INTRO

lular eu the est expans organ s the con the plan The tion an expans wall (G normal and have been likened to hoops around a barrel. Control of the orientation of the cellulose has been attributed to the cortical microtubules of the cytoskeleton. This is based on two types of evidence. First, cortical microtubules are normally found aligned in the same orientation as the cellulose microfibrils. Second, disruption of the microtubules has led to abnormal cellulose microfibril patterns and abnormal cell expansion (Giddings and Staehelin, 1991). A model has been proposed in which microtubules act like tracks between which the cellulose synthesis complex moves (Giddings and Staehelin, 1991). In this way the orientation of the microtubules dictates the orien-tation of the cellulose microfibrils without any direct contact between the two. Regulation of the extent of expansion is less well under-stood. The driving force for expansion is turgor pressure generated by the presence of a high osmotic potential in the vacuoles within the expanding cell. Water movement into the cell is thought to be passive, driven only by the difference in osmotic potential (Cosgrove, 1993). Expansion occurs by alon g a mer the es c s to f th ll co or p ressu he ugge but In this theory, expansion of individual cells is subject to a higher regulation that has predetermined the final shape of the organ (Kaplan and Hagemann, 1991). In animal cells, there is also little known about the regulation of cell expansion. Although most animal cells do not have cell walls, current studies of the primary plant cell wall indicate that it resembles the extracellular matrix surrounding animal cells (Roberts, 1989). Therefore, there are likely to be aspects of the regula-tion of cell expansion that are common to both animals and plants as well as some that are unique to each. The Arabidopsis root is particularly well suited to studies of cell expansion. Cell expansion in the body of the root is regulated so that the primary axis is normally parallel to the direction of growth. This makes for long cylindrical cells. Polarized expansion in the longitudinal orientation (which we will refer to as “elongation”) occurs primarily in a region at the tip of the root known as the elongation zone (Steeves an