Indole-3-acetic acid is a plant hormone that is classified as an auxin. Indole-3-acetic acid is the most active auxin. Auxins are synthesized in apical buds (at the tip) of growing shoots. They stimulate growth of the main shoot and inhibit lateral shoot development. A class of auxin-binding membrane proteins may represent auxin receptors. Auxin action involves pumping protons out of the cell, possibly in conjunction with a membrane ATPase.