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Gravity controls directional growth of plants, and the classical starch-statolith hypothesis proposed more than a century ago postulates that amyloplast sedimentation in specialized cells initiates gravity sensing, but the molecular mechanism remains uncharacterized. The LAZY proteins are known as key regulators of gravitropism, and lazy mutants show striking gravitropic defects. Here, we report that gravistimulation by reorientation triggers mitogen-activated protein kinase (MAPK) signaling-mediated phosphorylation of Arabidopsis LAZY proteins basally polarized in root columella cells. Phosphorylation of LAZY increases its interaction with several translocons at the outer envelope membrane of chloroplasts (TOC) proteins on the surface of amyloplasts, facilitating enrichment of LAZY proteins on amyloplasts. Amyloplast sedimentation subsequently guides LAZY to relocate to the new lower side of the plasma membrane in columella cells, where LAZY induces asymmetrical auxin distribution and root differential growth. Together, this study provides a molecular interpretation for the starch-statolith hypothesis: the organelle-movement-triggered molecular polarity formation. [Display omitted] • Gravistimulation triggers MAPK-mediated phosphorylation of LAZY proteins • Phosphorylation increases LAZY's interaction with TOCs on the surface of amyloplasts • Amyloplast sedimentation carries LAZY proteins to the new bottom of columella cells • LAZY translocates from amyloplasts to adjacent plasma membrane to form new polarity Amyloplast sedimentation guides LAZY to relocate to the new lower side of the plasma membrane in columella cells to induce gravitropic growth in plants. [ABSTRACT FROM AUTHOR]