ASGSB 1998 Annual Meeting Abstracts

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THREE-DIMENSIONAL ULTRASTRUCTURE OF LENTIL ROOT CAP STATOCYTES GROWN IN SPACE.    J.D. Smith¹, S. Burwen¹, N. Marinkovich¹, D. Driss-Ecole² and G. Perbal². ¹NASA Ames Research Center, Moffett Field, CA, ²Univ. Pierre et Marie Curie, Paris, France.

The gravity-dependent sedimentation of amyloplasts in the root caps of higher plants is thought to initiate gravity signaling events that direct plant growth. In a 1-g environment amyloplasts typically sediment atop a complex of Endoplasmic Reticulum (ER) located at the distal cell pole. When plants are gravistimulated (turned on their sides) amyloplasts sediment away from the distal ER complex, but as plant curvature progresses the plastids return to their positions atop the distal ER. Amyloplast sedimentation back onto the ER after gravicurvature may induce a signal to stop gravicurvature [Perbal and Driss-Ecole (1993) Acta Bot. Gallica 140:615-632]. Additionally, for seedlings raised in microgravity, amyloplasts are grouped together near the centers of root cap statocytes [Lorenzi and Perbal (1990) Physiol. Plant. 78:532-537; Smith et al. (1997) Plant J. 12:1361-1373]. The objective of this research is to determine if amyloplasts are closely associated with ER in a microgravity environment, even when plastids are grouped and/or located near the cell center, away from the distal ER complex. Lentil root caps from seedlings grown in 1-g on earth, in microgravity and in an onboard 1-g centrifuge in space (Spacelab IML 2 mission, 1994) were embedded in Araldite, serially sectioned and imaged by transmission electron microscopy. Three-dimensional reconstructions of whole amyloplasts and ER complexes were made using Reconstruction Of Serial Sections (ROSS), a software package developed by the Biocomputation Center at NASA Ames Research Center. With 3-D reconstructions, the absolute positions and proximities of amyloplasts to ER were visible, thus allowing for complete measurement of plastid/ER contacts. Quantitative analysis of the plastid/ER connections are under way to determine if gravity-dependent sedimentation of amyloplasts causes these organelles to physically interact. If plastid/ER contact is involved in the stop of the gravity-signal pathway then, in a microgravity environment where fewer contacts between these organelles are apparent, normal signaling events are disrupted which could explain that graviresponsiveness of the lentil root is greater in microgravity (Perbal and Driss-Ecole, S/MM-05 mission, 1997). (supported by NASA Ames Research Center.)

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