Leaf mineral concentrations of Erica arborea, Juniperus communis and Myrtus communis growing in the proximity of a natural CO2 spring

Leaf mineral concentrations of co‐occurring Erica arborea, Juniperus communis and Myrtus communis were measured at bimonthly intervals throughout a year in a natural CO₂ spring and in a nearby control site with similar soil chemistry in a Mediterranean environment. There were different responses to the elevated [CO₂] (c. 700 μL L^?1) of the spring site plants depending on the element and the species. In the CO₂ spring site K, Ca, Mg, Mn, Al, Fe, and Ti leaf concentrations and the ratio C/N showed significant greater values in at least one or two of the three species. Leaf S concentration were greater in all three species. Leaf concentrations of N, Sr, Co, and B were lower in at least one or two species, and those of C and Ba were lower in all the three studied species near the CO₂ spring. P, Na, Zn, Si, Cu, Ni, Cr, Pb, Mo, V and Cd leaf concentrations and the specific leaf area (SLA, measured in Myrtus communis) did not show any consistent or significant pattern in response to the elevated [CO₂] of the spring site. There was a slight trend towards maximum concentrations of most of these elements during autumn–winter and minimum values during the spring season, especially in Myrtus communis. Multivariate principal component analyses based on the leaf elemental concentrations clearly differentiated the two sites and the three species. Lower concentrations at the spring site were not the result of a dilution effect by increased structural or nonstructural carbon. In contrast to most experimental studies of CO₂ enrichment, mainly conducted for short periods, several of these elements had greater concentrations in the CO₂ spring site. Nutrient acclimation and possible causes including decreased nutrient export, increased nutrient uptake capacity, photosynthetic down‐regulation, Mediterranean water stress, and higher H₂S concentration in the spring site are discussed.     

Potential Role of Transposable Elements in the Rapid Reorganization of the Fusarium oxysporum Genome

The activity of several families of transposable elements (TEs) in the genome of Fusarium oxysporum represents a potential source of karyotypic instability. We investigated transposon-mediated chromosome rearrangements by analyzing the karyotypes of a set of strains in which transposition events had occurred. We uncovered exceptional electrophoretic karyotype (EK) variability, in both number and size of chromosomal bands. We showed that EK differences result from chromosomal translocations, large deletions, and even more complex rearrangements. We also revealed many duplicated chromosomal regions. By following transposition of two elements and analyzing the distribution of different families of TEs on whole chromosomes, we find (i) no evidence of chromosomal breakages induced by transposition, (ii) a clustering of TEs in some regions, and (iii) a correlation between the high level of chromosomal polymorphism and the concentration of TEs. These results suggest that chromosome length polymorphisms likely result from ectopic recombination between TEs that can serve as substrates for these changes.     

Absorption of Atmospheric NO2 by Spruce (Picea abies) Trees. III. Interaction with Nitrate Reductase Activity in the Needles and Phloem Transport

In order to compare the effects of excess pedospheric and atmospheric nitrogen supply on nitrate reductase activity (NR. EC 1.6.6.1) excised spruce branches were exposed to nitrate solutions or were fumigated with NO₂. Immersion of spruce branches in 6 mM nitrate caused an increase in NR activity by a factor of 14 or 19 in current-year and in one-year-old needles, respectively, as compared to controls incubated in tap water. Exposure to 65 nl I^?1 NO₂ increased NR activity by a factor of 1.5 in current-year needles and by a factor of 2.5 in one-year-old needles as compared to non-fumigated controls. Addition of cycloheximide (0.17 μM) or puromycin (200 μM) to the incubation solution prevented the induction of NR activity from both nitrate and NO₂ exposure. This finding indicates that induction of NR activity by both atmospheric NO₂ or increased nitrate supply of the needles is both caused by de-novo synthesis of NR protein. The increase in NR activity in needles of branches still attached to the tree as a consequence of exposure to 65 nl I^?1 NO₂ was found to be a transient phenomenon. The increase persisted for several days only and was no longer observed after one week of sustained NO₂ exposure. An interruption of phloem transport by girdling, applied subsequent to the induction of NR activity by atmospheric NO₂, prevented the decrease in NR activity. Apparently, export out of the exposed needles and phloem transport within the stem are involved in the regulation of NR activity upon NO₂ exposure.     

Dissection of cis-regulatory elements of the Drosophila gene Serrate

 The Drosophila gene Serrate encodes a membrane spanning protein, which is expressed in a complex pattern during embryogenesis and larval stages. Loss of Serrate function leads to larval lethality, which is associated with several morphogenetic defects, including the failure to develop wings and halteres. Serrate has been suggested to act as a short-range signal during wing development. It is required for the induction of the organising centre at the dorsal/ventral compartment boundary, from which growth and patterning of the wing is controlled. In order to understand the regulatory network required to control the spatially and temporally dynamic expression of Serrate, we analysed its cis-regulatory elements by fusing various genomic fragments upstream of the reporter gene lacZ. Enhancer elements reflecting the expression pattern of endogenous Serrate in embryonic and postembryonic tissues could be confined to 26 kb of genomic DNA, including 9 kb of transcribed region. Expression in some embryonic tissues is under the control of multiple enhancers located in the 5’ region and in intron sequences. The data presented here provide the tools to unravel the genetic network which regulates Serrate during different developmental stages in diverse tissues. Received: 27 March 1998 / Accepted: 17 May 1998     

The homeotic Macho mutant of Antirrhinum majus reverts to wild-type or mutates to the homeotic plena phenotype

Plants of Antirrhinum majus carrying the semidominant Macho alleles of the plena gene display carpelloid sepals and staminoid petals, but the two inner flower whorls of stamens and carpels are normal and produce fertile gametes. In the recessive plena mutant, in contrast, the two outer whorls are normal whereas the stamens are largely or entirely petaloid and the carpels sepaloid, thus producing weakly male-fertile or fully sterile lines. Two new plena and two new Macho alleles have been induced in transposon tagging experiments. Genetic and molecular analysis revealed that the two contrasting mutant phenotypes are caused by mutations in one and the same gene: Several wild-type plants appeared among 27 000 F1 plants of a cross between Macho female plants and wild-type males bearing the active transposons Taml and Tam3. One of these plants segregated plena mutants, three showed reversions to wild-type and another two segregated Macho plants, possibly representing somatic reversions. Additional evidence was provided by an allelism test of Macho × plena. Molecular analysis has independently corroborated the genetical results. Moreover, the double mutant Macho/deficiens shows only carpels and plena/deficiens only sepals, which is in accord with combinatorial models for homeotic flower formation presented recently.     

Regulation of nodulin gene expression

The expression of plant genes specifically induced during rhizobial infection and the early stages of nodule ontogeny (early nodulin genes) and those induced in the mature, nitrogen-fixing nodule (late nodulin genes) is differentially regulated and tissue/cell specific. We have been interested in the signal transduction pathway responsible for symbiotic, temporal and spatial control of expression of an early (Enod2) and a late (Leghemoglobin;lb) nodulin gene from the stem-nodulated legumeSesbania rostrata, and in identifying thecis-acting elements andtrans-acting factors involved in this process (De Bruijn and Schell, 1992). By introducing chimericS. rostrata lb promoter-gus reporter gene fusions into transgenicLotus corniculatus plants, we have been able to show that thelb promoter directs an infected-cell-specific expression pattern inLotus nodules. We have been able to delimit thecis-acting element responsible for nodule-infected-cell-expression to a 78 pb region of thelb promoter (NICE Element) and have analyzed this element in detail by site-specific mutagenesis. We have studied the interaction of the NICE element, and further upstreamcis-acting elements, withtrans-acting factors of both plant- and rhizobial origin. We have obtained evidence for the involvement of rhizobial proteins in infected-cell-specific plant gene expression (Welters et al., 1993). We have purified one of the bacterial binding proteins from theS. rostrata symbiontAzorhizobium caulinodans (AcBBP1), and cloned and mutated the corresponding gene, in order to examine its symbiotic phenotype. We have also found that theS. rostrata Enod2 gene is rapidly induced by physiologically significant concentrations of cytokinins, suggesting the role of cytokinin as a potential secondary signal involved in nodulation (Dehio and De Bruijn, 1992). We are examining whether the observed cytokinin induction, as well as the nodule-specific expression pattern, are modulated by theSrEnod2 promoter.     

Molecular evolutionary characterization of an Activator (Ac)-like transposable element sequence from pearl millet (Pennisetum glaucum) (Poaceae)

We present data on the evolution of the Ac/Ds family of transposable elements in select grasses (Poaceae). A defective Ac-like element was cloned from a DNA library of the grass Pennisetum glaucum (pearl millet) and its entire 4531 bp sequence has been determined. When the pearl millet Ac-like sequence is aligned with the maize Ac sequence, it is found that there is approximately 70% DNA similarity in the central region spanning most of maize Ac exon II and all of exon III. In addition, there are two smaller regions of similarity at the Ac terminii. Besides these three major structural similarities, Pennisetum Ac has two large regions, one 5 and one 3, that show little similarity to Zea Ac. Furthermore, most of the sequences corresponding to intron II in maize Ac are absent in pearl millet Ac. Kimura's evolutionary distance between the central region of maize and pearl millet Ac sequences is estimated to be 0.429±0.020 nucleotide substitutions per site. This value is not significantly different from the average number of synonymous substitutions for coding regions of the Adh1 gene between maize and pearl millet, which is 0.395±0.051 nucleotide substitutions per site. If we assume Ac and Adh1 divergence times are equivalent between maize and pearl millet, then the above calculations suggest Ac-like sequences have probably not been strongly constrained by natural selection. Conserved DNA and amino acid sequence motifs are also examined. The level of DNA sequence divergence between maize and pearl millet Ac sequences, the estimated date when maize and pearl millet diverged (25–40 million years ago), coupled with their reproductive isolation/lack of current genetic exchange, all support the theory that Ac-like sequences have not been recently introduced into pearl millet from maize. Instead, Ac-like sequences were probably present in the progenitor of maize and pearl millet and have thus existed in the grasses for at least 25 million years.     

Structural organisation of the rat genes encoding liver- and heart-type of cytochrome c oxidase subunit VIa and a pseudogene related to the COXVIa-L cDNA

To study the tissue-specific expression of the heart(H)- and liver(L)-type of rat cytochrome-c oxidase subunit VIa (rCOXVIa), we have screened and sequenced the genes for the two isoforms. Both genes contain three exons and two introns, spanning 880 bp (rCOXVIa-H) and 3089 bp (rCOXVIa-L), respectively. In both genes, exon I codes for the whole leader sequence comprising 12 (rCOXVIa-H) or 26 (rCOXVIa-L) amino acids and for 12 (rCOXVIa-H) or 10 (rCOXVIa-L) amino acids of the corresponding mature protein, while the remaining amino acids for the mature proteins are encoded by exons II and III. The 5′ region of the genes lack both TATA and CAAT boxes, but show a high G+C content in the early 5′-upstream region. We have identified in upstream regions and in the introns of both genes several putative binding sites associated with respiratory function, muscle gene activation and housekeeping function. In rCOXVIa-H, we identified a CCAC/Myo-D motif, known to be required for muscle-specific expression of the human myoglobin-encoding gene, which is not present in rCOXVIa-L. In addition, we have analyzed a pseudogene, showing 84% homology to the COXVIa-L cDNA sequence.     

动态神经元的网络模型——Ⅲ.电路实现

在动态神经元网络数学模型的基础上,利用模拟有源器件与数学开关电路组成的硬件系统来达到模拟生物神经元的目的。模拟的结果表明：这个硬件具有神经元脉冲发放的动态过程,系统中每部分的输出信号分别对应于突触后电位、感受器电位、始段分级电位和轴突上脉冲发放等波形,与生物实验资料相似,是一个比计算机仿真更接近实际的连续模型,它将为小型神经元网络的动态特性分析提供了更直观,更可靠的手段。