Starch-degrading enzymes during the induction of CAM in Mesembryanthemum crystallinum

Mesembryanthemum crystallinum plants were irrigated with 400 mol m^?3 NaCl to induce CAM and levels of leaf starch, and activities of starch-degrading enzymes were measured. During Crassulacean acid metabolism (CAM) induction, daily starch turnover gradually became more pronounced and was three- to four-fold greater than in leaves of C₃ plants after 3 weeks. Activities of α- and β-amylase, D-enzyme and starch phosphorylase all increased 10- to 20-fold within 3 weeks of the start of salt treatment. Activities of α- and β-amylase increased more than fourfold within the first 24 h of salt treatment, which is the fastest increase in enzyme activities so far measured during the induction of CAM with salt solution in intact plants of this species. Most enzyme activities were partially chloroplastic; however, the principal starch-degrading activity was constituted by an extra-chloroplastic β-amylase. CAM starch-phosphorylase activity, which was mainly chloroplastic, exhibited a two- to three-fold diurnal change in parallel with starch content. CAM induction in M. crystallinum is clearly associated with greater starch turnover and enhanced starch-degrading enzyme activities, which as catalysts of the initial reaction to release carbon for synthesis of phosphoenolpyruvate (PEP) appear highly significant for the functioning of the CAM pathway. The diurnal rhythm of phosphorylase activity may be of particular significance.     

The Irreversible C3 to CAM Shift in Well-watered and Salt-stressed Plants of Mesembryanthemum crystallinum is under Strict Ontogenetic Control

The shift from C₃ to CAM was investigated as a function of both leaf and plant age in well-watered and salt-stressed (300 mM NaCl solution) plants of Mesembryanthemum crystallinum. Initiation of a night-time accumulation of malic acid, the decisive criterion of CAM, was followed in plants that were continuously stressed at different points in their life cycle. The deinducibility of CAM was examined after the release from stress by extensively rinsing the potting soil with de-mineralized water. Our results show that in M. crystallinum CAM is under strict developmental control, since CAM appeared only when a certain stage of development of the whole plant was reached. CAM was not present in any plant before this threshold, which was the same in salt-stressed as well as in well-watered plants. The metabolic shift coincided with the change from the seedling to the juvenile growth phase, and not with that from vegetative to reproductive growth, represented by the start of branching. The latter is timed to the end of extension growth. In well-watered plants, after this decisive point in development, a weak nighttime accumulation of malic acid could be measured (? 0.05 mol kg_DW^?1) in the oldest, mature leaves but not in young, developing ones. This “CAM capacity” gradually increased up to 0.2 mol kg_DE^?1 with further plant ageing. Leaf senescence, characterized by wilting and yellowing, diminished the CAM activity. In mature leaves salt stress drastically enhanced the magnitude of diurnal fluctuation in malic acid content. Removal of salt stress did not deinduce CAM activity, but diminished the amplitude of malic acid oscillations to some extent in those plants which had been stressed from early in their life cycle. In these plants, salt stress delayed plant development and growth thus retarding the life cycle. Well-watered plants, for example, branched about three weeks earlier than those that had been stressed continuously from one week after germination. After removal of stress a quasi-preserved earlier developmental stage in relation to the control plants determined the weaker CAM expression.     

Investigation of phosphoeno/pyruvate carboxylase (PEPCase) in Mesembryanthemum crystallinum L. in C3 and CAM photosynthetic states

When exposed to osmotic stress, Mesembryanthemum crystallinum plants switch from C₃ to CAM photosynthesis. Phosphoenolpyruvate carboxylase (PEPCase) is a key enzyme in CAM plants, being responsible for the initial fixation of CO₂. In C₃ plants the enzyme has been shown to be involved in the replenishing of TCA cycle intermediates and in the operation of stomatal guard cells. Multiple PEPCase isoforms were observed in C₃-performing leaves with four isoelectric points of 5.2, 5.5, 5.6 and 5.9 and four apparent subunit molecular masses of 105, 108, 113 and 116 kDa. In some instances, subunits of different size possessed exactly the same pI. The induction of CAM led to the predominance of a new isoform of pI 6.5 with subunit molecular mass of 108 kDa, but in addition, changes were observed in some of the isoforms present in the C₃ plant. PEPCase subunits were purified from the C₃ and CAM forms of M. crystallinum and subjected to pep-tide mapping. Two distinct though similar sets of maps were obtained, one from the CAM isoform (pI 6.5) and C₃-associated subunits of pi 5.9 and another for C₃ subunits of pI 5.2 and 5.5. It was inferred from these data that the C₃ isoforms expressed in the leaf were derived from at least two genes. The C₃ isoform (pI 5.9) showing greatest similarity to the CAM isoform in terms of peptide mapping also increased in response to salt stress. It is speculated that the CAM isoform may have evolved from this enzyme.     

Quantification of Visible Structural Changes of the V0V1-ATPase in the Leaf-tonoplast of Mesembryanthemum crystallinum by Freeze-fracture Replicas Prepared During the C3-Photosynthesis to CAM Transition*

Plants of the annual facultative halophyte and facultative CAM-plant, Mesembryanthemum crystallinum L., were irrigated with a solution containing NaCl when they had developed 3 leaf pairs. This treatment induced CAM and the plants were then watered with 400 mM NaCl until the end of the experiment of 37 days. A separate set of plants was simultaneously maintained as non-salt treated controls. Tonoplast vesicles were prepared from the leaves at regular intervals during the time-course of the experiment. Three samples of each preparation were freezed fractured, and carbon/platinum-replicas taken. On a total of 1400 fracture faces the diameters and densities per unit area of intramembraneous particles were measured. The results show an increase in the average diameter of particles from 6.5 nm to 8.5 nm and an increase of the relative amount of fracture faces with high particle densities related to the total of fracture faces obtained; both of which kinetically correlated to CAM induction. This increase in size and density of particles, which are known to belong to the H⁺-transporting ATPase of the tonoplast. shows independently of and in addition to protein analyses, that an increased amount of ATPase-protein is incorporated into the membrane during CAM induction. Some possible explanations for the increase in ATPase particle size are discussed.     

Adaptation of the tonoplast V-type H+-ATPase of Mesembryanthemum crystallinum to salt stress, C3–CAM transition and plant age

Plants of the facultative halophyte and CAM species Mesembryanthemum crystallinum L. (Aizoaceae) were stressed for 8 d with 400 mol m⁻³ NaCl in the root medium. NaCl was then removed from the substratum, and the plants were watered again with NaCl-free solution. A second set of plants was maintained as controls. A small degree of CAM, as indicated by day-night changes in malate levels, was expressed during ageing of the plants. Salinity-stress-dependent CAM induction was reversible by the removal of salt, as indicated by similar Δ malate levels in previously salt-stressed plants and in non-stressed plants on day 19 of the experiment. Tonoplast vesicles were isolated from leaves during the time-course of stress application, stress removal and ageing. Parameters of the tonoplast H⁺-ATPase were correlated to the application of salinity, the expression of CAM and ageing. It was concluded, first, that a pronounced increase in the amount of tonoplast H⁺-ATPase is related to salinity per se and a smaller increase to ageing; secondly, that there is an increase in the specific activity of the enzyme related to ageing; thirdly, that the induction of two new polypeptides with molecular masses of 32 and 28 kDa is correlated in time with the expression of CAM, and, fourthly, that the two new polypeptides are part of the tonoplast H⁺-ATPase holoenzyme.     

Membrane Particles,Proteins and ATPase Activity of Tonoplast Vesicles of Mesembryanthemum crystallinum in the C-3 and CAM State*

Dimensions and area densities of membrane particles were studied by electron microscopy of replicas of freeze-fractured suspensions of tonoplast vesicles of Mesembryanthemum crystallinum L. in the C-3 state and after induction of crassulacean acid metabolism (CAM) by salinity. The results are compared with the relative contribution of tonoplast-ATPase protein to total membrane protein obtained from integration of elution profiles in size-exclusion chromatography. Coverage of tonoplast area by globular membrane particles was 20% and 36 % and ATPase in relation to total membrane protein was 33 % and 35 % in C-3 and CAM M. crystallinum, respectively. Thus, by order of magnitude, it is most likely that the particles represent the ATPase. In Kalanchoë daigremontiana Hamet et Perrier de la Bâthie the ATPase also constituted 36% of total tonoplast protein. Induction of CAM in M. crystallinum was associated with an increase in specific ATPase activity of the tonoplast and an increase of the size and area coverage of tonoplast particles, whereas the relative contribution of ATPase protein to total tonoplast protein and the molecular mass of the ATPase holoenzyme, as seen in size-exclusion chromatography, remained almost unchanged.     

Subcellular localization and stress responses of superoxide dismutase isoforms from leaves in the C3-CAM intermediate halophyte Mesembryanthemum crystallinum L.

BSA, bovine serum albumin
CAM, Crassulacean acid metabolism
DTT, dithiothreitol
EDTA, ethylenediaminetetraacetic acid
FPLCfast protein liquid chromatography
HEPES, N-(2-hydroxyethyl)piperazine-?-(ethanesulphonic acid)
ME, β-mercaptoethanol
NBT, nitro blue tetrazolium
PAGE, polyacrylamide gel electrophoresis
SDS, sodium dodecyl sulphate
SDS-PAGE, sodium dodecyl sulphate polyacrylamide gel electrophoresis
Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39)
SOD, superoxide dismutase (EC 1.15.1.1)
TEMED, N,N,?,?-tetramethylethylenediamine
Tris, Tris (hydroxymethyl) aminomethane
Tricine, N-Tris(hydroxymethyl)methylglycine

Treatment of Mesembryanthemum crystallinum for several days with 0·4 kmol m^–3 NaCl in the root medium, in parallel to an increase of the cell sap osmolarity enhances activity of important antioxidative enzymes, such as superoxide dismutases (SODs). M. crystallinum is equipped with three SOD isoforms. These isoforms were identified as Mn-, Fe-, and Cu/Zn-SODs, respectively. Mn-SOD was found in the mitochondrial fraction, Fe-SOD in the chloroplast fraction, and Cu/Zn-SOD is probably localized in the cytosol. The Fe-SOD found in M. crystallinum is the first iron-containing SOD enzyme to be characterized in the plant family Aizoaceae. Salt treatment increases the activity of this isoform earlier than the other SODs. Molecular masses of SOD isoforms were determined as 82, 48 and 34 kDa for Mn-, Fe-, Cu/Zn-SODs, respectively. Native Mn-SOD seems to be a tetramer, while Fe-SOD and Cu/Zn-SOD are dimers. All SOD isoforms show high thermal stability. Mn-SOD is active even after short heating at 90 °C and Fe-SOD at 70 °C. Moreover, high concentrations of β-mercaptoethanol used as a reducing agent did not destroy the function of all isoforms. With the salinity treatment in M. crystallinum, Crassulacean acid metabolism (CAM) is induced. Build-up of large stationary O₂ concentrations in the leaf air spaces is associated with the photosynthetic CO₂ reduction behind closed stomata in phase III of CAM. This illustrates why M. crystallinum may require higher antioxidative activities under NaCl stress and also explains earlier findings that CAM plants are more resistant than C₃ plants to environmental stress as imposed by, for example, SO₂ and O₃.     

Ultrastructure of Chloroplasts and Their Storage Inclusions in the Primary Leaves of Mesembryanthemum crystallinum Affected by Putrescine and NaCl

The effects of salinity (300 mM NaCl), putrescine (Put), and the combination of two agents on the structure of chloroplasts and storage deposits were studied in the third leaf pair of a facultative halophyte Mesembryanthemum crystallinum. Within 6 days, the common ice plants responded to NaCl and Put treatments by diminished chloroplast volumes and swollen grana. Different effects of the experimental treatments were primarily manifested in the chloroplast storage inclusions. Under the salinity conditions, the starch content dropped down almost threefold as compared to untreated plants (control), whereas the number of plastoglobules did not change. Put and Put + NaCl treatments further decreased the starch content per unit section area; in contrast, the plastoglobule area per chloroplast section increased eightfold and tenfold in Put and Put + NaCl treatments, respectively. The morphology and electronic density of plastoglobules changed in all treatments. In both Put treatments there ware no destructive changes in the chloroplasts, and therefore the authors presume that the increase in the numbers plastoglobules was related to the redirection of cell metabolism towards the products of the higher reduction potential. The ferritin deposits in the chloroplasts were observed in all treatments they were more abundant in the vascular parenchyma cells, especially under salinity. The ability of the common ice plants to accumulate large Fe quantities in their chloroplasts and the characteristic pectin-filled pockets, which were observed earlier, and intercellular spaces are probably related to the genetically determined traits of plant adaptation to salinity and water deficit.     

Effect of hypermethylation of CCWGG sequences in DNA of Mesembryanthemum crystallinum plants on their adaptation to salt stress

Under salt stress conditions, the level of CpNpG-methylation (N is any nucleoside) of the nuclear genome of the facultative halophyte Mesembryanthemum crystallinum in the CCWGG sequences (W = A or T) increases two-fold and is coupled with hypermethylation of satellite DNA on switching-over of C3-photosynthesis to the crassulacean acid metabolism (CAM) pathway of carbon dioxide assimilation. The methylation pattern of the CCWGG sequences is not changed in both the 5'-promoter region of the gene of phosphoenolpyruvate carboxylase, the key enzyme of C4-photosynthesis and CAM, and in the nuclear ribosomal DNA. Thus, a specific CpNpG-hypermethylation of satellite DNA has been found under conditions of expression of a new metabolic program. The functional role of the CpNpG-hypermethylation of satellite DNA is probably associated with formation of a specialized chromatin structure simultaneously regulating expression of a large number of genes in the cells of M. crystallinum plants on their adaptation to salt stress and switching-over to CAM metabolism.     

兼性CAM植物长叶景天叶片在C3和CAM型时的光氧化作用

通过停止浇水产生水分胁迫使兼性CAM植物长叶景天(Sedum spectabile Boreau)叶片光合途径由C3型转为CAM型.干旱15 d时观察到典型的CAM生理特征,且叶片的δ13C值与含水量成线性相关.水分胁迫改变了叶绿素荧光参数和抗氧化能力,ΦPSⅡ和qP降低50%和34%,NPQ提高约180%,SOD活性和清除DPPH@ 自由基能力也明显下降, 但膜半透性变化不大.当将处于C3(浇水)和诱导为CAM(缺水)型的叶圆片用外源甲基紫精(MV)和强光作光氧化处理后,与C3型叶片相比,诱导CAM型叶片的NPQ不能提高,qP和ΦPSⅡ降至很低水平,光系统处于高还原态,光能供给与消耗失衡(1-qP=0.86和(1-qP)/NPQ>1),膜系统几乎失去完整性.这种严重的光氧化损伤表明,与我们以前报告的专性CAM植物不同,以兼性CAM植物诱导表达的CAM型未能显示比C3型较强的耐光氧化优势.讨论了出现这种光氧化敏感性差别的可能原因.     

Regulation of CAM and Respiratory Recycling by Water Supply in Higher Poikilohydric Plants — Haberlea rhodopensis Friv. and Ramonda serbica Panč, at Transition from Biosis to Anabiosis and Vice Versa

In this paper the expression of C₃ and CAM in the resurrection plants Haberlea rhodopensis Friv. and Ramonda serbica Pan?, during the transition from biosis to anabiosis and Wee versa is reported for the first time. The transition from predominantly C₃ metabolism to net dark fixation of CO₂ occurred in leaves of R.serbica during desiccation. Desiccated plants of H. rhodopensis react by reducing light assimilation of CO₂. When watering was resumed night time fixation of CO₂ by R. serbica was observed within 24 hours. The recovery of CO₂ fixation by H. rhodopensis was not seen until the 8 th day. Desiccated and rehydrated plants of H. rhodopensis recapture a higher proportion of respiratory CO₂ than well-watered plants. Since both species have little capacity for water conservation in their tissues, the early onset of high recycling of CO₂ following drought could be an important mechanism for potentially saving water.