Meristems-4  
311 Syllabus

Control of the Cell Division Plane

Interphase Microtubules have a uniform distribution in the cortical (peripheral) Cytoplasm. Preprophase Band Prior to Prophase, Microtubules & Microfilaments accumulate and form a band around the circumference of the cell. This is called the Preprophase Band. Microtubules depolymerize and reassemble as the Spindle Fibers during Mitosis.

Phragmoplast At the end of Telophase the Spindle Microtubules depolymerize and new Microtubules form at the center of the Metaphase Plate where they guide Golgi Vesicles which fuse to form the Cell Plate. The MTs and Vesicles are called the Phragmoplast. The Microtubules depolymerize and repolymerize centrifugally so that the Cell Plate becomes continuous. The Phragmoplast MTs depolymerize following Cell Plate completion.

Ordered Planes of Cell Divisions NOT required -> Radial Tissue Organization

Mutant Arabidopsis (ton, fas barrel)

Lack precisely oriented cell divisions -> Abnormal Plants

Retain basic tissue patterns Axial & Radial

Mutant Arabidopsis (knolle)

Irregular Division Planes

Interfere with Cell Plate completion.

Binucleate Cells or cells with Large Plasmodesmata

Radial Tissue Pattern disrupted

Lack Epidermis

Lack Functional Apical Meristems

Suggests that Cells need to have physiological Isolation -> normal tissue patterns.

Formative Divisions may be required for Cell Differentiation

Microsporocytes -> Pollen

First Division is Asymmetric -> Large Vegetative (Tube) Cell & Small Generative Cell

Veg. Cell -> Pollen Tube & Gen. Cell -> Sperm

Block Cell Division with Colchicine (Causes Microtubule Depolarization)

Veg. Cell Develops & is Normal

No Generative Cell

Use Dilute Colchicine 

Symmetrical (Equal) Division

2 Veg. Cells

Production of the Gen. Cell requires Asymmetric Division

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Surprise Results

Microtubules depolymerize around 5 C.

French scientists wanted to culture Pollen.

They left their flowers in a refrigerator over the weekend.

When Pollen from these flowers were cultured, they produced Normal Haploid EMBRYOS!

This produced a new domain of Plant Research!!!

It would seem that Microtubules are necessary for the establishment of Polarity & normal Pollen development!!!!!

It would also seem that great discoveries can sometimes be happy accidents!!