The regulation of phosphoenolpyruvate carboxylase in CAM plants

Expression, purification, and initial characterization of a recombinant form of plant PEP-carboxylase kinase from CAM-induced Mesembryanthemum crystallinum with enhanced solubility in Escherichia coli

Plant phosphoenolpyruvate-carboxylase kinase (PEPC-kinase [PpcK]) is the smallest Ser/Thr kinase identified to date, having a molecular mass of approximately 32,000. This novel, monomeric kinase is dedicated to the phosphorylation of plant PEPC, thereby regulating this target enzyme's activity and allosteric properties. Although several recombinant, non-fusion PpcK proteins have been produced recently in Escherichia coli, these are plagued by their high degree of insolubility. Here, we report the use of the native, E. coli NusA protein and a related E. coli expression vector (pET-43a(+) [Novagen]) for enhancing the solubility of this recalcitrant Ser/Thr kinase at least 10-fold by its production as a dual 6xHis-tagged NusA/McPpcK1 fusion protein, which accounts for approximately 10% of the soluble protein fraction from induced cells. Capture of this fusion protein from the centrifuged cell extract by immobilized metal (Ni(2+)) affinity-chromatography, its "on-bead" cleavage by thrombin, and subsequent elution yielded milligram quantities of a "free," approximately 36-kDa form of PpcK for further purification by fast-protein liquid chromatography on blue dextran-agarose or preparative SDS-PAGE. Steady-state kinetic analysis of the former, active preparation revealed that this dedicated kinase discriminates against neither various isoforms of plant PEPC nor certain mutant forms of recombinant C(4) PEPC. Alternatively, the latter, electrophoretically homogeneous sample of the approximately 36-kDa polypeptide was used as antigen for polyclonal-antibody production in rabbits. The antibodies against the recombinant McPpcK1 from Mesembryanthemum crystallinum cross-reacted on Western blots with an enriched preparation of the maize-leaf kinase, but not with the parent crude extract, thus directly documenting this protein's extremely low abundance in vivo. However, these antibodies were effective in immunoprecipitating 32P-based PpcK activity from crude, desalted extracts of maize leaves and soybean root-nodules.

Identification and expression of a soybean nodule-enhanced PEP-carboxylase kinase gene (NE-PpcK) that shows striking up-/down-regulation in vivo

Various isoforms of plant phosphoenolpyruvate carboxylase (PEPC (Ppc)) are controlled post-translationally by an intricate interaction between allosteric regulation and reversible protein phosphorylation. In leaves and root nodules of legumes, these changes in PEPC phosphorylation state are governed primarily by PEPC-kinase (PpcK), a novel, 'minimal but functional' Ser/Thr kinase. To date, this plant-specific kinase has been investigated in molecular terms exclusively in non-leguminous plants, such as Crassulacean-acid-metabolism (CAM) species and Arabidopsis. As an important extension of our earlier biochemical studies on this dedicated kinase and PEPC phosphorylation in soybean (Glycine max) nodules, we now report the molecular cloning of the first legume PpcK from a soybean nodule cDNA library, which encodes a functional, 31.0 kDa PpcK polypeptide. Besides displaying organ, developmental, and spatial expression properties that are strikingly up-regulated in mature nodules, the expression pattern of this transcript is distinct from that of a second soybean PpcK isogene (GmPpcK). The steady-state abundance of this former, nodule-enhanced transcript (NE-PpcK) is markedly influenced by photosynthate supply from the shoots. This latter up-/down-regulation of NE-PpcK transcript level occurs in vivo in concert with the corresponding changes in the nodule PpcK activity, the phosphorylation-state of PEPC, and the abundance of a previously identified, nodule-enhanced transcript (GmPEPC7) that encodes the target enzyme (NE-Ppc). Furthermore, genomic Southern analysis and inspection of the public database indicate that there are at least three distinct PpcK and Ppc isogenes in soybean. Collectively, these and recent findings with Arabidopsis implicate the existence of multiple PpcK-Ppc'expression-partners' in plants, exemplified by NE-PpcK and NE-Ppc in the soybean nodule.

Identification of rhythmic subsystems in the circadian cycle of crassulacean acid metabolism under thermoperiodic perturbations

Leaves of the Crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perrier de la Bâthie show overt circadian rhythms in net CO2 uptake, leaf conductance to water and intercellular CO2 concentration, which are entrained by periodic temperature cycles. To probe their sensitivity to thermoperiodic perturbations, intact leaves were exposed to continuous light intensity and temperature cycles with a period of 16 h, applying a set of different baseline temperatures and thermodriver amplitudes. All three overt rhythms were analyzed with respect to their frequency spectra and their phase relations with the thermodriver. For most stimulation protocols, stomatal conductance and net CO2 change were fully or partially entrained by the temperature pulses, while the internal CO2 concentration remained dominated by oscillations in the circadian range. Prolonged time series recorded for up to 22 d in continuous light underline the robustness of these circadian oscillations. This suggests that the overt circadian rhythm of net CO2 uptake in CAM results from the interaction of two coupled original systems: (i) an endogenous cycle of CO2 fixation in the mesophyll, showing very robust periodic activity, and (ii) stomatal movements that respond to environmental stimuli independently of rhythmic processes in the mesophyll, and thus modulate the gas exchange amplitude.

Abiotic stresses affecting water balance induce phosphoenolpyruvate carboxylase expression in roots of wheat seedlings

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) plays an important role in CO(2) fixation in C4 and CAM plants. In C3 plants, PEPC is widely expressed in most organs; however, its function is not yet clearly established. With the aim of providing clues on the function of PEPC in C3 plants, we have analyzed its pattern of expression in wheat ( Triticum aestivum L.) seedlings. Roots showed almost double the level of PEPC activity of shoots. Further analysis of PEPC expression in roots by in situ localization techniques showed a high accumulation of PEPC transcripts and polypeptides in meristematic cells, whereas in the rest of the root PEPC localized preferentially to the vascular tissue. Treatment with NaCl and LiCl induced PEPC expression in roots. Similarly, other abiotic stresses affecting water status, such as drought or cold, induced PEPC expression. Induction was root-specific except for the cold treatment, which also induced PEPC in shoots, although to a lesser extent. In contrast, hypoxia, which does not affect water balance, did not promote any induction of PEPC expression. These results suggest an important role for this enzyme in the adaptation of plants to environmental changes.

Characterization and expression analysis of genes encoding phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxylase kinase of Lotus japonicus, a model legume

Phosphoenolpyruvate carboxylases (PEPCs), one form of which in each legume species plays a central role in the carbon metabolism in symbiotic root nodules, are activated through phosphorylation of a conserved residue by a specific protein kinase (PEPC-PK). We characterized the cDNAs for two PEPC isoforms of Lotus japonicus, an amide-translocating legume that forms determinate nodules. One gene encodes a nodule-enhanced form, which is more closely related to the PEPCs in amide-type indeterminate nodules than those in ureide-type determinate nodules. The other gene is expressed in shoots and roots at a low level. Both forms have the putative phosphorylation site, Ser11. We also isolated a cDNA and the corresponding genomic DNA for PEPC-PK of L. japonicus. The recombinant PEPC-PK protein expressed in Escherichia coli phosphorylated recombinant maize C4-form PEPC efficiently in vitro. The level of mRNA for PEPC-PK was high in root nodules, and those in shoots and roots were also significant. In situ hybridization revealed that the expression patterns of the transcripts for PEPC and PEPC-PK were similar in mature root nodules, but were different in emerging nodules. When L. japonicus seedlings were subjected to prolonged darkness and subsequent illumination, the activity of PEPC-PK and the mRNA levels of both PEPC and PEPC-PK in nodules decreased and then recovered, suggesting that they are regulated according to the amounts of photosynthates transported from shoots.

Photosynthetic flexibility in Pedilanthus tithymaloides poit, a CAM plant

The induction of CAM in Pedilanthus tithymaloides (Euphorbiaceae) under water-limited conditions was evaluated by following diurnal oscillations of CO2 fixation, titratable acidity and malic acid content in the leaf extracts. CAM induction was assessed by measuring the activities of phosphoenolpyruvate carboxylase (PEPC), NADH-malate dehydrogenase (MDH) and phosphoenolpyruvate caroxykinase (PEPCK) in the leaves as well. Drought resulted in large increases in the nocturnal acid accumulation and rates of CO2 uptake in the leaves of P. tithymaloides. The drought-induced CAM activity tended to be reversible after re-watering. Nevertheless, under well-watered conditions, plants of P. tithymaloides showed day time CO2 uptake patterns with less pronounced diurnal oscillations of organic acids. Our data indicate that although P. tithymaloides is a CAM plant, environmental variables like drought induce photosynthetic flexibility in this species. This type of plasticity in CAM and metabolic versatility in P. tithymaloides should be an adaptation for prolonged survival under natural adverse edaphic and microclimate situations.

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.

Crassulacean acid metabolism: plastic, fantastic

The occurrence, activity and plasticity of the CAM pathway is described from an introductory viewpoint, framed by the use of the four "Phases" of CAM as comparative indicators of the interplay between environmental constraints and internal molecular and biochemical regulation. Having described a number of "rules" which seem to govern the CAM cycle and apply uniformly to most species, a number of key regulatory points can then be identified. These include temporal separation of carboxylases, based on the circadian expression of key genes and their control by metabolites. The role of a circadian oscillator and interplay between tonoplast and nuclear control are central to maintaining the CAM cycle. Control of reserve carbohydrates is often neglected, but the importance of daily partitioning (for growth and the subsequent night-time CAM activity) and use at night is shown to drive the CAM cycle. Finally, it is shown that the genotypic and phenotypic plasticity in patterns of CAM expression is mediated partly by environmental conditions and molecular signalling, but also by diffusive constraints in succulent tissues. A transformation system is now required to allow these key areas of control to be elucidated.

The Arabidopsis circadian system

Rhythms with periods of approximately 24 hr are widespread in nature. Those that persist in constant conditions are termed circadian rhythms and reflect the activity of an endogenous biological clock. Plants, including Arabidopsis, are richly rhythmic. Expression analysis, most recently on a genomic scale, indicates that the Arabidopsis circadian clock regulates a number of key metabolic pathways and stress responses. A number of sensitive and high-throughput assays have been developed to monitor the Arabidopsis clock. These assays have facilitated the identification of components of plant circadian systems through genetic and molecular biological studies. Although much remains to be learned, the framework of the Arabidopsis circadian system is coming into focus.DedicationThis review is dedicated to the memory of DeLill Nasser, a wonderful mentor and an unwavering advocate of both Arabidopsis and circadian rhythms research.

Induction of Crassulacean acid metabolism by water limitation

Crassulacean acid metabolism (CAM), a key adaptation of photosynthetic carbon fixation to limited water availability, is characterized by nocturnal CO2 fixation and daytime CO2 re-assimilation, which generally results in improved water-use efficiency. However, CAM plants display a remarkable degree of photosynthetic plasticity within a continuum of diel gas exchange patterns. Genotypic, ontogenetic and environmental factors combine to govern the extent to which CAM is expressed. The ecological diversity of CAM is mirrored by plasticity in a range of biochemical and physiological attributes. In C3/CAM-intermediate plants, limited water availability can induce or enhance the expression of CAM. CAM induction is controlled by a combination of transcriptional, post-transcriptional and post-translational regulatory events. Early events in CAM induction point to a requirement for calcium and calcium-dependent protein kinase activities. Gene discovery efforts, improved transformation technologies and genetic models for CAM plants, coupled with detailed physiological investigations, will lead to new insights into the molecular genetic basis of induction processes and the circadian oscillator that governs carbon flux during CAM. Future integration of genomic, biochemical and physiological approaches in selected CAM models promise to provide a detailed view of the complex regulatory dynamics involved in CAM induction and modulation by water deficit. Such information is expected to have broad significance as the ecological and agricultural importance of CAM species increases in the face of global warming trends and the associated expansion of desertification in semi-arid regions around the world.

Phosphoenolpyruvate carboxylase kinase involved in C(4) photosynthesis in Flaveria trinervia: cDNA cloning and characterization

In C(4) plants, phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), a key enzyme in C(4) photosynthesis, is controlled by reversible phosphorylation of a conserved Ser residue near the N-terminus. We now report the first cloning of a cDNA from a C(4) plant, Flaveria trinervia, which encodes the specific protein kinase (FtPEPC-PK) involved in the phosphorylation of C(4)-form PEPC. Several lines of supportive evidence are: strict substrate specificity of the recombinant enzyme, prominent light/dark response of the transcript level and abundant expression in leaves of C(4) plant (F. trinervia) but very low expression in a C(3) plant of the same genus (Flaveria pringlei). We also discuss the possibility that the FtPEPC-PK gene has co-evolved with the PEPC gene to participate in C(4) photosynthesis.

Spatiotemporal variation of metabolism in a plant circadian rhythm: the biological clock as an assembly of coupled individual oscillators

The complex dynamic properties of biological timing in organisms remain a central enigma in biology despite the increasingly precise genetic characterization of oscillating units and their components. Although attempts to obtain the time constants from oscillations of gene activity and biochemical units have led to substantial progress, we are still far from a full molecular understanding of endogenous rhythmicity and the physiological manifestations of biological clocks. Applications of nonlinear dynamics have revolutionized thinking in physics and in biomedical and life sciences research, and spatiotemporal considerations are now advancing our understanding of development and rhythmicity. Here we show that the well known circadian rhythm of a metabolic cycle in a higher plant, namely the crassulacean acid metabolism mode of photosynthesis, is expressed as dynamic patterns of independently initiated variations in photosynthetic efficiency (phi(PSII)) over a single leaf. Noninvasive highly sensitive chlorophyll fluorescence imaging reveals randomly initiated patches of varying phi(PSII) that are propagated within minutes to hours in wave fronts, forming dynamically expanding and contracting clusters and clearly dephased regions of phi(PSII). Thus, this biological clock is a spatiotemporal product of many weakly coupled individual oscillators, defined by the metabolic constraints of crassulacean acid metabolism. The oscillators operate independently in space and time as a consequence of the dynamics of metabolic pools and limitations of CO(2) diffusion between tightly packed cells.

Phosphoenolpyruvate carboxylase kinase is controlled by a similar signaling cascade in CAM and C(4) plants

In Crassulacean acid metabolism (CAM) plants, phosphoenolpyruvate carboxylase (PEPC) is subject to day-night regulatory phosphorylation of a conserved serine residue in the plant enzyme's N-terminal domain. The dark increase in PEPC-kinase (PEPC-k) activity is under control of a circadian oscillator, via the enhanced expression of the corresponding gene (1). The signaling cascade leading to PEPC-k up-regulation was investigated in leaves and mesophyll cell protoplasts of the facultative, salt-inducible CAM species, Mesembryanthemum crystallinum. Mesophyll cell protoplasts had the same PEPC-k activity as leaves from which they were prepared (i.e., high at night, low during the day). However, unlike C(4) protoplasts (2), CAM protoplasts did not show marked PEPC-k up-regulation when isolated during the day and treated with a weak base such as NH(4)Cl. Investigations using various pharmacological reagents established the operation, in the darkened CAM leaf, of a PEPC-k cascade including the following components: a phosphoinositide-dependent phospholipase C (PI-PLC), inositol 1,4,5 P (IP(3))-gated tonoplast calcium channels, and a putative Ca(2+)/calmodulin protein kinase. These results suggest that a similar signaling machinery is involved in both C(4) (2, 3) and CAM plants to regulate PEPC-k activity, the phosphorylation state of PEPC, and, thus, carbon flux through this enzyme during CAM photosynthesis.

The CDPK superfamily of protein kinases

The CDPK superfamily consists of six types of protein kinases, which differ in the regulatory domains they contain. CDPKs (calcium-dependent protein kinases or calmodulin-like domain protein kinases) are activated by the binding of calcium to their calmodulin-like regulatory domains. The carboxyl terminal domains of CRKs (CDPK-related kinases) have sequence similarity to the regulatory domains of CDPKs, but do not bind calcium. PPCKs (PEP carboxylase kinases) contain only a catalytic domain. PRKs (PPCK-related kinases) have a carboxyl-terminal domain that has no similarity to that of any other member of the superfamily. CCaMKs (calcium and calmodulin regulated kinases) bind both calcium ions and the calcium/calmodulin complex, whereas CaMKs (calmodulin-dependent protein kinases) bind the calcium/calmodulin complex, but not calcium. Phylogenetic trees constructed from amino acid sequences of catalytic or regulatory domains show that CDPKs and CRKs are closely related and might share a common ancestor. Plant CCaMKs and CaMK form a group more closely related to protozoan, than to plant, CDPKs. Intron analysis of the 42 CDPK, CRK, PPCK, and PRK genes from Arabidopsis supports the structure of the gene trees, the possibility that PPCKs/PRKs belong to the CDPK superfamily, and suggests that several introns have been added during evolution of the family.

PEP carboxylase kinase is a novel protein kinase controlled at the level of expression

Phosphoenolpyruvate (PEP) carboxylase plays a number of key roles in the central metabolism of higher plants. The enzyme is regulated by reversible phosphorylation in response to a range of signals in many different plant tissues. The data discussed here illustrate several novel features of this system. The phosphorylation state of PEP carboxylase is controlled largely by the activity of PEP carboxylase kinase. This enzyme comprises a protein kinase catalytic domain with no regulatory regions. In many systems it is controlled at the level of expression. In C₄ plants, expression of PEP carboxylase kinase is light-regulated and involves changes in cytosolic pH, InsP₃ and Ca²⁺ levels. Expression of PEP carboxylase kinase in CAM plants is regulated by a circadian oscillator, perhaps via metabolite control. Some plants contain multiple PEP carboxylase kinase genes, probably with different expression patterns and roles. A newly discovered PEP carboxylase kinase inhibitor protein might facilitate the net dephosphorylation of PEP carboxylase under conditions in which flux through this enzyme is not required.

CIRCADIAN RHYTHMS IN PLANTS

Circadian rhythms, endogenous rhythms with periods of approximately 24 h, are widespread in nature. Although plants have provided many examples of rhythmic outputs and our understanding of photoreceptors of circadian input pathways is well advanced, studies with plants have lagged in the identification of components of the central circadian oscillator. Nonetheless, genetic and molecular biological studies, primarily in Arabidopsis, have begun to identify the components of plant circadian systems at an accelerating pace. There also is accumulating evidence that plants and other organisms house multiple circadian clocks both in different tissues and, quite probably, within individual cells, providing unanticipated complexity in circadian systems.

Day-to-night variations of cytoplasmic pH in a crassulacean acid metabolism plant

In crassulacean acid metabolism (CAM) large amounts of malic acid are redistributed between vacuole and cytoplasm in the course of night-to-day transitions. The corresponding changes of the cytoplasmic pH (pHcyt) were monitored in mesophyll protoplasts from the CAM plant Kalanchoe daigremontiana Hamet et Perrier by ratiometric fluorimetry with the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(and-6-)carboxyfluorescein as a pHcyt indicator. At the beginning of the light phase, pHcyt was slightly alkaline (about 7.5). It dropped during midday by about 0.3 pH units before recovering again in the late-day-to-early-dark phase. In the physiological context the variation in pHcyt may be a component of CAM regulation. Due to its pH sensitivity, phosphoenolpyruvate carboxylase appears as a likely target enzyme. From monitoring delta pHcyt in response to loading the cytoplasm with the weak acid salt K-acetate a cytoplasmic H(+)-buffer capacity in the order of 65 mM H+ per pH unit was estimated at a pHcyt of about 7.5. With this value, an acid load of the cytoplasm by about 10 mM malic acid can be estimated as the cause of the observed drop in pHcyt. A diurnal oscillation in pHcyt and a quantitatively similar cytoplasmic malic acid is predicted from an established mathematical model which allows simulation of the CAM dynamics. The similarity of model predictions and experimental data supports the view put forward in this model that a phase transition of the tonoplast is an essential functional element in CAM dynamics.

All in good time: the Arabidopsis circadian clock

Biological time-keeping mechanisms have fascinated researchers since the movement of leaves with a daily rhythm was first described >270 years ago. The circadian clock confers a approximately 24-hour rhythm on a range of processes including leaf movements and the expression of some genes. Molecular mechanisms and components underlying clock function have been described in recent years for several animal and prokaryotic organisms, and those of plants are beginning to be characterized. The emerging model of the Arabidopsis clock has mechanistic parallels with the clocks of other model organisms, which consist of positive and negative feedback loops, but the molecular components appear to be unique to plants.

A minimal serine/threonine protein kinase circadianly regulates phosphoenolpyruvate carboxylase activity in crassulacean acid metabolism-induced leaves of the common ice plant

Plant phosphoenolpyruvate carboxylase (PEPc) activity and allosteric properties are regulated by PEPc kinase (PPcK) through reversible phosphorylation of a specific serine (Ser) residue near the N terminus. We report the molecular cloning of PPcK from the facultative Crassulacean acid metabolism (CAM) common ice plant (Mesembryanthemum crystallinum), using a protein-kinase-targeted differential display reverse transcriptase-polymerase chain reaction approach. M. crystallinum PPcK encodes a minimal, Ca(2+)-independent Ser/threonine protein kinase that is most closely related to calcium-dependent protein kinases, yet lacks both the calmodulin-like and auto-inhibitory domains typical of plant calcium-dependent protein kinase. In the common ice plant PPcK belongs to a small gene family containing two members. McPPcK transcript accumulation is controlled by a circadian oscillator in a light-dependent manner. McPPcK encodes a 31.8-kD polypeptide (279 amino acids), making it among the smallest protein kinases characterized to date. Initial biochemical analysis of the purified, recombinant McPPcK gene product documented that this protein kinase specifically phosphorylates PEPc from CAM and C(4) species at a single, N-terminal Ser (threonine) residue but fails to phosphorylate mutated forms of C(4) PEPc in which this specific site has been changed to tyrosine or aspartate. McPPcK activity was specific for PEPc, Ca(2+)-insensitive, and displayed an alkaline pH optimum. Furthermore, recombinant McPPcK was shown to reverse the sensitivity of PEPc activity to L-malate inhibition in CAM-leaf extracts prepared during the day, but not at night, documenting that PPcK contributes to the circadian regulation of photosynthetic carbon flux in CAM plants.