MAPPING THE FUNCTIONAL ROLES OF CAP CELLS IN THE RESPONSE OF ARABIDOPSIS PRIMARY ROOTS TO GRAVITY

The cap is widely accepted to be the site of gravity sensing in roots because removal of the cap abolishes root curvature. Circumstantial ev idence favors the columella cells as the gravisensory cells because am yloplasts (and often other cellular components) are polarized with res pect to the gravity vector. However, there has been no functional conf irmation of their role. To address this problem, we used laser ablatio n 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 vert ical growth. Ablation of the peripheral cap cells and tip cells did no t alter root curvature. Ablation of the innermost columella cells caus ed the strongest inhibitory effect on root curvature without affecting growth rates. Many of these roots deviated significantly from vertica l growth and had a presentation time 6-fold longer than the controls. Among the two inner columella stories, the central cells of story 2 co ntributed the most to root gravitropism. These cells also exhibited th e largest amyloplast sedimentation velocities. Therefore, these result s are consistent with the starch-statolith sedimentation hypothesis fo r gravity sensing.