A down to earth model of gravisensing or Newton's Law of Gravitation from the apple's perspective

The physiology of gravity perception in plants is examined and a model of gravitational pressure is explained and compared to the statolith model. The gravitational pressure model is based on studies of tension and compression of the plasma membrane against the extracellular matrix. Further studies examine the role of peptides or enzymes that inhibit a compression receptor and calcium channels.     

Asymmetric competition between plant species

Despite extensive interest in the role of plant size in competition, few formal attempts have been made to quantify the magnitude of asymmetric competition, particularly for interactions between members of different species. This paper introduces the concept of asymmetric interspecific competition at the population livel (i.e. mean plant performance) in mixtures of species. It proposes an index of interspecific competitive asymmetry which allows for a progressively greater asymmetric effect as the average size differences between competing species increase, and allows for such an effect whether individuals of focal species are larger or smaller, on average, than competitors. This index of competitive asymmetry is evaluated in the study of interactions between two widely coexisting annuals of disturbed habitats, Stellaria media and Poa annua. An experiment was conducted in which the density, relative frequency and relative seedling sizes (emergence times) of Poa and Stellaria individuals were varied. The relative growth rate (RGR) for both species was measured over a 22-day period. An inverse linear model was fitted for each species, relating the RGR of the focal species to the initial biomass of each species. Each response model included an asymmetry coefficient () to assess whether the impact of a unit of initial biomass of the associate species changed with the relative sizes of seedlings of the two species. A zero value of implies symmetric competition between the two populations; i.e. the competitive effect of a unit of associate species biomass does not change with its initial seedling size. If is positive the smaller the initial relative size of seedlings of the associate species, the smaller their per unit biomass effect on the response of the focal species. The model fitted our data for Stellaria and Poa well and was validated by an alternative modelling approach. Asymmetry coefficients were estimated as 0.508 (P<0.05) for the effect of Poa in the Stellaria model, and 0.0001 (NS) for the effect of Stellaria in the Poa model; i.e. the effect of Poa on Stellaria was asymmetric while the effect of Stellaria on Poa was symmetric. Differences in interspecific species asymmetric competitive effects are discussed within the context of shoot architecture, and the relative importance of competition for light versus soil resources. Finally, we discuss the relationship of this model to earlier models of competitive asymmetry, and consider the implications of interspecific competitive asymmetry for a number of current theories of plant competition and community organisation.     

Side reactions in the synthesis of phosphotyrosine-containing peptides

Summary A series of phosphopeptides Tyr(PO₃H₂)-Val-Pro-Xxx-Leu (Xxx=Met, Met(O), Nle, Dab or Cys), derived from the native platelet-derived growth factor- receptor (PDGF-) sequence, has been prepared to study their interaction with the src-homology 2 (SH2) domains of the p85 subunit of PI3 kinase. The phosphopeptides were synthesized using Fmoc methodology incorporating N-Boc dibenzyl-protected phosphotyrosine (Boc-Tyr[PO₃(Bzl)₂]) as the N-terminal amino acid, since the benzyl groups can be removed during resin cleavage with TFA. Only peptides containing methionine were found to exist partially as S-benzyl sulfonium salts after TFA cleavage from the resin. The desired peptide could be obtained from the S-benzyl sulfonium salt by hydrogenolysis.     

The human glucagon receptor encoding gene: structure, cDNA sequence and chromosomal localization

Characterization of the human glucagon-receptor-encoding gene (GGR) should provide a greater understanding of blood glucose regulation and may reveal a genetic basis for the pathogenesis of diabetes. A cDNA encoding a complete functional human glucagon receptor (GGR) was isolated from a liver cDNA library by a combination of polymerase chain reaction and colony hybridization. The cDNA encodes a receptor protein with 80% identity to rat GGR that binds [¹²⁵I] glucagon and transduces a signal leading to increases in the concentration of intracellular cyclic adenosine 3′,5′-monophosphate. Southern blot analysis of human DNA reveals a hybridization pattern consistent with a single GGR locus. In situ hybridization to metaphase chromosome preparations maps the GGR locus to chromosome 17q25. Analysis of the genomic sequence shows that the coding region spans over 5.5 kb and is interrupted by 12 introns.     

A novel isotope labeling protocol for bacterially expressed proteins

Summary A novel protocol for isotopically labeling bacterially expressed proteins is presented. This method circumvents problems related to poor cell growth, commonly associated with the use of minimal labeled media, and problems with protein induction encountered, less commonly, when using enriched labeled media. The method involves initially growing the bacterial cells to high optical density in a commercially available enriched labeled medium. Following a suitable growth period, the cells are transferred to a different (minimal) labeled medium, appropriate for induction. The method is demonstrated using the protein melanoma growth stimulating activity (MGSA).     

Elevated CO2 and drought alter tissue water relations of birch (Betula populifolia Marsh.) seedlings

The effect of increasing atmospheric CO₂ concentrations on tissue water relations was examined in Betula populifolia, a common pioneer tree species of the northeastern U.S. deciduous forests. Components of tissue water relations were estimated from pressure volume curves of tree seedlings grown in either ambient (350 l l^–1) or elevated CO₂ (700 l l^–1), and both mesic and xeric water regimes. Both CO₂ and water treatment had significant effects on osmotic potential at full hydration, apoplasmic fractions, and tissue elastic moduli. Under xeric conditions and ambient CO₂ concentrations, plants showed a decrease in osmotic potentials of 0.15 MPa and an increase in tissue elastic moduli at full hydration of 1.5 MPa. The decrease in elasticity may enable plants to improve the soil-plant water potential gradient given a small change in water content, while lower osmotic potentials shift the zero turgor loss point to lower water potentials. Under elevated CO₂, plants in xeric conditions had osmotic potentials 0.2 MPa lower than mesic plants and decreased elastic moduli at full hydration. The increase in tissue elasticity at elevated CO₂ enabled the xeric plants to maintain positive turgor pressures at lower water potentials and tissue water contents. Surprisingly, the elevated CO₂ plants under mesic conditions had the most inelastic tissues. We propose that this inelasticity may enable plants to generate a favorable water potential gradient from the soil to the plant despite the low stomatal conductances observed under elevated CO₂ conditions.     

CO2-induced growth enhancements of co-occurring tree species decline at different rates

To elucidate how enriched CO₂ atmospheres, soil fertility, and light availability interact to influence the long-term growth of tree seedlings, six co-occurring members of temperate forest communities including ash (Fraxinus americana L.), gray birch (Betula populifolia), red maple (Acer rubrum), yellow birch (Betula alleghaniensis), striped maple (Acer pensylvanicum), and red oak (Quercus rubra L.) were raised in a glasshouse for three years in a complete factorial design. After three years of growth, plants growing in elevated CO₂ atmospheres were generally larger than those in ambient CO₂ atmospheres, however, magnitudes of CO₂-induced growth enhancements were contingent on the availability of nitrogen and light, as well as species identity. For all species, magnitudes of CO₂-induced growth enhancements after one year of growth were greater than after three years of growth, though species' growth enhancements over the three years declined at different rates. These results suggest that CO₂-induced enhancements in forest productivity may not be sustained for long periods of time. Additionally, species' differential growth responses to elevated CO₂ may indirectly influence forest productivity via long-term species compositional changes in forests.     

Interpreting the role of phosphorus and growth rate in enhanced fungal induction of sesquiterpenes from Hyoscyamus muticus root cultures

The effect of thiamine limitation in combination with fungal elicitation on sesquiterpene (solavetivone) production was studied in Agrobacterium-transformed hairy-root cultures of Hyoscyamus muticus as a potential means of manipulating the growth rate independent of phosphorus availability. Limiting the initial supply of thiamine did not affect the growth of these cultures compared to growth at the control level of thiamine (0.01 g/l). There was also no enhancement in sesquiterpene production when thiamine supply was limited. Serial culturing in thiamine-free media suggests that these root cultures are not strictly auxotrophic for thiamine, in contrast to previously published results for untransformed root culture. The effect of phosphate limitation combined with elicitation on the production of solavetivone was examined at constant media volume to provide a constant elicitor concentration and to eliminate feedback-inhibition effects. Limiting the initial supply of phosphate to elicited cultures resulted in a twofold increase in solavetivone production as compared to the elicitation at control media phosphate levels (1.1mm). Because growth was attenuated, production per unit cell mass increased 11-fold compared to the control. The effect of phosphate limitation on solavetivone production at constant cell mass and elicitor per root mass was studied. Limiting the initial supply of phosphate to elicited cultures under these conditions did not result in enhanced production of solavetivone. The initially observed enhanced production of solavetivone at limiting initial phosphate concentrations is therefore due to factors other than the growth rate or phosphate involvement in secondary metabolism. Correspondence to: W. R. Curtis     

Sequence analysis of sheep T-cell receptor β chains

The nucleotide sequences reported in this paper have been submitted to the GenBank database and have been assigned the accession numbers M94181-M94183.