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71.
Alison K. Skinner Ian D. Lunt Sue McIntyre Peter G. Spooner Sandra Lavorel 《Plant Ecology》2010,208(2):359-370
Grasses and forbs compete heavily with young tree seedlings for available resources, greatly reducing tree seedling establishment
success. Soil nutrient enrichment associated with agricultural intensification can increase the growth of both herbaceous
and woody lifeforms growing in isolation, but may change the balance of competitive advantage when growing together. The effects
of nitrogen and phosphorus enrichment on pasture biomass and competition with two Australian grassy woodland trees (Eucalyptus albens and Eucalyptus microcarpa) was investigated in a field plot trial. Soil nutrients increased pasture biomass, but had no measurable effect on tree growth
in our experiment. Competition from pasture species, even at low levels, led to high tree seedling mortality and greatly reduced
tree seedling growth compared with pasture-free plots. However, when pasture-free plots were excluded from the analysis, tree
seedling leaf area was not strongly correlated with herbaceous biomass. Tree seedling establishment was severely restricted
even at the lowest levels of pasture biomass. We conclude that increased soil fertility resulted in a competitive advantage
to the pasture, and does not improve tree seedling establishment when grown either with or without exotic herbaceous pasture
(grassy understorey) species. 相似文献
72.
Mechanical loading: biphasic osteocyte survival and targeting of osteoclasts for bone destruction in rat cortical bone 总被引:12,自引:0,他引:12
Noble BS Peet N Stevens HY Brabbs A Mosley JR Reilly GC Reeve J Skerry TM Lanyon LE 《American journal of physiology. Cell physiology》2003,284(4):C934-C943
Bone isremoved or replaced in defined locations by targeting osteoclasts andosteoblasts in response to its local history of mechanical loading.There is increasing evidence that osteocytes modulate this targeting bytheir apoptosis, which is associated with locally increasedbone resorption. To investigate the role of osteocytes in the controlof loading-related modeling or remodeling, we studied the effects onosteocyte viability of short periods of mechanical loading applied tothe ulnae of rats. Loading, which produced peak compressive strains of0.003 or 0.004, was associated with a 78% reduction in theresorption surface at the midshaft. The same loading regimen resultedin a 40% relative reduction in osteocyte apoptosis at the samesite 3 days after loading compared with the contralateral side(P = 0.01). The proportion of osteocytes that wereapoptotic was inversely related to the estimated local strain(P < 0.02). In contrast, a single short period ofloading resulting in strains of 0.008 engendered both tissuemicrodamage and subsequent bone remodeling and was associated with aneightfold increase in the proportion of apoptotic osteocytes(P = 0.02) at 7 days. This increase in osteocyteapoptosis was transient and preceded both intracorticalremodeling and death of half of the osteocytes (P < 0.01). The data suggest that osteocytes might use their U-shapedsurvival response to strain as a mechanism to influence boneremodeling. We hypothesize that this relationship reflects a causalmechanism by which osteocyte apoptosis regulates bone'sstructural architecture. 相似文献
73.
Knox PG Milner AE Green NK Eliopoulos AG Young LS 《Journal of immunology (Baltimore, Md. : 1950)》2003,170(2):677-685
The Fas ligand (FasL)/Fas receptor (CD95) pathway is an important mediator of apoptosis in the immune system and can also mediate cancer cell death. Soluble FasL (sFasL), shed from the membrane-bound form of the molecule by a putative metalloproteinase (MP), may function to locally regulate the activity of membrane-bound FasL. Using a replication-defective recombinant adenovirus-expressing FasL (RAdFasL), we identified a variable ability of different carcinoma cells to respond to FasL-induced cytotoxicity and to shed sFasL. Blockade of FasL cleavage with an MP inhibitor significantly enhanced RAdFasL-induced apoptosis suggesting that sFasL may antagonize the effect of membrane-bound FasL. In support of this concept, a recombinant adenovirus expressing a noncleavable form of FasL (RAdD4) was found to be a potent inducer of apoptosis even at very low virus doses. Our results highlight the therapeutic potential of noncleavable FasL as an antitumor agent and emphasize the important role of MP via the production of sFasL in regulating the response of the Fas pathway. Moreover, these findings have general implications for the therapeutic exploitation of TNF family ligands and for the possible impact of MP-based therapies on the normal physiology of Fas/TNF pathways. 相似文献
74.
Helicobacter pylori porCDAB and oorDABC Genes Encode Distinct Pyruvate:Flavodoxin and 2-Oxoglutarate:Acceptor Oxidoreductases Which Mediate Electron Transport to NADP 总被引:4,自引:0,他引:4 下载免费PDF全文
Nicky J. Hughes Chris L. Clayton Peter A. Chalk David J. Kelly 《Journal of bacteriology》1998,180(5):1119-1128
Helicobacter pylori, a major cause of human gastric disease, is a microaerophilic bacterium that contains neither pyruvate nor 2-oxoglutarate dehydrogenase activity. Previous studies (N. J. Hughes, P. A. Chalk, C. L. Clayton, and D. J. Kelly, J. Bacteriol. 177:3953–3959, 1995) have indicated that the major routes for the generation of acetyl coenzyme A (acetyl-CoA) and succinyl-CoA are via pyruvate:flavodoxin oxidoreductase (POR) and 2-oxoglutarate:acceptor oxidoreductase (OOR), respectively. The purified POR is a heterotetrameric protein, with subunits of 48 (PorA), 36 (PorB), 24 (PorC), and 14 (PorD) kDa. In this study OOR has been purified, and it is similarly composed of polypeptides of 43 (OorA), 33 (OorB), 24 (OorC), and 10 (OorD) kDa. Both POR and OOR are oxygen labile and are likely to be major contributors to the microaerophilic phenotype of H. pylori. Unlike POR, OOR was unable to use a previously identified flavodoxin (FldA) as an electron acceptor. Although the purified enzymes were unable to reduce NAD(P), electrons from both pyruvate and 2-oxoglutarate could reduce NADP in cell extracts, consistent with a role for these oxidoreductases in the provision of NADPH as a respiratory electron donor. The H. pylori por, oor, and fldA genes were cloned and sequenced. The deduced por gene products showed significant sequence similarity to archaeal four-subunit 2-oxoacid:acceptor oxidoreductases. However, the amino acid sequences of OorA and -B were more closely related to that of the two-subunit POR of the aerobic halophile Halobacterium halobium. Both porD and oorD encode integral ferredoxin-like subunits. POR and OOR are probably essential enzymes in H. pylori, as insertion inactivation of porB and oorA appeared to be lethal. 相似文献
75.
76.
Taylor NJ Hills PN Gold JD Stirk WA van Staden J 《Journal of plant physiology》2005,162(11):1270-1279
Thermoinhibition in Tagetes minuta achenes is tightly and rapidly regulated with regard to its imposition and release, with both processes occurring within 2-3h. Germination at high temperatures is almost exclusively regulated by the embryo, while the pericarp appears to play only a minor role. Thermoinhibition in T. minuta could not be alleviated by any single plant growth regulator application, but a combination of treatments that both reduced ABA levels and increased ethylene levels were able to restore germination at supraoptimal temperatures. This suggests a role for both ethylene and ABA in the imposition of thermoinhibition in this species. 相似文献
77.
Through the examination of past and present distributions of plants and animals, historical biogeographers have provided many insights on the dynamics of the massive organismal exchange between North and South America. However, relatively few phylogeographic studies have been attempted in the land bridge of Mesoamerica despite its importance to better understand the evolutionary forces influencing this biodiversity 'hotspot'. Here we use mitochondrial DNA sequence data from fresh samples and formalin-fixed museum specimens to investigate the genetic and biogeographic diversity of the threatened Mesoamerican spiny-tailed lizards of the Ctenosaura quinquecarinata complex. Species boundaries and their phylogeographic patterns are examined to better understand their disjunct distribution. Three monophyletic, allopatric lineages are established using mtDNA phylogenetic and nested clade analyses in (i) northern: México, (ii) central: Guatemala, El Salvador and Honduras, and (iii) southern: Nicaragua and Costa Rica. The average sequence divergence observed between lineages varied between 2.0% and 3.7% indicating that they do not represent a very recent split and the patterns of divergence support the recently established nomenclature of C. quinquecarinata, Ctenosaura flavidorsalis and Ctenosaura oaxacana. Considering the geological history of Mesoamerica and the observed phylogeographic patterns of these lizards, major evolutionary episodes of their radiation in Mesoamerica are postulated and are indicative of the regions' geological complexity. The implications of these findings for the historical biogeography, taxonomy and conservation of these lizards are discussed. 相似文献
78.
79.
‘Medical Tourism’ – the phenomenon of people travelling abroad to access medical treatment - has received increasing attention in academic and popular media. This paper reports findings from a study examining effect of inbound and outbound medical tourism on the UK NHS, by estimating volume of medical tourism and associated costs and benefits. A mixed methods study it includes analysis of the UK International Passenger Survey (IPS); interviews with 77 returning UK medical tourists, 63 policymakers, NHS managers and medical tourism industry actors policymakers, and a review of published literature. These informed costing of three types of treatments for which patients commonly travel abroad: fertility treatment, cosmetic and bariatric surgery. Costing of inbound tourism relied on data obtained through 28 Freedom-of-Information requests to NHS Foundation Trusts. Findings demonstrate that contrary to some popular media reports, far from being a net importer of patients, the UK is now a clear net exporter of medical travellers. In 2010, an estimated 63,000 UK residents travelled for treatment, while around 52,000 patients sought treatment in the UK. Inbound medical tourists treated as private patients within NHS facilities may be especially profitable when compared to UK private patients, yielding close to a quarter of revenue from only 7% of volume in the data examined. Costs arise where patients travel abroad and return with complications. Analysis also indicates possible savings especially in future health care and social costs averted. These are likely to be specific to procedures and conditions treated. UK medical tourism is a growing phenomenon that presents risks and opportunities to the NHS. To fully understand its implications and guide policy on issues such as NHS global activities and patient safety will require investment in further research and monitoring. Results point to likely impact of medical tourism in other universal public health systems. 相似文献
80.