VIB researchers at Ghent University have discovered the substance that
governs the formation of root offshoots in plants, and how it works.
Root offshoots are vitally important for plants ? and for farmers. Plants
draw the necessary nutrients from the soil through their roots. Because they
do this best with a well-branched root system, plants must form offshoots of
their roots at the right moment. The VIB researchers describe how this
process is controlled in the prominent professional journal Science. A key
player in this process is a protein called ACR4. Depending on the signals
that it receives from its environment, this protein triggers the formation
of a root offshoot. Now that we know the control mechanism, we can begin to
stimulate plant roots to form more, or fewer, offshoots. This can lead to a
more ecological agriculture and to the production of better crops at the
same time.

An efficient network

It is difficult to overstate the importance of plants in our lives - they
are responsible for our oxygen and for food, clothing, energy, and countless
other things. And in turn, the importance of a plant's roots is
unquestionable: they provide the plant with necessary nutrients and
moisture. The more the roots are subdivided, in breadth and depth, the
better they can do their work. So, a well-coordinated, controlled formation
of root offshoots is crucial to a plant. But, until now, how a plant
determines when and where an offshoot should be formed was unknown.

Asymmetric cell division

The presence of stem cells is very important in the development of plants
and animals. Stem cells are cells that can transform themselves into various
types of cells. In animals, tissues and organs are formed before birth; but
in fully-grown plants, stem cells continue to play a major role in the
formation of new organs or tissues, such as root offshoots.

These stem cells are found inside the root, and several of them will induce
the formation of an offshoot. These 'root-founder' cells undergo an
asymmetric cell division. In contrast to the usual cell division, which
gives rise to two identical cells, asymmetric cell division produces two
different cells: a stem cell that is identical to the original cell, and a
cell that is ready to become a specialized cell ? in this case, a secondary
root cell.

The decisive signal

With the aid of the mouse-ear cress (Arabidopsis thaliana), a frequently
used model plant, Ive De Smet and Valya Vassileva in Tom Beeckman's group
have been studying how a plant determines which cells will trigger
offshoots. To do this, the VIB researchers in Ghent have employed a special
technology that makes it possible to make synchronous offshoots develop at
different moments. This allowed them to isolate the cells that induce the
formation of offshoots. They found out which genes are active in these cells
and compared them with the genes that are crucial to normal cell division.
In this way, the researchers identified a specific set of genes that control
asymmetric cell division and send the signal for the formation of offshoots.

ACR4: control over asymmetric division

The researchers then examined one of these genes more closely. The ACR4 gene
contains the DNA code for a receptor, a protein that is often located on the
exterior of a cell to pick up signals from the outside and transmit them to
the controlling mechanisms within the cell. When the researchers disrupted
the function of ACR4 in plant cells, the precisely orchestrated asymmetric
cell division was also disturbed. From this finding, De Smet and Vassileva
inferred that ACR4 plays a key role in the creation of offshoots. Because
the protein has a receptor function, triggering the formation of offshoots
depends on its reaction to signals from the environment.

Desired or undesired

With this research, the scientists have discovered a fundamental mechanism -
fundamental for the plant, and very important for plant-breeders as well.
This new knowledge enables us to promote, or retard, the formation of
offshoots - both activities are useful in a large number of applications.

Promoting an extensive root system helps plants absorb nutrients more
readily, and thus they need less fertilizer. Such plants can also grow more
easily in dry or infertile soils. Furthermore, plants with a well-developed
root system are more firmly anchored in the soil and can be used to
counteract erosion.

On the other hand, slowing down secondary root formation can be advantageous
in tuberous plants, like potatoes or sugar beets. This enables these food
crops to invest all their energy in the production of nutrients. Fewer root
offshoots also makes it easier for farmers to harvest these crops.

Plant research with medical possibilities?

This plant research sheds light on the control of asymmetric cell division -
and this kind of cell division is similar to the cell division of stem cells
in animals, too. So, these results can also provide greater insight into how
animal stem cells specialize.

For example, irregular cell division plays a role in the development of
various types of cancer, and similar control mechanisms might underlie this
process as well. This is clearly an important area for future research.
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