ENT-KAURENE SYNTHASE IS LOCATED IN PROPLASTIDS OF MERISTEMATIC SHOOT TISSUES

Previous studies have indicated that entkaurene synthase (KS) is locat ed in the proplastid stroma of rapidly dividing plant tissues. Here we present further and more direct evidence for this hypothesis and foll ow the activity of KS throughout the entire vegetative growth period o f wheat plants. During germination of wheat caryopses, KS activity was maximal for a short period culminating on the third day in the scutel lum and on the forth day in the meristematic shoot base. Throughout fu rther development of the wheat plant., KS was found in the nodes but n ot in internodes or leaves. The activity of KS in each node increased when the internode above it was elongating and decreased again when th is internode had almost reached its final size. The correlation of KS activity with growth was particularly striking in the case of tiller d evelopment from the forth node: here KS activity had already declined, but was restored when the tiller began elongating. Electron micrograp hs of wheat seedling tissue with high KS activity (shoot base) showed the presence of proplastids, whereas electron micrographs of tissue wi thout such activity (primary leaves) showed only developing or mature chloroplasts. On density-gradient centrifugation, the plastids that yi elded stroma preparations with KS activity became distributed over a g reater density range and also had a lower NADP(+)-glyceraldehyde 3-pho sphate dehydrogenase:shikimate oxidoreductase ratio than plastids yiel ding KS-inactive stroma preparations. Pea shoot apices contain both pr oplastids and mature chloroplasts. Here also, KS activity was associat ed with the stroma of plastids with characteristics similar to those o f the wheat proplastids, indicating that KS is associated with proplas tids in pea shoot apices as well. We conclude that the stromal locatio n of KS may be a general feature of proplastids in rapidly dividing ti ssue.