Decrease of polygalacturonic acid synthase during xylem differentiation in sycamore

Abstract

The activity of a polygalacturonic acid synthase, a membrane bound enzyme, was measured in particulate preparations obtained from homogenates of cambial cells, differentiating xylem cells and differentiated xylem cells isolated from actively growing trees of sycamore ( Acer pseudoplatanus). The specific acitivity of the enzymic system fell at least 6–10-fold as cells differentiated from the vascular cambium to xylem. The percentage of radioactivity incorporated as galacturonic acid was 88% for cambium, 69% for differentiating xylem and 57% for differentiated xylem. The remainder of the radioactivity was in glucuronic acid. UDP- d-galacturonic acid-4-epimerase (EC 5.1.3.6) in the membrane preparation, therefore, probably enabled incorporation of radioactivity in the form of glucuronic acid by other synthases into polymers which are characteristic of secondary walls. Nevertheless, a considerable decrease of polygalacturonic acid synthase activity occurred which was correlated with the cessation of pectin deposition. This contrasts with the marked increase observed in xylan synthase activity as the cells differentiate.