PLANT PHYSIOLOGY , Vol 102, Issue 2 345-356, Copyright © 1993 by American Society of Plant Biologists
|
METABOLISM AND ENZYMOLOGY |
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
J. D. Everard, V. R. Franceschi and W. H. Loescher
Department of Horticulture, Michigan State University, East Lansing, Michigan 48824-1325 (J.D.E., W.H.L.)
Mannitol, a major photosynthetic product and transport carbohydrate in many
plants, accounts for approximately 50% of the carbon fixed by celery (Apium
graveolens L.) leaves. Previous subfractionation studies of celery leaves
indicated that the enzymes for mannitol synthesis were located in the
cytosol, but these data are inconsistent with that published for the sites
of sugar alcohol synthesis in other families and taxa, including apple
(Malus) and a brown alga (Fucus). Using antibodies to a key synthetic
enzyme, NADPH-dependent mannose-6-phosphate reductase (M6PR), and
immunocytochemical techniques, we have resolved both the inter-cellular and
intracellular sites of mannitol synthesis. In leaves, M6PR was found only
in cells containing ribulose-1,5-bisphosphate carboxylase/oxygenase. M6PR
was almost exclusively cytosolic in these 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 tissues or with the bundle sheath. These results
show that M6PR and, thus, 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 from the chloroplast, and it seems unlikely that this enzyme plays
even an indirect role in phloem loading of mannitol.
