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PHOSPHOINOSITIDE SIGNALING AND PLANT GRAVITROPISM.  I.Y. Perera, C.Y. Hung, B. Stapperfenne and W. F. Boss, Dept of Botany, North Carolina State University, Raleigh, NC

   The signaling processes linking gravity sensing to the initiation of a differential growth response in plants are not well understood. Using the pulvinus of cereal grasses as a model system, we have shown previously that both rapid and long term increases in inositol 1,4,5-trisphosphate (InsP₃) correlate positively with the gravitropic bending response of both oat and maize stems (Perera et al., 1999, Proc Nat Acad Sci 96:5838-5843; Perera et al., 2001, Plant Physiol 125:1499-1507). As an approach towards further delineating the role of InsP₃ in plant gravitropism, we have generated Arabidopsis plants constitutively expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase), an enzyme that specifically hydrolyzes InsP₃. The transgenic plants have no obvious phenotype under normal growth conditions. Strong expression of the InsP 5-ptase transgene is detectable in all tissues of the transgenic plants and basal InsP₃ levels are greatly reduced compared to wild type Arabidopsis. Significantly, the transgenic seedlings exhibit a reduced gravitropic response compared to wild type seedlings. Our results are consistent with the involvement of  InsP₃ signaling in the gravitropic response of plants.   This work is funded by NASA (# NAG 2-1502 to IYP and WFB).

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