Hydraulic properties of layered soils influence survival of Rhodes grass (<Emphasis Type="Italic">Chloris gayana</Emphasis> Kunth.) during water stress

Survival of vegetation on soil-capped mining wastes is often impaired during dry seasons due to the limited amount of water stored in the shallow soil capping. Growth and survival of Rhodes grass (Chloris gayana) during soil drying on various layered capping sequences constructed of combinations of topsoil, subsoil, seawater-neutralised residue sand and low grade bauxite was determined in a glasshouse. The aim was to describe the survival of Rhodes grass in terms of plant and soil water relationships. The soil water characteristic curve and soil texture analysis was a good predictor of plant survival. The combination of soil with a high water holding capacity and low soil water diffusivity (e.g. subsoil with high clay contents) with soil having a high water holding capacity and high diffusivity (e.g. residue sand) gave best survival during drying down (up to 88 days without water), whereas topsoil and low grade bauxite were unsuitable (plants died within 18–39 days). Clayey soil improved plant survival by triggering a water stress response during peak evaporative water demand once residue sand dried down and its diffusivity fell below a critical range. Thus, for revegetation in seasonally dry climates, soil capping should combine one soil with low diffusivity and one or more soils with high total water holding capacity and high diffusivity.     

The self-incompatibility (S) haplotypes of Brassica contain highly divergent and rearranged sequences of ancient origin.

In Brassica, the recognition of self-related pollen by the stigma is controlled by the highly polymorphic S locus that encodes several linked and coadapted genes and can span several hundred kilobases. We used pulsed-field gel electrophoresis to analyze the structure of different S haplotypes. We show that the S2 and S13 haplotypes of Brassica oleracea contain extensive sequence divergence and rearrangement relative to each other. In contrast, haplotypic configuration is more conserved between B. oleracea S13 and B. campestris S8, two haplotypes that have been proposed to be derived from a common ancestral haplotype based on sequence comparisons. These results support the view that extensive restructuring of the S locus preceded speciation in Brassica. This structural heteromorphism, together with haplotype-specific sequences, may suppress recombination within the S locus complex, potentially providing a mechanism for maintaining the linkage of coadapted allelic combinations of genes over time.     

Two modes of gating during late Na+ channel currents in frog sartorius muscle

Na+ currents were measured during 0.4-s depolarizing pulses using the cell-attached variation of the patch-clamp technique. Patches on Cs-dialyzed segments of sartorius muscle of Rana pipiens contained an estimated 25-500 Na+ channels. Three distinct types of current were observed after the pulse onset: a large initial surge of inward current that decayed within 10 ms (early currents), a steady "drizzle" of isolated, brief, inward unitary currents (background currents), and occasional "cloudbursts" of tens to hundreds of sequential unitary inward currents (bursts). Average late currents (background plus bursts) were 0.12% of peak early current amplitude at -20 mV. 85% of the late currents were carried by bursting channels. The unit current amplitude was the same for all three types of current, with a conductance of 10.5 pS and a reversal potential of +74 mV. The magnitudes of the three current components were correlated from patch to patch, and all were eliminated by slow inactivation. We conclude that all three components were due to Na+ channel activity. The mean open time of the background currents was approximately 0.25 ms, and the channels averaged 1.2 openings for each event. Neither the open time nor the number of openings of background currents was strongly sensitive to membrane potential. We estimated that background openings occurred at a rate of 0.25 Hz for each channel. Bursts occurred once each 2,000 pulses for each channel (assuming identical channels). The open time during bursts increased with depolarization to 1-2 ms at -20 mV, whereas the closed time decreased to less than 20 ms. The fractional open time during bursts was fitted with m infinity 3 using standard Na+ channel models. We conclude that background currents are caused by a return of normal Na+ channels from inactivation, while bursts are instances where the channel's inactivation gate spontaneously loses its function for prolonged periods.     

Kinetics of veratridine action on Na channels of skeletal muscle

Veratridine bath-applied to frog muscle makes inactivation of INa incomplete during a depolarizing voltage-clamp pulse and leads to a persistent veratridine-induced Na tail current. During repetitive depolarizations, the size of successive tail currents grows to a plateau and then gradually decreases. When pulsing is stopped, the tail current declines to zero with a time constant of approximately 3 s. Higher rates of stimulation result in a faster build-up of the tail current and a larger maximum value. I propose that veratridine binds only to open channels and, when bound, prevents normal fast inactivation and rapid shutting of the channel on return to rest. Veratridine-modified channels are also subject to a "slow" inactivation during long depolarizations or extended pulse trains. At rest, veratridine unbinds with a time constant of approximately 3 s. Three tests confirm these hypotheses: (a) the time course of the development of veratridine-induced tail currents parallels a running time integral of gNa during the pulse; (b) inactivating prepulses reduce the ability to evoke tails, and the voltage dependence of this reduction parallels the voltage dependence of h infinity; (c) chloramine-T, N-bromoacetamide, and scorpion toxin, agents that decrease inactivation in Na channels, each greatly enhance the tail currents and alter the time course of the appearance of the tails as predicted by the hypothesis. Veratridine-modified channels shut during hyperpolarizations from -90 mV and reopen on repolarization to -90 mV, a process that resembles normal activation gating. Veratridine appears to bind more rapidly during larger depolarizations.     

A highly conserved Brassica gene with homology to the S-locus-specific glycoprotein structural gene.

The S-locus-specific glycoprotein of Brassica and the gene encoding it (the SLG gene) are thought to be involved in determining self-incompatibility phenotype in this genus. It has been shown that the Brassica genome contains multiple SLG-related sequences. We report here the cloning and characterization of a Brassica oleracea gene, SLR1, which corresponds to one of these SLG-related sequences. Like the SLG gene, SLR1 is developmentally regulated. It is maximally expressed in the papillar cells of the stigma at the same stage of flower development as the onset of the incompatibility response. Unlike SLG, the SLR1 genes isolated from different S-allele homozygotes are highly conserved, and this gene, which appears to be ubiquitous in crucifers, is expressed in self-compatible strains as well as self-incompatible strains. Most importantly, we show that the SLR1 gene is not linked to the S-locus and therefore cannot be a determinant of S-allele specificity in Brassica.     

Using wintergreen oil for mounting mosquito larvae: a safer alternative to xylene

Permanent mounting of fourth instar mosquito larvae is essential for identifying Aedes spp. This procedure requires extensive exposure to xylene, a clearing agent in the mounting process. We investigated wintergreen oil as a substitute for xylene. Five hundred larvae were mounted on slides to evaluate shrinkage or expansion of specimens after clearing using xylene or wintergreen oil. We examined the ventral brush and siphonal hair tufts for species identification and for preservation of morphological characteristics after clearing specimens in xylene or wintergreen oil. Shrinkage of the length of whole larvae and width of the head, thorax and abdomen after mounting was significantly greater after clearing with xylene than with wintergreen oil. The length of the comb scale nearest the ventral brush was similar for both clearing agents. The clarity of the specimens after mounting was improved by clearing with wintergreen oil, but the integrity of the ventral brush and siphonal hair tufts were similar for both clearing agents.     

The regulation of RhoA at focal adhesions by StarD13 is important for astrocytoma cell motility

Malignant astrocytomas are highly invasive into adjacent and distant regions of the normal brain. Rho GTPases are small monomeric G proteins that play important roles in cytoskeleton rearrangement, cell motility, and tumor invasion. In the present study, we show that the knock down of StarD13, a GTPase activating protein (GAP) for RhoA and Cdc42, inhibits astrocytoma cell migration through modulating focal adhesion dynamics and cell adhesion. This effect is mediated by the resulting constitutive activation of RhoA and the subsequent indirect inhibition of Rac. Using Total Internal Reflection Fluorescence (TIRF)-based Förster Resonance Energy Transfer (FRET), we show that RhoA activity localizes with focal adhesions at the basal surface of astrocytoma cells. Moreover, the knock down of StarD13 inhibits the cycling of RhoA activation at the rear edge of cells, which makes them defective in retracting their tail. This study highlights the importance of the regulation of RhoA activity in focal adhesions of astrocytoma cells and establishes StarD13 as a GAP playing a major role in this process.