title: Gravitropism and starch statoliths in an Arabidopsis mutant.
abstract: The mutant TC 7 of Arabidopsis thaliana (L.) Heynh. has been reported to be starch-free and still exhibit root gravitropism (T. Caspar and B. G. Pickard 1989, Planta 177, 185-197). This is not consistent with the hypothesis that plastid starch has a statolith function in gravity perception. In the present study, initial light microscopy using the same mutant showed apparently starch-free statocytes. However, ultrastructural examination detected residues of amyloplast starch grains in addition to the starch-depleted amyloplasts. Applying a point-counting morphometric method, the starch grains in the individual amyloplasts in the mutant were generally found to occupy more than 20% and in a few cases up to 60% of the amyloplast area. In the wild type (WT) the starch occupied on average 98% of the amyloplast area and appeared as densely packed grains. The amyloplasts occupied 13.9% of the area of the statocyte in the mutant and 23.3% of the statocyte area in the WT. Sedimentation of starch-depleted amyloplasts in the mutant was not detected after 40 min of inversion while in the WT the amyloplasts sedimented at a speed of 6 micrometers h-1. The gravitropic reactivity and the curvature pattern were also examined in the WT and the mutant. The time-courses of root curvature in the WT and the mutant showed that when cultivated under standard conditions for 60 h in darkness, the curvatures were 83 degrees and 44 degrees, respectively, after 25 h of continuous stimulation in the horizontal position. The WT roots curved significantly more rapidly and with a more normal gravitropic pattern than those of the mutant. These results are discussed in relation to the results previously obtained with the mutant and with respect to the starch-statolith hypothesis.
authors.affiliation: Department of Botany, University of Trondheim, Dragvoll, Norway.
authors.forename: N
authors.initials: N
authors.lastname: Saether
authors.forename: T H
authors.initials: TH
authors.lastname: Iversen
mesh_terms: D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D004721:Endoplasmic Reticulum / Q000502:physiology / Q000648:ultrastructure; D006112:Gravitation; D018522:Gravitropism / Q000235:genetics / Q000502:physiology*; D018466:Gravity Sensing / Q000502:physiology; D008854:Microscopy, Electron; D009154:Mutation; D018518:Plant Root Cap / Q000235:genetics / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure*; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D018087:Plastids / Q000502:physiology* / Q000648:ultrastructure; D013213:Starch / Q000172:deficiency* / Q000502:physiology
chemical_list: D013213:Starch