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 (yielding)
Turgor Pressure -> Stress on Cell
Wall
Loosening of
Microfibrils ->
Wall Expansion ->
Reduces Stress
Control of Enlargement
Targeted Cellulose Secretion
Localized Acid Secretion
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 Microfibrils -> Wall Expansion
There are H+
ATPases present in Cell Wall
When they are Activated ->
H+ Ions -> Lower pH
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!
Catalyze pH dependent wall extension
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 
ATPases present in Cell Wall 