1. Physiological effects of auxin ¤ cell elongation ¤ phototropism: lateral distribution of auxin ¤ gravitropism: ¤ thigmotropism: less is known, probably involves auxin gradient

2. Evidence that the lateral redistribution of auxin The shaded side is more acidic

3. How do plant cells detect gravity  A gravity sensor: the large, dense amyloplasts  Statoliths; Statocytes – the specialized gravity-sensing cells in the central cylinder of root cap, columella In shoots: starch sheath a layer of cells that surrounds the vascular tissues of the shoot Fig. 19.13a

4. Starch-statolith hypothesis: columella Nodal ER (5~7 rough ER sheets) Starch-deficient mutants: no gravitropic response Starch-less mutants: some gravitropism  Exist some starch-independent gravity perception mechanism

5. Tensegrity model for gravitropism (02) Tensional integrity: structural integrity created by interactive tension between the structural components. Web Topic 19.10 The giant-celled fresh water alga Chara the entire protoplast acts like a statolith, and that gravity is perceived at the interface between plasma membrane and cell wall Plasmalemma Central Control (PCC) model, stretch-activated calcium channels are clustered around attachment centers connecting the cytoskeleton and the cell wall. Gravity sensing: calcium and pH dependent

6. Gravity sensing may involve pH changes A cytosolic pH-sensitive fluorescent dye The cytoplasmic pH , apoplastic pH  10 min delay tropic curvature (7.2  7.6) (5.5  4.5) columella  activate H + -ATPase

8. Gravity sensing – calcium and pH dependent ¤ + EGTA: agravitropism + Ca 2+ agar: bend to the agar side  intracellular [Ca 2+ ]  ¤ thigmotropism

9. Auxin is redistribution laterally in the root cap Bending occur Protection, perception Cap produces root growth inhibitor affect gravitropism