Interrelation between Lignin Deposition and Polysaccharide Matrices during the Assembly of Plant Cell Walls

Abstract: The modifications caused by genetic down‐regulation of the enzyme cinnamoyl CoA reductase (CCR) from monolignol biosynthetic pathways on tobacco and Arabidopsis thaliana were investigated at the ultrastructural level. A typical result was that the same transformation led to similar abnormality in secondary wall formation of fibres in both plants. The cell wall alterations mainly consisted in an important disorganization and loosening of cellulose microfibrils in the inner part of the S2 layer. This inability of the transformants to form a coherent cell wall coincided with a lack of synthesis of non‐condensed forms of lignin in this disorganized region of the wall, as demonstrated by immunolabelling of lignin subunits. A similar disorganization was observed during fibre wall formation in the differentiating tissues of young Populus and A. thaliana plants. The transitory lack of organization of cellulose microfibrils, also coincided with a depletion in non‐condensed forms of lignins. These results suggest that such lignin substructures may be involved in the cohesion of secondary walls during cell wall biogenesis. The mutual influence of the cellulose‐hemicellulose environment and monolignol local polymerization is discussed.