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Role of the Actin Related Protein (ARP) 2/3 Complex in Gravitropism.  J.C. Reboulet, P. Kumar, J.Z. Kiss Miami University, Oxford, OH 45056.

    Plants constantly modify their growth in response to environmental stimuli such as light and gravity.  The differential growth of roots and shoots in response to gravity is termed gravitropism.  In general, roots are positively gravitropic, (growing downward), while shoots are negatively gravitropic (growing upward).  The F-actin cytoskeleton has recently been hypothesized to play a major role in gravity perception through its interaction with starch-filled amyloplasts, as they travel through the cytoplasm in the direction of the gravity vector.  A variety of proteins are known to interact with the cytoskeleton and thus have potential roles in gravitropism.  The Actin Related Protein (ARP) 2/3 Complex, previously identified in bacteria and plants, is known to have prominent roles in cell locomotion and membrane protrusion.  In this study, we were interested in the distorted mutants (dis1 and dis2), that are characterized by the appearance of wavy trichomes and abnormal epidermal pavement cells of leaves, due to a mutation in ARP 2/3.  Since the ARP 2/3 complex is responsible for the nucleation and branching of F-actin, this study should provide adequate information to better understand the role of actin cytoskeleton in gravitropism.  We used 4-day old light- and dark-grown seedlings and mature stems to quantify growth and gravitropic curvature of dis1 and dis2 mutants and compared them with the wild type (Columbia).  We found that roots of dark grown plants of dis1 and dis2 are disoriented in growth with respect to the gravity vector, compared to the light-grown counterparts.  Thus, our results suggest a potential role for ARP 2/3 in the gravitropism pathway.

   (Financial support was provided by NASA Grant NCC2-1200).