Hyperbolic Modeling of Starburst Dendrimers

Starburst dendrimers are highly branched oligomers. A rigid dendritic hydrocarbon, C₁₁₃₄H₁₁₄₆, has recently been synthesized. It consists of 94 phenylacetylene units displayed in a self-similar two-dimensional skeleton isomorphous to the three-coordinated Bethe lattice. The three-dimensional representation of phenylacetylene dendrimer shows a globular architecture with large voids and niches in its interior, characteristic of hyperbolic surfaces. This work investigates the geometrical scaling behavior of this starburst dendrimer using the symmetry properties of a Bethe lattice embedded in the hyperbolic plane. The results for C₁₁₃₄H₁₁₄₆ provide its density profile and an upper bound for its macromolecular size. This revised version was published online in June 2006 with corrections to the Cover Date.     

Analysis of physiological and molecular changes in melon (Cucumis melo L.) varieties with different rates of ripening

Seven melon varieties (Alpha, Delada, Marygold, Sirio, Topper,Tornado, and Viva) known to exhibit differences in their ripeningbehaviour were used in this study. The expression of mRNAs forACC oxidase (MEL1) and phytoene synthase (MEL5), required forsynthesis of ethylene and carotenoids, respectively, and tworipening-related cDNAs (MEL2 and MEL7), of unknown function,was examined and correlated with the development of colour andsoftening of fruits. The MEL2 and MEL7 mRNAs were present andaccumulated in all varieties, indicating their importance inmelon fruit ripening. The fruits of Delada and Marygold didnot show any change in the colour of the flesh even at 50 daa(days after anthesis). All other varieties changed colour fromgreen to orange between 25–30 daa. The phytoene synthasemRNA levels in most varieties seemed to be unrelated to changein fruit flesh colour. The firmness of all the fruits was reducedsignificantly between 25 and 40 daa. The expression of ACC oxidasemRNA showed the most variation among the different varitiesand was delayed in Sirio and undetectable in Marygold fruitseven at 40 daa. Varieties with delayed expression of ACC oxidasemRNAs after anthesis also showed delayed softening during ripening.The prospects of genetic engineering and breeding for melonfruits with improved quality characteristics and extended storagelife are discussed. Key words: Cucumis melo, colour development, melon varieties, ripening genes, softening     

Delayed leaf senescence in ethylene-deficient ACC-oxidase antisense tomato plants: molecular and physiological analysis

To determine the role of ethylene during tomato (Lycopersicon esculentum Mill. cv. Alisa Craig) leaf senescence, transgenic ACC oxidase antisense plants were analysed. Northern analysis of wild-type plants indicated that ACC oxidase mRNA accumulation normally begins in pre-senescent green leaves but was severely reduced in the antisense plants. Although the levels of ethylene evolved by wild-type and transgenic leaves increased during the progression of senescence, levels were extremely low in transgenic leaves. Leaf senescence, as assessed by colour change from green to yellow, was clearly delayed by 10–14 days in the antisense plants when compared with wild-type plants. Northern analysis of the photosynthesis-associated genes, cab and rbcS, indicated that levels of the corresponding mRNAs were higher in transgenic leaves which were not yet senescing compared with senescing wild-type leaves of exactly the same age. Northern analysis using probes for tomato fruit ripening-related genes expressed during leaf senescence indicated that once senescence was initiated the expression pattern of these mRNAs was similar in transgenic and wild-type leaves. In the antisense plants chlorophyll levels, photosynthetic capacity and chlorophyll fluorescence were higher when compared with senescing wild-type plants of the same age. Photosynthetic capacity and the quantum efficiency of photosystem II were maintained for longer in the transformed plants at values close to those observed in wild-type leaves prior to the visible onset of senescence. These results indicate that inhibiting ACC oxidase expression and ethylene synthesis results in delayed leaf senescence, rather than inducing a stay-green phenotype. Once senescence begins, it progresses normally. Onset of senescence is not, therefore, related to a critical level of ethylene. The correlation between higher levels prior to senescence and early onset, however, suggests that ethylene experienced by the plant may be a significant contributing factor in the timing of senescence.     

The Viking mission: Implications for life on Mars

Summary The results of the Viking Biology experiments are best explained by non-biological phenomena: The interaction of the reagents with the materials comprising the regolith. Conditions of water activity, temperature, availability of carbon sources and others in most regions of the planet are too extreme for survival and growth of any known Earth microorganisms. Although the possibility persists that some very unusual form of life is somewhere on that planet the evidence is best interpreted as negative. Even though there is no evidence for current life on Mars, whether or not life ever originated there is not known.     

Effects of inorganic amendments (N,P and K) to soil on the rhizosphere microflora of antirrhinum plants infected withVerticillium dahliae Kleb

Summary A study of the inorganic amendments (N, P and K) to soil, and their effect on the rhizosphere microflora, as well as their relation to the control of wilt of antirrhinum plants caused byVerticillium dahliae Kleb. was done. Ammonium sulphate was the only chemical found to be significantly inhibitory toV. dahliae in vitro. Soil amendments (NPK) affected the rhizosphere microorganisms of the antirrhinum plants. Higher concentration of the chemicals were phytotoxic. It was further observed that ammonium sulphate, and the combined chemicals (NPK 25%) in soil delayed the senescence in healthy plants, suggests that chemical fertilisers affected the host plants directly. Addition of ammonium sulphate (0.25%), calcium nitrate (0.25%, 0.5%) combined NPK (0.25%) to soil caused considerable reduction in disease severity. It is assumed that this reduction may be caused by the (1) fungitoxic nature of the chemicali.e. ammonium sulphate, (2) antagonistic environment for the pathogen in the rhizosphere was boostedi.e. where calcium nitrate was added as soil amendments and (3) reduction in disease severity in soil-amended with combined NPK, may be due to the fact that antagonistic actinomycete population was boosted in the rhizosphere.     

Effects of organic amendments to soil on the rhizosphere microflora of antirrhinum infected withVerticillium dahliae Kleb.

Summary Organic (e.g. chitin, green manure, cellulose) amendments to soil induced quantitative and qualitative changes in the rhizosphere microflora of antirrhinum plants infected withVerticillium dahliae Kleb. Whereas reduction in disease severity occurred with chitin and green manure amendments, an increase in disease severity was observed with the application of cellulose. The reduction of the disease severity with chitin and green manure may be correlated with the increased population of actinomycetes in the antirrhinum rhizosphere.     

Molecular structure for microbial polysaccharides: conformation of the Klebsiella serotype K9 capsular polysaccharide

Fibre type X-ray diffraction patterns have been obtained from oriented, semicrystalline films prepared from the sodium salt form of the bacterial capsular polysaccharide of Klebsiella serotype K9. The molecule has a pentasaccharide repeating sequence, with four neutral residues in the backbone and a glucoronic acid side chain. A novel feature of the molecule is the incorporation of α-l-rhamnose residues, one 1,2 linked and two 1,3 linked in the backbone. Analysis of the X-ray diffraction results indicate an extended three-fold helical conformation with an axially projected chemical repeat of 1.377 nm. Both left and right handed helices have been examined using linked atom least squares techniques to optimize the stereochemistry while simultaneously meeting the observed helical parameters.     

An mRNA that specifically accumulates in maize roots delineates a novel subset of developing cortical cells

A near full-length cDNA clone (pZRP3) corresponding to an mRNA that accumulates specifically in roots of maize was isolated. The ZRP3 mRNA is ca. 600 nucleotides in length. The amino acid sequence of the predicted polypeptide is rich in leucine (16%), proline (11%), and cysteine (8.5%). The zrp3 gene appears to be expressed exclusively in roots, whereas other ZRP3-related genes are expressed in additional organs of the maize plant. In situ hybridization shows that ZRP3 mRNA accumulation is largely confined to the cells of the cortical ground meristem. Furthermore, accumulation of this mRNA occurs within a distinct subset of cortical cells, the inner three to four cell layers.Journal paper number J-14572 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa Project Number 2997.     

The “High Irradiance Responses” of plant photomorphogenesis

Plants growing in the natural environment are exposed daily to prolonged periods of high intensity irradiation. Many plant photomorphogenic responses are fully expressed only under prolonged exposures to high irradiances of light. The intensive study of these responses, the “High Irradiance Responses” (HIR) of plant photomorphogenesis, which started about 20 years ago, has been essentially directed—so far—toward the identification of the HIR photoreceptor, or photoreceptors, a problem that has not been satisfactorily and definitively solved, as yet. There is a great deal of evidence in support of the hypothesis that phytochrome, the pigment mediating the redfar red reversible plant photo-responses to low fluences of light, is involved in the photocontrol of the HIR. It seems likely that phytochrome may be the only photomorphogenic receptor responsible for the photocontrol of HIR responses brought about by irradiation at wavelengths longer than 600 nm. Phytochrome is probably also involved in the photocontrol of the HIR effects brought about by irradiation in the 350 to 500 nm region of the spectrum, but it cannot be excluded that other photochemical systems may also be involved. From a theoretical point of view, it does not seem unreasonable that the final expression of an HIR response may involve an interaction between phytochrome and other photochemical systems, with phytochrome probably playing the primary role and being responsible for the control of the activity of the other systems. Numerous “phytochrome only” interpretations (models) of the HIR have been proposed. Some of them have been developed to a fairly high degree of elaboration and have allowed the prediction of at least some of the features of the HIR. These “models,” although not rigorously and completely tested yet, seem to provide a reasonable interpretation for the HIR effects displayed under prolonged far red irradiation and for those HIR responses for which far red is the most effective spectral region. However, they do not provide a satisfactory explanation for the HIR responses for which blue is the most or the only effective spectral region, nor for the high effectiveness of white light. But, in spite of these problems, the “phytochrome only” interpretations of the HIR can be considered more satisfactory than those based on an interaction between phytochrome and other photochemical systems, especially in relation to the fact that the identity of these other photochemical systems has not been defined yet.