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   Arabidopsis roots respond to gravistimulation by developing a curvature that is modulated by a lateral gradient of auxin. This gradient originates in the columella statocytes, and is associated with a lateral repositioning of the PIN3 auxin efflux facilitator in these cells. We used genetics to identify proteins that contribute to gravity signal transduction in the statocytes.   ARG1 and ARL2 are needed for lateral auxin transport across the cap.  ARG1 is associated with the vesicular trafficking pathway, suggesting it regulates PIN3 function or trafficking. Indeed, immunolocalization studies confirm a lack of PIN3 relocalization in gravistimulated statocytes of arg1-2 and arl2-1 mutant root caps. Interestingly, arg1-2 and arl2-1 mutant seedlings still show significant gravitropic responses, as do starch-deficient mutants like pgm.  However, arg1-2 pgm and arl2-1 pgm double mutants display strongly enhanced gravitropic defects relative to each single mutant, suggesting a novel genetic approach to isolate new gravity signal transducers that function in the PGM pathway. Accordingly, we isolated and characterized two genetic enhancers of arg1-2, called mar1-1 and mar2-1. mar1-1 and mar2-1 mutant seedlings display almost wild type gravitropism in an ARG1 background. However, arg1-2 mar1-1 and arg1-2 mar2-1 double mutant seedlings are almost completely agravitropic while retaining wild-type root-growth responses to phytohormones and polar auxin transport inhibitors, remaining phototropism-competent, accumulating starch like wild type, and displaying seemingly wild-type amyloplast sedimentation in their statocytes. Hence, MAR1 and MAR2 appear to function in early phases of gravity signal transduction. The MAR loci were cloned and shown to encode proteins that are targeted to the plastid outer envelope, suggesting a more direct involvement of plastid-borne proteins in gravity signal transduction than originally anticipated by the classical starch-statolith hypothesis.

(Supported by NSF: Grant #MCB-0240084).