By Christopher Ortler, ACCESS! (Translation)

The development of a multicellular organism from of a single cell is one of
the classical and fascinating questions that have occupied scientists and
philosophers since the beginnings of science.
In the current issue of the research journal Developmental Cell, researchers
from the group of Professor Thomass Laux at the University of Freiburg have
published the discovery of genes that direct the earliest differentiation
steps in the life of a plant.

The first step on the way to being a multicellular organism is the dividing
of the zygote which results from the fusion of male and female germ cells.
In the higher plants, the upper daughter cell then becomes the embryo and
the lower daughter cell develops into the suspensor, a kind of umbilical
cord for plants. But why do the two daughter cells develop differently at
all?

Now, the research group under Professor Thoms Laux has been able to show
that in the model plant Arabidopsis each of the two daughter cells expresses
one of two related homeobox genes through which the development of each
daughter cell is directed.

The genes have been named WUSCHEL Homeobox genes, or WOX genes, a reference
to their similarity to the stem cell-regulating gene, WUSCHEL. WOX genes
code for a group of transcription factors that have a common DNA binding
domain.

In Arabidopsis, the two WOX genes responsible for directing development of
the two daughter cells are both expressed in the undivided zygote. Only
after the first cell division occurs is one or the other of the two genes
expressed in the two daughter cells. In spite of the apparent separation
between the two gene expression products, the factors somehow remain in
contact between the two daughter cells.

If expression of one of the factors fails or is prevented, then expression
of the other factor in the other daughter cell is also halted. The group of
Professor Laux believes that the development of the cells is coordinated via
this communication.

During embryo development and compartmentalization, further WOX genes become
expressed. The result is that each compartment of the developing embryo
exhibits a characteristic mix of WOX factors.

These cascades of WOX factors with their various functions are surprisingly
similar in the early developmental steps of plants and animals despite these
having diverged evolutionarily before the advent of multi-cellular
organisms.

Early decisions regarding cell differentiation in fruit flies and mammals
are also regulated by special families of homeobox-containing genes.
Interestingly, it has recently been found that the WOX genes that regulate
the early cell differentiation steps in Arabidopsis are closely related to
the genes which regulate stem cells at the apical meristem.

This finding renews interest in the theory that dominated the middle of the
last century, namely that stem cells are actually undifferentiated cells
?left over? from the embryo.

The economic value of plants is not to be disregarded. Plants provide us
with food, they produce the oxygen that we breath, wood, healing substances
and much more. For that reason, the processes that regulate embryonal
development in plants are not only interesting for plant developmental
biologists, but also for those researching practical applications.

Some of the genes newly identified at the University of Freiburg have
already been shown to be capable of transforming a normal plant cell into an
embryo. This opens up a new possibility.

Economically important plants can be propagated without having to go through
the process of sexual reproduction. This has been one of the biggest
problems for plant breeders because advantageous qualities are often lost
due to the genetic mixing that occurs with sexual reproduction.

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