Research note: Large gene family of phosphoenolpyruvate carboxylase in the crassulacean acid metabolism plant Kalanchoe pinnata (Crassulaceae) characterised by partial cDNA sequence analysis

Klaus Winter - the indefatigable CAM experimentalist

BACKGROUND

In January 1972, Klaus Winter submitted his first paper on crassulacean acid metabolism (CAM) whilst still an undergraduate student in Darmstadt. During the subsequent half-century, he passed his Staatsexamensarbeit, obtained his Dr. rer. nat. summa cum laude and Dr. rer. nat. habil., won a Heinz Maier-Leibnitz Prize and a Heisenberg Fellowship, and has occupied positions in Germany, Australia, the USA and Panama. Now a doyen in CAM circles, and a Senior Staff Scientist at the Smithsonian Tropical Research Institute (STRI), he has published over 300 articles, of which about 44 % are about CAM.

SCOPE

I document Winter's career, attempting to place his CAM-related scientific output and evolution in the context of factors that have influenced him as he and his science progressed from the 1970s to the 2020s.

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.

High-quality ice plant reference genome analysis provides insights into genome evolution and allows exploration of genes involved in the transition from C3 to CAM pathways

Ice plant (Mesembryanthemum crystallinum), a member of the Aizoaceae family, is a typical halophyte crop and a model plant for studying the mechanism of transition from C3 photosynthesis to crassulacean acid metabolism (CAM). Here, we report a high-quality chromosome-level ice plant genome sequence. This 98.05% genome sequence is anchored to nine chromosomes, with a total length of 377.97 Mb and an N50 scaffold of 40.45 Mb. Almost half of the genome (48.04%) is composed of repetitive sequences, and 24 234 genes have been annotated. Subsequent to the ancient whole-genome triplication (WGT) that occurred in eudicots, there has been no recent whole-genome duplication (WGD) or WGT in ice plants. However, we detected a novel WGT event that occurred in the same order in Simmondsia chinensis, which was previously overlooked. Our findings revealed that ice plants have undergone chromosome rearrangements and gene removal during evolution. Combined with transcriptome and comparative genomic data and expression verification, we identified several key genes involved in the CAM pathway and constructed a comprehensive network. As the first genome of the Aizoaceae family to be released, this report will provide a rich data resource for comparative and functional genomic studies of Aizoaceae, especially for studies on salt tolerance and C3-to-CAM transitions to improve crop yield and resistance.

Molecular Cloning of Novel-Type Phosphoenolpyruvate Carboxylase Isoforms in Pitaya (Hylocereus undatus)

Phosphoenolpyruvate carboxylase (PEPC) is an important enzyme involved in the initial CO₂ fixation of crassulacean acid metabolism (CAM) photosynthesis. To understand the cultivation characteristics of a CAM plant pitaya, it is necessary to clarify the characteristics of PEPC in this species. Here, we cloned three PEPC cDNAs in pitaya, HuPPC1, HuPPC2, and HuPPC3, which encode 942, 934, and 966 amino acid residues, respectively. Phylogenetic analysis indicated that these PEPC belonged to plant-type PEPC (PTPC), although HuPPC1 and HuPPC2 have no Ser-phosphorylation motif in N-terminal region, which is a crucial regulation site in PTPC and contributes to CAM periodicity. HuPPC1 and HuPPC2 phylogenetically unique to the Cactaceae family, whereas HuPPC3 was included in a CAM clade. Two isoforms were partially purified at the protein level and were assigned as HuPPC2 and HuPPC3 using MASCOT analysis. The most distinct difference in enzymatic properties between the two was sensitivity to malate and aspartate, both of which are allosteric inhibitors of PEPC. With 2 mM malate, HuPPC3 was inhibited to 10% of the initial activity, whereas HuPPC2 activity was maintained at 70%. Aspartate inhibited HuPPC3 activity by approximately 50% at 5 mM; however, such inhibition was not observed for HuPPC2 at 10 mM. These results suggest that HuPPC3 corresponds to a general CAM-related PEPC, whereas HuPPC1 and HuPPC2 are related to carbon and/or nitrogen metabolism, with a characteristic regulation mechanism similar to those of Cactaceae plants.

Crassulacean Acid Metabolism Abiotic Stress-Responsive Transcription Factors: a Potential Genetic Engineering Approach for Improving Crop Tolerance to Abiotic Stress

This perspective paper explores the utilization of abiotic stress-responsive transcription factors (TFs) from crassulacean acid metabolism (CAM) plants to improve abiotic stress tolerance in crop plants. CAM is a specialized type of photosynthetic adaptation that enhances water-use efficiency (WUE) by shifting CO2 uptake to all or part of the nighttime when evaporative water losses are minimal. Recent studies have shown that TF-based genetic engineering could be a useful approach for improving plant abiotic stress tolerance because of the role of TFs as master regulators of clusters of stress-responsive genes. Here, we explore the use of abiotic stress-responsive TFs from CAM plants to improve abiotic stress tolerance and WUE in crops by controlling the expression of gene cohorts that mediate drought-responsive adaptations. Recent research has revealed several TF families including AP2/ERF, MYB, WRKY, NAC, NF-Y, and bZIP that might regulate water-deficit stress responses and CAM in the inducible CAM plant Mesembryanthemum crystallinum under water-deficit stress-induced CAM and in the obligate CAM plant Kalanchoe fedtschenkoi. Overexpression of genes from these families in Arabidopsis thaliana can improve abiotic stress tolerance in A. thaliana in some instances. Therefore, we propose that TF-based genetic engineering with a small number of CAM abiotic stress-responsive TFs will be a promising strategy for improving abiotic stress tolerance and WUE in crop plants in a projected hotter and drier landscape in the 21st-century and beyond.

Laying the Foundation for Crassulacean Acid Metabolism (CAM) Biodesign: Expression of the C4 Metabolism Cycle Genes of CAM in Arabidopsis

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 hotter and drier climates. Introducing the CAM photosynthetic machinery into C3 (or C4) photosynthesis plants (CAM Biodesign) represents a potentially breakthrough strategy for improving WUE while maintaining high productivity. To optimize the success of CAM Biodesign approaches, the functional analysis of individual C4 metabolism cycle genes is necessary to identify the essential genes for robust CAM pathway introduction. Here, we isolated and analyzed the subcellular localizations of 13 enzymes and regulatory proteins of the C4 metabolism cycle of CAM from the common ice plant in stably transformed Arabidopsis thaliana. Six components of the carboxylation module were analyzed including beta-carbonic anhydrase (McBCA2), phosphoenolpyruvate carboxylase (McPEPC1), phosphoenolpyruvate carboxylase kinase (McPPCK1), NAD-dependent malate dehydrogenase (McNAD-MDH1, McNAD-MDH2), and NADP-dependent malate dehydrogenase (McNADP-MDH1). In addition, seven components of the decarboxylation module were analyzed including NAD-dependent malic enzyme (McNAD-ME1, McNAD-ME2), NADP-dependent malic enzyme (McNADP-ME1, NADP-ME2), pyruvate, orthophosphate dikinase (McPPDK), pyruvate, orthophosphate dikinase-regulatory protein (McPPDK-RP), and phosphoenolpyruvate carboxykinase (McPEPCK). Ectopic overexpression of most C4-metabolism cycle components resulted in increased rosette diameter, leaf area, and leaf fresh weight of A. thaliana except for McNADP-MDH1, McPPDK-RP, and McPEPCK. Overexpression of most carboxylation module components resulted in increased stomatal conductance and dawn/dusk titratable acidity (TA) as an indirect measure of organic acid (mainly malate) accumulation in A. thaliana. In contrast, overexpression of the decarboxylating malic enzymes reduced stomatal conductance and TA. This comprehensive study provides fundamental insights into the relative functional contributions of each of the individual components of the core C4-metabolism cycle of CAM and represents a critical first step in laying the foundation for CAM Biodesign.

Facultative crassulacean acid metabolism (CAM) plants: powerful tools for unravelling the functional elements of CAM photosynthesis

Facultative crassulacean acid metabolism (CAM) describes the optional use of CAM photosynthesis, typically under conditions of drought stress, in plants that otherwise employ C3 or C4 photosynthesis. In its cleanest form, the upregulation of CAM is fully reversible upon removal of stress. Reversibility distinguishes facultative CAM from ontogenetically programmed unidirectional C3-to-CAM shifts inherent in constitutive CAM plants. Using mainly measurements of 24h CO2 exchange, defining features of facultative CAM are highlighted in five terrestrial species, Clusia pratensis, Calandrinia polyandra, Mesembryanthemum crystallinum, Portulaca oleracea and Talinum triangulare. For these, we provide detailed chronologies of the shifts between photosynthetic modes and comment on their usefulness as experimental systems. Photosynthetic flexibility is also reviewed in an aquatic CAM plant, Isoetes howellii. Through comparisons of C3 and CAM states in facultative CAM species, many fundamental biochemical principles of the CAM pathway have been uncovered. Facultative CAM species will be of even greater relevance now that new sequencing technologies facilitate the mapping of genomes and tracking of the expression patterns of multiple genes. These technologies and facultative CAM systems, when joined, are expected to contribute in a major way towards our goal of understanding the essence of CAM.

Multiple isoforms of phosphoenolpyruvate carboxylase in the Orchidaceae (subtribe Oncidiinae): implications for the evolution of crassulacean acid metabolism

Phosphoenolpyruvate carboxylase (PEPC) catalyses the initial fixation of atmospheric CO2 into oxaloacetate and subsequently malate. Nocturnal accumulation of malic acid within the vacuole of photosynthetic cells is a typical feature of plants that perform crassulacean acid metabolism (CAM). PEPC is a ubiquitous plant enzyme encoded by a small gene family, and each member encodes an isoform with specialized function. CAM-specific PEPC isoforms probably evolved from ancestral non-photosynthetic isoforms by gene duplication events and subsequent acquisition of transcriptional control elements that mediate increased leaf-specific or photosynthetic-tissue-specific mRNA expression. To understand the patterns of functional diversification related to the expression of CAM, ppc gene families and photosynthetic patterns were characterized in 11 closely related orchid species from the subtribe Oncidiinae with a range of photosynthetic pathways from C3 photosynthesis (Oncidium cheirophorum, Oncidium maduroi, Rossioglossum krameri, and Oncidium sotoanum) to weak CAM (Oncidium panamense, Oncidium sphacelatum, Gomesa flexuosa and Rossioglossum insleayi) and strong CAM (Rossioglossum ampliatum, Trichocentrum nanum, and Trichocentrum carthagenense). Phylogenetic analysis revealed the existence of two main ppc lineages in flowering plants, two main ppc lineages within the eudicots, and three ppc lineages within the Orchidaceae. Our results indicate that ppc gene family expansion within the Orchidaceae is likely to be the result of gene duplication events followed by adaptive sequence divergence. CAM-associated PEPC isoforms in the Orchidaceae probably evolved from several independent origins.

The physiology of ex vitro pineapple (Ananas comosus L. Merr. var MD-2) as CAM or C3 is regulated by the environmental conditions: proteomic and transcriptomic profiles

KEY MESSAGE

Proteomic and transcriptomic profiles of key enzymes were monitored in pineapple plants propagated under C3 and CAM-inducing metabolisms to obtain insight into the CAM-facultative metabolism and the relationship of CAM plants with oxidative stress.

ABSTRACT

Pineapple is one of the most important tropical crops worldwide. The use of temporary immersion bioreactors for the first stages of pineapple propagation enables precise control of plant growth, increases the rate of plant multiplication, decreases space, energy and labor requirements for pineapple plants in commercial micropropagation. Once the plantlets are ready to be taken from the reactors, they are carefully acclimatized to natural environmental conditions, and a facultative C3/CAM metabolism in the first 2 months of growth is the characteristic of pineapple plants, depending on environmental conditions. We subjected two sets of micropropagated pineapple plants to C3 and CAM-inducing environmental conditions, determined by light intensity/relative humidity (respectively 40 μmol m−2 s−1/85 % and 260 μmol m−2 s−1/50 %). Leaves of pineapple plants grown under CAM-inducing conditions showed higher leaf thickness and more developed cuticles and hypodermic tissue. Proteomic profiles of several proteins, isoenzyme patterns and transcriptomic profiles were also measured. Five major spots were isolated and identified, two of them for the first time in Ananas comosus (OEE 1; OEE 2) and the other three corresponding to small fragments of the large subunit of Rubisco (LSU). PEPC and PEPCK were also detected by immunobloting of 2DE at the end of both ex vitro treatments (C3/CAM) during the dark period. Isoenzymes of SOD and CAT were identified by electrophoresis and the transcript levels of OEE 1 and CAT were associated with CAM metabolism in pineapple plants.

Crassulacean acid metabolism in the ZZ plant, Zamioculcas zamiifolia (Araceae)

Zamioculcas zamiifolia (Araceae), a terrestrial East African aroid, with two defining attributes of crassulacean acid metabolism (CAM) (net CO(2) uptake in the dark and diel fluctuations of titratable acidity) is the only CAM plant described within the Araceae, a mainly tropical taxon that contains the second largest number of epiphytes of any vascular plant family. Within the Alismatales, the order to which the Araceae belong, Z. zamiifolia is the only documented nonaquatic CAM species. Zamioculcas zamiifolia has weak CAM that is upregulated in response to water stress. In well-watered plants, day-night fluctuations in titratable acidity were 2.5 μmol H(+)·(g fresh mass)(-1), and net CO(2) uptake in the dark contributed less than 1% to daily carbon gain. Following 10 d of water stress, net CO(2) uptake in the light fell 94% and net CO(2) uptake in the dark increased 7.5-fold, such that its contribution increased to 19% of daily carbon gain. Following rewatering, dark CO(2) uptake returned to within 5% of prestressed levels. We postulate that CAM assists survival of Z. zamiifolia by reducing water loss and maintaining carbon gain during seasonal droughts characteristic of its natural habitat.

Environment or development? Lifetime net CO2 exchange and control of the expression of Crassulacean acid metabolism in Mesembryanthemum crystallinum

The relative influence of plant age and environmental stress signals in triggering a shift from C(3) photosynthesis to Crassulacean acid metabolism (CAM) in the annual halophytic C(3)-CAM species Mesembryanthemum crystallinum was explored by continuously monitoring net CO(2) exchange of whole shoots from the seedling stage until seed set. Plants exposed to high salinity (400 mm NaCl) in hydroponic culture solution or grown in saline-droughted soil acquired between 11% and 24% of their carbon via net dark CO(2) uptake involving CAM. In contrast, plants grown under nonsaline, well-watered conditions were capable of completing their life cycle by operating in the C(3) mode without ever exhibiting net CO(2) uptake at night. These observations are not consistent with the widely expressed view that the induction of CAM by high salinity in M. crystallinum represents an acceleration of preprogrammed developmental processes. Rather, our study demonstrates that the induction of the CAM pathway for carbon acquisition in M. crystallinum is under environmental control.

Phosphoenolpyruvate carboxylase genes in C3, crassulacean acid metabolism (CAM) and C3/CAM intermediate species of the genus Clusia: rapid reversible C3/CAM switches are based on the C3 housekeeping gene

The genus Clusia includes species that exhibit either the C3 or crassulacean acid metabolism (CAM) mode of photosynthesis, or those that are able to switch between both modes according to water availability. In order to screen for species-specific genetic variability, we investigated the key carboxylase for CAM, phosphoenolpyruvate carboxylase (PEPC). Sequence analysis of DNA isolated from the obligate CAM species, Clusia hilariana, the obligate C3 species, Clusia multiflora, and an intermediate species that can switch between C3 and CAM photosynthesis, Clusia minor, revealed three different isoforms for C. hilariana and one each for the other two species. Sequence alignments indicated that PEPC from the intermediate species had high homology with the C3 protein and with one of CAM plant proteins. These were assumed to constitute 'housekeeping' proteins, which can also support CAM in intermediate species. The other two isoforms of the CAM plant C. hilariana were either CAM-specific or showed homologies with PEPC from roots. Phylogenetic trees derived from neighbour-joining analysis of amino acid sequences from 13 different Clusia species resulted in two distinct groups of plants with either 'housekeeping' PEPC only, or additionally CAM-related isoforms. Only C. hilariana showed the third, probably root-specific isoform. The high homology of the PEPC from the intermediate species with the C3 protein indicates that for the reversible transition from the C3 to CAM mode of photosynthesis, the C3 type of PEPC is sufficient. Its expression, however, is strongly increased under CAM-inducing conditions. The use of the C3 isoform could have facilitated the evolution of CAM within the genus, which occurred independently for several times.

Photosynthetic flexibility and ecophysiological plasticity: questions and lessons from Clusia, the only CAM tree, in the neotropics

The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research 8 II. Phylogeny 8 III. Photosynthetic physiotypes 10 IV. Metabolic flexibility: organic acid variations 12 V. The environmental control of photosynthetic flexibility 13 VI. Phenotypic plasticity: physiotypes and morphotypes 16 VII. Ecological amplitude and habitat impact 16 VIII. Conclusions and outlook 21 Acknowledgements 22 References 22 Summary It is the aim of this review to present a monographic survey of the neotropical genus Clusia on scaling levels from molecular phylogeny, metabolism, photosynthesis and autecological environmental responses to ecological amplitude and synecological habitat impact. Clusia is the only dicotyledonous genus with real trees performing crassulacean acid metabolism (CAM). By way of introduction, a brief historical reminiscence describes the discovery of CAM in Clusia and the consequent increase in interest in studying this particular genus of tropical shrubs and trees. The molecular phylogeny of CAM in the genus is compared with that in Kalanchoë and the Bromeliaceae. At the level of metabolism and photosynthesis, the great plasticity of expression of photosynthetic physiotypes, i.e. (i) C(3) photosynthesis, (ii) CAM including CAM idling, (iii) CAM cycling and (iv) C(3)/CAM-intermediate behaviour, as well as metabolic flexibility in Clusia is illustrated. At the level of autecology, the factors water, irradiance and temperature, which control photosynthetic flexibility, are assessed. The phenotypic plasticity of physiotypes and morphotypes is described. At the level of synecology, the ecological amplitude of Clusia in the tropics and the relations to habitat are surveyed.