In comparing a material's resistance to distort under mechanical load rather than to alter in volume, Poisson's ratio offers the fundamental metric by which to compare the performance of any material ...when strained elastically. The numerical limits are set by ½ and -1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poisson's ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poisson's ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt.
Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly ...governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson’s ratio (ν), whereas shear is favored at large ν. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15⩽ν⩽0.20; Semi-Resilient, for 0.20⩽ν⩽0.25; and Easily-Damaged for 0.25<ν<0.30. Radial-median cracks occur at low load (⩽50mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for ν (∼0.22 depending on the Young’s modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small.
Copper–lead and copper–zinc borate glasses with different copper contents were prepared and studied for correlations between structure and mechanical properties, where the cations are used to tailor ...the intermediate-range speciation of borate groups. Structural characterization was done by optical absorption, electron spin resonance (ESR), Raman and infrared (IR) spectroscopy. The mechanical properties were investigated through in-depth instrumented indentation and mechanical resonance analyses. The zinc metaborate glass series shows a high disproportionation of metaborate into mainly trigonal pyroborate B2O54− and polyborate BØ30 units, while the lead borate glass series exhibits a network based on trigonal BØ2O− and tetrahedral BØ4− metaborate units and a minor PbO-pseudophase (Ø=bridging oxygen and O=non-bridging oxygen). For both glass types, the addition of copper oxide results in a more homogenous network containing BØ2O− and BØ4− metaborate units. This induces an enhancement of the elastic moduli and hardness in the lead borate glass series, but a decrease of these properties in the copper–zinc borate glasses, whereby copper–zinc borate glasses are stiffer and harder than copper–lead borate glasses.
•Copper–lead and copper–zinc borate glasses are studied for relations between structure and mechanical properties.•Cations are used to tailor the intermediate-range speciation of borate groups.•Zinc metaborates show a high disproportionation tendency of the metaborate group.•In both glass types, the addition of copper oxide results in a more homogenous network of BØ2O- and BØ4-metaborate units.
Map-based cloning of avirulence genes of the AvrLm1-2-6 cluster was recently undertaken in Leptosphaeria maculans and led to the identification of AvrLm1. The ensuing chromosome walk toward AvrLm6 ...resulted in the delineation of a 562-kb bacterial artificial chromosome (BAC) clone contig in an avirulent isolate. Following sequencing of the contig and sequence comparison with a virulent isolate, four AvrLm6 candidate genes were identified. Complementation of the virulent isolate with the four candidates was performed and one gene was found to fully restore the avirulent phenotype on Rlm6 oilseed rape genotypes. AvrLm6 was found to be located in the same genome context as AvrLm1, because it is a solo gene surrounded by 85 and 48 kb of degenerated repeats on its 5(prime) and 3(prime) sides, respectively. AvrLm6 is an orphan gene encoding a small, potentially secreted, cysteine-rich protein. Comparison of AvrLm1 and AvrLm6 expressions by quantitative reverse-transcription polymerase chain reaction revealed that both genes are highly overexpressed during primary leaf infection. Using RNA interference, decreasing expression of AvrLm6 was shown to result in virulence toward Rlm6 genotypes whenever the expression was reduced by more than 60% compared with the wild-type isolate.
Abstract
A glass containing mechanoluminescent crystalline particles behaves as a photonic sponge: that is to say it fills up with trapped electrons when exposed to UV light, and it emits light when ...submitted to a mechanical loading, similar to a sponge soaked with water that is wringed under mechanical action! A major finding of the present study is that the elasto-mechanoluminescence effect showing up on unloading is governed by the deviatoric part of the applied stress (no effect under hydrostatic pressure). Furthermore, the structural source for this phenomenon was elucidated by a detailed density functional theory analysis of the e
−
energetics at the possible oxygen vacancy sites within the crystalline phase. Both the e
−
trapping and detrapping processes under load could be explained. An analogy with hydraulic circuits and the rheology of viscoelastic media was successfully introduced to pave the way to a constitutive law for the mechano-optical coupling phenomenon.
Leptosphaeria maculans is the fungus responsible for the stem canker disease of oilseed rape (Brassica napus). AvrLm3 and AvrLm4‐7, two avirulence effector genes of L. maculans, are involved in an ...unusual relationship: AvrLm4‐7 suppresses the Rlm3‐mediated resistance. Here, we assessed AvrLm3 polymorphism in a collection of 235 L. maculans isolates. No field isolates exhibited deletion or inactivating mutations in AvrLm3, as observed for other L. maculans avirulence genes. Eleven isoforms of the AvrLm3 protein were found. In isolates virulent towards both Rlm3 and Rlm7 (a3a7), the loss of the Rlm3‐mediated resistance response was due to two distinct mechanisms. First, when AvrLm4‐7 was inactivated (deletion or inactivating mutations), amino acid substitutions in AvrLm3 generated virulent isoforms of the protein. Second, when only point mutations were observed in AvrLm4‐7, a3a7 isolates still contained an avirulent allele of AvrLm3. Directed mutagenesis confirmed that some point mutations in AvrLm4‐7 were sufficient for the fungus to escape Rlm7‐mediated resistance while maintaining the suppression of the AvrLm3 phenotype. Signatures of positive selection were also identified in AvrLm3. The complex evolutionary mechanisms enabling L. maculans to escape Rlm3‐mediated resistance while preserving AvrLm3 integrity, along with observed reduced aggressiveness of isolates silenced for AvrLm3, serves to emphasize the importance of this effector in pathogenicity towards B. napus. While the common response to resistance gene pressure is local selection of isolates depleted in the cognate avirulence gene, this example contributes to complexify the gene‐for‐gene concept of plant‐pathogen evolution with a ‘camouflaged’ model allowing retention of nondispensable avirulence effectors.
The mechanical response of amorphous silica (or silica glass) under hydrostatic compression for very high pressures up to 25GPa is modelled via an elastic–plastic constitutive equation (continuum ...mechanics framework). The material parameters appearing in the theory have been estimated from the ex situ experimental data of Rouxel et al. Rouxel T, Ji H, Guin JP, Augereau F, Rufflé B. J Appl Phys 2010;107(9):094903. The model is shown to capture the major features of the pressure–volume response changes from the in situ experimental work of Sato and Funamori Sato T, Funamori N. Phys Rev Lett 2008;101:255502 and Wakabayashi et al. Wakabayashi D, Funamori N, Sato T, Taniguchi T. Phys Rev B 2011;84(14):144103. In particular, the saturation of densification, the increase in elasticity parameters (bulk, shear and Young’s moduli) and Poisson’s ratio are found to be key parameters of the model.
SUMMARY
Leptosphaeria maculans is the most ubiquitous pathogen of Brassica crops, and mainly oilseed brassicas (oilseed rape, canola), causing the devastating ‘stem canker’ or ‘blackleg’. This review ...summarizes our current knowledge on the pathogen, from taxonomic issues to specific life traits. It mainly illustrates the importance of formal genetics approaches on the pathogen side to dissect the interaction with the host plants. In addition, this review presents the main current research topics on L. maculans and focuses on the L. maculans genome initiative recently begun, including its main research issues.
Taxonomy: Leptosphaeria maculans (Desm.) Ces. & de Not. (anamorph Phoma lingam Tode ex Fr.). Kingdom Fungi, Phylum Ascomycota, Class Dothideomycetes (Loculoascomycetes), Order Pleosporales, Genus Leptosphaeria, Species maculans.
Host range: cultivated Brassicas such as Brassica napus (oilseed rape, canola), B. rapa, B. juncea, B. oleracea, etc., along with numerous wild crucifers species. Arabidopsis thaliana was recently reported to be a potential host for L. maculans. Primary disease symptoms are greyish‐green collapse of cotyledon or leaf tissue, without a visible margin, bearing tiny black spots (pycnidia). The fungus then develops an endophytic symptomless growth for many months. Secondary symptoms, at the end of the growing season, are dry necroses of the crown tissues with occasional blackening (stem canker or blackleg) causing lodging of the plants. Pseudothecia differentiate on leftover residues. Seedling damping‐off and premature ripening are also reported under certain environmental conditions.
Useful websites: Leptosphaeria maculans sequencing project at Genoscope: http://www.genoscope.cns.fr/externe/English/Projets/Projet_DM/organisme_DM.html; the SECURE site: http://www.secure.rothamsted.ac.uk/ the ‘Blackleg’ group at the University of Melbourne: http://www.botany.unimelb.edu.au/blackleg/overview.htm
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