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CORTICAL ACTIN FILAMENTS IN PLANT CELLS: INVOLVEMENT IN ROOT GROWTH REGULATION.  E. B. Blancaflor and D. Mohamalawari.  Plant Biology Division, The Samuel Roberts Noble Foundation Inc., 2510 Sam Noble Parkway, Ardmore, Oklahoma, USA 73401

     Cortical microtubules have been implicated in the directional control of cell expansion. However, the role of actin filaments in this process is still uncertain. To determine the involvement of actin in cell elongation, the organization of actin filaments in primary roots was examined using an improved fluorochrome-conjugated phalloidin labeling method for fixed cells. Using this method, fine transversely oriented cortical actin was observed in cells of the elongation zone including the epidermis, cortex and vascular tissues. The orientation of cortical actin shifted from a predominantly transverse orientation to oblique, longitudinal and/or random arrangements as the cells matured. This behavior was similar to the behavior of cortical microtubules reported in previous studies. Roots treated with the microtubule stabilizing drug taxol improved the quality of actin preservation as evidenced by the thicker bundles of cortical actin suggesting that taxol was also capable of stabilizing the cortical actin networks. The elongation of roots exposed to 1 mM Latrunculin B (LB) was inhibited and after 24 hours roots exhibited moderate swelling along the transition and elongation zone. LB also caused the premature reorientation of microtubules from transverse to oblique arrays. These results indicate that cortical microtubules and actin filaments potentially interact in regulating cellular expansion. We are currently using green (GFP) and red (RFP) fluorescent-talin constructs and GFP-MAP4 to image the dynamics and possible interaction between actin and microtubules in living cells during root growth (Supported by NASA and the Noble Foundation).