[9]

   Plant gravitropism is a complex process involving different signaling pathways. We have shown previously that inositol 1,4,5-trisphosphate (InsP₃), a second messenger generated in the phosphoinositide (PI) pathway, increases in response to gravity in the pulvinus of inflorescence stems of cereal grass stems and in inflorescence stems of Arabidopsis.  Furthermore, transgenic Arabidopsis plants expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5- ptase), an enzyme which hydrolyzes InsP₃, showed a delayed gravitropic response compared to wild type plants (Perera et al., 2006 Plant Physiol. 140:746).  This work is focused on characterizing the role of InsP₃ and its connection to other events in the gravity signaling cascade such as amyloplast sedimentation and auxin redistribution.  To study the interaction between amyloplast signaling and the PI pathway we will use the starchless (pgm) mutants.  The pgm mutants have reduced root and hypocotyl gravitropism; however, the inflorescence response has not been well characterized (Caspar and Pickard 1989, Plant 177:185).  To compare pgm mutants to the wild type plants we are monitoring inflorescence bending using time lapse photography and InsP₃ levels will be measured during the gravitropic response.  In addition, we are generating pgm/ InsP 5-ptase double mutants to determine if the effects are additive.  In order to look at the dynamics of auxin redistribution we have generated transgenic plants expressing InsP 5-ptase and the auxin responsive promoter-reporter fusion; pIAA2-GUS.  Inflorescence bending and the appearance of asymmetric GUS staining in the inflorescence stem will be compared in InsP 5-ptase plants and plants expressing only the reporter gene. 

(Supported by Undergraduate Research Fellowship funded by the NC Space Grant)