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How to activate a plant gravireceptor?  C. Limbach, M. Braun   Institut fьr Molekulare Physiologie und Biotechnologie der Pflanzen, Gravitationsbiologie, Universitдt Bonn, Bonn, Germany.

   Although gravitropism has been studied for decades almost nothing is known about the early mechanisms of graviperception that transduce the positional information of sedimenting statoliths into a physiological signal and thereby initiate gravitropic signalling in gravisensing cells. Single celled model systems like rhizoids of characean algae are well suited for  nvestigating these cellular aspects of gravitropism since the gravitropic signalling pathway is rather short and locally confined to the tip region of one single cell. The actin cytoskeleton is the central keyplayer that determines structural organization of the cell and controls the mechanisms of gravity sensing and gravitropic reorientation. The actomyosin system is crucial for maintaining gravisensitivity since it precisely regulates statoliths' positioning and directs sedimenting statoliths to confined gravisensitive sites of the plasma membrane upon gravistimulation.     Centrifugation experiments during the microgravity phase of sounding rocket flights enabled us to estimate the forces of the molecular interactions between statoliths and the actin cytoskeleton in rhizoids by determining the threshold acceleration level of lateral statolith displacement. This value, which is close to the general threshold level of gravisensitivity, was measured to be in a range of 0.1 g which corresponds to a minimum force of 2 10-4 N that has to be exceeded by an acceleration stimulus in order to sediment a single statolith. Recent parabolic flight experiments aiming at the functional characterization of the gravireceptor provided unambiguous evidence that graviperception is not mediated mechanically by the pressure of sedimented statoliths. Even weightless statoliths are able to activate the receptor and increased statoliths' weight caused by centrifugation does not promote the graviresponse. We conclude that gravireceptor activation in characean rhizoids depends on direct contact with statoliths and is accomplished by molecular or electrochemical interactions.

(Financial support by Deutsches Zentrum fьr Luft- und Raumfahrt (DLR) on behalf of BMBF (50WB9998)).