• The genetic structure of bacterial and fungal communities was characterized in the rhizosphere of Medicago truncatula Gaertn. cv. Jemalong line J5 at five developmental stages (three vegetative and ...two reproductive stages), and in three compartments (bulk soil, rhizosphere soil and root tissues). • The genetic structure of microbial communities was determined by cultivation-independent methods using directly extracted DNA that was characterized by automated ribosomal intergenic spacer analysis (ARISA). • Principal component analyses (PCA) indicate that, for all developmental stages, the genetic structure of microbial communities differed significantly by compartment, with a major shift in the community in root tissues corresponding to the most intimate compartment with the plant. • Differences were also recorded during plant development, the most significant being observed during the transition between vegetative and reproductive stages. Throughout this period, plants were shown to establish the highest level of symbiotic association (mycorrhization, nodulation) with arbuscular mycorrhizal fungi and Rhizobia. During the reproductive stages, the dynamics of the genetic structure differed between bacterial and fungal communities. At the last reproductive stage, the genetic structure of bacterial communities became close to that recorded during the first vegetative stages, suggesting a resilience phenomenon, whereas the genetic structure of fungal communities remained different from the vegetative stages and also from the early reproductive stages, suggesting a persistence of the rhizosphere effect.
The genetic structures of bacterial communities associated with Medicago truncatula Gaertn. cv. Jemalong line J5 (Myc⁺ Nod⁺) and its symbiosis-defective mutants TRV48 (Myc⁺ Nod⁻) and TRV25 (Myc⁻ ...Nod⁻) were compared. Plants were cultivated in a fertile soil (Châteaurenard, France) and in soil from the Mediterranean basin showing a low fertility (Mas d'Imbert, France). Plant growth, root architecture, and the efficiency of root symbiosis of the three plant genotypes were characterized in the two soils. Structures of the bacterial communities were assessed by automated-ribosomal intergenic spacer analysis (A-RISA) fingerprinting from DNA extracted from the rhizosphere soil and root tissues. As expected, the TRV25 mutant did not develop endomycorrhizal symbiosis in any of the soils, whereas mycorrhization of line J5 and the TRV48 mutant occurred in both soils but at a higher intensity in the Mas d'Imbert (low fertility) than in the Châteaurenard soil. However, modifications of plant growth and root architecture, between mycorrhizal (J5 and TRV48) and nonmycorrhizal (TRV25) plants, were recorded only when cultivated in the Mas d'Imbert soil. Similarly, the genetic structures of bacterial communities associated with mycorrhizal and nonmycorrhizal plants differed significantly in the Mas d'Imbert soil but not in the Châteaurenard soil. Multivariate analysis of the patterns allowed the identification of molecular markers, explaining these differences, and markers were further sequenced. Molecular marker analysis allowed the delineation of 211 operational taxonomic units. Some of those belonging to the Comamonadaceae and Oxalobacteraceae (β-Proteobacteria) families were found to be significantly more represented within bacterial communities associated with the J5 line and the TRV48 mutant than within those associated with the TRV25 mutant, indicating that these bacterial genera were preferentially associated with mycorrhizal roots in the Mas d'Imbert soil.
Bacterial strains from mycorrhizal roots (three belonging to
Comamonadaceae
and one to
Oxalobacteraceae
) and from non-mycorrhizal roots (two belonging to
Comamonadaceae
) of
Medicago truncatula
and ...two reference strains (
Collimonas fungivorans
Ter331 and
Pseudomonas fluorescens
C7R12) were tested for their effect on the in vitro saprophytic growth of
Glomus mosseae
BEG12 and on its colonization of
M. truncatula
roots. Only the Oxalobacteraceae strain, isolated from barrel medic mycorrhizal roots, and the reference strain
P. fluorescens
C7R12 promoted both the saprophytic growth and root colonization of
G. mosseae
BEG12, indicating that they acted as mycorrhiza helper bacteria. Greatest effects were achieved by
P. fluorescens
C7R12 and its influence on the saprophytic growth of
G. mosseae
was compared to that on
Gigaspora rosea
BEG9 to determine if the bacterial stimulation was fungal specific. This fungal specificity, together with plant specificity, was finally evaluated by comparing bacterial effects on arbuscular mycorrhizal symbiosis when each of the fungal species was inoculated to two different plant species (
M. truncatula
and
Lycopersicon esculentum
). The results obtained showed that promotion of saprophytic growth by
P. fluorescens
C7R12 was expressed in vitro towards
G. mosseae
but not towards
G. rosea
. Bacterial promotion of mycorhization was also expressed towards
G. mosseae
, but not
G. rosea
, in roots of
M. truncatula
and
L. esculentum
. Taken together, results indicated that enhancement of arbuscular mycorrhiza development was only induced by a limited number of bacteria, promotion by the most efficient bacterial strain being fungal and not plant specific.
Abstract
The genetic diversity of bacterial communities associated with mycorrhizal and nonmycorrhizal roots of Medicago truncatula was characterized by two approaches. Firstly, phylogenetic analysis ...was performed on 164 partial 16S rRNA gene–intergenic spacer (IGS) sequences from operational taxonomic units previously shown to be preferentially associated with mycorrhizal roots. These sequences were distributed into three branches corresponding to Comamonadaceae, Oxalobacteraceae and Rubrivivax subgroups. Most sequences were obtained from mycorrhizal roots, indicating the preferential association of the corresponding families with mycorrhizal roots. A second phylogenetic analysis was performed on the partial 16S rRNA gene–IGS sequences of 173 isolates among a large collection of isolates, from mycorrhizal and nonmycorrhizal roots, belonging to Comamonadaceae and Oxalobacteraceae on the basis of their positive hybridization with a partial 16S rRNA gene–IGS probe obtained in this study. Sequence analysis confirmed the affiliation of 166 isolates to Comamonadaceae and seven to Oxalobacteraceae. Oxalobacteraceae isolates were more abundant in mycorrhizal (five) than in nonmycorrhizal (two) roots, whereas Comamonadaceae isolates were more abundant in nonmycorrhizal (109) than mycorrhizal roots (57). Further analysis of Comamonadaceae isolates by BOX-PCR showed that the genetic structure of culturable populations belonging to this family differed significantly in mycorrhizal and nonmycorrhizal roots, as indicated by distributions in different BOX types, differences being significantly explained by BOX types only including isolates from mycorrhizal roots. These data are discussed in an ecological context.
The superficial extensor muscles of the crayfish abdomen were examined as a possible site for modulation by DF
2 (Asp-Arg-Asn-Phe-Leu-Arg-Phe-NH
2), a FMRFamide-related neuropeptide found in crayfish ...pericardial organs (26). The superficial extensor muscles are of the tonic type and generate slow contractions that affect posture. DF
2, at concentrations of 10
−8 M or higher, increased muscle tonus in isolated, unstimulated neuromuscular preparations. In some preparations, the peptide also induced small, arrhythmic contractions or increased the amplitude of such contractions if they were already present. The spontaneous contractions were temperature-dependent and insensitive to 10
−7 M tetrodotoxin, indicating that they were myogenic. DF
2 increased muscle tonus in the presence of tetrodotoxin and when nerve-evoked contractions were blocked using Joro spider toxin (JSTX), a glutamate receptor antagonist. Thus, the effects of DF
2 on contraction appear to represent direct effects on the muscle and not changes in release of chemical transmitter from nerve terminals. DF
2 did not alter resting membrane potential or input resistance in the muscle fibres.
The effects of DF
2 on contraction were blocked by the Ca
2+ channel antagonists Mn
2+ Ni
2+ and Cd
2+ and nicardipine, and by replacing extracellular Ca
2+ with Mg
2+. This suggests that the peptide's effect may require an influx of extracellular Ca
2+ through dihydropyridine-sensitive Ca
2+ channels. The Ca
2+ channel antagonists also reduced muscle tonus on their own, suggesting that they may lower the intracellular calcium concentration. The peptide might act by enhancing Ca
2+ influx or by enhancing Ca
2+-dependent release of Ca
2+ ions from internal stores.
We report here on the design, fabrication and high-speed performance of a novel parallel optical module with sixteen 10-Gb/s transmitter and receiver channels for a 160-Gb/s bidirectional aggregate ...data rate. The module utilizes a single-chip CMOS optical transceiver containing both transmitter and receiver circuits. 16-channel high-speed photodiode (PD) and VCSEL arrays are flip-chip attached to the low-power CMOS IC. The substrate emitting/illuminated VCSEL and PD arrays operate at 985 nm and include collimating lenses integrated into the backside of the substrate. The IC-OE assembly is then flip-chip attached to a high density organic package forming the transceiver optical module. The exclusive use of flip-chip packaging for both the IC-to-optoelectronic (OE) devices and for the IC-to-organic package minimizes the module footprint and associated packaging parasitics. The OE-on-IC assembly achieves a high area efficiency of 9.4 Gb/s/mm 2 (Schow et al., 2007). The complete organic carrier transceiver package provides a low-cost, low-profile module similar to a conventional chip-carrier that can be directly surface mounted to a circuit board using a conventional BGA solder process. SLC transceiver modules with transmitter and receiver OE-IC arrays were assembled and characterized. Operation of all 16 transmitters in the transceiver module was demonstrated at data rates >10 Gb/s. Similarly, all 16 receiver channels operated error-free at >10 Gb/s. The receiver eye-diagrams were generated using a second transceiver source and therefore constitute a full transceiver optical link.
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