Plant Cell Walls - 20
311 Syllabus

Asymmetric Cell Division leads to Radically Different Developmental Events for many Taxa

The establishment of  Polarity, and the consequent developmental events seen with Fucus are seen in various cases with Flowering Plants.

The first division by Zygotes of most plant taxa produces two cells which follow radically different developmental paths.

In some cases the first divisions are unequal in size while others produce cells of equal size. In the latter case, the cells are unequal at the molecular level.

One reason why Fucus has been studied so extensively is because the production of gametes can be controlled such that large numbers can be used and experiments are easy to repeat.

It is highly probable that events like those documented with Fucus occur in other cases that are more difficult to investigate.

The Angiosperm Egg is Polar. There is a Large Vacuole near the base of the Egg Cell & most of the Plastids and Cytoplasm are located at the opposite end.

Following Fertilization an Asymmetric Division occurs.

This results in a Small, Densely Cytoplasmic cell which contains many Plastids & a Large Vacuolate Cell.

The Small Cell produces the Embryo while the Large Cell produces the Suspensor.

The Suspensor functions during embryo growth but contributes little to Embryo and the resulting Plant.

The initial Asymmetric Division produces two cells which have radically different fates.

This is very similar to what we saw with Fucus.

The Growth of Pollen Tubes has many of the features seen with Fucus Zygotes.

An Asymmetric Cell Division occurs in Pollen Development.

Pollen Develops in the Anthers of the Stamens
Following Meiosis the Uninucleate Haploid Microsporocytes Enter Mitosis
The First Division is Unequal and results in a Small Cell & a Large Cell.
The Large Cell becomes the Tube Cell and produces the Pollen Tube. The Small Cell is called the Generative Cell & it undergoes mitosis &  produces two Sperm.

There are significant pH Gradients in the growing Tip of Pollen Tubes. This is similar to Fucus.

Actin Microfilaments in the Pollen Tube have a Parallel Orientation with the Axis of Elongation.

They transport Golgi Vesicles towards the growing Tip.

Microtubules may also play a role in directional movements of Golgi Vesicles

Germinating Pollen Tube stained for Microtubules.

The Vesicles fuse with the Plasmalemma and provide new wall material for the growing point.

Formative vs Proliferative Divisions

Formative Divisions send the Daughter Cells along Different Developmental Paths. Remember the Big Cell and the Small Cell. Asymmetric Divisions are NOT the only kind that are Formative.

Proliferative Divisions increase the Number of Cells in One Path.