Photosynthetic Response of Barley Plants to Soil Flooding

72 to 120 h of soil flooding of barley plants (Hordeum vulgare L. cv. Alfa) led to a noticeable decrease in the rates of CO₂ assimilation and transpiration, and in chlorophyll and dry mass contents. Stomatal conductance decreased following flooding without appreciable changes in the values of intercellular CO₂ concentrations. A drop in the activity of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) and of the photorespiratory enzymes phosphoglycollate phosphatase (EC 3.1.3.18) and glycollate oxidase (EC 1.1.3.1) was observed, while the activity of phosphoenolpyruvate carboxylase (EC 4.1.1.31) increased in all flooded plants. Flooding of barley plants caused an increase in proline content and in leaf acidity.     

Changes in the Leaf Polypeptide Patterns of Barley Plants Exposed to Soil Flooding

Exposure of barley plants (Hordeum vulgare L.) to soil flooding for 72 – 120 h led to decrease in the content of the both subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase. The effect was more pronounced on the small subunit. Further, the changes in protein pattern were observed, mainly proteins with molecular masses 30 – 85 kD were down-regulated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Changes of photosynthetic activities of maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) seedlings in response to cadmium stress

Maize (Zea mays L.) seedlings were grown in nutrient solution culture containing 0, 5, and 20 μM cadmium (Cd) and the effects on various aspects of photosynthesis were investigated after 24, 48, 96 and 168 h of Cd treatments. Photosynthetic rate (P _N) decreased after 48 h of 20 μM Cd and 96 h of 5μM Cd addition, respectively. Chl a and total Chl content in leaves declined under 48 h of Cd exposure. Chl b content decreased on extending the period of Cd exposure to 96 h. The maximum quantum efficiency and potential photosynthetic capacity of PSII, indicated by F_v/F_m and F_v/F_o, respectively, were depressed after 96 h onset of Cd exposure. After 48 h of 5μM Cd and 24 h of 20 μM Cd treatments, the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.39) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in the leaves started to decrease, respectively. We found that the limitation of photosynthetic capacity in Cd stressed maize leaves was associated with Cd toxicity on the light and the dark stages. However, Cd stress initially reduced the activities of Rubisco and PEPC and subsequently affected the PSII electron transfer, suggesting that the Calvin cycle reactions in maize plants are the primary target of the Cd toxic effect rather than PSII.     

Influence of Root Oxygen Deficiency on Photosynthesis and Antioxidant Status in Barley Plants1

Roots of barley plants (Hordeum vulgare L., cv. Alfa) were subjected to hypoxia for 120 h. By 72 to 120 h of soil flooding, a noticeable decrease in the rate of CO₂ assimilation and transpiration was observed. A drop in the activities of Rubisco and photorespiratory enzymes was found. We examined the changes in the activities of enzymes involved in the antioxidative system and stress markers related to membrane integrity, namely, lipid peroxidation and electrolyte leakage. Catalase and peroxidase activities were increased during the experiment, whereas superoxide dismutase activity drastically decreased.     

Evidence for the expression of morphological and biochemical characteristics of C3-photosynthesis in chlorohyllous callus cultures of Zea mays

A Zea mays callus culture containing chlorophyll was established and grown photomixotrophically. Cell chloroplast structure, and pigment and soluble protein contents were examined. Expression of some key enzymes of C₄ carbon metabolism was compared with that of etiolated (heterotrophic) and green photoautotrophic leaves. Chlorophyll content of the callus was 15–20% that of green leaves. Soluble protein content of callus was half that of leaf cells. Electron microscopic observations showed that green callus cells contained only typical granal chloroplasts. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.38) activities in green callus were ca 30% those of green leaves but 2–3 times higher than in etiolated leaves. Quantitative enzyme protein determination, using antibodies specific to maize leaf Rubisco showed that the chloroplastic carboxylase represented about 7% of total soluble protein in green callus, in parallel to its low chlorophyll content. The specific activity of Rubisco in callus and leaves was unchanged. Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activity in green callus was about 20% that of green leaves and similar to that measured in etiolated leaves. Apparent K_m (PEP) values (0.08 mM) for PEPC isolated from green callus and etiolated leaves were very different from values (0.5 mM) obtained with PEPC from green leaves. These kinetic characteristics together with the absence of inhibition by malate and activation by glucose-6-phosphate suggest that the properties of PEPC isolated from green callus and etiolated maize leaves are very similar to those of PEPPC from C₃ plants. Using PEPC antibodies specific to green maize leaf enzyme, immunotitration of PEPC preparations containing identical enzyme units allowed complete precipitation of the green leaf enzyme with increasing antibody volumes. In contrast, 60–70% of the activity of PEPC from etiolated and green callus was inhibited, suggesting low affinity for the maize green leaf PEPC antiserum (typical C₄ form). Ouchterlony double diffusion tests revealed only partial recognition of PEPC in green callus and etiolated leaves. NAD-malate dehydrogenase (NAD-MDH, EC 1.1.1.37) activity in callus was 2 and 3 times higher, respectively, than in etiolated and green leaves. NADP-malic enzyme (NADP-ME, EC 1.1.1.40) activity in callus cultures was much lower than in green leaves. All our data support the hypothesis that cultures of fully dedifferentiated chlorophyllous tissues of Zea mays possess a C₃-like metabolism.     

Responses of Ribulose-1,5-Bisphosphate Carboxylase,Protein Content,and Stomatal Conductance to Water Deficit in Maize,Tomato, and Bean

We compared responses of maize, tomato, and bean plants to water stress. Maize reached a severe water deficit (leaf water potential –1.90 MPa) in a longer period of time as compared with tomato and bean plants. Maize stomatal conductance (g _s) decreased at mild water deficit. g _s of tomato and bean decreased gradually and did not reach values as low as in maize. The protein content was maintained in maize and decreased at low water potential (_w); in tomato it fluctuated and also decreased at low _w; in bean it gradually decreased. Ribulose-1,5-bisphosphate carboxylase/oxygenase activity remained high at mild and moderate stress in maize and tomato plants; in bean it remained high only at mild stress.     

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.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate     

Effect of LiCl on phosphoenolpyruvate carboxylase kinase and the phosphorylation of phosphoenolpyruvate carboxylase in leaf disks and leaves of <Emphasis Type="Italic">Sorghum vulgare</Emphasis>

In the present work, the effect of LiCl on phosphoenolpyruvate carboxylase kinase (PEPCase-k), C₄ phosphoenolpyruvate carboxylase (PEPCase: EC 4.1.1.31) and its phosphorylation process has been investigated in illuminated leaf disks and leaves of the C₄ plant Sorghum vulgare. Although this salt induced severe damages to older leaves, it did not significantly alter the physiological parameters (photosynthesis, transpiration rate, intercellular CO₂ concentration) of young leaves. An immunological approach was used to demonstrate that the PEPCase-k protein accumulated rapidly in illuminated leaf tissues, consistent with the increase in its catalytic activity. In vivo, LiCl was shown to strongly enhance the light effect on PEPCase-k protein content, this process being dependent on protein synthesis. In marked contrast, the salt was found to inhibit the PEPCase-k activity in reconstituted assays and to decrease the C₄ PEPCase content and phosphorylation state in LiCl treated plants. Short-term (15 min) LiCl treatment increased IP₃ levels, PPCK gene expression, and PEPCase-k accumulation. Extending the treatment (1 h) markedly decreased IP₃ and PPCK gene expression, while PEPCase-k activity was kept high. The cytosolic protein synthesis inhibitor cycloheximide (CHX), which blocked the light-dependent up-regulation of the kinase in control plants, was found not to be active on this process in preilluminated, LiCl-treated leaves. This suggested that the salt causes the kinase turnover to be altered, presumably by decreasing degradation of the corresponding polypeptide. Taken together, these results establish PEPCase-k and PEPCase phosphorylation as lithium targets in higher plants and that this salt can provide a means to investigate further the organization and functioning of the cascade controlling the activity of both enzymes.     

Hydroponic treatment with salicylic acid decreases the effects of chilling injury in maize (Zea mays L.) plants

The addition of 0.5 mM salicylic acid (SA) to the hydroponic growth solution of young maize (Zea mays L.) plants under normal growth conditions provided protection against subsequent low-temperature stress. This observation was confirmed by chlorophyll fluorescence parameters and electrolyte leakage measurements. In addition, 1 d of 0.5 mM SA pre-treatment decreased net photosynthesis, stomatal conductivity and transpiration at the growth temperature (22/20 °C). Since there was only a slight decrease in the ratio of variable to maximal fluorescence (F_v/F_m) the decrease in photosynthetic activity is not due to a depression in photosystem II. The analysis of antioxidant enzymes showed that whereas SA treatment did not cause any change in ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase (EC 1.15.1.1) activities, there was a decrease in catalase (EC 1.11.1.6) activity, and an increase in guaiacol peroxidase (EC 1.11.1.7) and glutathione reductase (EC 1.6.4.2) activities after the 1-d SA treatment at 22/20 °C. In native polyacrylamide gels there was, among the peroxidase isoenzymes, a band which could be seen only in SA-treated plants. It is suggested that the pre-treatment of maize plants with SA at normal growth temperature may induce antioxidant enzymes which lead to increased chilling tolerance. Received: 4 June 1998 / Accepted: 23 November 1998     

Water Stress and Day-To-Day Variation in Apparent Photosynthetic Acclimation of Field-Grown Soybeans to Elevated Carbon Dioxide Concentration

Midday measurements of single leaf gas exchange rates of upper canopy leaves of soybeans grown in the field at 350 (AC) and 700 (EC) µmol(CO₂) mol^–1 in open topped chambers sometimes indicated up to 50 % higher net photosynthetic rates (P _N) measured at EC in plants grown at AC compared to EC. On other days mean P _N were nearly identical in the two growth [CO₂] treatments. There was no seasonal pattern to the variable photosynthetic responses of soybean to growth [CO₂]. Even on days with significantly lower P _N in the plants grown at EC, there was no reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase, chlorophyll, or soluble protein contents per unit of leaf area. Over three years, gas exchange evidence of acclimation occurred on days when either soil was dry or the water vapor pressure deficit was high (n = 12 d) and did not occur on days after rain or on days with low water vapor pressure deficit (n = 9 d). On days when photosynthetic acclimation was evident, midday leaf water potentials were consistently 0.2 to 0.3 MPa lower for the plants grown at EC than at AC. This suggested that greater susceptibility to water stress in plants grown at EC cause the apparent photosynthetic acclimation. In other experiments, plants were grown in well-watered pots in field chambers and removed to the laboratory early in the morning for gas exchange measurements. In these experiments, the amount of photosynthetic acclimation evident in the gas exchange measurements increased with the maximum water vapor pressure deficit on the day prior to the measurements, indicating a lag in the recovery of photosynthesis from water stress. The apparent increase in susceptibility to water stress in soybean plants grown at EC is opposite to that observed in some other species, where photosynthetic acclimation was evident under wet but not dry conditions, and may be related to the observation that hydraulic conductance is reduced in soybeans when grown at EC. The day-to-day variation in photosynthetic acclimation observed here may account for some of the conflicting results in the literature concerning the existence of acclimation to EC in field-grown plants.     

Treatment with salicylic acid decreases the effect of cadmium on photosynthesis in maize plants

The present study investigated the possible mediatory role of salicylic acid (SA) in protecting photosynthesis from cadmium (Cd) toxicity. Seeds of maize (Zea mays L., hybrid Norma) were sterilized and divided into two groups. Half of the seeds were presoaked in 500muM SA solution for only 6h, after which both groups were allowed to germinate for 3d and were then grown for 14d in Hoagland solution at 22/18 degrees C in a 16/8-h light/dark period and 120mumolm(-2)s(-1) PAR. All seedlings (without H(2)O and SA controls) were transferred to Cd-containing solutions (10, 15, and 25muM) and grown for 14d. The rate of CO(2) fixation and the activity of ribulose 1,5-bisphosphate carboxylase (RuBPC, EC 4.1.1.39) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were measured. Changes in the levels of several important parameters associated with oxidative stress, namely H(2)O(2) and proline production, lipid peroxidation, electrolyte leakage, and the activities of antioxidative enzymes (superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), catalase (CAT, EC 1.11.1.6), and guaiacol peroxidase (POD, EC 1.11.1.7)) were measured. Exposure of the plants to Cd caused a gradual decrease in the shoot and root dry weight accumulation, with the effect being most pronounced at 25muM Cd. Seed pretreatment with SA alleviated the negative effect of Cd on plant growth parameters. The same tendency was observed for the chlorophyll level. The rate of CO(2) fixation was lower in Cd-treated plants, and the inhibition was partially overcome in SA-pretreated plants. A drop in the activities of RuBPC and PEPC was observed for Cd-treated plants. Pretreatment with SA alleviated the inhibitory effect of Cd on enzyme activity. Proline production and the rates of lipid peroxidation and electrolyte leakage increased in Cd-treated plants, whereas the values of these parameters were much lower in SA-pretreated plants. Treatment of plants with Cd decreased APX activity, but more than doubled SOD activity. Pretreatment with SA caused an increase in both APX and SOD activity, but caused a strong reduction in CAT activity. The data suggest that SA may protect cells against oxidative damage and photosynthesis against Cd toxicity.     

Regulation of glutathione S-transferases of Zea mays in plants and cell cultures

An antiserum to glutathione S-transferase (EC 2.5.1.18) from maize (Zea mays L.) responsible for herbicide detoxification has been raised in rabbit. The antiserum was specific to the M_r 26000 subunit of the enzyme from maize seedlings and suspension-cultured cells, and recognized the isoenzymes active toward both atrazine and metolachlor. When plants were treated for 24 h with the herbicide antidote N,N-diallyl-2-2-dich-loroacetamide (DDCA), enzyme activities toward metolachlor were doubled in the roots and this was associated with a 70% increase in immunodetectable protein. Translation of polysomal RNA in vitro showed that the increase in the transferase in root tissue was brought about by a ninefold increase in mRNA activity encoding the enzyme. Treatment of suspension-cultured cells with cinnamic acid, metolachlor and DDCA raised enzyme activities but did not increase synthesis of glutathione S-transferase. In cultured maize cells, enzyme synthesis was maximal in mid-logarithmic phase, coinciding with the highest levels of enzyme activity. When callus cultures were established from the shoots of a maize line known to conjugate chloro-s-triazines, enzyme activity towards atrazine was lost during primary dedifferentiation. However, levels of total immunodetectable enzyme and activity toward metolachlor were increased in cultured cells compared with the parent shoot tissue.     

Extent of ipt gene expression and resulting amount of cytokinins affect activities of carboxylation enzymes in transgenic plants

Three types of transgenic plants of Solanum tuberosum cvs. Kamyk and Oreb, and Nicotiana tabacum cvs. Maryland Mammoth and Trapezond were selected according to intensity of introduced ipt gene expression and resulting amount of synthesised cytokinins (CKs). In comparison with controls, original transgenic regenerants grown in vitro showed a massive increase of CK contents, in tobacco by 379 % and in potato by 159 % (MAS). Potato grown in soil from tubers of transgenic plants demonstrated a moderate increase (44 %) of CK contents (MOD). Transgenic tobacco grown from seeds in vitro did not show any significant change in CK contents (NOT). Initial (RuBPC_i and RuBPO_i) and total (RuBPC_t) activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and the activity of phosphoenolpyruvate carboxylase (PEPC) were not significantly affected by the transformation in the NOT plants. In the MOD plants, the RuBPCO activities were stimulated by up to 34 % whereas the PEPC activity was decreased by 17 %. On the other hand, all the measured enzyme activities were 32 – 91 % lower in the MAS. Leaf area, fresh and dry masses, and chlorophyll and soluble protein contents also went down with increasing CK amounts in the transformants. Dependence of RuBPC_i/RuBPO_i and RuBPC_t/PEPC ratios on the relative CK amounts in transgenic plants revealed that the individual enzyme activities were not affected uniformly. Endogenous CK contents in the MAS thus apparently exceeded an optimum needed for positive effects on many physiological traits and became a stress factor for such plants.     

Effect of growth conditions on carboxylating enzymes of Zea mays plants

Phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphospate (RuBP) carboxylase (EC 4.1.1.39) activities in leaves of different maize hybrids grown under field conditions (high light intensity) and in a growth chamber (low light intensity) were determined. Light intensity and leaf age affected PEP carboxylase activity, whereas RuBP carboxylase was affected by leaf age only at low light intensity. PEP carboxylase/RuBP carboxylase activity ratio decreased according to light intensity and leaf age. Results demonstrate that Zea mays grown under field conditions is a typical C₄ species in all leaves independently from their position on the stem, whereas it may be a C₃ plant when it is grown in a growth chamber at low light intensityAbbreviations PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate     

Effects of root flooding and stage of development on the growth and photosynthesis of field bean (Vicia faba L. minor)

Field bean plants were subjected to flooding stress for 7 days, during two stages of development: at the vegetative phase (4-week-old seedlings) and at the generative phase (8-week-old plants). The height of plants, total area of leaves, the number of undamaged leaves, dry plant matter, chlorophyll content, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity, the maximum quantum yield of PS2 photochemistry (Fv/Fm ratio), the photosynthesis rate (P _N) and stomatal conductance (g _s) were determined. A strong reduction in stem elongation and leaf area as well as in dry matter production was observed as a result of flooding. The responses from vegetative plants were greater than in generative plants. Waterlogging decreased chlorophyll a and b in leaves, notably at the vegetative stage, and persisted after cessation of flooding. After flooding, photosynthesis was strongly reduced and positively correlated with decreased stomatal conductance. Damage to the photosynthetic apparatus resulted in a lower Fv/Fm especially in young seedlings. In vegetative plants Fv/Fm quickly returned to the control levels after the soil was drained. The results show that an excess of water in the soil limits growth and injures the photosynthetic apparatus in field beans, but that the extent of the injury is strongly age dependent.     

Drought- and ABA-induced changes in photosynthesis of barley plants

The changes caused by drought stress and abscisic acid (ABA) on photosynthesis of barley plants (Hordeum vulgare. L. cv. Alfa) have been studied. Drought stress was induced by allowing the leaves to lose 12% of their fresh weight. Cycloheximide (CHI), an inhibitor of stress-induced ABA accumulation, was used to distinguish alterations in photosynthetic reactions that are induced after drought stress in response to elevated ABA levels from those that are caused directly by altered water relations. Four hoars after imposition of drought stress or 2 h after application of ABA, Ihe bulk of the leaf's ABA content measured by enzyme-amplified ELISA, increased 14- and 16-fold, respectively. CHI fully blocked the stress-induced ABA accumulation. Gas exchange measurements and analysis of enzyme activities were used to study the reactions of photosynthesis to drought stress and ABA. Leaf dehydration or ABA treatment led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate versus intercellular CO₂ concentration and the maximal rate of photosynthesis; dehydration of CHI-treated plants showed much slower inhibition of the latter. The calculated values of the intercellular CO₂ concentration, CO₂ compensation point and maximal carboxylating efficiency of ribulose 1,5-bisphosphate (RuBP) carboxylase support the suggestion that biochemical factors are involved in the response of photosynthesis to ABA and drought stress. RuBP carboxylase activity was almost unaffected in ABA- and CHI-treated, non-stressed plants. A drop in enzyme activity was observed after leaf dehydration of the control and ABA-treated plants. When barley plants were supplied with ABA, the activity of carbonic anhydrase (CA, EC 4.2.2.1) increased more than 2-fold. Subsequent dehydration caused an over 1.5-fold increase in CA activity of the control plants and a more than 2.5-fold increase in ABA-treated plants. Dehydration of CHI-treated plants caused no change in enzyme activity. It is suggested that increased activity of CA is a photosynthetic response to elevated ABA concentration.     

Effect of irradiance during growth of Glycine max on photosynthetic capacity and percent activation of ribulose 1,5-bisphosphate carboxylase

The initial (in vivo) and total (activity present after preincubation with CO₂ and Mg²⁺) activities of ribulose bisphosphate carboxylase were both assayed in extracts of leaves of soybean (Glycine max) plants which had been grown under 4 different irradiance levels. The total carboxylase activity per unit leaf area decreased with decreased irradiance during growth but was not different on a dry weight basis. The initial activity as a percentage of the total activity was unchanged (approximately 95%) except in leaves of plants grown at the lowest irradiance (74%). When the plants grown at the lowest irradiance were exposed to high irradiance, the initial activity was increased to 93% of the total. Light saturated rates of photosynthesis per unit leaf area were lower and saturated at lower irradiance for plants grown at lower irradiances. Initial carboxylase activity was correlated closely (r²=0.84) with leaf photosynthesis rate on a dry weight basis.     

Higher plant phosphoenolpyruvate carboxylase kinase is regulated at the level of translatable mRNA in response to light or a circadian rhythm

Phosphoenolpyruvate carboxylase is regulated by reversible phosphorylation in response to light in C₃ and C₄ plants and to a circadian oscillator in CAM plants. Increases in phosphoenolpyruvate carboxylase kinase activity require protein synthesis. This requirement has been analysed by quantifying translatable mRNA for this protein kinase using in vitro translation of isolated RNA followed by direct assay of kinase activity. In leaves of the CAM plant Bryophyllum (Kalanchoë) fedtschenkoi, in normal diurnal conditions, kinase mRNA was 20-fold more abundant at night than in the day. In constant environmental conditions (continuous darkness, CO₂-free air, 15°C) kinase mRNA exhibited circadian oscillations. The circadian disappearance of kinase mRNA and kinase activity was delayed by lowering the temperature to 4°C and accelerated by raising the temperature to 30°C. The appearance of kinase mRNA and activity was blocked by cordycepin and puromycin. In maize and barley, kinase mRNA increased in response to light. For all three plants, the phosphoenolpyruvate carboxylase kinase activity generated during in vitro translation was Ca²⁺-independent. These results demonstrate that phosphoenolpyruvate carboxylase kinase activity is regulated at the level of translatable mRNA in C₃, C₄ and CAM plants.     

Role of sulphur availability on cadmium-induced changes of nitrogen and sulphur metabolism in maize (Zea mays L.) leaves

The interactions between sulphur nutrition and Cd exposure were investigated in maize (Zea mays L.) plants. Plants were grown for 12 days in nutrient solution with or without sulphate. Half of the plants of each treatment were then supplied with 100 microM Cd. Leaves were collected 0, 1, 2, 3, 4 and 5 days from the beginning of Cd application and used for chemical analysis and enzyme assays. Cd exposure produced symptoms of toxicity (leaf chlorosis, growth reduction) and induced a noticeable accumulation of non-protein SH compounds. As phytochelatins are glutamate- and cysteine-rich peptides, the effect of cadmium on some enzyme activities involved in N and S metabolism of maize leaves was studied in relation to the plant sulphur supply. In vivo Cd application to S-sufficient plants resulted in a drop of all measured enzyme activities. On the other hand, S-deficient plants showed a decrease in nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2) activity, and an increase in NAD-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2) and phosphoenolpyruvate carboxylase (PEPc; EC 4.1.1.31) activity as a result of the Cd treatment. Furthermore, in the same plants ATP sulphurylase (ATPs; EC 2.7.7.4) and O-acetylserine sulphydrylase (OASs; EC 4.2.99.8) showed a particular pattern as both enzymes exhibited a transient maximum value of activity after 4 days from the beginning of Cd exposure. Results provide evidence that the increase of ATPs, OASs, GDH and PEPc activities, observed exclusively in S-deficient Cd-treated plants, may be part of the defence mechanism based on the production of phytochelatins.