| This study concerns the pathway of Ca2+ transport in onion (Allium cepa L.) roots. Ca2+ is an essential macronutrient, which must be present in ionic form in the soil solution to be absorbed by plant roots. To sustain their growth, plants require a continuous supply of Ca2+, as it is not redistributed from mature organs to the meristems. The pathway(s) and mechanism(s) of Ca2+ uptake and transport across the root have not been previously resolved. Because of low level of Ca2+ in the cell cytosol, the symplastic pathway may be insufficient to meet the Ca2+ requirements of the developing shoot. The apoplastic pathway, however, is obstructed by the deposition of hydrophobic materials (Casparian bands) in the walls of the endodermis and exodermis. It is commonly believed that Ca2+ follows an apoplastic pathway on its route to the xylem by entering the root tip, where the wall modifications in the endodermis and exodermis are not yet developed. In the present study, onion roots were used as a model system. These roots did not possess any measurable capacity for apoplastic transport, as determined by exclusion of 8-hydroxy-1-3-6-pyrenetrisuphonate (PTS) from the transpiration stream. Direct application of 45Ca2+ to discrete zones of the root revealed that Ca2+ applied to the root tip was not translocated to the shoot, ruling out the root tip as a point of entry for this ion. Further, an antimonate precipitation study ruled out the possibility that Ca2+ was delivered to the transpiration stream by maturation of tracheary elements. 45Ca2+ applied to more mature zones indicated that the ion moved radially through the mature endodermis and exodermis. The permeability of the exodermal Casparian bands to Ca 2+ was tested by means of compartmental elution, and the results indicated that these wall modifications hindered apoplastic Ca 2+ transport. Since Ca2+ was translocated from the oldest root zone, but could not pass through the exodermis apoplastically, its uptake into the symplast was investigated. Results of experiments with a number of inhibitors led to the hypothesis that lanthanum-sensitive Ca 2+ channels in the plasma membranes of the cells at the root margin mediate Ca2+ uptake while, at the other end of the radial path, P-type Ca2+-ATPases load Ca2+ into the xylem. A new model of radial Ca2+ transport is proposed, in which Ca2+ enters the cytoplasm of the cells at peripheral layer, is then extruded into the walls of the central cortex, where its transport occurs by cation exchange reaction, crosses the endodermis via the symplastic pathway, and is actively delivered to the apoplast of the xylem. |
