Mechanical characterization of disordered and anisotropic cellular monolayers

We consider a cellular monolayer, described using a vertex-based model, for which cells form a spatially disordered array of convex polygons that tile the plane. Equilibrium cell configurations are assumed to minimize a global energy defined in terms of cell areas and perimeters; energy is dissipated via dynamic area and length changes, as well as cell neighbour exchanges. The model captures our observations of an epithelium from a Xenopus embryo showing that uniaxial stretching induces spatial ordering, with cells under net tension (compression) tending to align with (against) the direction of stretch, but with the stress remaining heterogeneous at the single-cell level. We use the vertex model to derive the linearized relation between tissue-level stress, strain and strain-rate about a deformed base state, which can be used to characterize the tissue's anisotropic mechanical properties; expressions for viscoelastic tissue moduli are given as direct sums over cells. When the base state is isotropic, the model predicts that tissue properties can be tuned to a regime with high elastic shear resistance but low resistance to area changes, or vice versa.Comment: 9 figure

Molecular genetic tools for manipulation of the oleaginous yeast Rhodotorula toruloides

Rhodotorula (Rhodosporidium) toruloides is an oleaginous basidiomycete yeast with great biotechnological potential. Capable of accumulating lipid up to 76 % of its dry biomass and well suited to the metabolism of lignocellulosic hydrolysate, it is a good candidate for production of advanced biofuels as well as a host of other potential roles in industry. However, molecular genetic tools for manipulation of this yeast are lacking and its high genomic GC content can make routine cloning difficult. Agrobacterium tumefaciens-mediated transformation of R. toruloides CBS 14 was demonstrated, and plasmid vectors were developed for transformation of R. toruloides, including elements for Saccharomyces cerevisiae in-yeast assembly. In-yeast assembly is robust to the manipulation of GC-rich DNA and of large plasmids. Using these vectors and an EGFP reporter, a screen to identify inducible promoters was performed, and promoters from the genes NAR1, ICL1, CTR3, and MET16 identified. These promoters have independent induction/repression conditions and different levels and rates of induction. Minimal inducible promoters were determined, which are as small as 200 bp. As well as showing tight regulation of the EGFP marker, the NAR1 promoter was able to drive conditional rescue of a leu2 mutant strain. In parallel, as a proof of principle for production of advanced biofuels, hydrocarbon biosynthesis pathways were expressed in R. toruloides and analysed by GC-MS. After co-expression of Synechococcus elongatus fatty acyl-ACP reductase and fatty aldehyde decarbonylase, and E. coli ferredoxin and ferredoxin reductase, production of the alkane heptadecane was observed. To increase the availability of free fatty acids (FFA) for production of hydrocarbons by other pathways, Thermomyces lanuginosus lipase 2 was expressed, resulting in a 1.3-fold increase in the concentration of FFAs.BBSR

Validation of a Diagnostic Marker for Primocane-Fruiting in Blackberry

Typical blackberries (Rubus subgenus Rubus) have perennial crowns and roots and biennial canes. The first-year canes (primocanes) are usually vegetative, while second-year canes (floricanes) produce fruit. Primocane-fruiting blackberries produce fruit on first-year canes and are desirable to growers because they potentially allow for a longer harvest season in temperate regions and enable production in tropical areas where no natural chill hours are accumulated. The development of molecular markers for desirable traits can potentially increase efficiency in blackberry breeding. However, to date, there are no diagnostic molecular markers for economically important traits in blackberries. Primocane-fruiting is recessively inherited, and tetraploid blackberries must have four copies of the primocane allele for the trait to be expressed. A single locus strongly associated with primocane fruiting was recently identified on chromosome Ra03, and a new KASP marker (PF2) was developed within this locus. In this study, we validated the performance of the new PF2 marker in a seedling population (Population 1937) segregating for primocane-fruiting at the University of Arkansas System Division of Agriculture\u27s Fruit Breeding Program. In 2022, 170 seedlings in the population were evaluated for primocane-fruiting. Of these seedlings, 68 were floricane-fruiting, 86 were primocane-fruiting, and 16 could not be phenotyped due to poor plant growth. The PF2 marker correctly predicted the phenotype for 146 of 154 progeny that were scored in the 1937 population. Some of the inconsistencies between the marker prediction and the observed phenotypes could have been due to weak plants shaded out by neighbors or human error

Validation of a Diagnostic Marker for Primocane-Fruiting in Blackberry

Typical blackberries (Rubus subgenus Rubus) have perennial crowns and roots and biennial canes. The first-year canes (primocanes) are usually vegetative, while second-year canes (floricanes) produce fruit. Primocane-fruiting blackberries produce fruit on first-year canes and are desirable to growers because they potentially allow for a longer harvest season in temperate regions and enable production in tropical areas where no natural chill hours are accumulated. The development of molecular markers for desirable traits can potentially increase efficiency in blackberry breeding. However, to date, there are no diagnostic molecular markers for economically important traits in blackberries. Primocane-fruiting is recessively inherited, and tetraploid blackberries must have four copies of the primocane allele for the trait to be expressed. A single locus strongly associated with primocane fruiting was recently identified on chromosome Ra03, and a new KASP marker (PF2) was developed within this locus. In this study, we validated the performance of the new PF2 marker in a seedling population (Population 1937) segregating for primocane-fruiting at the University of Arkansas System Division of Agriculture\u27s Fruit Breeding Program. In 2022, 170 seedlings in the population were evaluated for primocane-fruiting. Of these seedlings, 68 were floricane-fruiting, 86 were primocane-fruiting, and 16 could not be phenotyped due to poor plant growth. The PF2 marker correctly predicted the phenotype for 146 of 154 progeny that were scored in the 1937 population. Some of the inconsistencies between the marker prediction and the observed phenotypes could have been due to weak plants shaded out by neighbors or human error

What's so Hot about Electrons in Metal Nanoparticles?

Metal nanoparticles are excellent light absorbers. The absorption processes create highly excited electron-hole pairs and recently there has been interest in harnessing these hot charge carriers for photocatalysis and solar energy conversion applications. The goal of this Perspectives article is to describe the dynamics and energy distribution of the charge carriers produced by photon absorption, and the implications for the photocatalysis mechanism. We will also discuss how spectroscopy can be used to provide insight into the coupling between plasmons and molecular resonances. In particular, the analysis shows that the choice of material and shape of the nanocrystal can play a crucial role in hot electron generation and coupling between plasmons and molecular transitions. The detection and even calculation of many-body hot-electron processes in the plasmonic systems with continuous spectra of electrons and short lifetimes are challenging, but at the same time very interesting from the point of view of both potential applications and fundamental physics. We propose that developing an understanding of these processes will provide a pathway for improving the efficiency of plasmon-induced photocatalysis.Comment: To be published in ACS Energy. 41 pages and 8 figures, including Supplementary Informatio

Implications of solar flare hard X-ray "knee" spectra observed by RHESSI

We analyse the RHESSI photon spectra of four flares that exhibit significant deviations from power laws - i.e. changes in the "local" Hard X-ray spectral index. These spectra are characterised by two regions of constant power law index connected by a region of changing spectral index - the "knee". We develop theoretical and numerical methods of describing such knees in terms of variable photon spectral indices and we study the results of their inversions for source mean thin target and collisional thick target injection electron spectra. We show that a particularly sharp knee can produce unphysical negative values in the electron spectra, and we derive inequalities that can be used to test for this without the need for an inversion to be performed. Such unphysical features would indicate that source model assumptions were being violated, particularly strongly for the collisional thick target model which assumes a specific form for electron energy loss. For all four flares considered here we find that the knees do not correspond to unphysical electron spectra. In the three flares that have downward knees we conclude that the knee can be explained in terms of transport effects through a region of non-uniform ionisation. In the other flare, which has an upward knee, we conclude that it is most likely a feature of the accelerated spectrum

Functional Expression of Parasite Drug Targets and Their Human Orthologs in Yeast

BACKGROUND: The exacting nutritional requirements and complicated life cycles of parasites mean that they are not always amenable to high-throughput drug screening using automated procedures. Therefore, we have engineered the yeast Saccharomyces cerevisiae to act as a surrogate for expressing anti-parasitic targets from a range of biomedically important pathogens, to facilitate the rapid identification of new therapeutic agents. METHODOLOGY/PRINCIPAL FINDINGS: Using pyrimethamine/dihydrofolate reductase (DHFR) as a model parasite drug/drug target system, we explore the potential of engineered yeast strains (expressing DHFR enzymes from Plasmodium falciparum, P. vivax, Homo sapiens, Schistosoma mansoni, Leishmania major, Trypanosoma brucei and T. cruzi) to exhibit appropriate differential sensitivity to pyrimethamine. Here, we demonstrate that yeast strains (lacking the major drug efflux pump, Pdr5p) expressing yeast ((Sc)DFR1), human ((Hs)DHFR), Schistosoma ((Sm)DHFR), and Trypanosoma ((Tb)DHFR and (Tc)DHFR) DHFRs are insensitive to pyrimethamine treatment, whereas yeast strains producing Plasmodium ((Pf)DHFR and (Pv)DHFR) DHFRs are hypersensitive. Reassuringly, yeast strains expressing field-verified, drug-resistant mutants of P. falciparum DHFR ((Pf)dhfr (51I,59R,108N)) are completely insensitive to pyrimethamine, further validating our approach to drug screening. We further show the versatility of the approach by replacing yeast essential genes with other potential drug targets, namely phosphoglycerate kinases (PGKs) and N-myristoyl transferases (NMTs). CONCLUSIONS/SIGNIFICANCE: We have generated a number of yeast strains that can be successfully harnessed for the rapid and selective identification of urgently needed anti-parasitic agents

On the origin of ionising photons emitted by T Tauri stars

We address the issue of the production of Lyman continuum photons by T Tauri stars, in an attempt to provide constraints on theoretical models of disc photoionisation. By treating the accretion shock as a hotspot on the stellar surface we show that Lyman continuum photons are produced at a rate approximately three orders of magnitude lower than that produced by a corresponding black body, and that a strong Lyman continuum is only emitted for high mass accretion rates. When our models are extended to include a column of material accreting on to the hotspot we find that the accretion column is extremely optically thick to Lyman continuum photons. Further, we find that radiative recombination of hydrogen atoms within the column is not an efficient means of producing photons with energies greater than 13.6eV, and find that an accretion column of any conceivable height suppresses the emission of Lyman continuum photons to a level below or comparable to that expected from the stellar photosphere. The photospheric Lyman continuum is itself much too weak to affect disc evolution significantly, and we find that the Lyman continuum emitted by an accretion shock is similarly unable to influence disc evolution significantly. This result has important consequences for models which use photoionisation as a mechanism to drive the dispersal of circumstellar discs, essentially proving that an additional source of Lyman continuum photons must exist if disc photoionisation is to be significant.Comment: 6 pages, 4 figures. Accepted for publication in MNRA