Plant Cell Walls - 19
311 Syllabus

Polarity in Tip Growth

Pollen Grains & Zygotes -> Apolar

Polarity required for Directional Growth

External Factors -> Light, Temp. pH   ->  Induce Polarity

Fucus (Phaeophyta- Brown Algae) Zygotes -> Extensively Studied

Zygotes = Apolar

Directional Light -> Polarity

Polarity Fixed after 14 hrs.

Rhizoid grows from dark side

Polar Axis Established  = Thallus vs Rhizoid

Unequal Cell Division -> Small Cell -> Rhizoid -> Holdfast -> Attach to Substrate

Large Cell -> Thallus ->   Photosynthetic Cells

Early Development of Fucus

Fluorescent Dye (FL-DHP) -> Blocks Ca++ Channels in Plasmalemma

First few hrs. -> Redistribution of Calcium Channels -> Dark Side of Zygote

Change light Direction -> Ca Channels Move -> Dark Side (Rhizoid Pole)

Positive Current (Ca++) flows into the Rhizoid Pole -> Out the Opposite Pole

Produces a Transcellular Current in Root Hairs & Pollen Tubes

Causes Redistribution of Membrane Proteins & Ionic composition of the Cytosol

Not Observed in Diffuse Growth

Calcium indicator dye (fura-2)

Fucus Rhizoids-> Ca++ gradient

Tip = 450 nM vs Base = 100 nM

Polarized assembly of Microfilaments

Polar migration and discharge of Golgi Vesicles

(Ca Ion Gradients are also present in Root Hairs and Pollen Tubes)

Actin Cytoskelleton becomes Polarized

Block Protein Synthesis NOT block establishment of Polarity

Therefore Protein synthesis is not required

Redistribution of Existing Components -> Polarity

Actin Filaments -> Localized at Rhizoid Pole

Extend from Nucleus -> Ca++ Channels in Plasmalemma

Fixation of Polarity BLOCKED by Cytochalazin B

Destroys Actin Microfilaments

Disrupts Ion Current

Actin Filaments Involved in movement of Secretory Vesicles -> Plasmalemma

Localized Secretion of Golgi Vesicles is Polar

Organelles -> Redistributed during establishment of Polarity

Golgi Vesicles -> Rhizoidal Pole

Vesicles = Redistributed in response to Light Reorientation

Vesicles Fuse with Plasmalemma -> Rhizoidal Growth

Ca++ Gradients involved in Vesicle Discharge

Cell wall Required for Fixation of polarity

Protoplasts -> Polarity Established but NOT Fixed

Chain of Events for Establishment of Polarity

Reorientation of  Ion Channels

Ca Gradient

Transcellular Current

Reorientation of Microfilaments

Redirect Vesicle Movements

Ca++  levels near Tip may cause Vesicle Fusion with PM

Cell Wall Deposition fixes Polarity

Asymmetric Cell Division

Rhizoid & Thallus Cells