Abstract
In higher plants, some organs such as roots, hypocotyls, and stems, can sense the direction of gravity to regulate their orientation. Gravitropic response is composed of four steps; 1. gravity sensing and conversion of physical stimuli to biochemical signals, 2. intracellular signal transduction in gravity sensing cells, 3. signal transmitting to responding tissues, 4. differential growth of organs. Here we focus on the former two steps. Recent studies using modern technique have gradually unveiled early events and mechanism of gravitropic response. Genetic approach provided evidences that strongly support the classical theory for gravity sensing (step 1). Computational analysis suggested the existence of another gravity sensing mechanism in roots. Spatial and temporal ion imaging in living organs in real time provided information on step2. In addition, reverse genetic approach suggested asymmetrical intracellular distribution of auxin transpoter is a possible link between step 2 and 3. However, molecular basis of the signaling mechanism remains unknown. We believe extensive molecular genetic approach combined with recent techniques cited here shed the light to this ambiguous area of research.
