Assessment of Elasticity and Topography of Aspergillus nidulans Spores via Atomic Force Microscopy

Previous studies have described both surface morphology and adhesive properties of fungal spores, but little information is currently available on their mechanical properties. In this study, atomic force microscopy (AFM) was used to investigate both surface topography and micromechanical properties of Aspergillus nidulans spores. To assess the influence of proteins covering the spore surface, wild-type spores were compared with spores from isogenic rodA⁺ and rodA⁻ strains. Tapping-mode AFM images of wild-type and rodA⁺ spores in air showed characteristic “rodlet” protein structures covering a granular spore surface. In comparison, rodA⁻ spores were rodlet free but showed a granular surface structure similar to that of the wild-type and rodA⁺ spores. Rodlets were removed from rodA⁺ spores by sonication, uncovering the underlying granular layer. Both rodlet-covered and rodlet-free spores were subjected to nanoindentation measurements, conducted in air, which showed the stiffnesses to be 110 ± 10, 120 ± 10, and 300 ± 20 N/m and the elastic moduli to be 6.6 ± 0.4, 7.0 ± 0.7, and 22 ± 2 GPa for wild-type, rodA⁺ and rodA⁻ spores, respectively. These results imply the rodlet layer is significantly softer than the underlying portion of the cell wall.     

Peroxisome fission in Hansenula polymorpha requires Mdv1 and Fis1, two proteins also involved in mitochondrial fission

We show that Mdv1 and Caf4, two components of the mitochondrial fission machinery in Saccharomyces cerevisiae , also function in peroxisome proliferation. Deletion of MDV1 , CAF4 or both, however, had only a minor effect on peroxisome numbers at peroxisome-inducing growth conditions, most likely related to the fact that Vps1 – and not Dnm1 – is the key player in peroxisome fission in this organism. In contrast, in Hansenula polymorpha , which has only a Dnm1-dependent peroxisome fission machinery, deletion of MDV1 led to a drastic reduction of peroxisome numbers. This phenotype was accompanied by a strong defect in mitochondrial fission. The MDV1 paralog CAF4 is absent in H. polymorpha . In wild-type H. polymorpha , cells Dnm1–mCherry and green fluorescent protein (GFP)–Mdv1 colocalize in spots that associate with both peroxisomes and mitochondria. Furthermore, Fis1 is essential to recruit Mdv1 to the peroxisomal and mitochondrial membrane. However, formation of GFP–Mdv1 spots – and related to this normal organelle fission – is strictly dependent on the presence of Dnm1. In dnm1 cells, GFP–Mdv1 is dispersed over the surface of peroxisomes and mitochondria. Also, in H. polymorpha mdv1 or fis1 cells, the number of Dnm1–GFP spots is strongly reduced. These spots still associate to organelles but are functionally inactive.     

Pulse-Driven Loss of Top-Down Control: The Critical-Rate Hypothesis

In systems ranging from lakes and woodlands to coral reefs, the long-term ecosystem state may often be determined largely by rare extreme events such as wet ENSO years, droughts, or disease outbreaks. Such events can flip these systems into a contrasting state that represents either an alternative attractor or a transient that is slow enough to persist even if the frequency of events that push the system to this state is low. Here we show that escape from herbivores is a mechanism that can play a role in several state shifts of this kind. This can happen if plants become less susceptible to herbivory as they grow. Using a model we show that, surprisingly, this mechanism can lead to a situation where a brief resource pulse for plants may invoke a persistent shift to a high biomass state whereas gradual enrichment to the same resource level is insufficient to allow such a change. This counterintuitive phenomenon occurs if the numerical response of herbivores is sufficiently slow to allow the plants to use the resource pulse to escape to a safe biomass at which herbivory is reduced. Our results imply that rates of environmental change can sometimes be more important than their magnitude. This has many ramifications. On the conceptual side, our findings suggest that key mechanisms that regulate long-term ecosystem dynamics are easily missed by the traditional focus of modelers on equilibria. A more practical corollary is that increased climatic variability may have more profound effects in some ecosystems than gradual change in conditions.     

Pex14 is the sole component of the peroxisomal translocon that is required for pexophagy

Pex14 was initially identified as a peroxisomal membrane protein that is involved in docking of the soluble receptor proteins Pex5 and Pex7, which are required for import of PTS1- or PTS2-containing peroxisomal matrix proteins. However, Hansenula polymorpha Pex14 is also required for selective degradation of peroxisomes (pexophagy). Previously we showed that Pex1, Pex4, Pex6 and Pex8 are not required for this process. Here we show that also in the absence of various other peroxins, namely Pex2, Pex10, Pex12, Pex13 and Pex17, pexophagy can normally occur. These peroxins are, like Pex14, components of the peroxisomal translocon. Our data confirm that Pex14 is the sole peroxin that has a unique dual function in two apparent opposite processes, namely peroxisome formation and selective degradation.     

Ca2+-dependent metarhodopsin inactivation mediated by calmodulin and NINAC myosin III

Phototransduction in flies is the fastest known G protein-coupled signaling cascade, but how this performance is achieved remains unclear. Here, we investigate the mechanism and role of rhodopsin inactivation. We determined the lifetime of activated rhodopsin (metarhodopsin = M( *)) in whole-cell recordings from Drosophila photoreceptors by measuring the time window within which inactivating M( *) by photoreisomerization to rhodopsin could suppress responses to prior illumination. M( *) was inactivated rapidly (tau approximately 20 ms) under control conditions, but approximately 10-fold more slowly in Ca2+-free solutions. This pronounced Ca2+ dependence of M( *) inactivation was unaffected by mutations affecting phosphorylation of rhodopsin or arrestin but was abolished in mutants of calmodulin (CaM) or the CaM-binding myosin III, NINAC. This suggests a mechanism whereby Ca2+ influx acting via CaM and NINAC accelerates the binding of arrestin to M( *). Our results indicate that this strategy promotes quantum efficiency, temporal resolution, and fidelity of visual signaling.     

A morphometric study of the Abies religiosa–hickelii–guatemalensis complex (Pinaceae) in Guatemala and Mexico

This morphometric study of the geographic variation in the Abies religiosa–hickelii–guatemalensis complex is based on samples from 15 Guatemalan and 12 Mexican populations, two populations of A. religiosa s.str. and A. hickelii s.str., and herbarium specimens of A. hickelii, A. vejarii and varieties of A. guatemalensis. The multivariate methods employed were principal components analysis, and UPGMA clustering. The multivariate and univariate analyses based on 231 operational taxonomic units imply that although morphological differences exist distinct morphospecies cannot be recognized within the A. religiosa–hickelii–guatemalensis complex. A Mantel’s test reports that taxonomic dissimilarities are significantly related to geographic distance. We suggest, therefore, that A. religiosa, A. hickelii and A. guatemalensis are merged so that A. hickelii is referred to as A. religiosa subsp. hickelii (Flous & Gaussen) U. Strandby, K.I. Chr. & M. Sørensen, comb. et stat. nov. and A. guatemalensis as A. religiosa subsp. mexicana (Martínez) U. Strandby, K.I. Chr. & M. Sørensen, comb. nov. According to our analyses A. vejarii cannot retain its status as a separate taxon as the material studied is nested within A. religiosa subsp. mexicana.     

Aurora1 phosphorylation activity on histone H3 and its cross-talk with other post-translational histone modifications in Arabidopsis

The enzymological properties of AtAurora1, a kinase responsible for the cell cycle-dependent phosphorylation of histone H3 at S10, and its cross-talk with other post-translational histone modifications, were determined. In vitro phosphorylation of H3S10 by AtAurora1 is strongly increased by K9 acetylation, and decreased by K14 acetylation and T11 phosphorylation. However, S10 phosphorylation activity is unaltered by mono-, di- or trimethylation of K9. An interference of H3K9 dimethylation by SUVR4 occurs by a pre-existing phosphorylation at S10. Hence, cross-talk in plants exists between phosphorylation of H3S10 and methylation, acetylation or phosphorylation of neighbouring amino acid residues. AtAurora1 undergoes autophosphorylation in vivo regardless of the presence of substrate, and forms dimers in planta . Of the three ATP-competitive Aurora inhibitors tested, Hesperadin was most effective in reducing the in vivo kinase activity of AtAurora1. Hesperadin consistently inhibited histone H3S10 phosphorylation during mitosis in Arabidopsis cells, but did not affect other H3 post-translational modifications, suggesting a specific inhibition of AtAurora in vivo . Inactivation of AtAurora also caused lagging chromosomes in a number of anaphase cells, but, unlike the situation in mammalian cells, Hesperadin did not influence the microtubule dynamics in dividing cells.