REGULATION OF TRANSCRIPTION IN ROOTS OF ARABIDOPSIS GRAVITY MUTANTS

Gravity is a constant force on Earth used by plants to guide their growth. However, the molecular mechanisms of sensing, transducing and responding to gravity in plants are poorly understood. Changes in the orientation towards the vector of gravity are perceived in plant roots by sedimentation of starch-containing amyloplasts (statolith) in the columella cells. Biochemical signal transduction elements involved in the gravitropic response are Inositol-1,4,5,- triphosphate (InsP3), Ca2+, and proton-transport leading to alkalization of the cytosol. Phytohormones (auxin, cytokinin, ethylene, brassinosteroids), reactive oxygen species, and nitric oxide have also been shown to mediate the gravity response.. How and when all these signal transduction changes occur and interact is unknown. Within one minute of reorientation, changes in gravity specific regulation of transcript abundance have been described (Kimbrough et al. 2004). In order to better understand how these different signals act and interact to regulate transcription, we investigated gravity induced transcript abundance changes in mutant plants defective in known signal transduction elements. Arabidopsis mutants defective in starch accumulation (pgm1), auxin redistribution (pin3), brassinosteroid receptor (bri1), along with a wild type control, were grown for seven days in the dark, reoriented (135?) and time-dependent changes in transcript abundance measured for the fast and transient gravity specific genes: cysteine protease (At4g11310), SAMtransferase (At5g38020), pentacyclic triterpene synthase (At5g48010), major latex protein (At4g23670), and expressed protein (At2g16005). The use of Arabidopsis mutants defective in specific sensing and signal transduction elements of root gravitropism will allow us to delineate the key factors involved in the regulation of the fastest transcriptional responses after reorientation.