Impact of cadmium on early stages of flax fibre differentiation: Ultrastructural aspects and pectic features of cell walls
Résumé
The effect of 0.5 mM cadmium (Cd) was studied on the ultrastructural aspects and pectin features of the walls of flax cellulosic fibres when the thickening of secondary wall had just started in the hypocotyl of 10-day old seedlings. As seen by PATAg staining in controls, cell-wall formation displayed two distinct steps, secretion and remodelling, which did not occur simultaneously for all the neighbouring fibres. The inner part of the secondary wall, where the cellulose molecules had just been synthesized, appeared very reactive to PATAg. The outer part, where the cellulose fibrils associated in larger microfibril complexes, became non-reactive to PATAg. Under Cd treatment, we noticed some acceleration of fibre differentiation in terms of fibre number, wall thickness and yield. As revealed by PATAg staining, treated fibres exhibited a disturbed cell-wall texture, indicating a modified adhesion between the matrix polysaccharides and the cellulose microfibrils. The Cd impact on the distribution of highly methylesterified homogalacturonans (recognized by JIM7 antibody) was moderate in the cell junctions and low in the primary wall and outer part of secondary wall. The data meant that no early deesterification occurred in these domains, a behaviour related to the specificity of the CW-II metabolism. No large redistribution of low esterified homogalacturonans (recognized by JIM5 antibody) happened either. In parallel, the amount of uronic acid significantly increased in the so-called H2SO4 cell-wall extract, indicating a Cd impact on pectin structure not detected by JIM5 or JIM7 antibodies.