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Investigating the Function of ARG1 and ARL2 in the Arabidopsis Root Gravitropic Response.  B.R. Harrison and P.H. Masson, Lab of Genetics, University of Wisconsin, Madison, WI.

 The Arabidopsis protein ARG1 (Altered Response to Gravity1) is a peripheral membrane DnaJ-like protein that is required for signal transduction in the early phases of the root and hypocotyl gravitropic responses.  Genetic investigation of the function of the ARG1 paralog ARL2 (Altered Response-Like2) indicates that it also functions in gravity signal transduction, most likely at the same step as ARG1 (1).  In order to determine the molecular function of ARG1 and ARL2 in gravity signal transduction we are studying the localization of the auxin efflux carrier PIN3 in arg1 and arl2 mutant roots before and after gravity stimulation.  The auxin efflux facilitator PIN3 has been shown to relocalize to the bottom side of gravity-sensing columella cells upon 90 gravity stimulation (2).  This relocalization is thought to facilitate the establishment of a lateral gradient of the growth regulator auxin across a stimulated root, which is essential for a full gravitropic response.  Preliminary immuno-localization experiments indicate that PIN3 is not properly localized after 20 minutes of gravity stimulation in the arg1 and arl2 mutants.  We are currently trying to determine if ARG1 and ARL2 function directly in the PIN3 localization mechanism.  We are also using the auxin responsive DR5-GUS and DR5-GFP reporters to analyze the distribution of auxin within arg1 and arl2 roots before and after 90 gravity stimulation.

(1) Guan C, Rosen E, Boosirichai K, Poff K, Masson P. Plant Physiology 133: 100-112 (2003)

(2) Friml J, Wisniewska J, Benkova E, Mendegen K, Palme K. Nature, 415: 806-809 (2002)