2,155 research outputs found
Herbicide mixtures at high doses slow the evolution of resistance in experimentally evolving populations of Chlamydomonas reinhardtii
The widespread evolution of resistance to herbicides is a pressing issue in global agriculture. Evolutionary principles and practices are key to the management of this threat to global food security. The application of mixtures of herbicides has been advocated as an anti-resistance strategy, without substantial empirical support for its validation.
We evolved experimentally populations of the unicellular green chlorophyte, Chlamydomonas reinhardtii, to minimum inhibitory concentrations (MICs) of single-herbicide modes of action and to pair-wise and three-way mixtures between different herbicides at various total combined doses.
Herbicide mixtures were most effective when each component was applied at or close to its MIC. When doses were high, increasing the number of mixture components was also effective in reducing the evolution of resistance. Employing mixtures at low combined doses did not retard resistance evolution, even accelerating the evolution of resistance to some components. At low doses, increasing the number of herbicides in the mixture tended to select for more generalist resistance (cross-resistance).
Our results reinforce findings from the antibiotic resistance literature and confirm that herbicide mixtures can be very effective for resistance management, but that mixtures should only be employed where the economic and environmental context permits the applications of high combined doses
Methionine adenosyltransferase S-nitrosylation is regulated by the basic and acidic amino acids surrounding the target thiol
S-Adenosylmethionine serves as the methyl donor for many biological methylation reactions and provides the propylamine group for the synthesis of polyamines. S-Adenosylmethionine is synthesized from methionine and ATP by the enzyme methionine adenosyltransferase. The cellular factors regulating S-adenosylmethionine synthesis have not been well defined. Here we show that in rat hepatocytes S-nitrosoglutathione monoethyl ester, a cell-permeable nitric oxide donor, markedly reduces cellular S-adenosylmethionine content via inactivation of methionine adenosyltransferase by S-nitrosylation. Removal of the nitric oxide donor from the incubation medium leads to the denitrosylation and reactivation of methionine adenosyltransferase and to the rapid recovery of cellular S-adenosylmethionine levels. Nitric oxide inactivates methionine adenosyltransferase via S-nitrosylation of cysteine 121. Replacement of the acidic (aspartate 355) or basic (arginine 357 and arginine 363) amino acids located in the vicinity of cysteine 121 by serine leads to a marked reduction in the ability of nitric oxide to S-nitrosylate and inactivate hepatic methionine adenosyltransferase. These results indicate that protein S-nitrosylation is regulated by the basic and acidic amino acids surrounding the target cysteine
Large ferroelectric polarization in the new double perovskite NaLaMnWO induced by non-polar instabilities
Based on density functional theory calculations and group theoretical
analysis, we have studied NaLaMnWO compound which has been recently
synthesized [Phys. Rev. B 79, 224428 (2009)] and belongs to the family of double perovskites. At low temperature, the structure has
monoclinic symmetry, with layered ordering of the Na and La ions and
rocksalt ordering of Mn and W ions. The Mn atoms show an antiferromagnetic
(AFM) collinear spin ordering, and the compound has been reported as a
potential multiferroic. By comparing the low symmetry structure with a parent
phase of symmetry, two distortion modes are found dominant. They
correspond to MnO and WO octahedron \textit{tilt} modes, often
found in many simple perovskites. While in the latter these common tilting
instabilities yield non-polar phases, in NaLaMnWO the additional presence
of the - cation ordering is sufficient to make these rigid unit modes
as a source of the ferroelectricity. Through a trilinear coupling with the two
unstable tilting modes, a significant polar distortion is induced, although the
system has no intrinsic polar instability. The calculated electric polarization
resulting from this polar distortion is as large as 16 . Despite its secondary character, this polarization is coupled with
the dominant tilting modes and its switching is bound to produce the switching
of one of two tilts, enhancing in this way a possible interaction with the
magnetic ordering. The transformation of common non-polar purely steric
instabilities into sources of ferroelectricity through a controlled
modification of the parent structure, as done here by the cation ordering, is a
phenomenon to be further explored.Comment: Physical Chemistry Chemical physics (in press
Spiral ground state against ferroelectricity in the frustrated magnet BiMnFe2O6
The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are
investigated by low-temperature neutron powder diffraction and density
functional theory band structure calculations. In spite of the random
distribution of the Mn3+ and Fe3+ cations, this compound undergoes a transition
into an incommensurate antiferromagnetically ordered state below TN ~ 220 K.
The magnetic structure is characterized by the propagation vector k=[0,beta,0]
with beta ~ 0.14 and the P22_12_11'(0 \beta 0)0s0s magnetic superspace
symmetry. It comprises antiferromagnetic helixes propagating along the b-axis.
The magnetic moments lie in the ac plane and rotate about pi*(1+beta) ~ 204.8
deg angle between the adjacent magnetic atoms along b. The spiral magnetic
structure arises from the peculiar frustrated arrangement of exchange couplings
in the ab plane. The antiferromagnetic coupling along the c-axis leads to the
cancellation of electric polarization, and results in the lack of
ferroelectricity in BiMnFe2O6.Comment: 11 pages, 8 figures, 8 table
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