Graviorientation in protists and plants

Gravitaxis, gravikinesis, and gravitropism are different graviresponses fou nd in protists and plants. The phenomena have been intensively studied unde r variable stimulations ranging from microgravity to hypergravity. A huge a mount of information is now available, e.g. about the time course of these events, their adaptation capacity, thresholds, and interaction between grav ity and other environmental stimuli. There is growing evidence that a pure physical mechanism can be excluded for orientation of protists in the gravi ty field. Similarly a physiological signal tranduction chain has been postu lated in plants. Current investigations focus on the question whether gravi ty is perceived by intracellular gravireceptors (e.g. the Muller organelle of the ciliate Loxodes, barium sulfate vacuoles in Chara rhizoids or starch statoliths in higher plants) or whether the whole cell acts as a sedimenti ng body exerting pressure on the lower membrane. Behavioral studies in dens ity adjusted media, effects of inhibitors of mechano-sensitive ion channels or manipulations of the proposed gravireceptor structures revealed that bo th mechanisms have been developed in protists and plants. The threshold val ues for graviresponses indicate that even 10 % of the normal gravitational field can be detected, which demands a focusing and amplifying system such as the cytoskeleton and second messengers.