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.
Each Pollen Grain develops from a
cell called a haploid Microsporocyte.
Its 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.
Actin Microfilaments in the
Pollen Tube have a parallel orientation with the axis of elongation.
They transport Golgi Vesicles
towards the growing Tip.
The Vesicles fuse with the
Plasmalemma and provide new wall material for the growing point.
Calcium plays an important role
in this system as well.
Plant Cell Walls - 20
