THE ORGANIZATION OF THE ACTIN CYTOSKELETON IN VERTICAL AND GRAVIRESPONDING PRIMARY ROOTS OF MAIZE

To determine whether actin microfilament (MF) organization is correlat ed with differential elongation, primary roots of Zea mays cv Merit ma intained vertically or reoriented horizontally for 15 to 120 min were stained with rhodamine phalloidin and examined with a confocal microsc ope. Root curvature was measured with a computer-controlled video digi tizer. In vertical roots bundles of MFs in the elongation and maturati on zone were oriented parallel to the longitudinal axis of cells. MFs in the vascular parenchyma cells were more abundant than in the cortex and epidermis. Epidermal and proendodermal cells in the meristematic region contained transverse cortical MFs. The organization of MFs of g raviresponding roots was similar to that of vertical roots. Applicatio n of cytochalasin B or cytochalasin D resulted in extensive disruption of MFs in the cortex and epidermis, but only partially affected MFs i n the stele. Despite the cytochalasin B-induced depolymerization of MF s, gravicurvature exceeded that of controls. In contrast, the auxin tr ansport inhibitor N-1 naphthylphthalamic acid suppressed root curvatur e but had no observable effect on the integrity of the MFs. The data i ndicate that MFs may not be involved in the graviresponse of maize roo ts.