Towards an understanding of photosynthetic acclimation

It has long been recognized that higher plants vary the composition and organization of the photosynthetic apparatus in response to the prevailing environmental conditions, with particular attention being paid to the responses to incident light. Under high light conditions there are increases in the amounts of photosystems, electron transport and ATP synthase complexes, and enzymes of the Calvin-Benson cycle; conversely, under low light there is an increase in the relative amounts of light-harvesting complexes (LHC) and in the stacking of thylakoid membranes to form grana. It is believed that these changes are of adaptive significance, and in a few instances evidence has been provided that this is indeed the case; an increase in photosynthetic capacity reduces susceptibility to photodamage, while changes in photosystem stoichiometry serve to optimize light utilization. By contrast, the potential benefit to the plant of other changes in chloroplast composition, such as in the levels of LHC, is far less clear. It is also believed that redox signals derived from photosynthetic electron transport play an important regulatory role in acclimation. However, while there is convincing evidence that such redox signals modulate the expression of many plastidic and nuclear genes encoding photosynthetic components, there is little to demonstrate that such changes are responsible for regulating chloroplast composition. This review discusses the evidence that particular aspects of acclimation are advantageous to the plant, and highlights the significant gaps in our understanding of the mechanisms underlying acclimation.     

Towards an understanding of the behavior of silanes on glass: an atomistic simulation study of glass surfaces

The present work attempts to develop a better understanding of the glass substrates upon which silane molecules are deposited. Experimentally, when a sodium silicate substrate is used, the silane layer does not bind as well compared to an alkaline earth borosilicate glass (AEBS). Molecular dynamics simulated annealing procedures were used to create atomistic models of these glasses and their surfaces. In sodium silicate glass, we see segregation of sodium ions to the surface and formation of sodium-rich regions. For the AEBS glass, we see a more homogenous distribution of ions. Boron, aluminium, silicon and oxygen form a connected framework structure. The barium and calcium ions are distributed throughout the structure and show no evidence of segregation. We speculate the poor performance of sodium silicate, compared to the AEBS, as a silane substrate could be attributed to its compositionally inhomogeneous surface.     

Endopolyploidy: Towards an understanding of its biological significance

There is a certain measure of perplexity concerning the significance of endopolyploidy. It seems that this results from a narrow frame of reference from which investigators view the phenomenon; that is, a predilection for emphasizing the specialized functional aspect of endopolyploidy as it operates in species at the present time overrides any consideration of the rôle that this state may play in the life of a species in its encounter with the forces of natural selection either in the past or in the future.There does not seem to be any obvious relationship between the degree of endopolyploidy that a species can exhibit and either its basic DNA content or the structure of its nucleus. The significance of endopolyploidy may reside not so much in any specialized function that the condition can support, but rather in the properties that are consequent upon the endopolyploid condition itself and which are distinct from those that apply to diploid cells. Some of the properties of the endopolyploid state, and examples of their manifestation in plants and animals, are discussed. The conclusion is that these properties have a potential that opens possibilities for new paths of development and serves as a factor upon which natural selection can operate.     

Towards an understanding of position effect variegation

Most variegating position effects are a consequence of placing a euchromatic gene adjacent to alpha-heterochromatin. In such rearrangements, the affected locus is inactivated in some cells, but not others, thereby giving rise to a mosaic tissue of mutant and wild-type cells. A detailed examination of the molecular structure of three variegating white mottled mutations of Drosophila melanogaster, all of which are inversions of the X chromosome, reveals that their euchromatic breakpoints are clustered and located approximately 25 kb downstream of the white promoter and that the heterochromatic sequences to which the white locus is adjoined are transposons. An analysis of three revertants of the wm4 mutation, created by relocating white to another euchromatic site, demonstrates that they also carry some heterochromatically derived sequences with them upon restoration of the wild-type phenotype. This suggests that variegation is not controlled from a heterochromatic sequence immediately adjacent to the variegating gene but rather from some site more internal to the heterochromatic domain itself. As a consequence of this observation we have proposed a boundary model for understanding how heterochromatic domains may be formed. It has been recognized for many years that the phenotype of variegating position effects may be altered by the presence of trans-acting dominant mutations that act to either enhance or suppress variegation. Using P-element mutagenesis, we have induced and examined 12 dominant enhancers of variegation that represent four loci on the second and third chromosomes. Most of these mutations are cytologically visible duplications or deficiencies. They exert their dominant effects through changes in the copy number of wild-type genes and can be divided into two reciprocally acting classes. Class I modifiers are genes that act as enhancers of variegation when duplicated and as suppressors when mutated or deficient. Conversely, class II modifiers are genes that enhance when mutated or deleted and suppress when duplicated. The available data indicate that, in Drosophila, there are 20-30 loci capable of dominantly modifying variegation. Of these, most appear to be of the class I type whereas only two class II modifiers have been identified so far.(ABSTRACT TRUNCATED AT 400 WORDS)     

Towards an understanding of dative objects in Basque: a logistic regression analysis

Most work on case marking has focussed on the standard or ??default?? case patterns, however more recent work has examined the marginal instances where non-standard cases are used to mark core grammatical relations. In this paper I investigate the use of logistic regression as a tool for validating competing analyses proposed to account for one such case pattern in Basque: transitive clauses where the object bears dative case. Several explanations for this dative marking have been proposed, appealing to notions such as telicity, animacy, and the ??person?? of the subject/object. To evaluate these different proposals a database of naturally occurring sentences was created from existing corpora and coded for these different possible variables. Following Bresnan et al (In: Bouma G, Kraemer I, Zwarts J (eds) Cognitive foundations of interpretation. Royal Netherlands Academy of Science, Amsterdam, 2007) and others, a logistic regression model was fit to the data using these predictor variables to ascertain the most important factors determining the use of the dative case for Basque objects.     

Towards an understanding of the mechanics underlying aortic dissection

Acute aortic dissection and associated aortic catastrophes are among the most devastating forms of cardiovascular disease, with a remarkably high morbidity and mortality despite current medical and surgical treatment. The mechanics underlying aortic dissection are incompletely understood, and a further understanding of the relevant fluid and solid mechanics may yield not only a better appreciation of its pathogenesis, but also the development of improved diagnostic and therapeutic strategies. After illustrating some of the inadequacies with respect to the extant work on the mechanics of aortic dissection, we alternatively postulate that the clinical hemodynamic disturbances that render the aorta susceptible to the initiation of dissection are principally elevated maximum systolic and mean aortic blood pressure, whereas the hemodynamic disturbances that facilitate propagation of dissection are principally elevated pulse pressure and heart rate. Furthermore, abnormal aortic mechanical properties and/or geometry are requisite for dissection to occur. Specifically, we propose that the degree of anisotropy will directly influence the probability of future aortic dissection. Imaging of the aorta may provide information regarding aortic anisotropy and geometry, and in combination with a hemodynamic risk assessment, has the potential to be able to prospectively identify patients at high risk for future aortic dissection thereby facilitating prophylactic intervention. The aim of the paper is to identify the main mechanical issues that have a bearing on aortic dissection, and to suggest an appropriate mathematical model for describing the problem.     

Towards an ecological understanding of biological nitrogen fixation

Biogeochemistry - N limitation to primary production and other ecosystem processes is widespread. To understand the causes and distribution of N limitation, we must understand the controls of...     

Towards a molecular understanding of cytokinesis

In this review, we focus on recent discoveries regarding the molecular basis of cleavage furrow positioning and contractile ring assembly and contraction during cytokinesis. However, some of these mechanisms might have different degrees of importance in different organisms. This synthesis attempts to uncover common themes and to reveal potential relationships that might contribute to the biochemical and mechanical aspects of cytokinesis. Because the information about cytokinesis is still fairly rudimentary, our goal is not to present a definitive model but to present testable hypotheses that might lead to a better mechanistic understanding of the process.     

Towards an understanding of microtubule function and cell organization: an overview.

Microtubules exhibit dynamic instability, converting abruptly between assembly and disassembly with continued growth dependent on the presence of a tubulin-GTP cap at the plus end of the organelle. Tubulin, the main structural protein of microtubules, is a heterodimer composed of related polypeptides termed alpha-tubulin and beta-tubulin. Most eukaryotic cells possess several isoforms of the alpha- and beta-tubulins, as well as gamma-tubulin, an isoform restricted to the centrosome. The isoforms of tubulin arise either as the products of different genes or by posttranslational processes and their synthesis is subject to regulation. Tubulin isoforms coassemble with one another and isoform composition does not appear to determine whether a microtubule is able to carry out one particular activity or another. However, the posttranslational modification of polymerized tubulin may provide chemical signals which designate microtubules for a certain function. Microtubules interact with proteins called microtubule-associated proteins (MAPs) and they can be divided into two groups. The structural MAPs stimulate tubulin assembly, enhance microtubule stability, and influence the spatial distribution of microtubules within cells. The dynamic MAPs take advantage of microtubule polarity and organization to vectorially translocate cellular components. The interactions between microtubules and MAPs contribute to the structural-functional integration that characterizes eukaryotic cells.     

Towards an understanding of semaphorin signalling in the spinal cord

Semaphorins are a large family of proteins that are classically associated with axon guidance. These proteins and their interacting partners, the neuropilins and plexins are now known to be key mediators in a variety of processes throughout the nervous system ranging from synaptic refinement to the correct positioning of neuronal and glial cell bodies. Recently, much attention has been given to the roles semaphorins play in other body tissues including the immune and vascular systems. This review wishes to draw attention back to the nervous system, specifically focusing on the role of semaphorins in the development of the spinal cord and their proposed roles in the adult. In addition, their functions in spinal cord injury at the glial scar are also discussed.     

Towards an understanding of the role of forces in carcinogenesis: a perspective with therapeutic implications.

Carcinogenesis may aberrantly have as its source an underlying dynamic which likely operates in accord with or is expressive of a universal principle of force. This drive can be seen as a type of necessary completion of force configurations giving forth a force-based, completing guidance or dyneotaxis. This completing guidance is a completing communication. Aberrations of this drive can manifest as incomplete communication arising from incomplete or non-uniform force-configurations throughout different levels of organization within the organism. Such non-uniformities of force-configuration manifest themselves through intra- and intercellular distortion. This is represented by changes or distortions within cellular and tissue architecture, or shape, occurring through and enabling a non-uniform loosening. Cycles of non-uniform loosening and distortion would generate a progressive disruption or aberration in an otherwise further adhesively completing, thereby further stabilizing, dyneotaxis. Carcinogenesis ensues as an aberrant means to re-establish a uniformity or symmetry of adhesive and mechanical forces in such tissues, and within the genome. However, it rather results in a further loosening-based reorganization or reshaping. This would be a manifestation of a still further non-uniformity of forces. Uncontrolled mutagenesis, itself stemming from genomic distortion, is also seen as a process that could lead to loosening and further distortion throughout the cell, and thereby, a contributor to carcinogenesis if a transcellular, globally completive or uniform adhesiveness cannot be maintained. A therapeutic approach could be employed to reverse such a cycle of loosening and distortion within cellular and tissue organization. The developmental reversal of carcinogenesis would be the consequence. Such an approach could be through the application of a coherent, uniform and united array of guiding mechanical and electromagnetic forces. The deeper consequence would be the re-establishment of a global, cancer-countering, uniform or symmetrical, guiding adhesiveness via and dependent on a complete and non-distorted network of cellular and intercellular structures, further completing, and thus further stabilizing, such structures in the process.