Gravitropism in the starch excess mutant of Arabidopsis thaliana.

Amyloplasts are hypothesized to play a key role in the cellular mechanisms of gravity perception in plants. While previous studies have examined the effects of starch deficiency on gravitropic sensitivity, in this paper, we report on gravitropism in plants with a greater amount of starch relative to the normal wild type. Thus, we have studied the sex1 (starch excess) mutant of Arabidopsis thaliana, which accumulates extra starch because it is defective in a protein involved in the regulation of starch mobilization. Compared to the wild type (WT), sex1 seedlings contained excess starch in cotyledons, hypocotyls, the root-hypocotyl transition zone, the body of the root, root hairs, and in peripheral rootcap cells. Sedimented amyloplasts were found in both the WT and in sex1 in the rootcap columella and in the endodermis of stems, hypocotyls, and petioles. In roots, the starch content and amyloplast sedimentation in central columella cells and the gravitropic sensitivity were comparable in sex1 and the WT. However, in hypocotyls, the sex1 mutant was much more sensitive to gravity during light-grown conditions compared to the WT. This difference was correlated to a major difference in size of plastids in gravity-perceiving endodermal cells between the two genotypes (i.e., sex1 amyloplasts were twice as big). These results are consistent with the hypothesis that only very large changes in starch content relative to the WT affect gravitropic sensitivity, thus indicating that wild-type sensing is not saturated.