Fate of the anterior neural ridge and the morphogenesis of the xenopus forebrain

The fate of the anterior neural ridge was studied by following the relative movements of simultaneous spot applications of DiI and DiO from stage 15 through stage 45. These dye movements were mapped onto the neuroepithelium of the developing brain whose shape was gleaned from whole-mount in situs to neural cell adhesion molecule and dissections of the developing nervous system. The result is a model of the cell movements that drive the morphogenesis of the forebrain. The midanterior ridge moves inside and drops down along the most anterior wall of the neural tube. It then pushes forward a bit, rotates ventrally during forebrain flexing, and gives rise to the chiasmatic ridge and anterior hypothalamus. The midanterior plate drops, forming the floor of the forebrain ventricle, and, keeping its place behind the ridge, it gives rise to the posterior hypothalamus or infundibulum. The midlateral anterior ridge slides into the lateral anterior wall of the neural tube and stretches laterally into the optic stalk and retina, and then rotates into a ventral position. The lateral anterior ridge converges to the most anterior part of the dorsal midline during neural tube closure, then rotates anteriorly, and gives rise to telencephalic structures. Whole-mount bromodeoxyuridine labeling at these stages showed that cell division is widespread and relatively uniform throughout the brain during the late neurula and early tailbud stages, but that during late tailbud stages cell division becomes restricted to specific proliferative zones. We conclude that the early morphogenesis of the brain is carried out largely by choreographed cell movements and that later morphogenesis depends on spatially restricted patterns of cell division. © 1995 John Wiley & Sons, Inc.     

Abscisic acid biosynthesis during corn embryo development

In this study we examined the biosynthesis of abscisic acid (ABA) by developing corn (Zea mays L.) embryos. Three comparisons were made: ABA biosynthesis in embryos isolated from kernels grown in vitro with those grown in the field; the developmental profile of ABA content with that of biosynthesis; and ABA biosynthesis in corn embryos lacking carotenoid precursors with ABA biosynthesis in normal embryos. Embryos were harvested at various times during seed development and divided into two groups. Endogenous levels of ABA were measured in one group of embryos and ABA biosynthetic capacity was measured in the other group. The ABA biosynthetic capacity was measured with and without tetcyclacis (an inhibitor of ABA degradation) in embryos from both field-grown and in-vitro-grown corn kernels. Reduced-carotenoid (either fluridone-treated or genetically viviparous) embryos were also included in the study. Corn kernels developing under field and in-vitro conditions differed from each other in their responses to tetcyclacis and in their profiles of ABA biosynthesis during development. Therefore, in-vitro kernel culture may not be an appropriate substitute for field conditions for studies of embryo development. The developmental profiles of endogenous ABA content differed from those of ABA biosynthesis in isolated embryos of both in-vitro-and field-grown kernels. This indicated that ABA levels in the developing embryos were determined by import from the maternal tissues available to the embryos rather than by in-situ biosynthesis. In embryos with reduced levels of carotenoids, either fluridone-treated or genetically viviparous embryos, ABA biosynthesis was low or nonexistent. This result is expected for the presence of an indirect pathway of ABA biosynthesis and in the absence of ABA precursors.Abbreviations ABA abscisic acid - DAP days after pollination     

The Casparian strip in pea epicotyls: Effects of light on its development

The Casparian strip, which is specific to roots, was studied in the epicotyls of dark-grown seedlings of pea (Pisum sativum L.) where it was found to have the same morphology and properties as the strip in roots. In dark-grown seedlings, the distance between the upper-most position of the Casparian strip and the bending point of the hook (about 37 mm) did not change during growth of the seedlings. In the uppermost 0.5-mm region of the region in which the Casparian strip could be detected by fluorescence microscopy, the plasma membrane was not firmly attached to the cell wall. The development of the Casparian strip continued for about 42 h after dark-grown seedlings were transferred to the light, indicating that (i) the cells that have been determined to form the Casparian strip in darkness form the strip in the light, and that (ii) it takes about 42 h for the cells to complete formation of the strip. Cells in the hook of dark-grown seedlings did not form a Casparian strip when such seedlings were transferred to the light. The Casparian strip was formed in rapidly elongating internodes of light-grown seedlings when the seedlings were transferred to darkness. Light did not control the formation of the Casparian strip in roots.Abbreviation PBS phosphate-buffered saline     

Changes in O horizon mass,thickness and carbon content following fire in northern hardwood forests

This study examines temporal changes in the thickness, mass, and organic carbon content of the O horizon (forest floor) of eight forested plots in northern Michigan, USA. Each plot had experienced a recent burn (prescribed or accidental); burn dates ranged from 1798 to 1980. The climax forest in this region is mixed Pinus-Acer-Betula-Tsuga, whereas the fire successional species are predominantly Populus spp. and Betula papyrifera. O horizon data were fit to logarithmic functions (chronofunctions) that depicted rapid accumulations of mass and thickness in the first years after the fire, followed by decreasing rates of increase after 100 years. Extension of the chronofunctions to 5000 years allowed for a theoretical examination of forest floor conditions, e.g., steady state and time to steady state, after long periods without disturbance. The models predicted greater O horizon thicknesses and slightly lower mass for steady state conditions than have been reported for old-growth stands elsewhere. Steady state accumulations of litter in these mixed, temperate forests requires at least 200 and possibly > 1000 years, which is markedly longer than most other estimates. Although frequent disturbance by fire in these forests would likely preclude such values from being attained, these data provide theoretical maximum values for forest floor conditions in these ecosystems.     

Seed development in Pisum

The Ih and lh _i alleles have been shown previously to reduce the level of endogenous gibberellin A₁ (GA₁) in shoots of pea (Pisum sativum L.), resulting in a dwarf phenotype compared with the wild type, cv. Torsdag (Lh). In addition, plants homozygous for the lh ⁱ allele have reduced seed yield compared with Lh (tall, wild type) and lh (dwarf) plants. In this paper we show that the lh ⁱ mutation is expressed in developing seeds and pods. Comparison of GA levels in young shoots and developing seeds of genotypes lh and lh ⁱ demonstrates that the relative severity of the two mutations varies in different tissues. Homozygous h ⁱ seeds have reduced GA levels, weigh less, and are less likely to develop to maturity when compared with Lh seeds. However, fertilization of lh ⁱ plants with Lh pollen increases seed GA levels, seed weight and seed survival, indicating that an increase in seed GA levels due to the presence of the Lh allele can restore normal seed growth. Pods developing on self-pollinated lh ⁱ plants are shorter than pods on Lh (wild type) plants, although this may be an indirect effect of the increased seed abortion of lh _i plants. Based on these results we suggest that endogenous GAs play an important role in the development of seeds of P. sativum L.Abbreviations GA_(n) gibberellin A_n We wish to thank Katherine McPherson, Peter Newman, Leigh Johnson and Peter Bobbi for technical assistance, Professor L. Mander (ANU, Canberra) and Professor B.O. Phinney (UCLA, USA) for labelled GA standards, and the Australian Research Council for financial support.     

Culture experiments with lichens

Fragments of several lichens were grown in the laboratory and developmental steps of growth and regeneration could be described. Different degrees of determination could be observed. The stages of podetial development inCladonia are easily distinguished and are reached after comparably short time. As they occur highly synchronized the results encourage further morphogenetic experiments.Dedicated to Prof. DrO. L. Lange on the occasion of his retirement as a tribute to his outstanding qualities as both a scientist and as a teacher and humane colleague.     

The role of meiofauna in the marine ecosystem: evolutionary considerations*

The meiobenthos are regarded as the first metazoans to appear on earth This paper considers some aspects of their role in the marine ecosystem over evolutionary time. It is speculatively suggested that they have had a profound influence on many structural and functional attributes of the marine ecosystem as we see it today, both in the benthic and pelagic realms. Certain features such as size spectra, the life-history characteristics of the macrobenthos, and the energetic balance between ecological compartments of the marine ecosystem are considered to result from evolution away from negative interactions with the meiobenthos.     

Between Xylem and Phloem: The Genetic Control of Cambial Activity in Plants

Abstract: Post-embryonic development is controlled by two types of meristems: apical and lateral. There has been considerable progress recently in understanding the function of root and shoot apical meristems at the molecular level. Knowledge of analogous processes in the lateral, or secondary, meristems, i.e. the vascular cambium or cork cambium, is, however, rudimentary. This is despite the fact that much of the diversity in the plant kingdom is based on the differential functions of these meristems, emphasizing the importance of lateral meristems in the development of different plant forms. The vascular cambium is particularly important for woody plants, but it also plays an important role during the development of various herbaceous species, such as Arabidopsis thaliana. In this review, we focus on the two basic functions of cambial activity: cell proliferation and pattern formation.     

Influence of mechanical impedance on root exudation of maize seedlings at two development stages

Studies were undertaken to evaluate the effects of mechanical impedance on root exudation by maize (Zea mays L., var Dea) and to examine the importance of these effects in relation to the stage of plant development. Plants were grown under sterile and hydroponic conditions. Mechanical impedance was simulated using glass beads of 1 mm diameter. This treatment was compared with a control without beads. Results demonstrated that plant growth was influenced by mechanical impedance. Mechanical impedance markedly affected the growth of the shoot, whether this was measured as leaf area or total dry matter. Besides increasing root/shoot biomass ratios, mechanical impedances also stimulated root exudation of organic and inorganic compounds. Stressed plants lost more nitrogenous compounds than control plants. Otherwise, the percentage of released carbon decreased. Depending on the developmental stage of the plant, there was a large variation in the magnitude and time course on mechanical impedance effects. The effects of mechanical impedance persist and accentuate with time.