Quantitative detection of _Potato virus Y_ in potato plants and aphids - Discussion of diverse applications in potato research

Every year potato growers worldwide complain about severe yield losses caused by _Potato virus Y_ (PVY). Therefore, PVY along with _Potato leafroll virus_ belongs to the most important potato viruses. There are three main strains of PVY: PVY^O^, PVY^N^ and PVY^C^. However, also recombinant forms exist such as PVY^N^Wilga and PVY^NTN^, both of which increase in importance due to their potential to displace the non-recombinant strains at a high percentage. They appear also in mixed infections. In recent years PCR and qPCR assays were developed to differentiate PVY isolates. In order to identify PVY isolates by PCR often large amplicons have to be generated which requires the input of expensive enzymes. On the other hand, qPCR assays until now do not allow the differentiation between PVY^N^Wilga and PVY^NTN^. 

For the discrimination between PVY^O^/PVY^N^Wilga and PVY^N^/PVY^NTN^ a qPCR assay was developed, which allows the differentiation and highly efficient quantification of both strains and recombinants, respectively. For this purpose dual-labeled hydrolysis probes tagged with different fluorophores were designed. The assay is suitable for many different applications, for example safety research on genetically modified (GM) potato plants. The goal of this research is to determine whether genetic modification causes changes in resistance to viruses. Two different GM cultivars were examined for signs of altered resistance to an infection with PVY in comparison to their near-isogenic lines and three reference cultivars. Reference cultivars are included to determine the baselines for resistance and thus to be able to decide if the changes could represent a biological risk. The plants to be investigated were mechanically inoculated with PVY^N^Wilga or PVY^NTN^ and analyzed by means of the developed assay after two weeks. The results of the experiment indicate that the differences in virus titer between the reference cultivars are higher than between the GM potatoes and their isogenic lines. Therefore, in our experiments the GM potato plants showed no alteration in PVY resistance to neither one of the tested strains.

Since _Myzus persicae_ is one of the most important vectors transmitting PVY, the developed assay will also be applied to the quantification of PVY particles in aphids. The displacement of PVY^O^ and PVY^N^ by PVY^N^Wilga and PVY^NTN^ may be due to a difference in efficiency of transmission by _M. persicae_. Therefore, the objective is to test whether more virus particles of the recombinant forms in comparison to the non-recombinant strains PVY^O^ and PVY^N^ bind in the stylets of _M. persicae_. 

A third possible application of the developed assay may be of interest in potato breeding. The exact quantification of PVY particles in plants allows the classification of resistance in potato plants. It is possible to estimate whether a resistance is extreme or not. Extreme resistance is characterized by the absence or presence of very low amounts of virus particles in plants several days after inoculation. When testing the plants for PVY infection by ELISA, often unspecific reactions occur which makes it difficult to differentiate between plants weakly infected and plants very weakly infected. An exact quantification of the PVY titer gives more certainty for the determination of the resistance type.

In conclusion, the developed assay is an efficient and low-cost method that allows the differentiation and quantification of PVY^O^/PVY^N^Wilga on the one hand and PVY^N^/PVY^NTN^ on the other hand with high throughput. The method can be utilized for a wide range of applications in potato research.
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Design and experimental validation of a compact collimated Knudsen source

In this paper we discuss the design and performance of a collimated Knudsen source which has the benefit of a simple design over recirculating sources. Measurements of the flux, transverse velocity distribution and brightness at different temperatures were conducted to evaluate the performance. The scaling of the flux and brightness with the source temperature follow the theoretical predictions. The transverse velocity distribution in the transparent operation regime also agrees with the simulated data. The source was found able to produce a flux of $10^{14}$ s$^{-1}$ at a temperature of 433 K. Furthermore the transverse reduced brightness of an ion beam with equal properties as the atomic beam reads $1.7 \times 10^2$ A/(m${}^2$ sr eV) which is sufficient for our goal: the creation of an ultra-cold ion beam by ionization of a laser-cooled and compressed atomic rubidium beam

Interpreting a conformally flat pure radiation space-time

A physical interpretation is presented of the general class of conformally flat pure radiation metrics that has recently been identified by Edgar and Ludwig. It is shown that, at least in the weak field limit, successive wave surfaces can be represented as null (half) hyperplanes rolled around a two-dimensional null cone. In the impulsive limit, the solution reduces to a pp-wave whose direction of propagation depends on retarded time. In the general case, there is a coordinate singularity which corresponds to an envelope of the wave surfaces. The global structure is discussed and a possible vacuum extension through the envelope is proposed.Comment: 9 pages, Plain TeX, 2 figures. To appear in Class. Quantum Grav. Reference adde

Invariant classification and the generalised invariant formalism: conformally flat pure radiation metrics, with zero cosmological constant

Metrics obtained by integrating within the generalised invariant formalism are structured around their intrinsic coordinates, and this considerably simplifies their invariant classification and symmetry analysis. We illustrate this by presenting a simple and transparent complete invariant classification of the conformally flat pure radiation metrics (except plane waves) in such intrinsic coordinates; in particular we confirm that the three apparently non-redundant functions of one variable are genuinely non-redundant, and easily identify the subclasses which admit a Killing and/or a homothetic Killing vector. Most of our results agree with the earlier classification carried out by Skea in the different Koutras-McIntosh coordinates, which required much more involved calculations; but there are some subtle differences. Therefore, we also rework the classification in the Koutras-McIntosh coordinates, and by paying attention to some of the subtleties involving arbitrary functions, we are able to obtain complete agreement with the results obtained in intrinsic coordinates. In particular, we have corrected and completed statements and results by Edgar and Vickers, and by Skea, about the orders of Cartan invariants at which particular information becomes available.Comment: Extended version of GRG publication, with some typos etc correcte

The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks

As stars evolve along the Asymptotic Giant Branch, strong winds are driven from the outer envelope. These winds form a shell, which may ultimately become a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at the presence of a binary companion. Some post-Asymptotic Giant Branch objects are surrounded by torii of crystalline dust, but there is no generally accepted mechanism for annealing the amorphous grains in the wind to crystals. In this Letter, we show that the shaping of the wind by a binary companion is likely to lead to the formation of crystalline dust in the orbital plane of the binary.Comment: Submitted to ApJ

Methane hydrate: shifting the coexistence temperature to higher temperatures with an external electric field

In the present work, we used molecular dynamic simulations of the equilibrium NPT ensemble to examine the effect of an external electric field on the three-phase coexistence temperature of methane gas, liquid water and methane hydrate. For these simulations, we used the TIP4P/Ice rigid water model and a single-site model for methane. The simulations were implemented at two pressures, 400 and 250bar, over temperatures ranging from 285 to 320K and from 280 to 315K, respectively. The application of an external electric field in the range of 0.1-0.9caused the effect of the thermal vibrations of the water molecules to become attenuated. This resulted in a shift of the three-phase coexistence temperature to higher temperatures. Electric fields below this range did not cause a difference in the coexistence temperature, and electric fields above this range enhanced the thermal effect. The shift had a magnitude of 22.5K on average.Peer ReviewedPostprint (author's final draft

Geodesic motion in the Kundt spacetimes and the character of envelope singularity

We investigate geodesics in specific Kundt type N (or conformally flat) solutions to Einstein's equations. Components of the curvature tensor in parallelly transported tetrads are then explicitly evaluated and analyzed. This elucidates some interesting global properties of the spacetimes, such as an inherent rotation of the wave-propagation direction, or the character of singularities. In particular, we demonstrate that the characteristic envelope singularity of the rotated wave-fronts is a (non-scalar) curvature singularity, although all scalar invariants of the Riemann tensor vanish there.Comment: 21 pages, 3 figures. To appear in Class. Quantum Gra