The machinery governing protein production from DNA is finely regulated. Activating the right gene at the right time, or keeping it inactive, is vital for all organisms. In plants, for example, when should the genes necessary for production of roots, or stems or leaves be chosen, and how? "Co-repressors help prevent inopportune gene activation" said François Parcy, team leader at BIG. They make it possible to create a physical link between other actors involved in regulation—transcription factors—and proteins that remodel the DNA package." This package, called chromatin, is decompacted to make genes accessible, and it is sometimes maintained in a dense form when the genes are silent.

In this study, the scientists have characterized the structure and function of one of these co-repressors, called Topless. The latter is the founding member of a small family of co-repressors found only in plants and involved in many signaling pathways that are essential for plant development and their response to stress. Its inactivation in model plant Arabidopsis thaliana induces the formation of a plant in which the stem is replaced by a root (which inspired the name 'Topless' for the corresponding mutant).

The scientists (from BIG, ENS in Lyon, and EMBL) have analyzed the part of the Topless protein that allows for the interaction with its protein partners. They discovered a structure similar to that of rice protein, which is also found in seaweed. The scientists have shown that the ability to interact is kept isolated in algae, which shows that this primordial function goes way back in evolution. "Comparisons between the structure of Topless with animal co-repressors have also revealed that these different co-repressors are made up of domains that are quasi-identical, yet assembled differently" Parcy said. "This study is a good example of what Nobel Prize winner François Jacob called 'the DIY of evolution' in the construction of protein function. This study also provides keys for manipulating this essential function to better respond to environmental stresses and improve the adaptive potential of crop plants." At least plants would avoid finding themselves topless—their roots in the open and stemless—due to stress.