Plant Cell Walls - 19
311 Syllabus

Polarity in Tip Growth

The term Polarity simply means that there are Regional Differences. This can be applied to a single Cell, an Organ or an Organism.

Mature plants are bipolar because they have a root and a shoot at opposite ends. This can be traced back to the embryo, the Zygote and the Egg.

Polarity is required for Directional Growth.

Polarity may preexist as you will see with the egg of some angiosperms.

External Factors like Light, Temp,  pH can  Induce Polarity

Fucus (Phaeophyta- Brown Algae) Zygotes have been Extensively Studied

The Zygotes are Apolar.

Directional Light can induce Polarity.

Polarity is Fixed after 14 hrs.

Rhizoid grows from dark side.

Photosynthetic Thallus grows from the opposite side.

A Polar Axis (Thallus vs Rhizoid) is thus Established.

This leads to the First Division which is Unequal

It produces a Small Cell which becomes a Rhizoid. This becomes a multicellular Holdfast that attaches the organism its Substrate.

The Large Cell produces the Photosynthetic Thallus.

Early Development of Fucus

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

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

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

Calcium influx and efflux at opposite sides produces a Transcellular Current. This also occurs in Root Hairs & Pollen Tubes.

This causes a Redistribution of Membrane Proteins & ions the Cytoplasm.

This makes the Zygote Polar at the Molecular Level.

This is 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 occurs.

Polar migration and discharge of Golgi Vesicles occurs.

Ca++ gradients are also present in Root Hairs and Pollen Tubes.

If you block Protein Synthesis, Polarity is still established.

Therefore, Protein synthesis is not required.

Fucus

The Redistribution of Existing Components is sufficient for the establishment of Polarity.

Actin Filaments are Localized at Rhizoid Pole.

They extend from Nucleus to the Ca++ Channels in Plasmalemma.

Fixation of Polarity BLOCKED by Cytochalazin B.

This Destroys Actin Microfilaments & Disrupts the Ion Current.

Actin Microfilaments are involved in movement of Secretory Vesicles to the Plasmalemma.

These Vesicles secrete Cell Wall materials at the growing Tip.

The cell can not grow without these vesicles.

The Localized Secretion of Golgi Vesicles is Polar.

Organelles are Redistributed during the establishment of Polarity.

Golgi Vesicles move towards the Rhizoidal Pole.

Vesicles are Redistributed in response to Light Reorientation.

Vesicles Fuse with Plasmalemma -> Rhizoidal Growth.

Ca++ Gradients are involved in Vesicle Discharge.

Calcium helps to stabilize Cell Wall components. This would be important for newly formed walls.

Chain of Events for Establishment of Polarity

Reorientation of  Ion Channels

Ca++ Gradient

Transcellular Current

Reorientation of Microfilaments

Redirection of  Vesicle Movements

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

Cell Wall Deposition fixes Polarity

Asymmetric Cell Division

Rhizoid & Thallus Cells