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INCREASED GRAVISENSITIVITY OF ROOTS WITH A DISRUPTED CAP ACTIN CYTOSKELETON.  E.B. Blancaflor, G.C. Hou, and D.R. Mohamalawari. Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, Oklahoma.

   The actin cytoskeleton has been proposed to be a major player in the gravitropic response of plants. However, there are still major gaps in our understanding of the role of actin in this process. To further address this problem, we analyzed the effect of various cytoskeletal inhibitors on root gravitropism. Latrunculin B (LB), an actin-disrupting drug promoted root curvature in a variety of plant species tested. More significantly, the gravitropic sensitivity of maize and Medicago truncatula roots was enhanced after LB-treatment as evident from the comparison of presentation times in LB-treated versus untreated roots. The microtubule inhibitor, oryzalin, did not have any significant effect on gravitropic sensitivity or root curvature. Interestingly, roots treated with LB consistently showed hyper-gravitropic responses even with short induction times. LB-treated roots gravistimulated for 5-10 min followed by rotation on a clinostat displayed extensive bending responses and often times the direction of root growth in the clinostat was characterized by 180 degree loops. These extensive bending responses occurred despite the strong growth inhibitory effects of LB. Application of LB specifically to maize root caps resulted in localized disruption of cap actin and this was sufficient to induce the increased gravitropic sensitivity of roots. On the other hand, disruption of actin in the elongation zone with LB did not affect gravisensitivity. Application of napthyphthalamic acid (NPA) to LB-treated roots abolished these extreme curvature responses of roots. Work on labeling of actin filaments in the columella is in progress to determine if gravistimulation results in actin reorganization. Our results indicate that while an intact actin network in the cap is not necessary for root curvature to occur, the increased sensitivity and response of roots treated with LB are consistent with a role for actin in modulating the activity of signaling molecules originating from the cap during root gravitropism.

(Supported by NASA grant NAG 2-1518).

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