Cortical actin filaments potentially interact with cortical microtubules in regulating polarity of cell expansion in primary roots of maize (Zea maysL.)

Evidence is accumulating implicating cortical microtubules in the direction al control of cell expansion. However, the role of actin filaments in this process is still uncertain. To determine the involvement of actin in cell e longation, the organization of actin filaments in primary roots of maize (Z ea mays L.) was examined by use of an improved fluorochrome-conjugated phal loidin-labeling method. With this method, a previously undetected state of actin organization was revealed in the elongation and maturation zone of ma ize roots. Fine transversely oriented cortical actin was observed in all ce lls of the elongation zone, including the epidermis, cortex, and vascular t issues. The orientation of cortical actin shifted from a predominantly tran sverse orientation to oblique, longitudinal, and/or random arrangements as the cells matured. The reorientation of cortical actin in maturing root cel ls mimics the behavior of cortical microtubules reported in other studies. Furthermore, roots treated with the microtubule-stabilizing drug taxol impr oved the quality of actin preservation as evidenced by the thicker bundles of cortical actin. This suggested that taxol was also capable of stabilizin g the cortical actin networks. The elongation of roots exposed to I muM Lat runculin B, an actin-disrupting drug, was inhibited, and after 24 h the roo ts exhibited moderate swelling particularly along the elongation zone. Latr unculin B also caused microtubules to reorient from transverse to oblique a rrays. The results from this study provide evidence that cortical microtubu les and actin filaments respond in a coordinated way to environmental signa ls and may well depend on both elements of the cytoskeleton.