131 research outputs found
Anatomia foliar comparativa de nove espécies do gênero Piper (Piperaceae).
As espécies de Piper são de grande interesse medicinal. Porém apresentam considerável desafio taxonômico, provavelmente pelo diminuto tamanho das partes florais. Por isso a morfologia externa da folha tem sido muito utilizada para taxonomia do grupo. Entretanto, há poucos trabalhos anatômicos para o gênero. Neste estudo, as folhas de nove espécies foram comparadas anatomicamente: Piper aduncum Vell.; P. cernuum Vell.; P. dilatatum Rich; P. gaudichaudianum Kunth; P. glabratum Kunth; P. hispidinervum C. DC.; P. lindbergii C. DC.; P. solmsianum C. DC. e P. umbellatum Jacq. O objetivo era avaliar o potencial dos caracteres anatômicos para separação de espécies. A maioria das espécies estudadas tem a epiderme do limbo constituÃda por células retangulares e arredondadas. Camadas subepidérmicas ocorrem em ambas à s faces do limbo, exceto em P. aduncum, P. cernuum e P. hispidinervum. Todas as espécies são hipoestomáticas, exceto P. hispidinervum. O mesofilo é dorsiventral na maioria das espécies, enquanto em P. solmsianum e P. umbellatum o mesofilo é homogêneo. O número de camadas dos tecidos paliçádico e esponjoso é variável. Também ocorrem variações no número e no tamanho dos feixes vasculares. Em geral, ocorrem células secretoras, idioblastos, tricomas tectores e glandulares, lipÃdios, compostos fenólicos e amido. A análise de agrupamento identificou três grupos distintos entre as espécies, com base nas caracterÃsticas anatômicas estudadas
Morfoanatomia foliar em mudas de Schinus terebinthifolius sob diferentes nÃveis de saturação hÃdrica.
As caracterÃsticas morfológicas e anatômicas foliares de espécies vegetais são importantes indicadores de sua ecologia e de seus hábitats. Objetivou-se caracterizar a plasticidade fenotÃpica de Schinus terebinthifolius Raddi em diferentes condições de saturação hÃdrica. As mudas foram produzidas em tubetes plásticos, permanecendo em estufa por quatro meses, sendo irrigadas normalmente. Em seguida foram submetidas aos tratamentos: T1- testemunha, T2- alagamento parcial e T3- alagamento total. Após três semanas sob alagamento, foram realizadas descrições anatômicas foliares comparativas e avaliadas as caracterÃsticas morfológicas área foliar, área especÃfica foliar, espessura foliar, teor de água e densidade estomática. Durante 10 semanas foram observadas as modificações fenotÃpicas adaptativas. Foi observado aumento da espessura da base do caule, clorose e abscisão foliar, surgimento de lenticelas e raÃzes adventÃcias. Após três semanas de alagamento, não foram verificadas grandes modificações na morfologia das folhas, principalmente para a AF e AEF. Em T3, o teor de água foi maior e a espessura foliar menor. Não houve diferença entre os tratamentos para DE. Quanto aos aspectos anatômicos, observou-se uma redução na espessura do mesofilo e das nervuras, assim como a produção de compostos fenólicos por alguns tipos celulares. Os espaços intercelulares são progressivamente mais amplos nas plantas submetidas ao alagament
Occlusion of Regulatory Sequences by Promoter Nucleosomes In Vivo
Nucleosomes are believed to inhibit DNA binding by transcription factors. Theoretical attempts to understand the significance of nucleosomes in gene expression and regulation are based upon this assumption. However, nucleosomal inhibition of transcription factor binding to DNA is not complete. Rather, access to nucleosomal DNA depends on a number of factors, including the stereochemistry of transcription factor-DNA interaction, the in vivo kinetics of thermal fluctuations in nucleosome structure, and the intracellular concentration of the transcription factor. In vitro binding studies must therefore be complemented with in vivo measurements. The inducible PHO5 promoter of yeast has played a prominent role in this discussion. It bears two binding sites for the transcriptional activator Pho4, which at the repressed promoter are positioned within a nucleosome and in the linker region between two nucleosomes, respectively. Earlier studies suggested that the nucleosomal binding site is inaccessible to Pho4 binding in the absence of chromatin remodeling. However, this notion has been challenged by several recent reports. We therefore have reanalyzed transcription factor binding to the PHO5 promoter in vivo, using ‘chromatin endogenous cleavage’ (ChEC). Our results unambiguously demonstrate that nucleosomes effectively interfere with the binding of Pho4 and other critical transcription factors to regulatory sequences of the PHO5 promoter. Our data furthermore suggest that Pho4 recruits the TATA box binding protein to the PHO5 promoter
Effect of promoter architecture on the cell-to-cell variability in gene expression
According to recent experimental evidence, the architecture of a promoter,
defined as the number, strength and regulatory role of the operators that
control the promoter, plays a major role in determining the level of
cell-to-cell variability in gene expression. These quantitative experiments
call for a corresponding modeling effort that addresses the question of how
changes in promoter architecture affect noise in gene expression in a
systematic rather than case-by-case fashion. In this article, we make such a
systematic investigation, based on a simple microscopic model of gene
regulation that incorporates stochastic effects. In particular, we show how
operator strength and operator multiplicity affect this variability. We examine
different modes of transcription factor binding to complex promoters
(cooperative, independent, simultaneous) and how each of these affects the
level of variability in transcription product from cell-to-cell. We propose
that direct comparison between in vivo single-cell experiments and theoretical
predictions for the moments of the probability distribution of mRNA number per
cell can discriminate between different kinetic models of gene regulation.Comment: 35 pages, 6 figures, Submitte
Stochastic Modeling for the Expression of a Gene Regulated by Competing Transcription Factors
It is widely accepted that gene expression regulation is a stochastic event. The common approach for its computer simulation requires detailed information on the interactions of individual molecules, which is often not available for the analyses of biological experiments. As an alternative approach, we employed a more intuitive model to simulate the experimental result, the Markov-chain model, in which a gene is regulated by activators and repressors, which bind the same site in a mutually exclusive manner. Our stochastic simulation in the presence of both activators and repressors predicted a Hill-coefficient of the dose-response curve closer to the experimentally observed value than the calculated value based on the simple additive effects of activators alone and repressors alone. The simulation also reproduced the heterogeneity of gene expression levels among individual cells observed by Fluorescence Activated Cell Sorting analysis. Therefore, our approach may help to apply stochastic simulations to broader experimental data
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