Nucleotide sugars and starch synthesis in spadix of Arum maculatum and suspension cultures of Glycine max

The aim of this work was to determine the relative contributions of ADPglucose and UDPglucose to starch synthesis in two non-photosynthetic tissues, the developing club of the spadix of Arum maculatum and suspension cultures of Glycine max. Rates of starch accumulation during growth are compared with estimates of the maximum catalytic activities in vitro of ADPglucose starch synthase, ADPglucose pyrophosphorylase, UDPglucose pyrophosphorylase and UDPglucose starch synthase. The latter could only be measured at high concentrations (10–30 mM) of UDPglucose. Clubs of Arum and cells of Glycine contained 292 and 6.8 nmol UDPglucose per gram fresh weight, respectively. The corresponding figures for ADPglucose were 29 and 0.4. From the above data it is argued that in both Arum club and Glycine cells the activity of UDPglucose starch synthase is too low to make any quantitatively significant contribution to starch synthesis. The activities of ADPglucose starch synthase and pyrophosphorylase were high enough to mediate the observed rates of starch accumulation. It is suggested that starch synthesis in these tissues is via ADPglucose.     

Cell surface binding of colloidal iron and Limax flavus agglutinin compared by X-ray microanalysis.

The surface characteristics of tumour cells and T-cell hybridoma variants were examined by quantitative X-ray microanalysis using metal-based probes. Comparisons of populations of cells differing in invasive potential revealed differences in the binding of positive colloidal iron hydroxide, a general probe for anionic sites. Substantial differences between populations were also detected by the binding of Limax flavus agglutinin, which was used in conjunction with fetuin-gold as a specific marker for sialic acid.     

Specific binding of anti-myosin and -actin γ-globulins to the surface of trypsin-dissociated embryonic chick cells

Summary ¹²⁵I-labelled sheep anti-rabbit -globulin antibodies were used to locate rabbit antibodies to smooth- and striated-muscle actomyosins at the surface of trypsin-dissociated embryonic chick cells. Statistical analysis of electron microscope autoradiographs revealed that the plasma membrane of these cells was significantly labelled with both antibodies. Further tests revealed that there were a significantly greater number of antigenic sites present on the cell surface for the gizzard smooth-muscle antibodies than for those against pectoralis striated-muscle actomyosin.It was further shown that both the rate and extent of binding of the ¹²⁵Ilabelled smooth-muscle actomyosin antibodies to the cells were greater than for anti-striated-muscle -globulins. Binding of the former was reduced to a level similar to that of ¹²⁵I-NIS conjugate by preincubation of the y-globulins with smooth-muscle heavy meromyosin, while a similar reduction was observed when anti-pectoralis actomyosin was treated with actin.It was concluded that actin- and myosin-like proteins must now be considered as integral components of the plasma membrane.The authors wish to thank Dr. W. Sinclair (Zoology) and Miss S. Lutkins (Statistics Department) for assistance with the statistical analysis and are grateful to Professor N. A. Mitchison (Zoology Department, University College London) for providing a control sample of ¹²⁵I-labelled sheep anti-rabbit -globulin, Dr. D. Catty (Experimental Pathology Department, Birmingham University) for donating sheep anti-rabbit serum and Dr. U. Gröschel-Stewart (Zoologisches Institut der TH., Darmstadt, Federal Republic of Germany) for the rabbit anti-actomyosin antibodies. Miss B. Morris and Messrs. P. C. Lloyd, D. Williams and J. Meredith gave skilled technical assistanceThis investigation was supported by grants from Science Research Council, Cancer Research Campaign and Deutsche Forschungsgemeinschaft.     

Pathway of starch breakdown in photosynthetic tissues of Pisum sativum.

1. The aim of this work was to discover the pathway of starch breakdown in the photosynthetic tissues of Pisum sativum. 2. Measurements of the starch in the leaves of plants grown in photoperiods of 12 or 18 h showed that starch, synthesized in the light, was rapidly metabolized in the dark at rates of 0.04--0.06 mumol glucose/min per g fresh weight. 3. The maximum catalytic activities of alpha-amylase, beta-amylase, hexokinase, alpha-glucan phosphorylase and phosphoglucomutase in extracts of leaves showed no diurnal variation in either photoperiod, and exceeded estimates of the rate of net starch breakdown in the dark. 4. Studies with intact chloroplasts, isolated from young shoots and from leaves, indicated that pea chloroplasts do not contain significant activities of alpha-amylase, beta-amylase and hexokinase, although some of the latter may be attached to the outside of the chloroplast envelope. These studies also showed that pea chloroplasts contained sufficient alpha-glucan phosphorylase and phosphoglucomutase to mediate the observed rates of starch breakdown. 5. It is proposed that starch breakdown in pea chloroplasts is phosphorolytic.     

The localisation of lead in the skin of light- and dark-adapted Xenopus laevis

Summary Toads pretreated for 2 months on either a dark or a light background were then exposed to lead nitrate at 50 ppm lead for 21 days, the illumination regimes being maintained. Metal analysis of dorsal skin showed significantly higher lead levels (p<0.01) in dark-adapted toads. No precipitated lead deposits were observed at the ultrastructural level, necessitating X-ray microanalysis of sections containing melanophores, gland cells and general (non-melanophore) cytoplasm. Analysis showed the lead to be concentrated within the melanosomes of the melanophores, and to be significantly higher (p<0.01) in individual melanosomes of dark-adapted toads than in light-adapted ones. Copper was also found to be concentrated in the melanosomes and was higher (p<0.01) in the melanosomes of the dark-adapted toads.The results are consistent with the known affinity of melanin for heavy metals and the documented increase in melanophore number under prolonged dark background regimes. Since all toads received the same lead exposure, the melanosome results give rise to speculation that higher melanin levels might occur in individual melanosomes of dark-adapted skin.     

Resistance is costly: trade-offs between immunity, fecundity and survival in the pea aphid

Parasitoids are among the most important natural enemies of insects in many environments. Acyrthosiphon pisum, the pea aphid, is a common pest of the leguminous crops in temperate regions. Pea aphids are frequently attacked by a range of endoparasitic wasps, including the common aphidiine, Aphidius ervi. Immunity to parasitoid attack is thought to involve secondary symbiotic bacteria, the presence of which is associated with the death of the parasitoid egg. It has been suggested that there is a fecundity cost of resistance, as individuals carrying the secondary symbionts associated with parasitoid resistance have fewer offspring. Supporting this hypothesis, we find a positive relationship between fecundity and susceptibility to parasitoid attack. There is also a negative relationship between fecundity and off-plant survival time (which positively correlates with resistance to parasitoid attack). Taken together, these results suggest that the aphids can either invest in defence (parasitoid resistance, increased off-plant survival time) or reproduction, and speculate that this may be mediated by changes in the aphids' endosymbiont fauna. Furthermore, there is a positive relationship between aphid size and resistance, suggesting that successful resistance to parasitoid attack may involve physical, as well as physiological, defences.     

The effect of sublethal lead exposure on the ultrastructure and on the distribution of acid phosphatase activity in chloragocytes of earthworms (Annelida, Oligochaeta)

Summary Laboratory experiments were conducted to study the effects of the exposure to a sublethal concentration (500 p.p.m.) of lead on the ultrastructure and acid phosphatase compartmentalization of the chloragogenous tissue of earthworms,Eisenia foetida. For the cytochemical demonstration of acid phosphatase activity, lead and cerium were used as capturing agents. In both cases there was a change in the compartmentalization of acid phosphatase, the enzyme activity being localized within the chloragosomes in controls, but distributed throughout the cytosol in treated animals. In addition, acid phosphatase activity increased following lead exposure. At the ultrastructural level, disruption of the chloragosomal membranes, an increase in chloragosomal fusion processes and vesiculation of the cytoplasm were evident. Moreover, an enhanced release of chloragosomes to the extracellular space was found in lead-exposed worms.     

Lysosomal origin of the chloragosomes in the chloragogenous tissue of the earthworm Eisenia foetida: cytochemical demonstration of acid phosphatase activity

Summary The cytochemical localization of the lysosomal marker enzyme acid phosphatase was studied in the chloragogenous tissue of earthworms. The Gomori lead technique and the cerium capture technique were utilized. Both techniques demonstrated the chloragosomal location of this enzyme. Only a small proportion of chloragosomes presented reactivity, which suggests that these organelles are distinctly heterogeneous. The reaction product was localized in the periphery of chloragosomes, suggesting a membrane-bound compartmentalization of acid phosphatase. In addition, degenerating mitochondria and membrane whorls were observed in some chloragosomes, indicating the possibility that these organelles perform autophagosomal functions.     

Metabolic control analysis of plant metabolism

Metabolic control analysis and its major coefficients are introduced. The importance of measuring both elasticity and concentration-control coefficients as well as flux-control coefficients is stressed. The conditions that need to be met before control analysis can be applied experimentally are emphasized. It is argued that successful application of this approach requires methods for the measurement of flux, maximum catalytic activities of enzymes and substrate contents. The measurement of flux by consumption of substrate, production of product, and the distribution of isotope after metabolism of labelled substrates is discussed. The need to ensure that measurements of enzymes and substrates are reliable and authenticated is stressed. Particular emphasis is placed on the ease with which such measurements can be invalidated in plant tissues, and ways for countering these difficulties are discussed.     

Sequence of the 18S-5S ribosomal gene region and the cytochrome oxidase II gene from mtDNA of Zea diploperennis

Summary The coding and flanking sequences of the 18S-5S ribosomal RNA genes and the cytochrome oxidase subunit II gene of Zea diploperennis mitochondrial DNA have been determined and compared to the corresponding sequences of normal maize (Zea mays L.) Both length and substitution mutations are found in the coding region of the 18S rRNA gene, whereas only one substitution mutation is found in the coding region of cytochrome oxidase II. Sequence divergence between maize and Zea diploperennis is about one-tenth of that between wheat and maize. The rate of nucleotide divergence by base substitution is less for plant mitochrondrial genes than for comparable genes in animal mitochondria.