The Evolution of Crassulacean Acid Metabolism. In: Winter K, Smith JAC, editors. Crassulacean Acid Metabolism. Berlin: Springer Berlin Heidelberg; 1996. pp. 360-85. [DOI: 10.1007/978-3-642-79060-7_25]

Functional traits of fossil plants

A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo-ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait-based ecology, to evaluate which well-established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait-based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi-quantitative ranking of 26 paleo-functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo-ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo-demography (life history), and Earth system history can be derived through the application of paleo-functional traits to fossil plants.

The CAM lineages of planet Earth

BACKGROUND AND SCOPE

The growth of experimental studies of crassulacean acid metabolism (CAM) in diverse plant clades, coupled with recent advances in molecular systematics, presents an opportunity to re-assess the phylogenetic distribution and diversity of species capable of CAM. It has been more than two decades since the last comprehensive lists of CAM taxa were published, and an updated survey of the occurrence and distribution of CAM taxa is needed to facilitate and guide future CAM research. We aimed to survey the phylogenetic distribution of these taxa, their diverse morphology, physiology and ecology, and the likely number of evolutionary origins of CAM based on currently known lineages.

RESULTS AND CONCLUSIONS

We found direct evidence (in the form of experimental or field observations of gas exchange, day-night fluctuations in organic acids, carbon isotope ratios and enzymatic activity) for CAM in 370 genera of vascular plants, representing 38 families. Further assumptions about the frequency of CAM species in CAM clades and the distribution of CAM in the Cactaceae and Crassulaceae bring the currently estimated number of CAM-capable species to nearly 7 % of all vascular plants. The phylogenetic distribution of these taxa suggests a minimum of 66 independent origins of CAM in vascular plants, possibly with dozens more. To achieve further insight into CAM origins, there is a need for more extensive and systematic surveys of previously unstudied lineages, particularly in living material to identify low-level CAM activity, and for denser sampling to increase phylogenetic resolution in CAM-evolving clades. This should allow further progress in understanding the functional significance of this pathway by integration with studies on the evolution and genomics of CAM in its many forms.

The diverse diaspora of CAM: a pole-to-pole sketch

BACKGROUND

Crassulacean acid metabolism (CAM) photosynthesis is a successful adaptation that has evolved often in angiosperms, gymnosperms, ferns and lycophytes. Present in ~5 % of vascular plants, the CAM diaspora includes all continents apart from Antarctica. Species with CAM inhabit most landscapes colonized by vascular plants, from the Arctic Circle to Tierra del Fuego, from below sea level to 4800 m a.s.l., from rainforests to deserts. They have colonized terrestrial, epiphytic, lithophytic, palustrine and aquatic systems, developing perennial, annual or geophyte strategies that can be structurally arborescent, shrub, forb, cladode, epiphyte, vine or leafless with photosynthetic roots. CAM can enhance survival by conserving water, trapping carbon, reducing carbon loss and/or via photoprotection.

SCOPE

This review assesses the phylogenetic diversity and historical biogeography of selected lineages with CAM, i.e. ferns, gymnosperms and eumagnoliids, Orchidaceae, Bromeliaceae, Crassulaceae, Euphorbiaceae, Aizoaceae, Portulacineae (Montiaceae, Basellaceae, Halophytaceae, Didiereaceae, Talinaceae, Portulacaceae, Anacampserotaceae and Cactaceae) and aquatics.

CONCLUSIONS

Most extant CAM lineages diversified after the Oligocene/Miocene, as the planet dried and CO2 concentrations dropped. Radiations exploited changing ecological landscapes, including Andean emergence, Panamanian Isthmus closure, Sundaland emergence and submergence, changing climates and desertification. Evidence remains sparse for or against theories that CAM biochemistry tends to evolve before pronounced changes in anatomy and that CAM tends to be a culminating xerophytic trait. In perennial taxa, any form of CAM can occur depending upon the lineage and the habitat, although facultative CAM appears uncommon in epiphytes. CAM annuals lack strong CAM. In CAM annuals, C3 + CAM predominates, and inducible or facultative CAM is common.

Atmospheric CO2 decline and the timing of CAM plant evolution

BACKGROUND AND AIMS

CAM photosynthesis is hypothesized to have evolved in atmospheres of low CO2 concentration in recent geological time because of its ability to concentrate CO2 around Rubisco and boost water use efficiency relative to C3 photosynthesis. We assess this hypothesis by compiling estimates of when CAM clades arose using phylogenetic chronograms for 73 CAM clades. We further consider evidence of how atmospheric CO2 affects CAM relative to C3 photosynthesis.

RESULTS

Where CAM origins can be inferred, strong CAM is estimated to have appeared in the past 30 million years in 46 of 48 examined clades, after atmospheric CO2 had declined from high (near 800 ppm) to lower (<450 ppm) values. In turn, 21 of 25 clades containing CAM species (but where CAM origins are less certain) also arose in the past 30 million years. In these clades, CAM is probably younger than the clade origin. We found evidence for repeated weak CAM evolution during the higher CO2 conditions before 30 million years ago, and possible strong CAM origins in the Crassulaceae during the Cretaceous period prior to atmospheric CO2 decline. Most CAM-specific clades arose in the past 15 million years, in a similar pattern observed for origins of C4 clades.

CONCLUSIONS

The evidence indicates strong CAM repeatedly evolved in reduced CO2 conditions of the past 30 million years. Weaker CAM can pre-date low CO2 and, in the Crassulaceae, strong CAM may also have arisen in water-limited microsites under relatively high CO2. Experimental evidence from extant CAM species demonstrates that elevated CO2 reduces the importance of nocturnal CO2 fixation by increasing the contribution of C3 photosynthesis to daily carbon gain. Thus, the advantage of strong CAM would be reduced in high CO2, such that its evolution appears less likely and restricted to more extreme environments than possible in low CO2.

Diversification Slowdown in the Cirrhopetalum Alliance (Bulbophyllum, Orchidaceae): Insights From the Evolutionary Dynamics of Crassulacean Acid Metabolism

Evolutionary slowdowns in diversification have been inferred in various plant and animal lineages. Investigation based on diversification models integrated with environmental factors and key characters could provide critical insights into this diversification trend. We evaluate diversification rates in the Cirrhopetalum alliance (Bulbophyllum, Orchidaceae subfam. Epidendroideae) using a time-calibrated phylogeny and assess the role of Crassulacean acid metabolism (CAM) as a hypothesised key innovation promoting the spectacular diversity of orchids, especially those with an epiphytic habit. An explosive early speciation in the Cirrhopetalum alliance is evident, with the origin of CAM providing a short-term advantage under the low atmospheric CO2 concentrations (pCO2) associated with cooling and aridification in the late Miocene. A subsequent slowdown of diversification in the Cirrhopetalum alliance is possibly explained by a failure to keep pace with pCO2 dynamics. We further demonstrate that extinction rates in strong CAM lineages are ten times higher than those of C3 lineages, with CAM not as evolutionarily labile as previously assumed. These results challenge the role of CAM as a "key innovation" in the diversification of epiphytic orchids.

A model of environmental limitations on production of Agave americana L. grown as a biofuel crop in semi-arid regions

Plants that use crassulacean acid metabolism (CAM) have the potential to meet growing agricultural resource demands using land that is considered unsuitable for many common crop species. Agave americana L., an obligate CAM plant, has potential as an advanced biofuel crop in water-limited regions, and has greater cold tolerance than other high-yielding CAM species, but physiological tolerances have not been completely resolved. We developed a model to estimate the growth responses of A. americana to water input, temperature, and photosynthetically active radiation (PAR). The photosynthetic response to PAR was determined experimentally by measuring the integrated leaf gas exchange over 24 h after acclimation to six light levels. Maximum CO2 fixation rates were observed at a PAR intensity of 1250 µmol photons m-2 s-1. Growth responses of A. americana to water and temperature were also determined, and a monthly environmental productivity index (EPI) was derived that can be used to predict biomass growth. The EPI was calculated as the product of water, temperature, and light indices estimated for conditions at a site in Maricopa (Arizona), and compared with measured biomass at the same site (where the first field trial of A. americana as a crop was completed). The monthly EPI summed over the lifetime of multi-year crops was highly correlated with the average measured biomass of healthy 2- and 3-year-old plants grown in the field. The resulting relationship between EPI and biomass provides a simple model for estimating the production of A. americana at a monthly time step according to light, temperature, and precipitation inputs, and is a useful tool for projecting the potential geographic range of this obligate CAM species in future climatic conditions.

Cystoliths of Parietaria judaica can serve as an internal source of CO2 for photosynthetic assimilation when stomata are closed

The recently reported 'alarm photosynthesis' acts as a biochemical process that assimilates CO2 derived from the decomposition of calcium oxalate crystals. This study examined whether CaCO3 cystoliths could also serve as CO2 pools, fulfilling a similar role. Shoots of Parietaria judaica were subjected to carbon starvation, abscisic acid (ABA), or bicarbonate treatments, and the volume of cystoliths and the photochemical parameters of photosystem II (PSII) were determined. The size of cystoliths was reduced under carbon starvation or ABA treatments, whereas it was restored by xylem-provided bicarbonate. Under carbon starvation, ABA, or bicarbonate treatments, the photochemical efficiency of PSII was higher, while non-photochemical quenching, representing the safe dissipation of excess PSII energy due to lack of electron sinks, was lower in treated samples compared with controls. This observation suggests the involvement of ABA or other carbon starvation cues in the release of subsidiary CO2 for photosynthesis, inevitably from an internal source, which could be the cystoliths. Carbon remobilized from cystoliths can be photosynthetically assimilated, thus acting as a safety valve under stress. Together with alarm photosynthesis, these results show a tight link between leaf carbon deposits and photosynthesis.

Evolutionary trajectories, accessibility and other metaphors: the case of C4 and CAM photosynthesis

Are evolutionary outcomes predictable? Adaptations that show repeated evolutionary convergence across the Tree of Life provide a special opportunity to dissect the context surrounding their origins, and identify any commonalities that may predict why certain traits evolved many times in particular clades and yet never evolved in others. The remarkable convergence of C₄ and Crassulacean Acid Metabolism (CAM) photosynthesis in vascular plants makes them exceptional model systems for understanding the repeated evolution of complex phenotypes. This review highlights what we have learned about the recurring assembly of C₄ and CAM, focusing on the increasingly predictable stepwise evolutionary integration of anatomy and biochemistry. With the caveat that we currently understand C₄ evolution better than we do CAM, I propose a general model that explains and unites C₄ and CAM evolutionary trajectories. Available data suggest that anatomical modifications are the 'rate-limiting step' in each trajectory, which in large part determines the evolutionary accessibility of both syndromes. The idea that organismal structure exerts a primary influence on innovation is discussed in the context of other systems. Whether the rate-limiting step occurs early or late in the evolutionary assembly of a new phenotype may have profound implications for its distribution across the Tree of Life.

Growth changes of eighteen herbaceous angiosperms induced by Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in soil

Study objectives were to describe and quantify growth responses (tolerance as shoot and root biomass accumulation) to soil-applied Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) treatments of eighteen terrestrial, herbaceous, angiospermous species and also; to determine how much of RDX, RDX transformation products, total N and RDX-derived N accumulated in the foliage. RDX altered growth of eighteen plant species or cultivars at levels of 100, 500, and 1,000 mg kg(-1)dry soil in a 75-d greenhouse study. Sixteen species or cultivars exhibited growth inhibition while two were stimulated in growth by RDX. A maximum amount of foliar RDX in a subset of three plant species was 36.0 mg per plant in Coronilla varia. Foliar concentrations of transformation products of RDX were low relative to RDX in the subset of three species. The proportion of RDX-N with respect to total N was constant, suggesting that foliar RDX transformation did not explain differences in tolerance. There was a δ (15)N shift towards that of synthetic RDX in foliage of the three species at a level of 1,000 mg kg(-1) RDX, proportional in magnitude to uptake of N from RDX and tolerance ranking.Reddened leaf margins for treated Sida spinosa indicate the potential of this species as a biosensor for RDX.

The function of the aerenchyma in arborescent lycopsids: evidence of an unfamiliar metabolic strategy

Most species of the modern family Isoëtaceae (Quillworts) some other modern hydrophytes, use a metabolic pathway for carbon fixation that involves uptake of sedimentary carbon and enrichment of CO(2) in internal gas spaces as a carbon-concentrating mechanism. This metabolism, which is related to 'aquatic CAM', is characterized by morphological, physiological and biochemical adaptations for decreasing photorespirative loss, aerating roots and maintaining high growth rates in anoxic, oligotrophic, stressed environments. Some of the closest relatives of the Isoëtaceae were the 'arborescent lycopsids', which were among the dominant taxa in the coal swamps found in lowland ecosystems during the Carboniferous and Permian periods (approx. 300 Ma). Morphological, ecological and geochemical evidence supports the hypothesis that the arborescent lycopsids had an unusual metabolism similar to that of modern Isoëtaceae and processed a biogeochemically significant proportion of organically fixed carbon over a period of about 100 million years in the late Palaeozoic. The temporal coincidence between the dominance of plants with this metabolism and an anomalous global atmosphere (high O(2); low CO(2)) supports the idea that biosphere feedbacks are important in regulating global climatic homeostasis. The potential influence of this metabolism on the global carbon cycle and its specific adaptive function suggest that it should perhaps be considered a fourth major photosynthetic pathway.

Carbon acquisition by diatoms

Diatoms are responsible for up to 40% of primary productivity in the ocean, and complete genome sequences are available for two species. However, there are very significant gaps in our understanding of how diatoms take up and assimilate inorganic C. Diatom plastids originate from secondary endosymbiosis with a red alga and their Form ID Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) from horizontal gene transfer, which means that embryophyte paradigms can only give general guidance as to their C acquisition mechanisms. Although diatom Rubiscos have relatively high CO(2) affinity and CO(2)/O(2) selectivity, the low diffusion coefficient for CO(2) in water has the potential to restrict the rate of photosynthesis. Diatoms growing in their natural aquatic habitats operate inorganic C concentrating mechanisms (CCMs), which provide a steady-state CO(2) concentration around Rubisco higher than that in the medium. How these CCMs work is still a matter of debate. However, it is known that both CO(2) and HCO (3) (-) are taken up, and an obvious but as yet unproven possibility is that active transport of these species across the plasmalemma and/or the four-membrane plastid envelope is the basis of the CCM. In one marine diatom there is evidence of C(4)-like biochemistry which could act as, or be part of, a CCM. Alternative mechanisms which have not been eliminated include the production of CO(2) from HCO (3) (-) at low pH maintained by a H(+) pump, in a compartment close to that containing Rubisco.

Are the metabolic components of crassulacean acid metabolism up-regulated in response to an increase in oxidative burden?

In the halophytic species Mesembryanthemum crystallinum, crassulacean acid metabolism (CAM) may be induced by a range of abiotic factors including drought, salinity, high light intensity, low temperature, and anoxia. A key biotic consequence of all these environmental changes is the generation of reactive oxygen species in planta that can elicit potentially damaging oxidative reactions and/or act as signals for engaging mechanisms that alleviate oxidative stress. However, induction of CAM per se also has the potential for increasing the oxidative burden via the enhanced internal O2 concentrations that develop behind closed stomata during daytime decarboxylation. The aim of this paper was to test two hypotheses. The first one, that reactive oxygen species are key signals for up-regulating the major genes and proteins required for the operation of CAM as part of an integrated strategy for alleviating oxidative burden, was tested using gaseous ozone to increase the oxidative burden at a cellular level. The second hypothesis, that CAM potentially increases oxidative load, was tested using a CAM-deficient mutant of M. crystallinum. The data indicate that ozone, like salinity, elicits an increase in the transcript and protein abundance of myo-inositol o-methyl transferase (a key enzyme of cyclitol synthesis), together with phosphoenolpyruvate carboxylase and other 'CAM-related' enzymes. However, ozone, unlike salinity, does not induce functional CAM, implying that the various metabolic components required for CAM respond to different signals. Comparing the activities of different subcellular isoforms of superoxide dismutase in wild-type and CAM-deficient mutants of M. crystallinum suggests that the induction of CAM potentially curtails the oxidative load in planta.

Crassulacean acid metabolism: plastic, fantastic

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

Induction of Crassulacean acid metabolism by water limitation

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

Molecular phylogeny of the genus Kalanchoe (Crassulaceae) inferred from nucleotide sequences of the ITS-1 and ITS-2 regions

The study presents an analysis of genotypic diversity in the genus Kalanchoe (Crassulaceae) on the level of Internal Transcribed Spacer (ITS) sequences and the attempt to correlate this diversity with previous findings on ecophysiological behavior, habitat preference, infrageneric taxonomic position of the species and DNA polymorphism derived from RAPD-PCR data. The Kalanchoe species are mainly abundant in Madagascar and eastern continental Africa and perform in situ diverse modes of crassulacean acid metabolism (CAM), an ecophysiologically relevant adaptation of photosynthesis. Total DNA was extracted from 68 Kalanchoe species and varieties. The ITS-1 and ITS-2 regions of the nuclear RNA genes were amplified by polymerase chain reaction, cloned and sequenced. The alignments of the sequences were evaluated by distance (neighbor joining) and character state (maximum parsimony) methods. The main topologies of the obtained ITS phylogenetic trees were quite similar irrespective of the mode of evaluation and show: (A) within the Crassulaceae the genus Kalanchoe forms a monophyletic clade; and (B) within the genus the species form three main clusters which coincide well with the previously reported three infrageneric sections of the species distinguishable by classical taxonomic criteria, the mode of in situ CAM performance, and DNA fragment pattern obtained by RAPD-PCR analyses. Moreover, the ITS phylogenetic trees show that all African Kalanchoe species form a distinct group within the most derived of the three main clusters. This is consistent with the view that the center of phylogenetic radiation of the genus is located in Madagascar from where the species have spread into the continental Africa.

CRASSULACEAN ACID METABOLISM: Molecular Genetics

▪ Abstract  Crassulacean acid metabolism (CAM) is an adaptation of photosynthesis to limited availability of water or CO2. CAM is characterized by nocturnal CO2 fixation via the cytosolic enzyme PEP carboxylase (PEPC), formation of PEP by glycolysis, malic acid accumulation in the vacuole, daytime decarboxylation of malate and CO2 re-assimilation via ribulose-1,5-bisphosphate carboxylase (RUBISCO), and regeneration of storage carbohydrates from pyruvate and/or PEP by gluconeogenesis. Within this basic framework, the pathway exhibits an extraordinary range of metabolic plasticity governed by environmental, developmental, tissue-specific, hormonal, and circadian cues. Characterization of genes encoding key CAM enzymes has shown that a combination of transcriptional, posttranscriptional, translational, and posttranslational regulatory events govern the expression of the pathway. Recently, this information has improved our ability to dissect the regulatory and signaling events that mediate the expression and operation of the pathway. Molecular analysis and sequence information have also provided new ways of assessing the evolutionary origins of CAM. Genetic and physiological analysis of transgenic plants currently under development will improve our further understanding of the molecular genetics of CAM.

Extrapolating feedback processes from the present to the past

Extant terrestrial vegetation alters its physical environment via its albedo, and its influence on immediate temperature via stomatal and boundary–layer influences of energy dissipation as sensible and latent heat; aquatic vegetation also controls albedo (e.g. coccolithophorids) and, by competing with water for electromagnetic energy absorption, the depth of the mixed layer and hence the quantity of nutrients trapped for the spring bloom. Both aquatic and terrestrial vegetation have had, together with microbial and geological processes, an influence on O 2 and CO 2 levels, and hence on the availability and biological functioning of Fe, Mn, Cu, Zn, Se and P, and the relative competitive advantage of C 3 versus C 4 , crassulacean acid metabolism (CAM) and carbon concentration mechanism (CCM) organisms. Less directly, changes in primary productivity impact on the production of CH 4 and N 2 O which, like CO 2 , are greenhouse gases, while some (marine) primary producers yield dimethyl sulphide (and hence cloud condensation nuclei, with effects on cloudiness) and halocarbons (via, in part, O 2 –dependent processes), partly negating the O 3 attenuation of UV–B radiation. These effects can be related to the terrestrial embryophytic vegetation back to ca. 450 Ma, and to eukaryotic marine vegetation back to at least 1.7, and probably 2.1 Ga, with implications for inter alia C 3 versus C4, CAM and CCM photosynthesis, and Fe acquisition mechanisms. Even earlier (3.8 Ga onwards) prokaryotes may have influenced CO 2 levels and hence controlled (as they did later) surface temperature. By producing O 2 , they led to decreasing availability of Fe, Mn and P (and utility of Se?), and increasing availability of Cu (and Zn?) that shaped the biochemistry on which later biogeochemistry was based.