Environmental Control of Phosphorus Acquisition: A Piece of the Molecular Framework Underlying Nutritional Homeostasis

Homeostasis of phosphorus (P), an essential macronutrient, is vital for plant growth under diverse environmental conditions. Although plants acquire P from the soil as inorganic phosphate (Pi), its availability is generally limited. Therefore, plants employ mechanisms involving various Pi transporters that facilitate efficient Pi uptake against a steep concentration gradient across the plant–soil interface. Among the different types of Pi transporters in plants, some members of the PHOSPHATE TRANSPORTER 1 (PHT1) family, present in the plasma membrane of root epidermal cells and root hairs, are chiefly responsible for Pi uptake from the rhizosphere. Therefore, accurate regulation of PHT1 expression is crucial for the maintenance of P homeostasis. Previous investigations positioned the Pi-dependent post-translational regulation of PHOSPHATE STARVATION RESPONSE 1 (PHR1) transcription factor activity at the center of the regulatory mechanism controlling PHT1 expression and P homeostasis; however, recent studies indicate that several other factors also regulate the expression of PHT1 to modulate P acquisition and sustain P homeostasis against environmental fluctuations. Together with PHR1, several transcription factors that mediate the availability of other nutrients (such as nitrogen and zinc), light, and stress signals form an intricate transcriptional network to maintain P homeostasis under highly diverse environments. In this review, we summarize this intricate transcriptional network for the maintenance of P homeostasis under different environmental conditions, with a main focus on the mechanisms identified in Arabidopsis.

Mathematical Modeling of Interface Movement during Diffusional Bonding of Surfaces

The concentration profile of Cu is modelled using semi-infinite geometry for diffusion couples of α and β phases in Cu-Al system. The dimensionless interface movement parameter γ is calculated, for various combinations of time and temperature, by root bracketing, bisection and inverse quadratic interpolation. A computational procedure is presented to calculate the concentration profile where the interface velocity (dε/dt) is high and/or with steep concentration gradient of the specie in the shrinking phase. In all cases the interface compositions are set at the equilibrium values given in the phase diagram with fixed composition of end members. The calculated profile match well with the experimental concentration profile as reported by Romig [3].

ATP-binding Cassette Transporters Are Required for Efficient RNA Interference in Caenorhabditis elegans

RNA interference (RNAi) is a conserved gene-silencing phenomenon that can be triggered by delivery of double-stranded RNA (dsRNA) to cells and is a widely exploited technology in analyses of gene function. Although a number of proteins that facilitate RNAi have been identified, current descriptions of RNAi and interrelated mechanisms are far from complete. Here, we report that the Caenorhabditis elegans gene haf-6 is required for efficient RNAi. HAF-6 is a member of the ATP-binding cassette (ABC) transporter gene superfamily. ABC transporters use ATP to translocate small molecule substrates across the membranes in which they reside, often against a steep concentration gradient. Collectively, ABC transporters are involved in a variety of activities, including protective or barrier mechanisms that export drugs or toxins from cells, organellar biogenesis, and mechanisms that protect against viral infection. HAF-6 is expressed predominantly in the intestine and germline and is localized to intracellular reticular organelles. We further demonstrate that eight additional ABC genes from diverse subfamilies are each required for efficient RNAi in C. elegans. Thus, the ability to mount a robust RNAi response to dsRNA depends upon the deployment of two ancient systems that respond to environmental assaults: RNAi mechanisms and membrane transport systems that use ABC proteins.

Interaction effects of clay swelling and dispersion and CaCO3 content on saturated hydraulic conductivity

Typic Chromoxeret-sand and Lithic Ruptic Xerochrept-sand mixtures, of high sodicity, and containing 8 and ~0% CaCO3, respectively, were packed in columns and leached with electrolyte solutions. When a reference solution of 500 mmol/L was replaced successively with solutions containing 100, 50, 10, 5, and 1 mmol/L and then with deionised water (gradual leaching), the saturated hydraulic conductivity (Ks) decreased gradually to steady-state values, and no clay was observed in the leachate. Under deionised leaching, the relative Ks (Ks/Ks0) (the ratio between the observed Ks and that under leaching with the reference solution) in the Chromoxeret mixture at exchangeable sodium percentages (ESPs) of 30, 38, and 63 was 0.83, 0.47, and 0.41, respectively, and that in the Xerochrept mixture at ESPs of 23, 28, and 36 were 0.82, 0.71, and 0.39 respectively. When a solution of 50 mmol/L was replaced directly with deionised water (abrupt leaching), the Ks/Ks0 of the Chromoxeret mixture at the above ESPs dropped sharply to minimum values of 0.25, 0.18, and 0.11, respectively, and that of the Xerochrept mixture to 0.42, 0.04, and 0.13, respectively, and dispersed clay was observed in the leachate. It is hypothesised that the abrupt leaching with deionised water generated a steep concentration gradient between the solutions within and around the aggregates, leading to clay dispersion and rapid Ks/Ks0 reduction; under gradual leaching, no steep concentration gradient developed, there was no clay dispersion, and clay swelling caused gradual Ks/Ks0 reduction. In the Chromoxeret, the CaCO3 decreased the clay swelling and dispersion, and probably acted as a cementing agent that stabilised the soil aggregates against slaking during the initial wetting of the mixture.

Self Organized Compound Semiconductor Nanocrystallite Distributions in SiO2 on Silicon Synthesized by Ion Implantation

ABSTRACTThe temporal and spatial evolution of sequentially and single ion implanted Cd and Se concentration distributions into thermally grown SiO2 on (001)-Si were investigated. Ex-situ rapid thermal annealing was performed to initiate the reaction-diffusion driven material transport, nanocrystal nucleation and growth. Finally this leads to the formation of buried distinct layers of CdSe nanocrystals. The spatiotemporal evolution of the concentration distributions were quantitatively analyzed by dynamic Secondary Ion Mass Spectrometry (SIMS). It will be shown, that there is a correlated diffusion of Cd and Se when both elements were implanted in overlapping concentration profiles, whereas the single implanted Cd and Se exhibit a completely different diffusion behaviour. In the region of the supersaturated solid solution reaction to thermodynamically stable CdSe clusters takes place. The steep concentration gradient provokes indiffusion of the stoichiometric compound. In the case of a surplus of Cd over Se and long annealing times self-organized, nearly periodic and correlated concentration variations of Cd and Se can be observed. Comparison of the profiles indicate that this pattern formation is controlled by the diffusion and precipitation of the over stoichiometric Cd.

Avoidance of Pentachlorophenol by Postlarval Brown Shrimp (Penaeus aztecus) (Decapoda, Penaeidae) in a Laminar-Flow Choice Chamber

Responses of postlarval brown shrimp (Penaeus aztecus) to pentachlorophenol (0–450 μg∙L−1) were measured in synthetic seawater and estuarine water using a laminar-flow choice chamber. This chamber provides individual postlarvae with equal exposure to two parallel olfactant streams separated by a steep concentration gradient. Shrimp detected and avoided pentachlorophenol concentrations above 91 μg∙L−1 in synthetic seawater. This detection threshold reflects limitations in statistical power, and with increased replication the physiological threshold could probably be resolved at a much lower concentration. Pentachlorophenol appeared to be more repellent when dissolved in estuarine water from Galveston Bay, Texas. The 96-h LC50 for pentachlorophenol was 317 μg∙L−1 which suggests that postlarvae are capable of avoiding acutely toxic concentrations of this pollutant. For postlarvae of this species, behavioral avoidance appears to provide a more sensitive indicator of pollutant responses than the conventional toxicity bioassay.

The Absorption and Metabolism of Some Sugars in the Locust, Schistocerca Gregaria (Forsk.)

1. The uptake of 14C-labelled glucose, mannose and fructose from the alimentary canal of Schistocerca gregaria (Forsk.) has been studied using the dye Amaranth as a reference substance. 2. Absorption was confined to the mid-gut, the proportion absorbed by the caeca depending on the type of sugar and its concentration in the gut lumen. 3. The absorbed sugars were converted, in varying degrees, to trehalose which accumulated in the haemolymph. The extent of the conversion appeared to parallel the rate of absorption of the sugars at the various concentrations. 4. The sugars passed through the gut wall at similar rates in experiments in which the isolated alimentary canal was suspended in a large volume of circulating poisoned saline. The passage through the gut wall under these conditions was equivalent to the rapid absorption obtained in vivo when there was a rapid conversion to trehalose. 5. It is suggested that these observations support the hypothesis that the sugars are absorbed by diffusion across the gut wall and that the process is facilitated by the rapid conversion to trehalose in the haemolymph, which tends to maintain a steep concentration gradient. 6. At very low concentrations in the gut lumen, glucose was at a similar level to the relatively small amount of glucose in equilibrium with the trehalose in the haemolymph. When the specific activity of injected 14C in the haemolymph approximated to that in the gut lumen the absorption of the labelled glucose from the mid-gut caeca was reduced at very low concentrations. Thus at these levels it is suggested that most of the absorption of labelled glucose can be attributed to an exchange with that in the haemolymph. At a concentration of 0.01 M/1. the proportion exchanging became negligibly small and the absorption of the labelled glucose molecules approximated to the net glucose absorption.

The Absorption of Glucose from the Alimentary Canal of the Locust Schistocerca Gregaria (Forsk.)

1. The absorption of glucose from the alimentary canal of Schistocerca gregaria has been studied by filling the gut with a saline solution containing 14C-labelled glucose together with a dye, Amaranth, which was used as a marker. The net percentage absorption was calculated from the glucose/dye ratio in the various parts of the alimentary canal. 2. The bulk of the glucose was absorbed from the mid-gut caeca, smaller amounts being absorbed by the ventriculus. 3. Glucose absorption was studied at concentrations of 0.002, 0.02 and 0.20M/l. in solutions in which the total osmolarity was maintained by altering the NaCl concentration. The percentage absorption was similar at concentrations of 0.002 and 0.02M/l., but was significantly less at 0.20M/l. 4. The fate of the 14C-labelled glucose was followed using paper chromatography. The glucose was shown to be rapidly converted to trehalose in the haemolymph. At a concentration of 0.20M/l. this mechanism became saturated and excess glucose accumulated in the haemolymph. 5. The absorption of glucose in vitro, from a gut suspended in a relatively large volume of poisoned saline, was found to be similar to that in the intact insect. 6. From these observations it is suggested that glucose is absorbed by diffusion across the gut wall and that the process is facilitated by the rapid conversion of glucose to trehalose in the haemolymph, which tends to maintain a steep concentration gradient across the gut wall.