Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity

Elison B. Blancaflor, Jeremiah M. Fasano, and Simon Gilroy
Biology Department, 208 Mueller Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802

Abstract.
The cap is widely accepted to be the site of gravity-sensing in roots since removal of the cap abolishes root curvature. Circumstantial evidence favors the columella cells as the gravisensory cells because amyloplasts (and often other cellular components) are polarized with respect to the gravity vector. However, there has been no functional confirmation of their role. To address this problem, we employed laser ablation to remove defined cells in the cap of Arabidopsis primary roots, and quantified the response of the roots to gravity using three parameters (time course of curvature, presentation time and deviation from vertical growth). Ablation of the peripheral cap cells and tip cells did not alter root curvature. Ablation of the innermost columella cells caused the strongest inhibitory effect on root curvature without affecting growth rates. Many of these roots deviated significantly from vertical growth, and had a presentation time six fold longer than the controls. Among the two inner columella stories, the central cells of story 2 contributed most to root gravitropism. These cells also exhibited the largest amyloplast sedimentation velocities. Therefore these results are consistent with the starch-statolith sedimentation hypothesis for gravity-sensing.

Back to publications