129 research outputs found
Experimental control of pattern formation by photonic lattices
We study the control of modulational instability and pattern formation in a nonlinear dissipative feedback
system with a periodic modulation of the material refractive index. We use a one-dimensional photonic lattice
in a single-mirror feedback configuration and identify three mechanisms for pattern control: bandgap
suppression of instability modes, periodicity induced pattern modes, and orientational pattern control.The authors acknowledge the support of the
Conseil RĂ©gional de Lorraine, the bilateral FrenchAustralian
Science and Technology program, and the
Australian Research Council through Discovery
projects
Roles of resonance and dark irradiance for infrared photorefractive self-focusing and solitons in bi-polar InP:Fe
This paper shows experimental evidence of photorefractive steady state
self-focusing in InP:Fe for a wide range of intensities, at both 1.06 and
1.55m. To explain those results, it is shown that despite the bi-polar
nature of InP:Fe where one photocarrier and one thermal carrier are to be
considered, the long standing one photocarrier model for photorefractive
solitons can be usefully applied. The relationship between the dark irradiance
stemming out of this model and the known resonance intensity is then discussed
Fast photorefractive self focusing in InP:Fe semiconductor at near infrared wavelengths
Self-trapping of optical beams in photorefractive (PR) materials at telecommunications wavelengths has been studied at steady state in insulators such as SBN [1] and in semiconductor InP:Fe [2], CdTe [3]. PR self-focusing and soliton interactions in semiconductors find interesting applications in optical communications such as optical routing and interconnections because of several advantages over insulators: their sensitivity to near-infrared wavelengths and shorter response time. Photorefractive self focusing in InP:Fe is characterized as a function of beam intensity and temperature. Transient self focusing is found to occur on two time scales for input intensities of tens of W/cm2 (one on the order of tens of ÎĽs, one on the order of milliseconds). A theory developed describes the photorefractive self focusing in InP:Fe and confirmed by steady state and transient regime measurements. PR associated phenomena (bending and self focusing) are taking place in InP:Fe as fast as a ÎĽs for intensities on the order of 10W/cm2 at 1.06 ÎĽm. Currently we are conducting more experiments in order to estimate the self focusing response time at 1.55ÎĽm, to clarify the temporal dynamic of the self focusing and to build up a demonstrator of fast optical routing by photorefractive spatial solitons interactions
Temporal behavior of two-wave-mixing in photorefractive InP:Fe versus temperature
The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc
electric field at different temperatures has been studied. In particular, the
temperature dependence of the characteristic time constant has been studied
both theoretically and experimentally, showing a strongly decreasing time
constant with increasing temperature
Experimental control of steady state photorefractive self-focusing in InP:Fe at infrared wavelengths
Dominant Negative Mutants of Bacillus thuringiensis Cry1Ab Toxin Function as Anti-Toxins: Demonstration of the Role of Oligomerization in Toxicity
BACKGROUND:Bacillus thuringiensis Cry toxins, that are used worldwide in insect control, kill insects by a mechanism that depends on their ability to form oligomeric pores that insert into the insect-midgut cells. These toxins are being used worldwide in transgenic plants or spray to control insect pests in agriculture. However, a major concern has been the possible effects of these insecticidal proteins on non-target organisms mainly in ecosystems adjacent to agricultural fields. METHODOLOGY/PRINCIPAL FINDINGS:We isolated and characterized 11 non-toxic mutants of Cry1Ab toxin affected in different steps of the mechanism of action namely binding to receptors, oligomerization and pore-formation. These mutant toxins were analyzed for their capacity to block wild type toxin activity, presenting a dominant negative phenotype. The dominant negative phenotype was analyzed at two levels, in vivo by toxicity bioassays against susceptible Manduca sexta larvae and in vitro by pore formation activity in black lipid bilayers. We demonstrate that some mutations located in helix alpha-4 completely block the wild type toxin activity at sub-stoichiometric level confirming a dominant negative phenotype, thereby functioning as potent antitoxins. CONCLUSIONS/SIGNIFICANCE:This is the first reported case of a Cry toxin dominant inhibitor. These data demonstrate that oligomerization is a fundamental step in Cry toxin action and represent a potential mechanism to protect special ecosystems from the possible effect of Cry toxins on non-target organisms
Binding Site Alteration Is Responsible for Field-Isolated Resistance to Bacillus thuringiensis Cry2A Insecticidal Proteins in Two Helicoverpa Species
Background
Evolution of resistance by target pests is the main threat to the long-term efficacy of crops expressing Bacillus thuringiensis (Bt) insecticidal proteins. Cry2 proteins play a pivotal role in current Bt spray formulations and transgenic crops and they complement Cry1A proteins because of their different mode of action. Their presence is critical in the control of those lepidopteran species, such as Helicoverpa spp., which are not highly susceptible to Cry1A proteins. In Australia, a transgenic variety of cotton expressing Cry1Ac and Cry2Ab (Bollgard II) comprises at least 80% of the total cotton area. Prior to the widespread adoption of Bollgard II, the frequency of alleles conferring resistance to Cry2Ab in field populations of Helicoverpa armigera and Helicoverpa punctigera was significantly higher than anticipated. Colonies established from survivors of F2 screens against Cry2Ab are highly resistant to this toxin, but susceptible to Cry1Ac.
Methodology/Principal Findings
Bioassays performed with surface-treated artificial diet on neonates of H. armigera and H. punctigera showed that Cry2Ab resistant insects were cross-resistant to Cry2Ae while susceptible to Cry1Ab. Binding analyses with 125I-labeled Cry2Ab were performed with brush border membrane vesicles from midguts of Cry2Ab susceptible and resistant insects. The results of the binding analyses correlated with bioassay data and demonstrated that resistant insects exhibited greatly reduced binding of Cry2Ab toxin to midgut receptors, whereas no change in 125I-labeled-Cry1Ac binding was detected. As previously demonstrated for H. armigera, Cry2Ab binding sites in H. punctigera were shown to be shared by Cry2Ae, which explains why an alteration of the shared binding site would lead to cross-resistance between the two Cry2A toxins.
Conclusion/Significance
This is the first time that a mechanism of resistance to the Cry2 class of insecticidal proteins has been reported. Because we found the same mechanism of resistance in multiple strains representing several field populations, we conclude that target site alteration is the most likely means that field populations evolve resistance to Cry2 proteins in Helicoverpa spp. Our work also confirms the presence in the insect midgut of specific binding sites for this class of proteins. Characterizing the Cry2 receptors and their mutations that enable resistance could lead to the development of molecular tools to monitor resistance in the [email protected]; [email protected]
Relação entre toxicidade de proteĂnas Vip3Aa e sua capacidade de ligação a receptores intestinais de lepidĂłpteros-praga
An Environmental Impact Statement: Designing Supportive Literacy Classrooms for Young Children
The physical environment of the classroom can be a powerful tool in support of literacy learning or an unrecognized and undirected influence (Loughlin and Martin, 1987). Teachers who organize, arrange, and dress up their classrooms intuitively understand that, Every home, every classroom, every school contains a certain atmosphere (Van Manen, 1986, p. 31). Although peripherally accepted as an important part of literacy instruction for many years, too little attention has been focused on what the literacy environment of the classroom brings to children and their learning. Recent research by Neuman and Roskos (1990, 1992) demonstrates a clear relationship between the quality of classroom environments and literacy related behaviors and learning
Enhanced complexity of optical chaos in a laser diode with phase-conjugate feedback
International audienceWe demonstrate numerically that a semiconductor laser subjected to phase-conjugate feedback (PCF) can exhibit an enhancement in the complexity of chaos by comparison to conventional optical feedback. Using quantifiers from spectral analysis and information theory, we demonstrate that under similar parametric conditions, PCF exhibits a larger chaotic bandwidth and higher spectral flatness and statistical complexity. These properties are of utmost importance for applications in secure communications and random number generation
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