FIGURE 1.

Root tissue sensing and response to gravistimulation by reorientation. (A) The statoliths of gravity-perceiving columella cells settle to the bottom of the cells relative to gravity, which results in a symmetrical distribution of auxin through all sides of the root cap and equal growth in the root elongation zone (apical of the root cap, not pictured). (B) Upon gravistimulation by reorientation, the statoliths settle at a new position toward the gravity vector. The new position is perceived by the cell, whereby a cascade of signals leads to an unequal distribution of auxin to the side of the root nearest the new gravity vector. This unequal distribution of auxin reduces cell growth where concentrations are high, resulting in growth-mediated bending of the root in the direction of the new gravity vector.