Jean Dénarié’s early career was a defining period.
First was the appeal of research, which he describes as a universal, unifying intellectual activity that offers experimentally validated answers to questions. Early in his education, he turned to the hard sciences, including biology, which study mechanisms at molecular level, where there is very little uncertainty in scientists’ interpretation.
Another major factor was Dénarié’s interest in a particular area: at the end of his Maîtrise postgraduate degree, he knew he wanted to work on nitrogen cycle genetics. The nitrogen cycle is fundamental to life on Earth; he thought genetics could provide insight into the workings of the cycle. Eventually the knowledge gained might be harnessed to improve human food and nutrition and preserve the environment. From that point on, with unwavering determination, Dénarié did everything possible to achieve these goals.
Having joined INRA at the age of 22 in the “Soil Microbiology” laboratory in Versailles, it was five years before he was able to introduce genetics into the design of his model: the symbiosis between legumes and nitrogen-fixing Rhizobium bacteria. He then began working with Pierre Boistard’s team at the “Plant Pathology” unit in Versailles, which was using molecular genetics to study another model: interactions between plants and pathogens. The approach and tools were the same, and so in 1981, a joint laboratory was set up by INRA and CNRS in Toulouse. Support was provided by directors from both establishments: Jacques Poly and André Berkaloff.
Thus began a long and fruitful collaboration with talented cytologist Georges Truchet. This interdisciplinary teamwork then extended to biochemistry, with Jean-Claude Promé, a mass spectrometry expert, and led to the major discovery of “Nod” factors. These factors are signalling molecules secreted by Rhizobium that cause the formation of outgrowths, or “nodules”, in legume roots, in which bacteria reproduce, feed off the plant and in return provide the plant with nitrogen in usable form. These molecules are responsible for the specific recognition of Rhizobium by host plants; they are the “key” that allows the bacterium to penetrate into the plant.
This research led to several surprising discoveries. The first was the structure of the bacterial factors, which does not resemble that of standard plant growth regulators, such as auxins or cytokinins, as was originally thought. Nod factors are lipid-chitin-oligosaccharides; the fact that they contain chitin is all the more surprising as it is produced neither by plants nor by bacteria but usually by fungi! As research was pursued, the scientists advanced the hypothesis that Nod factor structure had been “borrowed” by the Rhizobium-legume system from another, much older system in evolutionary terms: the symbiosis between endomycorrhizal fungi and plants. The research that followed confirmed the similarities between the two symbiotic systems.
The second discovery was that purified Nod factors can stimulate the development of root systems in crop-growing conditions. This observation led Dénarié and his colleagues to file several patents. Partnering with an industrial company made it possible to demonstrate that applying low concentrations of Nod factors to seeds (0.1 mg is sufficient for application to one hectare of seed) stimulates the formation of nitrogen-fixing nodules and increases soybean, peanut, pea and alfalfa yield. Production of Rhizobium inoculum enriched with Nod factors was launched in 2004 and 2006, for use on over one million hectares. The product was first marketed in the United States, where legumes are a major crop, and is now being developed in Europe. Nevertheless, says Dénarié, “legumes are under-used in Europe, where there is a long-established preference for cereals. Legumes, however, reduce the need for nitrogen fertiliser in crop rotations. Given that nitrogen fertilisation accounts for about 50% of the energy cost of crops, the advantages of legumes are obvious in terms of saving fossil energy and reducing global warming.”
To facilitate the study of legumes, Dénarié developed a model legume that has since been adopted by the international scientific community: Medicago truncatula, for which genome sequencing is underway. Research on this model should facilitate improvement of legumes in the near future. Dénarié’s team has already identified the major genes of the plant involved in the recognition of Nod factors.
Currently, Dénarié is leading research on the characterisation of other factors that carry major agricultural stakes. For example, “Myc” factors are molecular signals in endomycorrhizal symbiosis, which is important for plant phosphate and water nutrition. Dénarié has given himself a few years to meet this challenge and answer several new questions: are there structural similarities between Myc and Nod factors? Are Myc factors also growth regulators that could be used to stimulate the development of plant roots?
Jean Dénarié has contributed and continues to contribute to the recognition of INRA worldwide. He is an officer of the honorary Ordre du Mérite Agricole, has twice been awarded a Grand Prix de l’Académie des Sciences (1993 and 2005) and received a “French citation laureate” (1981-1998 period) from the ISI (Institute for Scientific Information). He has been elected a member of the EMBO (European Molecular Biology Organisation) and the Academia Europaea.
In 2007, he was distinguished by the INRA Agricultural Research Award for Scientific Excellence, attributed by an international jury for his work as a whole and for a career as an internationally renowned researcher who has made an exceptional contribution to the influence of agricultural research.
Jean Dénarié with his team, © Inra G. Cattiau
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