Photosynthetic enzyme activities and immunofluorescence studies on the localization of ribulose-1,5-bisphosphate carboxylase/oxygenase in leaves of C3, C4, and C3−C4 intermediate species of Flaveria (Asteraceae)

Tribulus (Zygophyllaceae) as a case study for the evolution of C2 and C4 photosynthesis

C2 photosynthesis is a photosynthetic pathway in which photorespiratory CO2 release and refixation are enhanced in leaf bundle sheath (BS) tissues. The evolution of C2 photosynthesis has been hypothesized to be a major step in the origin of C4 photosynthesis, highlighting the importance of studying C2 evolution. In this study, physiological, anatomical, ultrastructural, and immunohistochemical properties of leaf photosynthetic tissues were investigated in six non-C4 Tribulus species and four C4 Tribulus species. At 42°C, T. cristatus exhibited a photosynthetic CO2 compensation point in the absence of respiration (C*) of 21 µmol mol-1, below the C3 mean C* of 73 µmol mol-1. Tribulus astrocarpus had a C* value at 42°C of 55 µmol mol-1, intermediate between the C3 species and the C2 T. cristatus. Glycine decarboxylase (GDC) allocation to BS tissues was associated with lower C*. Tribulus cristatus and T. astrocarpus allocated 86% and 30% of their GDC to the BS tissues, respectively, well above the C3 mean of 11%. Tribulus astrocarpus thus exhibits a weaker C2 (termed sub-C2) phenotype. Increased allocation of mitochondria to the BS and decreased length-to-width ratios of BS cells, were present in non-C4 species, indicating a potential role in C2 and C4 evolution.

Photorespiration - Rubisco's repair crew

The photorespiratory repair pathway (photorespiration in short) was set up from ancient metabolic modules about three billion years ago in cyanobacteria, the later ancestors of chloroplasts. These prokaryotes developed the capacity for oxygenic photosynthesis, i.e. the use of water as a source of electrons and protons (with O₂ as a by-product) for the sunlight-driven synthesis of ATP and NADPH for CO₂ fixation in the Calvin cycle. However, the CO₂-binding enzyme, ribulose 1,5-bisphosphate carboxylase (known under the acronym Rubisco), is not absolutely selective for CO₂ and can also use O₂ in a side reaction. It then produces 2-phosphoglycolate (2PG), the accumulation of which would inhibit and potentially stop the Calvin cycle and subsequently photosynthetic electron transport. Photorespiration removes the 2-PG and in this way prevents oxygenic photosynthesis from poisoning itself. In plants, the core of photorespiration consists of ten enzymes distributed over three different types of organelles, requiring interorganellar transport and interaction with several auxiliary enzymes. It goes together with the release and to some extent loss of freshly fixed CO₂. This disadvantageous feature can be suppressed by CO₂-concentrating mechanisms, such as those that evolved in C₄ plants thirty million years ago, which enhance CO₂ fixation and reduce 2PG synthesis. Photorespiration itself provided a pioneer variant of such mechanisms in the predecessors of C₄ plants, C₃-C₄ intermediate plants. This article is a review and update particularly on the enzyme components of plant photorespiration and their catalytic mechanisms, on the interaction of photorespiration with other metabolism and on its impact on the evolution of photosynthesis. This focus was chosen because a better knowledge of the enzymes involved and how they are embedded in overall plant metabolism can facilitate the targeted use of the now highly advanced methods of metabolic network modelling and flux analysis. Understanding photorespiration more than before as a process that enables, rather than reduces, plant photosynthesis, will help develop rational strategies for crop improvement.

Metabolic profiles in C3, C3-C4 intermediate, C4-like, and C4 species in the genus Flaveria

C4 photosynthesis concentrates CO2 around Rubisco in the bundle sheath, favouring carboxylation over oxygenation and decreasing photorespiration. This complex trait evolved independently in >60 angiosperm lineages. Its evolution can be investigated in genera such as Flaveria (Asteraceae) that contain species representing intermediate stages between C3 and C4 photosynthesis. Previous studies have indicated that the first major change in metabolism probably involved relocation of glycine decarboxylase and photorespiratory CO2 release to the bundle sheath and establishment of intercellular shuttles to maintain nitrogen stoichiometry. This was followed by selection for a CO2-concentrating cycle between phosphoenolpyruvate carboxylase in the mesophyll and decarboxylases in the bundle sheath, and relocation of Rubisco to the latter. We have profiled 52 metabolites in nine Flaveria species and analysed 13CO2 labelling patterns for four species. Our results point to operation of multiple shuttles, including movement of aspartate in C3-C4 intermediates and a switch towards a malate/pyruvate shuttle in C4-like species. The malate/pyruvate shuttle increases from C4-like to complete C4 species, accompanied by a rise in ancillary organic acid pools. Our findings support current models and uncover further modifications of metabolism along the evolutionary path to C4 photosynthesis in the genus Flaveria.

Coleataenia prionitis, a C4-like species in the Poaceae

C₄-like plants represent the penultimate stage of evolution from C₃ to C₄ plants. Although Coleataenia prionitis (formerly Panicum prionitis) has been described as a C₄ plant, its leaf anatomy and gas exchange traits suggest that it may be a C₄-like plant. Here, we reexamined the leaf structure and biochemical and physiological traits of photosynthesis in this grass. The large vascular bundles were surrounded by two layers of bundle sheath (BS): a colorless outer BS and a chloroplast-rich inner BS. Small vascular bundles, which generally had a single BS layer with various vascular structures, also occurred throughout the mesophyll together with BS cells not associated with vascular tissue. The mesophyll cells did not show a radial arrangement typical of Kranz anatomy. These features suggest that the leaf anatomy of C. prionitis is on the evolutionary pathway to a complete C₄ Kranz type. Phosphoenolpyruvate carboxylase (PEPC) and pyruvate, Pi dikinase occurred in the mesophyll and outer BS. Glycine decarboxylase was confined to the inner BS. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) accumulated in the mesophyll and both BSs. C. prionitis had biochemical traits of NADP-malic enzyme type, whereas its gas exchange traits were close to those of C₄-like intermediate plants rather than C₄ plants. A gas exchange study with a PEPC inhibitor suggested that Rubisco in the mesophyll could fix atmospheric CO₂. These data demonstrate that C. prionitis is not a true C₄ plant but should be considered as a C₄-like plant.

Despite phylogenetic effects, C3-C4 lineages bridge the ecological gap to C4 photosynthesis

C4 photosynthesis is a physiological innovation involving several anatomical and biochemical components that emerged recurrently in flowering plants. This complex trait evolved via a series of physiological intermediates, broadly termed 'C3-C4', which have been widely studied to understand C4 origins. While this research program has focused on biochemistry, physiology, and anatomy, the ecology of these intermediates remains largely unexplored. Here, we use global occurrence data and local habitat descriptions to characterize the niches of multiple C3-C4 lineages, as well as their close C3 and C4 relatives. While C3-C4 taxa tend to occur in warm climates, their abiotic niches are spread along other dimensions, making it impossible to define a universal C3-C4 niche. Phylogeny-based comparisons suggest that, despite shifts associated with photosynthetic types, the precipitation component of the C3-C4 niche is particularly lineage specific, being highly correlated with that of closely related C3 and C4 taxa. Our large-scale analyses suggest that C3-C4 lineages converged toward warm habitats, which may have facilitated the transition to C4 photosynthesis, effectively bridging the ecological gap between C3 and C4 plants. The intermediates retained some precipitation aspects of their C3 ancestors' habitat, and likely transmitted them to their C4 descendants, contributing to the diversity among C4 lineages seen today.

Are changes in sulfate assimilation pathway needed for evolution of C4 photosynthesis?

C4 photosynthesis characteristically features a cell-specific localization of enzymes involved in CO2 assimilation in bundle sheath cells (BSC) or mesophyll cells. Interestingly, enzymes of sulfur assimilation are also specifically present in BSC of maize and many other C4 species. This localization, however, could not be confirmed in C4 species of the genus Flaveria. It was, therefore, concluded that the bundle sheath localization of sulfate assimilation occurs only in C4 monocots. However, recently the sulfate assimilation pathway was found coordinately enriched in BSC of Arabidopsis, opening new questions about the significance of such cell-specific localization of the pathway. In addition, next generation sequencing revealed expression gradients of many genes from C3 to C4 species and mathematical modeling proposed a sequence of adaptations during the evolutionary path from C3 to C4. Indeed, such gradient, with higher expression of genes for sulfate reduction in C4 species, has been observed within the genus Flaveria. These new tools provide the basis for reexamining the intriguing question of compartmentalization of sulfur assimilation. Therefore, this review summarizes the findings on spatial separation of sulfur assimilation in C4 plants and Arabidopsis, assesses the information on sulfur assimilation provided by the recent transcriptomics data and discusses their possible impact on understanding this interesting feature of plant sulfur metabolism to find out whether changes in sulfate assimilation are part of a general evolutionary trajectory toward C4 photosynthesis.

The evolutionary ecology of C4 plants

C4 photosynthesis is a physiological syndrome resulting from multiple anatomical and biochemical components, which function together to increase the CO2 concentration around Rubisco and reduce photorespiration. It evolved independently multiple times and C4 plants now dominate many biomes, especially in the tropics and subtropics. The C4 syndrome comes in many flavours, with numerous phenotypic realizations of C4 physiology and diverse ecological strategies. In this work, we analyse the events that happened in a C3 context and enabled C4 physiology in the descendants, those that generated the C4 physiology, and those that happened in a C4 background and opened novel ecological niches. Throughout the manuscript, we evaluate the biochemical and physiological evidence in a phylogenetic context, which demonstrates the importance of contingency in evolutionary trajectories and shows how these constrained the realized phenotype. We then discuss the physiological innovations that allowed C4 plants to escape these constraints for two important dimensions of the ecological niche--growth rates and distribution along climatic gradients. This review shows that a comprehensive understanding of C4 plant ecology can be achieved by accounting for evolutionary processes spread over millions of years, including the ancestral condition, functional convergence via independent evolutionary trajectories, and physiological diversification.

C2 photosynthesis generates about 3-fold elevated leaf CO2 levels in the C3-C4 intermediate species Flaveria pubescens

Formation of a photorespiration-based CO2-concentrating mechanism in C3-C4 intermediate plants is seen as a prerequisite for the evolution of C4 photosynthesis, but it is not known how efficient this mechanism is. Here, using in vivo Rubisco carboxylation-to-oxygenation ratios as a proxy to assess relative intraplastidial CO2 levels is suggested. Such ratios were determined for the C3-C4 intermediate species Flaveria pubescens compared with the closely related C3 plant F. cronquistii and the C4 plant F. trinervia. To this end, a model was developed to describe the major carbon fluxes and metabolite pools involved in photosynthetic-photorespiratory carbon metabolism and used quantitatively to evaluate the labelling kinetics during short-term (14)CO2 incorporation. Our data suggest that the photorespiratory CO2 pump elevates the intraplastidial CO2 concentration about 3-fold in leaves of the C3-C4 intermediate species F. pubescens relative to the C3 species F. cronquistii.

The role of photorespiration during the evolution of C4 photosynthesis in the genus Flaveria

C4 photosynthesis represents a most remarkable case of convergent evolution of a complex trait, which includes the reprogramming of the expression patterns of thousands of genes. Anatomical, physiological, and phylogenetic and analyses as well as computational modeling indicate that the establishment of a photorespiratory carbon pump (termed C2 photosynthesis) is a prerequisite for the evolution of C4. However, a mechanistic model explaining the tight connection between the evolution of C4 and C2 photosynthesis is currently lacking. Here we address this question through comparative transcriptomic and biochemical analyses of closely related C3, C3-C4, and C4 species, combined with Flux Balance Analysis constrained through a mechanistic model of carbon fixation. We show that C2 photosynthesis creates a misbalance in nitrogen metabolism between bundle sheath and mesophyll cells. Rebalancing nitrogen metabolism requires anaplerotic reactions that resemble at least parts of a basic C4 cycle. Our findings thus show how C2 photosynthesis represents a pre-adaptation for the C4 system, where the evolution of the C2 system establishes important C4 components as a side effect.

Evolution of the C4 photosynthetic pathway: events at the cellular and molecular levels

The biochemistry and leaf anatomy of plants using C4 photosynthesis promote the concentration of atmospheric CO2 in leaf tissue that leads to improvements in growth and yield of C4 plants over C3 species in hot, dry, high light, and/or saline environments. C4 plants like maize and sugarcane are significant food, fodder, and bioenergy crops. The C4 photosynthetic pathway is an excellent example of convergent evolution, having evolved in multiple independent lineages of land plants from ancestors employing C3 photosynthesis. In addition to C3 and C4 species, some plant lineages contain closely related C3-C4 intermediate species that demonstrate leaf anatomical, biochemical, and physiological characteristics between those of C3 plants and species using C4 photosynthesis. These groups of plants have been extremely useful in dissecting the modifications to leaf anatomy and molecular biology, which led to the evolution of C4 photosynthesis. It is now clear that great variation exists in C4 leaf anatomy, and diverse molecular mechanisms underlie C4 biochemistry and physiology. However, all these different paths have led to the same destination-the expression of a C4 CO2 concentrating mechanism. Further identification of C4 leaf anatomical traits and molecular biological components, and understanding how they are controlled and assembled will not only allow for additional insights into evolutionary convergence, but also contribute to sustainable food and bioenergy production strategies.

Strategies for engineering C(4) photosynthesis

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

Structural and biochemical characterization of the C₃-C₄ intermediate Brassica gravinae and relatives, with particular reference to cellular distribution of Rubisco

On the basis of its CO(2) compensation concentration, Brassica gravinae Ten. has been reported to be a C(3)-C(4) intermediate. This study investigated the structural and biochemical features of photosynthetic metabolism in B. gravinae. The cellular distribution of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) was also examined in B. gravinae, B. napus L. (C(3)), Raphanus sativus L. (C(3)), and Diplotaxis tenuifolia (L.) DC. (C(3)-C(4)) by immunogold electron microscopy to elucidate Rubisco expression during the evolution from C(3) to C(3)-C(4) intermediate plants. The bundle sheath (BS) cells of B. gravinae contained centrifugally located chloroplasts as well as centripetally located chloroplasts and mitochondria. Glycine decarboxylase P-protein was localized in the BS mitochondria. Brassica gravinae had low C(4) enzyme activities and high activities of Rubisco and photorespiratory enzymes, suggesting that it reduces photorespiratory CO(2) loss by the glycine shuttle. In B. gravinae, the labelling density of Rubisco was higher in the mesophyll chloroplasts than in the BS chloroplasts. A similar cellular pattern was found in other Brassicaceae species. These data demonstrate that, during the evolution from C(3) to C(3)-C(4) intermediate plants, the intercellular pattern of Rubisco expression did not change greatly, although the amount of chloroplasts in the BS cells increased. It also appears that intracellular variation in Rubisco distribution may occur within the BS cells of B. gravinae.

Water-use efficiency and nitrogen-use efficiency of C(3) -C(4) intermediate species of Flaveria Juss. (Asteraceae)

Plants using the C(4) pathway of carbon metabolism are marked by greater photosynthetic water and nitrogen-use efficiencies (PWUE and PNUE, respectively) than C(3) species, but it is unclear to what extent this is the case in C(3) -C(4) intermediate species. In this study, we examined the PWUE and PNUE of 14 species of Flaveria Juss. (Asteraceae), including two C(3) , three C(4) and nine C(3) -C(4) species, the latter containing a gradient of C(4) -cycle activities (as determined by initial fixation of (14) C into C-4 acids). We found that PWUE, PNUE, leaf ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) content and intercellular CO(2) concentration in air (C(i) ) do not change gradually with C(4) -cycle activity. These traits were not significantly different between C(3) species and C(3) -C(4) species with less than 50% C(4) -cycle activity. C(4) -like intermediates with greater than 65% C(4) -cycle activity were not significantly different from plants with fully expressed C(4) photosynthesis. These results indicate that a gradual increase in C(4) -cycle activity has not resulted in a gradual change in PWUE, PNUE, intercellular CO(2) concentration and leaf Rubisco content towards C(4) levels in the intermediate species. Rather, these traits arose in a stepwise manner during the evolutionary transition to the C(4) -like intermediates, which are contained in two different clades within Flaveria.

Functional analysis of corn husk photosynthesis

The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C(3) photosynthetic pathway, in contrast to its leaves, which use the C(4) photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO(2) compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO(2) assimilation rate in the husk was less than that in the leaves and did not saturate at high CO(2), indicating CO(2) diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO(2) compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C(4)-like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C(4) photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.

Sulfate assimilation and glutathione synthesis in C4 plants

Sulfate assimilation and glutathione synthesis were traditionally believed to be differentially compartmentalised in C4 plants with the synthesis of cysteine and glutathione restricted to bundle sheath and mesophyll cells, respectively. Recent studies, however, showed that although ATP sulfurylase and adenosine 5' phosphosulfate reductase, the key enzymes of sulfate assimilation, are localised exclusively in bundle sheath in maize and other C4 monocot species, this is not true for the dicot C4 species of Flaveria. On the other hand, enzymes of glutathione biosynthesis were demonstrated to be active in both types of maize cells. Therefore, in this review the recent findings on compartmentation of sulfate assimilation and glutathione metabolism in C4 plants will be summarised and the consequences for our understanding of sulfate metabolism and C4 photosynthesis will be discussed.

The evolution of C4 photosynthesis

C₄ photosynthesis is a series of anatomical and biochemical modifications that concentrate CO₂ around the carboxylating enzyme Rubisco, thereby increasing photosynthetic efficiency in conditions promoting high rates of photorespiration. The C₄ pathway independently evolved over 45 times in 19 families of angiosperms, and thus represents one of the most convergent of evolutionary phenomena. Most origins of C₄ photosynthesis occurred in the dicots, with at least 30 lineages. C₄ photosynthesis first arose in grasses, probably during the Oligocene epoch (24-35 million yr ago). The earliest C₄ dicots are likely members of the Chenopodiaceae dating back 15-21 million yr; however, most C₄ dicot lineages are estimated to have appeared relatively recently, perhaps less than 5 million yr ago. C₄ photosynthesis in the dicots originated in arid regions of low latitude, implicating combined effects of heat, drought and/or salinity as important conditions promoting C₄ evolution. Low atmospheric CO₂ is a significant contributing factor, because it is required for high rates of photorespiration. Consistently, the appearance of C₄ plants in the evolutionary record coincides with periods of increasing global aridification and declining atmospheric CO₂ . Gene duplication followed by neo- and nonfunctionalization are the leading mechanisms for creating C₄ genomes, with selection for carbon conservation traits under conditions promoting high photorespiration being the ultimate factor behind the origin of C₄ photosynthesis. Contents Summary 341 I. Introduction 342 II. What is C₄ photosynthesis? 343 III. Why did C₄ photosynthesis evolve? 347 IV. Evolutionary lineages of C₄ photosynthesis 348 V. Where did C₄ photosynthesis evolve? 350 VI. How did C₄ photosynthesis evolve? 352 VII. Molecular evolution of C₄ photosynthesis 361 VIII. When did C₄ photosynthesis evolve 362 IX. The rise of C₄ photosynthesis in relation to climate and CO₂ 363 X. Final thoughts: the future evolution of C₄ photosynthesis 365 Acknowledgements 365 References 365.

Assimilatory sulfate reduction in C(3), C(3)-C(4), and C(4) species of Flaveria

The activity of the enzymes catalyzing the first two steps of sulfate assimilation, ATP sulfurylase and adenosine 5'-phosphosulfate reductase (APR), are confined to bundle sheath cells in several C(4) monocot species. With the aim to analyze the molecular basis of this distribution and to determine whether it was a prerequisite or a consequence of the C(4) photosynthetic mechanism, we compared the intercellular distribution of the activity and the mRNA of APR in C(3), C(3)-C(4), C(4)-like, and C(4) species of the dicot genus Flaveria. Measurements of APR activity, mRNA level, and protein accumulation in six Flaveria species revealed that APR activity, cysteine, and glutathione levels were significantly higher in C(4)-like and C(4) species than in C(3) and C(3)-C(4) species. ATP sulfurylase and APR mRNA were present at comparable levels in both mesophyll and bundle sheath cells of C(4) species Flaveria trinervia. Immunogold electron microscopy demonstrated the presence of APR protein in chloroplasts of both cell types. These findings, taken together with results from the literature, show that the localization of assimilatory sulfate reduction in the bundle sheath cells is not ubiquitous among C(4) plants and therefore is neither a prerequisite nor a consequence of C(4) photosynthesis.

Compartmentation of photosynthesis in cells and tissues of C(4) plants

Critical to defining photosynthesis in C(4) plants is understanding the intercellular and intracellular compartmentation of enzymes between mesophyll and bundle sheath cells in the leaf. This includes enzymes of the C(4) cycle (including three subtypes), the C(3) pathway and photorespiration. The current state of knowledge of this compartmentation is a consequence of the development and application of different techniques over the past three decades. Initial studies led to some alternative hypotheses on the mechanism of C(4) photosynthesis, and some controversy over the compartmentation of enzymes. The development of methods for separating mesophyll and bundle sheath cells provided convincing evidence on intercellular compartmentation of the key components of the C(4) pathway. Studies on the intracellular compartmentation of enzymes between organelles and the cytosol were facilitated by the isolation of mesophyll and bundle sheath protoplasts, which can be fractionated gently while maintaining organelle integrity. Now, the ability to determine localization of photosynthetic enzymes conclusively, through in situ immunolocalization by confocal light microscopy and transmission electron microscopy, is providing further insight into the mechanism of C(4) photosynthesis and its evolution. Currently, immunological, ultrastructural and cytochemical studies are revealing relationships between anatomical arrangements and photosynthetic mechanisms which are probably related to environmental factors associated with evolution of these plants. This includes interesting variations in the C(4) syndrome in leaves and cotyledons of species in the tribe Salsoleae of the family Chenopodiaceae, in relation to evolution and ecology. Thus, analysis of structure-function relationships using modern techniques is a very powerful approach to understanding evolution and regulation of the photosynthetic carbon reduction mechanisms.

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.

Structure and expression analysis of the gdcsPA and gdcsPB genes encoding two P-isoproteins of the glycine-cleavage system from Flaveria pringlei

In Flaveria pringlei, a C3 plant, P protein of the glycine-cleavage system is encoded by a small gene family consisting of at least five transcriptionally active genes. We have cloned and sequenced two of these genes, gdcsPA and gdcsPB, and provide the first detailed report on the complete structure of eukaryotic gdcsP genes. Based on the lengths of exons and intervening sequences, the P-protein genes can be subdivided into two parts. In both cases the N-terminal region consists of one very long exon followed by a long intron. In contrast, the C-terminal parts show a complex mosaic structure of relatively small exons and introns. A highly conserved leucine-zipper motif was identified, which is supposed to participate in the assembly of the glycine decarboxylase multienzyme complex. The transcript derived from the gdcsPA sequence corresponds perfectly to a leaf cDNA isolated earlier. Reverse-transcriptase PCR experiments show that both genes are preferentially active in leaves. Stems contain distinctly less P protein mRNA and the relative level in roots is very low but still clearly detectable. In all three organs, but most significantly in roots, the gdcsPA transcript level is distinctly higher than that of gdcsPB. Analysis of promoter-beta-glucuronidase fusions in transgenic tobacco suggests that far-upstream elements enhance the transcriptional activity of both genes in leaves relative to stems. The analysis of distal gdcsPA promoter deletions reveals the presence of regulatory elements acting with a distinct organ preference and indicates their approximate location.

H-protein of glycine decarboxylase is encoded by multigene families in Flaveria pringlei and F. cronquistii (Asteraceae)

In Flaveria pringlei and F. cronquistii unlike other plants, H-protein of the glycine cleavage system is encoded by small multigene families. From leaf cDNA libraries and by reverse transcription of mRNA with subsequent polymerase chain reaction (PCR) amplification, we have obtained three different H-protein cDNA clones from each species. The relative levels of total H-protein mRNA, as well as of different H-protein transcripts, have been determined in leaves, stems, and roots of F. pringlei. Stems, with a total of 22% relative to leaves, contain substantial amounts of H-protein transcripts. The corresponding level in roots is relatively low (2.3% relative to leaves) but easily detectable. One of the transcripts occurs only in leaves (HFP20) and another one (HFP13) is present exclusively in photosynthesizing organs. Only one of the H-protein transcripts (HFP4) was found in all three organs, in leaves, stems, and roots of F. pringlei.

Interspecific variation in assimilation of (14)CO 2 into C 4 acids by leaves of C 3, C 4 and C 3-C 4 intermediate Flaveria species near the CO 2 compensation concentration

The assimilation of (14)CO2 into the C4 acids malate and aspartate by leaves of C3, C4 and C3-C4 intermediate Flaveria species was investigated near the CO2 compensation concentration Γ(*) in order to determine the potential role of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) in reducing photorespiration in the intermediates. Relative to air concentrations of CO2, the proportion of CO2 fixed by PEP carboxylase at Γ(*) increased in all six C3-C4 intermediate species examined. However, F. floridana J.R. Johnston and F. ramosissima Klatt were shown to be markedly less responsive to reduced external CO2, with only about a 1.6-fold enhancement of CO2 assimilation by PEP carboxylase, as compared to a 3.0- to 3.7-fold increase for the other C3-C4 species examined, namely, F. linearis Lag., F. anomala B.L. Robinson, F. chloraefolia A. Gray and F. pubescens Rydb. The C3 species F. pringlei Gandoger and F. cronquistii A.M. Powell exhibited a 1.5- and 2.9-fold increase in labeled malate and aspartate, respectively, at Γ(*). Assimilation of CO2 by PEP carboxylase in the C4 species F. trinervia (Spreng.) C. Mohr, F. australasica Hook., and the C4-like species F. brownii A.M. Powell was relatively insensitive to subatmospheric levels of CO2. The interspecific variation among the intermediate Flaverias may signify that F. floridana and F. ramosissima possess a more C4-like compartmentation of PEP carboxylase and ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) between the mesophyll and bundle-sheath cells. Chasing recently labeled malate and aspartate with (12)CO2 for 5 min at Γ(*) resulted in an apparent turnover of 25% and 30% of the radiocarbon in these C4 acids for F. ramosissima and F. floridana, respectively. No substantial turnover was detected for F. linearis, F. anomala, F. chloraefolia or F. pubescens. With the exception of F. floridana and F. ramosissima, it is unlikely that enhanced CO2 fixation by PEP carboxylase at the CO2 compensation concentration is a major mechanism for reducing photorespiration in the intermediate Flaveria species. Moreover, these findings support previous related (14)CO2-labeling studies at air-levels of CO2 which indicated that F. floridana and F. ramosissima were more C4-like intermediate species. This is further substantiated by the demonstration that F. floridana PEP carboxylase, like the enzyme in C4 plants, undergoes a substantial activation (2.2-fold) upon illuminating dark-adapted green leaves. In contrast, light activation was not observed for the enzyme in F. linearis or F. chloraefolia.

Gas-exchange of ears of cereals in response to carbon dioxide and light : II. Occurrence of a C3-C 4 intermediate type of photosynthesis

Data for the maximum carboxylation velocity of ribulose-1,5-biosphosphate carboxylase, Vm, and the maximum rate of whole-chain electron transport, Jm, were calculated according to a photosynthesis model from the CO2 response and the light response of CO2 uptake measured on ears of wheat (Triticum aestivum L. cv. Arkas), oat (Avena sativa L. cv. Lorenz), and barley (Hordeum vulgare L. cv. Aramir). The ratio Jm/Vm is lower in glumes of oat and awns of barley than it is in the bracts of wheat and in the lemmas and paleae of oat and barley. Light-microscopy studies revealed, in glumes and lemmas of wheat and in the lemmas of oat and barley, a second type of photosynthesizing cell which, in analogy to the Kranz anatomy of C4 plants, can be designated as a bundle-sheath cell. In wheat ears, the CO2-compensation point (in the absence of dissimilative respiration) is between those that are typical for C3 and C4 plants.A model of the CO2 uptake in C3-C4 intermediate plants proposed by Peisker (1986, Plant Cell Environ. 9, 627-635) is applied to recalculate the initial slopes of the A(p(c)) curves (net photosynthesis rate versus intercellular partial pressure of CO2) under the assumptions that the Jm/Vm ratio for all organs investigated equals the value found in glumes of oat and awns of barley, and that ribulose-1,5-bisphosphate carboxylase is redistributed from mesophyll to bundle-sheath cells. The results closely match the measured values. As a consequence, all bracts of wheat ears and the inner bracts of oat and barley ears are likely to represent a C3-C4 intermediate type, while glumes of oat and awns of barley represent the C3 type.

Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C3-and C 4-cycle co-function

Carbon-isotope ratios were examined as δ(13)C values in several C3, C4, and C3-C4 Flaveria species, and compared to predicted δ(13)C, values generated from theoretical models. The measured δ(13)C values were within 4‰ of those predicted from the models. The models were used to identify factors that contribute to C3-like δ(13)C values in C3-C4 species that exhibit considerable C4-cycle activity. Two of the factors contributing to C3-like δ(13)C values are high CO2 leakiness from the C4 pathway and pi/pa values that were higher than C4 congeners. A marked break occurred in the relationship between the percentage of atmospheric CO2 assimilated through the C4 cycle and the δ(13)C value. Below 50% C4-cycle assimialtion there was no significant relationship between the variables, but above 50% the δ(13)C values became less negative. These results demonstrate that the level of C4-cycle expression can increase from, 0 to 50% with little integration of carbon transfer from the C4 to the C3 cycle. As expression increaces above 50%, however, increased integration of C3- and C4-cycle co-function occurs.

Incorporation of (14)CO 2 into C 4 acids by leaves of C 3-C 4 intermediate and C 3 species of Moricandia and Panicum at the CO 2 compensation concentration

Comparative (14)CO2 pulse-(12)CO2 chase studies performed at CO2 compensation (Γ)-versus air-concentrations of CO2 demonstrated a four-to eightfold increase in assimilation of (14)CO2 into the C4 acids malate and aspartate by leaves of the C3-C4 intermediate species Panicum milioides Nees ex Trin., P. decipiens Nees ex Trin., Moricandia arvensis (L.) DC., and M. spinosa Pomel at Γ. Specifically, the distribution of (14)C in malate and aspartate following a 10-s pulse with (14)CO2 increases from 2% to 17% (P. milioides) and 4% to 16% (M. arvensis) when leaves are illuminated at the CO2 compensation concentration (20 μl CO2/l, 21% O2) versus air (340 μl CO2/l, 21% O2). Chasing recently incorporated (14)C for up to 5 min with (12)CO2 failed to show any substantial turnover of label in the C4 acids or in carbon-4 of malate. The C4-acid labeling patterns of leaves of the closely related C3 species, P. laxum Sw. and M. moricandioides (Boiss.) Heywood, were found to be relatively unresponsive to changes in pCO2 from air to Γ. These data demonstrate that the C3-C4 intermediate species of Panicum and Moricandia possess an inherently greater capacity for CO2 assimilation via phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) at the CO2 compensation concentration than closely related C3 species. However, even at Γ, CO2 fixation by PEP carboxylase is minor compared to that via ribulosebisphosphate carboxylase (EC 4.1.1.39) and the C3 cycle, and it is, therefore, unlikely to contribute in a major way to the mechanism(s) facilitating reduced photorespiration in the C3-C4 intermediate species of Panicum and Moricandia.

Reassimilation of carbon dioxide by Flaveria (Asteraceae) species representing different types of photosynthesis

The capability to reassimilate CO2 originating from intracellular decarboxylating processes connected with the photorespiratory glycolate pathway and-or decarboxylation of C4 acids during C4 photosynthesis has been investigated with four species of the genus Flaveria (Asteraceae). The C3-C4 intermediate species F. pubescens and F. anomala reassimilated CO2 much more efficiently than the C3 species F. cronquistii and, with respect to this feature, behaved similarly to the C4 species F. trinervia. Therefore, under atmospheric conditions the intermediate species photorespired with rates only between 10-20% of that measured with F. cronquistii. At low oxygen concentrations (1,5%) the reassimilation potential of F. anomala approached that of F. trinervia and was distinct from that found with F. pubescens. The data are discussed with respect to a possible sequence of events during evolution of C4 photosynthesis. If compared with related data for C3-C4 intermediate species from other genera they support the hypothesis that, during evolution of C4 photosynthesis, an efficient capacity for CO2 reassimilation evolved prior to a CO2-concentrating mechanism.

Co-function of C3-and C 4-photosynthetic pathways in C3, C 4 and C 3-C 4 intermediate Flaveria species

The potential for C4 photosynthesis was investigated in five C3-C4 intermediate species, one C3 species, and one C4 species in the genus Flaveria, using (14)CO2 pulse-(12)CO2 chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating (14)CO2 into the C4 acids malate and aspartate, following an 8-s pulse. The proportion of (14)C label in these C4 products ranged from 50-55% to 20-26% in the C3-C4 intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, (14)CO2 into aspartate as into malate. Generally, about 5-15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C4 photosynthesis among the intermediate species based on the apparent rate of conversion of (14)C label from the C4 cycle to the C3 cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C4-cycle products and an increase in percentage label in C3-cycle products during chase periods with (12)CO2, although the rate of change was slower than in the C4 species, F. palmeri. In these C3-C4 intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C3-C4 intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C4-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C4 and C3 cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C3 species, 7-18% of the initial label was in malate+aspartate. However, only 40-50% of this label was in the C-4 position, indicating C4-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O2, the absorbed quantum yields for CO2 uptake (in mol CO2·[mol quanta](-1)) averaged 0.053 in F. cronquistii (C3), 0.051 in F. trinervia (Spreng.) Mohr (C4), 0.052 in F. ramosissima (C3-C4), 0.051 in F. anomala (C3-C4), 0.050 in F. linearis (C3-C4), 0.046 in F. floridana (C3-C4), and 0.044 in F. pubescens (C3-C4). In 2% O2 an enhancement of the quantum yield was observed in all of the C3-C4 intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O2 were intermediate in value to the C3 and C4 species, indicating a co-function of the C3 and C4 cycles in CO2 assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O2 presumably reflect an ineffcient transfer of carbon from the C4 to the C3 cycle. The response of the quantum yield to four increasing O2 concentrations (2-35%) showed lower levels of O2 inhibition in the C3-C4 intermediate F. ramosissima, relative to the C3 species. This indicates that the co-function of the C3 and C4 cycles in this intermediate species leads to an increased CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O2.

Molecular properties of phosphoenolpyruvate carboxylase from C3, C 3-C 4 intermediate, and C 4 Flaveria species

Phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) from Flaveria trinervia Mohr (C4), F. floridana Johnston (C3-C4), and F. cronquistii Powell (C3) leaves were compared by electrotransfer blotting/enzyme-linked immunoassay (Western-blot analysis), mobility of the native enzyme in polyacrylamide gels and in isoelectric focusing (IEF) gels, peptide mapping, and in-vitro translation of RNA isolated from each plant. The PEPCases from the C3 and C3-C4 plants were very similar to each other in terms of electrophoretic mobilities on gels and isoenzyme patterns on IEF gels, and identical in peptide mapping. Quantitative differences were noted, however, in that the C3-C4 intermediate plant contained more PEPCase overall and that the relative activity of individual isoenzymes shifted between the C3 and C3-C4 intermediate PEPCases. The PEPCase from the C4 plant had a different isoenzyme pattern, a different peptide map, and was far more abundant than the other two enzymes. Western blot analysis demonstrated the cross-reactivity of PEPCases from all three Flaveria species with antibody raised against maize PEPCase. The results provide evidence, at the molecular level, that supports the view of C3-C4 intermediate species as C3-like plants with some C4-like photosynthetic characteristics, but there are differences from the C3 plant in the quantity and properties of the PEPCase from the C3-C4 intermediate plant.

Immunofluorescent localization of phosphoenolpyruvate carboxylase and ribulose 1,5-bisphosphate carboxylase/oxygenase proteins in leaves of C3, C 4 and C 3-C 4 intermediate Flaveria species

An indirect immunofluorescence technique was used to determine the intercellular compartmentation of phosphoenolpyruvate (PEP) carboxylase and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) proteins in leaves of five species of Flaveria, F. brownii A.M. Powell (C4), F. cronquistii A.M. Powell (C3), F. linearis Lag. (C3-C4 intermediate), F. floridana J.R. Johnston (C3-C4) and F. chloraefolia A. Gray (C3-C4). The results were compared with representative C3 and C4 plants. No strict intercellular compartmentation of either enzyme was observed in any of the five Flaveria species examined. Both carboxylase proteins were found throughout the leaf chlorenchyma of the C3 Flaveria, as was also the case in Nicotiana tabacum L. (C3). A similar distribution pattern was observed not only in the three C3-C4 intermediates, but in the C4 Flaveria as well. This distribution contrasted markedly with the localization patterns found in Zea mays L. (C4), where Rubisco was confined to bundle-sheath chloroplasts and PEP carboxylase to the mesophyll cytoplasm.