title: Tip-growing cells of the moss Ceratodon purpureus Are gravitropic in high-density media.
abstract: Gravity sensing in plants and algae is hypothesized to rely upon either the mass of the entire cell or that of sedimenting organelles (statoliths). Protonemata of the moss Ceratodon purpureus show upward gravitropism and contain amyloplasts that sediment. If moss sensing were whole-cell based, then media denser than the cell should prevent gravitropism or reverse its direction. Cells that were inverted or reoriented to the horizontal displayed distinct negative gravitropism in solutions of iodixanol with densities of 1.052 to 1.320 as well as in bovine serum albumin solutions with densities of 1.037 to 1.184 g cm(-3). Studies using tagged molecules of different sizes and calculations of diffusion times suggest that both types of media penetrate through the apical cell wall. Estimates of the density of the apical cell range from 1.004 to 1.085. Because protonemata grow upward when the cells have a density that is lower than the surrounding medium, gravitropic sensing probably utilizes an intracellular mass in moss protonemata. These data provide additional support for the idea that sedimenting amyloplasts function as statoliths in gravitropism.
authors.affiliation: Department of Plant Biology, Ohio State University, Columbus, Ohio 43210-1293, USA.
authors.forename: Jochen Michael
authors.initials: JM
authors.lastname: Schwuchow
authors.forename: Volker Dieter
authors.initials: VD
authors.lastname: Kern
authors.forename: Fred David
authors.initials: FD
authors.lastname: Sack
mesh_terms: D019068:Bryopsida / Q000166:cytology / Q000187:drug effects / Q000254:growth & development*; D002473:Cell Wall / Q000378:metabolism; D011232:Chemical Precipitation; D003287:Contrast Media / Q000494:pharmacology; D003470:Culture Media; D003720:Densitometry / Q000379:methods; D003911:Dextrans / Q000494:pharmacology; D018522:Gravitropism / Q000187:drug effects / Q000502:physiology*; D018466:Gravity Sensing / Q000187:drug effects / Q000502:physiology*; D018613:Microscopy, Confocal; D018087:Plastids / Q000187:drug effects / Q000378:metabolism; D012710:Serum Albumin, Bovine / Q000494:pharmacology; D014283:Triiodobenzoic Acids / Q000494:pharmacology
chemical_list: D003287:Contrast Media; D003470:Culture Media; D003911:Dextrans; D014283:Triiodobenzoic Acids; D012710:Serum Albumin, Bovine; C044834:iodixanol