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Complex cytoskeletal mechanisms underlying plant gravity sensing. A. Sievers and M. Braun, Botanisches Institut, Universitдt Bonn, Bonn, Germany.

   Positioning and gravity-induced sedimentation of statoliths are essential requirements for gravisensing in most higher and lower plants. Positively gravitropic rhizoids and negatively gravitropic protonemata of characean green algae are favorable cell types to study the role of the actin cytoskeleton in plant gravity responses. Using these cell types, statolith positioning by actomyosin forces was investigated in microgravity (<10-4g) during parabolic flights of rockets (TEXUS/MAXUS) and during the Space-Shuttle missions STS 65 and STS 81. In both cell types, the natural position of statoliths at 1g is established by basipetal and acropetal actomyosin forces which exactly compensate the statoliths' weight in this position. Regulated by several actin-associated proteins, actomyosin forces organize the tip growth machinery and act differently on statoliths in the different regions of both cell types in order to keep the statoliths close to the apex, the only position, where they actually function as gravity susceptors. Upon gravistimulation, statoliths are precisely directed to specific statolith-sensitive plasma membrane areas by the concerted action of gravity and the actomyosin forces. The perception site is confined to a small subapical region (10-35 µm) in positively gravitropic rhizoids, whereas in negatively gravitropic protonemata, this area is limited to the apical plasma membrane (0-10 µm). Thus, the actin cytoskeleton is a key element of the gravitropic signaling pathway, essentially required for the early process of graviperception, but also involved in the molecular mechanisms of the opposite graviresponses in characean rhizoids and protonemata. There is increasing evidence and it seems reasonable to assume that actin might play a similar role in higher plant gravity sensing.

(Supported by DLR on behalf of BMBF (50WB99998))