Regulation of Photosynthesis in C3 and C4 Plants: A Molecular Approach

Role of Circadian Rhythms in Major Plant Metabolic and Signaling Pathways

Plants require an endogenous regulatory network and mechanism to cope with diurnal environmental changes and compensate for their sessile nature. Plants use the circadian clock to anticipate diurnal changes. Circadian rhythm predicts a 24-h cycle with 16 h of light and 8 h of darkness in response to abiotic and biotic factors as well as the appropriate temperature. For a plant's fitness, proper growth, and development, these rhythms synchronize the diurnal photoperiodic changes. Input pathway, central oscillator, and output pathway are the three components that make up the endogenous clock. There are also transcriptional and translational feedback loops (TTFLs) in the clock, which are dependent on the results of gene expression. Several physiological processes, such as stress acclimatization, hormone signaling, morphogenesis, carbon metabolism, and defense response, are currently being investigated for their interactions with the circadian clock using phenotypic, genomic, and metabolic studies. This review examines the role of circadian rhythms in the regulation of plant metabolic pathways, such as photosynthesis and carbon metabolism, as well as developmental and degenerative processes, such as flowering and senescence. Furthermore, we summarized signaling pathways related to circadian rhythms, such as defense response and gene regulatory pathways.

The limiting factors and regulatory processes that control the environmental responses of C3, C3-C4 intermediate, and C4 photosynthesis

Here, we describe a model of C3, C3-C4 intermediate, and C4 photosynthesis that is designed to facilitate quantitative analysis of physiological measurements. The model relates the factors limiting electron transport and carbon metabolism, the regulatory processes that coordinate these metabolic domains, and the responses to light, carbon dioxide, and temperature. It has three unique features. First, mechanistic expressions describe how the cytochrome b6f complex controls electron transport in mesophyll and bundle sheath chloroplasts. Second, the coupling between the mesophyll and bundle sheath expressions represents how feedback regulation of Cyt b6f coordinates electron transport and carbon metabolism. Third, the temperature sensitivity of Cyt b6f is differentiated from that of the coupling between NADPH, Fd, and ATP production. Using this model, we present simulations demonstrating that the light dependence of the carbon dioxide compensation point in C3-C4 leaves can be explained by co-occurrence of light saturation in the mesophyll and light limitation in the bundle sheath. We also present inversions demonstrating that population-level variation in the carbon dioxide compensation point in a Type I C3-C4 plant, Flaveria chloraefolia, can be explained by variable allocation of photosynthetic capacity to the bundle sheath. These results suggest that Type I C3-C4 intermediate plants adjust pigment and protein distributions to optimize the glycine shuttle under different light and temperature regimes, and that the malate and aspartate shuttles may have originally functioned to smooth out the energy supply and demand associated with the glycine shuttle. This model has a wide range of potential applications to physiological, ecological, and evolutionary questions.

Understanding Maize Response to Nitrogen Limitation in Different Light Conditions for the Improvement of Photosynthesis

The photosynthetic capacity of leaves is determined by their content of nitrogen (N). Nitrogen involved in photosynthesis is divided between soluble proteins and thylakoid membrane proteins. In C4 plants, the photosynthetic apparatus is partitioned between two cell types: mesophyll cells and bundle sheath. The enzymes involved in the C4 carbon cycle and assimilation of nitrogen are localized in a cell-specific manner. Although intracellular distribution of enzymes of N and carbon assimilation is variable, little is known about the physiological consequences of this distribution caused by light changes. Light intensity and nitrogen concentration influence content of nitrates in leaves and can induce activity of the main enzymes involved in N metabolism, and changes that reduce the photosynthesis rate also reduce photosynthetic N use efficiency. In this review, we wish to highlight and discuss how/whether light intensity can improve photosynthesis in maize during nitrogen limitation. We described the general regulation of changes in the main photosynthetic and nitrogen metabolism enzymes, their quantity and localization, thylakoid protein abundance, intracellular transport of organic acids as well as specific features connected with C4 photosynthesis, and addressed the major open questions related to N metabolism and effects of light on photosynthesis in C4 plants.

Identification and characterization of a novel stay-green QTL that increases yield in maize

Functional stay-green is a valuable trait that extends the photosynthetic period, increases source capacity and biomass and ultimately translates to higher grain yield. Selection for higher yields has increased stay-green in modern maize hybrids. Here, we report a novel QTL controlling functional stay-green that was discovered in a mapping population derived from the Illinois High Protein 1 (IHP1) and Illinois Low Protein 1 (ILP1) lines, which show very different rates of leaf senescence. This QTL was mapped to a single gene containing a NAC-domain transcription factor that we named nac7. Transgenic maize lines where nac7 was down-regulated by RNAi showed delayed senescence and increased both biomass and nitrogen accumulation in vegetative tissues, demonstrating NAC7 functions as a negative regulator of the stay-green trait. More importantly, crosses between nac7 RNAi parents and two different elite inbred testers produced hybrids with prolonged stay-green and increased grain yield by an average 0.29 megagram/hectare (4.6 bushel/acre), in 2 years of multi-environment field trials. Subsequent RNAseq experiments, one employing nac7 RNAi leaves and the other using leaf protoplasts overexpressing Nac7, revealed an important role for NAC7 in regulating genes in photosynthesis, chlorophyll degradation and protein turnover pathways that each contribute to the functional stay-green phenotype. We further determined the putative target of NAC7 and provided a logical extension for the role of NAC7 in regulating resource allocation from vegetative source to reproductive sink tissues. Collectively, our findings make a compelling case for NAC7 as a target for improving functional stay-green and yields in maize and other crops.

Genome-Wide Identification and Characterization of FBA Gene Family in Polyploid Crop Brassica napus

Fructose-1,6-bisphosphate aldolase (FBA) is a versatile metabolic enzyme involved in multiple important processes of glycolysis, gluconeogenesis, and Calvin cycle. Despite its significance in plant biology, the identity of this gene family in oil crops is lacking. Here, we performed genome-wide identification and characterization of FBAs in an allotetraploid species, oilseed rape Brassica napus. Twenty-two BnaFBA genes were identified and divided into two groups based on integrative analyses of functional domains, phylogenetic relationships, and gene structures. Twelve and ten B. napus FBAs (BnaFBAs) were predicted to be localized in the chloroplast and cytoplasm, respectively. Notably, synteny analysis revealed that Brassica-specific triplication contributed to the expansion of the BnaFBA gene family during the evolution of B. napus. Various cis-acting regulatory elements pertinent to abiotic and biotic stresses, as well as phytohormone responses, were detected. Intriguingly, each of the BnaFBA genes exhibited distinct sequence polymorphisms. Among them, six contained signatures of selection, likely having experienced breeding selection during adaptation and domestication. Importantly, BnaFBAs showed diverse expression patterns at different developmental stages and were preferentially highly expressed in photosynthetic tissues. Our data thus provided the foundation for further elucidating the functional roles of individual BnaFBA and also potential targets for engineering to improve photosynthetic productivity in B. napus.

Computational modelling predicts substantial carbon assimilation gains for C3 plants with a single-celled C4 biochemical pump

Achieving global food security for the estimated 9 billion people by 2050 is a major scientific challenge. Crop productivity is fundamentally restricted by the rate of fixation of atmospheric carbon. The dedicated enzyme, RubisCO, has a low turnover and poor specificity for CO2. This limitation of C3 photosynthesis (the basic carbon-assimilation pathway present in all plants) is alleviated in some lineages by use of carbon-concentrating-mechanisms, such as the C4 cycle-a biochemical pump that concentrates CO2 near RubisCO increasing assimilation efficacy. Most crops use only C3 photosynthesis, so one promising research strategy to boost their productivity focuses on introducing a C4 cycle. The simplest proposal is to use the cycle to concentrate CO2 inside individual chloroplasts. The photosynthetic efficiency would then depend on the leakage of CO2 out of a chloroplast. We examine this proposal with a 3D spatial model of carbon and oxygen diffusion and C4 photosynthetic biochemistry inside a typical C3-plant mesophyll cell geometry. We find that the cost-efficiency of C4 photosynthesis depends on the gas permeability of the chloroplast envelope, the C4 pathway having higher quantum efficiency than C3 for permeabilities below 300 μm/s. However, at higher permeabilities the C4 pathway still provides a substantial boost to carbon assimilation with only a moderate decrease in efficiency. The gains would be capped by the ability of chloroplasts to harvest light, but even under realistic light regimes a 100% boost to carbon assimilation is possible. This could be achieved in conjunction with lower investment in chloroplasts if their cell surface coverage is also reduced. Incorporation of this C4 cycle into C3 crops could thus promote higher growth rates and better drought resistance in dry, high-sunlight climates.

Changes in northeast African hydrology and vegetation associated with Pliocene-Pleistocene sapropel cycles

East African climate change since the Late Miocene consisted of persistent shorter-term, orbital-scale wet-dry cycles superimposed upon a long-term trend towards more open, grassy landscapes. Either or both of these modes of palaeoclimate variability may have influenced East African mammalian evolution, yet the interrelationship between these secular and orbital palaeoclimate signals remains poorly understood. Here, we explore whether the long-term secular climate change was also accompanied by significant changes at the orbital-scale. We develop northeast African hydroclimate and vegetation proxy data for two 100 kyr-duration windows near 3.05 and 1.75 Ma at ODP Site 967 in the eastern Mediterranean basin, where sedimentation is dominated by eastern Sahara dust input and Nile River run-off. These two windows were selected because they have comparable orbital configurations and bracket an important increase in East African C4 grasslands. We conducted high-resolution (2.5 kyr sampling) multiproxy biomarker, H- and C-isotopic analyses of plant waxes and lignin phenols to document orbital-scale changes in hydrology, vegetation and woody cover for these two intervals. Both intervals are dominated by large-amplitude, precession-scale (approx. 20 kyr) changes in northeast African vegetation and rainfall/run-off. The δ(13)Cwax values and lignin phenol composition record a variable but consistently C4 grass-dominated ecosystem for both intervals (50-80% C4). Precessional δDwax cycles were approximately 20-30‰ in peak-to-peak amplitude, comparable with other δDwax records of the Early Holocene African Humid Period. There were no significant differences in the means or variances of the δDwax or δ(13)Cwax data for the 3.05 and 1.75 Ma intervals studied, suggesting that the palaeohydrology and palaeovegetation responses to precessional forcing were similar for these two periods. Data for these two windows suggest that the eastern Sahara did not experience the significant increase in C4 vegetation that has been observed in East Africa over this time period. This observation would be consistent with a proposed mechanism whereby East African precipitation is reduced, and drier conditions established, in response to the emergence of modern zonal sea surface temperature gradients in the tropical oceans between 3 and 2 Ma.This article is part of the themed issue 'Major transitions in human evolution'.

Comparative cell-specific transcriptomics reveals differentiation of C4 photosynthesis pathways in switchgrass and other C4 lineages

Almost all C4 plants require the co-ordination of the adjacent and fully differentiated cell types, mesophyll (M) and bundle sheath (BS). The C4 photosynthetic pathway operates through two distinct subtypes based on how malate is decarboxylated in BS cells; through NAD-malic enzyme (NAD-ME) or NADP-malic enzyme (NADP-ME). The diverse or unique cell-specific molecular features of M and BS cells from separate C4 subtypes of independent lineages remain to be determined. We here provide an M/BS cell type-specific transcriptome data set from the monocot NAD-ME subtype switchgrass (Panicum virgatum). A comparative transcriptomics approach was then applied to compare the M/BS mRNA profiles of switchgrass, monocot NADP-ME subtype C4 plants maize and Setaria viridis, and dicot NAD-ME subtype Cleome gynandra. We evaluated the convergence in the transcript abundance of core components in C4 photosynthesis and transcription factors to establish Kranz anatomy, as well as gene distribution of biological functions, in these four independent C4 lineages. We also estimated the divergence between NAD-ME and NADP-ME subtypes of C4 photosynthesis in the two cell types within C4 species, including differences in genes encoding decarboxylating enzymes, aminotransferases, and metabolite transporters, and differences in the cell-specific functional enrichment of RNA regulation and protein biogenesis/homeostasis. We suggest that C4 plants of independent lineages in both monocots and dicots underwent convergent evolution to establish C4 photosynthesis, while distinct C4 subtypes also underwent divergent processes for the optimization of M and BS cell co-ordination. The comprehensive data sets in our study provide a basis for further research on evolution of C4 species.

An integrated "omics" approach to the characterization of maize (Zea mays L.) mutants deficient in the expression of two genes encoding cytosolic glutamine synthetase

Background

To identify the key elements controlling grain production in maize, it is essential to have an integrated view of the responses to alterations in the main steps of nitrogen assimilation by modification of gene expression. Two maize mutant lines (gln1.3 and gln1.4), deficient in two genes encoding cytosolic glutamine synthetase, a key enzyme involved in nitrogen assimilation, were previously characterized by a reduction of kernel size in the gln1.4 mutant and by a reduction of kernel number in the gln1.3 mutant. In this work, the differences in leaf gene transcripts, proteins and metabolite accumulation in gln1.3 and gln1.4 mutants were studied at two key stages of plant development, in order to identify putative candidate genes, proteins and metabolic pathways contributing on one hand to the control of plant development and on the other to grain production.

Results

The most interesting finding in this study is that a number of key plant processes were altered in the gln1.3 and gln1.4 mutants, including a number of major biological processes such as carbon metabolism and transport, cell wall metabolism, and several metabolic pathways and stress responsive and regulatory elements. We also found that the two mutants share common or specific characteristics across at least two or even three of the “omics” considered at the vegetative stage of plant development, or during the grain filling period.

Conclusions

This is the first comprehensive molecular and physiological characterization of two cytosolic glutamine synthetase maize mutants using a combined transcriptomic, proteomic and metabolomic approach. We find that the integration of the three “omics” procedures is not straight forward, since developmental and mutant-specific levels of regulation seem to occur from gene expression to metabolite accumulation. However, their potential use is discussed with a view to improving our understanding of nitrogen assimilation and partitioning and its impact on grain production.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1005) contains supplementary material, which is available to authorized users.

Recent Progress in Advanced Nanobiological Materials for Energy and Environmental Applications

In this review, we briefly introduce our efforts to reconstruct cellular life processes by mimicking natural systems and the applications of these systems to energy and environmental problems. Functional units of in vitro cellular life processes are based on the fabrication of artificial organelles using protein-incorporated polymersomes and the creation of bioreactors. This concept of an artificial organelle originates from the first synthesis of poly(siloxane)-poly(alkyloxazoline) block copolymers three decades ago and the first demonstration of protein activity in the polymer membrane a decade ago. The increased value of biomimetic polymers results from many research efforts to find new applications such as functionally active membranes and a biochemical-producing polymersome. At the same time, foam research has advanced to the point that biomolecules can be efficiently produced in the aqueous channels of foam. Ongoing research includes replication of complex biological processes, such as an artificial Calvin cycle for application in biofuel and specialty chemical production, and carbon dioxide sequestration. We believe that the development of optimally designed biomimetic polymers and stable/biocompatible bioreactors would contribute to the realization of the benefits of biomimetic systems. Thus, this paper seeks to review previous research efforts, examine current knowledge/key technical parameters, and identify technical challenges ahead.

Photosynthetic gene expression in higher plants

Within the chloroplasts of higher plants and algae, photosynthesis converts light into biological energy, fueling the assimilation of atmospheric carbon dioxide into biologically useful molecules. Two major steps, photosynthetic electron transport and the Calvin-Benson cycle, require many gene products encoded from chloroplast as well as nuclear genomes. The expression of genes in both cellular compartments is highly dynamic and influenced by a diverse range of factors. Light is the primary environmental determinant of photosynthetic gene expression. Working through photoreceptors such as phytochrome, light regulates photosynthetic genes at transcriptional and posttranscriptional levels. Other processes that affect photosynthetic gene expression include photosynthetic activity, development, and biotic and abiotic stress. Anterograde (from nucleus to chloroplast) and retrograde (from chloroplast to nucleus) signaling insures the highly coordinated expression of the many photosynthetic genes between these different compartments. Anterograde signaling incorporates nuclear-encoded transcriptional and posttranscriptional regulators, such as sigma factors and RNA-binding proteins, respectively. Retrograde signaling utilizes photosynthetic processes such as photosynthetic electron transport and redox signaling to influence the expression of photosynthetic genes in the nucleus. The basic C3 photosynthetic pathway serves as the default form used by most of the plant species on earth. High temperature and water stress associated with arid environments have led to the development of specialized C4 and CAM photosynthesis, which evolved as modifications of the basic default expression program. The goal of this article is to explain and summarize the many gene expression and regulatory processes that work together to support photosynthetic function in plants.

Hydrogen peroxide functions as a secondary messenger for brassinosteroids-induced CO2 assimilation and carbohydrate metabolism in Cucumis sativus

Brassinosteroids (BRs) are potent regulators of photosynthesis and crop yield in agricultural crops; however, the mechanism by which BRs increase photosynthesis is not fully understood. Here, we show that foliar application of 24-epibrassinolide (EBR) resulted in increases in CO(2) assimilation, hydrogen peroxide (H(2)O(2)) accumulation, and leaf area in cucumber. H(2)O(2) treatment induced increases in CO(2) assimilation whilst inhibition of the H(2)O(2) accumulation by its generation inhibitor or scavenger completely abolished EBR-induced CO(2) assimilation. Increases of light harvesting due to larger leaf areas in EBR- and H(2)O(2)-treated plants were accompanied by increases in the photochemical efficiency of photosystem II (Φ(PSII)) and photochemical quenching coefficient (q(P)). EBR and H(2)O(2) both activated carboxylation efficiency of ribulose-1,5-bisphosphate oxygenase/carboxylase (Rubisco) from analysis of CO(2) response curve and in vitro measurement of Rubisco activities. Moreover, EBR and H(2)O(2) increased contents of total soluble sugar, sucrose, hexose, and starch, followed by enhanced activities of sugar metabolism such as sucrose phosphate synthase, sucrose synthase, and invertase. Interestingly, expression of transcripts of enzymes involved in starch and sugar utilization were inhibited by EBR and H(2)O(2). However, the effects of EBR on carbohydrate metabolisms were reversed by the H(2)O(2) generation inhibitor diphenyleneodonium (DPI) or scavenger dimethylthiourea (DMTU) pretreatment. All of these results indicate that H(2)O(2) functions as a secondary messenger for EBR-induced CO(2) assimilation and carbohydrate metabolism in cucumber plants. Our study confirms that H(2)O(2) mediates the regulation of photosynthesis by BRs and suggests that EBR and H(2)O(2) regulate Calvin cycle and sugar metabolism via redox signaling and thus increase the photosynthetic potential and yield of crops.

Antisense reductions in the PsbO protein of photosystem II leads to decreased quantum yield but similar maximal photosynthetic rates

Photosystem (PS) II is the multisubunit complex which uses light energy to split water, providing the reducing equivalents needed for photosynthesis. The complex is susceptible to damage from environmental stresses such as excess excitation energy and high temperature. This research investigated the in vivo photosynthetic consequences of impairments to PSII in Arabidopsis thaliana (ecotype Columbia) expressing an antisense construct to the PsbO proteins of PSII. Transgenic lines were obtained with between 25 and 60% of wild-type (WT) total PsbO protein content, with the PsbO1 isoform being more strongly reduced than PsbO2. These changes coincided with a decrease in functional PSII content. Low PsbO (less than 50% WT) plants grew more slowly and had lower chlorophyll content per leaf area. There was no change in content per unit area of cytochrome b6f, ATP synthase, or Rubisco, whereas PSI decreased in proportion to the reduction in chlorophyll content. The irradiance response of photosynthetic oxygen evolution showed that low PsbO plants had a reduced quantum yield, but matched the oxygen evolution rates of WT plants at saturating irradiance. It is suggested that these plants had a smaller pool of PSII centres, which are inefficiently connected to antenna pigments resulting in reduced photochemical efficiency.

Identification of C4 photosynthesis metabolism and regulatory-associated genes in Eleocharis vivipara by SSH

This is the first effort to investigate the candidate genes involved in kranz developmental regulation and C(4) metabolic fluxes in Eleocharis vivipara, which is a leafless freshwater amphibious plant and possesses a distinct culms anatomy structure and photosynthetic pattern in contrasting environments. A terrestrial specific SSH library was constructed to investigate the genes involved in kranz anatomy developmental regulation and C(4) metabolic fluxes. A total of 73 ESTs and 56 unigenes in 384 clones were identified by array hybridization and sequencing. In total, 50 unigenes had homologous genes in the databases of rice and Arabidopsis. The real-time quantitative PCR results showed that most of the genes were accumulated in terrestrial culms and ABA-induced culms. The C(4) marker genes were stably accumulated during the culms development process in terrestrial culms. With respect to C(3) culms, C(4) photosynthesis metabolism consumed much more transporters and translocators related to ion metabolism, organic acids and carbohydrate metabolism, phosphate metabolism, amino acids metabolism, and lipids metabolism. Additionally, ten regulatory genes including five transcription factors, four receptor-like proteins, and one BURP protein were identified. These regulatory genes, which co-accumulated with the culms developmental stages, may play important roles in culms structure developmental regulation, bundle sheath chloroplast maturation, and environmental response. These results shed new light on the C(4) metabolic fluxes, environmental response, and anatomy structure developmental regulation in E. vivipara.

Regulation of photosynthesis by sugars in sugarcane leaves

In sugarcane, increased sink demand has previously been shown to result in increased photosynthetic rates that are correlated with a reduction in leaf hexose concentrations. To establish whether sink limitation of photosynthesis is a result of sugar accumulation in the leaf, excision and cold-girdling techniques were used to modify leaf sugar concentrations in pot-grown sugarcane. In excised leaves that were preincubated in darkness for 3h, sucrose accumulation was reduced but accumulated again upon transfer to the light, while hexose concentrations remained lower than in controls (7.7 micromol mg(-1)FW versus 18.6 micromol mg(-1)FW hexose in controls). These results were associated with a 66% and 59% increase in photosynthetic assimilation (A) and electron transport rate (ETR), respectively, compared to controls maintained in the light. Similar increases in photosynthesis were observed when dark-treated leaves were supplied with 5mM sorbitol, but not when supplied with 5mM sucrose. Further analyses of (14)C-labeled sugars indicated rapid turnover between sucrose and hexose. Cold-girdling (5 degrees C) increased sucrose and hexose levels and resulted in a decline of photosynthetic rates over 5d (48% and 35% decline in assimilation rate and ETR, respectively). These sugar-induced changes in photosynthesis were independent of changes in stomatal conductance. This study demonstrates that the down-regulation of photosynthesis in response to culm sugar accumulation reported previously could be due to the knock-on effect of accumulation of sugar in leaf tissue, and supports the contention that hexose, rather than sucrose, is responsible for the modulation of photosynthetic activity.

Plant light interception can be explained via computed tomography scanning: demonstration with pyramidal cedar (Thuja occidentalis, Fastigiata)

BACKGROUND AND AIMS

Light interception by the leaf canopy is a key aspect of plant photosynthesis, which helps mitigate the greenhouse effect via atmospheric CO(2) recycling. The relationship between plant light interception and leaf area was traditionally modelled with the Beer-Lambert law, until the spatial distribution of leaves was incorporated through the fractal dimension of leafless plant structure photographed from the side allowing maximum appearance of branches and petioles. However, photographs of leafless plants are two-dimensional projections of three-dimensional structures, and sampled plants were cut at the stem base before leaf blades were detached manually, so canopy development could not be followed for individual plants. Therefore, a new measurement and modelling approach were developed to explain plant light interception more completely and precisely, based on appropriate processing of computed tomography (CT) scanning data collected for developing canopies.

METHODS

Three-dimensional images of canopies were constructed from CT scanning data. Leaf volumes (LV) were evaluated from complete canopy images, and fractal dimensions (FD) were estimated from skeletonized leafless images. The experimental plant species is pyramidal cedar (Thuja occidentalis, Fastigiata).

KEY RESULTS

The three-dimensional version of the Beer-Lambert law based on FD alone provided a much better explanation of plant light interception (R(2) = 0.858) than those using the product LV*FD (0.589) or LV alone (0.548). While values of all three regressors were found to increase over time, FD in the Beer-Lambert law followed the increase in light interception the most closely. The delayed increase of LV reflected the appearance of new leaves only after branches had lengthened and ramified.

CONCLUSIONS

The very strong correlation obtained with FD demonstrates that CT scanning data contain fundamental information about the canopy architecture geometry. The model can be used to identify crops and plantation trees with improved light interception and productivity.

Changes in photosynthetic rates and gene expression of leaves during a source-sink perturbation in sugarcane

BACKGROUND AND AIMS

In crops other than sugarcane there is good evidence that the size and activity of carbon sinks influence source activity via sugar-related regulation of the enzymes of photosynthesis, an effect that is partly mediated through coarse regulation of gene expression.

METHODS

In the current study, leaf shading treatments were used to perturb the source-sink balance in 12-month-old Saccharum spp. hybrid 'N19' (N19) by restricting source activity to a single mature leaf. Changes in leaf photosynthetic gas exchange variables and leaf and culm sugar concentrations were subsequently measured over a 14 d period. In addition, the changes in leaf gene response to the source-sink perturbation were measured by reverse northern hybridization analysis of an array of 128 expressed sequence tags (ESTs) related to photosynthetic and carbohydrate metabolism.

KEY RESULTS

Sucrose concentrations in immature culm tissue declined significantly over the duration of the shading treatment, while a 57 and 88% increase in the assimilation rate (A) and electron transport rate (ETR), respectively, was observed in the source leaf. Several genes (27) in the leaf displayed a >2-fold change in expression level, including the upregulation of several genes associated with C(4) photosynthesis, mitochondrial metabolism and sugar transport. Changes in gene expression levels of several genes, including Rubisco (EC 4.1.1.39) and hexokinase (HXK; EC 2.7.1.1), correlated with changes in photosynthesis and tissue sugar concentrations that occurred subsequent to the source-sink perturbation.

CONCLUSIONS

These results are consistent with the notion that sink demand may limit source activity through a kinase-mediated sugar signalling mechanism that correlates to a decrease in source hexose concentrations, which, in turn, correlate with increased expression of genes involved in photosynthesis and metabolite transport. The signal feedback system reporting sink sufficiency and regulating source activity may be a potentially valuable target for future genetic manipulation to increase sugarcane sucrose yield.

Characterization of photosynthesis of flag leaves in a wheat hybrid and its parents grown under field conditions

Two wheat cultivars, one with high yield and the other with a high tolerance against oxidative stress, were compared with a hybrid of these two cultivars by investigating their photosynthetic characteristics of flag leaves. From the beginning of flowering to the 17th day, CO(2) assimilation rate (P(max)) was maintained and there were no appreciable differences between the hybrid and its parents. P(max) showed no decrease at noon compared to that in the morning. From the 20th to the 30th day of flowering, P(max) decreased significantly, and this decrease was significantly less in the hybrid than in its parents. The actual photosystem II (PSII) efficiency (Phi(PSII)) and the maximal efficiency of PSII photochemistry (F(v)/F(m)) showed a significant decrease only on the 30th day after anthesis; this decline was much less marked in the hybrid relative to its parents, both in the morning and at noon. A decrease occurred in Phi(PSII) and F(v)/F(m) at noon when compared to that in the morning, but this decrease was less marked in the hybrid than in its parents. Rubisco activity decreased significantly from the 13th day of flowering and was higher in the morning than at noon both in the hybrid and its parents. However, the hybrid always showed a higher value of Rubisco activity. The activities of posphoenolpyruvate carboxylase and pyruvate phosphate dikinase showed similar changes to those in Rubisco activity, particularly from the 20th to 30th day. The results of this study suggest that the higher photosynthetic capacity of the flag leaf in the hybrid can help to accumulate more dry material, and may be the physiological basis for higher yield over its parents.

Binding of a transition state analog to newly synthesized Rubisco

Radioactive amino acids, when added to isolated pea chloroplasts or chloroplast extracts engaged in protein synthesis, are incorporated into Rubisco large subunits that co-migrate with native Rubisco during nondenaturing electrophoresis. We have added the transition state analog 2'-carboxyarabinitol bisphosphate (CABP) to chloroplast extracts after in organello or in vitro incorporation of radioactive amino acids into Rubisco large subunits. Upon addition of CABP the radioactive bands co-migrating with native Rubisco undergo a readily detected shift in electrophoretic mobility just as the native enzyme, thus demonstrating the ability of the newly assembled molecules to interact with this transition state analog.

Photosynthetic light and CO2 utilization and C4 traits of two novel super-rice hybrids

Characteristics of photosynthetic light and CO2 use efficiency from seedling to heading stage, and C4 pathway enzyme activities in both flag leaves and lemma were compared between two newly developed super-rice hybrids (Oryza sativa L.), Liangyoupeijiu and Hua-an 3, and a traditional rice hybrid, Shanyou 63. At seedling and tillering stages, Liangyoupeijiu and Hua-an 3 had higher net photosynthetic rates (Pn) and light saturated assimilation rates (Asat) than did Shanyou 63, at both normal (360 micromol mol(-1)) and doubled (720 micromol mol(-1)) CO2 concentrations. At the heading stage, the flag leaves of all three rice hybrids had similar Pn and Asat. However, the two super-rice hybrids had higher apparent quantum yield (AQY) and carboxylation efficiency (CE) during all three typical developmental stages, and higher quantum yield of CO2 fixation (PhiCO2) at the tillering and heading stages. In addition, Liangyoupeijiu showed significantly higher activities of the C(4) pathway enzymes in both flag leaves and lemmas than did Shanyou 63. As a result, flag leaves of the two super-rice hybrids had higher Pn at morning, noontime and late afternoon during the daily cycle. Since most of the grain yield of rice comes from the photosynthesis of flag leaves, the similar Asat and much higher AQY, CE and PhiCO2 at heading stage of the two super-rice hybrids indicates that higher photosynthetic efficiency rather than higher photosynthetic capacity may be the primary factor contributing to their higher grain yields.

The promoter for C4-type mitochondrial aspartate aminotransferase does not direct bundle sheath-specific expression in transgenic rice plants

For NAD-malic enzyme (NAD-ME)-type C4 photosynthesis, two types of aspartate aminotransferase (AAT) are involved. We examined the expression pattern of the Panicum miliaceum mitochondrial Aat gene (PmAat) and P. miliaceum cytosolic Aat gene (PcAat) in transgenic rice plants, which were specifically expressed in bundle sheath cells (BSCs) and mesophyll cells (MCs), respectively. Expression of a beta-glucuronidase (GUS) reporter gene under the control of the PcAat promoter was regulated in an organ-preferential and light-dependent manner in the transgenic rice plants. However, the PmAat promoter drove the GUS expression in all organs we tested without light dependency, and this non-preferential expression pattern was also observed in transgenic rice with introduction of the intact PmAat gene. The expression patterns of the rice counterpart Aat genes to PmAat or PcAat showed that the rice mitochondrial Aat (RmAat1) gene was expressed in all organs tested in a light-independent manner, while expression of the rice cytosolic Aat (RcAat1) gene showed an organ-preferential and light-dependent pattern. Taking these results together, we can generalize that the regulatory system of BSC-specific or light-dependent expression of mitochondrial Aat is not shared between P. miliaceum (C4) and rice (C3) and that the expression of the C4 genes introduced into rice mimics that of their counterpart genes in rice.

Untranslated regions from C4 amaranth AhRbcS1 mRNAs confer translational enhancement and preferential bundle sheath cell expression in transgenic C4 Flaveria bidentis

Many aspects of photosynthetic gene expression are posttranscriptionally regulated in C4 plants. To determine if RbcS mRNA untranslated regions (UTRs) in themselves could confer any characteristic C4 expression patterns, 5'- and 3'-UTRs of AhRbcS1 mRNA from the C4 dicot amaranth were linked to a gusA reporter gene. These were constitutively transcribed from a cauliflower mosaic virus promoter and assayed for posttranscriptional expression patterns in transgenic lines of the C4 dicot Flaveria bidentis. Three characteristic C4 expression patterns were conferred by heterologous AhRbcS1 UTRs in transgenic F. bidentis. First, the AhRbcS1 UTRs conferred strong translational enhancement of gusA expression, relative to control constructs lacking these UTRs. Second, while the UTRs did not appear to confer tissue-specific expression when analyzed by beta-glucuronidase activity assays, differences in gusA mRNA accumulation were observed in leaves, stems, and roots. Third, the AhRbcS1 UTRs conferred preferential gusA expression (enzyme activity and gusA mRNA accumulation) in leaf bundle sheath cells. AhRbcS1 UTR-mediated translational enhancement was also observed in transgenic C3 plants (tobacco [Nicotiana tabacum]) and in in vitro translation extracts. These mRNAs appear to be translated with different efficiencies in C4 versus C3 plants, indicating that processes determining overall translational efficiency may vary between these two categories of higher plants. Our findings suggest that the AhRbcS1 5'-UTR functions as a strong translational enhancer in leaves and other tissues, and may work synergistically with the 3'-UTR to modulate overall levels of Rubisco gene expression in different tissues and cell types of C4 plants.

The C(4) pathway: an efficient CO(2) pump

The C(4) pathway is a complex combination of both biochemical and morphological specialisation, which provides an elevation of the CO(2) concentration at the site of Rubisco. We review the key parameters necessary to make the C(4) pathway function efficiently, focussing on the diffusion of CO(2) out of the bundle sheath compartment. Measurements of cell wall thickness show that the thickness of bundle sheath cell walls in C(4) species is similar to cell wall thickness of C(3) mesophyll cells. Furthermore, NAD-ME type C(4) species, which do not have suberin in their bundle sheath cell walls, do not appear to compensate for this with thicker bundle sheath cell walls. Uncertainties in the CO(2) diffusion properties of membranes, such as the plasmalemma, choroplast and mitochondrial membranes make it difficult to estimate bundle sheath diffusion resistance from anatomical measurements, but the cytosol itself may account for more than half of the final calculated resistance value for CO(2) leakage. We conclude that the location of the site of decarboxylation, its distance from the mesophyll interface and the physical arrangement of chloroplasts and mitochondria in the bundle sheath cell are as important to the efficiency of the process as the properties of the bundle sheath cell wall. Using a mathemathical model of C(4) photosynthesis, we also examine the relationship between bundle sheath resistance to CO(2) diffusion and the biochemical capacity of the C(4) photosynthetic pathway and conclude that bundle sheath resistance to CO(2) diffusion must vary with biochemical capacity if the efficiency of the C(4) pump is to be maintained. Finally, we construct a mathematical model of single cell C(4) photosynthesis in a C(3) mesophyll cell and examine the theoretical efficiency of such a C(4) photosynthetic CO(2) pump.

The Calvin cycle revisited

The sequence of reactions in the Calvin cycle, and the biochemical characteristics of the enzymes involved, have been known for some time. However, the extent to which any individual enzyme controls the rate of carbon fixation has been a long standing question. Over the last 10 years, antisense transgenic plants have been used as tools to address this and have revealed some unexpected findings about the Calvin cycle. It was shown that under a range of environmental conditions, the level of Rubisco protein had little impact on the control of carbon fixation. In addition, three of the four thioredoxin regulated enzymes, FBPase, PRKase and GAPDH, had negligible control of the cycle. Unexpectedly, non-regulated enzymes catalysing reversible reactions, aldolase and transketolase, both exerted significant control over carbon flux. Furthermore, under a range of growth conditions SBPase was shown to have a significant level of control over the Calvin cycle. These data led to the hypothesis that increasing the amounts of these enzymes may lead to an increase in photosynthetic carbon assimilation. Remarkably, photosynthetic capacity and growth were increased in tobacco plants expressing a bifunctional SBPase/FBPase enzyme. Future work is discussed which will further our understanding of this complex and important pathway, particularly in relation to the mechanisms that regulate and co-ordinate enzyme activity.

The Calvin cycle revisited

The sequence of reactions in the Calvin cycle, and the biochemical characteristics of the enzymes involved, have been known for some time. However, the extent to which any individual enzyme controls the rate of carbon fixation has been a long standing question. Over the last 10 years, antisense transgenic plants have been used as tools to address this and have revealed some unexpected findings about the Calvin cycle. It was shown that under a range of environmental conditions, the level of Rubisco protein had little impact on the control of carbon fixation. In addition, three of the four thioredoxin regulated enzymes, FBPase, PRKase and GAPDH, had negligible control of the cycle. Unexpectedly, non-regulated enzymes catalysing reversible reactions, aldolase and transketolase, both exerted significant control over carbon flux. Furthermore, under a range of growth conditions SBPase was shown to have a significant level of control over the Calvin cycle. These data led to the hypothesis that increasing the amounts of these enzymes may lead to an increase in photosynthetic carbon assimilation. Remarkably, photosynthetic capacity and growth were increased in tobacco plants expressing a bifunctional SBPase/FBPase enzyme. Future work is discussed which will further our understanding of this complex and important pathway, particularly in relation to the mechanisms that regulate and co-ordinate enzyme activity.

Distinct but conserved functions for two chloroplastic NADP-malic enzyme isoforms in C3 and C4 Flaveria species

In the most common C4 pathway for carbon fixation, an NADP-malic enzyme (NADP-ME) decarboxylates malate in the chloroplasts of bundle sheath cells. Isoforms of plastidic NADP-ME are encoded by two genes in all species of Flaveria, including C3, C3-C4 intermediate, and C4 types. However, only one of these genes, ChlMe1, encodes the enzyme that functions in the C4 pathway. We compared the expression patterns of the ChlMe1 and ChlMe2 genes in developing leaves of Flaveria pringlei (C3) and Flaveria trinervia (C4) and in transgenic Flaveria bidentis (C4). ChlMe1 expression in C4 species increases in leaves with high C4 pathway activity. In the C3 species F. pringlei, ChlMe1 expression is transient and limited to early leaf development. In contrast, ChlMe2 is expressed in C3 and C4 species concurrent with stages in chloroplast biogenesis. Because previous studies suggest that NADP-ME activities generally reflect the level of its mRNA abundance, we discuss possible roles of ChlMe1 and ChlMe2 based on these expression patterns.

MOLECULAR ENGINEERING OF C4 PHOTOSYNTHESIS

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

Mesophyll-specific, light and metabolic regulation of the C4 PPCZm1 promoter in transgenic maize

To play an essential role in C4 photosynthesis, the maize C4 phosphoenolpyruvate carboxylase gene (PPCZm1) acquired many new expression features, such as leaf specificity, mesophyll specificity, light inducibility and high activity, that distinguish the unique C4 PPC from numerous non-C4 PPC genes in maize. We present here the first investigation of the developmental, cell-specific, light and metabolic regulation of the homologous C4 PPCZm1 promoter in stable transgenic maize plants. We demonstrate that the 1.7 kb of the 5'-flanking region of the PPCZm1 gene is sufficient to direct the C4-specific expression patterns of beta-glucuronidase (GUS) activity, as a reporter, in stable transformed maize plants. In light-grown shoots, GUS expression was strongest in all developing and mature mesophyll cells in the leaf, collar and sheath. GUS activity was also detected in mesophyll cells in the outer husks of ear shoots and in the outer glumes of staminate spikelets. We did not observe histological localization of GUS activity in light- or dark-grown callus, roots, silk, developing or mature kernels, the shoot apex, prop roots, or pollen. In addition, we used the stable expressing transformants to conduct and quantify physiological induction studies. Our results indicate that the expression of the C4 PPCZm1-GUS fusion gene is mesophyll-specific and influenced by development, light, glucose, acetate and chloroplast biogenesis in transgenic maize plants. These studies suggest that the adoption of DNA regulatory elements for C4-specific gene expression is a crucial step in C4 gene evolution.

Binding of cell type-specific nuclear proteins to the 5'-flanking region of maize C4 phosphoenolpyruvate carboxylase gene confers its differential transcription in mesophyll cells

C4-type phosphenolpyruvate carboxylase (C4PEPC) acts as a primary carbon assimilatory enzyme in the C4 photosynthetic pathway. The maize C4PEPC gene (C4Ppc1) is specifically expressed in mesophyll cells (MC) of light-grown leaves, but the molecular mechanism responsible for its cell type-specific expression has not been characterized. In this study, we introduced a chimeric maize C4Ppc1 5'-flanking region/beta-glucuronidase (GUS) gene into maize plants by Agrobacterium-mediated transformation. Activity assay and histochemical staining showed that GUS is almost exclusively localized in leaf MC of transgenic maize plants. This observation suggests that the introduced 5' region of maize C4Ppc1 contains the necessary cis element(s) for its specific expression in MC. Next, we investigated whether the 5' region of the maize gene interacts with nuclear proteins in a cell type-specific manner. By gel shift assays with nuclear extracts prepared from MC or bundle sheath cells (BSC), cell type-specific DNA-protein interactions were detected: nuclear factors PEP(Ib) and PEP(Ic) are specific to MC whereas PEP(Ia) and PEP(IIa) are specific to BSC. Light alters the binding activity of these factors. These interactions were not detected in the assay with nuclear extract prepared from root, or competed out by oligonucleotides corresponding to the binding sites for the maize nuclear protein, PEP-I, which is known to bind specifically to the promoter region of C4Ppc1. The results suggest that novel cell type-specific positive and negative nuclear factors bind to the maize C4Ppc1 5'-flanking region and regulate its differential transcription in MC in a light-dependent manner.

Complementary expression of two plastid-localized sigma-like factors in maize

The eubacterial-like RNA polymerase of plastids is composed of organelle-encoded core subunits and nuclear-encoded sigma-factors. Families of sigma-like factors (SLFs) have been identified in several plants, including maize (Zea mays) and Arabidopsis. In vitro import assays determined that at least two of the maize sigma-like proteins have functional chloroplast transit peptides and thus are likely candidates for chloroplast transcriptional regulators. However, the roles of individual SLFs in chloroplast transcription remain to be determined. We have raised antibodies against the unique amino-terminal domains of two maize SLFs, ZmSig1 and ZmSig3, and have used these specific probes to examine the accumulation of each protein in different maize tissues and during chloroplast development. The expression of ZmSig1 is tissue specific and parallels the light-activated chloroplast development program in maize seedling leaves. Its accumulation in mature chloroplasts however, is not affected by subsequent changes in the light regime. It is interesting that the expression profile of ZmSig3 is complementary to that of ZmSig1. It accumulates in non-green tissues, including roots, etiolated seedling leaves, and the basal region of greening seedling leaves. The nonoverlapping expression patterns of these two plastid-localized SLFs suggest that they may direct differential expression of plastid genes during chloroplast development.

Responses of wild watermelon to drought stress: accumulation of an ArgE homologue and citrulline in leaves during water deficits

Wild watermelon from the Botswana desert had an ability to survive under severe drought conditions by maintaining its water status (water content and water potential). In the analysis by two-dimensional electrophoresis of leaf proteins, seven spots were newly induced after watering stopped. One with the molecular mass of 40 kilodaltons of the spots was accumulated abundantly. The cDNA encoding for the protein was cloned based on its amino-terminal sequence and the amino acid sequence deduced from the determined nucleotide sequences of the cDNA exhibited homology to the enzymes belong to the ArgE/DapE/Acy1/Cpg2/YscS protein family (including acetylornithine deacetylase, carboxypeptidase and aminoacylase-1). This suggests that the protein is involved in the release of free amino acid by hydrolyzing a peptidic bond. As the drought stress progressed, citrulline became one of the major components in the total free amino acids. Eight days after withholding watering, although the lower leaves wilted significantly, the upper leaves still maintained their water status and the content of citrulline reached about 50% in the total free amino acids. The accumulation of citrulline during the drought stress in wild watermelon is an unique phenomenon in C3-plants. These results suggest that the drought tolerance of wild watermelon is related to (1) the maintenance of the water status and (2) a metabolic change to accumulate citrulline.

Post-transcriptional regulation prevents accumulation of glutathione reductase protein and activity in the bundle sheath cells of maize

Glutathione reductase (GR; EC 1.6.4.2) activity was assayed in bundle sheath and mesophyll cells of maize (Zea mays L. var H99) from plants grown at 20 degrees C, 18 degrees C, and 15 degrees C. The purity of each fraction was determined by measuring the associated activity of the compartment-specific marker enzymes, Rubisco and phosphoenolpyruvate carboxylase, respectively. GR activity and the abundance of GR protein and mRNA increased in plants grown at 15 degrees C and 18 degrees C compared with those grown at 20 degrees C. In all cases GR activity was found only in mesophyll fractions of the leaves, with no GR activity being detectable in bundle sheath extracts. Immunogold labeling with GR-specific antibodies showed that the GR protein was exclusively localized in the mesophyll cells of leaves at all growth temperatures, whereas GR transcripts (as determined by in situ hybridization techniques) were observed in both cell types. These results indicate that post-transcriptional regulation prevents GR accumulation in the bundle sheath cells of maize leaves. The resulting limitation on the capacity for regeneration of reduced glutathione in this compartment may contribute to the extreme chilling sensitivity of maize leaves.

Low temperature-induced changes in the distribution of H2O2 and antioxidants between the bundle sheath and mesophyll cells of maize leaves

The distribution of antioxidants between bundle sheath and mesophyll cells of maize leaves was analysed in plants grown at 20 degrees C, 18 degrees C and 15 degrees C. The purity of the isolated bundle sheath and mesophyll fractions was determined using compartment-specific marker enzymes. In plants grown at 15 degrees C, ascorbate peroxidase, CuZn-superoxide dismutase (CuZn-SOD) and monodehydroascorbate reductase activities were increased in the bundle sheath cells, and glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were enhanced in the mesophyll cells. SOD was absent from the mesophyll of plants grown at 20 degrees C but an Fe-SOD activity was found in the mesophyll of plants grown at 15 degrees C. Foliar Mn-SOD activities were decreased at 15 degrees C compared to 20 degrees C. Catalase was undetectable in the mesophyll extracts of plants grown at 15 degrees C. Ascorbate and glutathione contents were considerably higher in the mesophyll than the bundle sheath fractions of plants grown at 20 degrees C. The ratios of reduced to oxidized forms of these antioxidants were significantly decreased in the bundle sheath, but increased in the mesophyll of leaves grown at 15 degrees C. Foliar H2O2 accumulated at 15 degrees C compared to 20 degrees C. Most of the foliar H2O2 was localized in the mesophyll tissues at all growth temperatures. The differential distribution of antioxidants between leaf bundle sheath and mesophyll tissues, observed at 20 degrees C, is even more pronounced when plants are grown at 15 degrees C and may contribute to the extreme sensitivity of maize to low temperatures.

Light induction of cell type differentiation and cell-type-specific gene expression in cotyledons of a C(4) plant, Flaveria trinervia

In Flaveria trinervia (Asteraceae) seedlings, light-induced signals are required for differentiation of cotyledon bundle sheath cells and mesophyll cells and for cell-type-specific expression of Rubisco small subunit genes (bundle sheath cell specific) and the genes that encode pyruvate orthophosphate dikinase and phosphoenolpyruvate carboxylase (mesophyll cell specific). Both cell type differentiation and cell-type-specific gene expression were complete by d 7 in light-grown seedlings, but were arrested beyond d 4 in dark-grown seedlings. Our results contrast with those found for another C(4) dicot, Amaranthus hypochondriacus, in which light was not required for either process. The differences between the two C(4) dicot species in cotyledon cell differentiation may arise from differences in embryonic and post-embryonic cotyledon development. Our results illustrate that a common C(4) photosynthetic mechanism can be established through different developmental pathways in different species, and provide evidence for independent evolutionary origins of C(4) photosynthetic mechanisms within dicotyledonous plants.

cDNA cloning and characterization of maize phosphoenolpyruvate carboxykinase, a bundle sheath cell-specific enzyme

We isolated a full-length cDNA that encodes ATP-dependent phosphoenolpyruvate carboxykinase (EC 4.1.1.49, PCK) from leaves of maize, an NADP-malic enzyme type C4 plant. The mRNA was specifically and rather abundantly expressed in bundle sheath cells in accordance with the recent finding of cell-type-specific localization of PCK protein in maize, which has been detected with antibodies against cucumber PCK protein. The predicted protein had an N-terminal extension, which is characteristic of plant PCKs. The transcript level was much higher in the daytime than at night in 14-day old seedlings. However, in 42-day old plants the extent of diurnal change decreased. The maize PCK was expressed in Escherichia coli with the pET32 plasmid and purified to homogeneity. Through digestion with enterokinase, two types of enzyme were prepared; one with an intact N-terminus and the other lacking its N-terminal 77 amino acid residues due to over-digestion. The truncated protein had about 2-fold higher specific activity than the intact one, and was inhibited by 3-phosphoglycerate (3-PGA) with an I0.5 of 17.5 mM. In contrast, the intact protein was almost insensitive to 3-PGA. These results strongly suggest that the intact N-terminal extension may be involved in the regulation of PCK activity in vivo through some modification such as reversible phosphorylation.

Regulation of the Calvin cycle for CO2 fixation as an example for general control mechanisms in metabolic cycles

The theory of a metabolic cycle with the main portion of its intermediates remaining inside the cycle during one turnover has been developed. On this basis, the regulation of the Calvin cycle is analyzed. It is demonstrated that not only the reactions of non-equilibrium enzymes, as the carboxylation of ribulose 1,5-bisphosphate, but reactions that operate close to a thermodynamic equilibrium, especially the reduction of 3-phosphoglycerate and the transketolase reaction can significantly influence the total turnover period in the Calvin cycle. The role of compensating mechanisms in the maintenance of the photosynthesis rate upon changes of environmental conditions and of enzyme contents is analyzed for the Calvin cycle. It is shown that the change of the total quantity of the metabolites is one of the main self-regulated mechanisms in the Calvin cycle. A change of the ATP/ADP ratio can be used by the cell to maintain the CO2 assimilation rate, when the total quantity of the metabolites is changed. The developed analysis permits to explain some experimental data obtained with transgenic plants with restricted efflux of carbon from the chloroplasts.

C4 GENE EXPRESSION

C4 plants, including maize, Flaveria, amaranth, sorghum, and an amphibious sedge Eleocharis vivipara, have been employed to elucidate the molecular mechanisms and signaling pathways that control C4 photosynthesis gene expression. Current evidence suggests that pre-existing genes were recruited for the C4 pathway after acquiring potent and surprisingly diverse regulatory elements. This review emphasizes recent advances in our understanding of the creation of C4 genes, the activities of the C4 gene promoters consisting of synergistic and combinatorial enhancers and silencers, the use of 5' and 3' untranslated regions for transcriptional and posttranscriptional regulations, and the function of novel transcription factors. The research has also revealed new insights into unique or universal mechanisms underlying cell-type specificity, coordinate nuclear-chloroplast actions, hormonal, metabolic, stress and light responses, and the control of enzymatic activities by phosphorylation and reductive processes.

The molecular basis of C4 photosynthesis in sorghum: isolation, characterization and RFLP mapping of mesophyll- and bundle-sheath-specific cDNAs obtained by differential screening

C4 photosynthesis depends upon the strict compartmentalization of the CO2-assimilatory enzymes of the C4 and Calvin cycle in two different cell types, mesophyll and bundle-sheath cells. A differential accumulation is also observed for enzymes of other metabolic pathways, and mesophyll and bundle-sheath chloroplasts of NADP-malic enzyme type C4 plants differ even in their photosynthetic electron transport chains. A large number of studies indicate that this division of labour between mesophyll and bundle-sheath cells is the result of differential gene expression. To investigate the extent of this differential gene expression and thus gain insight into the genetic basis of C4 photosynthesis, genes that are differentially expressed in the mesophyll and bundle-sheath cells were catalogued in the NADP-malic enzyme type C4 grass Sorghum bicolor. A total of 58 cDNAs were isolated by differential screening. Using a tenfold difference in transcript abundance between mesophyll and bundle-sheath cells as a criterion, 25 cDNAs were confirmed to encode mesophyll-specific gene sequences and 8 were found to encode bundle-sheath-specific sequences. Eight mesophyll-specific cDNAs showed no significant similarities within GenBank and may therefore represent candidates for the elucidation of hitherto unknown functions in the differentiation of mesophyll and bundle-sheath cells. The chromosomal location of 50 isolated cDNAs was determined by RFLP mapping using an interspecific sorghum cross.

Induction of kranz anatomy and C4-like biochemical characteristics in a submerged amphibious plant by abscisic acid

The amphibious leafless sedge Eleocharis vivipara develops C4-like traits as well as Kranz anatomy under terrestrial conditions, but it develops C3-like traits without Kranz anatomy under submerged conditions. When submerged plants are exposed to aerial conditions, they rapidly produce new photosynthetic tissues with C4-like traits. In this study, experiments were performed to determine whether abscisic acid (ABA), a plant stress hormone, could induce the formation of photosynthetic tissues with Kranz anatomy and C4-like biochemical traits under water in the submerged form. When the submerged plants were grown in water containing 5 &mgr;M ABA, they developed new photosynthetic tissues with Kranz anatomy, forming well-developed Kranz (bundle sheath) cells that contained many organelles. The ABA-induced tissues accumulated large amounts of phosphoenolpyruvate carboxylase, pyruvate orthophosphate dikinase, and NAD-malic enzyme at the appropriate cellular sites. The tissues had 3.4 to 3.8 times more C4 enzyme activity than did tissues of the untreated submerged plants. Carbon-14 pulse and carbon-12 chase experiments revealed that the ABA-induced tissues fixed higher amounts of carbon-14 into C4 compounds and lower amounts of carbon-14 into C3 compounds as initial products than did the submerged plants and that they exhibited a C4-like pattern of carbon fixation under aqueous conditions of low carbon, indicating enhanced C4 capacity in the tissues. This report provides an example of the hormonal control of the differentiation of the structural and functional traits required for the C4 pathway.

Expression of the C4 Me1 Gene from Flaveria bidentis Requires an Interaction between 5[prime] and 3[prime] Sequences

The efficient functioning of C4 photosynthesis requires the strict compartmentation of a suite of enzymes in either mesophyll or bundle sheath cells. To determine the mechanism controlling bundle sheath cell-specific expression of the NADP-malic enzyme, we made a set of chimeric constructs using the 5[prime] and 3[prime] regions of the Flaveria bidentis Me1 gene fused to the [beta]-glucuronidase gusA reporter gene. The pattern of GUS activity in stably transformed F. bidentis plants was analyzed by histochemical and cell separation techniques. We conclude that the 5[prime] region of Me1 determines bundle sheath specificity, whereas the 3[prime] region contains an apparent enhancer-like element that confers high-level expression in leaves. The interaction of 5[prime] and 3[prime] sequences was dependent on factors that are present in the C4 plant but not found in tobacco.

Expression of the C4 Me1 Gene from Flaveria bidentis Requires an Interaction between 5[prime] and 3[prime] Sequences

The efficient functioning of C4 photosynthesis requires the strict compartmentation of a suite of enzymes in either mesophyll or bundle sheath cells. To determine the mechanism controlling bundle sheath cell-specific expression of the NADP-malic enzyme, we made a set of chimeric constructs using the 5[prime] and 3[prime] regions of the Flaveria bidentis Me1 gene fused to the [beta]-glucuronidase gusA reporter gene. The pattern of GUS activity in stably transformed F. bidentis plants was analyzed by histochemical and cell separation techniques. We conclude that the 5[prime] region of Me1 determines bundle sheath specificity, whereas the 3[prime] region contains an apparent enhancer-like element that confers high-level expression in leaves. The interaction of 5[prime] and 3[prime] sequences was dependent on factors that are present in the C4 plant but not found in tobacco.

Differential Localization of Antioxidants in Maize Leaves

The aim of this work was to determine the compartmentation of antioxidants between the bundle-sheath and mesophyll cells of maize (Zea mays L.) leaves. Rapid fractionation of the mesophyll compartment was used to minimize modifications in the antioxidant status and composition due to extraction procedures. The purity of the mesophyll isolates was assessed via the distribution of enzyme and metabolite markers. Ribulose-1,5 bisphosphate and ribulose-1,5-bisphosphate carboxylase/oxygenase were used as bundle-sheath markers and phosphoenolpyruvate carboxylase was used as the mesophyll marker enzyme. Glutathione reductase and dehydroascorbate reductase were almost exclusively localized in the mesophyll tissue, whereas ascorbate, ascorbate peroxidase, and superoxide dismutase were largely absent from the mesophyll fraction. Catalase, reduced glutathione, and monodehydroascorbate reductase were found to be approximately equally distributed between the two cell types. It is interesting that, whereas H2O2 levels were relatively high in maize leaves, this oxidant was largely restricted to the mesophyll compartment. We conclude that the antioxidants in maize leaves are partitioned between the two cell types according to the availability of reducing power and NADPH and that oxidized glutathione and dehydroascorbate produced in the bundle-sheat tissues have to be transported to the mesophyll for re-reduction to their reduced forms.

C4 Gene Expression in Photosynthetic and Nonphotosynthetic Leaf Regions of Amaranthus tricolor

Throughout most of its growth and development, Amaranthus tricolor produces fully green leaves. However, near the onset of flowering, unique leaves emerge that consist of three distinct color regions: green apices, yellow middle regions, and red basal regions. The green apices are identical to fully green leaves in terms of pigment composition, photosynthetic function, and C4 gene expression. The yellow and red regions possess greatly reduced levels of chlorophyll and they lack photosynthetic activity. The absence of photosynthetic capacity in the nongreen leaf regions was associated with three distinct alterations in C4 gene expression. First, there was a reduction in the translation of C4 polypeptides, and in the yellow regions synthesis of the ribulose-1,5-bisphosphate carboxylase small subunit occurred in the absence of large subunit synthesis. Second, there was a reduction in the relative transcription rates of two plastid-encoded photosynthetic genes, rbcL and psbA. Third, there was a loss of bundle-sheath cell-specific accumulation of the rbcL and RbcS mRNAs (but not the polypeptides, which remained bundle-sheath-specific). This study indicates that alterations in photosynthetic activity or developmental processes responsible for the loss of activity can influence C4 gene expression at multiple regulatory levels.

Does Decrease in Ribulose-1,5-Bisphosphate Carboxylase by Antisense RbcS Lead to a Higher N-Use Efficiency of Photosynthesis under Conditions of Saturating CO2 and Light in Rice Plants?

Rice (Oryza sativa L.) plants with decreased ribulose-1,5-bisphosphate carboxylase (Rubisco) were obtained by transformation with the rice rbcS antisense gene under the control of the rice rbcS promoter. The primary transformants were screened for the Rubisco to leaf N ratio, and the transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content at saturating CO2 partial pressures for photosynthesis under conditions of high irradiance and 25[deg]C. This optimal Rubisco content was estimated from the amounts and kinetic constants of Rubisco and the gas-exchange data. The R1 selfed progeny of the selected transformant were grown hydroponically with different N concentrations. Rubisco content in the R1 population was distributed into two groups: 56 plants had about 65% wild-type Rubisco, whereas 23 plants were very similar to the wild type. Although the plants with decreased Rubisco showed 20% lower rates of light-saturated photosynthesis in normal air (36 Pa CO2), they had 5 to 15% higher rates of photosynthesis in elevated partial pressures of CO2, (100-115 Pa CO2) than the wild-type plants for a given leaf N content. We conclude that the rice plants with 65% wild-type Rubisco show a higher N-use efficiency of photosynthesis under conditions of saturating CO2 and high irradiance.