Breakthrough in understanding plant growth and development

Together with colleagues from Wageningen and Helsinki, researchers from the Theoretical Biology & Bioinformatics group at Utrecht University have discovered how plants can quickly react to stimuli, such as gravity or light, while maintaining their structure. This is an important fundamental discovery that could be useful for agriculture, according to the primary author from Utrecht, Dr. Kirsten ten Tusscher. The results of their research have recently been published in Nature.

In order to meet the world’s increasing demand for food, the agriculture branch is constantly looking for ways to develop crops that produce more and are better adapted to diseases and climate change. According to the researchers, a proper understanding of plant growth and development are essential in order to achieve real breakthroughs in this field. Thanks to a combination of experiments and computer simulations, scientists now finally understand how the hormone auxin works with regulatory proteins to organise the plant’s quick reactions while maintaining its structure. 

Auxin: Jack of all trades

Plants grow through processes such as cell division, cell elongation and the differentiation of cells and tissues. These processes also give the plant its structure. But plants can also react quickly to stimuli: the shoots bend towards the light, while roots follow gravity.
The plant hormone auxin is a true Jack of all trades: it influences the development of stem cells into differentiated tissues, but it also plays a role in the plant’s quick reactions to changes in the environment. The question that the researchers wanted to answer was how a single signal substance was able to control these very different processes. 

Interplay with regulatory proteins

Plant growth and development appears to be the result of an interplay between auxin and four regulatory proteins (transcription factors) that are known by the collective name of PLETHORA (PLT). These proteins switch genes on or off.
In this interplay, auxin directly controls the plant’s rapid reactions. If the direction of gravity changes, then a root will bend to point the other direction within just a few minutes.
The regulatory proteins influence the location and size of the zones in which the processes of cell division, cell elongation and differentiation take place. This occurs at a much slower tempo.

Computer simulations essential

The distribution of the zones depends on the gradient in the concentration of the PLTs. “We once thought that the gradient was a direct result of the gradient of auxin. But that would not explain the stable structure of the various zones under rapidly changing conditions. Now it seems that the entire process is full of feedbacks and dependent factors. The growth influences the gradient, which in turn influences subsequent growth. We had already charted out parts of this process, but now we understand the complete picture”, according to co-author Prof. Ben Scheres, former professor at Utrecht University, now affiliated with Wageningen University.

Computer simulations by the biologists from Utrecht played an important part in deciphering this process. “Computer models allow you to separate the processes that are actually closely connected in real plants in order to study the role that feedback plays in these processes”, explains the fellow author Dr. Kirsten ten Tusscher. “Our models have been essential in showing that the old ideas could not be correct and in illustrating the separation between the slow and fast processes.”

Future Food Utrecht

The insight into this complete picture may contribute to improvements in crop farming and amelioration. “The roots have only a small zone in which they can grow lateral roots. If we know the switches for that zone assignment, then we may be able to change the architecture. This may produce plants with better and more productive root systems”, explains Prof. Ben Scheres.
Researchers at Utrecht University collaborate in the Future Food Utrecht project to help ensure a sustainable food supply for the future. 

Publication

PLETHORA gradient formation mechanism separates auxin responses
Ari Pekka Mähönen, Kirsten ten Tusscher, Riccardo Siligato, Ondřej Smetana, Sara Díaz-Triviño, Jarkko Salojärvi, Guy Wachsman, Kalika Prasad, Renze Heidstra, Ben Scheres

Nature, doi:10.1038/nature13663