ALBERT

Description: When Arabidopsis thaliana seedlings grow embedded in an agar-based medium, their roots grow vertically downward. This reflects their ability to sense the gravity vector and to position their tip parallel to it (gravitropism). We have isolated a number of mutations affecting root gravitropism in Arabidopsis thaliana. One of these mutations, named arg1, affects root and hypocotyl gravitropism without promoting defects in starch content or in the ability of seedlings' organs to respond to plant hormones. The ARG1 gene was cloned and shown to code for a protein with a J domain at its amino terminus and a second sequence motif found in several cytoskeleton binding proteins. Mutations in the AGR1 locus promote a strong defect in root gravitropism. Some alleles also confer an increased root resistance to exogenous ethylene and an increased sensitivity to auxin. AGR1 was cloned and found to encode a putative transmembrane protein which might be involved in polar auxin transport, or in regulating the differential growth response to gravistimulation. When Arabidopsis seedlings grow on the surface of agar-based media tilted backward, their roots wave. That wavy pattern of root growth derives from a combined response to gravity, touch and other surface-derived stimuli. It is accompanied by a reversible rotation of the root tip about its axis. A number of mutations affect the presence or the shape of root waves on tilted agar-based surfaces. One of them, wvc1, promotes the formation of compressed root waves under these conditions. The physiological and molecular analyses of this mutant suggest that a tryptophan-derived molecule other than IAA might be an important regulator of the curvature responsible for root waving.

Description: The ability of plant organs to use gravity as a guide for growth, named gravitropism, has been recognized for over two centuries. This growth response to the environment contributes significantly to the upward growth of shoots and the downward growth of roots commonly observed throughout the plant kingdom. Root gravitropism has received a great deal of attention because there is a physical separation between the primary site for gravity sensing, located in the root cap, and the site of differential growth response, located in the elongation zones (EZs). Hence, this system allows identification and characterization of different phases of gravitropism, including gravity perception, signal transduction, signal transmission, and curvature response. Recent studies support some aspects of an old model for gravity sensing, which postulates that root-cap columellar amyloplasts constitute the susceptors for gravity perception. Such studies have also allowed the identification of several molecules that appear to function as second messengers in gravity signal transduction and of potential signal transducers. Auxin has been implicated as a probable component of the signal that carries the gravitropic information between the gravity-sensing cap and the gravity-responding EZs. This has allowed the identification and characterization of important molecular processes underlying auxin transport and response in plants. New molecular models can be elaborated to explain how the gravity signal transduction pathway might regulate the polarity of auxin transport in roots. Further studies are required to test these models, as well as to study the molecular mechanisms underlying a poorly characterized phase of gravitropism that is independent of an auxin gradient.

Description: A root gravitropism mutant was isolated from the DuPont Arabidopsis thaliana T-DNA insertional mutagenesis collection. This mutant has reduced root gravitropism, hence the name rgr1. Roots of rgr1 are shorter than those of wild-type, and they have reduced lateral root formation. In addition, roots of rgr1 coil clockwise on inclined agar plates, unlike wild-type roots which grow in a wavy pattern. The rgr1 mutant has increased resistance, as measured by root elongation, to exogenously applied auxins (6-fold to indole-3-acetic acid, 3-fold to 2,4-dichlorophenoxyacetic acid, and 2-fold to napthyleneacetic acid). It is also resistant to polar auxin transport inhibitors (2-fold to triiodobenzoic acid and 3- to 5-fold to napthylphthalamic acid). The rgr1 mutant does not appear to be resistant to other plant hormone classes. When grown in the presence of 10(-7) M 2,4-dichlorophenoxyacetic acid, rgr1 roots have fewer root hairs than wild type. All these rgr1 phenotypes are Mendelian recessives. Complementation tests indicate that rgr1 is not allelic to previously characterized agravitropic or auxin-resistant mutants. The rgr1 locus was mapped using visible markers to 1.4 +/- 0.6 map units from the CH1 locus at 1-65.4. The rgr1 mutation and the T-DNA cosegregate, suggesting that rgr1 was caused by insertional gene inactivation.

Description: No abstract available

Description: The surficial morphology of Callisto is dominated by large concentric patterns of ridges, scarps, and furrows. The largest one, Valhalla, shows a smooth central area 350 km in radius, surrounded by concentric sinuous ridges which extend as far as 600km from the center. Beyond the ridged area, scarps and furrows are observed as far as 2000 km. The global azimuthal distribution of the scarps and furrows of the outer ring (beyond 700 km from the center) was analyzed. It is proposed that the local geometry and the global distribution of scarps and furrows around Valhalla are the results of the reactivation of an old pattern by the Valhalla event. The coincidence of these local directions of a preexisting pattern (NW-SE and NE-SW) with the directions of the grids observed on most bodies observed so far in the solar system would indicate that this local pattern is a part of a global calistean grid.

Description: Voyager close up views of Ganymede reveal some provinces of old, cratered, dark terrains that are broken into polygons and cut through by light stripes of grooved terrains. The groove terrain unit and its relationships with the other units was studied at various scales because these features seem to relate at least to two main scales of processes. Minor displacements concerning only local features, i.e., large scale processes and major displacements concerning usually the major evolution of the surface, i.e., small scale processes. It appears that tectonic processes, at least in the studied areas, if playing a role in the formation of local or regional features, are very limited. The conclusions reached agree with those of Thomas et al. who show that post grooves basins reveal the presence of a primitive grid that is not disturbed by the grooved terrains.

Description: The surface of Ganymede consists of dark cratered terrain, and groved terrain. The dark cratered terrains form polygonal units, the largest of which is Galileo Regio, the surface of which is transected by furrows, smooth floored valleys bounded by relatively sharp parallel ridges. The most apparent of them are grouped together and form an apparently arcuate system of subparallel furrows which was mapped using Voyager pictures and plotted on a map using a stereographic projection. With this kind of projection, the main furrow system is not arcuate, but rectilinear. Observations strongly suggest that the Galileo Regio furrow systems are not of impact origin and appear to be irrelevant to discussions about the basins' morphology or evolution of planetary lithosphere determined from multiring structures.

Description: Three large basin surroundings on Ganymede located on grooved terrains, which are supposed to be intensely tectonized areas were studied. All rectilinear morphological elements such as ridges, block edges; and parts of scarps were mapped. The geometric properties (grid pattern) characteristics were determined and a history for the formation of the Ganymede basins studied is proposed. Results indicate that the grooved terrains are very surficial layers, and that their formation does not significantly affect, disturb or rotate the basement. This is in agreement with the conclusion obtained from completely different data (crosscutting relationships between groove sets and their basements).

Description: A systematic study of Martian wrinkle ridges was performed to synthesize a theory of the planetary interior forces which produced such surface features. The survey was carried out using Mariner-9 and Viking orbiter imagery. Attention was given to the global distributions of ridge types in terms of geologic and tectonic surrounds, and to the orientation of ridges relative to impact basins. High/low relief ridges, ridge wings and rectilinear ridges were examined. Ridge orientation and distribution were found to be controlled by the forces of formation and modification of impact basins. Several other conclusions were reached regarding basin-concentric ridge patterns, regional stress patterns, regions of the most numerous ridges, and the location of a major compressive zone.

Description: No abstract available