Tracking ectopic lignification in flax stems following scarification

When flax (Linum usitatissimum L.) stems are scarified, major changes occur in the organization of cell walls within the tissues that border the wound. We sought to characterize the plant's response using a variety of approaches, with a particular focus on lignin deposition within the peripheral fiber cell walls of the stem. Raman spectroscopy and imaging first showed that changes occurred in the polysaccharide matrix of the parenchyma and fiber cell walls. These changes were accompanied by rapid deposition of lignin which initially diffuses centripetally and then, once the vascular cambium was reached, propagates in a periclinal manner until 150 μm from the edges of the wounded zone. Lignin biosynthesis appears to be the result of a de novo activity, as demonstrated by the concomitant accumulation of transcripts corresponding to lignin biosynthesis genes. In addition, using bioorthogonal chemistry approaches, we showed that wounding had enhanced the capacity of fiber cell walls to incorporate modified lignin precursors, in parallel with an increase in transcripts corresponding to peroxidases in the cortical tissues. This incorporation potential was identical for the 3 different types of reporters tested. Our findings demonstrated that mechanical stress can trigger lignification, in a polarized manner within the bast fibers, providing insights into the plasticity of cell wall composition and the potential for modulating fiber properties in flax.