Enzymes of C4 Photosynthesis

The Evolution of C4 Photosynthesis in Flaveria (Asteraceae): Insights from the Flaveria linearis Complex

Flaveria is a leading model for C4 plant evolution due to the presence of a dozen C3-C4 intermediate species, many of which are associated with a phylogenetic complex centered around Flaveria linearis. To investigate C4 evolution in Flaveria, we updated the Flaveria phylogeny and evaluated gas exchange, starch δ13C, and activity of C4 cycle enzymes in 19 Flaveria species and 28 populations within the F. linearis complex. A principal component analysis identified six functional clusters: (1) C3, (2) sub-C2, (3) full C2, (4) enriched C2, (5) sub-C4, and (6) fully C4 species. The sub-C2 species lacked a functional C4 cycle, while a gradient was present in the C2 clusters from little to modest C4 cycle activity as indicated by δ13C and enzyme activities. Three Yucatan populations of F. linearis had photosynthetic CO2 compensation points equivalent to C4 plants but showed little evidence for an enhanced C4 cycle, indicating they have an optimized C2 pathway that recaptures all photorespired CO2 in the bundle sheath (BS) tissue. All C2 species had enhanced aspartate aminotransferase activity relative to C3 species and most had enhanced alanine aminotransferase activity. These aminotransferases form aspartate and alanine from glutamate and in doing so could help return photorespiratory nitrogen (N) from BS to mesophyll cells, preventing glutamate feedback onto photorespiratory N assimilation. Their use requires upregulation of parts of the C4 metabolic cycle to generate carbon skeletons to sustain N return to the mesophyll, and thus could facilitate the evolution of the full C4 photosynthetic pathway.

Distinct C4 sub-types and C3 bundle sheath isolation in the Paniceae grasses

In C4 plants, the enzymatic machinery underpinning photosynthesis can vary, with, for example, three distinct C4 acid decarboxylases being used to release CO2 in the vicinity of RuBisCO. For decades, these decarboxylases have been used to classify C4 species into three biochemical sub-types. However, more recently, the notion that C4 species mix and match C4 acid decarboxylases has increased in popularity, and as a consequence, the validity of specific biochemical sub-types has been questioned. Using five species from the grass tribe Paniceae, we show that, although in some species transcripts and enzymes involved in multiple C4 acid decarboxylases accumulate, in others, transcript abundance and enzyme activity is almost entirely from one decarboxylase. In addition, the development of a bundle sheath isolation procedure for a close C3 species in the Paniceae enables the preliminary exploration of C4 sub-type evolution.

Redox regulation of NADP-malate dehydrogenase is vital for land plants under fluctuating light environment

Many enzymes involved in photosynthesis possess highly conserved cysteine residues that serve as redox switches in chloroplasts. These redox switches function to activate or deactivate enzymes during light-dark transitions and have the function of fine-tuning their activities according to the intensity of light. Accordingly, many studies on chloroplast redox regulation have been conducted under the hypothesis that "fine regulation of the activities of these enzymes is crucial for efficient photosynthesis." However, the impact of the regulatory system on plant metabolism is still unclear. To test this hypothesis, we here studied the impact of the ablation of a redox switch in chloroplast NADP-malate dehydrogenase (MDH). By genome editing, we generated a mutant plant whose MDH lacks one of its redox switches and is active even in dark conditions. Although NADPH consumption by MDH in the dark is expected to be harmful to plant growth, the mutant line did not show any phenotypic differences under standard long-day conditions. In contrast, the mutant line showed severe growth retardation under short-day or fluctuating light conditions. These results indicate that thiol-switch redox regulation of MDH activity is crucial for maintaining NADPH homeostasis in chloroplasts under these conditions.

Evolutionary Convergence of C4 Photosynthesis: A Case Study in the Nyctaginaceae

C4 photosynthesis evolved over 65 times, with around 24 origins in the eudicot order Caryophyllales. In the Caryophyllales family Nyctaginaceae, the C4 pathway is known in three genera of the tribe Nyctagineae: Allionia, Okenia and Boerhavia. Phylogenetically, Allionia and Boerhavia/Okenia are separated by three genera whose photosynthetic pathway is uncertain. To clarify the distribution of photosynthetic pathways in the Nyctaginaceae, we surveyed carbon isotope ratios of 159 species of the Nyctaginaceae, along with bundle sheath (BS) cell ultrastructure, leaf gas exchange, and C4 pathway biochemistry in five species from the two C4 clades and closely related C3 genera. All species in Allionia, Okenia and Boerhavia are C4, while no C4 species occur in any other genera of the family, including three that branch between Allionia and Boerhavia. This demonstrates that C4 photosynthesis evolved twice in Nyctaginaceae. Boerhavia species use the NADP-malic enzyme (NADP-ME) subtype of C4 photosynthesis, while Allionia species use the NAD-malic enzyme (NAD-ME) subtype. The BS cells of Allionia have many more mitochondria than the BS of Boerhavia. Bundle sheath mitochondria are closely associated with chloroplasts in Allionia which facilitates CO2 refixation following decarboxylation by mitochondrial NAD-ME. The close relationship between Allionia and Boerhavia could provide insights into why NADP-ME versus NAD-ME subtypes evolve, particularly when coupled to analysis of their respective genomes. As such, the group is an excellent system to dissect the organizational hierarchy of convergent versus divergent traits produced by C4 evolution, enabling us to understand when convergence is favored versus when divergent modifications can result in a common phenotype.

Mesophyll CO2 conductance and leakiness are not responsive to short- and long-term soil water limitations in the C4 plant Sorghum bicolor

Breeding economically important C₄ crops for enhanced whole-plant water-use efficiency (WUE_plant ) is needed for sustainable agriculture. WUE_plant is a complex trait and an efficient phenotyping method that reports on components of WUE_plant , such as intrinsic water-use efficiency (WUE_i , the rate of leaf CO₂ assimilation relative to water loss via stomatal conductance), is needed. In C₄ plants, theoretical models suggest that leaf carbon isotope composition (δ¹³ C), when the efficiency of the CO₂ -concentrating mechanism (leakiness, ϕ) remains constant, can be used to screen for WUE_i . The limited information about how ϕ responds to water limitations confines the application of δ¹³ C for WUE_i screening of C₄ crops. The current research aimed to test the response of ϕ to short- or long-term moderate water limitations, and the relationship of δ¹³ C with WUE_i and WUE_plant , by addressing potential mesophyll CO₂ conductance (g_m ) and biochemical limitations in the C₄ plant Sorghum bicolor. We demonstrate that g_m and ϕ are not responsive to short- or long-term water limitations. Additionally, δ¹³ C was not correlated with gas-exchange estimates of WUE_i under short- and long-term water limitations, but showed a significant negative relationship with WUE_plant . The observed association between the δ¹³ C and WUE_plant suggests an intrinsic link of δ¹³ C with WUE_i in this C₄ plant, and can potentially be used as a screening tool for WUE_plant in sorghum.

Biochemical and genetic analyses of N metabolism in maize testcross seedlings: 2. Roots

Intracellular factors differentially affected enzyme activities of N assimilation in the roots of maize testcrosses where alanine aminotransferase and glutamate synthase were the main enzymes regulating the levels of glutamate. N is a key macronutrient for plant growth and development. Breeding maize with improved efficiency in N use could help reduce environmental contamination as well as increase profitability for the farmers. Quantitative trait loci (QTL) mapping of traits related to N metabolism in the root tissue was undertaken in a maize testcross mapping population grown in hydroponic cultures. N concentration was negatively correlated with root and total dry mass. Neither the enzyme activities nor metabolites were appreciably correlated between the root and leaf tissues. Repeatability measures for most of the enzymes were lower than for dry mass. Weak negative correlations between most of the enzymes and dry mass resulted likely from dilution and suggested the presence of excess of enzyme activities for maximal biomass production. Glutamate synthase and alanine aminotransferase each explained more variation in glutamate concentration than either aspartate aminotransferase or asparagine synthetase whereas glutamine synthetase was inconsequential. Twenty-six QTL were identified across all traits. QTL models explained 7-43% of the variance with no significant epistasis between the QTL. Thirteen candidate genes were identified underlying QTL within 1-LOD confidence intervals. All the candidate genes were located in trans configuration, unlinked or even on different chromosomes, relative to the known genomic positions of the corresponding structural genes. Our results have implications in improving NUE in maize and other crop plants.

Shade compromises the photosynthetic efficiency of NADP-ME less than that of PEP-CK and NAD-ME C4 grasses

The high energy cost and apparently low plasticity of C4 photosynthesis compared with C3 photosynthesis may limit the productivity and distribution of C4 plants in low light (LL) environments. C4 photosynthesis evolved numerous times, but it remains unclear how different biochemical subtypes perform under LL. We grew eight C4 grasses belonging to three biochemical subtypes [NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PEP-CK)] under shade (16% sunlight) or control (full sunlight) conditions and measured their photosynthetic characteristics at both low and high light. We show for the first time that LL (during measurement or growth) compromised the CO2-concentrating mechanism (CCM) to a greater extent in NAD-ME than in PEP-CK or NADP-ME C4 grasses by virtue of a greater increase in carbon isotope discrimination (∆P) and bundle sheath CO2 leakiness (ϕ), and a greater reduction in photosynthetic quantum yield (Φmax). These responses were partly explained by changes in the ratios of phosphoenolpyruvate carboxylase (PEPC)/initial Rubisco activity and dark respiration/photosynthesis (Rd/A). Shade induced a greater photosynthetic acclimation in NAD-ME than in NADP-ME and PEP-CK species due to a greater Rubisco deactivation. Shade also reduced plant dry mass to a greater extent in NAD-ME and PEP-CK relative to NADP-ME grasses. In conclusion, LL compromised the co-ordination of the C4 and C3 cycles and, hence, the efficiency of the CCM to a greater extent in NAD-ME than in PEP-CK species, while CCM efficiency was less impacted by LL in NADP-ME species. Consequently, NADP-ME species are more efficient at LL, which could explain their agronomic and ecological dominance relative to other C4 grasses.

Photosynthesis in C3-C4 intermediate Moricandia species

Evolution of C4 photosynthesis is not distributed evenly in the plant kingdom. Particularly interesting is the situation in the Brassicaceae, because the family contains no C4 species, but several C3-C4 intermediates, mainly in the genus Moricandia Investigation of leaf anatomy, gas exchange parameters, the metabolome, and the transcriptome of two C3-C4 intermediate Moricandia species, M. arvensis and M. suffruticosa, and their close C3 relative M. moricandioides enabled us to unravel the specific C3-C4 characteristics in these Moricandia lines. Reduced CO2 compensation points in these lines were accompanied by anatomical adjustments, such as centripetal concentration of organelles in the bundle sheath, and metabolic adjustments, such as the balancing of C and N metabolism between mesophyll and bundle sheath cells by multiple pathways. Evolution from C3 to C3-C4 intermediacy was probably facilitated first by loss of one copy of the glycine decarboxylase P-protein, followed by dominant activity of a bundle sheath-specific element in its promoter. In contrast to recent models, installation of the C3-C4 pathway was not accompanied by enhanced activity of the C4 cycle. Our results indicate that metabolic limitations connected to N metabolism or anatomical limitations connected to vein density could have constrained evolution of C4 in Moricandia.

Phosphorylation of phosphoenolpyruvate carboxykinase (PEPCK) and phosphoenolpyruvate carboxylase (PEPC) in the flesh of fruits

This study determined whether phosphoenolpyruvate carboxykinase (PEPCK) and phosphoenolpyruvate carboxylase (PEPC) are phosphorylated in the flesh of a range of fruits. This was done by incubating fruit flesh with ³²P[P] (where ³²P[P] = ³²PO₄^3-), then PEPCK and PEPC were immunoprecipitated from extracts using specific antisera. The incorporation of ³²P[P] into these enzymes was then determined by autoradiography of SDS-PAGE gels. Both enzymes were subject to phosphorylation in vivo in the flesh of grape, tomato, cherry and plum. PEPCK was also subject to phosphorylation in vivo in developing grape seeds. Proteolytic cleavage of PEPCK showed that it was phosphorylated at a site(s) located on its N-terminal extension. Potentially phosphorylation of these enzymes could contribute to the coordinate regulation of their activities in the flesh of fruits and in developing seeds.

Photosynthetic responses to chilling in a chilling-tolerant and chilling-sensitive Miscanthus hybrid

Miscanthus is a C4 perennial grass being developed for bioenergy production in temperate regions where chilling events are common. To evaluate chilling effects on Miscanthus, we assessed the processes controlling net CO2 assimilation rate (A) in Miscanthus x giganteus (M161) and a chilling-sensitive Miscanthus hybrid (M115) before and after a chilling treatment of 12/5 °C. The temperature response of A and maximum Rubisco activity in vitro were identical below 20 °C in chilled and unchilled M161, demonstrating Rubisco capacity limits or co-limits A at cooler temperatures. By contrast, A in M115 decreased at all measurement temperatures after growth at 12/5 °C. Rubisco activity in vitro declined in proportion to the reduction in A in chilled M115 plants, indicating Rubisco capacity is responsible in part for the decline in A. Pyruvate orthophosphate dikinase activities were also reduced by the chilling treatment when assayed at 28 °C, indicating this enzyme may also contribute to the reduction in A in M115. The maximum extractable activities of PEPCase and NADP-ME remained largely unchanged after chilling. The carboxylation efficiency of the C4 cycle was depressed in both genotypes to a similar extent after chilling. ΦP :ΦCO2 remained unchanged in both genotypes indicating the C3 and C4 cycles decline equivalently upon chilling.

Improved analysis of C4 and C3 photosynthesis via refined in vitro assays of their carbon fixation biochemistry

Plants operating C3 and C4 photosynthetic pathways exhibit differences in leaf anatomy and photosynthetic carbon fixation biochemistry. Fully understanding this underpinning biochemical variation is requisite to identifying solutions for improving photosynthetic efficiency and growth. Here we refine assay methods for accurately measuring the carboxylase and decarboxylase activities in C3 and C4 plant soluble protein. We show that differences in plant extract preparation and assay conditions are required to measure NADP-malic enzyme and phosphoenolpyruvate carboxylase (pH 8, Mg(2+), 22 °C) and phosphoenolpyruvate carboxykinase (pH 7, >2mM Mn(2+), no Mg(2+)) maximal activities accurately. We validate how the omission of MgCl2 during leaf protein extraction, lengthy (>1min) centrifugation times, and the use of non-pure ribulose-1,5-bisphosphate (RuBP) significantly underestimate Rubisco activation status. We show how Rubisco activation status varies with leaf ontogeny and is generally lower in mature C4 monocot leaves (45-60% activation) relative to C3 monocots (55-90% activation). Consistent with their >3-fold lower Rubisco contents, full Rubisco activation in soluble protein from C4 leaves (<5min) was faster than in C3 plant samples (<10min), with addition of Rubisco activase not required for full activation. We conclude that Rubisco inactivation in illuminated leaves primarily stems from RuBP binding to non-carbamylated enzyme, a state readily reversible by dilution during cellular protein extraction.

Variations in nitrogen use efficiency reflect the biochemical subtype while variations in water use efficiency reflect the evolutionary lineage of C4 grasses at inter-glacial CO2

C4 photosynthesis evolved multiple times in diverse lineages. Most physiological studies comparing C4 plants were not conducted at the low atmospheric CO2 prevailing during their evolution. Here, 24 C4 grasses belonging to three biochemical subtypes [nicotinamide adenine dinucleotide malic enzyme (NAD-ME), phosphoenolpyruvate carboxykinase (PCK) and nicotinamide adenine dinucleotide phosphate malic enzyme (NADP-ME)] and six major evolutionary lineages were grown under ambient (400 μL L(-1) ) and inter-glacial (280 μL L(-1) ) CO2 . We hypothesized that nitrogen-related and water-related physiological traits are associated with subtypes and lineages, respectively. Photosynthetic rate and stomatal conductance were constrained by the shared lineage, while variation in leaf mass per area (LMA), leaf N per area, plant dry mass and plant water use efficiency were influenced by the subtype. Subtype and lineage were equally important for explaining variations in photosynthetic nitrogen use efficiency (PNUE) and photosynthetic water use efficiency (PWUE). CO2 treatment impacted most parameters. Overall, higher LMA and leaf N distinguished the Chloridoideae/NAD-ME group, while NADP-ME and PCK grasses were distinguished by higher PNUE regardless of lineage. Plants were characterized by high photosynthesis and PWUE when grown at ambient CO2 and by high conductance at inter-glacial CO2 . In conclusion, the evolutionary and biochemical diversity among C4 grasses was aligned with discernible leaf physiology, but it remains unknown whether these traits represent ecophysiological adaptation.

Physiological and transcriptional analyses of developmental stages along sugarcane leaf

BACKGROUND

Sugarcane is one of the major crops worldwide. It is cultivated in over 100 countries on 22 million ha. The complex genetic architecture and the lack of a complete genomic sequence in sugarcane hamper the adoption of molecular approaches to study its physiology and to develop new varieties. Investments on the development of new sugarcane varieties have been made to maximize sucrose yield, a trait dependent on photosynthetic capacity. However, detailed studies on sugarcane leaves are scarce. In this work, we report the first molecular and physiological characterization of events taking place along a leaf developmental gradient in sugarcane.

RESULTS

Photosynthetic response to CO2 indicated divergence in photosynthetic capacity based on PEPcase activity, corroborated by activity quantification (both in vivo and in vitro) and distinct levels of carbon discrimination on different segments along leaf length. Additionally, leaf segments had contrasting amount of chlorophyll, nitrogen and sugars. RNA-Seq data indicated a plethora of biochemical pathways differentially expressed along the leaf. Some transcription factors families were enriched on each segment and their putative functions corroborate with the distinct developmental stages. Several genes with higher expression in the middle segment, the one with the highest photosynthetic rates, were identified and their role in sugarcane productivity is discussed. Interestingly, sugarcane leaf segments had a different transcriptional behavior compared to previously published data from maize.

CONCLUSION

This is the first report of leaf developmental analysis in sugarcane. Our data on sugarcane is another source of information for further studies aiming to understand and/or improve C4 photosynthesis. The segments used in this work were distinct in their physiological status allowing deeper molecular analysis. Although limited in some aspects, the comparison to maize indicates that all data acquired on one C4 species cannot always be easily extrapolated to other species. However, our data indicates that some transcriptional factors were segment-specific and the sugarcane leaf undergoes through the process of suberizarion, photosynthesis establishment and senescence.

Changes in C-N metabolism under elevated CO2 and temperature in Indian mustard (Brassica juncea L.): an adaptation strategy under climate change scenario

The present study was performed to investigate the possible role of carbon (C) and nitrogen (N) metabolism in adaptation of Indian mustard (Brassica juncea L.) growing under ambient (370 ± 15 ppm) and elevated CO2 (700 ± 15 ppm), and jointly in elevated CO2 and temperature (30/22 °C for day/night). The key enzymes responsible for C-N metabolism were studied in different samples of Brassica juncea L. collected from ambient (AMB), elevated (ELE) and ELExT growth conditions. Total percent amount of C and N in leaves were particularly estimated to establish a clear understanding of aforesaid metabolism in plant adaptation. Furthermore, key morphological and physiological parameters such as plant height, leaf area index, dry biomass, net photosynthetic rate, stomatal conductance, transpiration, total protein and chlorophyll contents were also studied in relation to C/N metabolism. The results indicated that the C-metabolizing enzymes, such as (ribulose-1,5-bisphosphate carboxylase/oxygenase, phosphoenolpyruvate carboxylase, malate dehydrogenase, NAD-malic enzyme, NADP-malic enzyme and citrate synthase) and the N-metabolizing enzymes, such as (aspartate amino transferase, glutamine synthetase, nitrate reductase and nitrite reductase) showed significantly (P < 0.05) higher activities along with the aforesaid physiological and biochemical parameters in order of ELE > ELExT > AMB growth conditions. This is also evident by significant (P < 0.05) increase in percent contents of C and N in leaves as per said order. These findings suggested that improved performance of C-N metabolism could be a possible approach for CO2 assimilation and adaptation in Brassica juncea L. against elevated CO2 and temperature prevailing in climate change scenarios.

Delivery of multiple transgenes to plant cells by an improved version of MultiRound Gateway technology

At present, only few methods for the effective assembly of multigene constructs have been described. Here we present an improved version of the MultiRound Gateway technology, which facilitates plant multigene transformation. The system consists of two attL-flanked entry vectors, which contain an attR cassette, and a transformation-competent artificial chromosome based destination vector. By alternate use of the two entry vectors, multiple transgenes can be delivered sequentially into the Gateway-compatible destination vector. Multigene constructs that carried up to seven transgenes corresponding to more than 26 kb were assembled by seven rounds of LR recombination. The constructs were successfully transformed into tobacco plants and were stably inherited for at least two generations. Thus, our system represents a powerful, highly efficient tool for multigene plant transformation and may facilitate genetic engineering of agronomic traits or the assembly of genetic pathways for the production of biofuels, industrial or pharmaceutical compounds in plants.

The role of C4 metabolism in the marine diatom Phaeodactylum tricornutum

Diatoms are important players in the global carbon cycle. Their apparent photosynthetic affinity for ambient CO(2) is much higher than that of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), indicating that a CO(2)-concentrating mechanism (CCM) is functioning. However, the nature of the CCM, a biophysical or a biochemical C(4), remains elusive. Although (14)C labeling experiments and presence of complete sets of genes for C(4) metabolism in two diatoms supported the presence of C(4), other data and predicted localization of the decarboxylating enzymes, away from Rubisco, makes this unlikely. We used RNA-interference to silence the single gene encoding pyruvate-orthophosphate dikinase (PPDK) in Phaeodactylum tricornutum, essential for C(4) metabolism, and examined the photosynthetic characteristics. The mutants possess much lower ppdk transcript and PPDK activity but the photosynthetic K(1/2) (CO(2)) was hardly affected, thus clearly indicating that the C(4) route does not serve the purpose of raising the CO(2) concentration in close proximity of Rubisco in P. tricornutum. The photosynthetic V(max) was slightly reduced in the mutant, possibly reflecting a metabolic constraint that also resulted in a larger lipid accumulation. We propose that the C(4) metabolism does not function in net CO(2) fixation but helps the cells to dissipate excess light energy and in pH homeostasis.

A possible role for C4 photosynthetic enzymes in tolerance of Zea mays to NaCl

Treatment of 14-day-old maize cultivars (Hybrid351 and Giza2) with 250 mM NaCl significantly reduced shoot fresh and dry weights and protein content during the subsequent 12 days. The magnitude of reduction was more pronounced in Giza than Hybrid. Both cultivars contained converging levels of protein for the enzymes phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), pyruvate phosphate dikinase (PPDK) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) under normal conditions; however, NaCl led to increase these levels in Hybrid and decrease them in Giza. Moreover, NaCl significantly inhibited the activities of PEPC, MDH and PPDK in both cultivars during the first 2 days, thereafter the inhibition nullified only in Hybrid; nonetheless, Rubisco was the least affected enzyme in both cultivars. In addition, NaCl slightly increased V (max) of PEPC, MDH and PPDK in Hybrid with no change in K (m); nevertheless V (max) dropped in Giza with an increase in K (m) of only PEPC and MDH. Also K (cat), K (cat)/K (m) and V (max)/K (m) of all enzymes were lower in treated Giza than in treated Hybrid. The increased V (max) of all enzymes in only Hybrid by NaCl confirms that they were synthesised more in Hybrid than in Giza. However, the decreased V (max) in Giza concomitant with the increased K (m) points to an interference of salinity with synthesis of enzymes and their structural integrity. This would lead to a noncompetitive inhibition for the enzymes. These findings declare that maize tolerance to NaCl was larger in Hybrid compared to Giza due to a role for C4 enzymes.

Phylogeny and photosynthetic pathway distribution in Anticharis Endl. (Scrophulariaceae)

C(4) photosynthesis independently evolved >62 times, with the majority of origins within 16 dicot families. One origin occurs in the poorly studied genus Anticharis Endl. (Scrophulariaceae), which consists of ~10 species from arid regions of Africa and southwest Asia. Here, the photosynthetic pathway of 10 Anticharis species and one species from each of the sister genera Aptosimum and Peliostomum was identified using carbon isotope ratios (δ(13)C). The photosynthetic pathway was then mapped onto an internal transcribed spacer (ITS) phylogeny of Anticharis and its sister genera. Leaf anatomy was examined for nine Anticharis species and plants from Aptosimum and Peliostomum. Leaf ultrastructure, gas exchange, and enzyme distributions were assessed in Anticharis glandulosa collected in SE Iran. The results demonstrate that C(3) photosynthesis is the ancestral condition, with C(4) photosynthesis occurring in one clade containing four species. C(4) Anticharis species exhibit the atriplicoid type of C(4) leaf anatomy and the NAD-malic enzyme biochemical subtype. Six Anticharis species had C(3) or C(3)-C(4) δ(13)C values and branched at phylogenetic nodes that were sister to the C(4) clade. The rest of Anticharis species had enlarged bundle sheath cells, close vein spacing, and clusters of chloroplasts along the centripetal (inner) bundle sheath walls. These traits indicate that basal-branching Anticharis species are evolutionary intermediates between the C(3) and C(4) conditions. Anticharis appears to be an important new group in which to study the dynamics of C(4) evolution.

Photosynthetic responses of a C(3) and three C(4) species of the genus Panicum (s.l.) with different metabolic subtypes to drought stress

Young plants of Panicum bisulcatum (C(3)), Zuloagaea bulbosa [NADP-malic enzyme (ME)-C(4)], P. miliaceum (NAD-ME-C(4)) and Urochloa maxima [phosphoenolpyruvate carboxykinase (PCK)-C(4)] were subjected to drought stress (DS) in soil for 6 days. The C(3) species showed severe wilting symptoms at higher soil water potential (-1.1 MPa) and relative leaf water content (77 %) than in the case of the C(4) species (-1.5 to -1.7 MPa; 58-64 %). DS decreased photosynthesis, both under atmospheric and under saturating CO(2). Stomatal limitation of net photosynthesis (P(N)) in the C(3), but not in the C(4) species was indicated by P(N)/C(o) curves. Chlorophyll fluorescence of photosystem II, resulting from different cell types in the four species, indicated NADPH accumulation and non-stomatal limitation of photosynthesis in all four species, even under high CO(2). In the NAD-ME-C(4) and the PCK-C(4) species, DS plants showed increased violaxanthin de-epoxidase rates. Biochemical analyses of carboxylating enzymes and in vitro enzyme activities of the C(4) enzymes identified the most likely non-stomatal limiting steps of photosynthesis. In P. bisulcatum, declining RubisCO content and activity would explain the findings. In Z. bulbosa, all photosynthesis enzymes declined significantly; photosynthesis is probably limited by the turnover rate of the PEPC reaction. In P. miliaceum, all enzyme levels remained fairly constant under DS, but photosynthesis can be limited by feedback inhibition of the Calvin cycle, resulting in asp inhibition of PEPC. In U. maxima, declines of in vivo PEPC activity and feedback inhibition of the Calvin cycle are the main candidates for non-stomatal limitation of photosynthesis under DS.

Development and metabolism of the fruit and seed of the Japanese plum Ozark premier (Rosaceae)

The growth characteristics of some plums and their component parts have been previously studied, as have some aspects of their developmental anatomy and composition. However, little is known about either their metabolism or about the interactions between the metabolism of their component parts. In this study we investigated these aspects in the Japanese plum Ozark Premier. Throughout fruit and seed development, changes in sugar and organic acid contents, protein composition and abundance of selected enzymes were determined. In the stone, there was a transient accumulation of vegetative storage proteins. These were subsequently mobilized and this coincided with the onset of the lignification of the stone and the start of storage protein accumulation in the seed. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was present in the seeds, even though they lacked chlorophyll, and its presence may be related to limited gas exchange. In the flesh of some fruits, phosphoenolpyruvate carboxykinase (PEPCK) and NADP malic enzyme (NADP-ME) are thought to function in the dissimilation of malate and/or citrate during ripening. However, PEPCK and NADP-ME were present in plum flesh for most of its development, although there was no net dissimilation of malate until the latter stages of ripening. There is an interaction between the developing seed and endocarp with respect to the utilization of imported sugars and amino acids. An hypothesis is presented to account for the presence of PEPCK and NADP-ME enzyme in plum flesh when there was no net dissimilation of organic acids.

The occurrence of C(2) photosynthesis in Euphorbia subgenus Chamaesyce (Euphorbiaceae)

This study investigated whether Euphorbia subgenus Chamaesyce subsection Acutae contains C(3)-C(4) intermediate species utilizing C(2) photosynthesis, the process where photorespired CO(2) is concentrated into bundle sheath cells. Euphorbia species in subgenus Chamaesyce are generally C(4), but three species in subsection Acutae (E. acuta, E. angusta, and E. johnstonii) have C(3) isotopic ratios. Phylogenetically, subsection Acutae branches between basal C(3) clades within Euphorbia and the C(4) clade in subgenus Chamaesyce. Euphorbia angusta is C(3), as indicated by a photosynthetic CO(2) compensation point (Г) of 69 μmol mol(-1) at 30 °C, a lack of Kranz anatomy, and the occurrence of glycine decarboxylase in mesophyll tissues. Euphorbia acuta utilizes C(2) photosynthesis, as indicated by a Г of 33 μmol mol(-1) at 30 °C, Kranz-like anatomy with mitochondria restricted to the centripetal (inner) wall of the bundle sheath cells, and localization of glycine decarboxlyase to bundle sheath mitochondria. Low activities of PEP carboxylase, NADP malic enzyme, and NAD malic enzyme demonstrated no C(4) cycle activity occurs in E. acuta thereby classifying it as a Type I C(3)-C(4) intermediate. Kranz-like anatomy in E. johnstonii indicates it also utilizes C(2) photosynthesis. Given the phylogenetically intermediate position of E. acuta and E. johnstonii, these results support the hypothesis that C(2) photosynthesis is an evolutionary intermediate condition between C(3) and C(4) photosynthesis.

Carbon concentrating mechanisms in eukaryotic marine phytoplankton

The accumulation of inorganic carbon from seawater by eukaryotic marine phytoplankton is limited by the diffusion of carbon dioxide (CO2) in water and the dehydration kinetics of bicarbonate to CO2 and by ribulose-1,5-bisphosphate carboxylase/oxygenase's (RubisCO) low affinity for its inorganic carbon substrate, CO2. Nearly all marine phytoplankton have adapted to these limitations and evolved inorganic carbon (or CO2) concentrating mechanisms (CCMs) to support photosynthetic carbon fixation at the concentrations of CO2 present in ocean surface waters (< 10-30 microM). The biophysics and biochemistry of CCMs vary within and among the three dominant groups of eukaryotic marine phytoplankton and may involve the activity of external or intracellular carbonic anhydrase, HCO3- transport, and perhaps a C4 carbon pump. In general, coccolithophores have low-efficiency CCMs, and diatoms and the haptophyte genus Phaeocystis have high-efficiency CCMs. Dinoflagellates appear to possess moderately efficient CCMs, which may be necessitated by the very low CO2 affinity of their form II RubisCO. The energetic and nutrient costs of CCMs may modulate how variable CO2 affects primary production, element composition, and species composition of phytoplankton in the ocean.

Modelling temperature-compensated physiological rates, based on the co-ordination of responses to temperature of developmental processes

Temperature fluctuates rapidly and affects all developmental and metabolic processes. This often obscures the effects of developmental trends or of other environmental conditions when temperature fluctuates naturally. A method is proposed for modelling temperature-compensated rates, based on the coordination of temperature responses of developmental processes. In a data set comprising 41 experiments in the greenhouse, growth chamber, or the field, the temperature responses in the range of 6-36 degrees C for different processes were compared in three species, maize, rice, and Arabidopsis thaliana. Germination, cell division, expansive growth rate, leaf initiation, and phenology showed coordinated temperature responses and followed common laws within each species. The activities of 10 enzymes involved in carbon metabolism exhibited monotonous exponential responses across the whole range 10-40 degrees C. Hence, the temperature dependence of developmental processes is not explained by a simple relationship to central metabolism. Temperature-compensated rates of development were calculated from the equations of response curve, by expressing rates per unit equivalent time at 20 degrees C. This resulted in stable rates when temperatures fluctuated over a large range (for which classical thermal time was inefficient), and in time courses of leaf development which were common to several experiments with different temperature scenarios.

Stem juice production of the C4 sugarcane (Saccharum officinarum) is enhanced by growth at double-ambient CO2 and high temperature

Two cultivars of sugarcane (Saccharum officinarum cv. CP73-1547 and CP88-1508) were grown for 3 months in paired-companion, temperature-gradient, sunlit greenhouses under daytime [CO2] of 360 (ambient) and 720 (double ambient) micromol mol(-1) and at temperatures of 1.5 degrees C (near ambient) and 6.0 degrees C higher than outside ambient temperature. Leaf area and biomass, stem biomass and juice and CO2 exchange rate (CER) and activities of ribulose bisphosphate carboxylase-oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) of fully developed leaves were measured at harvest. On a main stem basis, leaf area, leaf dry weight, stem dry weight and stem juice volume were increased by growth at doubled [CO2] or high temperature. Such increases were even greater under combination of doubled [CO2]/high temperature. Plants grown at doubled [CO2]/high temperature combination averaged 50%, 26%, 84% and 124% greater in leaf area, leaf dry weight, stem dry weight and stem juice volume, respectively, compared with plants grown at ambient [CO2]/near-ambient temperature combination. In addition, plants grown at doubled [CO2]/high temperature combination were 2-3-fold higher in stem soluble solids than those at ambient [CO2]/near-ambient temperature combination. Although midday CER of fully developed leaves was not affected by doubled [CO2] or high temperature, plants grown at doubled [CO2] were 41-43% less in leaf stomatal conductance and 69-79% greater in leaf water-use efficiency, compared with plants grown at ambient [CO2]. Activity of PEPC was down-regulated 23-32% at doubled [CO2], while high temperature did not have a significant impact on this enzyme. Activity of Rubisco was not affected by growth at doubled [CO2], but was reduced 15-28% at high temperature. The increases in stem juice production and stem juice soluble solids concentration for sugarcane grown at doubled [CO2] or high temperature, or at doubled [CO2]/high temperature combination, were partially the outcome of an increase in whole plant leaf area. Such increase would enhance the ongoing and cumulative photosynthetic capability of the whole plant. The results indicate that a doubling of [CO2] would benefit sugarcane production more than the anticipated 10-15% increase for a C4 species.

The activities of PEP carboxylase and the C4 acid decarboxylases are little changed by drought stress in three C4 grasses of different subtypes

The C4 photosynthetic pathway involves the assimilation of CO2 by phosphoenolpyruvate carboxylase (PEPC) and the subsequent decarboxylation of C4 acids. The enzymes of the CO2 concentrating mechanism could be affected under water deficit and limit C4 photosynthesis. Three different C4 grasses were submitted to gradually induced drought stress conditions: Paspalum dilatatum (NADP-malic enzyme, NADP-ME), Cynodon dactylon (NAD-malic enzyme, NAD-ME) and Zoysia japonica (PEP carboxykinase, PEPCK). Moderate leaf dehydration affected the activity and regulation of PEPC in a similar manner in the three grasses but had species-specific effects on the C4 acid decarboxylases, NADP-ME, NAD-ME and PEPCK, although changes in the C4 enzyme activities were small. In all three species, the PEPC phosphorylation state, judged by the inhibitory effect of L-malate on PEPC activity, increased with water deficit and could promote increased assimilation of CO2 by the enzyme under stress conditions. Appreciable activity of PEPCK was observed in all three species suggesting that this enzyme may act as a supplementary decarboxylase to NADP-ME and NAD-ME in addition to its role in other metabolic pathways.

Comparison of the biological properties of several marine sponge-derived sesquiterpenoid quinones

Eight naturally occurring marine-sponge derived sesquiterpenoid quinones were evaluated as potential inhibitors of pyruvate phosphate dikinase (PPDK), a C4 plant regulatory enzyme. Of these, the hydroxyquinones ilimaquinone, ethylsmenoquinone and smenoquinone inhibited PPDK activity with IC50's (reported with 95% confidence intervals) of 285.4 (256.4-317.7), 316.2 (279.2-358.1) and 556.0 (505.9-611.0) microM, respectively, as well as being phytotoxic to the C4 plant Digitaria ciliaris. The potential anti-inflammatory activity of these compounds, using bee venom phospholipase A2 (PLA2), was also evaluated. Ethylsmenoquinone, smenospongiarine, smenospongidine and ilimaquinone inhibited PLA2 activity (% inhibition of 73.2 +/- 4.8 at 269 microM, 61.5 +/- 6.1 at 242 microM, 41.0 +/- 0.6 at 224 microM and 36.4 +/- 8.2 at 279 microM, respectively). SAR analyses indicate that a hydroxyquinone functionality and a short, hydroxide/alkoxide side-chain atC-20 is preferred for inhibition of PPDK activity, and that a larger amine side-chain at C-20 is tolerated for PLA2 inhibitory activity.

Combined transcript and metabolite profiling of Arabidopsis leaves reveals fundamental effects of the thiol-disulfide status on plant metabolism

In this study, we used gas chromatography-mass spectrometry analysis in combination with flux analysis and the Affymetrix ATH1 GeneChip to survey the metabolome and transcriptome of Arabidopsis (Arabidopsis thaliana) leaves in response to manipulation of the thiol-disulfide status. Feeding low concentrations of the sulfhydryl reagent dithiothreitol for 1 h at the end of the dark period led to posttranslational redox activation of ADP-glucose pyrophosphorylase and major alterations in leaf carbon partitioning, including an increased flux into major respiratory pathways, starch, cell wall, and amino acid synthesis, and a reduced flux to sucrose. This was accompanied by a decrease in the levels of hexose phosphates, while metabolites in the second half of the tricarboxylic acid cycle and various amino acids increased, indicating a stimulation of anaplerotic fluxes reliant on alpha-ketoglutarate. There was also an increase in shikimate as a precursor of secondary plant products and marked changes in the levels of the minor sugars involved in ascorbate synthesis and cell wall metabolism. Transcript profiling revealed a relatively small number of changes in the levels of transcripts coding for components of redox regulation, transport processes, and cell wall, protein, and amino acid metabolism, while there were no major alterations in transcript levels coding for enzymes involved in central metabolic pathways. These results provide a global picture of the effect of redox and reveal the utility of transcript and metabolite profiling as systemic strategies to uncover the occurrence of redox modulation in vivo.

Cloning and expression of maize-leaf pyruvate, Pi dikinase regulatory protein gene

Pyruvate, orthophosphate dikinase (PPDK; E.C. 2.7.9.1) catalyzes the synthesis of the primary inorganic carbon acceptor, phosphoenolpyruvate in the C4 photosynthetic pathway and is reversibly regulated by light. PPDK regulatory protein (RP), a bifunctional serine/threonine kinase-phosphatase, catalyzes both the ADP-dependent inactivation and the Pi-dependent activation of PPDK. Attempts to clone the RP have to date proven unsuccessful. A bioinformatics approach was taken to identify the nucleotide and amino acid sequence of the protein. Based on previously established characteristics including molecular mass, known inter- and intracellular location, functionality, and low level of expression, available databases were interrogated to ultimately identify a single candidate gene. In this paper, we describe the nucleotide and deduced amino acid sequence of this gene and establish its identity as maize PPDK RP by in vitro analysis of its catalytic properties via the cloning and expression of the recombinant protein.

Effect of altitude on the primary products of photosynthesis and the associated enzymes in barley and wheat

There is little information available on the primary products of photosynthesis and the change in the activity of the associated enzymes with altitude. We studied the same in varieties of barley and wheat grown at 1300 (low altitude, LA) and 4200 m (high altitude, HA) elevations above mean sea level in the western Himalayas. Plants at both the locations had similar photosynthetic rates, leaf water potential and the chlorophyll fluorescence kinetics. The short-term radio-labelling experiments in leaves showed appearance of (14)CO(2) in phosphoglyceric acid and sugar phosphates in plants at both the LA and HA, suggesting a major role of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in CO(2) fixation in the plants at two altitudes, whereas the appearance of labelled carbon in aspartate (Asp) and glutamate (Glu) at HA suggested a role of phosphoenolpyruvate carboxylase (PEPCase) in photosynthesis metabolism. Plants at HA had significantly higher activities of PEPCase, carboxylase and oxygenase activity of Rubisco, aspartate aminotransferase (AspAT), and glutamine synthetase (GS). However, the activities of malate dehydrogenase, NAD-malic enzyme and citrate synthase were similar at the two locations. Such an altered metabolism at HA suggested that PEPCase probably captured CO(2) directly from the atmosphere and/or that generated metabolically e.g. from photorespiration at HA. Higher oxygenase activity at HA suggests high photorespiratory activity. OAA thus produced could be additionally channelised for Asp synthesis using Glu as a source of ammonia. Higher GS activity ensures higher assimilation rate of NH(3) and the synthesis of Glu through GS-GOGAT (glutamine:2-oxoglutarate aminotransferase) pathway, also as supported by the appearance of radiolabel in Glu at HA. Enhanced PEPCase activity coupled with higher activities of AspAT and GS suggests a role in conserving C and N in the HA environment.

Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2 concentration in the absence of drought

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 micromol mol(-1)) and elevated [CO2] (550 micromol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.

Phosphoenolpyruvate carboxykinase and its potential role in the catabolism of organic acids in the flesh of soft fruit during ripening

Previous studies of grapes and tomatoes have shown that the abundance of phosphoenolpyruvate carboxykinase (PEPCK) increases in their flesh at the start of ripening, and that this coincides with a decrease in its citrate and/or malate content. Thus, PEPCK might function in the catabolism of organic acid anions during the ripening of these fruits. In the present study, the abundance of PEPCK was determined in the flesh of blueberries, raspberries, red currants, and strawberries at different stages of their development. In addition, changes in the amounts of citrate, malate, soluble sugars, isocitrate lyase, NADP-malic enzyme, phosphoenolpyruvate carboxylase, and pyruvate, orthophosphate dikinase in the flesh were determined. PEPCK was not detected in strawberry flesh, in which there was no dissimilation of malate or citrate. In the flesh of the other fruits, the abundance of PEPCK increased during ripening to an amount that was similar to that in grapes and tomatoes. In the flesh of blueberries and red currants, PEPCK was most abundant when there was dissimilation of malate. In the flesh of raspberries, PEPCK was most abundant when there was dissimilation of malate and citrate. These results are consistent with PEPCK playing a role in the dissimilation of citrate and/or malate in the flesh of these fruits during ripening. However, PEPCK was also present in the flesh of blueberries, raspberries, and red currants when there was no dissimilation of malate or citrate, and this raises the possibility that PEPCK might have additional functions. Dissection of blueberries provided evidence that both PEPCK and phosphoenolpyruvate carboxylase were present in the same cells, and possible functions for this are discussed.

A rapid screening method to detect specific inhibitors of pyruvate orthophosphate dikinase as leads for C4 plant-selective herbicides

Plants using the C(4) photosynthetic pathway are highly represented among the world's worst weeds, with only 4 C(4) species being agriculturally productive (maize, sorghum, millet, and sugar cane). With the C(4) acid cycle operating as a biochemical appendage of C(3) photosynthesis, the additional enzymes involved in C(4) photosynthesis represent an attractive target for the development of weed-specific herbicides. The rate-limiting enzyme of this metabolic pathway is pyruvate orthophosphate dikinase (PPDK). PPDK, coupled with phosphoenolpyruvate carboxylase and nicotinamide adenine dinucleotide-malate dehydrogenase, was used to develop a microplate-based assay to detect inhibitors of enzymes of the C(4) acid cycle. The resulting assay had a Z' factor of 0.61, making it a high-quality assay able to reliably identify active test samples. Organic extracts of 6679 marine macroscopic organisms were tested within the assay, and 343 were identified that inhibited the 3 enzyme-coupled reaction. A high confirmation rate was achieved, with 95% of these hit extracts proving active again upon retesting. Sequential addition of phosphoenolpyruvate and oxaloacetate to the assay facilitated identification of 83 extracts that specifically inhibited PPDK.

Sensitivity of photosynthesis in a C4 plant, maize, to heat stress

Our objective was to determine the sensitivity of components of the photosynthetic apparatus of maize (Zea mays), a C4 plant, to high temperature stress. Net photosynthesis (Pn) was inhibited at leaf temperatures above 38 degrees C, and the inhibition was much more severe when the temperature was increased rapidly rather than gradually. Transpiration rate increased progressively with leaf temperature, indicating that inhibition was not associated with stomatal closure. Nonphotochemical fluorescence quenching (qN) increased at leaf temperatures above 30 degrees C, indicating increased thylakoid energization even at temperatures that did not inhibit Pn. Compared with CO(2) assimilation, the maximum quantum yield of photosystem II (F(v)/F(m)) was relatively insensitive to leaf temperatures up to 45 degrees C. The activation state of phosphoenolpyruvate carboxylase decreased marginally at leaf temperatures above 40 degrees C, and the activity of pyruvate phosphate dikinase was insensitive to temperature up to 45 degrees C. The activation state of Rubisco decreased at temperatures exceeding 32.5 degrees C, with nearly complete inactivation at 45 degrees C. Levels of 3-phosphoglyceric acid and ribulose-1,5-bisphosphate decreased and increased, respectively, as leaf temperature increased, consistent with the decrease in Rubisco activation. When leaf temperature was increased gradually, Rubisco activation acclimated in a similar manner as Pn, and acclimation was associated with the expression of a new activase polypeptide. Rates of Pn calculated solely from the kinetics of Rubisco were remarkably similar to measured rates if the calculation included adjustment for temperature effects on Rubisco activation. We conclude that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30 degrees C.

Pyruvate,orthophosphate dikinase in leaves and chloroplasts of C(3) plants undergoes light-/dark-induced reversible phosphorylation

Pyruvate,orthophosphate (Pi) dikinase (PPDK) is best recognized as a chloroplastic C(4) cycle enzyme. As one of the key regulatory foci for controlling flux through this photosynthetic pathway, it is strictly and reversibly regulated by light. This light/dark modulation is mediated by reversible phosphorylation of a conserved threonine residue in the active-site domain by the PPDK regulatory protein (RP), a bifunctional protein kinase/phosphatase. PPDK is also present in C(3) plants, although it has no known photosynthetic function. Nevertheless, in this report we show that C(3) PPDK in leaves of several angiosperms and in isolated intact spinach (Spinacia oleracea) chloroplasts undergoes light-/dark-induced changes in phosphorylation state in a manner similar to C(4) dikinase. In addition, the kinetics of this process closely resemble the reversible C(4) process, with light-induced dephosphorylation occurring rapidly (< or =15 min) and dark-induced phosphorylation occurring much more slowly (> or =30-60 min). In intact spinach chloroplasts, light-induced dephosphorylation of C(3) PPDK was shown to be dependent on exogenous Pi and photosystem II activity but independent of electron transfer from photosystem I. These in organello results implicate a role for stromal pools of Pi and adenylates in regulating the reversible phosphorylation of C(3)-PPDK. Last, we used an in vitro RP assay to directly demonstrate ADP-dependent PPDK phosphorylation in desalted leaf extracts of the C(3) plants Vicia faba and rice (Oryza sativa). We conclude that an RP-like activity mediates the light/dark modulation of PPDK phosphorylation state in C(3) leaves and chloroplasts and likely represents the ancestral isoform of this unusual and key C(4) pathway regulatory "converter" enzyme.

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.

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.

Further analysis of maize C(4) pyruvate,orthophosphate dikinase phosphorylation by its bifunctional regulatory protein using selective substitutions of the regulatory Thr-456 and catalytic His-458 residues

In C(4) plants such as maize, pyruvate,orthophosphate dikinase (PPDK) catalyzes the regeneration of the initial carboxylation substrate during C(4) photosynthesis. The primary catalytic residue, His-458 (maize C(4) PPDK), is involved in the ultimate transfer of the beta-phosphate from ATP to pyruvate. C(4) PPDK activity undergoes light-dark regulation in vivo by reversible phosphorylation of a nearby active-site residue (Thr-456) by a single bifunctional regulatory protein (RP). Using site-directed mutagenesis of maize recombinant C(4) dikinase, we made substitutions at the catalytic His residue (H458N) and at this regulatory target Thr (T456E, T456Y, T456F). Each of these affinity-purified mutant enzymes was assayed for changes in dikinase activity. As expected, substituting His-458 with Asn results in a catalytically incompetent enzyme. Substitutions of the Thr-456 residue with Tyr and Phe reduced activity by about 94 and 99%, respectively. Insertion of Glu at this position completely abolished activity, presumably by the introduction of negative charge proximal to the catalytic His. Furthermore, neither the T456Y nor inactive H458N mutant enzyme was phosphorylated in vitro by RP. The inability of the former to serve as a phosphorylation substrate indicates that RP is functionally a member of the Ser/Thr family of protein kinases rather than a "dual-specificity" Ser-Thr/Tyr kinase, since our previous work showed that RP effectively phosphorylated Ser inserted at position 456. The inability of RP to phosphorylate its native target Thr residue when Asn is substituted for His-458 documents that RP requires the His-P catalytic intermediate form of PPDK as its protein substrate. For these latter studies, synthetic phosphopeptide-directed antibodies specific for the Thr(456)-P form of maize C(4) PPDK were developed and characterized.

Control of C4 photosynthesis: effects of reduced activities of phosphoenolpyruvate carboxylase on CO2 assimilation in Amaranthus edulis L

Heterozygous mutants of Amaranthus edulis deficient in PEP carboxylase (PEPC) have been used to study the control of photosynthetic carbon assimilation. A reduction in PEPC activity led to a decrease in the initial slope of the relationship between the CO2 assimilation rate and the intercellular CO2 concentration and to a decrease in photosynthesis at high light intensities, consistent with a decrease in the capacity of the C4 cycle in high light. PEPC exerted appreciable control on photosynthetic flux in the wild-type, with a relatively high flux control coefficient of 0.35 in saturating light and ambient CO2. The flux control coefficient was decreased in low light or increased in low CO2 or in plants containing lower PEPC activity. However, the rate of CO2 assimilation decreased down to about 55% PEPC, followed by an up-turn in the light-saturated photosynthetic rate as PEPC was further reduced, suggesting the existence of a mechanism that compensates for the loss of PEPC activity. The amounts of photosynthetic metabolites, including glycine and serine, also showed a biphasic response to decreasing PEPC. There was a linear relationship between the activity of PEPC and the activation state of the enzyme. A possible mechanism of compensation involving photorespiratory intermediates is discussed.

Analysis of promoter activity for the gene encoding pyruvate orthophosphate dikinase in stably transformed C4 flaveria species

The C4 enzyme pyruvate orthophosphate dikinase is encoded by a single gene, Pdk, in the C4 plant Flaveria trinervia. This gene also encodes enzyme isoforms located in the chloroplast and in the cytosol that do not have a function in C4 photosynthesis. Our goal is to identify cis-acting DNA sequences that regulate the expression of the gene that is active in the C4 cycle. We fused 1.5 kb of a 5' flanking region from the Pdk gene, including the entire 5' untranslated region, to the uidA reporter gene and stably transformed the closely related C4 species Flaveria bidentis. beta-Glucuronidase (GUS) activity was detected at high levels in leaf mesophyll cells. GUS activity was detected at lower levels in bundle-sheath cells and stems and at very low levels in roots. This lower-level GUS expression was similar to the distribution of mRNA encoding the nonphotosynthetic form of the enzyme. We conclude that cis-acting DNA sequences controlling the expression of the C4 form in mesophyll cells and the chloroplast form in other cells and organs are co-located within the same 5' region of the Pdk gene.

A simple, single-tube radioisotopic assay for the phosphorylation/inactivation activity of the pyruvate,orthophosphate dikinase regulatory protein

A simple, single-tube radiolsotopic method has been developed to assay the relative phosphorylation (inaetivation) activity of the bifunctional regulatory protein (RP) of C4-leaf pyruvate,orthophosphate dikinase (PPDK) in desalted leaf homogenates and partially purified preparations. RP catalyzes the inactivation of maize PPDK by phosphorylation of Thr-456, utilizing [β-P]ADP as the specific phosphoryl donor. Existing spectrophotometric and radioisotopic assays for the detection of RP activity are either relatively insensitive or labor-intensive and timeconsuming. We describe a modified radioisotopic assay that couples the synthesis of [β-(32)P]ADP by exogenous adenylate kinase with the subsequent RP-catalyzed [β-(32)P]ADP-dependent phosphorylation of exogenous maize PPDK. The incorporation of [β-(32)P] is dependent on the initial concentrations of ATP and PPDK, as well as the presence of active RP. Desalted leaf homogenates of C3 species fail to catalyze (32)P incorporation into exogenous maize PPDK. Conversely, heterologous systems containing the maize target enzyme and leaf homogenats of other C4 species result in PPDK-specific (32)P-incorporation. This simple radioisotopic assay is at least 40-times more sensitive than the routine spectrophotometric assay, and qualitatively exhibits comparable sensitivity and requires significantly less time than the currently available radioisotopic RP assay. The present assay reliably generates [β-(32)P]ADP and as such may be useful for studies of other systems requiring β-labeled ADP, which is not commercially available.