Published on 6 July 2015

The study of the regulation of cell proliferation in plants opens up new perspectives.

Interview with Cécile Raynaud, a researcher at CNRS, Institute of Biology of plants
The cell cycle is the process by which cells multiply. This cycle consists of the interphase during which the cell grows and duplicates its genetic material and a phase in which the cell divides (mitosis or meiosis) to give rise to two identical daughter cells (in the case of mitosis). Daughter cells reproduce this cycle, and so on. Interphase is divided into three phases: the S phase (S for synthesis), during which the cell doubling its DNA quantity and the G1 and G2 phases (Gap) before and after the S phase during which checkpoints help to stop the cycle if errors occurred during the previous phase, and integrate external signals that can positively or negatively regulate the cell cycle. If not repairable defect of DNA, the cell evolves to death, although the plants seem able to manage the anomalies differently. Autopsy of a complex mechanism.
You have received the bronze medal of the CNRS in 2014 for your work on cell division. What are the research issues around this concept?
Issues related to cell proliferation are multiple. First, it is fundamental to understand how the balance between cell proliferation and differentiation is regulated, allowing the harmonious development of plants. During the formation of organs, cells lose their ability to divide in order to specialize in certain functions, it is the phenomenon of differentiation. Plants maintain throughout their lives cells capable of dividing in two types of cells, they are called meristems: stem meristems that are causing stems, leaves and flower and root meristems.

This balance between division and differentiation is also the basis for the incredible plasticity of plant development: Although the basic organization of the plant remains invariant, its architecture can profoundly change depending on light conditions, temperature etc ... The plants are sessile organisms, they can not move to escape adverse conditions, and their survival depends on the ability to adapt to changes around them. The regulation of cell proliferation is one of the key elements of this process. On the other hand, the meristematic cells that ensure continuous development of new organs, retain their ability to proliferate throughout the life of the plant, which can last up to thousands of years for some species, and gametes, these reproductive cells from meiosis late differ from these cells. The plants thus appear to have an exceptional ability to protect the integrity of their genome over the cell divisions, although each replication step is prone to errors. One of the mechanisms proposed to explain this observation is that the meristems, cells contain their center tanks that divide much more slowly than the cells of the periphery from which to initiate new organs such as leaves. This division idling would protect their genome too many injuries. Another possibility is that specific mechanisms exist to ensure the plants a particular loyalty in the DNA replication.

Finally, over the divisions, the cells acquire a specific identity although they all have the same genome. This is based on the establishment of an own gene expression program for each cell type. This program depends on the presence of specific proteins called transcription factors that directly regulate gene transcription but also changes in chromatin organization. Indeed, the DNA is associated with proteins known as histones that allow its compaction in the nucleus. Changes in these histones or in DNA itself called chromatin markings may promote the release and condensation of a given locus, and thus play a fundamental role in regulating gene expression. These markings must be reproduced in part during DNA replication, but the S phase of the cell cycle could also be favorable to the deposition of new brands.

You participate in ICAR2015 Congress. What can we learn plants on replication and cell death?
The molecular mechanisms that govern the replication are well known in animals and yeast, and very poorly in plants. Indeed, most of the proteins involved seem preserved, and it is tempting to assume that the mechanisms are perfectly preserved. However, a number of observations suggest that replication in plants may have specific, so it is important to deepen its analysis.
Regarding cell death, it appears to involve widely own mechanisms to plants, and plays a crucial role in their response to virtually any stress. Understanding its regulation is therefore not only a fundamental interest but also obvious applications in agronomy.
What do you bring back into the field (problematic, experimental methods, collaborations ...)
Our group has obtained results that indicate that the assumption that proteins involved in replication function in plants just like in other eukaryotes is perhaps not fully justified. Moreover, the reproduction of chromatin marks during replication remains rather mysterious. The close interaction with Mr. Benhamed who is a specialist of epigenetics (mechanisms of reversible DNA modification) and co-leads the group with me offers us a unique opportunity to address these issues.
Can you explore fundamental mechanisms be useful for example to therapies?
Our research project is very fundamental and speak therapy would be largely premature. The study replication in plants, however, can provide valuable information on identifying new mechanisms maintaining genome integrity during the S phase. In addition, it is often possible to obtain viable mutants in plants that are lethal in animals, giving access to a more detailed functional study.
Finally unlike humans, plants do not develop cancer in the absence of infection with pathogens, the study of the regulation of cell proliferation in plants can therefore create opportunities in this area. Long-term agronomic applications can also be envisaged, while advances are made in understanding the mechanisms that regulate proliferation in response to stress, or links between response to DNA damage and stress resistance which are becoming highlighted.