Dynamic changes in cell wall polysaccharides during wheat seedling development.

Changes in arabinoxylan content and composition during development of wheat seedlings were investigated. The cell walls isolated from the seedlings showed an increasing content of arabinoxylan during development, which could be correlated to increased activity of xylan synthase and arabinoxylan arabinosyltransferase. Arabinoxylan changed from initially having a high degree of arabinose substitution to a much lower degree of substitution. beta-Glucan was present in the walls at the early stages of development, but was actively degraded after day 4. Increased deposition of arabinoxylan did not take place until beta-glucan had been fully degraded. Ferulic and p-coumaric acid esters were present at all points but increased significantly from day 3 to 6, where lignification began. Ferulic acid dimers did not appear in the cell wall until day three and the different ferulic acid dimers varied in the course of accumulation. The ratio of ferulic acid dimers to free ferulic acid was maximal at the time when the wall had been depleted for beta-glucan, which had not yet been fully replaced by arabinoxylan. This pattern suggests a role for ferulic acid dimers in stabilizing the wall during the transition from a flexible to a more rigid structure. To investigate if the same changes could be observed within a single seedling, 7 day old seedlings were divided into four sections and the walls were analyzed. Some of the changes observed during the seedling development could also be observed within a single seedling, when analyzing the segments from the elongation zone at the base to the top of the leaf. However, the expanding region of older seedlings was much richer in hydroxycinnamates than the expanding region of younger seedlings. Diferulic acids are stabilizing the wall in the transition phase from an expanding to a mature wall. This transition can take place in different manners depending on the cell and tissue type.