bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cytokinin oxidase: Biochemical features and physiological significance
Authors: Hare P. D. , van Staden J. 
Published: May 2006

The catabolism of cytokinin in plant tissues appears to be due, in large part, to the activity of a specific enzyme, cytokinin oxidase. This enzyme catalyses the oxidation of cytokinin substrates bearing unsaturated isoprenoid side chains, using molecular oxygen as the oxidant. In general, substrate specificity is highly conserved and cytokinin substrates bearing saturated or cyclic side chains do not serve as substrates for most cytokinin oxidases tested to date. Despite variation in molecular properties of the enzyme from a number of higher plants, oxygen is always required for the reaction. Cytokinin oxidases from several sources have been shown to be glycosylated. Cytokinin oxidase activity appears to be universally inhibited by cytokinin‐active urea derivatives. Auxin has been reported to act as an allosteric regulator which increases activity of the enzyme.Cytokinin oxidase activity is subject to tight regulation. Levels of the enzyme are controlled by a mechanism sensitive to cytokinin supply. The up‐regulation of cytokinin oxidase expression in response to exogenous application of cytokinin suggests that the metabolic fate of exogenously applied cytokinins may not accurately mimic that of the endogenous compounds.Cytokinin oxidase is believed to be a copper‐containing amine oxidase (EC 1.4.3.6). Considerable evidence strongly supports a common mechanism for amine oxidases. It is possible that advances in understanding of other amine oxidases could be extrapolated to increase our understanding of cytokinin oxidase at the molecular level. This is discussed with reference to what is currently known about the catalytic mechanism of the enzyme. The possibility of pyrroloquinoline quinone, or a closely related compound, as a redox cofactor of cytokinin oxidase is considered, as are the implications of the glycosylated nature of the enzyme for its regulation and compartmentalisation within the cell.
Carbohydrate and mineral nutrition of orange fruitlets in relation to growth and abscission
Authors: Ruíz R. , Guardiola J. L. 
Published: May 2006

Carbohydrate and mineral nutrition was studied in relation to abscission in fruitlets from leafy inflorescences of the Washington navel orange (Citrus sinensis [L.] Osbeck). Differences in the growth rate of the fruitlets permitted to predict abscission several weeks in advance. This allowed characterization of early differences in composition and behaviour of persisting and abscising fruitlets.Inflorescences with persisting fruitlets accumulated more mineral elements than inflorescences with abscising fruitlets, and for the phloem‐mobile elements the excess accumulation was allocated to the fruitlets. Starch accumulated in the inflorescence leaves during early fruitlet growth, and this accumulation was enhanced by the persisting fruitlets despite their higher growth rate and mobilizing ability. The relations between the fruitlets and the inflorescence leaves cannot be explained totally in terms of source sink relationships; a hormonal regulation of the leaves by the fruitlets is postulated.Acid invertase activities and hexose concentration in the pericarp were higher in the abscising fruitlets. The lower early growth rate of these fruitlets is thus not caused by a limitation in carbohydrate supply. It seems more related to carbohydrate utilization, probably hormonally mediated, as demonstrated by the higher dependence on hormone supply for the growth in vitro of the endocarp explants.
Vascular occlusion in cut flowering rose stems exposed to air: role of the xylem wall pathway for water
Authors: van Doorn Wouter G. 
Published: May 2006

Upon exposure to air at 20°C and 60% relative humidity (RH), an occlusion to water uptake developed in the basal stem segment of cut, flowering Sonia roses. The development of the occlusion was delayed by removing the leaves before storage, indicating an effect of transpiration. Placing the stem ends at 100% RH during exposure to air had no effect, which shows that water loss through the cut end is not the cause. Scanning electron microscopy did not show plant gums or tyloses in the water conducting elements.The hypothesis by Scholander et al. (1955) that water uptake into stems held in air and then placed in water occurs through conduit walls rather than the lumen was tested by using stems of which the cut surface was covered with laboratory grease and a ring of bark was removed (girdling). Girdled stems placed in water remained fully turgid when the girdled area was about 60 mm2 or more. Water uptake was strongly inhibited when the girdled stems were exposed to air for 24–36 h, as in non‐girdled controls exposed to air for the same period. Inclusion of a surfactant (nonylphenoxypolyethoxy ethanol) in the vase water, however, facilitated water uptake after dry storage of normal cut flowering stems but did not improve water uptake into the girdled stem system, which is inconsistent with the hypothesis.It is concluded that the vascular occlusion developing upon exposure to air cannot be explained by decreased flow in the wall pathway for water.
Changes in protein kinase and protein phosphatase properties during the cycle of asparaginase activity in Leptosphaeria michotti
Authors: Jerebzoff‐Quintin Simonne , Jerebzoff Stéphan 
Published: May 2006

Regulation of the cyclic activity of asparaginase (obtained as a purified protein complex) by a reversible auto‐phosphorylation process has been previously reported in the fungus Leptosphaeria michotii (West) Sacc.In the present study, the protein complex was purified in the presence of either a mixture of 3 protein phosphatase inhibitors (fluoride, vanadate and molybdate) or EGTA, during the cycle of asparaginase activity, and the protein kinase and protein phosphatase activities characterized. (I) At the phase of increasing asparaginase activity, a Ca2+/calmodulin‐dependent kinase activity was identified by (a) its inhibition by calmidazolium, reversed by calmodulin, and its inhibition by EGTA, but not by poly(Glu/Tyr 4:1)n. dichloro‐(ribofuranosyl)‐benzimidazole or polylysine (b) an increasing level of calmodulin bound to the complex, as estimated by enzyme‐linked immunosorbent assay (ELISA). (2) At the phase of decreasing asparaginase activity, the Ca2+‐calmodulin‐dependent kinase activity disappeared and a little calmodulin remained associated with the complex: phosphorylation of the complex was increased several‐fold by 1 nM okadaic acid and 25 nM inhibitor‐2, and was not affected by EGTA, indicating a protein phosphatase‐2A‐like activity. (3) When asparaginase activity was low, a little calmodulin was bound to the complex. The kinase could phosphorylate casein and phosvitin. was inhibited by poly(Glu/Tyr 4:1)n. dichloro‐(ribofuranosyl)‐benzimidazole and heparin, stimulated by polylysine and not affected by calmidazolium or EGTA, just as a casein kinase 2. A Ca2+‐dependent but calmodulin‐independent protein phosphatase activity, not affected by okadaic acid and inhibitor‐2. was then identified.We postulate the presence in the complex, of (a) only one protein kinase and one protein phosphatase, whose properties could change during the cycle of asparaginase activity: (b) two Ca2+/‐binding proteins: first calmodulin, which could bind to Ca2+ and the casein kinase‐2 form to give a Ca2+/calmodulin‐dependent kinase, which could become Ca2+/calmodulin‐independent following an auto‐phosphorylation process: second a protein homologous to calmodulin, able to bind to the protein phosphatase‐2A catalytic subunit to give a protein phosphatase‐2B catalytic subunit.
Influence of selenium on uptake and toxicity of copper and cadmium in pea (Pisum sativum) and wheat (Triticum aestivum)
Authors: Landberg Tommy , Greger Maria 
Published: May 2006

The detoxifying effect of selenium on animals toxicated with heavy metals is well known. In this study we examine if there is a similar effect in plants. Wheat (Triticum aestivum L. cv. Sunny) and pea (Pisum sativum L. cv. Fenomen) were grown for 21 days on a nutrient solution based on the nutrient proportions in healthy plants. Nutrients along with cadmium, copper, selenite, selenate or selenite and selenate in combinations with copper or cadmium were supplied in small amounts with a daily incremental increase of 0.12 (wheat) and 0.20 (pea). The metal and selenium uptake and distribution in the plants as well as the effects on growth were investigated.The results show that selenium does not reduce the toxicity of heavy metals to plants. Instead, selenium enhances metal uptake and toxicity, especially in peas grown in the presence of metal and selenate. Selenite increased cadmium concentrations of pea roots up to 300% and selenate that of wheat shoots up to 50%.
The accumulation of solutes and water binding strength in durum wheat
Authors: Rascio A. , Platani C. , Scalfati G. , Tonti A. , Di Fonzo N. 
Published: May 2006

Variations in water binding strength, water relations and the accumulation of solutes during water stress of three durum wheat (Triticum durum Desf.) cultivars are reported and discussed. Water binding strength was determined by constructing adsorption isotherms at 5 and 20°C and by calculating the differential enthalpy (ΔH) after van't Hoff.Reducing sugars, proline, K+ and Cl− were the major contributors to osmotic adjustment. Solutes, such as quaternary ammonium compounds, and non‐reducing sugars contributed to osmotic potential at full turgor, but did not increase in proportion to water stress. Genotypic differences have been observed for K+ accumulation capability, the water‐stressed leaves of cv. Capeiti 8 showing the largest increase. The same cultivar demonstrated the most negative ΔH values, indicative of strongly bound water, and the highest integrated enthalpy (ΔHinf) values for leaf moisture below 0.1 g H2O per g dry weight, i.e. in the isotherm region where water was presumably chemisorbed to the charged groups of macromolecules. The accumulation of ions (Cl−, K+) and proline was concurrent with an increase in the binding‐strength of tightly and weakly bound water, respectively.
Plasma membrane‐bound H+‐ATPase and reductase activities in Fe‐deficient cucumber roots
Authors: Rabotti Giuseppina , Zocchi Graziano 
Published: May 2006

Physiological and biochemical modifications induced by Fe‐deficiency have been studied in cucumber (Cucumis sativus L. cv. Marketer) roots, a Strategy I plant that initiates a rapid acidification of the medium and an increase in the electric potential difference when grown under Fe‐deficiency. Using the aqueous two‐phase partitioning method, a membrane fraction which has the plasmalemma characteristics was purified from roots of plants grown in the absence and in the presence of iron. The plasma membrane vesicles prepared from Fe‐deficient plants showed an H+‐ATPase activity (EC 3.6.1.35) that is twice that of the non‐deficient control. Furthermore, membranes from Fe‐deficient plants showed a higher capacity to reduce Fe3+‐chelates. The difference observed in the reductase activity was small with ferricyanide (only 30%) but was much greater with Fe3‐EDTA and Fe3‐citrate (210 and 250%, respectively). NADH was the preferred electron donor for the reduction of Fe3+ compounds. Fe3+ reduction in plasma membrane from cucumber roots seems to occur with utilisation of superoxide anion, since addition of superoxide dismutase (SOD; EC 1.15.1.1) “in vitro” decreased Fe3+ reduction by 60%.The response and the difference induced by iron starvation on these two plasma membrane activities together with a possible involvement of O2 in controlling the Fe3+/Fe2+ ratio in the rhizosphere are discussed.
NaCl‐induced changes in protoplasmic characteristics of Hordeum vulgare cultivars differing in salt tolerance
Authors: Mansour M. M. F. , Stadelmann E. J. 
Published: May 2006

Water permeability and cytoplasmic viscosity and streaming were investigated in seedlings of two Hordeum vulgare cultivars differing in salt tolerance. Six‐day‐old seedlings were grown for 4 additional days in Hoagland solution with and without 100 mM NaCl added.Observations and measurements were made in subepidermal cells of the coleoptile using plasmolytic and centrifugation methods and recordings of the speed of movement of microsomes.Water permeability was about the same in controls of both cultivars, and was decreased by NaCl stress, but decreased less in the tolerant cultivar. Cells from control plants of the stress tolerant variety had a higher cytoplasmic viscosity than cells from the moderately sensitive cultivar. Cytoplasmic viscosity in both cultivars decreased due to NaCl stress, and more so in the sensitive one. Cytoplasmic streaming was faster in the controls of the salt sensitive cultivar than in controls of the salt tolerant cultivar; NaCl had no significant effect on cytoplasmic streaming in both cultivars.The specific responses of the cytoplasm of the sensitive and tolerant cultivars to the salt treatment reflect differences in its structure and composition. These differences in the cytoplasm already exist before exposure to salt stress but some alterations of cytoplasmic parameters (e.g. water permeability) were induced by the saline environment.
Inhibition of the degradation of chloroplast membranes during senescence in nuclear ‘stay green’ mutants of soybean
Authors: Guiamét Juan J. , Giannibelli M. Cristina 
Published: May 2006

Near‐isogenic lines of soybean (Glycine max [L.] Merr.) cv. Clark carrying nuclear ‘stay green’ genes were examined to determine the effects of these genes on the breakdown of thylakoid membranes during senescence. In order to accelerate their senescence, mature leaves were excised and incubated in darkness for 7 days. The homozygous combination of the recessive alleles d1 and d2 (at two different nuclear loci), with or without G (a dominant allele in another locus that causes green seed coat) inhibited the loss of chlorophyll and thylakoid proteins during senescence. Electron micrographs of leaves of cv. Clark during the yellowing process showed chloroplasts in various stages of disintegration; their thylakoid network was disrupted and abundant osmiophilic globuli formed. These senescent leaves also showed evident signs of deterioration of the plasma membrane, including discontinuities, invaginations and membrane ‘whorls’. In contrast, leaves carrying d1d1d2d2 and GGd1d1d2d2 did not show signs of plasma membrane degradation, and their chloroplasts appeared intact, with a continuous, unbroken thylakoid network and tightly stacked grana.Exogenous applications of abscisic acid (1 and 10 μM), methyl jasmonate (10 μM) or ethylene (1 and 10 μl]−1) accelerated chlorophyll degradation in cv. Clark, but had no appreciable effect in d1d1d2d2 and GGd1d1d2d2, which indicates that their pheno‐types are not due to a deficiency in any of these hormones. The nuclear ‘stay green’ genotypes d1d1d2d2 and GGd1d1d2d2 exhibit a general incompetence for the degradation of chloroplast membranes and, thus, they may constitute useful tools in the study of the biochemistry and regulation of leaf senescence.
Photosynthesis in cold acclimated leaves of plants with various degrees of freezing tolerance
Authors: Bauer Helmut , Nagele Monika , Comploj Monica , Galler Verena , Mair Monika , Unterpertinger Edith 
Published: May 2006

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire different degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and – after determination of their frost resistance (LT50) – the net photosynthetic rate at an ambient CO2 of 33 Pa (Pn33), the dependencies of Pn to light and to CO2 and the room temperature chlorophyll a fluorescence were recorded under optimal conditions.Herbaceous plants acquired freezing tolerances to temperatures between −10 and −15°C when hardened at temperatures around 0°C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence after frost hardening. In non‐senescing leaves Pn33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around −10°C Brassica survived down to −24°C, but Pn33 was almost abolished as a result of disturbances in the chloroplasts. After transferring the plants to 20/15°C Pn33 recovered completely within a few days. Woody plants hardened at temperatures around 0°C tolerated – 15 to −36°C: Pn33 was reduced by 25 to 60% and hardly recovered at 20/15°C. Hardening at −10°C induced a tolerance of −32 to <−80°C. Pn33 was almost totally blocked, but at 20/15°C it returned to the values of the plants hardened at 0°C within a few days. In woody plants disturbances were invariably localized in the chloroplasts.Thus, conifers, and especially Pinus cembra, can survive much lower temperatures than herbaceous plants and, at the same level of freezing tolerance, exhibit appreciably less restriction in relative Pn33.
Action spectra of phenylalanine ammonia‐lyase and chalcone synthase expression in carrot cells in suspension
Authors: Takeda Junko , Obi Ichiro , Yoshida Kazuichi 
Published: May 2006

The fluence‐response relationships and action spectra of light‐induced mRNA accumulation of phenylalanine ammonia‐lyase (PAL, EC 4.3.1.5) and chalcone synthase (CHS, EC 2.3.1.74) were investigated in light‐responsive, suspension‐cultured carrot cells after 2,4‐D‐depletion.The action spectrum for PAL mRNA accumulation (determined 6 h after a 1‐h light treatment) had a single peak around 290 nm and was similar to that of anthocyanin formation. The action spectrum for CHS mRNA accumulation, determined simultaneously, had two peaks, a sharp peak at 290 nm and a broad peak at 330 to 360 nm. Below 320 nm, the peak wavelength of the spectrum and the shape of the fluence‐response curve for CHS were similar to those of anthocyanin formation. The Bunsen‐Roscoe law of reciprocity in the response to UV‐B light was verified for PAL transcript accumulation, but not for that of CHS, suggesting that the system leading to the expression after UV‐B light perception was different between PAL and CHS.
A morphological and cytological study of Petunia hybrida exposed to UV‐B radiation
Authors: Staxén Irina , Bornman Janet F. 
Published: May 2006

The aim of this study was to investigate whether the cytoskeleton, and in particular the microtubular system, is affected by enhanced levels of ultraviolet‐B (280–320 nm, 9 kJ m−2 day−1 biologically effective UV‐B radiation) radiation in epidermal cells of Petunia x hybrida Vilm, isolated from leaves of plants grown under UV‐B radiation and visible light. In addition, morphological changes during development were monitored. In a previous study microtubules were depolymerized and delays in the different stages of the cell cycle were found when protoplasts of Petunia were irradiated with UV‐B radiation (Staxén et al. 1993. Protoplasma 173: 70–76). Thus it was of interest to ascertain whether the cytoskeleton would be similarly affected in an intact system. Assuming an effect of UV‐B radiation on the microtubular system, we wished to determine whether this could be correlated to concomitant changes in leaf morphology. Plants of Petunia hybrida were grown in greenhouse conditions in the presence or absence of UV‐B radiation. During the course of the experiment, samples were taken from young, expanding leaves and from older, fully expanded leaves and prepared for localization and analysis of microtubules from the adaxial epidermal cells. Morphology rather than the cytoskeleton was affected by UV radiation, despite the fact that the epidermal cytoskeleton would most likely be affected, since it is located in the cells which form the first intercepting layer for incident radiation.Morphological changes under UV‐B radiation, as compared to those under control conditions, were reflected in earlier flowering and an increase in leaf number. Cell division was thus stimulated as was also evidenced from the increased leaf area. Our results indicate that the number of stomata differentiated on a leaf area basis was not altered although the number of stomata per epidermal cell was reduced.
Respiratory energy requirements and rate of protein turnover in vivo determined by the use of an inhibitor of protein synthesis and a probe to assess its effect
Authors: Bouma T. J. , De Visser R. , Janssen J.H.J.A. , De Kock M.J. , Van Leeuwen P.H. , Lambers H. 
Published: May 2006

Protein turnover is generally regarded as a major maintenance process, but experimental evidence to support this contention is scarce. Here we quantify the component of dark respiration rate associated with overall protein turnover of tissues in vivo. The effect of an inhibitor of cytosolic protein synthesis (cycloheximide, CHM) on dark respiration was tested on a cell suspension from potato (Solanum tuberosum L.) and quantified on leaf discs of expanding and full‐grown primary leaves of bean (Phaseolus vulgaris L.). The in vivo effect of CHM on protein biosynthesis was assessed by monitoring the inhibition of the induction of the ethylene‐forming enzyme (EFE) activity. The present method yields the energy costs of turnover of the total pool of proteins irrespective of their individual turnover rates. Average turnover rates were derived from the respiratory costs and the specific costs for turnover.Inhibition of respiration by CHM was readily detectable in growing‐cell suspensions and discs of expanding leaves, The derived respiratory costs of protein turnover in expanding leaves were maximally 17–37% of total respiration. Turnover costs in full‐grown primary leaves of bean amounted to 17–21% of total dark respiration. The maximum degradation constants (i.e. Kd‐values) derived for growing and full‐grown leaves were up to 2.42 × 10−6 and 1.12 × l0−6 s−1, respectively.
Occurrence of polygodial in plant organs and tissue culture of Polygonum hydropiper
Authors: Hagendoorn Marc J.M. , Geelen Threes A.M. , van Beek Teris A. , Jamar Diaan C.L. , Tetteroo Frans A.A. , van der Plas Linus H.W. 
Published: May 2006

Shoots of Polygonum hydropiper L. (waterpepper), especially in the leaves and flower‐heads, contain significant amounts of the sesquiterpenoid polygodial, a compound with a potential use as a natural pesticide. The polygodial content of the tepals is particularly high: up to 8.5% of the dry weight. Roots and seeds do not contain detectable amounts of polygodial. Polygodial containing organs e.g. leaves and tepals, were found to contain cavities. Fourier transform infra‐red (FTIR) microspectroscopy demonstrated that polygodial or its congeners are found in these cavities but not in other tissues or cells. Comparable cavities containing polygodial‐like compounds were absent in the closely related species Polygonum persicaria L.Callus cultures and cell suspensions as well as root‐ and shoot cultures were initiated from mature P. hydropiper plants. Polygodial could be detected only in shoot cultures. Our results indicate that functioning plastids may be essential for polygodial production and cavities for its accumulation.