The cell and subcellular localization of plasma membrane P-type H + -ATPase in root apices from Zea mays L. (maize) seedlings was investigated by immunofluorescence microscopy. H-ATPase was highly abundant in cells of epidermal and endodermal tissues as well as in phloem companion cells. Strong immunodecoration was also observed in a subset of xylem parenchyma cells forming a connection between the endodermis and metaxylem. Evidence that these cells are equipped for active membrane transport raises the potential that they play a special role in xylem loading. Significant amounts of H + -ATPase were also observed in outer cortical cells. Progressively less H + -ATPase was seen in cortical cells further away from the root-soil interface. The H 1 -ATPase was asymmetrically localized within both epidermal and outer cortical cells, with higher levels detected on cell surfaces closest to the root-soil interface. This asymmetric localization of H + -ATPase is consistent with the hypothesis that transport systems for uptake of nutrients from the soil are selectively targeted to cell surfaces most exposed to nutrients.