Question: Understanding the mechanisms underlying how habitat degradation, topography and rainfall variability interactively affect seed distribution and seedling recruitment is crucial for explaining plant community patterns and dynamics. Interactions between these major factors were studied together in a semiarid sand dune grassland. Location: Eastern Inner Mongolia, China. Methods: The study system used four sites of fixed, semifixed, semishifting and shifting sand dune grasslands, representing a gradient of habitat degradation. We investigated the density of germinable seeds deposited in the top 5 cm of soil and in situ seedling emergence (number of seedlings emerging early in the growing season) and establishment (number of plants recruited at the end of the growing season) at three topographic positions (dune top, windward and leeward sides) within each site over 2 years that differed in rainfall. Habitat characteristics (i.e. vegetation cover, plant species composition and diversity, soil moisture and nutrient availability and soil erodibility) of the four sites were also measured. Results: Habitat degradation (i.e. decreased vegetation cover and enhanced wind erosion rate) significantly reduced the size of the germinable soil seed bank. On average, germinable seed number from the high‐vegetation cover fixed dune was 36‐fold larger than the low‐vegetation cover shifting dune, and eight‐ and two‐fold larger, respectively, than the semishifting and semifixed dunes with intermediate vegetation cover. We observed within‐habitat variability in seed distribution, but among‐topographic position variation differed among habitats. Seedling recruitment showed large between‐year, and among‐ and within‐habitat variability, but these variations varied significantly depending on the response variables evaluated (i.e. initial seedling density, final plant density, emergence rate and recruitment rate). Path analysis revealed complex density‐dependent positive and negative, direct and indirect effects of germinable seed density and initial seedling density on recruitment, but the relative importance of these density‐dependent effects varied depending on habitat type and rainfall availability. Conclusion: Our results suggest that habitat degradation, microtopography and rainfall availability interact in shaping sand dune seed bank and plant community recruitment patterns and dynamics. Their effects were mainly mediated through changes in both the biotic and abiotic environment during the process of habitat deterioration. 相似文献
In Saccharomyces cerevisiae, the sequence-specific binding of the negative regulator Rap1p provides a mechanism to measure telomere length: as the telomere length increases, the binding of additional Rap1p inhibits telomerase activity in cis. We provide evidence that the association of Rap1p with telomeric DNA in vivo occurs in part by sequence-independent mechanisms. Specific mutations in EST2 (est2-LT) reduce the association of Rap1p with telomeric DNA in vivo. As a result, telomeres are abnormally long yet bind an amount of Rap1p equivalent to that observed at wild-type telomeres. This behavior contrasts with that of a second mutation in EST2 (est2-up34) that increases bound Rap1p as expected for a strain with long telomeres. Telomere sequences are subtly altered in est2-LT strains, but similar changes in est2-up34 telomeres suggest that sequence abnormalities are a consequence, not a cause, of overelongation. Indeed, est2-LT telomeres bind Rap1p indistinguishably from the wild type in vitro. Taken together, these results suggest that Est2p can directly or indirectly influence the binding of Rap1p to telomeric DNA, implicating telomerase in roles both upstream and downstream of Rap1p in telomere length homeostasis. 相似文献
Akr1p, which contains six ankyrin repeats, was identified during a screen for mutations that displayed synthetic lethality with a mutant allele of the bud emergence gene BEM1. Cells from which AKR1 had been deleted were alive but misshapen at 30 degrees C and inviable at 37 degrees C. During a screen for mutants that required one or more copies of wild-type AKR1 for survival at 30 degrees C, we isolated mutations in GPA1, which encodes the G alpha subunit of the pheromone receptor-coupled G protein. (The active subunit of this G protein is G beta gamma, and G alpha plays an inhibitory role in G beta gamma-mediated signal transduction.) AKR1 could serve as a multicopy suppressor of the lethality caused by either loss of GPA1 or overexpression of STE4, which encodes the G beta subunit of this G protein, suggesting that pheromone signaling is inhibited by overexpression of Akr1p. Mutations in AKR1 displayed synthetic lethality with a weak allele of GPA1 and led to increased expression of the pheromone-inducible gene FUS1, suggesting that Akr1p normally (and not just when overexpressed) inhibits signaling. In contrast, deletion of BEM1 resulted in decreased expression of FUS1, suggesting that Bem1p normally facilitates pheromone signaling. During a screen for proteins that displayed two-hybrid interactions with Akr1p, we identified Ste4p, raising the possibility that an interaction between Akr1p and Ste4p contributes to proper regulation of the pheromone response pathway. 相似文献
Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non-stationarity, that is, a decoupling of tree growth responses from climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non-stationarity. To systematically assess potential drivers of non-stationarity, we compiled tree-ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non-linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate–growth models were tested in independent verification periods to quantify their non-stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non-stationarity varied between species, site climatic conditions, and models. Chronologies of P. sylvestris showed stronger non-stationarity compared with Picea abies stands with a high degree of stationarity. Sites with mixed climatic signals were most affected by non-stationarity compared with sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non-stationarity compared with monthly-resolved non-linear models. We conclude that non-stationarity in climate–growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non-stationarity, we recommend that temporal non-stationarity rather than stationarity should be considered as the baseline model of climate–growth response for temperate forests. 相似文献
Background: Transplantation of bone marrow-derived mesenchymal stem cells (BMSCs) is a potential therapy for cerebral ischemia. However, the underlying protective mechanism remains undetermined. Here, we tested the hypothesis that transplantation of BMSCs via intravenous injection can alleviate neurological functional deficits through activating PI3K/AKT signaling pathway after cerebral ischemia in rats.
Methods: A cerebral ischemic rat model was established by the 2 h middle cerebral artery occlusion (MCAO). Twenty-four hours later, BMSCs (1?×?106 in 1 ml PBS) from SD rats were injected into the tail vein. Neurological function was evaluated by modified neurological severity score (mNSS) and modified adhesive removal test before and on d1, d3, d7, d10 and d14 after MCAO. Protein expressions of AKT, GSK-3β, CRMP-2 and GAP-43 were detected by Western-bolt. NF-200 was detected by immunofluorescence.
Results: BMSCs transplantation did not only significantly improve the mNSS score and the adhesive-removal somatosensory test after MCAO, but also increase the density of NF-200 and the expression of p-AKT, pGSK-3β and GAP-43, while decrease the expression of pCRMP-2. Meanwhile, these effects can be suppressed by LY294002, a specific inhibitor of PI3K/AKT.
Conclusion: These data suggest that transplantation of BMSCs could promote axon growth and neurological deficit recovery after MCAO, which was associated with activation of PI3K/AKT /GSK-3β/CRMP-2 signaling pathway.