The role of calcium ions in gravity signal perception and transduction.

Ca2+ is implicated as a messenger in coupling various environmental stimuli, such as gravity and light, to response. In recent years, it has become evident that Ca2+ plays a central role in all three phases of gravitropism--perception, transduction and response. The root cap, which is known to contain high amounts of Ca2+ and calmodulin, is the primary site of gravity perception. The possible role of phosphoinositide turnover and Ca(2+) - and Ca(2+) -calmodulin-dependent enzymes such as Ca(2+) -ATPase and protein kinases in gravitropism is discussed. A model is proposed to describe the role of Ca2+ in both normal and light-dependent gravity response in roots.