eskimo1 mutants of Arabidopsis are constitutively freezing-tolerant

Temperate plants develop a greater ability to withstand freezing in response to a period of low but nonfreezing temperatures through a complex, adaptive process of cold acclimation. Very little is known about the signaling processes by which plants perceive the low temperature stimulus and transduce it into the nucleus to activate genes needed for increased freezing tolerance. To help understand the signaling processes, we have isolated mutants of Arabidopsis that are constitutively freezing-tolerant in the absence of cold acclimation. Freezing tolerance of wild-type Arabidopsis was increased from −5.5°C to −12.6°C by cold acclimation whereas the freezing tolerance of 26 mutant lines ranged from −6.8°C to −10.6°C in the absence of acclimation. Plants with mutations at the eskimo1 (esk1) locus accumulated high levels of proline, a compatible osmolyte, but did not exhibit constitutively increased expression of several cold-regulated genes involved in freezing tolerance. RNA gel blot analysis suggested that proline accumulation in esk1 plants was mediated by regulation of transcript levels of genes involved in proline synthesis and degradation. The characterization of esk1 mutants and results from other mutants suggest that distinct signaling pathways activate different aspects of cold acclimation and that activation of one pathway can result in considerable freezing tolerance without activation of other pathways.

Echo-delay resolution in sonar images of the big brown bat, Eptesicus fuscus

Echolocating big brown bats (Eptesicus fuscus) broadcast ultrasonic frequency-modulated (FM) biosonar sounds (20–100 kHz frequencies; 10–50 μs periods) and perceive target range from echo delay. Knowing the acuity for delay resolution is essential to understand how bats process echoes because they perceive target shape and texture from the delay separation of multiple reflections. Bats can separately perceive the delays of two concurrent electronically generated echoes arriving as little as 2 μs apart, thus resolving reflecting points as close together as 0.3 mm in range (two-point threshold). This two-point resolution is roughly five times smaller than the shortest periods in the bat’s sounds. Because the bat’s broadcasts are 2,000–4,500 μs long, the echoes themselves overlap and interfere with each other, to merge together into a single sound whose spectrum is shaped by their mutual interference depending on the size of the time separation. To separately perceive the delays of overlapping echoes, the bat has to recover information about their very small delay separation that was transferred into the spectrum when the two echoes interfered with each other, thus explicitly reconstructing the range profile of targets from the echo spectrum. However, the bat’s 2-μs resolution limit is so short that the available spectral cues are extremely limited. Resolution of delay seems overly sharp just for interception of flying insects, which suggests that the bat’s biosonar images are of higher quality to suit a wider variety of orientation tasks, and that biosonar echo processing is correspondingly more sophisticated than has been suspected.

Continuous and lurching traveling pulses in neuronal networks with delay and spatially decaying connectivity

Propagation of discharges in cortical and thalamic systems, which is used as a probe for examining network circuitry, is studied by constructing a one-dimensional model of integrate-and-fire neurons that are coupled by excitatory synapses with delay. Each neuron fires only one spike. The velocity and stability of propagating continuous pulses are calculated analytically. Above a certain critical value of the constant delay, these pulses lose stability. Instead, lurching pulses propagate with discontinuous and periodic spatio-temporal characteristics. The parameter regime for which lurching occurs is strongly affected by the footprint (connectivity) shape; bistability may occur with a square footprint shape but not with an exponential footprint shape. For strong synaptic coupling, the velocity of both continuous and lurching pulses increases logarithmically with the synaptic coupling strength gsyn for an exponential footprint shape, and it is bounded for a step footprint shape. We conclude that the differences in velocity and shape between the front of thalamic spindle waves in vitro and cortical paroxysmal discharges stem from their different effective delay; in thalamic networks, large effective delay between inhibitory neurons arises from their effective interaction via the excitatory cells which display postinhibitory rebound.

Residual delay maps unveil global patterns of atmospheric nonlinearity and produce improved local forecasts

We use residual-delay maps of observational field data for barometric pressure to demonstrate the structure of latitudinal gradients in nonlinearity in the atmosphere. Nonlinearity is weak and largely lacking in tropical and subtropical sites and increases rapidly into the temperate regions where the time series also appear to be much noisier. The degree of nonlinearity closely follows the meridional variation of midlatitude storm track frequency. We extract the specific functional form of this nonlinearity, a V shape in the lagged residuals that appears to be a basic feature of midlatitude synoptic weather systems associated with frontal passages. We present evidence that this form arises from the relative time scales of high-pressure versus low-pressure events. Finally, we show that this nonlinear feature is weaker in a well regarded numerical forecast model (European Centre for Medium-Range Forecasts) because small-scale temporal and spatial variation is smoothed out in the grided inputs. This is significant, in that it allows us to demonstrate how application of statistical corrections based on the residual-delay map may provide marked increases in local forecast accuracy, especially for severe weather systems.

Change in Apoplastic Aluminum during the Initial Growth Response to Aluminum by Roots of a Tolerant Maize Variety1

Root elongation, hematoxylin staining, and changes in the ultrastructure of root-tip cells of an Al-tolerant maize variety (Zea mays L. C 525 M) exposed to nutrient solutions with 20 μm Al (2.1 μm Al3+ activity) for 0, 4, and 24 h were investigated in relation to the subcellular distribution of Al using scanning transmission electron microscopy and energy-dispersive x-ray microanalysis on samples fixed by different methods. Inhibition of root-elongation rates, hematoxylin staining, cell wall thickening, and disturbance of the distribution of pyroantimoniate-stainable cations, mainly Ca, was observed only after 4 and not after 24 h of exposure to Al. The occurrence of these transient, toxic Al effects on root elongation and in cell walls was accompanied by the presence of solid Al-P deposits in the walls. Whereas no Al was detectable in cell walls after 24 h, an increase of vacuolar Al was observed after 4 h of exposure. After 24 h, a higher amount of electron-dense deposits containing Al and P or Si was observed in the vacuoles. These results indicate that in this tropical maize variety, tolerance mechanisms that cause a change in apoplastic Al must be active. Our data support the hypothesis that in Al-tolerant plants, Al can rapidly cross the plasma membrane; these data clearly contradict the former conclusions that Al mainly accumulates in the apoplast and enters the symplast only after severe cell damage has occurred.

Molecular and Physiological Responses to Water Deficit in Drought-Tolerant and Drought-Sensitive Lines of Sunflower1 : Accumulation of Dehydrin Transcripts Correlates with Tolerance

To investigate correlations between phenotypic adaptation to water limitation and drought-induced gene expression, we have studied a model system consisting of a drought-tolerant line (R1) and a drought-sensitive line (S1) of sunflowers (Helianthus annuus L.) subjected to progressive drought. R1 tolerance is characterized by the maintenance of shoot cellular turgor. Drought-induced genes (HaElip1, HaDhn1, and HaDhn2) were previously identified in the tolerant line. The accumulation of the corresponding transcripts was compared as a function of soil and leaf water status in R1 and S1 plants during progressive drought. In leaves of R1 plants the accumulation of HaDhn1 and HaDhn2 transcripts, but not HaElip1 transcripts, was correlated with the drought-adaptive response. Drought-induced abscisic acid (ABA) concentration was not associated with the varietal difference in drought tolerance. Stomata of both lines displayed similar sensitivity to ABA. ABA-induced accumulation of HaDhn2 transcripts was higher in the tolerant than in the sensitive genotype. HaDhn1 transcripts were similarly accumulated in the tolerant and in the sensitive plants in response to ABA, suggesting that additional factors involved in drought regulation of HaDhn1 expression might exist in tolerant plants.

Light-induced phase-delay of the chicken pineal circadian clock is associated with the induction of cE4bp4, a potential transcriptional repressor of cPer2 gene

The chicken pineal gland contains the autonomous circadian oscillator together with the photic-input pathway. We searched for chicken pineal genes that are induced by light in a time-of-day-dependent manner, and isolated chicken homolog of bZIP transcription factor E4bp4 (cE4bp4) showing high similarity to vrille, one of the Drosophila clock genes. cE4bp4 was expressed rhythmically in the pineal gland with a peak at very early (subjective) night under both 12-h light/12-h dark cycle and constant dark conditions, and the phase was nearly opposite to the expression rhythm of cPer2, a chicken pineal clock gene. Luciferase reporter gene assays showed that cE4BP4 represses cPer2 promoter through a E4BP4-recognition sequence present in the 5′-flanking region, indicating that cE4BP4 can down-regulate the chick pineal cPer2 expression. In vivo light-perturbation studies showed that the prolongation of the light period to early subjective night maintained the high level expression of the pineal cE4bp4, and presumably as a consequence delayed the onset of the induction of the pineal cPer2 expression in the next morning. These light-dependent changes in the mRNA levels of the pineal cE4bp4 and cPer2 were followed by a phase-delay of the subsequent cycles of cE4bp4/cPer2 expression, suggesting that cE4BP4 plays an important role in the phase-delaying process as a light-dependent suppressor of cPer2 gene.

Viral dynamics in vivo: limitations on estimates of intracellular delay and virus decay.

Anti-viral drug treatment of human immunodeficiency virus type I (HIV-1) and hepatitis B virus (HBV) infections causes rapid reduction in plasma virus load. Viral decline occurs in several phases and provides information on important kinetic constants of virus replication in vivo and pharmacodynamical properties. We develop a mathematical model that takes into account the intracellular phase of the viral life-cycle, defined as the time between infection of a cell and production of new virus particles. We derive analytic solutions for the dynamics following treatment with reverse transcriptase inhibitors, protease inhibitors, or a combination of both. For HIV-1, our results show that the phase of rapid decay in plasma virus (days 2-7) allows precise estimates for the turnover rate of productively infected cells. The initial quasi-stationary phase (days 0-1) and the transition phase (days 1-2) are explained by the combined effects of pharmacological and intracellular delays, the clearance of free virus particles, and the decay of infected cells. Reliable estimates of the first three quantities are not possible from data on virus load only; such estimates require additional measurements. In contrast with HIV-1, for HBV our model predicts that frequent early sampling of plasma virus will lead to reliable estimates of the free virus half-life and the pharmacological properties of the administered drug. On the other hand, for HBV the half-life of infected cells cannot be estimated from plasma virus decay.

Characterization of Arabidopsis thaliana cDNAs that render yeasts tolerant toward the thiol-oxidizing drug diamide.

Diamide oxidizes cellular thiols and induces oxidative stress. To isolate plant genes which may, when overexpressed, increase tolerance of plants toward oxidative damage, an in vivo diamide tolerance screening in yeasts was used. An Arabidopsis cDNA library in a yeast expression vector was used to transform a yeast strain with intact antioxidant defense. Cells from approximately 10(5) primary transformants were selected for resistance to diamide. Three Arabidopsis cDNAs which confer diamide tolerance were isolated. This drug tolerance was specific and no cross tolerance toward hydroperoxides was found. One cDNA (D3) encodes a polypeptide which has an amino-terminal J domain characteristic of a divergent family of DnaJ chaperones. Another (D18) encodes a putative dTDP-D-glucose 4,6-dehydratase. Surprisingly, the third cDNA (D22) encodes a plant homolog of gamma-glutamyltransferases. It would have been difficult to predict that the expression of those genes would lead to an improved survival under conditions of depletion of cellular thiols. Hence, we suggest that this cloning approach may be a useful contribution to the isolation of plant genes that can help to cope with oxidative stress.

A heme-protein-based oxygen-sensing mechanism controls the expression and suppression of multiple proteins in anoxia-tolerant turtle hepatocytes.

The O2 sensitivity of protein expression was assessed in hepatocytes from the western painted turtle. Anoxic cells consistently expressed proteins of 83.0, 70.4, 42.5, 35.3, and 16.1 kDa and suppressed proteins of 63.7, 48.2, 36.9, 29.5, and 17.7 kDa. Except for the 70.4-kDa protein, this pattern was absent during aerobic incubation with 2 mM NaCN, suggesting a specific requirement for O2. Aerobic incubation with Co2+ or Ni2+ increased expression of the 42.5-, 35.3-, and 16.1-kDa protein bands which was diminished with the heme synthesis inhibitor 4,6-dioxoheptanoic acid. Proteins suppressed in anoxia were also suppressed during aerobic incubation with Co2+ or Ni2+ but this was not relieved by 4,6-dioxoheptanoic acid. The anoxia- and Co2+/Ni2+-induced expression of the 42.5-, 35.3-, and 16.1-kDa protein bands was antagonized by 10% CO; however, with the exception of the 17.7-kDa protein, this was not found for any of the O2- or Co2+/Ni2+-suppressed proteins. Anoxia-induced proteins were compared with proteins expressed during heat shock. Heat shock proteins appeared at 90.2, 74.8, 63.4, 25, and 15.5 kDa and were of distinct molecular masses compared with the anoxia-induced proteins. These results suggest that O2-sensing mechanisms are active in the control of protein expression and suppression during anoxia and that, in the case of the 42.5-, 35.3-, 17.7-, and 16.1-kDa proteins, a conformational change in a ferro-heme protein is involved in transducing the O2 signal.