ASYMMETRIC DISTRIBUTION OF THE PLASMA-MEMBRANE H-ATPASE IN EMBRYOS OFVICIA-FABA L WITH SPECIAL REFERENCE TO TRANSFER CELLS()

The ultrastructural localization of the plasma-membrane H+-ATPase by i mmunocytochemistry was studied in Vicia faba embryos which absorb nutr ients from the maternal organism through the transfer cells of their e xternal epidermis. The samples were embedded in LR White resin and the specificity of immunolabelling was checked by inhibition in the prese nce of purified H ATPase. The following results were obtained: (i) The H ATPase density varied according to the cell type, being higher in t ransfer cells than in other cell types, especially the non-modified ce lls of the internal epidermis. (ii) There was a marked polarity in tra nsfer cells as proton pumps were more numerous in the area of plasmale mma infoldings where active nutrient uptake is assumed to take place. (iii) No clear immunolabelling occurred on the plasma membrane of plas modesmata. These results demonstrate that in transfer cells the area o f plasmalemma infoldings is highly specialized for active solute trans port; they also support the idea of specific structural properties of the plasmalemma in plasmodesmata.