IAA causes phototropism response in shoot tips

1.Cells of the shoot tip make and release IAA, which diffuses down the shoot through cells. 2. Light causes IAA to move (active transport) from the lit side to the shaded side of the shoot, so a greater concentration of IAA builds up in the cells on the shaded side. 3. IAA causes shoot cells to elongate so the cells on the shaded side lengthen more than cells on the lit side 4. Due to this, the shaded side of the shoot grows faster, causing the shoot to bend TOWARDS the light source