Etude de l'organisation des lignines, du réseau polysaccharidique et des complexes lignines-polysaccharides des parois de graminées – MAGIC

In March 2007, the European Council announced a 10% incorporation rate of biofuels. Such an objective will be impossible to reach if 2nd generation biofuels are not rapidly effective. It becomes then imperative to resolve the technical locks that restrict the use of lignocellulosic biomass. Improving the whole process efficiency requires a better knowledge of the key structural factors governing enzyme accessibility to cellulose within the plant cell wall. Most bioenergetics resources are grasses. That's why our present project focuses on grass cell wall structural specificities using maize as model. Cell wall is composed by two main classes of constituents: polysaccharides (cellulose and hemicelluloses) and phenolic compounds (lignin and p-hydroxycinnamic acids). Most past studies devoted to lignified walls have been carried out with deciduous or coniferous wood species as lignins detrimentally affect the industrial conversion of wood to pulp. However, grass cell walls display specific structural features that make them quite distinct from non grass cell walls. Accordingly, to fully capitalize the potential of these abundant and renewable resources for conventional uses or innovative future ones requires an in-depth understanding of the mechanisms leading to the complex wall assembly from both the lignin and polysaccharidic point of view. The proposed program aims at providing such an in-depth insight into the biochemical processes critical to the formation and fractionation of lignified grass cell walls. Lignified grass cell walls are typified by a sophisticated organization of wall polymers. It specifically involves p-hydroxycinnamic acids, namely ferulic and p-coumaric acids. Ferulic acid, ester-linked to arabinoxylans, is incorporated into lignins at the onset of lignification, thereby acting as a covalent cross-link between lignins and hemicelluloses. p-Coumaric acid specifically acylates syringyl lignin units, which probably affects the lignin interacting capabilities with polysaccharides. Many questions remain to be addressed on the specific and probably major interplay between p-hydroxycinnamic acids and cell wall polymers and on the resulting organization and properties of grass lignocelluloses. We propose herein to combine biochemical analyses of lignified cell walls, molecular modelling of their interacting components and in vitro synthesis of artificial lignins, to better assess the complex assembly of these resources. The project will underpin the kinetic of the lignification process. It will rely on the comprehensive use of maize as a plant model devoted with a wide genetic variability susceptible to affect the plant cell wall organization and properties. It will involve the development of new methods for the characterization of lignin-polysaccharide complexes. Maize with distinct degradabilities will be analyzed by biochemical methods to identify structural traits that play a major role in cell wall organization and properties. The molecular mechanisms that control the formation of lignified cell walls will be studied by in vitro synthesis of artificial lignins, with a kinetic LC-MS and SEC monitoring of the oligomers formed upon peroxidasic oxidation. These strategies will provide original information as a basis of molecular modelling studies aiming at a novel and comprehensive view of the grass cell wall assembly. This multidisciplinary approach will allow us to evaluate to which extent and how the specific organization of grass lignocelluloses control their current and future uses.