ent-Kaurene synthase is located in proplastids of meristematic shoot tissues

Previous studies have indicated that entkaurene synthase (KS) is located in the proplastid stroma of rapidly dividing plant tissues. Here we present further and more direct evidence for this hypothesis and follow the activity of KS throughout the entire vegetative growth period of wheat plants. During germination of wheat caryopses, KS activity was maximal for a short period culminating on the third day in the scutellum and on the fourth day in the meristematic shoot base. Throughout further development of the wheat plant, KS was found in the nodes but not in the internodes or leaves. The activity of KS in each node increased when the internode above it was elongating and decreased again when this internode had almost reached its final size. The correlation of KS activity with growth was particularly striking in the case of tiller development from the fourth node: here KS activity had already declined, but was restored when the tiller began elongating. Electron micrographs of wheat seedling tissue with high KS activity (shoot base) showed the presence of proplastids, whereas electron micrographs of tissue without such activity (primary leaves) showed only developing or mature chloroplasts. On density-gradient centrifugation, the plastids that yielded stroma preparations with KS activity became distributed over a greater density range and also had a lower NADP(+)- glyceraldehyde 3-phosphate dehydrogenase: shikimate oxidoreductase ratio than plastids yielding KS-inactive stroma preparations. Pea shoot apices contain both proplastids and mature chloroplasts. Here also, KS activity was associated with the stroma of plastids with characteristics similar to those of the wheat proplastids, indicating that KS is associated with proplastids in pea shoot apices as well. We conclude that the stromal location of KS may be a general feature of proplastids in rapidly dividing tissue.