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 gene transcription. Now, scientists led by Jiří 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. Jiří 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.

For more details, read the news release from IST Austria.

Figure: A growing root observed by the research team. Colors are caused by a fluorescent auxin reporter in the nuclei of the cells. Red indicates high amounts of auxin, green indicates low amounts. Credit: Matouš Glanc