[CO2-fixation metabolism in the halophytic species Mesembryanthemum crystallinum grown under different environmental conditions]

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.

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.

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.

The effects of salinity, crassulacean acid metabolism and plant age on the carbon isotope composition of Mesembryanthemum crystallinum L., a halophytic C(3)-CAM species

The carbon isotope composition of the halophyte Mesembryanthemum crystallinum L. (Aizoaceae) changes when plants are exposed to environmental stress and when they shift from C(3) to crassulacean acid metabolism (CAM). We examined the coupling between carbon isotope composition and photosynthetic pathway by subjecting plants of different ages to salinity and humidity treatments. Whole shoot delta(13)C values became less negative in plants that were exposed to 400 mM NaCl in the hydroponic solution. The isotopic change had two components: a direct NaCl effect that was greatest in plants still operating in the C(3) mode and decreased proportionally with increasing levels of dark fixation, and a second component related to the degree of CAM expression. Ignoring the presumably diffusion-related NaCl effect on carbon isotope ratios results in an overestimation of nocturnal CO(2) gain in comparison to an isotope versus nocturnal CO(2) gain calibration established previously for C(3) and CAM species grown under well-watered conditions. It is widely taken for granted that the shift to CAM in M. crystallinum is partially under developmental control and that CAM is inevitably expressed in mature plants. Plants, cultivated under non-saline conditions and high relative humidity (RH) for up to 63 days, maintained diel CO(2) gas-exchange patterns and delta(13)C values typical of C(3) plants. However, a weak CAM gas-exchange pattern and an increase in delta(13)C value were observed in non-salt-treated plants grown at reduced RH. These observations are consistent with environmental control rather than developmental control of the induction of CAM in mature M. crystallinum under non-saline conditions.

Distinctive diel growth cycles in leaves and cladodes of CAM plants: differences from C3 plants and putative interactions with substrate availability, turgor and cytoplasmic pH

Distinct diel rhythms of leaf and cladode expansion growth were obtained in crassulacean acid metabolism (CAM) plants under water-limited conditions, with maxima at mid-day during phase III of CO₂ assimilation. This pattern coincided with the availability of CO₂ for photosynthesis and growth during the decarboxylation of malic acid, with maximum cell turgor due to the nocturnally accumulated malic acid, and with the period of low cytoplasmic pH associated with malic acid movement from vacuole to cytosol. Maximum growth rates were generally only 20% of those in C₃ plants and were reached at a different time of the day compared with C₃ plants. The results suggest that malic acid, as a source of carbohydrates, and a determinant of turgor and cytoplasmic pH, plays a major role in the control of diel growth dynamics in CAM plants under desert conditions. The observed plasticity in phasing of growth rhythms under situations of differing water availability suggests that a complex network of factors controls the diel growth patterns in CAM plants and needs to be investigated further.

How closely do the delta(13)C values of Crassulacean Acid metabolism plants reflect the proportion of CO(2) fixed during day and night?

The extent to which Crassulacean acid metabolism (CAM) plant delta(13)C values provide an index of the proportions of CO(2) fixed during daytime and nighttime was assessed. Shoots of seven CAM species (Aloe vera, Hylocereus monocanthus, Kalanchoe beharensis, Kalanchoe daigremontiana, Kalanchoe pinnata, Vanilla pauciflora, and Xerosicyos danguyi) and two C(3) species (teak [Tectona grandis] and Clusia sp.) were grown in a cuvette, and net CO(2) exchange was monitored for up to 51 d. In species exhibiting net dark CO(2) fixation, between 14% and 73.3% of the carbon gain occurred in the dark. delta(13)C values of tissues formed inside the cuvette ranged between -28.7 per thousand and -11.6 per thousand, and correlated linearly with the percentages of carbon gained in the light and in the dark. The delta(13)C values for new biomass obtained solely during the dark and light were estimated as -8.7 per thousand and -26.9 per thousand, respectively. For each 10% contribution of dark CO(2) fixation integrated over the entire experiment, the delta(13)C content of the tissue was, thus, approximately 1.8 per thousand less negative. Extrapolation of the observations to plants previously surveyed under natural conditions suggests that the most commonly expressed version of CAM in the field, "the typical CAM plant," involves plants that gain about 71% to 77% of their carbon by dark fixation, and that the isotopic signals of plants that obtain one-third or less of their carbon in the dark may be confused with C(3) plants when identified on the basis of carbon isotope content alone.

Influence of leaf water content on the C3 -CAM transition in Mesembryanthemum crystallinum

Changes in leaf water content, night-time accumulation of malic (δ-malate) and citric acid (δ-citrate) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activity were followed for 60 d after germination in well watered and salt-stressed plants of the facultatively halophytic ephemeral Mesembryanthemum crystallinum L. To separate the effects of development, salt stress and water deficit on crassulacean acid metabolism (CAM) induction plants were stressed initially 10 d after germination and then successively at 1-wk intervals (five sets). Related to dry mass or organic matter (i.e. dry mass corrected for the mass of inorganic ions) water content started to decrease during the late embryonal phase of the life cycle. Water content on a dry mass basis was always lower in salt-stressed than in well watered individuals. However, on an organic matter basis no difference was detectable. This indicated that salt treatment did not reduce leaf water content but falsified the basis (dry mass). Increases in leaf succulence and in pressure potential prevented long-term water deficit in well watered and in salt-stressed plants. Instead, these changes displayed enhanced vacuolisation, which is an essential prerequisite for the development of CAM. The end of that differentiation process might allow the initiation of nocturnal malic acid accumulation in a threshold response. At the onset of each salt treatment, short-term water deficits occurred due to an incomplete osmotic adaptation independent of plant age. As δ-malate only appeared when plants were c. 35 d old this water deficit was unlikely to be a decisive CAM-inducing factor. About 2 wk after germination water content began to decline during the light periods in plants of all treatments. This pattern disappeared again when CAM had been fully established. Daytime transpirational water loss is therefore unlikely to be the decisive factor because it failed to induce the metabolic shift in young plants. Environmental stress (e.g. salt or drought) can therefore only induce δ-malate when leaf and plant differentiation has reached a certain stage.

The role of crassulacean acid metabolism (CAM) in the adaptation of plants to salinity

Two case studies are presented illustrating how the behaviour of plants using crassulacean acid metabolism (CAM) provides adaptation to salinity. Perennial cacti having constitutive CAM show adaptation at the whole-plant level, engaging regulation of stomata, internal CO₂ -recycling and root physiology with salt exclusion. They are stress avoiders. Annual plants such as Mesembryanthemum crystallinum, with inducible CAM, are salt includers. They are stress-tolerant and show reactions at an array of levels: (i) regulation of turgor and gas exchange at the whole-plant level; (ii) metabolic adjustments at the cellular level; (iii) adapptive transport proteins at the membrane level and also (iv) at the macromolecular level; and (v) inductive changes at the gene expression level of the enzyme complement for metabolism (in particular involving glycolysis and malic-acid synthesis with phosphoenolpyruvate carboxylase (PEPC) as the key enzyme, and gluconeogenesis (with pyruvate-phosphate dikinase (PPDK) as a key enzyme) and membrane transport (in particular involving the tonoplast ATPase).

Nuclear-magnetic-resonance imaging of leaves ofMesembryanthemum crystallinum L. plants grown at high salinity

Differences in water binding were measured in the leaf cells ofMesembryanthemum crystallinum L. plants grown under high-salinity conditions by using nuclear-magnetic-resonance (NMR) imaging. The 7-Tesla proton NMR imaging system yielded a spatial resolution of 20·20·100 μm(3). Images recorded with different spin-echo times (4.4 ms to 18 ms) showed that the water concentrations in the bladder cells (located on the upper and lower leaf surface), in the mesophyll cells and in the water-conducting vessels were nearly identical. All of the water in the bladder cells and in the water-conducting vessels was found to be mobile, whilst part of the water in the mesophyll cells was bound. Patches of mesophyll cells could be identified which bound water more strongly than the surrounding mesophyll cells. Optical investigations of leaf cross-sections revealed two types of mesophyll cells of different sizes and chloroplast contents. It is therefore likely that in the small-sized mesophyll cells water is strongly bound. A long-term asymmetric water exchange between the mesophyll cells and the bladder cells during Crassulacean acid metabolism has been described in the literature. The high density of these mesophyll cells in the lower epidermis is a possible cause of this asymmetry.

Characteristics of MgATP(2-)-dependent electrogenic proton transport in tonoplast vesicles of the facultative crassulacean-acid-metabolism plant Mesembryanthemum crystallinum L

Membrane vesicles were isolated from mesophyll cells of Mesembryanthemum crystallinum in the C3 state and in the crassulacean acid metabolism (CAM) state. The distribution of ATP-hydrolysis and H(+)-transport activities, and the activities of hydroxypyruvate reductase and Antimycin-insensitive cytochrome-c-reductase on continuous sucrose gradients was studied. For isolations carried out routinely a discontinuous sucrose gradient (24%/37%/50%) was used. Nitrate-sensitive ATP-hydrolysis and H(+)-transport activities increased several-fold during the transition from C3 photosynthesis to CAM. Nitrate-sensitive ATPase showed a substrate preference for ATP with an apparent Km (MgATP(2-)) of 0.19-0.37 mM. In both C3 and CAM states the ATPase showed a concentration-dependent stimulation by the anions chloride and malate. However, the pH optima of the two states were different: the ATPase of C3- M. crystallinum had an optimum of pH 7.4 and that of CAM-M. crystallinum an optimum of pH 8.4. The optical probe oxonol-VI was used to demonstrate the formation of MgATP(2-)-dependent electric-potential gradients in tonoplast vesicles.

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.

The influence of inorganic phosphate on photosynthesis in intact chloroplasts from Mesembryanthemum crystallinum L. plants exhibiting C3 photosynthesis or crassulacean acid metabolism

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C3 or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O2 evolution and CO2 fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75-85% of the (14)CO2 fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of (14)CO2 fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.

CAM-idling in Hoya carnosa (Asclepiadaceae)

In the leaf succulent Asclepiad Hoya carnosa (L.) R. Br., CAM photosynthesis occurred under well-watered conditions, as characterized by diurnal gas exchange and changes in titratable acidity. Following 10-12 days of severe water stress, the plants shifted from CAM to a modified CAM-idling mode of metabolism. CAM-idling was characterized by complete or almost complete stomatal closure accompanied by CAM-like diurnal changes in titratable acidity. H. carnosa plants maintained this CAM-idling mode of photosynthesis for at least 8 weeks. Upon reirrigation, the plants returned to the original CAM mode within 1 week. These results suggested that CAM-idling is a reversible, intermediate form of sustained metabolism which enables plant survival under conditions of extended drought.

CAM-idling in Hoya carnosa (Asclepiadaceae)

In the leaf sueculent Asclepial Hoya carnosa (L.) R. Br., CAM photosynthesis occurred under well-watered conditions, as characterized by diurnal gas exchange and changes in titratable acidity. Following 10-12 days of severe water stress, the plants shifted from CAM to a modified CAM-idling mode of metabolism. CAM-idling was characterized by complete or almost complete stomatal closure accompanied by CAM-like diurnal changes in titratable acidity. H. carnosa plants maintained this CAM-idling mode of photosynthesis for at least 8 weeks. Upon reirrigation, the plants returned to the original CAM mode within 1 week. These results suggested that CAM-idling is a reversible, intermediate form of sustained metabolism which enables plant survival under conditions of extended drought.

Activity of enzymes of carbon metabolism during the induction of Crassulacean acid metabolism in Mesembryanthemum crystallinum L

The maximum extractable activities of twenty-one photosynthetic and glycolytic enzymes were measured in mature leaves of Mesembryanthemum crystallinum plants, grown under a 12 h light 12 h dark photoperiod, exhibiting photosynthetic characteristics of either a C3 or a Crassulacean acid metabolism (CAM) plant. Following the change from C3 photosynthesis to CAM in response to an increase in the salinity of in the rooting medium from 100 mM to 400 mM NaCl, the activity of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) increased about 45-fold and the activities of NADP malic enzyme (EC 1.1.1.40) and NAD malic enzyme (EC 1.1.1.38) increased about 4- to 10-fold. Pyruvate, Pi dikinase (EC 2.7.9.1) was not detected in the non-CAM tissue but was present in the CAM tissue; PEP carboxykinase (EC 4.1.1.32) was detected in neither tissue. The induction of CAM was also accompanied by large increases in the activities of the glycolytic enzymes enolase (EC 4.2.1.11), phosphoglyceromutase (EC 2.7.5.3), phosphoglycerate kinase (EC 2.7.2.3), NAD glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12), and glucosephosphate isomerase (EC 2.6.1.2). There were 1.5- to 2-fold increases in the activities of NAD malate dehydrogenase (EC 1.1.1.37), alanine and aspartate aminotransferases (EC 2.6.1.2 and 2.6.1.1 respectively) and NADP glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13). The activities of ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39), fructose-1,6-bisphosphatase (EC 3.1.3.11), phosphofructokinase (EC 2.7.1.11), hexokinase (EC 2.7.1.2) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) remained relatively constant. NADP malate dehydrogenase (EC 1.1.1.82) activity exhibited two pH optima in the non-CAM tissue, one at pH 6.0 and a second at pH 8.0. The activity at pH 8.0 increased as CAM was induced. With the exceptions of hexokinase and glucose-6-phosphate dehydrogenase, the activities of all enzymes examined in extracts from M. crystallinum exhibiting CAM were equal to, or greater than, those required to sustain the maximum rates of carbon flow during acidification and deacidification observed in vivo. There was no day-night variation in the maximum extractable activities of phosphoenolpyruvate carboxylase, NADP malic enzyme, NAD malic enzyme, fructose-1,6-bisphosphatase and NADP malate dehydrogenase in leaves of M. crystallinum undergoing CAM.

[Effect of water stress on phosphoenolpyruvate carboxylase activity in Mesembryanthemum crystallinum (L.)]

Four-and-a-half-week-old plants of Mesembryanthemum crystallinum (L.) were transferred to cooled nutrient solution or to nutrient solution of low oxygen content. These stress treatments are known to reduce the absorption of water by plants. Examination of the second foliar leaves showed that the stress treatments result in a malate accumulation in the leaf mesophyll during the night which is paralleled by a drastically increased activity of phosphoenolpyruvate carboxylase. The results are discussed in relation to alterations of leaf growth, leaf water content and leaf water saturation deficit of the plants treated as described.

[Discovery of NaCl induced crassulacean acid metabolism in a second member of the Aizoaceae family: Carpobrotus edulis]

Carpobrotus edulis grown for 24 days in nutrient solution plus 400 mM of NaCl shows the typical CO2 gas exchange reactions observed in CAM plants. Control plants grown in nutrient solution alone exhibit CO2 gas exchange reactions typical for C3 plants.