Bioelectricity, gravity and plants.

This brief review summarizes gravity-induced changes in bioelectric parameters and evaluates their contribution to our understanding of the sensing of gravity, and the transduction and transmission of the gravity stimulus in plants. During the last few decades, information has accumulated demonstrating gravity-induced changes in surface potentials, membrane voltages, endogenous electric currents and ion fluxes. These changes point to the plasma membrane as the site of perception and transduction of the gravity signal. To date, it is reasonable to assume that gravity affects the state of ion channels (in particular, Ca2+ channels) and the activity of ion pumps (in particular, the electrogenic H(+)-ATPase) in the plasma membrane leading to intracellular and apoplasmic changes in ion activities and in membrane voltages. The flow of H+ and Ca2+ currents is probably the means by which information about gravity is amplified and transmitted from sensing to responding cells. No data are available so far about the effect of microgravity on bioelectric parameters. However, it would be interesting to learn if plants become hypersensitive to gravity during a prolonged stay in microgravity. If so, such plants might fire action potentials after return to earth, because more Ca2+ channels than usual may be activated by 1 g in microgravity-adapted plants.