ASGSB 1999 Annual Meeting Abstracts

COLUMELLA CELLS REVISITED: NOVEL STRUCTURES, NOVEL PROPERTIES. L.A.Staehelin¹, T.L. Yoder², H.-Q. Zheng¹ and P. Todd². ¹Dept. MCD Biology, and ²Dept. Chem. Engineering, Univ. of Colorado, Boulder CO 80309-0347.

Despite 100 years of research, the mechanism that transduces the energy associated with the sedimentation of starch-filled amyloplasts (statoliths) in root cap columella cells into a growth response is still poorly understood. Most models postulate that the statoliths interact with microfilaments (MFs) to transmit signals to the plasma membrane (or ER) or that statolith sedimentation onto these organelles produces the signals. However, no direct evidence for statolith-MF links has been reported, and no asymmetric structures of columella cells have been identified that might explain how a root turned by 90 degrees knows which side is up. To address the asymmetry question we have re-examined the ultrastructure of columella tissues preserved by cryofixation and discovered a novel, columella-specific type of asymmetrically organized ER, termed "nodal ER" based on its structure. To detect possible statolith-MF interactions we have quantitatively analyzed the movement of sedimenting statoliths in living cells. These results have led to an alternative hypothesis of gravity sensing in which the statoliths function by locally disrupting the attachment of the 3D actin-based MF network to the plasma membrane of columella cells.

(Supported by NASA: NAG5-3967)

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