A tilted seedling redistributes indole acetic acid to its lower side. What happens in the root and in the shoot?

1. Settling amyloplasts contact the endoplasmic reticulum and release calcium ions, which drives polar transport of the hormone to the bottom of the cell. 2. In roots a high concentration of that hormone inhibits cell elongation, so growth slows on the lower side while the upper side develops normally. 3. The slowed lower side makes the root curve downward, which is positive gravitropism. 4. The hormone has the opposite effect in shoots, where a higher concentration on the lower side stimulates cell expansion. 5. That expansion lifts the shoot upward, which is negative gravitropism. 6. Any option that gives both organs the same response cannot explain why roots go down while shoots go up from one signal. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.6 Plant Sensory Systems and Responses_