The structure of plasmodesmata as revealed by plasmolysis, detergent extraction, and protease digestion.

Plasmodesmata or intercellular bridges that connect plant cells are cylindrical channels approximately 40 nm in diameter. Running through the center of each is a dense rod, the desmotubule, that is connected to the endoplasmic reticulum of adjacent cells. Fern, Onoclea sensibilis, gametophytes were cut in half and the cut surfaces exposed to the detergent, Triton X 100, then fixed. Although the plasma membrane limiting the plasmodesma is solubilized partially or completely, the desmotubule remains intact. Alternatively, if the cut surface is exposed to papain, then fixed, the desmotubule disappears, but the plasma membrane limiting the plasmodesmata remains intact albeit swollen and irregular in profile. Gametophytes were plasmolyzed, and then fixed. As the cells retract from their cell walls they leave behind the plasmodesmata still inserted in the cell wall. They can break cleanly when the cell proper retracts or can pull away portions of the plasma membrane of the cell with them. Where the desmotubule remains intact, the plasmodesma retains its shape. These images and the results with detergents and proteases indicate that the desmotubule provides a cytoskeletal element for each plasmodesma, an element that not only stabilizes the whole structure, but also limits its size and porosity. It is likely to be composed in large part of protein. Suggestions are made as to why this structure has been selected for in evolution.

Leaf dimorphism in aquatic angiosperms: significance of turgor pressure and cell expansion.

Depending on environmental conditions, the aquatic angiosperm Callitriche heterophylia can develop two different leaf types with distinctive morphological characteristics. Cellular turgor pressure seems to act as the biophysical mechanism responsible for the selection of leaf form in control conditions designed to mimic the natural habitat of this plant. The experimental induction of leaf form involves the ability of various treatments to mediate cell expansion through their effects on turgor pressure or wall extensibility.

The Effects of Exogenous Auxins on Endogenous Indole-3-Acetic Acid Metabolism (The Implications for Carrot Somatic Embryogenesis).

The effect of auxin application on auxin metabolism was investigated in excised hypocotyl cultures of carrot (Daucus carota). Concentrations of both free and conjugated indole-3-acetic acid (IAA), [2H4]IAA, 2,4-dichlorophenoxyacetic acid, and naphthaleneacetic acid (NAA) were measured by mass spectroscopy using stable-isotope-labeled internal standards. [13C1]NAA was synthesized for this purpose, thus extending the range of auxins that can be assayed by stable-isotope techniques. 2,4-Dichlorophenoxyacetic acid promoted callus proliferation of the excised hypocotyls, accumulated as the free form in large quantities, and had minor effects on endogenous IAA concentrations. NAA promoted callus proliferation and the resulting callus became organogenic, producing both roots and shoots. NAA was found mostly in the conjugated form and had minor effects on endogenous IAA concentrations. [2H4]IAA had no visible effect on the growth pattern of cultured hypocotyls, possibly because it was rapidly metabolized to form inactive conjugates or possibly because it mediated a decrease in endogenous IAA concentrations by an apparent feedback mechanism. The presence of exogenous auxins did not affect tryptophan labeling of either the endogenous tryptophan or IAA pools. This suggested that exogenous auxins did not alter the IAA biosynthetic pathway, but that synthetic auxins did appear to be necessary to induce callus proliferation, which was essential for excised hypocotyls to gain the competence to form somatic embryos.

Hormonal Studies of fass, an Arabidopsis Mutant That Is Altered in Organ Elongation.

We have isolated an allele of fass, an Arabidopsis thaliana mutation that separates plant development and organ differentiation from plant elongation, and studied its hormonal regulation. Micro-surgically isolated fass roots elongate 2.5 times as much as the roots on intact mutant plants. Wild-type heart embryos, when cultured with a strong auxin, naphthaleneacetic acid, phenocopy fass embryos. fass seedlings contain variable levels of free auxin, which average 2.5 times higher than wild-type seedling levels, and fass seedlings evolve 3 times as much ethylene as wild-type seedlings on a per-plant basis over a 24-h period. The length-to-width ratios of fass seedlings can be changed by several compounds that affect their endogenous ethylene levels, but fass is epistatic to etr1, an ethylene-insensitive mutant. fass's high levels of free auxin may be inducing its high levels of ethylene, which may, in turn, result in the fass phenotype. We postulate that FASS may be acting as a negative regulator to maintain wild-type auxin levels and that the mutation may be in an auxin-conjugating enzyme.

Family migration and the employment of married women and men.

"This research reconsiders the human capital hypothesis that married women have a lower probability of employment after family migration. The empirical analysis focuses on a sample of married parents in the economically active population residing in the midwestern United States in 1980. Our analysis establishes that, after controlling for the effects of migration self-selection bias, family migration increases the probability of employment among married women by 9 percent but has no effect on the probability of employment among married men. This research demonstrates the limitations of the human capital model of family migration and indicates the need for reconceptualizing family migration behavior."

Infective arthritis secondary to bladder outflow obstruction.

We describe two cases of septic arthritis occurring in association with lower urinary tract infection in elderly men. In both cases the organism isolated from both the joint and the urine was Staphylococcus aureus. Further investigation of the urinary tract in both individuals identified bladder outflow obstruction secondary to benign prostatic hyperplasia predisposing them to infection. The urinary tract should be suspected as a focus of infection in septic arthritis in elderly men and further investigation of the urinary system may disclose a surgically correctable lesion.

Unusual patterns of somatic embryogenesis in the domesticated carrot: developmental effects of exogenous auxins and auxin transport inhibitors.

The effects of various exogenous auxins and polar auxin transport inhibitors on somatic embryogenesis in carrot cultures were investigated. Indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid do not disrupt the sequence or the polarity of individual stages in embryo development, but tend to cause developing embryos to revert to undifferentiated callus, with increasing frequency in later embryo stages. The transport inhibitors, N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid, block morphological transitions to the subsequent stage; for example, they cause the formation of enlarged globular and oblong embryos. Heart embryos in these treatments usually develop additional lateral growth axes. These results shed light on the role of auxin and its polar transport in somatic embryogenesis.

The role of electrical phenomena in tip growth, with special reference to the developmental plasticity of filamentous fern gametophytes.

Cell expansion in many plant structures, including algal rhizoids, fungal hyphae, root hairs, and pollen tubes, is restricted to their apical tips. Endogenous electric fields are seen to accompany polarized growth in all tip-growing cells studied to date. The extensive studies on absorptive tip-growing structures have established that positive currents enter their elongating tips, with a portion of the entry current being carried by a localized calcium influx into the extreme tip. The resulting tip-to-base gradient in calcium concentration appears to be responsible for maintaining polarized growth in these systems, although it is uncertain whether this calcium effect is mediated via either electrophoretic or cytoskeletal mechanisms. In contrast, the few electrical measurements made on photosynthetic cells suggest that the orientation of their transcellular fields is transiently or permanently reversed relative to the fields in absorptive structures. In darkness, microelectrode measurements indicate that the apical tip of the fern filament is 5 mV electronegative relative to the base of the apical cell. This cellular dipole is perceived with the vibrating probe as a focused outward current that departs from the tip region and a more diffuse inward current that enters the lateral sides of the apical cell. The tip current is predominantly composed of protons, as can be identified with various cation-selective electrodes. This proton current is thought to help maintain localized wall expansion in the filament tip. Blue light mediates the major morphogenetic transition in fern gametophytes, i.e. the transition from the tip-growing filament to the planar prothallus. All the above electrical and ionic parameters change in the few minutes of irradiation before the filament tip starts lateral swelling. The plasma membrane at the extreme tip begins to hyperpolarize within 3 s, while the basal region shows a delayed, but greater response. The cellular dipole that had existed in darkness is thus abolished in 10 to 15 min after the start of irradiation. With the vibrating probe a more diffuse pattern of positive currents is observed to emerge from the tip as well as the subapical regions of the apical cell. Simultaneously, proton efflux increases in the subapical region; the resulting decrease in cell wall pH should help plasticize the lateral walls, which may, in turn, facilitate the process of lateral swelling over the next few hours.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical Changes in the Apical Cell of the Fern Gametophyte during Irradiation with Photomorphogenetically Active Light.

Electrophysiological procedures were used to evaluate cellular responses of fern (Onoclea sensibilis L.) gametophytes to photomorphogenetically active light. Red, far red, and blue light caused rapid changes in the membrane potential of the apical cell of the gametophyte filament; other cells in the filament were not similarly responsive. Measurements made with one electrode in the apical cell revealed that the membrane potential depolarized in red light and repolarized in far red light. Irradiation with blue light caused a hyperpolarization, the rapidity of which was dependent on a red light pretreatment. More refined measurements with one electrode in the tip of the apical cell and another in the base of the cell showed that both red and blue light treatments cause the apical cell to behave as a dipole. Because of the profound, long-term morphological changes that follow light irradiation in this organism, it was hoped to use it to elucidate the role that electrical parameters play in determining subsequent developmental events.

Cell expansion in the filamentous gametophyte of the fernOnoclea sensibilis L.

Filamentous gametophytes of the fernO. sensibilis were exposed to paired combinations of light of different qualities, hormones and cations in the attempt to elucidate the underlying processes that regulate cell expansion. Simultaneous treatments with high-pH buffers or the auxin antagonistp-chlorophenoxyisobutyric acid abolished blue-light-mediated expansion but did not influence growth in red light. In contrast, the red-light response was preferentially altered by the ethylene absorbant KMnO4 or the Ca(2+) chelator ethyleneglycol-bis(ß-aminoethyl ether) N,N'-tetraacetic acid. The Ca(2+) ionophore A23187 caused a significant reduction in cell expansion under both blue and red irradiation. A marked promotion of expansion was mediated by high concentrations of indole-3-acetic acid, but this effect was dependent on the presence of low-pH buffers. The ethylene-generating agent 2-chloroethylphosphonic acid decreased the magnitudes of both photoresponses; this inhibition was further enhanced by high Ca(2+) concentrations. These findings and those with other plants are interpreted in terms of two independent control mechanisms for cell expansion: 1) a blue light photoreceptor-auxin-hydrogen ion system, and 2) a phytochrome-ethylene-calcium ion system.

Family migration and employment: the importance of migration history and gender.

"This article uses event history data to specify a model of employment returns to initial migration, onward migration, and return migration among newly married persons in the U.S. Husbands are more likely to be full-time employed than wives, and being a parent reduces the employment odds among married women. Employment returns to repeated migration differ by gender, with more husbands full-time employed after onward migration and more wives full-time employed after return migration events. We interpret these empirical findings in the context of family migration theory, segmented labor market theory, and gender-based responsibilities." Data are from the National Longitudinal Survey of Youth from 1979 to 1991.

Gender and ethnic concentration and employment prospects for Mexican-American migrants.

PIP: Mexican-Americans comprise the US's second largest minority group and one of its fastest growing minorities. Between 1980 and 1990, there was a 54% increase in the number of Mexican-Americans in the US, with half of the increase due to births and the remainder from immigration. The authors analyzed the effects of migration upon the employment of Mexican-Americans using US Census Bureau 1990 Public Use Microsample (PUMS) data. The effects were then differentiated by gender and studied with regard to how localized concentrations of coethnics affect the returns to migration, after controlling for migration self-selection bias. Results from the analysis of the data suggest that migration decreases the employment probabilities for married women with no significant effect for single men or women. A greater percentage of coethnics increases employment for all groups except single women.^ieng

Initial events in the tip-swelling response of the filamentous gametophyte of Onoclea sensibilis L. to blue light.

Several inhibitors were applied to filamentous gametophytes of the fern Onoclea sensibilis in the attempt to characterize how electrical phenomena might be involved in the tip-swelling response to blue light (BL). The membrane potential of the apical cell in the typical fern filament rests near-120 mV in darkness, but irradiation with blue light causes the membrane to hyperpolarize at a steady rate of 2.6 mV min(-1) until it reaches a new stable value between-130 and-135 mV. In darkness, 10(-4)M salicylhydroxamic acid (SHAM), an inhibitor of BL-mediated absorbance changes in putative plasma-membrane fractions from maize coleoptiles, has no observable effects on the membrane potential or on filamentous growth. A SHAM pretreatment before BL irradiation causes approx. 70% inhibition of the membrane hyperpolarization as well as a comparable reduction in the growth response; however, SHAM has no effect in experiments where its application follows the onset of the electrical response. Exposing the filaments to 10(-5)M Na3VO4, an inhibitor of the plasma-membrane ATPase, depresses the membrane potential in darkness. Depending on the timing of application, Na3VO4 prevents the initiation of or blocks further increases in the BL-mediated hyperpolarization. Application of Na3VO4 causes an immediate cessation of growth in both darkness and BL. These findings implicate the involvement of a plasmalemma-bound flavin-cytochrome complex and ATP-driven proton pump in the initial events of this growth response to blue light.

Modifications of extracellular electric and ionic gradients preceding the transition from tip growth to isodiametric expansion in the apical cell of the fern gametophyte.

Fern (Onoclea sensibilis L.) gametophytes exposed to blue light are induced to undergo a morphological transition from a tip-growing filament to a planar prothallus. Extracellular measurements of electric currents and localized ion activities around the apical cell of 8 to 10 day-old gametophytes were made with a vibrating probe and ion selective electrodes. In darkness, we observed exit current densities of an average of 75 nanoamperes per square centimeter near the tip and 2 to 15 nanoamperes per square centimeter along the lateral walls of this cell. Measurements with ion selective electrodes for H(+), K(+), and Ca(2+) showed that this cell was bounded by a thin layer of medium that was depleted in K(+) and Ca(2+) and exhibited a lower pH than the bulk solution. Both the K(+) and Ca(2+) depletion zones and the zone of higher acidity were particularly pronounced at the tip end of the cell; the pH at 2 micrometers from the tip was nearly 0.5 units more acid than the bulk medium at pH 6. Disruption of steady state, external gradients with media that contained lower concentrations of H(+), K(+), Ca(2+), or Cl(-) produced certain differences in the rates of restoration of particular ion zones, raising the possibility that some of the ion migrations are interdependent. Within 15 minutes after irradiation with blue light, current leaving the tip declined to levels which were indistinguishable from those leaving the lateral walls and there was a rapid lowering in the rates of tip acidification and K(+) depletion near the tip. The rapid dissipation of both the longitudinally aligned electrical field and the tip-localized asymmetries in external cation distribution in blue light suggest that loss of electrical polarity in this tip growing cell may be an initial step in the chain of events which govern changes in cell shape.

Anterior fontanelle size in the neonate.

A simple method is described for measuring the area of the anterior fontanelle at birth. Normal values in preterm and term infants suggest enlargement of the fontanelle with gestational age. Small-for-dates infants have significantly larger anterior fontanelles than either preterm or term infants. Knowledge of the normal variation in anterior fontanelle size may be of help in the early diagnosis of congenital hypothyroidism or provide a clue to other disorders of skeletal development.

A microtechnique for the analysis of free and conjugated indole-3-acetic acid in milligram amounts of plant tissue using a benchtop gas chromatograph-mass spectrometer.

A microtechnique was developed for the quantification of indole-3-acetic acid (IAA) in plant samples of one milligram fresh weight or less. The method permitted quantification of both free and conjugated IAA using a benchtop gas chromatograph-mass spectrometer. New methods for sample purification with high recovery at microscale levels, together with simple changes that result in enhanced sensitivity of the instrumentation, allowed for a significant reduction in the amount of plant material required for analysis. Single oat (Avena sativa L.) coleoptile tips could be studied with this method and were found to contain free and total IAA levels of 137 and 399 pg.mg-1 fresh weight, respectively. A single 5-d-old Arabidopsis thaliana (L.) Heynh. seedling was shown to contain 61 pg.mg-1 fresh weight free IAA and 7850 pg.mg-1 fresh weight of total IAA following basic hydrolysis. This microtechnique provides a way to accurately measure IAA levels in very small structures and individual seedlings, thus making it a valuable research tool for elucidating the role and distribution of auxin in relation to growth and development.

A cross-national comparison of the impact of family migration on women's employment status.

In this paper we consider the effects of family migration on women's employment status, using census microdata from Great Britain and the United States. We test a simple hypothesis that families tend to move long distances in favor of the male's career and that this can have a detrimental effect on women's employment status. Unlike many previous studies of this question, our work emphasizes the importance of identifying couples that have migrated together, rather than simply comparing long-distance (fe)male migrants with nonmigrant (fe)males individually. We demonstrate that women's employment status is harmed by family migration; the results we present are surprisingly consistent for Great Britain and the United States, despite differing economic situations and cultural norms regarding gender and migration. We also demonstrate that studies that fail to identify linked migrant couples are likely to underestimate the negative effects of family migration on women's employment status.

The distribution of plasmodesmata and its relationship to morphogenesis in fern gametophytes.

Fern (Onoclea sensibilis) gametophytes when grown in the dark form a linear file of cells (one-dimensional) called a protonema. In the light two-dimensional growth occurs which results in a heart-shaped prothallus one cell thick. The objective of this paper is to relate the most common pattern of cell division observed in developing gametophytes to the formation of the plasmodesmatal network. Since the prothalli are only two dimensional, we can easily determine from thin sections the total number and the density (number per unit surface area) of plasmodesmata at each developmental stage. As the prothallus grows the number of plasmodesmata increases 50-fold in the apical or meristematic cell. This number eventually reaches a plateau even though the density continues to increase with each new cell division. What is particularly striking is that both the number and density of plasmodesmata between adjacent cells is precisely determined. Furthermore, the pattern of plasmodesmata distribution is predictable so that (1) we can identify the apical meristematic cells by their plasmodesmata number, or density, as well as by their size, shape and location, (2) we can predict, again from plasmodesmata number, the location of a future wall of the apical cell prior to its actual formation, (3) we can show that the density of plasmodesmata in the triangular apical cell of the prothallus (14 plasmodesmata microns-2) is comparable to those reported for secretory glands which are known to have high rates of plasmodesmatal transport and (4) we can show that once the plasmodesmata have been formed during division, no subsequent change in the number of plasmodesmata occurs following cell plate formation.