Mucilage secretion: an adaptive mechanism to reduce seed removal by soil erosion?

Diaspores of many plant species inhabiting open vegetation in semi‐arid environments secrete mucilage after wetting (myxospermy) that glues the diaspores to the ground and prevents movement when the mucilage dries. In the present study, we test whether mucilage secretion can be considered as a selective response to soil erosion in plant species inhabiting semi‐arid environments. We relate the amount and type of mucilage secretion by seeds of Helianthemum violaceum and Fumana ericifolia (Cistaceae) to the number of raindrop impacts needed to remove these seeds after gluing them with their own mucilage to the ground and also the time that these seeds resist water run‐off without detaching. We also compare the amount of seed mucilage production by plants growing in habitats without erosion and plants affected by severe erosion by fitting mixed effect models. Our results show an important phenotypic variation in the amount of mucilage secretion in both species, although it is suggested that the effect of mucilage secretion in the rate of seed removal by erosion is species‐ and mechanism‐dependent. For F. ericifolia, the amount of mucilage secreted by the seeds is directly proportional to their resistance to raindrop impacts and is positively related to the intensity of the erosive processes that the plants experience. Nevertheless, all the seeds resist the force of run‐off during 60 min, irrespective of the amount of mucilage they produce. In H. violaceum, mucilage secretion per se, and not the amount of mucilage produced by the seeds, has an effect on the rate of seed removal by erosive processes. Furthermore, cellulosic fibrils were found only in the mucilage of F. ericifolia but not in H. violaceum. Overall, our results only partially support the hypothesis that a selective response to soil erosion exists. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 241–251.     

Fibrotic enzymes modulate wound‐induced skin tumorigenesis

Fibroblasts are a major component of the microenvironment of most solid tumours. Recent research elucidated a large heterogeneity and plasticity of activated fibroblasts, indicating that their role in cancer initiation, growth and metastasis is complex and context‐dependent. Here, we performed genome‐wide expression analysis comparing fibroblasts in normal, inflammatory and tumour‐associated skin. Cancer‐associated fibroblasts (CAFs) exhibit a fibrotic gene signature in wound‐induced tumours, demonstrating persistent extracellular matrix (ECM) remodelling within these tumours. A top upregulated gene in mouse CAFs encodes for PRSS35, a protease capable of collagen remodelling. In human skin, we observed PRSS35 expression uniquely in the stroma of high‐grade squamous cell carcinomas. Ablation of PRSS35 in mouse models of wound‐ or chemically‐induced tumorigenesis resulted in aberrant collagen composition in the ECM and increased tumour incidence. Our results indicate that fibrotic enzymes expressed by CAFs can regulate squamous tumour initiation by remodelling the ECM.     

Shrub cover in northern Nunavik: can herbivores limit shrub expansion?

Recent climate changes have increased the primary productivity of many Arctic and subarctic regions. Erected shrub has been shown to increase in abundance over the last decades in northern regions in response to warmer climate. At the same time, caribou herds are declining throughout the circumboreal regions. Based on observation of heavy browsing on shrubs at Deception Bay (Nunavik, Canada), we hypothesized that the densification of shrubs observed in nearby locations did not occur at our study site despite of observed warming because of a recent peak of the Rivière-aux-Feuilles caribou herd. To assess shrub cover changes, we compared a 1972 mosaic of aerial photos to a 2010 satellite image over a 5 km² area, divided into 56 grids of 100 30 m × 30 m cells. Most cells (n = 4,502) did not show any changes in the cover of shrubs but those who did were as likely to increase as to decrease. The relative cover of shrubs in cells who changed was not higher in 2010 (6.1 ± 0.2 %) than in 1972 (7.3 ± 0.4 %). More than 70 % of birch and willow had more than 50 % of their shoot browsed, suggesting that caribou may limit shrub expansion at this site. We cannot rule out that abiotic factors also contribute to the inertia in shrub cover. Increases in shrub abundance reported in Nunavik and elsewhere were located closer to the tree line or in discontinuous permafrost, whereas our site is characterized by herbaceous arctic tundra, continuous permafrost and relatively low annual precipitation.     

Retinoid Binding Properties of Nucleotide Binding Domain 1 of the Stargardt Disease-associated ATP Binding Cassette (ABC) Transporter,ABCA4

The retina-specific ATP binding cassette transporter, ABCA4 protein, is associated with a broad range of inherited macular degenerations, including Stargardt disease, autosomal recessive cone rod dystrophy, and fundus flavimaculatus. In order to understand its role in retinal transport in rod out segment discs, we have investigated the interactions of the soluble domains of ABCA4 with both 11-cis- and all-trans-retinal. Using fluorescence anisotropy-based binding analysis and recombinant polypeptides derived from the amino acid sequences of the four soluble domains of ABCA4, we demonstrated that the nucleotide binding domain 1 (NBD1) specifically bound 11-cis-retinal. Its affinity for all-trans-retinal was markedly reduced. Stargardt disease-associated mutations in this domain resulted in attenuation of 11-cis-retinal binding. Significant differences in 11-cis-retinal binding affinities were observed between NBD1 and other cytoplasmic and lumenal domains of ABCA4. The results suggest a possible role of ABCA4 and, in particular, the NBD1 domain in 11-cis-retinal binding. These results also correlate well with a recent report on the in vivo role of ABCA4 in 11-cis-retinal transport.     

Serum non-transferrin-bound iron and low-density lipoprotein oxidation in heterozygous hemochromatosis

Non-transferrin-bound iron (NTBI) is implicated in lipid peroxidation but the relation with oxidative modification of low-density lipoprotein (LDL) is not known. We assessed variables reflecting in vitro and in vivo LDL oxidation in two age- and sex-matched groups (n=23) of hereditary hemochromatosis heterozygotes (C282Y), characterized by a clear difference in mean serum NTBI (1.55+/-0.57 micromol/L vs 3.70+/-0.96 micromol/L). Plasma level of oxidized LDL (absolute and relative to plasma apolipoprotein B), and IgG and IgM antibodies to oxidized LDL, markers of in vivo LDL oxidation, did not differ between the groups with low and high serum NTBI. Mean lag-phase of in vitro LDL oxidation was also not significantly different between both study groups. Conclusion: these findings do not support the hypothesis that NTBI promotes oxidative modification of plasma LDL.     

Relationship between the functional regions of the RecA protein and ATP hydrolysis in UV-irradiated Escherichia coli cells.

The time course of the intracellular ATP concentration in several UV-irradiated RecA protease constitutive (Cptc) mutants of E. coli has been studied. All Cptc mutants harboring a mutation in region 3 of the RecA protein (including amino acid residues 298-301) increased ATP after UV damage but without any subsequent decrease. Nevertheless, these mutants induced the SOS response after UV irradiation. Likewise, truncated RecA proteins lacking region 3 are also unable to carry out massive ATP hydrolysis in UV-irradiated cells. On the other hand, mutants in region 1 (including amino acids 25-39) or 2 (amino acids 157-184) of the RecA protein showed an increase in ATP concentration during the first 20 min following UV irradiation, which dropped afterwards to the basal level. All these data indicate that region 3 of the RecA protein must be involved in the ATP hydrolysis process. Furthermore, a relationship between the quantity of the UV-mediated ATP produced and the strength of the different RecA Cptc mutants has also been found. Accordingly, both lexA71::Tn5 and null lexA mutants of E. coli only show a cellular ATP increase after UV irradiation when containing a multicopy plasmid carrying either a wild-type lexA or a lexA (Ind-) gene.     

Holocephalan (Chondrichthyes) dental plates with hypermineralized dentine as a substitute for missing teeth through developmental plasticity

All extant holocephalans (Chimaeroidei) have lost the ability to make individual teeth, as tooth germs are not part of the embryonic development of the dental plates or of their continuous growth. Instead, a hypermineralized dentine with a unique mineral, whitlockin, is specifically distributed within a dentine framework into structures that give the dental plates their distinctive, species-specific morphology. Control of the regulation of this distribution must be cellular, with a dental epithelium initiating the first outer dentine, and via contact with ectomesenchymal tissue as the only embryonic cell type that can make dentine. Chimaeroids have three pairs of dental plates within their mouth, two in the upper jaw and one in the lower. In the genera Chimaera, Hydrolagus and Harriotta, the morphology and distribution of this whitlockin within each dental plate differs both between different plates in the same species and between species. Whitlockin structures include ovoids, rods and tritoral pads, with substantial developmental changes between these. For example, rods appear before the ovoids and result from a change in the surrounding trabecular dentine. In Harriotta, ovoids form separately from the tritoral pads, but also contribute to tritor development, while in Chimaera and Hydrolagus, tritoral pads develop from rods that later are perforated to accommodate the vasculature. Nevertheless, the position of these structures, secreted by the specialized odontoblasts (whitloblasts), appears highly regulated in all three species. These distinct morphologies are established at the aboral margin of the dental plate, with proposed involvement of the outer dentine. We observe that this outer layer forms into serially added lingual ridges, occurring on the anterior plate only. We propose that positional, structural specificity must be contained within the ectomesenchymal populations, as stem cells below the dental epithelium, and a coincidental occurrence of each lingual, serial ridge with the whitlockin structures that contribute to the wear-resistant oral surface.     

SHORT‐TERM TEMPORAL VARIABILITY OF AMMONIUM AND UREA UPTAKE BY ALEXANDRIUM CATENELLA (DINOPHYTA) IN CULTURES1

In batch cultures of four Mediterranean strains (from France, Italy, and Spain) of Alexandrium catenella (Whedon et Kof.) Balech growing on a daily light cycle, ammonium and urea uptake were estimated by the ¹⁵N tracer technique. Ammonium uptake could be described by Michaelis–Menten kinetics along a substrate gradient of 0.1–10 μgat N · L^?1 for the four strains, while two different patterns were observed for urea uptake with Michaelis–Menten kinetics for one strain and linear kinetics for the others. In all cases, an increase in uptake rates with time was noted over the daylight period. This trend led to a net increase in the maximum uptake rate (V_max; for saturable kinetics) and in the initial slope α. For ammonium, V_max increased by a factor of 2–10 depending on the strain, and, for urea, the maximal uptake rates measured increased by a factor of 2–18. Temporal variations of half‐saturation constants (K_s) for both nutrients did not show a clear trend. Increases in V_max and α showed an acclimation of the cells’ uptake system over time to a N pulse, which may be explained by the light periodicity. For two strains, extensive ammonium release was observed during urea assimilation. This mechanism removes urea from the medium, so it is no longer available to other potential competitors, but supplies N back to the medium in the form of ammonium. From a methodological point of view, the phenomenon leads to considerable underestimates of the contribution of urea to phytoplankton growth.