Sedimentation and movement of plastids in columella cells of the root cap were measured in seedlings of wild-type, a reduced starch mutant, and a starchless mutant of Arabidopsis. To assay for sedimentation, we used both linear measurements and the change of angle from the cell center as indices in vertical and reoriented plants with the aid of computer-assisted image analysis. Seedlings were fixed at short periods after reorientation, and plastid sedimentation correlated with starch content in the three strains of Arabidopsis. Amyloplasts of wild-type seedlings showed the greatest sedimentation, whereas plastids of the starchless mutant showed no significant sedimentation in the vertically grown and reoriented seedlings. Because previous research has shown that a full complement of starch is needed for full gravitropic sensitivity, this study correlates increased sensitivity with plastid sedimentation. However, although plastid sedimentation contributed to gravisensitivity, it was not required, because the gravitropic starchless mutant had plastids that did not sediment. This is the first study, to our knowledge, to measure plastid sedimentation in Arabidopsis roots after reorientation of seedlings. Taken together, the results of this study are consistent with the classic plastid-based and protoplast-based models of graviperception and suggest that multiple systems of perception exist in plant cells.Of the many different environmental cues that any organism uses for orientation, gravity is the most constant and pervasive. All life has evolved in its presence. Since the emergence of plants from an aqueous environment, gravity has been an essential factor in plant development (Barlow, 1995). Gravitropism can be divided into three phases, distinct in time and, in higher plants, space: perception, transduction, and response (Evans et al., 1986;Salisbury, 1993). It has long been hypothesized that the central columella cells of the root cap are the graviperceptive cells, i.e. the statocytes, in plant roots (Volkmann and Sievers, 1979). Direct evidence supporting the role of columella cells as statocytes includes work by Behrens et al. (1985), who found that only the columella cells experienced rapid membrane potential change after reorientation, and work by Blancaflor et al. (1998), who showed that selective laser ablation of columella cells reduced gravitropic sensitivity.When a plant is reoriented with respect to a gravitational force, the amyloplasts in the columella cells settle on the new "lower" cell wall. This original observation by Haberlandt (1914) implicated amyloplasts in graviperception and still is a good indication of a graviperceptive organ (Volkmann and Sievers, 1979;Sack, 1991;Salisbury, 1993). On theoretical grounds, Bjö rkman (1988) calculated that amyloplasts have the mass and mobility to activate a receptor with enough energy to make it reliable (250 times thermal noise and 15 times activation energy), whereas other organelles, including mitochondria and the entire protoplast, do not. But even ...