Photosynthetic characteristics of chloroplasts isolated fromMesembryanthemum crystallinum L., a halophilic plant capable of Crassulacean acid metabolism

Shifting photosynthesis between the fast and slow lane: Facultative CAM and water-deficit stress

Five decades ago, the first report of a shift from C3 to CAM (crassulacean acid metabolism) photosynthesis following the imposition of stress was published in this journal. The annual, Mesembryanthemum crystallinum (Aizoaceae), was shown to be a C3 plant when grown under non-saline conditions, and a CAM plant when exposed to high soil salinity. This observation of environmentally triggered CAM eventually led to the introduction of the term facultative CAM, which categorises CAM that is induced or upregulated in response to water-deficit stress and is lost or downregulated when the stress is removed. Reversibility of C3-to-CAM shifts distinguishes stress-driven facultative-CAM responses from purely ontogenetic increases of CAM activity. We briefly review how the understanding of facultative CAM has developed, evaluate the current state of knowledge, and highlight questions of continuing interest. We demonstrate that the long-lived leaves of a perennial facultative-CAM arborescent species, Clusia pratensis, can repeatedly switch between C3 and CAM in response to multiple wet-dry-wet cycles. Undoubtedly, this is a dedicated response to environment, independent of ontogeny. We highlight the potential for engineering facultative CAM into C3 crops to provide a flexible capacity for drought tolerance.

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.

The Investigation of the Impact of Toxicity of Metals on Oxygen-Evolving Complex in Spinacia oleracea

The current article reported the investigation of metal toxicity on the oxygen-evolving complex (OEC) in Spinacia oleracea related to depletion in chloride ion concentration, an essential part of the photosystem (II). The greenhouse experiment was conducted where S. oleracea was cultivated in three replicates with control plants (plants “a”) treated with tap water. Moreover, 30 ppm of Cu²⁺ ion solution and Pb²⁺ ion solution was used to irrigate the rest of the plants, labeled as plants “b” and “c”, respectively, on alternative days. Advanced technologies such as Atomic Absorption Spectrophotometry (AAS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and UV-visible Spectrophotometry were used to monitor the essential nutrients in leaves to validate the function of the photosystem (I and II). Reduced Cl⁻ ions contents showed that both metals (Cu²⁺ and Pb²⁺) altered the essential elements of the oxygen-evolving complex (OEC) of photosystem (II), required to maintain the coordination structure of the Mn₄CaO₅ cluster. SEM analysis revealed the modified leaf structure of the S. oleracea under Cu²⁺ and Pb²⁺ accumulation due to which distorted cellular structure, reduced surface area, and the (shattered) stomatal opening compared to the plants “a” were observed. The EDS analysis of plants “b” and “c” showed high oxygen contents followed by reduced chloride contents over plants “a”, reflecting the infirmity of OEC to push out oxygen, which leads to generating oxidative stress. The lower pigment concentration in leaves of metal-contaminated plants “b” and “c” impacts carbon assimilation, which is linked to the reduced stomatal opening and influences the gaseous exchange rates. Additionally, increased contents of K⁺ and Ca²⁺ may be due to self-defense mechanisms under low chloride contents to speed up oxygen evolution to protect plants against oxidative stress. It was concluded that Cu²⁺ and Pb²⁺ metal toxicity influences essential Cl⁻ and K⁺ contents, which modify the photosystem II system; subsequently, a reduced growth rate was observed.

Crassulacean Acid Metabolism Induction in Mesembryanthemum crystallinum Can Be Estimated by Non-Photochemical Quenching upon Actinic Illumination During the Dark Period

Mesembryanthemum crystallinum, which switches the mode of photosynthesis from C3 to crassulacean acid metabolism (CAM) upon high salt stress, was shown here to exhibit diurnal changes in not only the CO2 fixation pathway but also Chl fluorescence parameters under CAM-induced conditions. We conducted comprehensive time course measurements of M. crystallinum leaf Chl fluorescence using the same leaf throughout the CAM induction period. By doing so, we were able to distinguish the effect of CAM induction from that of photoinhibition and avoid the possible effects of differences in foliar age. We found that the diurnal change in the status of electron transfer could be ascribed to the formation of a proton gradient across thylakoid membranes presumably resulting from diurnal changes in the ATP/ADP ratio reported earlier. The electron transport by actinic illumination thus became limited at the step of plastoquinol oxidation by the Cyt b6/f complex in the 'night' period upon CAM induction, resulting in high levels of non-photochemical quenching. The actinically induced non-photochemical quenching in the 'night' period correlated well with the degree of CAM induction. Chl fluorescence parameters, such as NPQ or qN, could be used as a simple indexing system for the CAM induction.

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.

Structural-functional role of chloride in photosystem II

Chloride binding in photosystem II (PSII) is essential for photosynthetic water oxidation. However, the functional roles of chloride and possible binding sites, during oxygen evolution, remain controversial. This paper examines the functions of chloride based on its binding site revealed in the X-ray crystal structure of PSII at 1.9 Å resolution. We find that chloride depletion induces formation of a salt bridge between D2-K317 and D1-D61 that could suppress the transfer of protons to the lumen.

Na+/H+ antiporters are differentially regulated in response to NaCl stress in leaves and roots of Mesembryanthemum crystallinum

Salinity tolerance in plants involves controlled Na(+) transport at the site of Na(+) accumulation and intracellular Na(+) compartmentation. The focus of this study was the identification and analysis of the expression of Na(+)/H(+) antiporters in response to NaCl stress in one particular plant, the facultative halophyte Mesembryanthemum crystallinum Na(+)/H(+) antiporters of M. crystallinum were cloned by RACE-PCR from total mRNA of leaf mesophyll cells. Functional complementation of Saccharomyces cerevisiae and Escherichia coli mutants was performed. The kinetics of changes in the expression of antiporters were quantified by real-time PCR in leaves and roots. Five Na(+)/H(+) antiporters (McSOS1, McNhaD, McNHX1, McNHX2 and McNHX3) were cloned, representing the entire set of these transporters in M. crystallinum. The functionality of McSOS1, McHX1 and McNhaD was demonstrated in complementation experiments. Quantitative analysis revealed a temporal correlation between salt accumulation and expression levels of genes in leaves, but not in roots, which was most pronounced for McNhaD. Results suggest a physiological role of McSOS1, McNhaD and McNHX1 in Na(+) compartmentation during plant adaptation to high salinity. The study also provides evidence for salt-induced expression and function of the Na(+)/H(+) antiporter McNhaD in chloroplasts and demonstrates that the chloroplast is one of the compartments involved in the response of cells to salt stress.

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.

Intracellular transport and pathways of carbon flow in plants with crassulacean acid metabolism

The massive daily reciprocal transfer of carbon between acids and carbohydrates that is unique to crassulacean acid metabolism (CAM) involves extensive and regulated transport of metabolites between chloroplasts, vacuoles, the cytosol and mitochondria. In this review of the CAM pathways of carbon flow and intracellular transport, we highlight what is known and what has been postulated. For three of the four CAM pathway variants currently known (malic enzyme- or PEP carboxykinase-type decarboxylase, and starch- or soluble sugar-type carbohydrate storage), the mechanisms of intracellular transport are still hypothetical and have yet to be demonstrated experimentally. Even in malic enzyme starch-storing species such as Kalanchoë daigremontiana Hamet et Perr. and Mesembryanthemum crystallinum L., the best-described variants of plants with the second-most common mode of photosynthetic carbon metabolism known, no tonoplast or mitochondrial transporter has been functionally described at a molecular level.

Phosphoribulokinase from ice plant: Transcription, transcripts and protein expression during environmental stress

The expression of PRK (phosphoribulokinase, E.C.2.7.1.19) in ice plant (Mesembryanthemum crystallinum) during development and under environmental stress was studied. cDNA clones were isolated and full-length cDNAs were characterized. Ice plant PRK is contained in a 1520 nucleotide transcript including a 126 nucleotide leader sequence, a 175 nucleotide 3'-end and a 20-30 nucleotide polyA(+)-stretch. The coding region, 397 codons, specifies a protein of Mr 44 064. The mature sequence is preceded by a transit peptide of approximately 46 amino acids. The mature portion of ice plant PRK is 86.4% identical to that of spinach and, e.g., 16.2% identical to PRK from Xanthomonas flavus. Under salt stress or cold adaptation conditions, the amount of mRNA declined by a factor of approximately three within days, followed by an increase to approximately pre-stress levels. The fluctuation in mRNA amount is not reflected on the level of transcription of the gene, suggesting post-transcriptional control, nor is PRK protein amount affected significantly over the short stress period. The recovery of transcript levels for photosynthesis-related proteins after stress appears to be a general response to environmental stresses that affect water status in ice plant. We suggest that the photosynthetic machinery in this facultative halophyte is effectively buffered from damage caused by such environmental stress.

The photosynthetic apparatus of C3 and CAM-induced Mesembryanthemum crystallinum L

Accompanying the CAM induction of Mesembryanthemum crystallinum L. grown in high salinity there are changes in the enzymes of carbon metabolism. However, there are no changes in the electron transport activities, Chla/b ratios or in the distribution of chlorophyll amongst the various pigment-protein complexes of isolated thylakoids. Hence with CAM induction there are no changes in the photochemical apparatus of M. crystallinum thylakoids.Despite comparable amounts of chlorophylla/b-proteins of photosystem II to those found in typical C3 sun plants, both the C3 and CAM M. crystallinum chloroplasts have relatively more photosystem II, and, concommitantly, less photosystem I complex. This is consistent with greater fluorescence emission at 685 and 695 nm, and lower emission at 735 nm (measured at 77 K) than typically found for C3 plants, whether sun or shade species.Photoinhibition of isolated C3 and CAM thylakoids by white light led to comparable decreases in electron transport capacities and fluorescence emission at 77 K with photosystem II being more affected than PSI. We suggest however, that the presence of more core PSII complexes relative to PSI complexes in this CAM-inducible plant, may provide an additional strategy to mitigate photoinhibition in the short-term.

Sodium, potassium, chloride and proline concentrations of chloroplasts isolated from a halophyte, Mesembryanthemum crystallinum L

Concentrations of four major solutes (Na(+), K(+), Cl(-), proline) were determined in isolated, intact chloroplasts from the halophyte Mesembryanthemum crystallinum L. following long-term exposure of plants to three levels of NaCl salinity in the rooting medium. Chloroplasts were obtained by gentle rupture of leaf protoplasts. There was either no or only small leakage of inorganic ions from the chloroplasts to the medium during three rapidly performed washing steps involving precipitation and re-suspension of chloroplast pellets. Increasing NaCl salinity of the rooting medium resulted in a rise of Na(+) und Cl(-) in the total leaf sap, up to approximately 500 and 400 mM, respectively, for plants grown at 400 mM NaCl. However, chloroplast levels of Na(+) und Cl(-) did not exceed 160-230 and 40-60 mM, respectively, based upon a chloroplast osmotic volume of 20-30 μl per mg chlorophyll. At 20 mM NaCl in the rooting medium, the Na(+)/K(+) ratio of the chloroplasts was about 1; at 400 mM NaCl the ratio was about 5. Growth at 400 mM NaCl led to markedly increased concentrations of proline in the leaf sap (8 mM) compared with the leaf sap of plants grown in culture solution without added NaCl (proline 0.25 mM). Although proline was fivefold more concentrated in the chloroplasts than in the total leaf sap of plants treated with 400 mM NaCl, the overall contribution of proline to the osmotic adjustment of chloroplasts was small. The capacity to limit chloroplast Cl(-) concentrations under conditions of high external salinity was in contrast to an apparent affinity of chloroplasts for Cl(-) under conditions of low Cl(-) availability.