REST GRAVITROPISM

When a plant root is reoriented within the gravity field, it responds by initiating a curvature which eventually results in vertical growth. Gravity sensing occurs primarily in the root tip. It may involve amyl oplast sedimentation in the columella cells of the root cap, or the de tection of forces exerted by the mass of the protoplast on opposite si des of its cell wall. Gravisensing activates a signal transduction cas cade which results in the asymmetric redistribution of auxin and apopl astic Ca2+ across the root tip, with accumulation at the bottom side. The resulting lateral asymmetry in Ca2+ and auxin concentration is pro bably transmitted to the elongation zone where differential cellular e longation occurs until the tip resumes vertical growth. The Cholodny-W ent theory proposes that gravity-induced auxin redistribution across a gravistimulated plant organ is responsible for the gravitropic respon se. However, recent data indicate that the gravity-induced reorientati on is more complex, involving both auxin gradient-dependent and auxin gradient-independent events.