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

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