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Unraveling the Role of the Actin Cytoskeleton in Root Growth and Gravitropism Using Green Fluorescent Protein (GFP) Reporters and Knockouts to Actin Regulatory Proteins. C.-M. Yoo, Y-S. Wang and E.B. Blancaflor, Plant Biology Division, Noble Foundation, Ardmore, OK

   The cytoskeleton is a network of filamentous proteins found in all eukaryotic cells. In plants, it plays important roles in a variety of plant growth and developmental processes including cell division, cell elongation, organelle positioning and cell signaling. To function properly, the cytoskeleton must undergo dynamic rearrangements through a process regulated by several cytoskeletal modulating proteins. These regulatory proteins include various actin binding proteins and microtubule associated proteins. Through database searches of the Arabidopsis genome, a multitude of candidate genes encoding proteins with cytoskeletal binding motifs have been identified thus paving the way for a complete analysis of cytoskeletal function during plant growth and development. Previously, we have shown that disrupting the actin cytoskeleton with actin disrupting drugs enhances the gravity response of roots possibly by downregulating components of the gravitropic signaling machinery (Hou et al., Plant Physiol 131: 1360-1373; Plant J 31: 113-125). To identify specific components of the actin cytoskeleton that contribute to this phenomenon, we have identified Arabidopsis knockouts to actin regulatory proteins expressed in roots including fimbrins and actin related proteins (ARPs). Furthermore, we have created Arabidopsis plants overexpressing some of these actin regulatory proteins. The responses of these knockouts and overexpressing lines to gravity are being analyzed using a number of parameters including presentation time calculations and kinetics of root curvature. We have also created a number of green fluorescent protein (GFP) reporters that would allow us to determine auxin response and auxin gradient development in roots, monitor amyloplast dynamics in the root cap and visualize F-actin organization. The combination of molecular, cellular and physiological approaches should allow us to gain deeper insights into the function of actin in root gravitropism.

(Supported by NASA grant NAG 2-1518 to EBB).