Arabidopsis roots stained and visualized under a confocal microscope. The root cells contour is shown in pink. In yellow, the stem cells from the quiescent centre. In the left, a wild-type root, and in the right, a root with mutated BRAVO and WOX5 genes. These mutation produces the division of the cells of the quiescent centre, which are more abundant in the right image. (Credit: CRAG).
A multidisciplinary research team led by the CSIC biologist at CRAG Ana I. Caño Delgado and the physicist from the University of Barcelona Marta Ibañes, found that two plant stem cell proteins, known for their role in the proper development of the root, interact physically and regulate each other to avoid cellular division. The study, which results from fifteen years of research conducted by both researchers, reveals these two proteins, called BRAVO and WOX5, act in a certain manner on a small group of stem cells, and their interaction is key for the survival of the plant regarding genomic and environmental stress factors, such as extreme heat or cold, or floods. A recently published article in the journal Molecular Systems Biology gathers these results, obtained with the model plant Arabidopsis thaliana.
The findings would have been possible without joining the knowledge and academic disciplines of the teams of both scientists: on the one hand, biochemistry, genetics, and cellular biology, and on the other, mathematical modelling.
"Previous studies by our team and others from other teams had shown that the loss of one of these proteins (BRAVO or WOX) generates the division of the root's stem cells. However, the molecular relation was not understood yet", notes Ana I. Caño Delgado.
"In general, genic regulations involve a complexity that is not very intuitive most of the times, and which is only understood by means of mathematical models and computing simulations. The mathematical models we created could provide a sense to the great amount of gathered data by the CRAG team", notes Marta Ibañes.
These mathematical models will allow researchers to experiment virtually, creating hypothetical situations that could occur in the root's stem cells, such as the effect of applying hormones or the response in stress situations.
The quiescent centre: a stem cell insurance policy
Plants have a set of stem cells at the tip of the taproot that give the taproot the ability to grow indefinitely. Most of these cells divide rapidly, giving rise to other stem cells and the various cells that make up the root tissues, such as the epidermis or vascular tissue. However, at one end of this niche are a few stem cells that divide much more slowly, which is why the area they occupy has been called the quiescent centre.
Every time a cell duplicates its genetic material in order to divide itself, it runs the risk of incorporating errors, mutations that can have negative consequences for the organism. To deal with this, the stem cells in the quiescent centre build a safeguard, a reservoir of genetically safe cells. If necessary, these cells can "wake up" and divide to fill the stem cell niche.
It is precisely in these few cells in the quiescent centre that the BRAVO and WOX5 proteins use their important function by repressing cell division. Isabel Betegón-Putze, first author of the article, explains the experiments she carried out during her doctoral thesis to reach this conclusion: "We created Arabidopsis plants with the BRAVO and WOX5 genes mutated simultaneously and observed that they had less capacity to regenerate roots, which were shorter and less abundant".
Under situations of severe or prolonged stress, two types of response occur in the stem cell niche: the death of fast-dividing cells and the activation of quiescent centre cells. Thus, for example, the cells of the quiescent centre are activated after a cut in the root cap, or after freezing or lead poisoning of the root. When doing so, it is possible to replace the dead stem cells and still ensure the growth and proper development of the root, which in turn ensures the nutrition and support of the plant.
Understanding the molecular mechanisms that regulate these processes is key to obtain more resilient crops, especially in the current situation, when climate is getting more extreme every time.
An extraordinary source of youth
Plants, unlike animals, can create new organs, (leaves, flowers, etc.) at an adult age, and they keep growing during their whole life, which can last more than 2,000 years. Animal and plant stem cells seem to use similar strategies to solve similar biological problems. However, molecular processes that regulate these seem to be different. Understanding these differences can be useful to design strategies in medicine and cosmetics that slow down ageing and promote the regeneration of the damaged tissue. This study and others will shed light on advancing in this direction.
