Isolation of casparian strips from pea roots

Casparian strips were isolated from roots of pea seedlings. The plasma membrane was firmly attached to the wall of the individual cells in the isolated Casparian strip. The outer electrondense lamella of the plasma membrane, which is characteristic of the membrane of the Casparian strip, was not removed from the isolated Casparian strip by treatment with either 1 M NaCl or 2 M sodium thiocyanate. Although treatment with 1% Triton X-100 disrupted the lamellar structure of the membrane and removed most of the electron-dense material of the outer lamella, no detectable polypeptides were extracted by treatment of isolated Casparian strips with 1% Triton X-100. Small amounts of electron-dense material remained after treatment with Triton X-100 and this material seemed to be removed almost entirely by subsequent treatment of the same sample with 2% SDS. Since polypeptides of 46, 30 and 20 kDa, which were detected in the extract obtained by treatment with SDS, were not detected in an extract of total cell walls of pea roots, it seems possible that these polypeptides are specific to the Casparian strip and, therefore, that they may be responsible singly or together for the adhesion of the plasma membrane to the cell wall at the Casparian strip.