Possible control sites of polysaccharide synthesis during cell growth and wall expansion of pea seedlings (Pisum sativum L.)

The activities of the enzymes of uridine diphosphate sugar interconversions (UDP-D-glucose 4-epimerase, UDP-D-glucuronate 4-epimerase, UDP-D-xylose 4-epimerase, UDP-D-glucose dehydrogenase and UDP-D-glucuronate decarboxylase) were measured by using enzymic preparations (protein precipitated between 40–65% (NH₄)₂SO₄ saturation) isolated from segments at different stages of elongation of the third internode of pea seedlings. All enzymic activities increased from dividing and non-elongated cells to fully elongated cells. At all stages of growth, the specific activity or the activity per cell of UDP-D-glucose dehydrogenase was much lower than that of UDP-D-glucuronate decarboxylase and this may represent a controlling step in the formation of UDP-D-xylose. During elongation, changes were also found in the activities of the epimerases. These could be correlated with the corresponding variations which occur in the chemical structure and physical properties of pectins during cell wall extension. However, the high levels of the epimerases present in cells which have completed elongation growth suggest that pectin synthesis is mainly controlled at the sites of the synthetase reactions.