Links between Polysaccharide side chains and mechanical properties
Résumé
Plant cell walls are made of interacting networks of polysaccharides (cellulose, hemicelluloses, pectins) and some structural proteins that greatly contribute to cell and organ mechanical properties. Various enzymes remodel these networks with consequences on the wall rheology during cell growth and development. In fleshy fruits, these remodeling together with turgor pressure and spatial organization of cells in tissues determine texture. Fruit genetic, physiology and development modulate the cell wall composite structure and enzyme remodeling and are at the origin of a large range of textures hardly controlled. To date, beside the concurrent pectin degradation and fruit softening, relationships between cell wall assemblies and fruit texture are scarcely known. To establish roles of specific cell wall polysaccharides on the cell wall mechanical properties, spatially homogeneous parenchyma from contrasted texture Golden Delicious (Go) and Granny Smith (Gr) apples were sampled and vacuum infused with enzymes in buffer solution aimed at maintaining and homogenising turgor pressure and limiting oxidation. Glycoside hydrolases (a-fucosidase, a- and ß- galactosidase, a-arabinofuranosidase and combinations of fucosidase and galactosidases) were used to study the impact polysaccharide side chains on parenchyma mechanical properties. The mechanical assay consisted in a sinusoidal deformation in elastic domain at 1 Hz frequency to yield the storage modulus (representing the elastic behaviour) and phase angle (describing the viscous behaviour) of apple samples. The temporal evolution of mechanical properties was measured every hour over 5 h of enzymatic degradation and modelled by second order polynomials. The equations coefficients of the curves described the change in samples mechanical parameters during enzymatic treatments. Comparing the chemical composition and structure of cell wall polysaccharides in treated and control samples assessed the enzyme action. Principal Component Analysis (PCA) was applied on all data to identify relations between specific polysaccharide networks and structures with cell wall mechanical properties. Enzymatic treatments induce modifications of mechanical parameters leading to time dependant modifications in viscous and elastic behaviour. Due to different cell wall polysaccharide compositions and structures, the extent of conversion from elastic to viscous material differed for Go and Gr. In particular, altering ß-galactose decreased storage modulus and increased phase angle for Go, but for Gr, it increased the storage modulus but kept constant phase angle. Altering fucose, decreased storage modulus and increased phase angle for Go and Gr. Altering fucose and ß-galactose simultaneously emphasised fucose induced effects. These results emphasize the key roles of side chains in the regulation of mechanical properties whether directly in changing polysaccharides conformations and interactions or indirectly in regulating endogenous enzymes involved in the remodelling of networks assemblies. With these contrasted effects in Go and Gr, these results illustrate the diversity and complexity of apple cell walls with regard to their role on texture. The experimental framework developed in this study opens the way to explore the functions of cell wall polysaccharide structure on mechanical properties emerging at the tissue scale.