Plants come in all shapes and sizes, from grand Redwood trees to the common
Snowdrop. Although we cannot see them, under the ground plants rely on a
complex network of roots. What determines the pattern of root growth has
been a mystery, but a new paper published this week in PLoS Biology shows
that the shape of the existing root can determine how further roots branch
from it ? because shape determines hormone concentration. The work also
suggests that the root-patterning system shares a deep evolutionary
relationship to the patterning system of plant shoots, something that had
not been realized previously.

The paper, by Laskowski, Grieneisen, Hofhuis, et al, explores the
architecture of the root system of the model organism Arabidopsis thaliana,
a plant with the unusual common name "mouse-ear cress." The authors show
that the curve of the root is key in provoking new growth. They used
computational modeling of the transport of a well-known plant hormone,
auxin, and by following the diffusion of this hormone they reveal that its
accumulation leads to the specification of new growth regions in the root
structure.

In particular, the initial trigger of this accumulation is a difference in
cell size between the inner and outer sides of a root curve, which is then
amplified by feedback responses from the hormone transport system.

Surprisingly, this new model on root architecture is reminiscent of the way
leaves develop around growing tips in plant ? a key feature of shoot
architecture. This is exciting because it suggests that a deep connection
exists between both root and shoot architectures ? which have hitherto been
viewed upon as being entirely separate. The work further shows that a new
kind of biology, involving complete mixing of experiments and computer
modeling, is a very powerful tool in probing organismal architecture.
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