Graviorientation in protists and plants.

Gravitaxis, gravikinesis, and gravitropism are different graviresponses found in protists and plants. The phenomena have been intensively studied under variable stimulations ranging from microgravity to hypergravity. A huge amount of information is now available, e.g. about the time course of these events, their adaptation capacity, thresholds, and interaction between gravity and other environmental stimuli. There is growing evidence that a pure physical mechanism can be excluded for orientation of protists in the gravity field. Similarly, a physiological signal transduction chain has been postulated in plants. Current investigations focus on the question whether gravity 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 sedimenting body exerting pressure on the lower membrane. Behavioral studies in density adjusted media, effects of inhibitors of mechano-sensitive ion channels or manipulations of the proposed gravireceptor structures revealed that both mechanisms have been developed in protists and plants. The threshold values 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.