Plasma membrane H+‐ATPase in the root apex: Evidence for strong expression in xylem parenchyma and asymmetric localization within cortical and epidermal cells

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⁺‐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.