A Pathway for Photosynthetic Carbon Flow to Mannitol in Celery Leaves

In the polyol producing plant, celery (Apium graveolens L.), mannitol is a major photosynthetic product and a form in which carbohydrate is translocated. Measurements of whole leaf extracts of celery indicated substantial activity of the following enzymes: mannose-6-P reductase, mannose-6-P isomerase, mannitol-1-P phosphatase, and nonreversible glyceraldehyde-3-P dehydrogenase. The activities of these enzymes were either undetectable or very low in the nonpolyol producing plants, Secale cereale L. (rye) and Vigna mungo (L.) Hepper (black gram).Mesophyll protoplasts were enzymically isolated from celery leaves, broken with a Yeda press and the intracellular localization of the above enzymes for mannitol synthesis studied following differential and/or sucrose density gradient centrifugation of the protoplast extract. These data suggested the enzymes involved in mannitol synthesis are exclusively localized in the cytoplasm. Ninety-five to 100% of the activity of these enzymes, along with the cytoplasmic marker enzyme phosphoenolpyruvate carboxylase, was found in the cytosolic fraction.We propose the pathway of photosynthetic carbon flow from triose-P to mannitol in celery occurs via fructose-6-P, mannose-6-P, and mannitol-1-P; these final reactions being catalyzed by the cytoplasmic enzymes, mannose-6-P isomerase, NADPH-dependent mannose-6-P reductase, and mannitol-1-P phosphatase, respectively. The requirement for NADPH may be met via the cytoplasmically located NADP-linked nonreversible glyceraldehyde-3-P dehydrogenase.