Zeitschrift für Pflanzenphysiologie最新文献

Mitochondrial MDH (EC 1.1.1.37) is synthesized in watermelon cotyledons as a cytosolic, higher molecular weight precursor (41 kd). We now show that the precursor produced in a reticulocyte lysate in the presence of watermelon mRNA, is posttranslationally sequestered by a crude mitochondrial fraction or by mitochondria purified on Percoll® gradients. It is processed to a proteinase resistant form corresponding to the mature subunit (38 kd). Optimal incorporations are obtained with organelles from early stages of seedling development. An uptake of the mature MDH dimer to a proteolytically protected form could not be detected.

The effect of additional fertilization (NPK) on metabolism was studied in the first three leaves of barley. Activities of ribulose-1,5-bisphosphate carboxylase, alkaline fructose-1,6-bisphosphatase, nitrate reductase and protein content were markedly higher at elevated nutrient levels, but content of main non-structural carbohydrates were lower under these conditions. Nutrient limited plants accumulated large amounts of carbohydrates, but their dry weight was reduced, indicating a limited capacity for photosynthesis and utilization of assimilates. The specific activity of RuBPCase increased at low nutrient levels during development of the second leaf, but not at elevated nutrient levels. Therefore, it is likely that RuBPCase protein is degraded less efficiently than other protein. Alternatively, these findings could be interpreted as an altered response to activation of RuBPCase by bicarbonate.

In cotyledons of sunflower (Helianthus annuus L.) seeds glyoxysomal enzyme activities were shown to develop already during seed maturation. Glyoxysomes isolated from cotyledons 30 d after anthesis exhibited activities of monoacylglycerol lipase, hydroxyacyl-CoA dehydrogenase, catalase, and malate synthase, while apparent isocitrate lyase activity could be measured only in the supernatant fluid of the sucrose density gradients yet not in the crude homogenates. This indicates the presence of a probably unspecific inhibitor especially since glyoxysomes from cotyledons 40 d after anthesis showed isocitrate lyase activity although still no activity could be detected in the crude homogenates. After dehydration of the mature seeds a glyoxysomal fraction could not be isolated which presumably is due to damage of the organelles by the strong forces required for tissue disintegration, for low activities of all but one glyoxysomal enzyme activities measured were present in the crude homogenates. Comparison of glyoxysomes from maturing seeds to those from germinating seeds revealed, in contrast to earlier reports in the literature, that during seed ripening low but substantial activities of all key enzymes for glyoxysomes are present in the organelles. It is suggested that these glyoxysomes function as intermediate forms immediately after seed imbibition and start of germination until the pleomorphic population of germination related glyoxysomes is developed.

An enzyme, UDP-galactose : sucrose 6^fru-α-galactosyltransferase, which synthesizes planteose (6^fru-α-galactosylsucrose), has been purified 55-fold from seeds of Sesamum indicum L. (Pedaliaceae) using ammonium sulfate and protamine sulfate precipitation. The partially purified enzyme fraction showed only very low α-galactosidase and β-fructosidase activity but significant UDP-glucose 4-epimerase, phosphodiesterase, phosphatase and sucrose synthase activity which interfere with planteose synthesis. UDP-glucose 4-epimerase activity could be inhibited by about 50% by the addition of 10 mmol/l Mn²⁺, whereas phosphodiesterase activity could not be reduced by the addition of ATP, NAD or UDP-glucose, potentially alternative substrates for UDP-galactose. The optimal activity of the planteose synthesizing enzyme was obtained at pH 6.2. The enzyme was stable for at least six months at -20°C. It did not need sulfhydryl reagents for full activity. The enzyme displayed a high specificity for UDP-galactose (K_m 0.2–0.5 mmol/l), the galactosyl donor, and for sucrose (K_m 3.6–6.0 mmol/l), the acceptor. 5′-UMP was inhibitory, while galactose 1-phosphate, glucose l-phosphate and UDP-glucose did not affect the enzyme activity.

A rabbit antiserum against spinach calmodulin was prepared and characterized. In Ouchterlony's immunodiffusion test, the antiserum gave with spinach calmodulin rise to a single precipitin line. The antiserum crossreacted with pea, wheat and corn calmodulin with single precipitin lines which completely fused. However, it did not react with bovine brain calmodulin. NAD kinase purified from pea seedlings was inhibited when calmodulin was removed by treatment with the anti-calmodulin serum. A specific radioimmunoassay for plant calmodulin was developed to determine calmodulin content in pea plant tissues. Calmodulin contents were high in growing tissues such as shoot apex and hook in etiolated plants. In green plants, calmodulin content was high in leaf tissues.

Two treatments affecting the functioning of barley (Hordeum vulgare L.) plastids were used to study the localisation and control of ABA accumulation in response to partial dehydration. The X-ray-induced barley mutant albostrians was used to compare ABA accumulation in green, white and striped mutant leaves, and in green leaves of albostrians which had been treated with the herbicide norflurazon. NF inhibits carotenoid synthesis and in high light results in photo-bleaching of chlorophyll.

ABA accumulation was severely inhibited in completely white mutant leaves, in white portions of striped leaves, and in NF-treated leaves which had been photo-bleached. These leaves also had much lower stomatal conductances than those of green leaves, and plastid ribosomes were completely absent. NF had no effect on ABA accumulation in leaves grown in the dark, in low light (where NF reduced carotenoid but not chlorophyll levels) and in high light prior to the onset of photo-bleaching. In all these treatments plastid ribosomes were still present. Although hardly any ABA accumulation took place in white leaves, the presence of low levels of ABA in these leaves was indicated by UV isomerisation of assay samples.

It was concluded firstly, that protein synthesis on plastid ribosomes was necessary at some stage for drought stress to elicit a rise in ABA synthesis, and secondly, that enzymes for every step in the synthesis of ABA are encoded on nuclear DNA.

In winter rape (Brassica napus, L., var. oleifera, cv. Górczański) and cucumber (Cucumis sativus, L., var. Inspektowy Warszawski) plants we found two types of pyruvate kinase. These were distinguished by different pH, ion and substrate requirements, as well as by their response to such negative modulators as ATP and alanine. Their response to temperature change was also different: the Arrhenius plot for the cucumber enzyme was linear over the temperature range 30°C to 0°C whereas that for the winter rape enzyme showed a break in the slope of the plot at 8°C. The cucumber enzyme extracted from plants exposed to chilling temperature (3°C) showed a transient increase of activity during the first days of cold treatment. The activity markedly decreased only after a pronounced (80%) protein degradation had occurred. Cold (2°C) acclimation of winter rape plants resulted in lower sensitivity of the enzyme to ATP, loss of sensitivity to alanine and decreased sensitivity to temperature below 10°C. The results are discussed in terms of plant adaptation to different temperature conditions of the environment.

The aim of this work was to study the effects of metabolic intermediates on pyrophosphate: fructose 6-phosphate phosphotransferase (PFP) from endosperm of germinating castor bean (Ricinus communis). Glucose 1-phosphate, glucose 6-phosphate, UDPglucose, glucose, fructose, sucrose and pyruvate, each at 1 mM, and malate, succinate and glycolate, each at up to 5mM, had no significant effect on enzyme activity. PFP was inhibited by 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, citrate and all nucleotides tested. Inhibition by MgATP, 3-phosphoglycerate, phosphoenolpyruvate and citrate was due primarily to their effect on the affinity of PFP for fructose 2,6-bisphosphate, a potent activator of the enzyme. These inhibitors each increased the concentration of fructose 2,6-bisphosphate required for half maximum activation of PFP up to 30-fold, from 6 to 180 nanomolar. The possible roles of these compounds in the control of PFP are discussed.