Abstract
Little information exists concerning the biochemical route of mannitol catabolism in higher plant cells. In this study, the role of a recently discovered mannitol 1-oxidoreductase (MDH) in mannitol catabolism was investigated. Suspension cultures of celery (Apium graveolens L. var dulce [Mill.] Pers.) were successfully grown on nutrient media with either mannitol, mannose, or sucrose as the sole carbon source. Cell cultures grown on any of the three carbon sources did not differ in relative growth rate, as measured by packed cell volume, but differed drastically in internal carbohydrate concentration. Mannitol-grown cells contained high concentrations of mannitol and extremely low concentrations of sucrose, fructose, glucose, and mannose. Sucrose-grown cells had high concentrations of sucrose early in the growth cycle and contained a substantial hexose pool. Mannose-grown cells had a high mannose concentration early in the cycle, which decreased during the growth cycle, whereas their internal sucrose concentrations remained relatively constant during the entire growth cycle. Celery suspension cultures on all three carbon substrates contained an NAD-dependent MDH. Throughout the growth cycle, MDH activity was 2- to 4-fold higher in mannitol-grown cells compared with sucrose- or mannose-grown cells, which did not contain detectable levels of mannitol, indicating that MDH functions pre-dominantly in an oxidative capacity in situ. The MDH activity observed in celery cells was 3-fold higher than the minimum amount required to account for the observed rate of mannitol utilization from the media. Cultures transferred from mannitol to mannose underwent a decrease in MDH activity over a period of days, and transfer from mannose to mannitol resulted in an increase in MDH activity. These data provide strong evidence that MDH plays an important role in mannitol utilization in celery suspension cultures.