Calcium in gravitropism. A re-examination

Abstract.

For more than a decade it has been assumed that there is a strong relationship between Ca²⁺ and gravitropism. There is evidence to suggest that the movement of Ca²⁺ in the wall might regulate extension growth and that free intracellular Ca²⁺ might mediate signalling in statocytes. However, it is unlikely that changes in either wall Ca²⁺ or the concentration of free intracellular calcium, [Ca²⁺]_i, act independently of each other. This results in ambiguity in both experimentation and interpretation of experimental information. It is also uncertain that the observed redistributions of wall Ca²⁺ are sufficient to induce bending and there is at present no direct evidence from measurement or imaging that changes in [Ca²⁺]_i initiate or transduce gravitropic signals. The evidence relating Ca²⁺ to gravitropism is therefore substantial but still circumstantial. The involvement of free intracellular Ca²⁺ might best be tested using the new technologies of transgenic aequorin targeted to columella cells and thus resolve this important question. New information which relates control or orientation of tip-growing cells, such as pollen tubes, to cytosolic Ca²⁺ manipulation is presented. This information may be directly applicable to a gravitropic model system studied for many years, the Chara rhizoid.