HIGH EXPRESSION OF THE TONOPLAST AQUAPORIN ZMTIP1 IN EPIDERMAL AND CONDUCTING TISSUES OF MAIZE

Aquaporins are integral membrane proteins of the tonoplast and the pla sma membrane that facilitate the passage of water through these membra nes. Because of their potentially important role in regulating water f low in plants, studies documenting aquaporin gene expression in specia lized tissues involved in water and solute transport are important. We used in situ hybridization to examine the expression pattern of the t onoplast aquaporin ZmTIP1 in different organs of maize (Zea mays L.). This tonoplast water channel is highly expressed in the root epidermis , the root endodermis, the small parenchyma cells surrounding mature x ylem vessels in the root and the stem, phloem companion cells and a ri ng of cells around the phloem strand in the stem and the leaf sheath, and the basal endosperm transfer cells in developing kernels. We postu late that the high level of expression of ZmTIP1 in these tissues faci litates rapid flow of water through the tonoplast to permit osmotic eq uilibration between the cytosol and the vacuolar content, and to permi t rapid transcellular water flow through living cells when required.