Mechanisms and function of flower and inflorescence reversion

Flower and inflorescence reversion involve a switch from floral development back to vegetative development, thus rendering flowering a phase in an ongoing growth pattern rather than a terminal act of the meristem. Although it can be considered an unusual event, reversion raises questions about the nature and function of flowering. It is linked to environmental conditions and is most often a response to conditions opposite to those that induce flowering. Research on molecular genetic mechanisms underlying plant development over the last 15 years has pinpointed some of the key genes involved in the transition to flowering and flower development. Such investigations have also uncovered mutations which reduce floral maintenance or alter the balance between vegetative and floral features of the plant. How this information contributes to an understanding of floral reversion is assessed here. One issue that arises is whether floral commitment (defined as the ability to continue flowering when inductive conditions no longer exist) is a developmental switch affecting the whole plant or is a mechanism which assigns autonomy to individual meristems. A related question is whether floral or vegetative development is the underlying default pathway of the plant. This review begins by considering how studies of flowering in Arabidopsis thaliana have aided understanding of mechanisms of floral maintenance. Arabidopsis has not been found to revert to leaf production in any of the conditions or genetic backgrounds analysed to date. A clear-cut reversion to leaf production has, however, been described in Impatiens balsamina. It is proposed that a single gene controls whether Impatiens reverts or can maintain flowering when inductive conditions are removed, and it is inferred that this gene functions to control the synthesis or transport of a leaf-generated signal. But it is also argued that the susceptibility of Impatiens to reversion is a consequence of the meristem-based mechanisms controlling development of the flower in this species. Thus, in Impatiens, a leaf-derived signal is critical for completion of flowering and can be considered to be the basis of a plant-wide floral commitment that is achieved without accompanying meristem autonomy. The evidence, derived from in vitro and other studies, that similar mechanisms operate in other species is assessed. It is concluded that most species (including Arabidopsis) are less prone to reversion because signals from the leaf are less ephemeral, and the pathways driving flower development have a high level of redundancy that generates meristem autonomy even when leaf-derived signals are weak. This gives stability to the flowering process, even where its initiation is dependent on environmental cues. On this interpretation, Impatiens reversion appears as an anomaly resulting from an unusual combination of leaf signalling and meristem regulation. Nevertheless, it is shown that the ability to revert can serve a function in the life history strategy (perenniality) or reproductive habit (pseudovivipary) of many plants. In these instances reversion has been assimilated into regular plant development and plays a crucial role there.

Convergence analysis of stochastic diffusion search

In this paper we present a connectionist searching technique - the Stochastic Diffusion Search (SDS), capable of rapidly locating a specified pattern in a noisy search space. In operation SDS finds the position of the pre-specified pattern or if it does not exist - its best instantiation in the search space. This is achieved via parallel exploration of the whole search space by an ensemble of agents searching in a competitive cooperative manner. We prove mathematically the convergence of stochastic diffusion search. SDS converges to a statistical equilibrium when it locates the best instantiation of the object in the search space. Experiments presented in this paper indicate the high robustness of SDS and show good scalability with problem size. The convergence characteristic of SDS makes it a fully adaptive algorithm and suggests applications in dynamically changing environments.

The potential mechanisms involved in the anti-carcinogenic action of probiotics

Probiotic bacteria are live microbial food ingredients that provide a health benefit to the consumer. In the past it was suggested that they served to benefit the host primarily through the prevention of intestinal infections. More recent studies have implicated probiotic bacteria in a number of other beneficial effects within the host including: *The suppression of allergies. *Control of blood cholesterol levels. *Modulation of immune function. *And the prevention of cancers of the colon. The reputed anti-carcinogenic effect of probiotics arises from in vivo studies in both animals and to a limited extent in man; this evidence is supported by in vitro studies with carcinoma cell lines and anti-mutagenicity assays. However, the mechanisms involved in any effect have thus far been difficult to elucidate; studies offer evidence for a variety of mechanisms; we have reviewed these and come to the opinion that, the anti-carcinogenic effect may not be attributable to a single mechanism but rather to a combination of events not yet fully elucidated or understood.

Optimal convergence estimates for the trace of the polynomial L2-projection operator on a simplex

In this paper we study convergence of the L2-projection onto the space of polynomials up to degree p on a simplex in Rd, d >= 2. Optimal error estimates are established in the case of Sobolev regularity and illustrated on several numerical examples. The proof is based on the collapsed coordinate transform and the expansion into various polynomial bases involving Jacobi polynomials and their antiderivatives. The results of the present paper generalize corresponding estimates for cubes in Rd from [P. Houston, C. Schwab, E. Süli, Discontinuous hp-finite element methods for advection-diffusion-reaction problems. SIAM J. Numer. Anal. 39 (2002), no. 6, 2133-2163].

Towards a resolution of ‘the paradox of the plankton’: a brief overview of the proposed mechanisms

In plankton ecology, it is a fundamental question as to how a large number of competing phytoplankton species coexist in marine ecosystems under a seemingly-limited variety of resources. This ever-green question was first proposed by Hutchinson [Hutchinson, G.E., 1961. The paradox of the plankton. Am. Nat. 95, 137–145] as ‘the paradox of the plankton’. Starting from Hutchinson [Hutchinson, G.E., 1961. The paradox of the plankton. Am. Nat. 95, 137–145], over more than four decades several investigators have put forward varieties of mechanisms for the extreme diversity of phytoplankton species. In this article, within the boundary of our knowledge, we review the literature of the proposed solutions and give a brief overview of the mechanisms proposed so far. The proposed mechanisms that we discuss mainly include spatial and temporal heterogeneity in physical and biological environment, externally imposed or self-generated spatial segregation, horizontal mesoscale turbulence of ocean characterized by coherent vortices, oscillation and chaos generated by several internal and external causes, stable coexistence and compensatory dynamics under fluctuating temperature in resource competition, and finally the role of toxin-producing phytoplankton in maintaining the coexistence and biodiversity of the overall plankton population that we have proposed recently. We find that, although the different mechanisms proposed so far is potentially applicable to specific ecosystems, a universally accepted theory for explaining plankton diversity in natural waters is still an unachieved goal.

Kinetic and mechanistic investigation of the ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution

The ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution was investigated by determining the major reaction products and their evolution as a function of the reaction time and their dependence on the pH of the reaction system. 2,4-Dimethyl-nitrobenzene and 2,4-dimethyl-phenol were found to be primary reaction products; their formation might be explained by electron transfer and substitution reactions. 2,4-Dimethyl-phenol was further oxidized yielding 2,4-dimethyl- and/or 4,6-dimethyl-resorcinol by electrophilic addition of HO(center dot) radicals. The best fitting phenomenological kinetic model and the good convergence of calculated and experimentally determined rate constants imply two additional competitive pathways of substrate oxidation: (i) electrophilic addition of HO(center dot) radicals and fast subsequent substitution would also yield the resorcinol derivatives. (ii) Substrate and isolated products are thought to be oxidized by hydrogen abstraction at the benzylic sites, but the corresponding products (alcohols, aldehydes, and carboxylic acids) could not be identified. Fe(II) was added to probe for the presence of H(2)O(2), but had no or only a minor effect on the kinetics of the ozonolysis. (c) 2009 Elsevier B.V. All rights reserved.

Convergence analysis of sampling-based decomposition methods for risk-averse multistage stochastic convex programs

We consider a class of sampling-based decomposition methods to solve risk-averse multistage stochastic convex programs. We prove a formula for the computation of the cuts necessary to build the outer linearizations of the recourse functions. This formula can be used to obtain an efficient implementation of Stochastic Dual Dynamic Programming applied to convex nonlinear problems. We prove the almost sure convergence of these decomposition methods when the relatively complete recourse assumption holds. We also prove the almost sure convergence of these algorithms when applied to risk-averse multistage stochastic linear programs that do not satisfy the relatively complete recourse assumption. The analysis is first done assuming the underlying stochastic process is interstage independent and discrete, with a finite set of possible realizations at each stage. We then indicate two ways of extending the methods and convergence analysis to the case when the process is interstage dependent.

Stability and convergence analysis of a neural model applied in nonlinear systems optimization

A neural model for solving nonlinear optimization problems is presented in this paper. More specifically, a modified Hopfield network is developed and its internal parameters are computed using the valid-subspace technique. These parameters guarantee the convergence of the network to the equilibrium points that represent an optimal feasible solution. The network is shown to be completely stable and globally convergent to the solutions of nonlinear optimization problems. A study of the modified Hopfield model is also developed to analyze its stability and convergence. Simulation results are presented to validate the developed methodology.

Convergence and Asymmetry in Sanitary and Phytosanitary Measures in the Region's Trade Agreements

Sanitary and phytosanitary matters have acquired greater significance in the region's trade, as reflected in the significant number of complaints brought before the various dispute settlement mechanisms pertaining to the regional integration schemes. This may be attributed to the importance of the Latin American countries in world agricultural trade and to different phytosanitary and zoosanitary standards required by each. Given the multiplication of bilateral and plurilateral agreements in Latin America and the Caribbean, convergence on the sanitary standards required under such accords is crucial for the trade integration of a region that is an agro-exporter par excellence. Convergence is essential to facilitate market access and expedite trade flows. This bulletin assesses convergence of standards in the bilateral and plurilateral trade agreements signed by the countries of the region, the treatment afforded to the principles contained in the World Trade Organization (WTO) Agreement on the Application of Sanitary and Phytosanitary Measures (SPS Agreement) and the progress the region has made relative to that Agreement.

A model of the roles of essential kinases in the induction and expression of late long-term potentiation.

The induction of late long-term potentiation (L-LTP) involves complex interactions among second-messenger cascades. To gain insights into these interactions, a mathematical model was developed for L-LTP induction in the CA1 region of the hippocampus. The differential equation-based model represents actions of protein kinase A (PKA), MAP kinase (MAPK), and CaM kinase II (CAMKII) in the vicinity of the synapse, and activation of transcription by CaM kinase IV (CAMKIV) and MAPK. L-LTP is represented by increases in a synaptic weight. Simulations suggest that steep, supralinear stimulus-response relationships between stimuli (e.g., elevations in [Ca(2+)]) and kinase activation are essential for translating brief stimuli into long-lasting gene activation and synaptic weight increases. Convergence of multiple kinase activities to induce L-LTP helps to generate a threshold whereby the amount of L-LTP varies steeply with the number of brief (tetanic) electrical stimuli. The model simulates tetanic, -burst, pairing-induced, and chemical L-LTP, as well as L-LTP due to synaptic tagging. The model also simulates inhibition of L-LTP by inhibition of MAPK, CAMKII, PKA, or CAMKIV. The model predicts results of experiments to delineate mechanisms underlying L-LTP induction and expression. For example, the cAMP antagonist RpcAMPs, which inhibits L-LTP induction, is predicted to inhibit ERK activation. The model also appears useful to clarify similarities and differences between hippocampal L-LTP and long-term synaptic strengthening in other systems.

The Immune Inhibitor A1 Protease of Bacillus anthracis

Bacillus anthracis, an organism ubiquitous in the soil and the causative agent of anthrax, utilizes multiple mechanisms to regulate secreted factors; one example is the activity of secreted proteases. One of the most abundant proteins in the culture supernates of B. anthracis is the Immune Inhibitor A1 (InhA1) protease. Here, I demonstrate that InhA1 modulates the abundance of approximately half of the proteins secreted into the culture supernates, including substrates that are known to contribute to the ability of the organism to cause virulence. For example, InhA1 cleaves the anthrax toxin proteins, PA, LF, and EF. InhA1 also targets a number of additional proteases, including Npr599, contributing to a complex proteolytic regulatory cascade with far-reaching affects on the secretome. Using an intra-tracheal mouse model of infection, I found that an inhA-null strain is attenuated in relation to the parent strain. The data indicate that reduced virulence of the inhA mutant strain may be the result of toxin protein deregulation, decreased association with macrophages, and/or the inability to degrade host antimicrobial peptides. Given the significant modulation of the secretome by InhA1, it is likely that expression of the protease is tightly regulated. To test this I examined inhA1 transcript and protein levels in the parent and various isogenic mutant strains and found that InhA1 expression is regulated by several mechanisms. First, the steady state levels of inhA1 transcript are controlled by the regulatory protein SinR, which inhibits inhA1 expression. Second, InhA1 abundance is inversely proportional to the SinR-regulated protease camelysin, indicating the post-transcriptional regulation of InhA1 by camelysin. Third, InhA1 activity is dependent on a conserved zinc binding motif, suggesting that zinc availability regulates InhA1 activity. The convergence of these regulatory mechanisms signifies the importance of tight regulation of InhA1 activity, activity that substantially affects how B. anthracis interacts with its environment.

Dissection of floral pollination syndromes in petunia

Animal-mediated pollination is essential in the reproductive biology of many flowering plants and tends to be associated with pollination syndromes, sets of floral traits that are adapted to particular groups of pollinators. The complexity and functional convergence of various traits within pollination syndromes are outstanding examples of biological adaptation, raising questions about their mechanisms and origins. In the genus Petunia, complex pollination syndromes are found for nocturnal hawkmoths (P. axillaris) and diurnal bees (P. integrifolia), with characteristic differences in petal color, corolla shape, reproductive organ morphology, nectar quantity, nectar quality, and fragrance. We dissected the Petunia syndromes into their most important phenotypic and genetic components. They appear to include several distinct differences, such as cell-growth and cell-division patterns in the basal third of the petals, elongation of the ventral stamens, nectar secretion and nectar sugar metabolism, and enzymatic differentiation in the phenylpropanoid pathway. In backcross-inbred lines of species-derived chromosome segments in a transposon tagging strain of P. hybrida, one to five quantitative trait loci were identified for each syndrome component. Two loci for stamen elongation and nectar volume were confirmed in introgression lines and showed large allelic differences. The combined data provide a framework for a detailed understanding of floral syndromes from their developmental and molecular basis to their impact on animal behavior. With its molecular genetic tools, this Petunia system provides a novel venue for a pattern of adaptive radiation that is among the most characteristic of flowering plants.

Three Mechanisms for the Calcium Alleviation of Mineral Toxicities

Ca2+ in rooting medium is essential for root elongation, even in the absence of added toxicants. In the presence of rhizotoxic levels of Al3+, H+, or Na+ (or other cationic toxicants), supplementation of the medium with higher levels of Ca2+ alleviates growth inhibition. Experiments to determine the mechanisms of alleviation entailed measurements of root elongation in wheat (Triticum aestivum L. cv Scout 66) seedlings in controlled medium. A Gouy-Chapman-Stern model was used to compute the electrical potentials and the activities of ions at the root-cell plasma membrane surfaces. Analysis of root elongation relative to the computed surface activities of ions revealed three separate mechanisms of Ca2+ alleviation. Mechanism I is the displacement of cell-surface toxicant by the Ca2+-induced reduction in cell-surface negativity. Mechanism II is the restoration of Ca2+ at the cell surface if the surface Ca2+ has been reduced by the toxicant to growth-limiting levels. Mechanism III is the collective ameliorative effect of Ca2+ beyond mechanisms I and II, and may involve Ca2+-toxicant interactions at the cell surface other than the displacement interactions of mechanisms I and II. Mechanism I operated in the alleviation of all of the tested toxicities; mechanism II was generally a minor component of alleviation; and mechanism III was toxicant specific and operated strongly in the alleviation of Na+ toxicity, moderately in the alleviation of H+ toxicity, and not at all in the alleviation of Al3+ toxicity.

Uniparental inheritance of mitochondrial and chloroplast genes: mechanisms and evolution.

In nearly all eukaryotes, at least some individuals inherit mitochondrial and chloroplast genes from only one parent. There is no single mechanism of uniparental inheritance: organelle gene inheritance is blocked by a variety of mechanisms and at different stages of reproduction in different species. Frequent changes in the pattern of organelle gene inheritance during evolution suggest that it is subject to varying selective pressures. Organelle genes often fail to recombine even when inherited biparentally; consequently, their inheritance is asexual. Sexual reproduction is apparently less important for genes in organelles than for nuclear genes, probably because there are fewer of them. As a result organelle sex can be lost because of selection for special reproductive features such as oogamy or because uniparental inheritance reduces the spread of cytoplasmic parasites and selfish organelle DNA.

Mechanisms underlying the sensitivity of songbird forebrain neurons to temporal order.

Neurons in the songbird forebrain area HVc (hyperstriatum ventrale pars caudale or high vocal center) are sensitive to the temporal structure of the bird's own song and are capable of integrating auditory information over a period of several hundred milliseconds. Extracellular studies have shown that the responses of some HVc neurons depend on the combination and temporal order of syllables from the bird's own song, but little is known about the mechanisms underlying these response properties. To investigate these mechanisms, we recorded intracellular responses to a set of auditory stimuli designed to assess the degree of dependence of the responses on temporal context. This report provides evidence that HVc neurons encode information about temporal structure by using a variety of mechanisms including syllable-specific inhibition, excitatory postsynaptic potentials with a range of different time courses, and burst-firing nonlinearity. The data suggest that the sensitivity of HVc neurons to temporal combinations of syllables results from the interactions of several cells and does not arise in a single step from afferent inputs alone.