C4 Syndrome Research Articles

Global distribution, climatic preferences and photosynthesis-related traits of C4 eudicots and how they differ from those of C4 grasses.

C₄ is one of three known photosynthetic processes of carbon fixation in flowering plants. It evolved independently more than 61 times in multiple angiosperm lineages and consists of a series of anatomical and biochemical modifications to the ancestral C3 pathway increasing plant productivity under warm and light-rich conditions. The C4 lineages of eudicots belong to seven orders and 15 families, are phylogenetically less constrained than those of monocots and entail an enormous structural and ecological diversity. Eudicot C4 lineages likely evolved the C4 syndrome along different evolutionary paths. Therefore, a better understanding of this diversity is key to understanding the evolution of this complex trait as a whole. By compiling 1207 recognised C4 eudicots species described in the literature and presenting trait data among these species, we identify global centres of species richness and of high phylogenetic diversity. Furthermore, we discuss climatic preferences in the context of plant functional traits. We identify two hotspots of C4 eudicot diversity: arid regions of Mexico/Southern United States and Australia, which show a similarly high number of different C4 eudicot genera but differ in the number of C4 lineages that evolved insitu. Further eudicot C4 hotspots with many different families and genera are in South Africa, West Africa, Patagonia, Central Asia and the Mediterranean. In general, C4 eudicots are diverse in deserts and xeric shrublands, tropical and subtropical grasslands, savannas and shrublands. We found C4 eudicots to occur in areas with less annual precipitation than C4 grasses which can be explained by frequently associated adaptations to drought stress such as among others succulence and salt tolerance. The data indicate that C4 eudicot lineages utilising the NAD-ME decarboxylating enzyme grow in drier areas than those using the NADP-ME decarboxylating enzyme indicating biochemical restrictions of the later system in higher temperatures. We conclude that in most eudicot lineages, C4 evolved in ancestrally already drought-adapted clades and enabled these to further spread in these habitats and colonise even drier areas.

Adaptive diversity in structure and function of C4 photosynthetic components.

Many tropical and subtropical plant lineages have independently evolved C4 photosynthesis. The convergent evolution of this complex functional trait from different ancestors is reflected in variations in the structural and biochemical characteristics of C4 components such as enzymes and cellular specializations. The mechanism of C4 carbon concentration mostly involves coordinated function of mesophyll and bundle sheath cells. Important adaptations of the C4 syndrome include increased vein density and the development of photosynthetic bundle sheath cells with low gas conductance. In addition, the enzymes and transporters of the C4 pathway evolved via the co-option of multiple genes, each derived from a specific lineage of isoforms present in nonC4-ancestors. In particular, the adaptation of C4 enzymes resulted in a variety of structural and biochemical modifications, generally leading to increased catalytic efficiency and regulation by metabolites and post-translational modifications. Differences in these adaptations are particularly evident in the C4-acid decarboxylation step, which can be catalyzed by three decarboxylases that define the C4 subtypes. Associated with the biochemical subtypes, there are also differences in the extend of grana staking and localization of bundle sheath cells chloroplasts. The presence of a suberin layer and symplastic connections also likely vary among the different C4-subtypes. This review examines the current understanding of the diversity of structural and functional changes in key components of the C4 carbon concentration mechanism. This knowledge is necessary not only to identify divergent solutions for convergent optimization of C4 components in different C4 lineages, but also to guide their creation for rational synthetic biology approaches.

The evolution of stomatal traits along the trajectory toward C4 photosynthesis.

C4 photosynthesis optimizes plant carbon and water relations, allowing high photosynthetic rates with low stomatal conductance. Stomata have long been considered a part of the C4 syndrome. However, it remains unclear how stomatal traits evolved along the path from C3 to C4. Here, we examined stomata in the Flaveria genus, a model used for C4 evolutionary study. Comparative, transgenic, and semi-in vitro experiments were performed to study the molecular basis that underlies the changes of stomatal traits in C4 evolution. The evolution from C3 to C4 species is accompanied by a gradual rather than an abrupt change in stomatal traits. The initial change appears near the Type I intermediate stage. Co-evolution of the photosynthetic pathway and stomatal traits is supported. On the road to C4, stomata tend to be fewer in number but larger in size and stomatal density dominates changes in anatomical maximum stomatal conductance (gsmax). Reduction of FSTOMAGEN expression underlies decreased gsmax in Flaveria and likely occurs in other C4 lineages. Decreased gsmax contributes to the increase in intrinsic water-use efficiency in C4 evolution. This work highlights the stomatal traits in the current C4 evolutionary model. Our study provides insights into the pattern, mechanism, and role of stomatal evolution along the road toward C4.

Using breeding and quantitative genetics to understand the C4 pathway.

Reducing photorespiration in C3 crops could significantly increase rates of photosynthesis and yield. One method to achieve this would be to integrate C4 photosynthesis into C3 species. This objective is challenging as it involves engineering incompletely understood traits into C3 leaves, including complex changes to their biochemistry, cell biology, and anatomy. Quantitative genetics and selective breeding offer underexplored routes to identify regulators of these processes. We first review examples of natural intraspecific variation in C4 photosynthesis as well as the potential for hybridization between C3 and C4 species. We then discuss how quantitative genetic approaches including artificial selection and genome-wide association could be used to better understand the C4 syndrome and in so doing guide the engineering of the C4 pathway into C3 crops.

The Developmental Enhancement of a C4 System With Non-Typical C4 Physiological Characteristics in Salsola ferganica (Kranz Anatomy), an Annual Desert Halophyte.

Variations of photosynthetic structures in different tissues or cells are in coordination with changes in various aspects, e.g. physiology, biochemistry, gene expression, etc. Most C4 plant species undergo developmental enhancement of the photosynthetic system, which may present different modes of changes between anatomy and physiology/biochemistry. In the current study, we investigated a Kranz-type C4 species Salsola ferganica with the progressive development of photosynthetic (PS) structure, performance of PS physiology, induction of PS enzymes, and transcriptional and translational regulation of PS genes, results revealed that S. ferganica presented C3 type anatomy in cotyledons but C4 type in leaves (C3/L4), with the C4 system separation of initial carbon fixation in the palisade mesophyll (M) cells and the following incorporation into triosephosphates and sugars in the bundle sheath (BS) cells, respectively. The BS cells continuously surrounded the vascular bundles and water storage cells in leaf anatomic structure. Compared to the single-cell C4 species Suaeda aralocaspica, S. ferganica exhibited similar developmental enhancement of C4 syndrome temporally and spatially in anatomic structures, enzyme activities, and gene expression, which suggests that completion of differentiation of the photosynthetic system is necessary for a C4 assimilation pathway. Besides, S. ferganica also displayed some different characteristics compared to S. aralocaspica in photosynthetic physiology, e.g. a more flexible δ13C value, much lower phosphoenolpyruvate carboxylase (PEPC) activity, and an insensitive response to stimuli, etc., which were not typical C4 characteristics. We speculate that this may suggest a different status of these two species in the evolutionary process of the photosynthesis pathway. Our findings will contribute to further understanding of the diversity of photosynthesis systems in Kranz-type C4 species and the Salsola genus.

De novo Transcriptome Assembly and Comparison of C3, C3-C4, and C4 Species of Tribe Salsoleae (Chenopodiaceae).

C4 photosynthesis is a carbon-concentrating mechanism that evolved independently more than 60 times in a wide range of angiosperm lineages. Among other alterations, the evolution of C4 from ancestral C3 photosynthesis requires changes in the expression of a vast number of genes. Differential gene expression analyses between closely related C3 and C4 species have significantly increased our understanding of C4 functioning and evolution. In Chenopodiaceae, a family that is rich in C4 origins and photosynthetic types, the anatomy, physiology and phylogeny of C4, C2, and C3 species of Salsoleae has been studied in great detail, which facilitated the choice of six samples of five representative species with different photosynthetic types for transcriptome comparisons. mRNA from assimilating organs of each species was sequenced in triplicates, and sequence reads were de novo assembled. These novel genetic resources were then analyzed to provide a better understanding of differential gene expression between C3, C2 and C4 species. All three analyzed C4 species belong to the NADP-ME type as most genes encoding core enzymes of this C4 cycle are highly expressed. The abundance of photorespiratory transcripts is decreased compared to the C3 and C2 species. Like in other C4 lineages of Caryophyllales, our results suggest that PEPC1 is the C4-specific isoform in Salsoleae. Two recently identified transporters from the PHT4 protein family may not only be related to the C4 syndrome, but also active in C2 photosynthesis in Salsoleae. In the two populations of the C2 species S. divaricata transcript abundance of several C4 genes are slightly increased, however, a C4 cycle is not detectable in the carbon isotope values. Most of the core enzymes of photorespiration are highly increased in the C2 species compared to both C3 and C4 species, confirming a successful establishment of the C2 photosynthetic pathway. Furthermore, a function of PEP-CK in C2 photosynthesis appears likely, since PEP-CK gene expression is not only increased in S. divaricata but also in C2 species of other groups.

Metabolic Network Constrains Gene Regulation of C4 Photosynthesis: The Case of Maize.

Engineering C3 plants to increase their efficiency of carbon fixation as well as of nitrogen and water use simultaneously may be facilitated by understanding the mechanisms that underpin the C4 syndrome. Existing experimental studies have indicated that the emergence of the C4 syndrome requires co-ordination between several levels of cellular organization, from gene regulation to metabolism, across two co-operating cell systems—mesophyll and bundle sheath cells. Yet, determining the extent to which the structure of the C4 plant metabolic network may constrain gene expression remains unclear, although it will provide an important consideration in engineering C4 photosynthesis in C3 plants. Here, we utilize flux coupling analysis with the second-generation maize metabolic models to investigate the correspondence between metabolic network structure and transcriptomic phenotypes along the maize leaf gradient. The examined scenarios with publically available data from independent experiments indicate that the transcriptomic programs of the two cell types are co-ordinated, quantitatively and qualitatively, due to the presence of coupled metabolic reactions in specific metabolic pathways. Taken together, our study demonstrates that precise quantitative coupling will have to be achieved in order to ensure a successfully engineered transition from C3 to C4 crops.

Developmental genetic mechanisms of C4 syndrome based on transcriptome analysis of C3 cotyledons and C4 assimilating shoots in Haloxylon ammodendron.

It is believed that transferring the C4 engine into C3 crops will greatly increase the yields of major C3 crops. Many efforts have been made since the 1960s, but relatively little success has been achieved because C4plant traits, referred to collectively as C4 syndrome, are very complex, and little is known about the genetic mechanisms involved. Unfortunately, there exists no ideal genetic model system to study C4 syndrome. It was previously reported that the Haloxylon species have different photosynthetic pathways in different photosynthetic organs, cotyledons and assimilating shoots. Here, we took advantage of the developmental switch from the C3 to the C4 pathway to study the genetic mechanisms behind this natural transition. We compared the transcriptomes of cotyledons and assimilating shoots using mRNA-Seq to gain insight into the molecular and cellular events associated with C4 syndrome. A total of 2959 differentially expressed genes [FDR≤0.001 and abs (|log2(Fold change)|≥1)] were identified, revealing that the transcriptomes of cotyledons and assimilating shoots are considerably different. We further identified a set of putative regulators of C4 syndrome. This study expands our understanding of the development of C4 syndrome and provides a new model system for future studies on the C3-to- C4 switch mechanism.

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.

C4 photosynthesis promoted species diversification during the Miocene grassland expansion.

Identifying how organismal attributes and environmental change affect lineage diversification is essential to our understanding of biodiversity. With the largest phylogeny yet compiled for grasses, we present an example of a key physiological innovation that promoted high diversification rates. C4 photosynthesis, a complex suite of traits that improves photosynthetic efficiency under conditions of drought, high temperatures, and low atmospheric CO2, has evolved repeatedly in one lineage of grasses and was consistently associated with elevated diversification rates. In most cases there was a significant lag time between the origin of the pathway and subsequent radiations, suggesting that the ‘C4 effect’ is complex and derives from the interplay of the C4 syndrome with other factors. We also identified comparable radiations occurring during the same time period in C3 Pooid grasses, a diverse, cold-adapted grassland lineage that has never evolved C4 photosynthesis. The mid to late Miocene was an especially important period of both C3 and C4 grass diversification, coincident with the global development of extensive, open biomes in both warm and cool climates. As is likely true for most “key innovations”, the C4 effect is context dependent and only relevant within a particular organismal background and when particular ecological opportunities became available.

When do different C4 leaf anatomies indicate independent C4 origins? Parallel evolution of C4 leaf types in Camphorosmeae (Chenopodiaceae).

Broad-scale phylogenetic studies give first insights in numbers, relationships, and ages of C4 lineages. They are, however, generally limited to a model that treats the evolution of the complex C4 syndrome in different lineages as a directly comparable process. Here, we use a resolved and well-sampled phylogenetic tree of Camphorosmeae, based on three chloroplast and one nuclear marker and on leaf anatomical traits to infer a more detailed picture of C4 leaf-type evolution in this lineage. Our ancestral character state reconstructions allowed two scenarios: (i) Sedobassia is a derived C3/C4 intermediate, implying two independent gains of C4 in Bassia and Camphorosma; or (ii) Sedobassia is a plesiomorphic C3/C4 intermediate, representing a syndrome ancestral to the Bassia/Camphorosma/Sedobassia lineage. In Bassia, a kochioid leaf type (Bassia muricata and/or Bassia prostrata type) is ancestral. At least three independent losses of water-storage tissue occurred, resulting in parallel shifts towards an atriplicoid leaf type. These changes in leaf anatomy are adaptations to different survival strategies in steppic or semi-desert habitats with seasonal rainfall. In contrast, Camphorosma shows a fixed C4 anatomy differing from Bassia types in its continuous Kranz layer, which indeed points to an independent origin of the full C4 syndrome in Camphorosma, either from an independent C3 or from a common C3/C4 intermediate ancestor, perhaps similar to its C3/C4 intermediate sister genus Sedobassia. The enlarged bundle sheath cells of Sedobassia might represent an important early step in C4 evolution in Camphorosmeae.

Gisekia (Gisekiaceae): Phylogenetic relationships, biogeography, and ecophysiology of a poorly known C4 lineage in the Caryophyllales

Gisekiaceae are a monogeneric family of the core Caryophyllales distributed in arid regions of Africa and Asia. The only widespread species of the genus, Gisekia pharnaceoides, performs C4 photosynthesis based on CO2 compensation point measurements. This study investigates the C4 syndrome and its evolution in Gisekia. The infrageneric relationships, distribution and bioclimatic preferences of Gisekia are also investigated. Leaf gas exchange characteristics, activity of Rubisco and major C4 cycle enzymes, and ultrastructural characteristics of mesophyll and bundle sheath cells are studied for Gisekia pharnaceoides. δ(13)C values and leaf anatomy are analyzed for all species. A dated molecular phylogeny of 39 accessions representing all species of Gisekiaceae and 14 representatives of closely related core Caryophyllales families is generated using four cp markers and ITS. The precise current distribution and bioclimatic niche of Gisekia is assessed on the basis of 520 georeferenced specimen localities. All traditionally recognized species of Gisekia are C4 plants with atriplicoid Kranz anatomy. Gisekia pharnaceoides uses the NAD-ME biochemical type. The molecular phylogeny demonstrated two East African clades nested within South African clades, demonstrating migration along the arid areas of eastern Africa during the Late Miocene/Pliocene Epochs. Most traditionally defined species are polyphyletic. Gisekia represents an isolated C4 lineage within core Caryophyllales dating back to the Miocene Epoch and probably spread along the African arid corridor from a South African center of origin. The seven currently recognized species should be treated as one polymorphic species or species complex, Gisekia pharnaceoides agg.

Systems biology or the biology of systems: routes to reducing hunger

Systems biology or the biology of systems: routes to reducing hunger

A multi-treatment experimental system to examine photosynthetic differentiation in the maize leaf.

BackgroundThe establishment of C4 photosynthesis in maize is associated with differential accumulation of gene transcripts and proteins between bundle sheath and mesophyll photosynthetic cell types. We have physically separated photosynthetic cell types in the leaf blade to characterize differences in gene expression by microarray analysis. Additional control treatments were used to account for transcriptional changes induced by cell preparation treatments. To analyse these data, we have developed a statistical model to compare gene expression values derived from multiple, partially confounded, treatment groups.ResultsDifferential gene expression in the leaves of wild-type maize seedlings was characterized using the latest release of a maize long-oligonucleotide microarray produced by the Maize Array Project consortium. The complete data set is available through the project web site. Data is also available at the NCBI GEO website, series record GSE3890. Data was analysed with and without consideration of cell preparation associated stress.ConclusionEmpirical comparison of the two analyses suggested that consideration of stress helped to reduce the false identification of stress responsive transcripts as cell-type enriched. Using our model including a stress term, we identified 8% of features as differentially expressed between bundle sheath and mesophyll cell types under control of false discovery rate of 5%. An estimate of the overall proportion of differentially accumulating transcripts (1-π0) suggested that as many as 18% of the genes may be differentially expressed between B and M. The analytical model presented here is generally applicable to gene expression data and demonstrates the use of statistical elimination of confounding effects such as stress in the context of microarray analysis. We discuss the implications of the high degree of differential transcript accumulation observed with regard to both the establishment and engineering of the C4 syndrome.

Light-dependent development of single cell C4 photosynthesis in cotyledons of Borszczowia aralocaspica (Chenopodiaceae) during transformation from a storage to a photosynthetic organ.

Previous work has shown that Borszczowia aralocaspica (Chenopodiaceae) accomplishes C4 photosynthesis in a unique, polarized single-cell system in leaves. Mature cotyledons have the same structure as leaves, with chlorenchyma cells having biochemical polarization of dimorphic chloroplasts and C4 functions at opposite ends of the cell. Development of the single-celled C4 syndrome in cotyledons was characterized. In mature seeds, all cell layers are already present in the cotyledons, which contain mostly lipids and little starch. The incipient chlorenchyma cells have a few plastids towards the centre of the cell. Eight days after germination and growth in the dark, small plastids are evenly distributed around the periphery of the expanding cells. Immunolocalization studies show slight labelling of Rubisco in plastids in seeds, including chlorenchyma, hypodermal and water storage, but not epidermal, cells. After imbibition and 8 d of growth in the dark labelling for Rubisco progressively increased, being most prominent in chlorenchyma cells. There was no immunolabelling for the plastid C4 enzyme pyruvate, Pi dikinase under these conditions. Cotyledons developing in light show formation of chlorenchyma tissue, induction of the cytosolic enzyme phosphoenolpyruvate carboxylase and development of dimorphic chloroplasts at opposite ends of the cells. Proximal chloroplasts have well-developed grana, store starch and contain Rubisco; those located distally have reduced grana, lack starch and contain pyruvate, Pi dikinase. The results show cotyledons developing in the dark have a single structural plastid type which expresses Rubisco, while light induces formation of dimorphic chloroplasts from the single plastid pool, synthesis of C4 enzymes, and biochemical and structural polarization leading to the single-cell C4 syndrome.

Development of C4 photosynthesis in sorghum leaves grown under free-air CO2 enrichment (FACE).

The developmental pattern of C4 expression has been well characterized in maize and other C4 plants. However, few reports have explored the possibility that the development of this pathway may be sensitive to changes in atmospheric CO2 concentrations. Therefore, both the structural and biochemical development of leaf tissue in the fifth leaf of Sorghum bicolor plants grown at elevated CO2 have been characterized. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities accumulate rapidly as the leaf tissue differentiates and emerges from the surrounding whorl. Rubisco was not expressed in a cell-specific manner in the youngest tissue at the base of the leaf, but did accumulate before PEPC was detected. This suggests that the youngest leaf tissue utilizes a C3-like pathway for carbon fixation. However, this tissue was in a region of the leaf receiving very low light and so significant rates of photosynthesis were not likely. Older leaf tissue that had emerged from the surrounding whorl into full sunlight showed the normal C4 syndrome. Elevated CO2 had no effect on the cell-specific localization of Rubisco or PEPC at any stage of leaf development, and the relative ratios of Rubisco to PEPC remained constant during leaf development. However, in the oldest tissue at the tip of the leaf, the total activities of Rubisco and PEPC were decreased under elevated CO2 implying that C4 photosynthetic tissue may acclimate to growth under elevated CO2.

Development of the C4Photosynthetic Apparatus in Cotyledons and Leaves ofSalsola richteri(Chenopodiaceae)

Development of characteristics of the C4 syndrome was studied in cotyledons and leaves of NADP‐ME type C4Salsola richteri, a desert shrub. This species has seeds in which the cotyledons contain chloroplasts, storage proteins, and lipid bodies but no starch. Following imbibition (day 0), tissue types are already apparent in the cotyledons, and the chloroplasts have extensive grana stacking, but Kranz type anatomy and C4 photosynthesis have not developed. At day 0, there is high Rubisco and low phosphoenolpyruvate carboxylase (PEPC) content, distributed throughout all tissue. After 15 d of development in the dark, the cotyledons were in a C3‐like default condition with three distinct layers of chlorenchyma (hypodermis, mesophyll, and bundle sheath [BS]); all contained Rubisco in their chloroplasts. Light was required for development of the C4 syndrome, including differentiation of chloroplasts in mesophyll and BS cells, development of thick BS cell walls, selective compartmentation of Rubisco in BS cells and PEPC in mesophyll cells, and development of intercellular air spaces around hypodermal and mesophyll cells. During cotyledon development in the light, Rubisco content decreased and PEPC content increased, and starch synthesis was associated with tissue‐specific compartmentation of Rubisco. There was also a C3 to C4 transition during leaf development. After 3 d of leaf development in the light, cell division was still occurring, the tissue was not fully differentiated, and there was no cell‐specific compartmentation of Rubisco or PEPC. There was the beginning of tissue and organelle differentiation together with Rubisco and PEPC compartmentation in 5‐d leaves. In mature leaves, Kranz anatomy was fully developed with selective compartmentation of PEPC in mesophyll.

Origin and evolution of C 4 photosynthesis in the tribe Salsoleae (Chenopodiaceae) based on anatomical and biochemical types in leaves and cotyledons

The Chenopodiaceae genus Salsola contains a large number of species with C4 photosynthesis. Along with derivative genera they have a prominent position among the desert vegetation of Asia and Africa. About 130 species from Asia and Africa were investigated to determine the occurrence of C3 versus C4 syndrome in leaves and cotyledons, and to study specific anatomical and biochemical features of photosynthesis in both photosynthetic organs. The species studied belong to all six previously identified sections of the tribe Salsoleae based on morphological characters. Types of photosynthesis were identified using carbon 13C/12C isotope fractionation. The representatives of all systematic groups were investigated for mesophyll anatomy and biochemical subtypes by determination of enzyme activity (RUBPC, PEPC, NAD- and NADP-ME and AAT) and primary photosynthetic products. Two photosynthetic types (C3 and C4) and two biochemical subtypes (NAD- and NADP-ME) were identified in both leaves and cotyledons. Both Kranz and non-Kranz type anatomy were found in leaves and cotyledons, but cotyledons had more diversity in anatomical structure. Strong relationships between anatomical types and biochemical subtypes in leaves and cotyledons were shown. We found convincing evidence for a similar pattern of structural and biochemical features of photosynthesis in leaves and cotyledons within systematic groups, and evaluated their relevance at the evolutionary level. We identified six groups in tribe Salsoleae with respect to photosynthetic types and mesophyll structure in leaves and cotyledons. Two separate lineages of biochemical and anatomical evolution within Salsoleae were demonstrated based on studies of leaves and cotyledons. The sections Caroxylon, Malpighipila, Cardiandra and Belanthera have no C3 species and only the NAD-ME C4 subtype has been found in leaves. We suggest the C4 species in the NADP-ME lineage evolved in Coccosalsola and Salsola sections, and originated in the subsection Arbuscula. Coccosalsola contains many species with C3 and/or C3-C4 intermediate photosynthesis. Within these main evolutionary lineages, species of different taxonomic groups (sections and subsections) had differences in anatomical or/and biochemical features in leaves and cotyledons. We conclude that structural and biochemical changes in the photosynthetic apparatus in species of the tribe Salsoleae were a key factor in their evolution and broad distribution in extreme desert environments.

Environmental and Evolutionary Preconditionsfor the Origin and Diversification of the C4 PhotosyntheticSyndrome

Abstract: C4 photosynthesis is an evolutionary solution to high rates of photorespiration and low kinetic efficiency of Rubisco in CO2‐depleted atmospheres of recent geologic time. About 7500 plant species are C4, in contrast to 30 000 CAM and 250 000 C3 species. All C4 plants occur in approximately 90 genera from 18 angiosperm families. In all of these families, the C4 pathway evolved independently. In many, multiple independent origins have occurred, such that over 30 distinct evolutionary origins of the C4 pathway are recognized. Fossil and carbon isotope evidence show that the C4 syndrome is at least 12 to 15 million years old, although estimates based on molecular sequence comparisons indicate it is over 20 million years old. The evolutionary radiation of herbaceous angiosperms may have been required for C4 plant evolution. All C4 species occur in advanced angiosperm families that appeared in the fossil record in the past 70 million years. Most of these families diversified in terms of genera and species numbers between 20 to 40 million years ago, during a period of global cooling, atmospheric CO2 reduction and aridification. During the period of diversification, numerous traits arose in the C3 flora that enhanced their performance in arid environments and atmospheres of reduced CO2. Some of these traits may have predisposed certain taxa to develop the C4 pathway once atmospheric CO2 levels declined to a point where the ability to concentrate CO2 had a selective advantage. Leading traits in C3 plants that may have facilitated the initial transition to C4 photosynthesis include close vein spacing and an enlargement of the bundle sheath cell layer to form a Kranz‐like anatomy. Ecological factors not directly connected with photosynthesis probably also played a role. For example, extensive ecological disturbance may have been needed to convert C3‐dominated woodlands into open, high‐light habitats where herbaceous C4 plants could succeed. Disturbances in the form of fire, and browsing by large mammals, increase during the time of C4 plant evolution and diversification. Fire increased because of the drying climate, while browsing increased with the evolutionary diversification of the mammalian megafauna in the Oligocene and Miocene epochs. In summary, the origin of C4 plants is hypothesized to have resulted from a novel combination of environmental and phylogenetic developments that, for the first time, established the preconditions required for C4 plant evolution.

A manual for the C4 fan club

Although plants possessing the C4 photosynthesis pathway account for less than 5% of the world's higher plant species (-8000 of 250 000), their economic and ecological importance, particularly in the tropics, is disproportionately large. This volume, C4 Plant Biology edited by Rowan E Sage and Russell K. Monson, is a testament to that importance and its impact on fields as diverse as plant physiology, ecology, geology, and anthropology, to name a few. Indeed, readers from the entire spectrum of plant biology are likely to find at least a few chapters of interest in this book, which is divided into five sections on various aspects of C4 plant biology: perspectives, structure-function, ecology, evolution, and human interactions. The perspectives section contains an excellent chapter by Sage on the evolutionary constraints that were likely important drivers in the evolution of this pathway. The other chapter in this section, a historical overview by Marshall Hatch, reviews the events and key players in the discovery of C4 photosynthesis. Hatch also provides a context for much of the subsequent biochemical research into the pathway he helped elucidate in the 1960s. Some of the individuals he cites for their key roles also contributed chapters to this volume. The structure-function section focuses on the biochemical and morphological features of the C4 syndrome. Gerald Edwards, one of the workers highlighted by Hatch, co-authored a chapter with Ryuzi Kanai on the biochemistry of C4 photosynthesis. They describe the biochemical workings that underpin the whole-plant responses to CO, light, and temperature, which play a central role in the book's subsequent chapters on the ecology of C4 plants. Readers who are not expert in the details of C4 photosynthesis should start here, as a careful reading will enhance an understanding of the concepts advanced in later chapters. Other chapters in this section address regulation of the pathway (Leegood and Walker), leaf structure and development (Dengler and Nelson), and modeling C4 photosynthesis. The last chapter, by Suzanne von Caemmerer and Robert Furbank, is a nice summary of the key aspects of this pathway and how they can be captured in a small set of equations. Though much of the material in this section of the book concerns topics that were generally understood by about 1980, all of the chapters effectively integrate older and newer information, establishing a strong background for the book's other sections. The next section of the book concerns the ecology of C4 photosynthesis, in particular how the biochemical and anatomical adaptations of this pathway influence plant performance in the environment. A chapter by Steve