A team of scientists from the John Innes Centre (JIC) in the United Kingdom
and Sapienza University in Rome, Italy, combined mathematical and computer
modelling with molecular genetics to show how roots regulate their growth
through the interactions of two antagonistic hormones, auxin and cytokinin.
As roots grow and meristem cells at the tip continuously divide, they are
left behind in relation to the moving root tip. When these cells reach a
certain distance from the tip, called the transition position, they stop
dividing and instead, start elongating until reaching their maximum lengths.
According to Dr. Veronica Grieneisen, cells "know" that they have reached
the transition position because of positional information.
The teams of Dr. Grieneisen and Dr. Stan Marée at JIC have shown that the
hormone auxin was present at very high levels at the root tip to maintain
certain cells as stem cells, as a result of fast dynamics of auxin swirling
around. These currents of auxin allowed the auxin maximum and its associated
gradient to move together with the growing root. Further work showed that
auxin does not regulate the transition alone, but antagonistic cross-talk
between auxin and another hormone, cytokinin, stabilize the size of the
meristem zone, and even change it, either stabilizing root growth, or
changing its velocity. They found that cytokinin's influence generated a
very typical pattern of auxin concentrations in the root.