Control of Cell Elongation
Typical Enlargement = 10 - 100 X
- Maximal = 10k X
Cell Wall
must be Loosened -> Expansion
Stress
Relaxation &
Acid Growth Hypothesis
Cell Wall is very Hydrated
-> Intermediate Properties Solid vs Liquid
Growing Cells = Less Rigid
Growth = Long-term irreversible
stretching
Turgor Pressure -> Stress on Cell Wall
Loosening of Cellulose
Fibrils -> Reduces Stress
-> Wall Expansion
Control of Enlargement
Targeted Cellulose
Secretion
Localized Acid
Secretion
Remember
that Hydrogen Bonds stabilize Wall Components
Changes in pH -> Changes in
Hydrogen Ion Concentration
Reduced pH
-> More Free H+ Ions which have a + Charge
These disrupt
existing H-Bonds -> Loosening of Hemicelluloses ->
Slipping of Cellulose Fibrils ->
Wall Expansion
There are H+ ATPases
present in Cell Wall
When they are Activated -> they secrete
H+ Ions -> Lower pH
This is Common to all
Land Plants!!!
Observed in Isolated Cell Walls (Lasts
for Hours)
Not Found in
Mature (Nongrowing) Walls which
have more stable covalent linkages.
Acid-Induced Growth
Mediated by Expansins
Heat - Inactivated Walls -> Restored
Extension when supplied with Proteins from Growing Walls. This indicates that
Proteins have a role in this process.
These Proteins = Expansins!
They catalyze pH dependent wall extension.
They loosen H-Bonds at
the Interface of Cellulose and Hemicellulose.
Glucanases & Other Hydrolytic Enzymes
Matrix Glucans -> Increased
hydrolysis during growth
Apply Auxin -> Increased Enzyme
activity -> Increased growth
Interfere with Antibodies to Enzyme
-> Less Growth
Add Glucanases to cultured cells
-> Increased Growth
Glucanases Modulate
Expansin-mediated Growth
Plant Cell Walls - 18 
