Molecular and physiological evaluation of transgenic tobacco plants expressing a maize phosphoenolpyruvate carboxylase gene under the control of the cauliflower mosaic virus 35S promoter

Laying the Foundation for Crassulacean Acid Metabolism (CAM) Biodesign: Expression of the C4 Metabolism Cycle Genes of CAM in Arabidopsis

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that exploits a temporal CO2 pump with nocturnal CO2 uptake and concentration to reduce photorespiration, improve water-use efficiency (WUE), and optimize the adaptability of plants to hotter and drier climates. Introducing the CAM photosynthetic machinery into C3 (or C4) photosynthesis plants (CAM Biodesign) represents a potentially breakthrough strategy for improving WUE while maintaining high productivity. To optimize the success of CAM Biodesign approaches, the functional analysis of individual C4 metabolism cycle genes is necessary to identify the essential genes for robust CAM pathway introduction. Here, we isolated and analyzed the subcellular localizations of 13 enzymes and regulatory proteins of the C4 metabolism cycle of CAM from the common ice plant in stably transformed Arabidopsis thaliana. Six components of the carboxylation module were analyzed including beta-carbonic anhydrase (McBCA2), phosphoenolpyruvate carboxylase (McPEPC1), phosphoenolpyruvate carboxylase kinase (McPPCK1), NAD-dependent malate dehydrogenase (McNAD-MDH1, McNAD-MDH2), and NADP-dependent malate dehydrogenase (McNADP-MDH1). In addition, seven components of the decarboxylation module were analyzed including NAD-dependent malic enzyme (McNAD-ME1, McNAD-ME2), NADP-dependent malic enzyme (McNADP-ME1, NADP-ME2), pyruvate, orthophosphate dikinase (McPPDK), pyruvate, orthophosphate dikinase-regulatory protein (McPPDK-RP), and phosphoenolpyruvate carboxykinase (McPEPCK). Ectopic overexpression of most C4-metabolism cycle components resulted in increased rosette diameter, leaf area, and leaf fresh weight of A. thaliana except for McNADP-MDH1, McPPDK-RP, and McPEPCK. Overexpression of most carboxylation module components resulted in increased stomatal conductance and dawn/dusk titratable acidity (TA) as an indirect measure of organic acid (mainly malate) accumulation in A. thaliana. In contrast, overexpression of the decarboxylating malic enzymes reduced stomatal conductance and TA. This comprehensive study provides fundamental insights into the relative functional contributions of each of the individual components of the core C4-metabolism cycle of CAM and represents a critical first step in laying the foundation for CAM Biodesign.

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.

Physiological characteristics and metabolomics of transgenic wheat containing the maize C4 phosphoenolpyruvate carboxylase (PEPC) gene under high temperature stress

In this paper, two transgenic wheat lines, PC27 and PC51, containing the maize PEPC gene and its wild-type (WT) were used as experimental material to study the effects of high temperature on their photosynthetic physiological characteristics and metabolome. The results showed that transgenic wheat lines had higher photosynthetic rate (P _n) than WT under non-stress treatment (NT) and high temperature stress treatment (HT), and more significantly under HT. The change trends of F _v/F _m, Ф _PSII, and q _P were similar to P _n, whereas that of non-photochemical quenching (NPQ) was the opposite. Compared with WT, no differences in chlorophyll content between the transgenic wheat and WT were observed under NT, but two transgenic lines had relatively higher contents than WT under HT. The change trends of Chlorophyll a/b radio, the decreased values of F _m, W_k, and V_j, and the activity of the antioxidant enzyme were consistent with the chlorophyll content. Compared with WT, transgenic wheat lines exhibited lower rate of superoxide anion production, H₂O₂ and malondialdehyde content under HT, and no significant differences were observed under NT. The expression pattern of the ZmPEPC gene and wheat endogenous photosynthesis-related genes were in agreement with that of P _n. Compared with WT, about 13 different metabolites including one organic acid, six amino acids, four sugars, and two polyols were identified under NT; 25 different metabolites including six organic acids, 12 amino acids, four sugars, and three polyols were identified under HT. Collectively, our results indicate that ZmPEPC gene can enhance photochemical and antioxidant enzyme activity, upregulate the expression of photosynthesis-related genes, delay degradation of chlorophyll, change contents of proline and other metabolites in wheat, and ultimately improves its heat tolerance.

Phosphoenolpyruvate carboxylase regulation in C4-PEPC-expressing transgenic rice during early responses to drought stress

Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) has important functions in C4 photosynthesis and biosynthesis of intermediate metabolites. In this study, the drought resistance of C4-PEPC-expressing transgenic rice (Oryza sativa, line PC) plants was assessed using simulated drought conditions [i.e. polyethylene glycol (PEG)-6000 treatment]. The dry weight of PC plants was higher than that of wild-type (WT) plants following treatment with 15% PEG-6000 for 16 days. Furthermore, the water use efficiency, relative water content and proline content in PC plants were higher than those of WT plants, as were C4-PEPC activity and transcript levels following treatment with 5% PEG-6000 for 2 h. The protein kinase activities and transcript levels of sucrose non-fermenting-1-related protein kinases (SnRKs) genes, such as SnRK1a, OsK24 and OsK35 were also higher in PC plants than in WT plants following treatment with 5% PEG-6000 for 2 h. Additionally, phosphoenolpyruvate carboxylase kinase (PPCK, EC 4.1.1.32) activities and transcript levels (e.g. PPCK1 and PPCK2) increased following drought treatment. These changes were regulated by signaling molecules, such as calcium, nitric oxide and hydrogen peroxide. Furthermore, the -1095 to -416 region of the C4-PEPC promoter in PC plants was demethylated following exposure to drought conditions for 1 h. The demethylation coincided with an increase in C4-PEPC expression. Our data suggest that the demethylation of the C4-PEPC promoter and the phosphorylation catalyzed by PPCK have key roles in conferring drought tolerance to the transgenic rice plants.

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.

Pyramiding expression of maize genes encoding phosphoenolpyruvate carboxylase (PEPC) and pyruvate orthophosphate dikinase (PPDK) synergistically improve the photosynthetic characteristics of transgenic wheat

Using particle bombardment transformation, we introduced maize pepc cDNA encoding phosphoenolpyruvate carboxylase (PEPC) and ppdk cDNA encoding pyruvate orthophosphate dikinase (PPDK) into the C3 crop wheat to generate transgenic wheat lines carrying cDNA of pepc (PC lines), ppdk (PK lines) or both (PKC lines). The integration, transcription, and expression of the foreign genes were confirmed by Southern blot, Real-time quantitative reverse transcription PCR (Q-RT-PCR), and Western blot analysis. Q-RT-PCR results indicated that the average relative expression levels of pepc and ppdk in the PKC lines reached 10 and 4.6, respectively, compared to their expressions in untransformed plants (set to 1). The enzyme activities of PEPC and PPDK in the PKC lines were 4.3- and 2.1-fold higher, respectively, than in the untransformed control. The maximum daily net photosynthetic rates of the PKC, PC, and PK lines were enhanced by 26.4, 13.3, and 4.5%, respectively, whereas the diurnal accumulations of photosynthesis were 21.3, 13.9, and 6.9%, respectively, higher than in the control. The Fv/Fm of the transgenic plants decreased less than in the control under high temperature and high light conditions (2 weeks after anthesis), suggesting that the transgenic wheat transports more absorbed light energy into a photochemical reaction. The exogenous maize C4-specific pepc gene was more effective than ppdk at improving the photosynthetic performance and yield characteristics of transgenic wheat, while the two genes showed a synergistic effect when they were transformed into the same genetic background, because the PKC lines exhibited improved photosynthetic and physiological traits.

Strategies for engineering C(4) photosynthesis

C(3) photosynthesis is an inefficient process, because the enzyme that lies at the heart of the Benson-Calvin cycle, ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) is itself a very inefficient enzyme. The oxygenase activity of Rubisco is an unavoidable side reaction that is a consequence of its reaction mechanism. The product of oxygenation, glycollate 2-P, has to be retrieved by photorespiration, a process which results in the loss of a quarter of the carbon that was originally present in glycollate 2-P. Photorespiration therefore reduces carbon gain. Purely in terms of carbon economy, there is, therefore, a strong selection pressure on plants to reduce the rate of photorespiration so as to increase carbon gain, but it also improves water- and nitrogen-use efficiency. Possibilities for the manipulation of plants to decrease the amount of photorespiration include the introduction of improved Rubisco from other species, reconfiguring photorespiration, or introducing carbon-concentrating mechanisms, such as inorganic carbon transporters, carboxysomes or pyrenoids, or engineering a full C(4) Kranz pathway using the existing evolutionary progression in C(3)-C(4) intermediates as a blueprint. Possible routes and progress to suppressing photorespiration by introducing C(4) photosynthesis in C(3) crop plants will be discussed, including whether single cell C(4) photosynthesis is feasible, how the evolution of C(3)-C(4) intermediates can be used as a blueprint for engineering C(4) photosynthesis, which pathway for the C(4) cycle might be introduced and the extent to which processes and structures in C(3) plant might require optimisation.

Engineering nitrogen use efficient crop plants: the current status

In the last 40 years the amount of synthetic nitrogen (N) applied to crops has risen drastically, resulting in significant increases in yield but with considerable impacts on the environment. A requirement for crops that require decreased N fertilizer levels has been recognized in the call for a 'Second Green Revolution' and research in the field of nitrogen use efficiency (NUE) has continued to grow. This has prompted a search to identify genes that improve the NUE of crop plants, with candidate NUE genes existing in pathways relating to N uptake, assimilation, amino acid biosynthesis, C/N storage and metabolism, signalling and regulation of N metabolism and translocation, remobilization and senescence. Herein is a review of the approaches taken to determine possible NUE candidate genes, an overview of experimental study of these genes as effectors of NUE in both cereal and non-cereal plants and the processes of commercialization of enhanced NUE crop plants. Patents issued regarding increased NUE in plants as well as gene pyramiding studies are also discussed as well as future directions of NUE research.

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.

A novel mechanism of allosteric regulation of archaeal phosphoenolpyruvate carboxylase: a combined approach to structure-based alignment and model assessment

Phosphoenolpyruvate carboxylase (PEPC) catalyzes the irreversible carboxylation of phosphoenolpyruvate (PEP) and plays a crucial role in fixing atmospheric CO(2) in C(4) and CAM plants. The enzyme is widespread in plants and bacteria and mostly regulated allosterically by both positive and negative effectors. Archaeal PEPCs (A-PEPCs) have unique characteristics in allosteric regulation and molecular mass, distinct from their bacterial and eukaryote homologues, and their amino acid sequences have become available only recently. In this paper, we generated a structure-based alignment of archaeal, bacterial and eukaryote PEPCs and built comparative models using a combination of fold recognition, sequence and structural analysis tools. Our comparative modeling analysis identified A-PEPC-specific strong interactions between the two loops involved in both allostery and catalysis, which explained why A-PEPC is not influenced by any allosteric activators. We also found that the side-chain located three residues before the C-terminus appears to play a key role in determining the sensitivity to allosteric inhibitors. In addition to these unique features, we revealed how archaeal, bacterial and eukaryote PEPCs would share a common catalytic mechanism and adopt a similar mode of tetramer formation, despite their divergent sequences. Our novel observations will help design more efficient molecules for ecological and industrial use.

Metabolic engineering with Dof1 transcription factor in plants: Improved nitrogen assimilation and growth under low-nitrogen conditions

Utilization of transcription factors might be a powerful approach to modification of metabolism for a generation of crops having superior characteristics because a single transcription factor frequently regulates coordinated expression of a set of key genes for respective pathways. Here, we apply the plant-specific Dof1 transcription factor to improve nitrogen assimilation, the essential metabolism including the primary assimilation of ammonia to carbon skeletons to biosynthesize amino acids and other organic compounds involving nitrogen in plants. Expressing Dof1 induced the up-regulation of genes encoding enzymes for carbon skeleton production, a marked increase of amino acid contents, and a reduction of the glucose level in transgenic Arabidopsis. The results suggest cooperative modification of carbon and nitrogen metabolisms on the basis of their intimate link. Furthermore, elementary analysis revealed that the nitrogen content increased in the Dof1 transgenic plants (approximately 30%), indicating promotion of net nitrogen assimilation. Most significantly, the Dof1 transgenic plants exhibit improved growth under low-nitrogen conditions, an agronomically important trait. These results highlight the great utility of transcription factors in engineering metabolism in plants.

Metabolic consequences of overproduction of phosphoenolpyruvate carboxylase in C3 plants

Phosphoenolpyruvate carboxylase (PEPC) has a variety of functions in plants, including a major anaplerotic role in replenishing the tricarboxylic acid cycle with intermediates to meet the demand of carbon skeletons for synthesis of organic acids and amino acids. Various transgenic C3 plants that overproduce PEPC have been produced and analyzed in detail. The results indicate that foreign PEPC is under the control of the regulatory mechanisms intrinsic to the host plant and down-regulated so as not to cause detrimental metabolic effects, although the anaplerotic reaction is slightly enhanced by the foreign PEPC. By use of foreign PEPCs that can avert such regulation, metabolic flow is largely directed toward synthesis of organic acids and amino acids. Observations with transgenic C3 plants also shed light on the interrelation among various metabolic pathways inside the cell.

Crystal structures of C4 form maize and quaternary complex of E. coli phosphoenolpyruvate carboxylases

Phosphoenolpyruvate carboxylase (PEPC) catalyzes the first step in the fixation of atmospheric CO(2) during C(4) photosynthesis. The crystal structure of C(4) form maize PEPC (ZmPEPC), the first structure of the plant PEPCs, has been determined at 3.0 A resolution. The structure includes a sulfate ion at the plausible binding site of an allosteric activator, glucose 6-phosphate. The crystal structure of E. coli PEPC (EcPEPC) complexed with Mn(2+), phosphoenolpyruvate analog (3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate), and an allosteric inhibitor, aspartate, has also been determined at 2.35 A resolution. Dynamic movements were found in the ZmPEPC structure, compared with the EcPEPC structure, around two loops near the active site. On the basis of these molecular structures, the mechanisms for the carboxylation reaction and for the allosteric regulation of PEPC are proposed.

An engineered phosphoenolpyruvate carboxylase redirects carbon and nitrogen flow in transgenic potato plants

Phosphoenolpyruvate carboxylase (PEPC) plays a central role in the anaplerotic provision of carbon skeletons for amino acid biosynthesis in leaves of C3 plants. Furthermore, in both C4 and CAM plants photosynthetic isoforms are pivotal for the fixation of atmospheric CO2. Potato PEPC was mutated either by modifications of the N-terminal phosphorylation site or by an exchange of an internal cDNA segment for the homologous sequence of PEPC from the C4 plant Flaveria trinervia. Both modifications resulted in enzymes with lowered sensitivity to malate inhibition and an increased affinity for PEP. These effects were enhanced by a combination of both mutated sequences and pulse labelling with 14CO2 in vivo revealed clearly increased fixation into malate for this genotype. Activity levels correlated well with protein levels of the mutated PEPC. Constitutive overexpression of PEPC carrying both N-terminal and internal modifications strongly diminished plant growth and tuber yield. Metabolite analysis showed that carbon flow was re-directed from soluble sugars and starch to organic acids (malate) and amino acids, which increased four-fold compared with the wild type. The effects on leaf metabolism indicate that the engineered enzyme provides an optimised starting point for the installation of a C4-like photosynthetic pathway in C3 plants.

Overexpression of C(4)-cycle enzymes in transgenic C(3) plants: a biotechnological approach to improve C(3)-photosynthesis

The process of photorespiration diminishes the efficiency of CO(2) assimilation and yield of C(3)-crops such as wheat, rice, soybean or potato, which are important for feeding the growing world population. Photorespiration starts with the competitive inhibition of CO(2) fixation by O(2) at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and can result in a loss of up to 50% of the CO(2) fixed in ambient air. By contrast, C(4) plants, such as maize, sugar cane and Sorghum, possess a CO(2) concentrating mechanism, by which atmospheric CO(2) is bound to C(4)-carbon compounds and shuttled from the mesophyll cells where the prefixation of bicarbonate occurs via phosphoenolpyruvate carboxylase (PEPC) into the gas-tight bundle-sheath cells, where the bound carbon is released again as CO(2) and enters the Calvin cycle. However, the anatomical division into mesophyll and bundle-sheaths cells ("Kranz"-anatomy) appears not to be a prerequisite for the operation of a CO(2) concentrating mechanism. Submerged aquatic macrophytes, for instance, can induce a C(4)-like CO(2) concentrating mechanism in only one cell type when CO(2) becomes limiting. A single cell C(4)-mechanism has also been reported recently for a terrestrial chenopod. For over 10 years researchers in laboratories around the world have attempted to improve photosynthesis and crop yield by introducing a single cell C(4)-cycle in C(3) plants by a transgenic approach. In the meantime, there has been substantial progress in overexpressing the key enzymes of the C(4) cycle in rice, potato, and tobacco. In this review there will be a focus on biochemical and physiological consequences of the overexpression of C(4)-cycle genes in C(3) plants. Bearing in mind that C(4)-cycle enzymes are also present in C(3) plants, the pitfalls encountered when C(3) metabolism is perturbed by the overexpression of individual C(4) genes will also be discussed.

Introduction of genes encoding C4 photosynthesis enzymes into rice plants: physiological consequences

Transgenic rice plants expressing the maize phosphoeno/pyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) exhibit a higher photosynthetic capacity (up to 35%) than untransformed plants. The increased photosynthetic capacity in these plants is mainly associated with an enhanced stomatal conductance and a higher internal CO2 concentration. Plants simultaneously expressing high levels of both enzymes also have a higher photosynthetic capacity. The results suggest that both PEPC and PPDK play a key role in organic acid metabolism in the guard cells to regulate stomatal opening. Under photoinhibitory and photooxidative conditions, PEPC transgenic rice plants are capable of maintaining a higher photosynthetic rate, a higher photosynthetic quantum yield by PSII and a higher capacity to dissipate excess energy photochemically and non-photochemically than untransformed plants. Preliminary data from field trials show that relative to untransformed plants, the grain yield is about 10-20% higher in selected PEPC and 30-35% higher in PPDK transgenic rice plants, due to increased tiller number. Taken together, these results suggest that introduction of C4 photosynthesis enzymes into rice has a good potential to enhance its tolerance to stress, photosynthetic capacity and yield.

Single and double overexpression of C(4)-cycle genes had differential effects on the pattern of endogenous enzymes, attenuation of photorespiration and on contents of UV protectants in transgenic potato and tobacco plants

To improve the efficiency of CO(2) fixation in C(3) photosynthesis, C(4)-cycle genes were overexpressed in potato and tobacco plants either individually or in combination. Overexpression of the phosphoenolpyruvate carboxylase (PEPC) gene (ppc) from Corynebacterium glutamicum (cppc) or from potato (stppc, deprived of the phosphorylation site) in potato resulted in a 3-6-fold induction of endogenous cytosolic NADP malic enzyme (ME) and an increase in the activities of NAD-ME (3-fold), NADP isocitrate dehydrogenase (ICDH), pyruvate kinase (PK), NADP glycerate-3-P dehydrogenase (NADP-GAPDH), and PEP phosphatase (PEPP). In double transformants overexpressing cppc and chloroplastic NADP-ME from Flaveria pringlei (fpMe1), cytosolic NADP-ME was less induced and pleiotropic effects were diminished. There were no changes in enzyme pattern in single fpMe1 overexpressors. In cppc overexpressors of tobacco, the increase in endogenous cytosolic NADP-ME activity was small and changes in other enzymes were less pronounced. Determinations of the CO(2) compensation point (Gamma*) as well as temperature and oxygen effects on photosynthesis produced variational data suggesting that the desired decline in photorespiration occurred only under certain experimental conditions. Double transformants of potato (cppc/fpMe1) exhibited the most consistent attenuating effect on photorespiration. In contrast, photorespiration in tobacco plants appeared to be diminished most in single cppc overexpressors rather than in double transformants (cppc/fpMe1). In tobacco, introduction of the PEP carboxykinase (PEPCK) gene from the bacterium Sinorhizobium meliloti (pck) had little effect on photosynthetic parameters in single (pck) and double transformants (cppc/pck). In transgenic potato plants, increased PEPC activities resulted in a decline in UV protectants (flavonoids) in single cppc or stppc transformants, but not in double transformants (cppc/fpMe1). PEP provision to the shikimate pathway inside the plastids, from which flavonoids derive, might be restricted only in single PEPC overexpressors.

MOLECULAR ENGINEERING OF C4 PHOTOSYNTHESIS

The majority of terrestrial plants, including many important crops such as rice, wheat, soybean, and potato, are classified as C3 plants that assimilate atmospheric CO2 directly through the C3 photosynthetic pathway. C4 plants such as maize and sugarcane evolved from C3 plants, acquiring the C4 photosynthetic pathway to achieve high photosynthetic performance and high water- and nitrogen-use efficiencies. The recent application of recombinant DNA technology has made considerable progress in the molecular engineering of C4 photosynthesis over the past several years. It has deepened our understanding of the mechanism of C4 photosynthesis and provided valuable information as to the evolution of the C4 photosynthetic genes. It also has enabled us to express enzymes involved in the C4 pathway at high levels and in desired locations in the leaves of C3 plants for engineering of primary carbon metabolism.

High level expression of C4-specific NADP-malic enzyme in leaves and impairment of photoautotrophic growth in a C3 plant, rice

The chloroplastic NADP-malic enzyme (NADP-ME) is a key enzyme of the C4 photosynthesis pathway in NADP-ME type C4 plants such as maize. To express the chloroplastic NADP-ME in leaves of a C3 plant, rice, full-length cDNAs encoding the rice C3-specific isoform and the maize C4-specific isoform of the enzyme were expressed under the control of the rice CAB: promoter. Transformants carrying the rice cDNA showed the NADP-ME activities in the leaves less than several-fold that of non-transformants, while those carrying the maize cDNA showed activities up to 30-fold that of non-transformants or about 60% of the NADP-ME activity of maize leaves. These results indicate that expression of the rice C3-specific NADP-ME is suppressed at co- and/or post-transcriptional levels by some regulation mechanisms intrinsic to rice, while that of the foreign C4-specific isoform can escape from such suppression. The accumulation of the maize C4-specific NADP-ME led to bleaching of leaf color and growth hindrance in rice plants under natural light. These deteriorative effects resulted from enhanced photoinhibition of photosynthesis due to an increase in the level of NADPH inside the chloroplast by the action of the maize enzyme.

Changes in photosynthetic carbon flow in transgenic rice plants that express C4-type phosphoenolpyruvate carboxykinase from Urochloa panicoides

A cDNA encoding phosphoenolpyruvate carboxykinase (PCK) of Urochloa panicoides (a PCK-type C4 plant) was expressed in rice (Oryza sativa cv Tsukinohikari) plants under the control of the promoter of a maize (Zea mays) gene for phosphoenolpyruvate carboxylase or pyruvate, orthophosphate dikinase with the transit peptide of the small subunit of Rubisco. Crude extracts prepared from the green leaves of transgenic plants had high PCK activity and the newly expressed PCK was localized in chloroplasts. In labeling experiments with (14)CO(2) up to 20% of the radioactivity was incorporated into 4C compounds (malate, oxaloacetate, and aspartate) in excised leaves of transgenic plants, as compared with about 1% in excised leaves of control plants. There was a positive correlation between PCK activity and the extent of labeling of 4C compounds. When L-[4-(14)C]malate was fed to excised leaves the extent of incorporation of radioactivity into sucrose was 3-fold greater in transgenic plants than in control plants and the level of radiolabeled aspartate was significantly lower in transgenic plants. These results indicate that the ectopic expression of PCK in rice chloroplasts was able partially to change the carbon flow in mesophyll cells into a C4-like photosynthetic pathway. Such a strategy appears to provide a possible method for enhancing the photosynthetic capacity of C3 plants.

High-level expression of maize phosphoenolpyruvate carboxylase in transgenic rice plants

Using an Agrobacterium-mediated transformation system, we have introduced the intact gene of maize phosphoenolpyruvate carboxylase (PEPC), which catalyzes the initial fixation of atmospheric CO2 in C4 plants into the C3 crop rice. Most transgenic rice plants showed high-level expression of the maize gene; the activities of PEPC in leaves of some transgenic plants were two- to threefold higher than those in maize, and the enzyme accounted for up to 12% of the total leaf soluble protein. RNA gel blot and Southern blot analyses showed that the level of expression of the maize PEPC in transgenic rice plants correlated with the amount of transcript and the copy number of the inserted maize gene. Physiologically, the transgenic plants exhibited reduced O2 inhibition of photosynthesis and photosynthetic rates comparable to those of untransformed plants. The results demonstrate a successful strategy for installing the key biochemical component of the C4 pathway of photosynthesis in C3 plants.

Effects of altered phosphoenolpyruvate carboxylase activities on transgenic C3 plant Solanum tuberosum

Phosphoenolpyruvate carboxylase (PEPC) genes from Corynebacterium glutamicum (cppc), Escherichia coli (eppc) or Flaveria trinervia (fppc) were transferred to Solanum tuberosum. Plant regenerants producing foreign PEPC were identified by Western blot analysis. Maximum PEPC activities measured in eppc and fppc plants grown in the greenhouse were doubled compared to control plants. For cppc a transgenic plant line could be selected which exhibited a fourfold increase in PEPC activity. In the presence of acetyl-CoA, a known activator of the procaryotic PEPC, a sixfold higher activity level was observed. In cppc plants grown in axenic culture PEPC activities were even higher. There was a 6-fold or 12-fold increase in the PEPC activities compared to the controls measured in the absence or presence of acetyl-CoA, respectively. Comparable results were obtained by transient expression in Nicotiana tabacum protoplasts. PEPC of C. glutamicum (PEPC C.g.) in S. tuberosum leaf extracts displays its characteristic K(m) (PEP) value. Plant growth was examined with plants showing high expression of PEPC and, moreover, with a plant cell line expressing an antisense S. tuberosum (anti-sppc) gene. In axenic culture the growth rate of a cppc plant cell line was appreciably diminished, whereas growth rates of an anti-sppc line were similar or slightly higher than in controls. Malate levels were increased in cppc plants and decreased in antisense plants. There were no significant differences in photosynthetic electron transport or steady state CO2 assimilation between control plants and transformants overexpressing PEPC C.g. or anti-sppc plants. However, a prolonged dark treatment resulted in a delayed induction of photosynthetic electron transport in plants with less PEPC. Rates of CO2 release in the dark determined after a 45 min illumination period at a high proton flux density were considerably enhanced in cppc plants and slightly diminished in anti-sppc plants. When CO2 assimilation rates were corrected for estimated rates of mitochondrial respiration in the light, the electron requirement for CO2 assimilation determined in low CO2 was slightly lower in transformants with higher PEPC, whereas transformants with decreased PEPC exhibited an appreciably elevated electron requirement. The CO2 compensation point remained unchanged in plants (cppc) with high PEPC activity, but might be increased in an antisense plant cell line. Stomatal opening was delayed in antisense plants, but was accelerated in plants overexpressing PEPC C.g. compared to the controls.

PHOSPHOENOLPYRUVATE CARBOXYLASE: A Ubiquitous, Highly Regulated Enzyme in Plants

Since plant phosphoenolpyruvate carboxylase (PEPC) was last reviewed in the Annual Review of Plant Physiology over a decade ago (O'Leary 1982), significant advances have been made in our knowledge of this oligomeric, cytosolic enzyme. This review highlights this exciting progress in plant PEPC research by focusing on the three major areas of recent investigation: the enzymology of the protein; its posttranslational regulation by reversible protein phosphorylation and opposing metabolite effectors; and the structure, expression, and molecular evolution of the nuclear PEPC genes. It is hoped that the next ten years will be equally enlightening, especially with respect to the three-dimensional structure of the plant enzyme, the molecular analysis of its highly regulated protein-Ser/Thr kinase, and the elucidation of its associated signal-transduction pathways in various plant cell types.