Practice question

Root gravitropic perception is highly localized to central columella cells within root cap containing dense starch-filled amyloplasts functioning as statoliths sedimenting according to gravity vector. Upon reorientation, statolith sedimentation triggers relocalization of LAZY/NGR proteins to lower membrane, D6PK-dependent polarization of PIN3 and PIN7 efflux carriers toward lower side, elevated auxin accumulation in lower elongation zone flank inhibiting cell elongation causing downward curvature. Laser ablation or starchless pgm mutants lacking amyloplasts abolish response, confirming cap as exclusive site for gravity perception initiating signal transduction toward differential growth.