Crassulacean Acid Metabolism Photosynthesis Research Articles

Water exchange between the Chlorenchyma and the Hydrenchyma and its physiological role in leaves with Crassulacean acid metabolism.

Direct and non-destructive measurements of plant-water relations of plants exhibiting the Crassulacean acid metabolism (CAM) photosynthetic pathway are seldom addressed, with most findings inferred from gas exchange measurements. The main focus of this paper was to study how the water exchange between the chlorenchyma and the hydrenchyma depends on and follows the CAM photosynthetic diel pattern using non-invasive and continuous methods. Gas exchange and leaf patch clamp pressure probe (LPCP) measurements were performed on Aloe vera (L.) Burm f., a CAM species, and compared to measurements on banana (Musa acuminata Colla), a C3 species. The LPCP output pressure, Pp , of Aloe vera plants follows its diel CAM photosynthetic cycle, reversed to that observed in banana and other C3 species. The four phases of CAM photosynthesis can also be identified in the diel LPCP output pressure, Pp , cycle. The Pp values in Aloe vera are determined by the hydrenchyma turgor pressure, with both parameters being reversely related. A non-invasive and continuous assessment of the water exchange between the chlorenchyma and the hydrenchyma in CAM plants, namely, by following the changes in the hydrenchyma turgor pressure, is presented. However, showing once more how the LPCP output pressure, Pp , depends on the leaf structure, such an approach can be used to study plant-water relations in other CAM species with a leaf structure similar to Aloe vera, with the hydrenchyma composing most of the leaf volume.

Sensitivity and Regulation of Diel Photosynthesis in Red-Fleshed Pitaya (Hylocereus polyrhizus) Micropropagules under Mannitol-Induced Water Stress/Rehydration Cycle In Vitro

Climate change-induced prolonged water stress (WS) affects crassulacean acid metabolism photosynthesis in pitaya (Hylocereus), limiting crop productivity through insufficient photosynthate. To document how WS/rehydration affects diel photosynthesis, red-fleshed pitaya (H. polyrhizus) micropropagules were studied for 5 weeks in a mannitol-induced water potential gradient replaced with moderate (MWS; −1.0 MPa in week 2; −0.5 MPa for the rest) or intensified (IWS; −1.0 and −1.5 MPa in weeks 2 and 3; −0.5 MPa for the rest) WS in vitro. Net photosynthetic rate (Pn) and integrated net CO2 uptake (INCU) were measured using an Arduino-based photosynthesis system. Micropropagules under MWS had similar Pn in weeks 5 and 1, whereas the control (−0.5 MPa) increased. Pn recovery did not occur after IWS. The average relative INCU was similar in the control and MWS, but lower in IWS. The Pn difference increased with WS, becoming more evident at dawn (Phase II), evening (Phase IV), and predawn the next day (Phase I), and occurred earlier in Phases IV and I under IWS. MWS did not reduce photosynthesis, demonstrating that the photosynthetic regulation could respond to short-term WS in pitaya and indicating the potential of watering for Pn recovery at evening and predawn under IWS.

Engineering Crassulacean Acid Metabolism in C3 and C4 Plants.

Carbon dioxide (CO2) is a major greenhouse gas contributing to changing climatic conditions, which is a grand challenge affecting the security of food, energy, and environment. Photosynthesis plays the central role in plant-based CO2 reduction. Plants performing CAM (crassulacean acid metabolism) photosynthesis have a much higher water use efficiency than those performing C3 or C4 photosynthesis. Therefore, there is a great potential for engineering CAM in C3 or C4 crops to enhance food/biomass production and carbon sequestration on arid, semiarid, abandoned, or marginal lands. Recent progresses in CAM plant genomics and evolution research, along with new advances in plant biotechnology, have provided a solid foundation for bioengineering to convert C3/C4 plants into CAM plants. Here, we first discuss the potential strategies for CAM engineering based on our current understanding of CAM evolution. Then we describe the technical approaches for engineering CAM in C3 and C4 plants, with a focus on an iterative four-step pipeline: (1) designing gene modules, (2) building the gene modules and transforming them into target plants, (3) testing the engineered plants through an integration of molecular biology, biochemistry, metabolism, and physiological approaches, and (4) learning to inform the next round of CAM engineering. Finally, we discuss the challenges and future opportunities for fully realizing the potential of CAM engineering.

Defining Mechanisms of C3 to CAM Photosynthesis Transition toward Enhancing Crop Stress Resilience.

Global climate change and population growth are persistently posing threats to natural resources (e.g., freshwater) and agricultural production. Crassulacean acid metabolism (CAM) evolved from C3 photosynthesis as an adaptive form of photosynthesis in hot and arid regions. It features the nocturnal opening of stomata for CO2 assimilation, diurnal closure of stomata for water conservation, and high water-use efficiency. To cope with global climate challenges, the CAM mechanism has attracted renewed attention. Facultative CAM is a specialized form of CAM that normally employs C3 or C4 photosynthesis but can shift to CAM under stress conditions. It not only serves as a model for studying the molecular mechanisms underlying the CAM evolution, but also provides a plausible solution for creating stress-resilient crops with facultative CAM traits. This review mainly discusses the recent research effort in defining the C3 to CAM transition of facultative CAM plants, and highlights challenges and future directions in this important research area with great application potential.

Genome-wide mapping of DNase I hypersensitive sites in pineapple leaves.

Pineapple [Ananas comosus (L.) Merr.] is the most economically important crop possessing crassulacean acid metabolism (CAM) photosynthesis which has a higher water use efficiency by control of nocturnal opening and diurnal closure of stomata. To provide novel insights into the diel regulatory landscape in pineapple leaves, we performed genome-wide mapping of DNase I hypersensitive sites (DHSs) in pineapple leaves at day (2a.m.) and night (10a.m.) using a simplified DNase-seq method. As a result, totally 33340 and 28753 DHSs were found in green-tip tissue, and 29597 and 40068 were identified in white-base tissue at 2a.m. and 10a.m., respectively. We observed that majority of the pineapple genes occupied less than two DHSs with length shorter than 1kb, and the promotor DHSs showed a proximal trend to the transcription start site (>77% promotor DHSs within 1kb). In addition, more intergenic DHSs were identified around transcription factors or transcription co-regulators (TFs/TCs) than other functional genes, indicating complex regulatory contexts around TFs/TCs. Through combined analysis of tissue preferential DHSs and genes, we respectively found 839 and 888 coordinately changed genes in green-tip at 2a.m. and 10a.m. (AcG2 and AcG10). Furthermore, AcG2-specific, AcG10-specific and common accessible DHSs were dissected from the total photosynthetic preferential DHSs, and the regulatory networks indicated dynamic regulations with multiple cis-regulatory elements occurred to genes preferentially expressed in photosynthetic tissues. Interestingly, binding motifs of several cycling TFs were identified in the DHSs of key CAM genes, revealing a circadian regulation to CAM coordinately diurnal expression. Our results provide a chromatin regulatory landscape in pineapple leaves during the day and night. This will provide important information to assist with deciphering the circadian regulation of CAM photosynthesis.

Diel dynamics of multi-omics in elkhorn fern provide new insights into weak CAM photosynthesis

Crassulacean acid metabolism (CAM) has high water-use efficiency (WUE) and is widely recognized to have evolved from C3 photosynthesis. Different plant lineages have convergently evolved CAM, but the molecular mechanism that underlies C3-to-CAM evolution remains to be clarified. Platycerium bifurcatum (elkhorn fern) provides an opportunity to study the molecular changes underlying the transition from C3 to CAM photosynthesis because both modes of photosynthesis occur in this species, with sporotrophophyll leaves (SLs) and cover leaves (CLs) performing C3 and weak CAM photosynthesis, respectively. Here, we report that the physiological and biochemical attributes of CAM in weak CAM-performing CLs differed from those in strong CAM species. We investigated the diel dynamics of the metabolome, proteome, and transcriptome in these dimorphic leaves within the same genetic background and under identical environmental conditions. We found that multi-omic diel dynamics in P. bifurcatum exhibit both tissue and diel effects. Our analysis revealed temporal rewiring of biochemistry relevant to the energy-producing pathway (TCA cycle), CAM pathway, and stomatal movement in CLs compared with SLs. We also confirmed that PHOSPHOENOLPYRUVATE CARBOXYLASE KINASE (PPCK) exhibits convergence in gene expression among highly divergent CAM lineages. Gene regulatory network analysis identified candidate transcription factors regulating the CAM pathway and stomatal movement. Taken together, our results provide new insights into weak CAM photosynthesis and new avenues for CAM bioengineering.

Mechanisms Underlying the C3–CAM Photosynthetic Shift in Facultative CAM Plants

Crassulacean acid metabolism (CAM), one of three kinds of photosynthesis, is a water-use efficient adaptation to an arid environment. CAM is characterized by CO2 uptake via open stomata during the nighttime and refixation CO2 via the Calvin cycle during the daytime. Facultative CAM plants can shift the photosynthesis from C3 to CAM and exhibit greater plasticity in CAM expression under different environments. Though leaf thickness is an important anatomical feature of CAM plants, there may be no anatomical feature changes during the C3–CAM transition for all facultative CAM plants. The shift from C3 photosynthesis to CAM in facultative CAM plants is accompanied by significant changes in physiology including stomata opening, CO2 gas exchange and organic acid fluxes; the activities of many decarboxylating enzymes increase during the shift from C3 to CAM; the molecular changes occur during the photosynthesis C3–CAM shift involved DNA hypermethylation, transcriptional regulation, post-transcriptional regulation and protein level regulation. Recently, omics approaches were used to discover more proceedings underling the C3–CAM transition. However, there are few reviews on the mechanisms involved in this photosynthetic shift in facultative CAM plants. In this paper, we summarize the progress in the comparative analysis of anatomical, physiological, metabolic and molecular properties of facultative CAM plants between C3 and CAM photosynthesis. Facultative CAM plants also show the potential for sustainable food crop and biomass production. We also discuss the implications of the photosynthesis transition from C3 to CAM on horticultural crops and address future directions for research.

Leaf vein density correlates with crassulacean acid metabolism, but not hydraulic capacitance, in the genus Clusia.

Many succulent species are characterized by the presence of Crassulacean acid metabolism (CAM) and/or elevated bulk hydraulic capacitance (CFT). Both CAM and elevated CFT substantially reduce the rate at which water moves through transpiring leaves. However, little is known about how these physiological adaptations are coordinated with leaf vascular architecture. The genus Clusia contains species spanning the entire C3-CAM continuum, and also is known to have >5-fold interspecific variation in CFT. We used this highly diverse genus to explore how interspecific variation in leaf vein density is coordinated with CAM and CFT. We found that constitutive CAM phenotypes were associated with lower vein length per leaf area (VLA) and vein termini density (VTD), compared to C3 or facultative CAM species. However, when vein densities were standardized by leaf thickness, this value was higher in CAM than C3 species, which is probably an adaptation to overcome apoplastic hydraulic resistance in deep chlorenchyma tissue. In contrast, CFT did not correlate with any xylem anatomical trait measured, suggesting CAM has a greater impact on leaf transpiration rates than CFT. Our findings strongly suggest that CAM photosynthesis is coordinated with leaf vein densities. The link between CAM and vascular anatomy will be important to consider when attempting to bioengineer CAM into C3 crops.

High-quality Cymbidium mannii genome and multifaceted regulation of crassulacean acid metabolism in epiphytes

Epiphytes with crassulacean acid metabolism (CAM) photosynthesis are widespread among vascular plants, and repeated evolution of CAM photosynthesis is a key innovation for micro-ecosystem adaptation. However, we lack a complete understanding of the molecular regulation of CAM photosynthesis in epiphytes. Here, we report a high-quality chromosome-level genome assembly of a CAM epiphyte, Cymbidium mannii (Orchidaceae). The 2.88-Gb orchid genome with a contig N50 of 22.7 Mb and 27 192 annotated genes was organized into 20 pseudochromosomes, 82.8% of which consisted of repetitive elements. Recent expansions of long terminal repeat retrotransposon families have made a major contribution to the evolution of genome size in Cymbidium orchids. We reveal a holistic scenario of molecular regulation of metabolic physiology using high-resolution transcriptomics, proteomics, and metabolomics data collected across a CAM diel cycle. Patterns of rhythmically oscillating metabolites, especially CAM-related products, reveal circadian rhythmicity in metabolite accumulation in epiphytes. Genome-wide analysis of transcript and protein level regulation revealed phase shifts during the multifaceted regulation of circadian metabolism. Notably, we observed diurnal expression of several core CAM genes (especially βCA and PPC) that may be involved in temporal fixation of carbon sources. Our study provides a valuable resource for investigating post-transcription and translation scenarios in C. mannii, an Orchidaceae model for understanding the evolution of innovative traits in epiphytes.

Carbon and nitrogen metabolic regulation in freshwater plant Ottelia alismoides in response to carbon limitation: A metabolite perspective.

Carbon and nitrogen metabolism are basic, but pivotal metabolic pathways in plants and are tightly coupled. Maintaining the balance of carbon and nitrogen metabolism is critical for plant survival. Comprehensively revealing the metabolic balance of carbon–nitrogen interactions is important and helpful for understanding the adaptation of freshwater plants to CO2 limited aqueous environment. A comprehensive metabolomics analysis combined with physiological measurement was performed in the freshwater plant Ottelia alismoides acclimated to high and low CO2, respectively, for a better understanding of how the carbon and nitrogen metabolic adjustment in freshwater plants respond to carbon limitation. The present results showed that low CO2 acclimated O. alismoides exhibited significant diurnal titratable acidity and malate fluctuations, as well as an opposite diel pattern of starch change and high enzymatic activities required for crassulacean acid metabolism (CAM) photosynthesis, which indicates that CAM was induced under low CO2. Moreover, the metabolomic analysis showed that most intermediates of glycolysis, pentose phosphate pathway (PPP) and tricarboxylic acid (TCA) cycle, were increased under low CO2, indicative of active respiration in low-CO2-treated O. alismoides. Meanwhile, the majority of amino acids involved in pathways of glutamate and arginine metabolism, aspartate metabolism, and the branched-chain amino acids (BCAAs) metabolism were significantly increased under low CO2. Notably, γ-aminobutyric acid (GABA) level was significantly higher in low CO2 conditions, indicating a typical response with GABA shunt compensated for energy deprivation at low CO2. Taken together, we conclude that in low-CO2-stressed O. alismoides, CAM photosynthesis was induced, leading to higher carbon and nitrogen as well as energy requirements. Correspondingly, the respiration was greatly fueled via numerous starch degradation to ensure CO2 fixation in dark, while accompanied by linked promoted N metabolism, presumably to produce energy and alternative carbon sources and nitrogenous substances for supporting the operation of CAM and enhancing tolerance for carbon limitation. This study not only helps to elucidate the regulating interaction between C and N metabolism to adapt to different CO2 but also provides novel insights into the effects of CO2 variation on the metabolic profiling of O. alismoides.

Comparative genomics analysis of drought response between obligate CAM and C3 photosynthesis plants

Crassulacean acid metabolism (CAM) plants exhibit elevated drought and heat tolerance compared to C3 and C4 plants through an inverted pattern of day/night stomatal closure and opening for CO2 assimilation. However, the molecular responses to water-deficit conditions remain unclear in obligate CAM species. In this study, we presented genome-wide transcription sequencing analysis using leaf samples of an obligate CAM species Kalanchoë fedtschenkoi under moderate and severe drought treatments at two-time points of dawn (2-h before the start of light period) and dusk (2-h before the dark period). Differentially expressed genes were identified in response to environmental drought stress and a whole genome wide co-expression network was created as well. We found that the expression of CAM-related genes was not regulated by drought stimuli in K. fedtschenkoi. Our comparative analysis revealed that CAM species (K. fedtschenkoi) and C3 species (Arabidopsis thaliana, Populus deltoides ‘WV94’) share some common transcriptional changes in genes involved in multiple biological processes in response to drought stress, including ABA signaling and biosynthesis of secondary metabolites.

Updating the occurrence of crassulacean acid metabolism (CAM) in the genus Clusia through carbon isotope analysis of species from Colombia

Clusia is a widely distributed neotropical genus with 321 currently described species. This remarkable genus is the only one known to contain trees sensu stricto with CAM photosynthesis. To survey the occurrence of CAM in Clusia species from Colombia, we determined the leaf stable carbon isotope composition (δ13C) of 568 specimens from 114 species deposited in 12 Colombian herbaria. In the vast majority of specimens, δ13C values indicated that C3 photosynthesis was the principal contributor to carbon gain. δ13C values typical of strong CAM (less negative than -20‰) were observed in only five species, in four of them for the first time. All samples with CAM-type isotopic signatures were collected below 1,000 m a.s.l., whereas species with predominantly C3 occurred from sea level to 3,500 m a.s.l. Together with information already available in the literature, we conclude that CAM is present in 22% (35/156) of the species of Clusia investigated thus far.

Functional relationship between leaf/stem pseudobulb size and photosynthetic pathway in the Orchidaceae

Water storage has been commonly cited as an important function of orchid pseudobulbs, and it is reasonable to assume that orchids which utilize crassulacean acid metabolism (CAM) photosynthesis have larger pseudobulbs than those of C3 orchid taxa, because these foliar (or stem) structures may reflect another type of succulent tissue in CAM plants. On the other hand, it is equally plausible that C3 orchids have larger pseudobulbs, as they lack succulent tissue, as well as the water-conservative CAM pathway. The goal of this study was to compare pseudobulb size in over 100 living orchid species growing at the Missouri Botanical Garden by measurement. Pseudobulb volumes of C3 species did not differ from those of species with CAM photosynthesis in a family-wide comparison of all sampled species, as well as in comparisons of taxa with these two photosynthetic pathways among three subtribes and within one genus. The results did not support a functional relationship between pseudobulb volume and photosynthetic pathway in the Orchidaceae. Pseudobulbs are equally important structures in C3 and CAM orchid taxa, and may function similarly in water, carbohydrates, and (or) essential elements storage in the two groups of orchids. This study lays a foundation for further research into pseudobulb evolution in orchids.

Salinity and Salt-Priming Impact on Growth, Photosynthetic Performance, and Nutritional Quality of Edible Mesembryanthemum crystallinum L.

Mesembryanthemum crystallinum L. is a nutritious edible facultative halophyte. This study aimed to investigate the physiology and quality of M. crystallinum L. grown under different salinities and salt-priming conditions. All plants were first grown in 10% artificial seawater (ASW) for 10 days. After that, some plants remained in 10% ASW while the others were transferred to 20%, 30%, 40%, or 50% ASW for another 10 days. Some plants also underwent a salt priming by transferring them gradually from 10% to 100% ASW over a span of 10 days (defined as salt primed). All plants were green and healthy. However, there were reductions in shoot and root productivity, leaf growth, and water content, but also an increase in leaf succulence after transferring plants to higher salinities. The salt-primed plants showed higher photosynthetic light use efficiency with higher chlorophyll concentration compared to other plants. The concentrations of proline, ascorbic acid (ASC), and total phenolic compounds (TPC) increased as percentages of ASW increased. The salt-primed plants switched from C3 to crassulacean acid metabolism photosynthesis and accumulated the greatest amounts of proline, ASC, and TPC. In conclusion, higher salinities and salt priming enhance the nutritional quality of M. crystallinum L. but compromises productivity.

Transcriptome profiling reveals that foliar water uptake occurs with C3 and crassulacean acid metabolism facultative photosynthesis in Tamarix ramosissima under extreme drought.

Tamarix ramosissima is a typical desert plant species that is widely distributed in the desert areas of Northwest China. It plays a significant role in sand fixation and soil water conservation. In particular, how it uses water to survive in the desert plays an important role in plant growth and ecosystem function. Previous studies have revealed that T. ramosissima can alleviate drought by absorbing water from its leaves under extreme drought conditions. To date, there is no clear molecular regulation mechanism to explain foliar water uptake (FWU). In the present study, we correlated diurnal meteorological data, sap flow and photosynthetic parameters to determine the physical and biological characteristics of FWU. Our results suggested that the lesser the groundwater, the easier it is for T. ramosissima to absorb water via the leaves. Gene ontology annotation and Kyoto Encyclopaedia of Genes and Genomes pathway analysis of the transcriptome profile of plants subjected to high humidity suggested that FWU was highly correlated to carbohydrate metabolism, energy transfer, pyruvate metabolism, hormone signal transduction and plant–pathogen interaction. Interestingly, as a C3 plant, genes such as PEPC, PPDK, MDH and RuBP, which are involved in crassulacean acid metabolism (CAM) photosynthesis, were highly upregulated and accompanied by FWU. Therefore, we proposed that in the case of sufficient water supply, C3 photosynthesis is used in T. ramosissima, whereas in cases of extreme drought, starch is degraded to provide CO2 for CAM photosynthesis to make full use of the water obtained via FWU and the water that was transported or stored to assimilating branches and stems. This study may provide not only an important theoretical foundation for FWU and conversion from C3 plants to CAM plants but also for engineering improved photosynthesis in high-yield drought-tolerant plants and mitigation of climate change-driven drought.

Evolutionary history of CAM photosynthesis in NeotropicalClusia: insights from genomics, anatomy, physiology and climate

AbstractClusia is a remarkable genus of Neotropical woody plants as its members engage in either C3 photosynthesis or employ, to varying degrees, crassulacean acid metabolism (CAM) photosynthesis. Information about the evolutionary history of CAM in Clusia is scarce. Restriction site-associated sequencing of 64 species (20% of the genus) provided strong support for most of the previously recognized nine lineages. Ancestral reconstruction using maximum parsimony or maximum likelihood under a one-rate model suggested that CAM evolved at least four times independently from a most recent common ancestor (MRCA) with C3, whereas a maximum likelihood two-rate model suggested that CAM was already present in the MRCA followed by reversions to C3 in several lineages. Phylogenetic generalized least square analysis assessed variation in seven leaf anatomical traits and CAM activity measured as δ 13C. Results indicate that CAM is highly correlated with palisade mesophyll layer thickness and cell size. In addition, correlation between 19 bioclimatic variables and δ 13C was evaluated. It was found that CAM is positively correlated with habitats with a more severe dry season and greater precipitation seasonality. Since CAM is weakly and/or only periodically expressed in many Clusia spp., and thus not readily reflected in δ 13C, future analysis of phylogenetically-informed CAM expression in Clusia must include physiological measurements such as CO2 exchange and/or diel changes in leaf acidity for each species under investigation.

CAM photosynthesis: the acid test.

SummaryThere 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.

Underwater CAM photosynthesis elucidated by Isoetes genome

To conserve water in arid environments, numerous plant lineages have independently evolved Crassulacean Acid Metabolism (CAM). Interestingly, Isoetes, an aquatic lycophyte, can also perform CAM as an adaptation to low CO2 availability underwater. However, little is known about the evolution of CAM in aquatic plants and the lack of genomic data has hindered comparison between aquatic and terrestrial CAM. Here, we investigate underwater CAM in Isoetes taiwanensis by generating a high-quality genome assembly and RNA-seq time course. Despite broad similarities between CAM in Isoetes and terrestrial angiosperms, we identify several key differences. Notably, Isoetes may have recruited the lesser-known ‘bacterial-type’ PEPC, along with the ‘plant-type’ exclusively used in other CAM and C4 plants for carboxylation of PEP. Furthermore, we find that circadian control of key CAM pathway genes has diverged considerably in Isoetes relative to flowering plants. This suggests the existence of more evolutionary paths to CAM than previously recognized.

At the Edges of Photosynthetic Metabolic Plasticity-On the Rapidity and Extent of Changes Accompanying Salinity Stress-Induced CAM Photosynthesis Withdrawal.

The common ice plant (Mesembryanthemum crystallinum L.) is a facultative crassulacean acid metabolism (CAM) plant, and its ability to recover from stress-induced CAM has been confirmed. We analysed the photosynthetic metabolism of this plant during the 72-h response period following salinity stress removal from three perspectives. In plants under salinity stress (CAM) we found a decline of the quantum efficiencies of PSII (Y(II)) and PSI (Y(I)) by 17% and 15%, respectively, and an increase in nonphotochemical quenching (NPQ) by almost 25% in comparison to untreated control. However, 48 h after salinity stress removal, the PSII and PSI efficiencies, specifically Y(II) and Y(I), elevated nonphotochemical quenching (NPQ) and donor side limitation of PSI (YND), were restored to the level observed in control (C3 plants). Swelling of the thylakoid membranes, as well as changes in starch grain quantity and size, have been found to be components of the salinity stress response in CAM plants. Salinity stress induced an over 3-fold increase in average starch area and over 50% decline of average seed number in comparison to untreated control. However, in plants withdrawn from salinity stress, during the first 24 h of recovery, we observed chloroplast ultrastructures closely resembling those found in intact (control) ice plants. Rapid changes in photosystem functionality and chloroplast ultrastructure were accompanied by the induction of the expression (within 24 h) of structural genes related to the PSI and PSII reaction centres, including PSAA, PSAB, PSBA (D1), PSBD (D2) and cp43. Our findings describe one of the most flexible photosynthetic metabolic pathways among facultative CAM plants and reveal the extent of the plasticity of the photosynthetic metabolism and related structures in the common ice plant.

Plastid RNA editing reduction accompanied with genetic variations in Cymbidium, a genus with diverse lifestyle modes

Recent sequencing efforts have broadly uncovered the evolutionary trajectory of plastid genomes (plastomes) of flowering plants in diverse habitats, yet our knowledge of the evolution of plastid posttranscriptional modifications is limited. In this study, we generated 11 complete plastomes and performed ultra-deep transcriptome sequencing to investigate the co-evolution of plastid RNA editing and genetic variation in Cymbidium, a genus with diverse trophic lifestyles. Genome size and gene content is reduced in terrestrial and green mycoheterotrophic orchids relative to their epiphytic relatives. This could be partly due to extensive losses and pseudogenization of ndh genes for the plastid NADH dehydrogenase-like complex, but independent pseudogenization of ndh genes has also occurred in the epiphyte C. mannii, which was reported to use strong crassulacean acid metabolism photosynthesis. RNA editing sites are abundant but variable in number among Cymbidium plastomes. The nearly twofold variation in editing abundance is mainly due to extensive reduction of ancestral editing sites in ndh transcripts of terrestrial, mycoheterotrophic, and C. mannii plastomes. The co-occurrence of editing reduction and pseudogenization in ndh genes suggests functional constraints on editing machinery may be relaxed, leading to nonrandom loss of ancestral edited sites via reduced editing efficiency. This study represents the first systematic examination of RNA editing evolution linked to plastid genome variation in a single genus. We also propose an explanation for how genomic and posttranscriptional variations might be affected by lifestyle-associated ecological adaptation strategies in Cymbidium.