-translational controls of non-photosynthetic PEPCs

PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled enzyme located at the core of plant C-metabolism that catalyses the irreversible β-carboxylation of PEP to form oxaloacetate and Pi. The critical role of PEPC in assimilating atmospheric CO2 during C4 and Crassulacean acid metabolism photosynthesis has been studied extensively. PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis and nitrogen assimilation. An impressive array of strategies has evolved to co-ordinate in vivo PEPC activity with cellular demands for C4–C6 carboxylic acids. To achieve its diverse roles and complex regulation, PEPC belongs to a small multigene family encoding several closely related PTPCs (plant-type PEPCs), along with a distantly related BTPC (bacterial-type PEPC). PTPC genes encode ~110-kDa polypeptides containing conserved serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist as homotetrameric Class-1 PEPCs. In contrast, BTPC genes encode larger ~117-kDa polypeptides owing to a unique intrinsically disordered domain that mediates BTPC's tight interaction with co-expressed PTPC subunits. This association results in the formation of unusual ~900-kDa Class-2 PEPC hetero-octameric complexes that are desensitized to allosteric effectors. BTPC is a catalytic and regulatory subunit of Class-2 PEPC that is subject to multi-site regulatory phosphorylation in vivo. The interaction between divergent PEPC polypeptides within Class-2 PEPCs adds another layer of complexity to the evolution, physiological functions and metabolic control of this essential CO2-fixing plant enzyme. The present review summarizes exciting developments concerning the functions, post-translational controls and subcellular location of plant PTPC and BTPC isoenzymes.

What can enzymes of C₄ photosynthesis do for C₃ plants under stress?

Phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), and pyruvate, phosphate dikinase (PPDK) participate in the process of concentrating CO₂ in C₄ photosynthesis. Non-photosynthetic counterparts of these enzymes, which are present in all plants, play important roles in the maintenance of pH and replenishment of Krebs cycle intermediates, thereby contributing to the biosynthesis of amino acids and other compounds and providing NADPH for biosynthesis and the antioxidant system. Enhanced activities of PEPC and/or NADP-ME and/or PPDK were found in plants under various types of abiotic stress, such as drought, high salt concentration, ozone, the absence of phosphate and iron or the presence of heavy metals in the soil. Moreover, the activities of all of these enzymes were enhanced in plants under biotic stress caused by viral infection. The functions of PEPC, NADP-ME and PPDK appear to be more important for plants under stress than under optimal growth conditions.

RNA interference-based suppression of phosphoenolpyruvate carboxylase results in susceptibility of rapeseed to osmotic stress

The diverse functions of phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) in C(3) plants are not as well understood as in C(4) plants. To investigate the functions of PEPCase in C(3) plants, rapeseed (Brassica napus L.) PEPCase gene (referred to as BNPE15) was silenced by the RNA interference (RNAi) technique. Under normal growth conditions, no significant difference in lipid content and fatty acid composition were found between wild-type (WT) and transgenic rapeseed plants. However, when these plants were subjected to osmotic stress induced by osmoticum polyethylene glycol (PEG-6000), membrane permeability and membrane lipid peroxidization in roots and leaves of transgenic plants were higher than those of WT plants. It suggested that transgenic plants are more susceptible to osmotic stress than WT plants. Taken together, the results showed that the suppression of PEPCase by RNAi leads to susceptibility to osmotic stress in rapeseed, and PEPCase is involved in the response of C(3) plants to environmental stress.

Regulation of phosphoenolpyruvate carboxylase in PVY(NTN)-infected tobacco plants

The effect of viral infection on the regulation of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in Nicotiana tabacum L. leaves was studied. PEPC activity was 3 times higher in infected plant leaves compared to healthy plants. Activity of plant PEPC can be regulated, e.g., by de novo synthesis or reversible phosphorylation. The reason for the increase of PEPC activity as a consequence of PVY(NTN) infection was studied. The amount of PEPC determined by Western blot analysis or by relative estimation of PEPC mRNA by real-time PCR did not differ in control and PVY(NTN)-infected plants. Changes in posttranslational modification of PEPC by phosphorylation were evaluated by comparing activity of the native and the dephosphorylated enzyme. The infected plants were characterized by a higher decrease of the enzyme activity after its dephosphorylation, which indicated a higher phosphorylation level. Immunochemical detection of phosphoproteins by Western blot analysis showed a more intensive band corresponding to PEPC from the infected material. This strengthens the hypothesis of an infection-related phosphorylation, which could be part of the plant's response to pathogen attack. The physiological implications of the increase in PEPC activity during PVY(NTN) infection are discussed.

How stable isotopes may help to elucidate primary nitrogen metabolism and its interaction with (photo)respiration in C3 leaves

Intense efforts are currently devoted to elucidate the metabolic networks of plants, in which nitrogen assimilation is of particular importance because it is strongly related to plant growth. In addition, at the leaf level, primary nitrogen metabolism interacts with photosynthesis, day respiration, and photorespiration, simply because nitrogen assimilation needs energy, reductant, and carbon skeletons which are provided by these processes. While some recent studies have focused on metabolomics and genomics of plant leaves, the actual metabolic fluxes associated with nitrogen metabolism operating in leaves are not very well known. In the present paper, it is emphasized that (12)C/(13)C and (14)N/(15)N stable isotopes have proved to be useful tools to investigate such metabolic fluxes and isotopic data are reviewed in the light of some recent advances in this area. Although the potential of stable isotopes remains high, it is somewhat limited by our knowledge of some isotope effects associated with enzymatic reactions. Therefore, this paper should be viewed as a call for more fundamental studies on isotope effects by plant enzymes.

The Importance of the Strictly Conserved, C-terminal Glycine Residue in Phosphoenolpyruvate Carboxylase for Overall Catalysis

Phosphoenolpyruvate carboxylase (PEPC) is a "multifaceted," allosteric enzyme involved in C4 acid metabolism in green plants/microalgae and prokaryotes. Before the elucidation of the three-dimensional structures of maize C4 leaf and Escherichia coli PEPC, our truncation analysis of the sorghum C4 homologue revealed important roles for the enzyme's C-terminal alpha-helix and its appended QNTG961 tetrapeptide in polypeptide stability and overall catalysis, respectively. Collectively, these functional and structural observations implicate the importance of the PEPC C-terminal tetrapeptide for both catalysis and negative allosteric regulation. We have now more finely dissected this element of PEPC structure-function by modification of the absolutely conserved C-terminal glycine of the sorghum C4 isoform by site-specific mutagenesis (G961(A/V/D)) and truncation (DeltaC1/C4). Although the C4 polypeptide failed to accumulate in a PEPC- strain (XH11) of E. coli transformed with the Asp mutant, the other variants were produced at wild-type levels. Although neither of these four mutants displayed an apparent destabilization of the purified PEPC homotetramer, all were compromised catalytically in vivo and in vitro. Functional complementation of XH11 cells under selective growth conditions was restricted progressively by the Ala, DeltaC1 and Val, and DeltaC4 modifications. Likewise, steady-state kinetic analysis of the purified mutant enzymes revealed corresponding negative trends in kcat and kcat/K0.5 (phosphoenolpyruvate) but not in K0.5 or the Hill coefficient. Homology modeling of these sorghum C-terminal variants against the structure of the closely related maize C4 isoform predicted perturbations in active-site molecular cavities and/or ion-pairing with essential, invariant Arg-638. These collective observations reveal that even a modest, neutral alteration of the PEPC C-terminal hydrogen atom side chain is detrimental to enzyme function.

Arabidopsis phosphoenolpyruvate carboxylase genes encode immunologically unrelated polypeptides and are differentially expressed in response to drought and salt stress

The phosphoenolpyruvate carboxylase (PEPC) gene family of Arabidopsis is composed of four genes. Based on sequence analysis it was deduced that Atppc1, Atppc2 and Atppc3 genes encode plant-type PEPCs, whereas Atppc4 encodes a PEPC without phosphorylation motif, but no data at the protein level have been reported. Here, we describe the analysis of the four Arabidopsis PEPC polypeptides, which were expressed in Escherichia coli. Immunological characterization with anti plant-type PEPC and an anti-AtPPC4 antibody, raised in this work, showed that the bacterial-type PEPC is unrelated with plant-type PEPCs. Western-blot analysis of different Arabidopsis organs probed with anti plant-type PEPC antibodies detected a double band, the one with low molecular weight corresponding to the three plant-type PEPCs. The high molecular weight subunit is not encoded by any of the Arabidopsis PEPC genes. No bands were detected with the anti-AtPPC4 antibody. PEPC genes show differential expression in Arabidopsis organs and in response to environmental stress. Atppc2 transcripts were found in all Arabidopsis organs suggesting that it is a housekeeping gene. In contrast, Atppc3 gene was expressed in roots and Atppc1 in roots and flowers, as Atppc4. Highest PEPC activity was found in roots, which showed expression of the four PEPC genes. Salt and drought exerted a differential induction of PEPC gene expression in roots, Atppc4 showing the highest induction in response to both stresses. These results show that PEPC is part of the adaptation of the plant to salt and drought and suggest that this is the function of the new bacterial-type PEPC.

Maize Phosphoenolpyruvate Carboxylase

Phosphoenolpyruvate carboxylases (PEPC, EC 4.1.1.31) from higher plants are regulated by both allosteric effects and reversible phosphorylation. Previous x-ray crystallographic analysis of Zea mays PEPC has revealed a binding site for sulfate ion, speculated to be the site for an allosteric activator, glucose 6-phosphate (Glc-6-P) (Matsumura, H., Xie, Y., Shirakata, S., Inoue, T., Yoshinaga, T., Ueno, Y., Izui, K., and Kai, Y. (2002) Structure (Lond.) 10, 1721-1730). Because kinetic experiments have also supported this notion, each of the four basic residues (Arg-183, -184, -231, and -372' on the adjacent subunit) located at or near the binding site was replaced by Gln, and the kinetic properties of recombinant mutant enzymes were investigated. Complete desensitization to Glc-6-P was observed for R183Q, R184Q, R183Q/R184Q (double mutant), and R372Q, as was a marked decrease in the sensitivity for R231Q. The heterotropic effect of Glc-6-P on an allosteric inhibitor, l-malate, was also abolished, but sensitivity to Gly, another allosteric activator of monocot PEPC, was essentially not affected, suggesting the distinctness of their binding sites. Considering the kinetic and structural data, Arg-183 and Arg-231 were suggested to be involved directly in the binding with phosphate group of Glc-6-P, and the residues Arg-184 and Arg-372 were thought to be involved in making up the site for Glc-6-P and/or in the transmission of an allosteric regulatory signal. Most unexpectedly, the mutant enzymes had almost lost responsiveness to regulatory phosphorylation at Ser-15. An apparent lack of kinetic competition between the phosphate groups of Glc-6-P and of phospho-Ser at 15 suggested the distinctness of their binding sites. The possible roles of these Arg residues are discussed.

Phosphoenolpyruvate carboxylase: a new era of structural biology

There have been remarkable advances in our knowledge of this important enzyme in the last decade. This review focuses on three recent topics: the three-dimensional structure of the protein, molecular mechanisms of catalytic and regulatory functions, and the molecular cloning and characterization of PEPC kinases, which are Ser/Thr kinases involved specifically in regulatory phosphorylation of vascular plant PEPC. Analysis by X-ray crystallography and site-directed mutagenesis for E. coli and maize PEPC identified the catalytic site and allosteric effector binding sites, and revealed the functional importance of mobile loops. We present the reaction mechanism of PEPC in which we assign the roles of individual amino acid residues. We discuss the unique molecular property of PEPC kinase and its possible regulation at the post-translational level.