Crassulacean acid metabolism: a continuous or discrete trait?

Flexible oviposition behavior enabled the evolution of terrestrial reproduction

Among vertebrates, nearly all oviparous animals are considered to have either obligate aquatic or terrestrial oviposition, with eggs that are specialized for developing in those environments. The terrestrial environment has considerably more oxygen but is dry and thus presents both opportunities and challenges for developing embryos, particularly those adapted for aquatic development. Here, we present evidence from field experiments examining egg-laying behavior, egg size, and egg jelly function of 13 species of Central and South American treefrogs in the genus Dendropsophus, which demonstrates that flexible oviposition (individuals laying eggs both in and out of water) and eggs capable of both aquatic and terrestrial development are the likely factors which enable the transition from aquatic to terrestrial reproduction. Nearly half of the species we studied had previously undescribed degrees of flexible oviposition. Species with obligate terrestrial reproduction have larger eggs than species with aquatic reproduction, and species with flexible reproduction have eggs of intermediate sizes. Obligate terrestrial breeding frogs also have egg masses that absorb water more quickly than those with flexible oviposition. We also examined eight populations of a single species, Dendropsophus ebraccatus, and document substantial intraspecific variation in terrestrial oviposition; populations in rainy, stable climates lay fewer eggs in water than those in drier areas. However, no differences in egg size were found, supporting the idea that the behavioral component of oviposition evolves before other adaptations associated with obligate terrestrial reproduction. Collectively, these data demonstrate the key role that behavior can have in facilitating major evolutionary transitions.

Multiomics unravels potential molecular switches in the C3 to CAM transition of Mesembryanthemum crystallinum

Mesembryanthemum crystallinum (common ice plant), a facultative CAM plant, shifts from C3 to CAM photosynthesis under salt stress, enhancing water use efficiency. Here we used transcriptomics, proteomics, and targeted metabolomics to profile molecular changes during the diel cycle of C3 to CAM transition. The results confirmed expected changes associated with CAM photosynthesis, starch biosynthesis and degradation, and glycolysis/gluconeogenesis. Importantly, they yielded new discoveries: 1) Transcripts displayed greater circadian regulation than proteins. 2) Oxidative phosphorylation and inositol methylation may play important roles in initiating the transition. 3) V-type H+-ATPases showed consistent transcriptional regulation, aiding in vacuolar malate uptake. 4) A protein phosphatase 2C, a major component in the ABA signaling pathway, may trigger the C3 to CAM transition. Our work highlights the potential molecular switches in the C3 to CAM transition, including the potential role of ABA signaling. SIGNIFICANCE: The common ice plant is a model facultative CAM plant, and under stress conditions it can shift from C3 to CAM photosynthesis within a three-day period. However, knowledge about the molecular changes during the transition and the molecular switches enabling the transition is lacking. Multi-omic analyses not only revealed the molecular changes during the transition, but also highlighted the importance of ABA signaling, inositol methylation, V-type H+-ATPase in initiating the shift. The findings may explain physiological changes and nocturnal stomatal opening, and inform future synthetic biology effort in improving crop water use efficiency and stress resilience.

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.

CAM plants: their importance in epiphyte communities and prospects with global change

BACKGROUND AND SCOPE

The epiphytic life form characterizes almost 10 % of all vascular plants. Defined by structural dependence throughout their life and their non-parasitic relationship with the host, the term epiphyte describes a heterogeneous and taxonomically diverse group of plants. This article reviews the importance of crassulacean acid metabolism (CAM) among epiphytes in current climatic conditions and explores the prospects under global change.

RESULTS AND CONCLUSIONS

We question the view of a disproportionate importance of CAM among epiphytes and its role as a 'key innovation' for epiphytism but do identify ecological conditions in which epiphytic existence seems to be contingent on the presence of this photosynthetic pathway. Possibly divergent responses of CAM and C3 epiphytes to future changes in climate and land use are discussed with the help of experimental evidence, current distributional patterns and the results of several long-term descriptive community studies. The results and their interpretation aim to stimulate a fruitful discussion on the role of CAM in epiphytes in current climatic conditions and in altered climatic conditions in the future.

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.

Brief reflections on 50 years as a plant ecophysiologist

SCOPE

This paper is a short biographical sketch of my life as a plant ecophysiologist in which serendipity and outstanding collaborators have been key allies.

CAM photosynthesis in Bulnesia retama (Zygophyllaceae), a non-succulent desert shrub from South America

BACKGROUND AND AIMS

Bulnesia retama is a drought-deciduous, xerophytic shrub from arid landscapes of South America. In a survey of carbon isotope ratios (δ13C) in specimens from the field, B. retama exhibited less negative values, indicative of CAM or C4 photosynthesis. Here, we investigate whether B. retama is a C4 or CAM plant.

METHODS

Gas-exchange responses to intercellular CO2, diurnal gas-exchange profiles, δ13C and dawn vs. afternoon titratable acidity were measured on leaves and stems of watered and droughted B. retama plants. Leaf and stem cross-sections were imaged to determine whether the tissues exhibited succulent CAM or C4 Kranz anatomy.

KEY RESULTS

Field-collected stems and fruits of B. retama exhibited δ13C between -16 and -19 ‰. Plants grown in a glasshouse from field-collected seeds had leaf δ13C values near -31 ‰ and stem δ13C values near -28 ‰. The CO2 response of photosynthesis showed that leaves and stems used C3 photosynthesis during the day, while curvature in the nocturnal response of net CO2 assimilation rate (A) in all stems, coupled with slightly positive rates of A at night, indicated modest CAM function. C4 photosynthesis was absent. Succulence was absent in all tissues, although stems exhibited tight packing of the cortical chlorenchyma in a CAM-like manner. Tissue titratable acidity increased at night in droughted stems.

CONCLUSIONS

Bulnesia retama is a weak to modest C3 + CAM plant. This is the first report of CAM in the Zygophyllaceae and the first showing that non-succulent, xerophytic shrubs use CAM. CAM alone in B. retama was too limited to explain less negative δ13C in field-collected plants, but combined with effects of low stomatal and mesophyll conductance it could raise δ13C to observed values between -16 and -19 ‰. Modest CAM activity, particularly during severe drought, could enable B. retama to persist in arid habitats of South America.

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.

Developmental dynamics of crassulacean acid metabolism (CAM) in Opuntia ficus-indica

BACKGROUND AND AIMS

The relative contributions of C3 photosynthesis and crassulacean acid metabolism (CAM) during the earliest stages of development were investigated to assess how much each might contribute to cactus pear (Opuntia ficus-indica) productivity.

METHODS

The developmental progression of C3 photosynthesis and CAM was assessed in seedlings and daughter cladodes of mature plants by titratable acidity, δ13C isotopic values and diel gas exchange measurements.

KEY RESULTS

Nocturnal acidification was observed in seedling cladodes and cotyledons at the earliest stages of development and became highly significant by 75 days of development. Seedling cotyledons showed mean δ13C values of -21.4 and -17.1 ‰ at 30 and 100 days of age, respectively. Seedling cladodes showed mean δ13C values of -19.4 and -14.5 ‰ at 30 and 100 days of age, respectively. These values are typical of CAM plants. Net CO2 assimilation was negative, then occurred in both the day and the night, with nighttime fixation becoming predominant once the primary cladode reached 5 cm in size. Emergent daughter cladodes growing on mature plants showed nocturnal titratable acidity at the earliest stages of development, which became significant when daughter cladodes were >2.5-5 cm in height. Emergent daughter cladodes showed mean δ13C values of -14.5 to -15.6 ‰, typical of CAM plants. CO2 assimilation studies revealed that net CO2 uptake was negative in daughter cladodes <12 cm in length, but then exhibited net positive CO2 assimilation in both the day and the night, with net nocturnal CO2 assimilation predominating once the daughter cladode grew larger.

CONCLUSIONS

Developing O. ficus-indica primary and daughter cladodes begin as respiring sink tissues that transition directly to performing CAM once net positive CO2 fixation is observed. Overall, these results demonstrate that CAM is the primary form of photosynthetic carbon assimilation for O. ficus-indica even at the earliest stages of seedling or daughter cladode development.

Reconciling continuous and discrete models of C4 and CAM evolution

BACKGROUND

A current argument in the CAM biology literature has focused on the nature of the CAM evolutionary trajectory: whether there is a smooth continuum of phenotypes between plants with C3 and CAM photosynthesis or whether there are discrete steps of phenotypic evolutionary change such as has been modelled for the evolution of C4 photosynthesis. A further implication is that a smooth continuum would increase the evolvability of CAM, whereas discrete changes would make the evolutionary transition from C3 to CAM more difficult.

SCOPE

In this essay, I attempt to reconcile these two viewpoints, because I think in many ways this is a false dichotomy that is constraining progress in understanding how both CAM and C4 evolved. In reality, the phenotypic space connecting C3 species and strong CAM/C4 species is both a continuum of variably expressed quantitative traits and yet also contains certain combinations of traits that we are able to identify as discrete, recognizable phenotypes. In this sense, the evolutionary mechanics of CAM origination are no different from those of C4 photosynthesis, nor from the evolution of any other complex trait assemblage.

CONCLUSIONS

To make progress, we must embrace the concept of discrete phenotypic phases of CAM evolution, because their delineation will force us to articulate what aspects of phenotypic variation we think are significant. There are some current phenotypic gaps that are limiting our ability to build a complete CAM evolutionary model: the first is how a rudimentary CAM biochemical cycle becomes established, and the second is how the 'accessory' CAM cycle in C3+CAM plants is recruited into a primary metabolism. The connections to the C3 phenotype we are looking for are potentially found in the behaviour of C3 plants when undergoing physiological stress - behaviour that, strangely enough, remains essentially unexplored in this context.

Diversification of flowering plants in space and time

The rapid diversification and high species richness of flowering plants is regarded as 'Darwin's second abominable mystery'. Today the global spatiotemporal pattern of plant diversification remains elusive. Using a newly generated genus-level phylogeny and global distribution data for 14,244 flowering plant genera, we describe the diversification dynamics of angiosperms through space and time. Our analyses show that diversification rates increased throughout the early Cretaceous and then slightly decreased or remained mostly stable until the end of the Cretaceous-Paleogene mass extinction event 66 million years ago. After that, diversification rates increased again towards the present. Younger genera with high diversification rates dominate temperate and dryland regions, whereas old genera with low diversification dominate the tropics. This leads to a negative correlation between spatial patterns of diversification and genus diversity. Our findings suggest that global changes since the Cenozoic shaped the patterns of flowering plant diversity and support an emerging consensus that diversification rates are higher outside the tropics.

Weak crassulacean acid metabolism and other xerophytic adaptive traits in the genus Beaucarnea Lem. (Asparagaceae)

The genus Beaucarnea (Asparagaceae) has economic value as an ornamental plant but also has ecological importance. Nonetheless, habitat, physiological traits and growth parameters of this genus remain largely unknown. We characterized the environmental ranges of Beaucarnea inermis, B. gracilis and B. pliabilis; and screened for the presence of physiological adaptations to drought (biomass allocation, presence of crassulacean acid metabolism [CAM] and its effect on plant water use). We performed experiments in 3- and 5-year-old nursery-grown plants of the three species, measured dry mass accumulation in leaves/stems/roots and screened for CAM using gas exchange, titratable acidity and δ¹³ C. We performed a second experiment on the water and light use responses of B. pliabilis under drought treatment. We found that B. gracilis was limited to xerophytic scrubs (precipitation >400 mm yr^-1 ), while B. pliabilis and B. inermis (precipitation >500 and 700 mm year^-1 , respectively) inhabited dry forests. Beaucarnea gracilis had the lowest dry mass and allocation to leaves, while B. inermis showed the opposite pattern. Only B. pliabilis exhibited small but significant acid fluctuations, characterized as weak CAM, along with high proline content. Acid concentration contributed in 2.7% of the daily carbon during the wet season but represented most of the carbon in the dry season, under closed stomata and had an important contribution to osmolality. Thus, CAM is described for the first time in the genus Beaucarnea, but was only present in one of three species, warranting exploration of this metabolism in the remaining species of this genus.

Differential timing of gene expression and recruitment in independent origins of CAM in the Agavoideae (Asparagaceae)

Crassulacean acid metabolism (CAM) photosynthesis has evolved repeatedly across the plant tree of life, however our understanding of the genetic convergence across independent origins remains hampered by the lack of comparative studies. Here, we explore gene expression profiles in eight species from the Agavoideae (Asparagaceae) encompassing three independent origins of CAM. Using comparative physiology and transcriptomics, we examined the variable modes of CAM in this subfamily and the changes in gene expression across time of day and between well watered and drought-stressed treatments. We further assessed gene expression and the molecular evolution of genes encoding phosphoenolpyruvate carboxylase (PPC), an enzyme required for primary carbon fixation in CAM. Most time-of-day expression profiles are largely conserved across all eight species and suggest that large perturbations to the central clock are not required for CAM evolution. By contrast, transcriptional response to drought is highly lineage specific. Yucca and Beschorneria have CAM-like expression of PPC2, a copy of PPC that has never been shown to be recruited for CAM in angiosperms. Together the physiological and transcriptomic comparison of closely related C₃ and CAM species reveals similar gene expression profiles, with the notable exception of differential recruitment of carboxylase enzymes for CAM function.

Elastic and collapsible: current understanding of cell walls in succulent plants

Succulent plants represent a large functional group of drought-resistant plants that store water in specialized tissues. Several co-adaptive traits accompany this water-storage capacity to constitute the succulent syndrome. A widely reported anatomical adaptation of cell walls in succulent tissues allows them to fold in a regular fashion during extended drought, thus preventing irreversible damage and permitting reversible volume changes. Although ongoing research on crop and model species continuously reports the importance of cell walls and their dynamics in drought resistance, the cell walls of succulent plants have received relatively little attention to date, despite the potential of succulents as natural capital to mitigate the effects of climate change. In this review, we summarize current knowledge of cell walls in drought-avoiding succulents and their effects on tissue biomechanics, water relations, and photosynthesis. We also highlight the existing knowledge gaps and propose a hypothetical model for regulated cell wall folding in succulent tissues upon dehydration. Future perspectives of methodological development in succulent cell wall characterization, including the latest technological advances in molecular and imaging techniques, are also presented.

CAM photosynthesis: the acid test

There is currently considerable interest in the prospects for bioengineering crassulacean acid metabolism (CAM) photosynthesis - or key elements associated with it, such as increased water-use efficiency - into C₃ plants. Resolving how CAM photosynthesis evolved from the ancestral C₃ pathway could provide valuable insights into the targets for such bioengineering efforts. It has been proposed that the ability to accumulate organic acids at night may be common among C₃ plants, and that the transition to CAM might simply require enhancement of pre-existing fluxes, without the need for changes in circadian or diurnal regulation. We show, in a survey encompassing 40 families of vascular plants, that nocturnal acidification is a feature entirely restricted to CAM species. Although many C₃ species can synthesize malate during the light period, we argue that the switch to night-time malic acid accumulation requires a fundamental metabolic reprogramming that couples glycolytic breakdown of storage carbohydrate to the process of net dark CO₂ fixation. This central element of the CAM pathway, even when expressed at a low level, represents a biochemical capability not seen in C₃ plants, and so is better regarded as a discrete evolutionary innovation than as part of a metabolic continuum between C₃ and CAM.

Cadmium-Tolerant Rhizospheric Bacteria of the C3/CAM Intermediate Semi-Halophytic Common Ice Plant (Mesembryanthemum crystallinum L.) Grown in Contaminated Soils

The common ice plant, Mesembryanthemum crystallinum L., has recently been found as a good candidate for phytoremediation of heavy-metal polluted soils. This semi-halophyte is a C₃/CAM (Crassulacean acid metabolism) intermediate plant capable of tolerating extreme levels of cadmium in the soil. The aim of the work was to obtain and characterize novel, Cd-tolerant microbial strains that populate the root zone of M. crystallinum performing different types of photosynthetic metabolism and growing in Cd-contaminated substrates. The plants exhibiting either C₃ or CAM photosynthesis were treated for 8 days with different CdCl₂ doses to obtain final Cd concentrations ranging from 0.82 to 818 mg⋅kg^-1 of soil d.w. The CAM phase was induced by highly saline conditions. After treatment, eighteen bacterial and three yeast strains were isolated from the rhizosphere and, after preliminary Cd-resistance in vitro test, five bacterial strains were selected and identified with a molecular proteomics technique. Two strains of the species Providencia rettgeri (W6 and W7) were obtained from the C₃ phase and three (one Paenibacillus glucanolyticus S7 and two Rhodococcus erythropolis strains: S4 and S10) from the CAM performing plants. The isolates were further tested for Cd-resistance (treatment with either 1 mM or 10 mM CdCl₂) and salinity tolerance (0.5 M NaCl) in model liquid cultures (incubation for 14 days). Providencia rettgeri W7 culture remained fully viable at 1 mM Cd, whereas Rh. erythropolis S4 and S10 together with P. glucanolyticus S7 were found to be resistant to 10 mM Cd in the presence of 0.5 M NaCl. It is suggested that the high tolerance of the common ice plant toward cadmium may result from the synergic action of the plant together with the Cd/salt-resistant strains occurring within rhizospheral microbiota. Moreover, the isolated bacteria appear as promising robust microorganisms for biotechnological applications in bio- and phytoremediation projects.

Evolution of crassulacean acid metabolism (CAM) as an escape from ecological niche conservatism in Malagasy Bulbophyllum (Orchidaceae)

Despite growing evidence that niche shifts are more common in flowering plants than previously thought, little is known of whether such shifts are promoted by changes in photosynthetic pathways. Here we combine the most complete phylogeny for epiphytic Malagasy Bulbophyllum orchids (c. 210 spp.) with climatic niche and carbon isotope ratios to infer the group's spatial-temporal history, and the role of strongly expressed crassulacean acid metabolism (CAM) in facilitating niche shifts and diversification. We find that most extant species still retain niche (Central Highland) and photosynthesis (C₃ ) states as present in the single mid-Miocene (c. 12.70 million yr ago (Ma)) ancestor colonizing Madagascar. However, we also infer a major transition to CAM, linked to a late Miocene (c. 7.36 Ma) invasion of species from the sub-humid highland first into the island's humid eastern coastal, and then into the seasonally dry 'Northwest Sambirano' rainforests, yet without significant effect on diversification rates. These findings indicate that CAM in tropical epiphytes may be selectively advantageous even in high rainfall habitats, rather than presenting a mere adaptation to dry environments or epiphytism per se. Overall, our study qualifies CAM as an evolutionary 'gateway' trait that considerably widened the spatial-ecological amplitude of Madagascar's most species-rich orchid genus.

Engineering of Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) has evolved from a C₃ ground state to increase water use efficiency of photosynthesis. During CAM evolution, selective pressures altered the abundance and expression patterns of C₃ genes and their regulators to enable the trait. The circadian pattern of CO₂ fixation and the stomatal opening pattern observed in CAM can be explained largely with a regulatory architecture already present in C₃ plants. The metabolic CAM cycle relies on enzymes and transporters that exist in C₃ plants and requires tight regulatory control to avoid futile cycles between carboxylation and decarboxylation. Ecological observations and modeling point to mesophyll conductance as a major factor during CAM evolution. The present state of knowledge enables suggestions for genes for a minimal CAM cycle for proof-of-concept engineering, assuming altered regulation of starch synthesis and degradation are not critical elements of CAM photosynthesis and sufficient malic acid export from the vacuole is possible.

Transcriptome and Degradome Profiling Reveals a Role of miR530 in the Circadian Regulation of Gene Expression in Kalanchoë marnieriana

Crassulacean acid metabolism (CAM) is an important photosynthetic pathway for plant adaptation to dry environments. CAM plants feature a coordinated interaction between mesophyll and epidermis functions that involves refined regulations of gene expression. Plant microRNAs (miRNAs) are crucial post-transcription regulators of gene expression, however, their roles underlying the CAM pathway remain poorly investigated. Here, we present a study characterizing the expression of miRNAs in an obligate CAM species Kalanchoë marnieriana. Through sequencing of transcriptome and degradome in mesophyll and epidermal tissues under the drought treatments, we identified differentially expressed miRNAs that were potentially involved in the regulation of CAM. In total, we obtained 84 miRNA genes, and eight of them were determined to be Kalanchoë-specific miRNAs. It is widely accepted that CAM pathway is regulated by circadian clock. We showed that miR530 was substantially downregulated in epidermal peels under drought conditions; miR530 targeted two tandem zinc knuckle/PLU3 domain encoding genes (TZPs) that were potentially involved in light signaling and circadian clock pathways. Our work suggests that the miR530-TZPs module might play a role of regulating CAM-related gene expression in Kalanchoë.

Low-level CAM photosynthesis in a succulent-leaved member of the Urticaceae, Pilea peperomioides

Pilea peperomioides Diels (Urticaceae) is a semi-succulent herbaceous species native to south-western China that has become popular in cultivation as an ornamental plant. To investigate whether this species possesses the capacity for CAM photosynthesis, measurements were made of CO2 gas exchange and titratable acidity in plants under both well-watered and water-deficit conditions. Plants were found to assimilate CO2 almost exclusively in the light via C3 photosynthesis. However, distinct transient reductions in the rate of net nocturnal CO2 release were consistently observed during the course of the dark period, and under water-deficit conditions one plant exhibited a brief period of net nocturnal CO2 uptake, providing unequivocal evidence of CAM activity. Furthermore, nocturnal increases in titratable acidity in both leaf laminas and petioles were observed in all plants exposed to wet-dry-wet cycles. This is the first report of CAM in the family Urticaceae. The results are discussed in relation to the phylogenetic position of Pilea and the partially shaded montane habitats in which this species is typically found. An updated list of all plant families currently known to contain species with CAM is presented.

Diversity of CAM plant photosynthesis (crassulacean acid metabolism): a tribute to Barry Osmond

This special issue is a tribute to the Australian plant biologist Professor Charles Barry Osmond - Fellow of the Australian Academy of Sciences, the Royal Society of London, and Leopoldina, the German National Academy of Sciences - and his many contributions to our understanding of the biochemistry and physiological ecology of CAM (crassulacean acid metabolism) photosynthesis. This water-conserving photosynthetic pathway is characterised by nocturnal uptake of atmospheric CO2 and typically enables succulent plants to perform and survive in warm semiarid terrestrial and epiphytic habitats. The idea for this issue is to mark the occasion of Barry's 80th birthday in 2019. The foreword highlights some of his outstanding contributions and introduces the research papers of the special issue.

Crassulacean acid metabolism (CAM) supersedes the turgor loss point (TLP) as an important adaptation across a precipitation gradient, in the genus Clusia

As future climates continue to change, precipitation deficits are expected to become more severe across tropical ecosystems. As a result, it is important that we identify plant physiological traits that act as adaptations to drought, and determine whether these traits act synergistically or independently of each other. In this study, we assessed the role of three leaf-level putative adaptations to drought: crassulacean acid metabolism (CAM), the turgor loss point (TLPΨ) and water storage hydrenchyma tissue. Using the genus Clusia as a model, we were able to explore the extent to which these leaf physiological traits co-vary, and also how they contribute to species' distributions across a precipitation gradient in Central and South America. We found that CAM is independent of the TLPΨ and hydrenchyma depth in Clusia. In addition, we provide evidence that constitutive CAM is an adaptation to year-long water deficits, whereas facultative CAM appears to be more important for surviving acute dry seasons. Finally, we find that the other leaf traits tested did not correlate with environmental precipitation, suggesting that the reduced transpirational rates associated with CAM obviate the need to adapt the TLPΨ and hydrenchyma depth in this genus.

Constitutive and facultative crassulacean acid metabolism (CAM) in Cuban oregano, Coleus amboinicus (Lamiaceae)

Plants exhibiting the water-conserving crassulacean acid metabolism (CAM) photosynthetic pathway provide some of the most intriguing examples of photosynthetic diversity and plasticity. Here, a largely unnoticed facet of CAM-plant photosynthesis is highlighted: the co-occurrence of ontogenetically controlled constitutive and environmentally controlled facultative CAM in a species. Both forms of CAM are displayed in leaves of Coleus amboinicus Lour. (Lamiaceae), a semi-succulent perennial plant with oregano-like flavour that is native to southern and eastern Africa and naturalised elsewhere in the tropics. Under well-watered conditions, leaves assimilate CO2 predominantly by the C3 pathway. They also display low levels of CO2 uptake at night accompanied by small nocturnal increases in leaf tissue acidity. This indicates the presence of weakly expressed constitutive CAM. CAM expression is strongly enhanced in response to drought stress. The drought-enhanced component of CAM is reversible upon rewatering and thus considered to be facultative. In contrast to C. amboinicus, the thin-leaved closely related Coleus scutellarioides (L.) Benth. exhibits net CO2 fixation solely in the light via the C3 pathway, both under well-watered and drought conditions. However, low levels of nocturnal acidification detected in leaves and stems indicate that the CAM cycle is present. The highly speciose mint family, which contains few known CAM-exhibiting species and is composed predominantly of C3 species, appears to be an excellent group of plants for studying the evolutionary origins of CAM and for determining the position of facultative CAM along the C3-full CAM trajectory.

Crassulacean acid metabolism species differ in the contribution of C3 and C4 carboxylation to end of day CO2 fixation

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO₂ uptake, via phosphoenolpyruvate carboxylase (PEPC, C₄ carboxylation), from the diurnal refixation by Rubisco (C₃ carboxylation). At the end of the day (CAM-Phase IV), when nocturnally stored CO₂ has depleted, stomata reopen and allow additional CO₂ uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO₂ uptake via C₃ and C₄ carboxylation in phase IV in the CAM species Phalaenopsis "Sacramento" and Kalanchoe blossfeldiana "Saja." Short blackout periods during phase IV caused a sharp drop in CO₂ uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O₂ indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.

The eco-evolutionary significance of rainfall constancy for facultative CAM photosynthesis

A flexible use of the crassulacean acid metabolism (CAM) has been hypothesised to represent an intermediate stage along a C₃ to full CAM evolutionary continuum, when relative contributions of C₃ vs CAM metabolism are co-determined by evolutionary history and prevailing environmental constraints. However, evidence for such eco-evolutionary interdependencies is lacking. We studied these interdependencies for the leaf-succulent genus Drosanthemum (Aizoaceae, Southern African Succulent Karoo) by testing for relationships between leaf δ¹³ C diagnostic for CAM dependence (i.e. contribution of C₃ and CAM to net carbon gain), and climatic variables related to temperature and precipitation and their temporal variation. We further quantified the effects of shared phylogenetic ancestry on CAM dependence and its relation to climate. CAM dependence is predicted by rainfall and its temporal variation, with high predictive power of rainfall constancy (temporal entropy). The predictive power of rainfall seasonality and temperature-related variables was negligible. Evolutionary history of the tested clades significantly affected the relationship between rainfall constancy and CAM dependence. We argue that higher CAM dependence might provide an adaptive advantage in increasingly unpredictable rainfall environments when the anatomic exaptation (succulence) is already present. These observations might shed light on the evolution of full CAM.

The photosynthetic pathways of plant species surveyed in Australia's national terrestrial monitoring network

The photosynthetic pathway of plants is a fundamental trait that influences terrestrial environments from the local to global level. The distribution of different photosynthetic pathways in Australia is expected to undergo a substantial shift due to climate change and rising atmospheric CO2; however, tracking change is hindered by a lack of data on the pathways of species, as well as their distribution and relative cover within plant communities. Here we present the photosynthetic pathways for 2428 species recorded across 541 plots surveyed by Australia's Terrestrial Ecosystem Research Network (TERN) between 2011 and 2017. This dataset was created to facilitate research exploring trends in vegetation change across Australia. Species were assigned a photosynthetic pathway using published literature and stable carbon isotope analysis of bulk tissue. The photosynthetic pathway of species can be extracted from the dataset individually, or used in conjunction with vegetation surveys to study the occurrence and abundance of pathways across the continent. This dataset will be updated as TERN's plot network expands and new information becomes available.

CAM-physiology and carbon gain of the orchid Phalaenopsis in response to light intensity, light integral and CO2

The regulation of photosynthesis and carbon gain of crassulacean acid metabolism (CAM) plants has not yet been disclosed to the extent of C3-plants. In this study, the tropical epiphyte Phalaenopsis cv. "Sacramento" was subjected to different lighting regimes. Photosynthesis and biochemical measuring techniques were used to address four specific questions: (1) the response of malate decarboxylation to light intensity, (2) the malate carboxylation pathway in phase IV, (3) the response of diel carbon gain to the light integral and (4) the response of diel carbon gain to CO₂ . The four CAM-phases were clearly discernable. The length of phase III and the malate decarboxylation rate responded directly to light intensity. In phase IV, CO₂ was initially mainly carboxylated via Rubisco. However, at daylength of 16 h, specifically beyond ±12 h, it was mainly phosphoenolpyruvate carboxylase (PEP-C) carboxylating CO₂ . Diel carbon gain appeared to be controlled by the light integral during phase III rather than the total daily light integral. Elevated CO₂ further enhanced carbon gain both in phase IV and phase I. This establishes that neither malate storage capacity, nor availability of PEP as substrate for nocturnal CO₂ carboxylation were limiting factors for carbon gain enhancement. These results advance our understanding of CAM-plants and are also of practical importance for growers.

Phylogenetic signal and climatic niche of stem photosynthesis in the mediterranean and desert regions of California and Baja California Peninsula

PREMISE

Woody plants with photosynthetic stems are common in the drylands of the world; however, we know little about the origin(s) and geographical distribution of photosynthetic stems. Therefore, we set to answer the following questions: (1) Is stem photosynthesis phylogenetically conserved? (2) Do green-stemmed and fleshy-stemmed species have identifiable climatic niches?

METHODS

We mapped the photosynthetic stem trait onto a phylogeny of 228 mediterranean and desert species and calculated indices of phylogenetic signal and created climatic niche models of 28 species belonging to three groups: green, fleshy, and green-and-fleshy stemmed species.

RESULTS

We found phylogenetic signal in the fleshy stem trait, but not in the green stem trait. Fleshy-stemmed species occupy areas associated with high isothermality, high precipitation seasonality, and high mean temperature of the wettest quarter, whereas green-stemmed species occupy areas associated with high precipitation of driest month, high precipitation of coldest quarter, high mean diurnal temperature range and high maximum temperature of the warmest month.

CONCLUSIONS

Despite the fact that both photosynthetic stem types help cope with water shortage, having fleshy stems allows plants to cope with greater precipitation seasonality than is possible with green stems. Green stems require a lot of water to be stored in the soil to maintain net photosynthesis during the dry season, so they inhabit areas with higher and more predictable precipitation.

Genomic footprints of repeated evolution of CAM photosynthesis in a Neotropical species radiation

The adaptive radiation of Bromeliaceae (pineapple family) is one of the most diverse among Neotropical flowering plants. Diversification in this group was facilitated by shifts in several adaptive traits or "key innovations" including the transition from C₃ to CAM photosynthesis associated with xeric (heat/drought) adaptation. We used phylogenomic approaches, complemented by differential gene expression (RNA-seq) and targeted metabolite profiling, to address the mechanisms of C₃ /CAM evolution in the extremely species-rich bromeliad genus, Tillandsia, and related taxa. Evolutionary analyses of whole-genome sequencing and RNA-seq data suggest that evolution of CAM is associated with coincident changes to different pathways mediating xeric adaptation in this group. At the molecular level, C₃ /CAM shifts were accompanied by gene expansion of XAP5 CIRCADIAN TIMEKEEPER homologs, a regulator involved in sugar- and light-dependent regulation of growth and development. Our analyses also support the re-programming of abscisic acid-related gene expression via differential expression of ABF2/ABF3 transcription factor homologs, and adaptive sequence evolution of an ENO2/LOS2 enolase homolog, effectively tying carbohydrate flux to abscisic acid-mediated abiotic stress response. By pinpointing different regulators of overlapping molecular responses, our results suggest plausible mechanistic explanations for the repeated evolution of correlated adaptive traits seen in a textbook example of an adaptive radiation.

Crassulacean acid metabolism

Gilman and Edwards introduce crassulacean acid metabolism and highlight how recent advances in molecular biology are deepening our knowledge of CAM evolution.

Comparative proteomics of Mesembryanthemum crystallinum guard cells and mesophyll cells in transition from C3 to CAM

Salinity can induce Mesembryanthemum crystallinum to shift its photosynthesis from C₃ to crassulacean acid metabolism (CAM), leading to enhanced plant water use efficiency. Studying how M. crystallinum changes its carbon fixation pathways is important for potential translation into crops and enhancing crop resilience. In this study, we examined proteomic changes in guard cells and mesophyll cells in the course of the C₃ to CAM transition. We collected enriched guard cells and mesophyll cells during a short period of transition. A total of 1153 proteins were identified and quantified in the two cell-types. During the transition, proteins in the guard cells and mesophyll cells exhibited differential changes. For example, we observed nocturnal carbon fixation in mesophyll cells and proteins involved in cell growth in the two cell-types. Proteins involved in osmotic adjustment, ion transport, energy metabolism and light response may play important roles in the C₃ to CAM transition. Real-time PCR experiments were conducted to determine potential correlations between transcript and protein levels. These results have highlighted potential molecular mechanisms underlying the C₃ to CAM transition of guard cells and mesophyll cells of the important facultative CAM plant. BIOLOGICAL SIGNIFICANCE: Fresh water resource for agricultural food production is a global challenge. Nature has evolved crassulacean acid metabolism (CAM) plants with enhanced water use efficiency. Using single cell-type proteomics, this study revealed molecular changes taking place in guard cells and mesophyll cells during the shift of ice plant photosynthesis from C3 to CAM. The results have provided important insights into the CAM transition and may facilitate effort toward enhancing crop resilience for global food security.

Biosystems Design to Accelerate C3-to-CAM Progression

Global demand for food and bioenergy production has increased rapidly, while the area of arable land has been declining for decades due to damage caused by erosion, pollution, sea level rise, urban development, soil salinization, and water scarcity driven by global climate change. In order to overcome this conflict, there is an urgent need to adapt conventional agriculture to water-limited and hotter conditions with plant crop systems that display higher water-use efficiency (WUE). Crassulacean acid metabolism (CAM) species have substantially higher WUE than species performing C3 or C4 photosynthesis. CAM plants are derived from C3 photosynthesis ancestors. However, it is extremely unlikely that the C3 or C4 crop plants would evolve rapidly into CAM photosynthesis without human intervention. Currently, there is growing interest in improving WUE through transferring CAM into C3 crops. However, engineering a major metabolic plant pathway, like CAM, is challenging and requires a comprehensive deep understanding of the enzymatic reactions and regulatory networks in both C3 and CAM photosynthesis, as well as overcoming physiometabolic limitations such as diurnal stomatal regulation. Recent advances in CAM evolutionary genomics research, genome editing, and synthetic biology have increased the likelihood of successful acceleration of C3-to-CAM progression. Here, we first summarize the systems biology-level understanding of the molecular processes in the CAM pathway. Then, we review the principles of CAM engineering in an evolutionary context. Lastly, we discuss the technical approaches to accelerate the C3-to-CAM transition in plants using synthetic biology toolboxes.

Exploring C4-CAM plasticity within the Portulaca oleracea complex

Portulaca oleracea is a C₄ herb capable of performing CAM under drought stress. It is distributed worldwide and is either considered a polymorphic species or a complex of subspecies, due to its numerous morphological variations. We evaluated CAM plasticity within P. oleracea genotypes since the complexity surrounding this species may be reflected in intraspecific variations in photosynthetic behavior. Eleven subspecies of P. oleracea from distant geographical locations and one cultivar were morphologically and physiologically characterized. C₄ and CAM photosynthesis were monitored in plants exposed to well-watered, droughted and rewatered treatments, and data obtained were compared among individual genotypes. All subspecies expressed CAM in a fully-reversible manner. Transcript abundance of C₄–CAM signature genes was shown to be a useful indicator of the C₄–CAM–C₄ switches in all genotypes. C₄-related genes were down-regulated and subsequently fully expressed upon drought and rewatering, respectively. CAM-marker genes followed the opposite pattern. A gradient of morphological traits and drought-induced nighttime malate accumulation was observed across genotypes. Therefore, different combinations of CAM expression levels, plant sizes and shapes are available within the P. oleracea complex, which can be a valuable tool in the context of C₄/CAM photosynthesis research.

Physiological Changes in Mesembryanthemum crystallinum During the C3 to CAM Transition Induced by Salt Stress

Salt stress impedes plant growth and development, and leads to yield loss. Recently, a halophyte species Mesembryanthemum crystallinum has become a model to study plant photosynthetic responses to salt stress. It has an adaptive mechanism of shifting from C3 photosynthesis to crassulacean acid metabolism (CAM) photosynthesis under stresses, which greatly enhances water usage efficiency and stress tolerance. In this study, we focused on investigating the morphological and physiological changes [e.g., leaf area, stomatal movement behavior, gas exchange, leaf succulence, and relative water content (RWC)] of M. crystallinum during the C3 to CAM photosynthetic transition under salt stress. Our results showed that in M. crystallinum seedlings, CAM photosynthesis was initiated after 6 days of salt treatment, the transition takes place within a 3-day period, and plants became mostly CAM in 2 weeks. This result defined the transition period of a facultative CAM plant, laid a foundation for future studies on identifying the molecular switches responsible for the transition from C3 to CAM, and contributed to the ultimate goal of engineering CAM characteristics into C3 crops.

Model approaches to advance crassulacean acid metabolism system integration

This review summarises recent progress in understanding crassulacean acid metabolism (CAM) systems and the integration of internal and external stimuli to maximise water-use efficiency. Complex CAM traits have been reduced to their minimum and captured as computational models, which can now be refined using recently available data from transgenic manipulations and large-scale omics studies. We identify three key areas in which an appropriate choice of modelling tool could help capture relevant comparative molecular data to address the evolutionary drivers and plasticity of CAM. One focus is to identify the environmental and internal signals that drive inverse stomatal opening at night. Secondly, it is important to identify the regulatory processes required to orchestrate the diel pattern of carbon fluxes within mesophyll layers. Finally, the limitations imposed by contrasting succulent systems and associated hydraulic conductance components should be compared in the context of water-use and evolutionary strategies. While network analysis of transcriptomic data can provide insights via co-expression modules and hubs, alternative forms of computational modelling should be used iteratively to define the physiological significance of key components and informing targeted functional gene manipulation studies. We conclude that the resultant improvements of bottom-up, mechanistic modelling systems can enhance progress towards capturing the physiological controls for phylogenetically diverse CAM systems in the face of the recent surge of information in this omics era.

Shared expression of crassulacean acid metabolism (CAM) genes pre-dates the origin of CAM in the genus Yucca

Crassulacean acid metabolism (CAM) is a carbon-concentrating mechanism that has evolved numerous times across flowering plants and is thought to be an adaptation to water-limited environments. CAM has been investigated from physiological and biochemical perspectives, but little is known about how plants evolve from C3 to CAM at the genetic or metabolic level. Here we take a comparative approach in analyzing time-course data of C3, CAM, and C3+CAM intermediate Yucca (Asparagaceae) species. RNA samples were collected over a 24 h period from both well-watered and drought-stressed plants, and were clustered based on time-dependent expression patterns. Metabolomic data reveal differences in carbohydrate metabolism and antioxidant response between the CAM and C3 species, suggesting that changes to metabolic pathways are important for CAM evolution and function. However, all three species share expression profiles of canonical CAM pathway genes, regardless of photosynthetic pathway. Despite differences in transcript and metabolite profiles between the C3 and CAM species, shared time-structured expression of CAM genes in both CAM and C3Yucca species suggests that ancestral expression patterns required for CAM may have pre-dated its origin in Yucca.

Facultative crassulacean acid metabolism in a C3-C4 intermediate

The Portulacaceae enable the study of the evolutionary relationship between C4 and crassulacean acid metabolism (CAM) photosynthesis. Shoots of well-watered plants of the C3-C4 intermediate species Portulaca cryptopetala Speg. exhibit net uptake of CO2 solely during the light. CO2 fixation is primarily via the C3 pathway as indicated by a strong stimulation of CO2 uptake when shoots were provided with air containing 2% O2. When plants were subjected to water stress, daytime CO2 uptake was reduced and CAM-type net CO2 uptake in the dark occurred. This was accompanied by nocturnal accumulation of acid in both leaves and stems, also a defining characteristic of CAM. Following rewatering, net CO2 uptake in the dark ceased in shoots, as did nocturnal acidification of the leaves and stems. With this unequivocal demonstration of stress-related reversible, i.e. facultative, induction of CAM, P. cryptopetala becomes the first C3-C4 intermediate species reported to exhibit CAM. Portulaca molokiniensis Hobdy, a C4 species, also exhibited CAM only when subjected to water stress. Facultative CAM has now been demonstrated in all investigated species of Portulaca, which are well sampled from across the phylogeny. This strongly suggests that in Portulaca, a lineage in which species engage predominately in C4 photosynthesis, facultative CAM is ancestral to C4. In a broader context, it has now been demonstrated that CAM can co-exist in leaves that exhibit any of the other types of photosynthesis known in terrestrial plants: C3, C4 and C3-C4 intermediate.

Ecophysiology of constitutive and facultative CAM photosynthesis

In plants exhibiting crassulacean acid metabolism (CAM), CAM photosynthesis almost always occurs together with C3 photosynthesis, and occasionally with C4 photosynthesis. Depending on species, ontogeny, and environment, CAM input to total carbon gain can vary from values of <1% to 100%. The wide range of CAM phenotypes between and within species is a fascinating example of functional diversity and plasticity, but poses a significant challenge when attempting to define CAM. CO2 gas exchange experiments designed for this review illustrate key patterns of CAM expression and highlight distinguishing features of constitutive and facultative CAM. Furthermore, they help to address frequently recurring questions on CAM terminology. The functional and evolutionary significance of contrasting CAM phenotypes and of intermediate states between extremes is discussed. Results from a study on nocturnal malate accumulation in 50 species of Aizoaceae exposed to drought and salinity stress suggest that facultative CAM is more widespread amongst vascular plants than previously thought.

Operating at the very low end of the crassulacean acid metabolism spectrum: Sesuvium portulacastrum (Aizoaceae)

Demonstration of crassulacean acid metabolism (CAM) in species with low usage of this system relative to C3-photosynthetic CO2 assimilation can be challenging experimentally but provides crucial information on the early steps of CAM evolution. Here, weakly expressed CAM was detected in the well-known pantropical coastal, leaf-succulent herb Sesuvium portulacastrum, demonstrating that CAM is present in the Sesuvioideae, the only sub-family of the Aizoaceae in which it had not yet been shown conclusively. In outdoor plots in Panama, leaves and stems of S. portulacastrum consistently exhibited a small degree of nocturnal acidification which, in leaves, increased during the dry season. In potted plants, nocturnal acidification was mainly facultative, as levels of acidification increased in a reversible manner following the imposition of short-term water-stress. In drought-stressed plants, nocturnal net CO2 exchange approached the CO2-compensation point, consistent with low rates of CO2 dark fixation sufficient to eliminate respiratory carbon loss. Detection of low-level CAM in S. portulacastrum adds to the growing number of species that cannot be considered C3 plants sensu stricto, although they obtain CO2 principally via the C3 pathway. Knowledge about the presence/absence of low-level CAM is critical when assessing trajectories of CAM evolution in lineages. The genus Sesuvium is of particular interest because it also contains C4 species.

A perspective on crassulacean acid metabolism photosynthesis evolution of orchids on different continents: Dendrobium as a case study

Members of the Orchidaceae, one of the largest families of flowering plants, evolved the crassulacean acid metabolism (CAM) photosynthesis strategy. It is thought that CAM triggers adaptive radiation into new niche spaces, yet very little is known about its origin and diversification on different continents. Here, we assess the prevalence of CAM in Dendrobium, which is one of the largest genera of flowering plants and found in a wide range of environments, from the high altitudes of the Himalayas to relatively arid habitats in Australia. Based on phylogenetic time trees, we estimated that CAM, as determined by δ 13C values less negative than -20.0‰, evolved independently at least eight times in Dendrobium. The oldest lineage appeared in the Asian clade during the middle Miocene, indicating the origin of CAM was associated with a pronounced climatic cooling that followed a period of aridity. Divergence of the four CAM lineages in the Asian clade appeared to be earlier than divergence of those in the Australasian clade. However, CAM species in the Asian clade are much less diverse (25.6%) than those in the Australasian clade (57.9%). These findings shed new light on CAM evolutionary history and the aridity levels of the paleoclimate on different continents.

The Evolution of CAM Photosynthesis in Australian Calandrinia Reveals Lability in C3+CAM Phenotypes and a Possible Constraint to the Evolution of Strong CAM

Australian Calandrinia has radiated across the Australian continent during the last 30 Ma, and today inhabits most Australian ecosystems. Given its biogeographic range and reports of facultative Crassulacean acid metabolism (CAM) photosynthesis in multiple species, we hypothesized (1) that CAM would be widespread across Australian Calandrinia and that species, especially those that live in arid regions, would engage in strong CAM, and (2) that Australian Calandrinia would be an important lineage for informing on the CAM evolutionary trajectory. We cultivated 22 Australian Calandrinia species for a drought experiment. Using physiological measurements and δ13C values we characterized photosynthetic mode across these species, mapped the resulting character states onto a phylogeny, and characterized the climatic envelopes of species in their native ranges. Most species primarily utilize C3 photosynthesis, with CAM operating secondarily, often upregulated following drought. Several phylogenetically nested species are C3, indicating evolutionary losses of CAM. No strong CAM was detected in any of the species. Results highlight the limitations of δ13C surveys in detecting C3+CAM phenotypes, and the evolutionary lability of C3+CAM phenotypes. We propose a model of CAM evolution that allows for lability and reversibility among C3+CAM phenotypes and C3 and suggest that an annual life-cycle may preclude the evolution of strong CAM.

Understanding trait diversity associated with crassulacean acid metabolism (CAM)

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that exploits a temporal CO₂ pump with nocturnal CO₂ uptake and concentration to reduce photorespiration, improve water-use efficiency (WUE), and optimize the adaptability of plants to climates with seasonal or intermittent water limitations. CAM plants display a plastic continuum in the extent to which species engage in net nocturnal CO₂ uptake that ranges from 0 to 100%. CAM plants also display diverse enzyme and organic acid and carbohydrate storage systems, which likely reflect the multiple, independent evolutionary origins of CAM. CAM is often accompanied by a diverse set of anatomical traits, such as tissue succulence and water-storage and water-capture strategies to attenuate drought. Other co-adaptive traits, such as thick cuticles, epicuticular wax, low stomatal density, high stomatal responsiveness, and shallow rectifier-like roots limit water loss under conditions of water deficit. Recommendations for future research efforts to better explore and understand the diversity of traits associated with CAM and CAM Biodesign efforts are presented.

Effect of Light/Dark Cycle on Photosynthetic Pathway Switching and CO2 Absorption in Two Dendrobium Species

Many Dendrobium species are both ornamental and medicinal plants in China. Several wild species have been exploited to near extinction, and facility cultivation has become an important way to meet the great market demand. Most Dendrobium species have evolved into crassulacean acid metabolism (CAM) pathways in adapting to harsh epiphytic environment, leading to low daily net CO₂ absorption. Photosynthetic pathways of many facultative CAM plants are regulated by various environmental factors. Light/dark cycle plays an important role in regulating the photosynthetic pathway of several CAM species. The aims of this study were to investigate whether the photosynthetic pathway of Dendrobium species could be regulated between C3 and CAM by changing light/dark cycles and the daily net CO₂ absorption could be enhanced by shortening light/dark cycle. In this study, net CO₂ exchange rates of D. officinale and D. primulinum were monitored continuously during two different light/dark cycles conversion compared to Kalanchoe daigremontiana as an obligate CAM plant. The net CO₂ exchange pattern and stomatal behavior of D. officinale and D. primulinum were switched from CAM to C3-like by changing the light/dark cycle from 12/12 h to 4/4 h. However, this switching was not completely reversible. Compared to the original 12/12 h light/dark cycle, the dark, light, and daily net CO₂ exchange amount of D. officinale were significantly increased after the light/dark cycle was changed from 4/4 h to 12/12 h, but those in D. primulinum was opposite and those in K. daigremontiana was not affected. Daily net CO₂ exchange amount of D. officinale increased by 47% after the light/dark cycle was changed from 12/12 h to 4/4 h, due to the sharp increase of light net CO₂ exchange amount. However, the large decrease of dark net CO₂ exchange amount could not be offset by increased light net CO₂ exchange amount, leading to reduced daily net CO₂ exchange amount of D. primulinum. In conclusion, the 4/4 h light/dark cycle can induce the photosynthetic pathway of D. officinale and D. primulinum to C3-like, and improve the daily CO₂ absorption of D. officinale.

Altered Gene Regulatory Networks Are Associated With the Transition From C3 to Crassulacean Acid Metabolism in Erycina (Oncidiinae: Orchidaceae)

Crassulacean acid metabolism (CAM) photosynthesis is a modification of the core C3 photosynthetic pathway that improves the ability of plants to assimilate carbon in water-limited environments. CAM plants fix CO2 mostly at night, when transpiration rates are low. All of the CAM pathway genes exist in ancestral C3 species, but the timing and magnitude of expression are greatly altered between C3 and CAM species. Understanding these regulatory changes is key to elucidating the mechanism by which CAM evolved from C3. Here, we use two closely related species in the Orchidaceae, Erycina pusilla (CAM) and Erycina crista-galli (C3), to conduct comparative transcriptomic analyses across multiple time points. Clustering of genes with expression variation across the diel cycle revealed some canonical CAM pathway genes similarly expressed in both species, regardless of photosynthetic pathway. However, gene network construction indicated that 149 gene families had significant differences in network connectivity and were further explored for these functional enrichments. Genes involved in light sensing and ABA signaling were some of the most differently connected genes between the C3 and CAM Erycina species, in agreement with the contrasting diel patterns of stomatal conductance in C3 and CAM plants. Our results suggest changes to transcriptional cascades are important for the transition from C3 to CAM photosynthesis in Erycina.

Transcriptomic profiling for prolonged drought in Dendrobium catenatum

Orchid epiphytes, a group containing at least 18,000 species, thrive in habitats that often undergo periodic drought stress. However, few global gene expression profiling datasets have been published for studies addressing the drought-resistant mechanism of this special population. In this study, an experiment involving the effect of continuous drought treatments on an epiphytic orchid, Dendrobium catenatum, was designed to generate 39 mature-leaf-tissue RNA-seq sequencing datasets with over two billion reads. These datasets were validated by a series of quality assessments including RNA sample quality, RNA-seq read quality, and global gene expression profiling. We believe that these comprehensive transcriptomic resources will allow a better understanding of the drought-resistant mechanisms of orchid epiphytes.