A Role for Calcium in Auxin Transport

Abstract

The basipetal transport of the auxin, indoleacetic acid, in sunflower stem sections is markedly suppressed by washing the tissue in ethylenediaminetetraacetate, and transport is restored by subsequent application of calcium solutions. The ethylenedi-aminetetraacetate treatment is shown to result in the removal of substantial amounts of calcium from the tissue, and the res-toration of transport is distinctive for calcium solutions, lesser effects being observed for magnesium and lanthanum, and little effect for monovalent cations. The calcium effects are inter-preted as indicating that the auxin transport system depends upon structural or functional features of cellular membranes which involve calcium in a manner analogous to the transport of inorganic ions. The transport of auxin in plant stems and coleoptiles shows all of the characteristics of an active transport system: sub-strate specificity, velocity greater than diffusion, dependence upon metabolism, and apparent movement against a concen-tration gradient (12). After the movement of this hormone was found to be secretive (8), a model of transport as a function of permeases located in the plasmalemma was suggested (13). Os-borne (15) suggested the necessity for the synthesis of a carrier protein for the maintenance of auxin transport. Recent experi-ments on the site of auxin attachment within the cell infer that auxin becomes attached to membranes (9), and this may be to the plasmalemma (11). If the transport of this hormone may occur at the plasmalemma, one might expect that factors which would alter membrane structure and function could alter the transport of auxin. In view of the well documented evidence that calcium ions play a major role in the structure and function of membranes (2), we have selected this ion as a physiological variable, and we can show that it has dis-tinctive effects on the transport of auxin in sunflower stems.