A Novel Role for MAPKAPK2 in Morphogenesis during Zebrafish Development

One of the earliest morphogenetic processes in the development of many animals is epiboly. In the zebrafish, epiboly ensues when the animally localized blastoderm cells spread, thin over, and enclose the vegetally localized yolk. Only a few factors are known to function in this fundamental process. We identified a maternal-effect mutant, betty boop (bbp), which displays a novel defect in epiboly, wherein the blastoderm margin constricts dramatically, precisely when half of the yolk cell is covered by the blastoderm, causing the yolk cell to burst. Whole-blastoderm transplants and mRNA microinjection rescue demonstrate that Bbp functions in the yolk cell to regulate epiboly. We positionally cloned the maternal-effect bbp mutant gene and identified it as the zebrafish homolog of the serine-threonine kinase Mitogen Activated Protein Kinase Activated Protein Kinase 2, or MAPKAPK2, which was not previously known to function in embryonic development. We show that the regulation of MAPKAPK2 is conserved and p38 MAP kinase functions upstream of MAPKAPK2 in regulating epiboly in the zebrafish embryo. Dramatic alterations in calcium dynamics, together with the massive marginal constrictive force observed in bbp mutants, indicate precocious constriction of an F-actin network within the yolk cell, which first forms at 50% epiboly and regulates epiboly progression. We show that MAPKAPK2 activity and its regulator p38 MAPK function in the yolk cell to regulate the process of epiboly, identifying a new pathway regulating this cell movement process. We postulate that a p38 MAPKAPK2 kinase cascade modulates the activity of F-actin at the yolk cell margin circumference allowing the gradual closure of the blastopore as epiboly progresses

Quantitative Cryo-Analytical Scanning ElectronMicroscopy (CEDX): An Important Technique Useful for Cell-Speci fi c Localization of Salt

Advances in the techniques required for the X-ray microanalysis of cryo-fixed, naturally hydrated plant tissues in the cryo-scanning electron microscope have reached the stage that accurate, cell-specific localization and quantification of the nutrient a

Daily embolism and refilling of root xylem vessels in three dicotyledonous crop plants

Embolisms were observed directly in the xylem vessels of roots of field-grown buckwheat (Fagopyrum esculentum Moench.), sunflower (Helianthus annuus L.) and soybean (Glycine max (L.) Merr.) by cryo-microscopy. No vessels were embolized at dawn, but in all three species some embolisms had formed by 0900 hours. The percentage of vessels embolized reached 40-60 % between 0900 and 1200 hours, then declined to near 0 % by sunset. The plants were still transpiring when embolisms were refilling. There was no correlation of percentage embolism with rate of transpiration, but a small positive correlation with balance pressure. Daily embolism and refilling of root vessels were observed previously in maize, and also appear to be a normal process in dicotyledonous crop plants. (© Inra/Elsevier, Paris.)Cycle journalier de l'embolie et remplissage des vaisseaux du xylème des racines chez trois dicotylédones. De l'embolie a été observée par cryo-microscopie dans les vaisseaux du xylème racinaire du blé noir (Fagopyrum esculentum Moench.), du tournesol (Helianthus annuus L.) et du soja (Glycine max (L.) Merr.) cultivés en champ. Aucun vaisseau n'était embolisé à l'aube, mais pour chacune des trois espèces étudiées, l'embolie est apparue vers 9h. Le pourcentage de vaisseaux embolisés a atteint 40 à 60% entre 9 et 12h, puis est retombé à un taux presque nul avant le coucher du soleil. Les plantes transpiraient encore lorsque les vaisseaux se réemplirent. On n'a pas observé de corrélation entre le pourcentage de vaisseaux embolisés et le taux de transpiration, mais la corrélation était légèrement positive avec la pression d'équilibre. L'embolie diurne et le réemplissage des vaisseaux racinaires ont déjà été observés sur maïs et semblent être aussi un processus normal chez les plantes Dicotylédones cultivées. (© Inra/Elsevier, Paris.

Cryo-scanning electron microscopy (CSEM) in the advancement of functional plant biology: Morphological and anatomical applications

Cryo-scanning electron microscopy (CSEM) is reviewed by exploring how the images obtained have changed paradigms of plant functions and interactions with their environment. Its power to arrest and stabilise plant parts in milliseconds, and to preserve them at full hydration for examination at micrometre resolution has changed many views of plant function. For example, it provides the only feasible way of accurately measuring stomatal aperture during active transpiration, and volume and shape changes in guard cells, or examining the contents of laticifers. It has revealed that many xylem conduits contain gas, not liquid, during the day, and that they can be refilled with sap and resume water transport. It has elucidated the management of ice to prevent cell damage in frost tolerant plants and has revealed for the first time inherent biological and physical features of root/soil interactions in the field. CSEM is increasingly used to reveal complementary structural information in studies of metabolism, fungal infection and symbiosis, molecular and genetic analysis

The management of extracellular ice by petioles of frost resistant herbaceous plants

• Background and Aims: Some frost-tolerant herbaceous plants droop and wilt during frost events and recover turgor and posture on thawing. It has long been known that when plant tissues freeze, extracellular ice forms. Distributions of ice and water i

Cryo-scanning electron microscopy observations of vessel content durng transpiration in walnut petioles. Facts or artefacts

The reliability of the cryo-SEM technique for stabilizing and quantifying embolisms in vessels of transpiring plants has recently been criticized, on the grounds that the embolisms observed are artefacts of the freezing. One of the tests used was a comparison of the embolisms found in rachises of transpiring walnut leaves (Juglans regia L.) frozen intact on the tree, with both cryo-SEM images of vessel contents and the measured hydraulic conductivity of similar samples whose xylem pressure had been returned to atmospheric pressure by cutting the leaves off under water. Embolisms found in intact frozen rachises were not present in cut rachises, and the high rachis hydraulic conductivities indicated a similar absence of embolisms. We show that the authors' conclusions are wrong because their test produces a different artefact. When the petiole is cut under dye solution it is obvious that the immersing solution is drawn into the embolized vessels and fills them. Thus, the cryo-SEM images and the high hydraulic conductivities of specimens prepared by this technique do not indicate the contents of the xylem in the intact plant. In fact, this artefact may perhaps be used (with the dye) to measure the embolisms in the intact plant. The embolisms seen in the cryo-SEM are most unlikely to be artefacts. The published work that also shows embolisms and their refilling in transpiring plants by techniques involving no freezing is reviewed

Branch roots of young maize seedlings, their production, growth, and phloem supply from the main root

Branch root development on the primary root of maize (Zea mays L.) seedlings was followed for 9 d after planting. This period includes the shift from seedling heterotrophy to autotrophy. Linear density of branches in the basal region ranged from ∼38 cm

The vascular system of maize stems revisited: Implications for water transport and xylem safety.

The plexus of vascular bundles in the nodes of grasses is notoriously complex, where long axial bundles pass through a network of transverse bundles. The xylem pathways for water in maize stems have been investigated anatomically and with dye and particulate tracers, revealing some of the details of this complexity. Only approx. 3 % of axial vessels pass through nodes without being interrupted by end walls. Axial bundles at nodes differ from those in internodes in having the metaxylem and protoxylem vessels connected by small tracheary elements. So it is only at nodes that exchange of sap occurs between the large vessels within a bundle. End walls, acting as filters for particles and gas bubbles, always separate axial vessels from vessels in transverse bundles. The high redundancy of bundle connections in the nodal plexus is interpreted as providing alternative water pathways to bypass embolisms and damaged or diseased sections of the xylem. The pores in the filters at the base of nodes and between axial and transverse vessels within nodes are < 20 nm in diameter. Where axial vessels connect to transverse vessels, a variety of unusual shapes of vessel elements mediate two- and three-way connections within the plexus. (C) 2000 Annals of Botany Company