A specific protein complex from plant stem cells regulates their division and response to stressBRAVO and WOX5 proteins, which are involved in the control of quiescent centre stem cells' division in plant roots, need each other for these cells to function properly
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, has discovered that two plant stem cell proteins, known for their role in the correct development of the root, physically interact and regulate each other to avoid cellular division. The study, result of fifteen years of continued research carried out by the two researchers, reveals that these two proteins, known as BRAVO and WOX5, act in a specific manner in a small group of stem cells, and that their interaction is key to the plant’s survival under genomic and environmental stress factors like extreme cold, heat, or floods. The results, obtained with the model plant Arabidopsis thaliana, have recently been published in the high impact journal Molecular Systems Biology.
In the same way that the proteins BRAVO and WOX5 need each other to function properly, this discovery could not have been possible without uniting the knowledge and the academic disciplines provided by both research teams: the biochemistry, genetics and cellular biology, on one hand, and the mathematical modelling, on the other.
“Previous work from my team and others had demonstrated that the loss of either one of the proteins, BRAVO or WOX5, produced the root stem cells division. However, their molecular connection was not yet understood,” explains Ana I. Caño Delgado.
“In general, genetic regulations involve a complexity that is often not intuitive and which is only understood by means of mathematical models and computer simulations. The mathematical models we created could provide a sense of the great amount of gathered data by the CRAG team”, adds Marta Ibañes, researcher at the UB Institute of Complex Systems.
These new mathematical models will allow now the in silico experimentation, creating hypothetical situations that could occur in the root’s stem cells, such as the effect of applying hormones or the possible responses during stress situations.
The quiescent centre: a reservoir of stem cells
Plants have a set of stem cells in the tip of the primary root that gives them the ability to grow indefinitely. Most of these cells divide at a fast pace, giving rise to other stem cells and the various cells that form 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 replication errors, mutations that can have negative consequences for the organism. To deal with this, the stem cells of 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 of the quiescent centre that the BRAVO and WOX5 proteins exercise their important function: suppressing cell division.
“We created arabidopsis plants with simultaneous mutations in BRAVO and WOX5 genes and we observed that they had less capacity to regenerate the roots, which were shorter and less abundant”, explains Isabel Betegón-Putze, first author of the article, who carried out these experiments during her PhD.
Under situations of severe or prolonged stress, two types of response occur in the stem cell niche: the death of the fast-dividing stem cells and the activation of the quiescent centre cells. Thus, for example, the cells of the quiescent centre are activated after a cut at the root apex, or after freezing or lead poisoning of the root. When doing so, they replace the dead stem cells allowing the root to keep growing and developing correctly, which in turn guarantees the nutrition and support of the plant.
Understanding the molecular mechanisms that regulate these processes is key to obtaining 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 form new organs (leaves, flowers, etc.) at an adult age and, furthermore, they keep growing during their whole life (which can last more than 2000 years!). Animal and plant stem cells seem to use similar strategies to solve similar biological problems. However, the 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 cellular aging and promote the regeneration of damaged tissue. This study and others led by Ana I. Caño-Delgado are one step forward in this direction.
Reference article: Isabel Betegón-Putze, Josep Mercadal, Nadja Bosch, Ainoa Planas-Riverola, Mar Marquès-Bueno, Josep Vilarrasa-Blasi, David Frigola, Rebecca C Burkart, Cristina Martínez, Ana Conesa, Rosangela Sozzani, Yvonne Stahl, Salomé Prat, Marta Ibañes, Ana I Caño-Delgado. Precise transcriptional control of cellular quiescence by BRAVO/WOX5 complex in Arabidopsis roots Mol Syst Biol (2021)17:e9864 https://doi.org/10.15252/msb.20209864
About the authors and the funding of the study: In addition to the authors from CRAG and from the University of Barcelona, authors from Heinrich-Heine University (Düsseldorf, Germany), Centro Nacional de Biotecnologı́a (Madrid, Spain); University of Florida (Gainesville, USA), and North Carolina State University, (Raleigh, USA) have also collaborated in the study. The research done in the Caño-Delgado and Ibañes’ laboratories has been funded by projects from the Spanish Government, the European Regional Development Funds (ERDF), the European Research Council and the Government of Catalonia.