Résumé
Enzymatic saccharification of lignocellulosic biomass is a bottleneck for its use in the green chemistry. Differences in degradability were observed according to plant species but also according to cellular type within a plant. Such variability was related to heterogeneity in composition. Lignin amount or lignin structure as well as its interaction with polysaccharides were pointed out. Considering anatomical plant heterogeneity, lignin structure must be analysed at different scale (from cell wall to tissue or organ). Spectral methods, in particular Raman spectroscopy, are of particular interest due to (i) the high Raman scattering of phenolic compounds and (ii) the ability to get spatially resolved information at micrometer scale. The objective of this study was first to create a sample data set allowing spectral assignment of lignin structural features and secondly to evaluate the ability of Raman spectroscopy to characterize lignin at tissue scale in planta. Plant cell wall materials were prepared from different sources in order to cover a wide compositional range and dioxane lignins were isolated from these cell walls. All these samples were analyzed by wet chemistry (sugar analysis, lignin amount and composition, hydroxycinnamic acid content) and by Raman spectroscopy. Syringyl to guaiacyl ratio could be evaluated in the 1270-1330 cm-1 spectral range. In cell wall material, while the lignin amount was lower than 24%, spectra were dominated by lignin scattering in the 1200-1800 cm-1 spectral range. Polysaccharides fingerprint was observed in the 800-1100 cm-1 region. Syringyl to guaiacyl ratio was still detectable. Cell wall compositional changes of ear-bearing maize internodes were then analyzed at three developmental stages. Lignification occurred between the 9-leaf stage and female flowering stage in parenchyma cell walls. Difference in cell wall composition in different tissues (phloem, sclerenchyma) was also pointed out.