Despite the fact that plants do not have brains or a nervous system, their roots know in which direction to grow. Several environmental factors influence the direction of growth, such as salt, water, light and gravity. Theoretical biologists Dr. Kirsten ten Tusscher and Thea van den Berg at Utrecht University have studied halotropism, which is the reaction of the plant’s roots to an environment with a salt gradient, together with their fellow botanists from the University of Amsterdam. As a result, they discovered a new aspect of the story.
Kirsten ten Tusscher en Thea van den Berg on their publication in Plant Development
“We have increased the knowledge of the mechanism behind the sense of direction in plant roots”
It is well know that growth is coordinated by the plant hormone auxin. An asymmetrical distribution of this hormone results in asymmetrical growth, causing the root to bend away from the highest concentration of salt. In a special Plant Development issue of the journal Development released on 15 September, the researchers used a detailed model to reveal three important aspects relevant this phenomenon. “Our model indicates that the shape of the point of the root tip plays an important role. Moreover, auxin apparently initiates a feedback mechanism that encourages asymmetrical growth. But another protein appeared to play an important role as well”, according to Van den Berg. An illustration of the model adorns the cover of the special issue.
Salt stress in plants is a popular subject for research among botanists, due to its importance to agriculture. “In 2013, a publication by Christa Testerink (UvA) and other botanists described the subtle way by which plants grow away from salty soil. So I wondered how the salt gradient caused the plant to bend, exactly”, says Ten Tusscher. One side of the plant root transports less auxin, while the other transports more. The models used by Ten Tusscher and Van den Berg showed how this process is regulated, and whether the mechanism alone is sufficient. Their results shed new light on the subject. Ten Tusscher: “We discovered a level of complexity that we had not suspected in advance.”
The model shows that the wedge-shaped taper of the root’s tip is important in ensuring that the decrease of a protein that transports the auxin, called PIN2, at the side exposed to the largest concentration of salt can result in a slight increase in auxin levels on the other side.
The researchers also showed that auxin has a positive feedback on the proteins that transport the auxin, reinforcing the initial asymmetry in auxin levels. Moreover, the level of another protein, called PIN1, increases under the influence of salt. This increase in PIN1 then accelerates the auxin amplification mechanism. “The latter protein proved to be more important than we had thought, because although it has no effect on the long term, in the short term it accelerates the auxin’s positive feedback mechanism”, according to Ten Tusscher.
It was no accident that the two biologists from Utrecht decided to work together with Christa Testerink and Ruud Korver from the University of Amsterdam. Van de Berg: “I knew Christa from my Bachelor’s in Biology in Amsterdam. Thanks to her enthusiastic lectures about salt stress, I gained an affinity with the subject as well. And when I had the opportunity to do my internship with Kirsten’s research group during my Master’s in Molecular and Cellular Life Sciences, I immediately jumped at the chance. In Amsterdam, they conduct experiments on the Arabidopsis plant, and we model the data obtained. On our side, we use our models to predict additional factors that might also play a role, and then they test and confirm these predictions with their experiments. That way, all of the pieces of the puzzle fall into place.”
Communications in plants
Van den Berg has since completed her Master’s studies, and she began her PhD research in September to follow up on her research. Van de Berg: “We eventually want to uncover how plants decide the best direction for them to grow. For example, how do they know which side is less salty, as the salt concentration often only differs by around 5%. Plus, gravity also plays an important role. Somehow, the plant cells can communicate with one another, and we hope to use fundamental research to find out how they do it. Once we understand this communication, we will understand how plants can integrate the information and make a decision.”
Modeling halotropism: a key role for root tip architecture and reflux loop remodeling in redistributing auxin
Thea van den Berg*, Ruud A. Korver, Christa Testerink & Kirsten H. W. J. ten Tusscher*
Development (2016); doi:10.1242/dev.135111
* affiliated with Utrecht University