Can Ca^2+ Fluxes to the Root Xylem Be Sustained by Ca^2+-ATPases in Exodermal and Endodermal Plasma Membranes?

The pathway of Ca^2+ movement from the soil solution into the root stele has been a subject of controversy. If transport through the endodermis is assumed to be through the cytoplasm, the limiting factor is believed to be the active pumping of Ca^2+ from the cytoplasm into the stele apoplast through the plasma membrane lying on the stele side of the Casparian band. By analogy, for similar transport through the exodermis, the limiting step would be the active pumping into the apoplast on the central cortical side of the layer. Such effluxes are mediated by Ca^2+ATPases. To assess whether or not known Ca^2+ fluxes to the stele in onion (Allium cepa) roots could be supported by Ca^2+-ATPases, the percentages of total membrane protein particles required to effect the transport were calculated using measured values of membrane surface areas, an animal literature value for Ca^2+-ATPase Vmax, plant literature values for Ca^2+-ATPase Km, and protein densities of relevant membranes. Effects of a putative symplastic movement of Ca^2+ from the exo- or endodermis into the next cell layer, which would increase the surface areas available for pumping, were also considered. Depending on the assumptions applied, densities of Ca^2+ pumps, calculated as a percentage of total membrane protein particles, varied tremendously between three and 1,600 for the endodermis, and between 0.94 and 1,900 for the exodermis. On the basis of the data, the possibility of Ca^2+ transport through the cytoplasm and membranes of the exodermis and endodermis cannot be discounted. Thus, it is premature to assign an entirely apoplastic pathway for Ca^2+ movement from the soil solution to the tracheary elements of the xylem. To verify any conclusion with certainty, more detailed data are required for the characteristics of exo- and endodermal Ca^2+-ATPases.