Abstract
Mannitol represents a major carbon storage compound in celery petioles. Recently, a novel mannitol dehydrogenase was isolated from celeriac (Apium graveoiens L. var. rapaceum) root tips and partially purified to a specific activity of 210 μmol/h/mg protein. This NAD-dependent enzyme catalyzes the oxidation of mannitol to mannose. not fructose, and this is uniquely different from the mannitol oxidation in lower organisms. Data are presented that show that the enzyme is also present in celery (Apium graveoiens L. var. dulce) sink tissue. The product of the mannitol oxidation in celery was determined to be mannose. Previous studies suggested that differences in fertilizer concentration in the root zone of celery could alter carbon partitioning between mannitol and hexose sugars and change enzyme activities. In this study, plants were grown hydroponically at two different salt concentrations, E.C.=2.7 mS/cm and E.C.=6.0 mS/cm. A high mannitol to hexose ratio was observed in celery petioles of plants grown at high salt concentration, supporting the hypothesis that the salt environment might alter both mannitol and hexose pools in a coordinate way. Plants grown at both salt concentrations did not differ in root or shoot fresh weight. These data support the hypothesis that mannitol might be involved in osmoprotection by acting as a compatible solute. Mannitol accumulation in celery petioles might be, in part, regulated through a mannitol catabolizing pathway involving mannitol oxidation to mannose by the NAD-dependent 1-oxidoreductase, mannose phosphorylation by mannose kinase, mannose-6-P conversion to fructose-6-P by phosphomannose isomerase and further conversion to hexoses through a sucrose intermediate. The novel mannitol 1-oxidoreductase might therefore be a key enzyme in controlling mannitol levels in sink tissues.
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