Auxin is a key plant hormone controlling a number of processes ranging from
shaping the embryo in the seed to branching of the growing plant. It is
previously believed that the main signaling mechanism of auxin operated in
the cell nucleus and acted only by regulating
[www.isaaa.org] gene
transcription. Now, scientists led by Ji?í Friml at the Institute of Science
and Technology Austria (IST Austria) showed that another mechanism exists
which enables rapid adaption of root growth direction.

When seeds germinate, their roots bend to grow deeper into the soil, and in
order to do this, cell growth continues on one side of the root while being
inhibited on the other. This inhibition was known to be triggered by auxin
and to happen very quickly, but the exact reaction times were difficult to
measure. Using an innovative setup, the researchers could now measure the
time roots need to react to changes in the auxin concentration precisely.
They concluded that the extremely rapid adaption of growth rate was far too
fast to be explained by the gene transcription mechanism, and therefore must
involve a correspondingly rapid perception mechanism.

According to the researchers, the new mechanism is not entirely unknown.
Components of the well-studied pathway, the TIR1 receptor, are needed for
the newly discovered mechanism. Ji?í Friml explains that they were able to
prove that the signaling is non-transcriptional, and they have seen that
components of the original transcriptional pathway are needed. "This means
that we are not looking at an entirely new pathway, but at a new branch of
the canonical pathway," he adds.



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