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Symbiotic nitrogen (N2) fixation in legumes may give the host plant a distinct competitive advantage; at the same time it is mainly responsible for introducing N into terrestrial ecosystems which may ultimately benefit all organisms. Depending on environmental conditions, symbiotic N2 fixation may be tuned to the plant's N demand or specifically inhibited (a disadvantage for plants which depend mainly on symbiotic N2 fixation), or even prevented. Thus, the ecological range for symbiotic N2 fixation can be narrower than that of the host plants. A shortage of mineral N is the only case in which adverse environmental conditions clearly favour symbiotic N2 fixation. Variations in number or mass of nodules or nodule morphology are persistent features, that may represent one kind of regulation of N2 fixation. In addition, varying O2 permeability of nodules functions as a rapid and reversible control of N2 fixation which may compensate partially or fully for poor nodulation. The plant's demand for symbiotically fixed N is thought to play a central role in modulating both nodulation and N2 fixation activity; an N feedback mechanism is assumed. The control of symbiotic N2 fixation operates through a series of ecophysiological triggers which are also influenced by complex interactions between legume plants and other organisms in the ecosystem. The proportion of legume biomass and the performance of symbiotic N2 fixation in each individual legume are the main parameters which determine the amount of symbiotically fixed N introduced into a terrestrial ecosystem. The various triggers and N feedback mechanisms from the whole ecosystem to the gene expression level which regulate symbiotic N2 fixation in terrestrial ecosystems are reviewed and discussed in terms of a conceptual model. Although the presented model is based primarily on our knowledge about the physiology of a few leguminous crop species and of ecosystem processes in managed, perennial grassland in temperate climatic conditions, it may stimulate thinking about functional relationships between symbiotic N2 fixation and terrestrial ecosystems at various system levels. 相似文献
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SETH1 and SETH2, two components of the glycosylphosphatidylinositol anchor biosynthetic pathway, are required for pollen germination and tube growth in Arabidopsis 总被引:8,自引:0,他引:8 下载免费PDF全文
Lalanne E Honys D Johnson A Borner GH Lilley KS Dupree P Grossniklaus U Twell D 《The Plant cell》2004,16(1):229-240
76.
Genetic interaction of an origin recognition complex subunit and the Polycomb group gene MEDEA during seed development 总被引:2,自引:0,他引:2 下载免费PDF全文
The eukaryotic origin recognition complex (ORC) is made up of six subunits and functions in nuclear DNA replication, chromatin structure, and gene silencing in both fungi and metazoans. We demonstrate that disruption of a plant ORC subunit homolog, AtORC2 of Arabidopsis (Arabidopsis thaliana), causes a zygotic lethal mutant phenotype (orc2). Seeds of orc2 abort early, typically producing embryos with up to eight cells. Nuclear division in the endosperm is arrested at an earlier developmental stage: only approximately four nuclei are detected in orc2 endosperm. The endosperm nuclei in orc2 are dramatically enlarged, a phenotype that is most similar to class B titan mutants, which include mutants in structural maintenance of chromosomes (SMC) cohesins. The highest levels of ORC2 gene expression were found in preglobular embryos, coinciding with the stage at which homozygous orc2 mutant seeds arrest. The homologs of the other five Arabidopsis ORC subunits are also expressed at this developmental stage. The orc2 mutant phenotype is partly suppressed by a mutation in the Polycomb group gene MEDEA. In double mutants between orc2 and medea (mea), orc2 homozygotes arrest later with a phenotype intermediate between those of mea and orc2 single mutants. Either alterations in chromatin structure or the release of cell cycle checkpoints by the mea mutation may allow more cell and nuclear divisions to occur in orc2 homozygous seeds. 相似文献
77.
Jagged1 signals in the postnatal subventricular zone are required for neural stem cell self-renewal 下载免费PDF全文
Nyfeler Y Kirch RD Mantei N Leone DP Radtke F Suter U Taylor V 《The EMBO journal》2005,24(19):3504-3515
Neural stem cells (NSCs) in the postnatal mammalian brain self-renew and are a source of neurons and glia. To date, little is known about the molecular and cellular mechanisms regulating the maintenance and differentiation of these multipotent progenitors. We show that Jagged1 is required by mitotic cells in the subventricular zone (SVZ) and stimulates self-renewal of multipotent epidermal growth factor-dependent NSCs. Jagged1-expressing cells line the adult SVZ and are juxtaposed to Notch1-expressing cells, some of which are putative NSCs. In vitro, endogenous Jagged1 acts through Notch1 to promote NSC maintenance and multipotency. In vivo, reducing Jagged1/Notch1 signaling decreases the number of proliferating cells in the SVZ. In addition, soluble Jagged1 promotes self-renewal and neurogenic potential of multipotent neural progenitors in vitro. Our findings suggest a central role for Jagged1 in the NSC niche in the SVZ for maintaining a population of NSCs in the postnatal brain. 相似文献
78.
Thomann A Brukhin V Dieterle M Gheyeselinck J Vantard M Grossniklaus U Genschik P 《The Plant journal : for cell and molecular biology》2005,43(3):437-448
Cullin (CUL)-dependent ubiquitin ligases form a class of structurally related multisubunit enzymes that control the rapid and selective degradation of important regulatory proteins involved in cell cycle progression and development, among others. The CUL3-BTB ligases belong to this class of enzymes and despite recent findings on their molecular composition, our knowledge on their functions and substrates remains still very limited. In contrast to budding and fission yeast, CUL3 is an essential gene in metazoans. The model plant Arabidopsis thaliana encodes two related CUL3 genes, called CUL3A and CUL3B. We recently reported that cul3a loss-of-function mutants are viable but exhibit a mild flowering and light sensitivity phenotype. We investigated the spatial and temporal expression of the two CUL3 genes in reproductive tissues and found that their expression patterns are largely overlapping suggesting possible functional redundancy. Thus, we investigated the consequences on plant development of combined Arabidopsis cul3a cul3b loss-of-function mutations. Homozygous cul3b mutant plants developed normally and were fully fertile. However, the disruption of both the CUL3A and CUL3B genes reduced gametophytic transmission and caused embryo lethality. The observed embryo abortion was found to be under maternal control. Arrest of embryogenesis occurred at multiple stages of embryo development, but predominantly at the heart stage. At the cytological level, CUL3 loss-of-function mutations affected both embryo pattern formation and endosperm development. 相似文献
79.
Mycorrhizas improve nitrogen nutrition of Trifolium repens after 8 yr of selection under elevated atmospheric CO2 partial pressure 总被引:1,自引:0,他引:1
Altered environmental conditions may change populations of arbuscular mycorrhizal fungi and thereby affect mycorrhizal functioning. We investigated whether 8 yr of free-air CO2 enrichment has selected fungi that differently influence the nutrition and growth of host plants. In a controlled pot experiment, two sets of seven randomly picked single spore isolates, originating from field plots of elevated (60 Pa) or ambient CO2 partial pressure (pCO2), were inoculated on nodulated Trifolium repens (white clover) plants. Fungal isolates belonged to the Glomus claroideum or Glomus intraradices species complex, and host plants were clonal micropropagates derived from nine genets. Total nitrogen (N) concentration was increased in leaves of plants inoculated with fungal isolates from elevated-pCO2 plots. These isolates took up nearly twice as much N from the soil as isolates from ambient-pCO2 plots and showed much greater stimulation of biological N2 fixation. The morpho-species identity of isolates had a more pronounced effect on N2 fixation and on root length colonized than isolate identity. We conclude that rising atmospheric pCO2 may select for fungal strains that will help their host plants to meet increased N demands. 相似文献
80.
Analyses of isolated pre-ribosomes yielded biochemical "snapshots" of the dynamic, nascent 60S and 40S subunits during their path from the nucleolus to the cytoplasm. Here, we present the structure of a pre-60S ribosomal intermediate located in the nucleoplasm. A huge dynein-related AAA-type ATPase (Rea1) and the Rix1 complex (Rix1-Ipi1-Ipi3) are components of an extended (approximately 45 nm long) pre-60S particle. Antibody crosslinking in combination with electron microscopy revealed that the Rea1 localizes to the "tail" region and ribosomal proteins to the "head" region of the elongated "tadpole-like" structure. Furthermore, in vitro treatment with ATP induces dissociation of Rea1 from the pre-60S subunits. Rea1 and the Rix1 complex could mediate ATP-dependent remodeling of 60S subunits and subsequent export from the nucleoplasm to the cytoplasm. 相似文献