MANNOSE-6-PHOSPHATE REDUCTASE, A KEY ENZYME IN PHOTOASSIMILATE PARTITIONING, IS ABUNDANT AND LOCATED IN THE CYTOSOL OF PHOTOSYNTHETICALLY ACTIVE-CELLS OF CELERY (APIUM-GRAVEOLENS L) SOURCE LEAVES

Mannitol, a major photosynthetic product and transport carbohydrate in many plants, accounts for approximately 50% of the carbon fixed by ce lery (Apium graveolens L.) leaves. Previous subfractionation studies o f celery leaves indicated that the enzymes for mannitol synthesis were located in the cytosol, but these data are inconsistent with that pub lished for the sites of sugar alcohol synthesis in other families and taxa, including apple (Malus) and a brown alga (Fucus). Using antibodi es to a key synthetic enzyme, NADPH-dependent mannose-6-phosphate redu ctase (M6PR), and immunocytochemical techniques, we have resolved both the intercellular and intracellular sites of mannitol synthesis. In l eaves, M6PR was found only in cells containing ribulose-1,5-bisphospha te carboxylase/oxygenase. M6PR was almost exclusively cytosolic in the se cells, with the nucleus being the only organelle to show labeling. The key step in transport carbohydrate biosynthesis that is catalyzed by M6PR displays no apparent preferential association with vascular ti ssues or with the bundle sheath. These results show that M6PR and, thu s, mannitol synthesis are closely associated with the distribution of photosynthetic carbon metabolism in celery leaves. The principal role of M6PR is, therefore, in the assimilation of carbon being exported fr om the chloroplast, and it seems unlikely that this enzyme plays even an indirect role in phloem loading of mannitol.