Physiological effects of auxin

¤ cell elongation

¤ phototropism:

lateral distribution of auxin

¤ gravitropism:

¤ thigmotropism:

less is known, probably involves auxin gradient

Evidence that the lateral redistribution of auxin

The shaded side is more acidic

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

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

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

Gravity sensing may involve pH changes

A cytosolic pH-sensitive fluorescent dye

The cytoplasmic pH , apoplastic pH 10 min delay tropic curvature

(7.27.6) (5.54.5)

columella

activate H+-ATPase

Gravity sensing – calcium and pH dependent

¤ + EGTA: agravitropism

+ Ca2+ agar: bend to the agar side

intracellular [Ca2+]

¤ thigmotropism

Auxin is redistribution laterally in the root cap

Bending occur

Protection, perception

Cap produces root growth inhibitor affect gravitropism